Sloshing Assessment Guidance Document for Membrane · PDF fileAdditional Design Procedures...

Additional DesignProcedures

Sloshing Assessment GuidanceDocument for Membrane TankLNG Operations

Version 2.0May 2009

ShipRightDesign and construction

Additional DesignProcedures

Sloshing Assessment GuidanceDocument for Membrane TankLNG Operations

Version 2.0May 2009

ShipRightDesign and construction

Lloyd’s Register, 71 Fenchurch Street, London EC3M 4BS.

Switchboard: +44 (0)20 7709 9166, Fax: +44 (0)20 7488 4796, Web site: www.lr.org

Direct line: +44 (0)20 7423 + extension no., Fax: +44 (0)20 7423 2061, E-mail: [email protected]

Lloyd’s Register Marine Business Stream 71 Fenchurch Street London EC3M 4BS Telephone 020 7709 9166 Telex 888379 LR LON G Fax 020 7488 4796

Document History

Document Date Notes February 2009 Draft version 1.5 released for industry comment May 2009 General release – version 2.0 Lloyd's Register, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as the ‘Lloyd's Register Group’. The Lloyd's Register Group assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Lloyd's Register Group entity for the provision of this information or advice and in that case any responsibility. Lloyd’s Register Marine Business Stream is a part of Lloyd’s Register. Lloyd’s Register is an exempt charity under the UK Charities Act 1993. © Lloyd’s Register, 2009

Sloshing Assessment Guidance Document for Membrane Tank LNG Operations, May 2009

LLOYD’S REGISTER i

Contents

Nomenclature...............................................................................................................................................................vi

Introduction.......................................................................................................................................... 1

Chapter 1: Introduction......................................................................................................................................... 1 ■ Section 1: Guidance Document............................................................................................................................ 1 ■ Section 2: Application ........................................................................................................................................... 1 ■ Section 3: Assessment Procedures ...................................................................................................................... 2

3.1 Initial Screening Phase ........................................................................................................................ 2 3.2 Comparative Procedure...................................................................................................................... 3 3.3 Proven Good Service Record ............................................................................................................. 4 3.4 Enhanced Comparative Procedure ................................................................................................... 5 3.5 Absolute Procedure............................................................................................................................. 5 3.6 Simplified Absolute Procedure.......................................................................................................... 6

■ Section 4: Sloshing Phenomena ........................................................................................................................... 6 ■ Section 5: Documentation..................................................................................................................................... 8

Chapter 2: Sloshing Assessment Procedure ...................................................................................................... 9 ■ Section 1: Introduction.......................................................................................................................................... 9 ■ Section 2: Part A: Ship Motions Analysis........................................................................................................... 9 ■ Section 3: Part A: Determination of the Design Sloshing Loads................................................................... 11 ■ Section 4: Part B: Structural Assessment .......................................................................................................... 11 ■ Section 5: Part C: Acceptance Criteria .............................................................................................................. 13

Part A: Design Sloshing Loads ....................................................................................................... 15

Chapter 1: Design Sloshing Loads Overview ................................................................................................. 15 ■ Section 1: Introduction........................................................................................................................................ 15 ■ Section 2: Design Basis........................................................................................................................................ 15

Chapter 2: Ship Motions ..................................................................................................................................... 16 ■ Section 1: Introduction........................................................................................................................................ 16 ■ Section 2: Seakeeping Analysis.......................................................................................................................... 16 ■ Section 3: Tank Selection .................................................................................................................................... 16 ■ Section 4: Loading Conditions........................................................................................................................... 16 ■ Section 5: Wave Environment............................................................................................................................ 17 ■ Section 6: Ship Speeds and Wave Headings.................................................................................................... 19

Chapter 3: Model Tests ....................................................................................................................................... 20 ■ Section 1: Introduction........................................................................................................................................ 20 ■ Section 2: Model Testing Phases........................................................................................................................ 21

2.1 Initial Screening Phase ...................................................................................................................... 21


ii LLOYD’S REGISTER

2.2 Initial Screening Phase (Wave Height Sensitivity)........................................................................ 22 2.3 Refined Screening Phase................................................................................................................... 23 2.4 Design Phase ...................................................................................................................................... 23

■ Section 3: Sloshing Model Tank Set-Up ........................................................................................................... 23 3.1 Tank Model......................................................................................................................................... 24 3.2 Test Rig and Motion Generation ..................................................................................................... 24 3.3 Video Recording ................................................................................................................................ 24 3.4 Ullage Gas........................................................................................................................................... 24 3.5 Sensors ................................................................................................................................................ 25 3.6 Sensor Placement ............................................................................................................................... 25 3.7 Sensor Density.................................................................................................................................... 25

■ Section 4: Results Data Processing.................................................................................................................... 27 4.1 Introduction........................................................................................................................................ 27 4.2 Identification of Sloshing Impact Pressures................................................................................... 27 4.3 Pressure Curves and Scaling Laws ................................................................................................. 27 4.4 Data Processing of Sloshing Impact Pressures .............................................................................. 28 4.5 Load Area Processing........................................................................................................................ 29 4.6 Statistical Analysis Methods ............................................................................................................ 31 4.7 Derivation of Statistical Values for Extreme Sea States................................................................ 31 4.8 Derivation of the Statistical Values for Non-Extreme Sea States ................................................ 31 4.9 Design Sloshing Load........................................................................................................................ 35

Part B: Structural Assessment......................................................................................................... 38

Chapter 1: Structural Assessment Overview................................................................................................... 38 ■ Section 1: Introduction........................................................................................................................................ 38 ■ Section 2: Strength Assessment by Finite Element Analysis ......................................................................... 38 ■ Section 3: Strength Assessment by Experimental Methods........................................................................... 39 ■ Section 4: Review of Major Containment Systems ......................................................................................... 40

4.1 Plywood Box Type Containment System....................................................................................... 40 4.2 Layered Foam Type Containment System..................................................................................... 40

■ Section 5: Scope of Assessment ......................................................................................................................... 41 ■ Section 6: Documentation................................................................................................................................... 41

Chapter 2: Finite Element Analysis .................................................................................................................. 43 ■ Section 1: Introduction........................................................................................................................................ 43 ■ Section 2: General Finite Element Modelling .................................................................................................. 43 ■ Section 3: NO96 Containment System Finite Element Modelling ................................................................ 44

3.1 Box Covers and Bulkheads............................................................................................................... 44 3.2 Invar Primary and Secondary Membranes .................................................................................... 45 3.3 Mastic .................................................................................................................................................. 45 3.4 Connection between the Primary and Secondary Boxes.............................................................. 46 3.5 Spring Elements ................................................................................................................................. 46 3.6 Connection Rod to Hull Plating....................................................................................................... 46


LLOYD’S REGISTER iii

3.7 Connection between Adjacent Primary and Secondary Boxes.................................................... 47 3.8 Detailed Analysis Finite Element Models ...................................................................................... 49

■ Section 4: Mark III Containment System Finite Element Modelling............................................................ 50 4.1 Top and Bottom Plywood Covers ................................................................................................... 50 4.2 Primary and Secondary Insulation Layers of R-PUF.................................................................... 50 4.3 Slots in the R-PUF.............................................................................................................................. 50 4.4 Mastic Ropes ...................................................................................................................................... 50 4.5 Triplex Layer (Secondary Membrane Barrier)............................................................................... 51 4.6 Primary Membrane ........................................................................................................................... 51 4.7 Connectivity of the Containment System Components ............................................................... 53

■ Section 5: Hull Structure..................................................................................................................................... 53 5.1 Extent of the Hull Structure to be Modelled.................................................................................. 53 5.2 Modelling Details .............................................................................................................................. 55

■ Section 6: Materials ............................................................................................................................................. 55 ■ Section 7: Boundary Conditions........................................................................................................................ 55

7.1 NO96 Containment System.............................................................................................................. 55 7.2 Mark III Containment System.......................................................................................................... 56 7.3 Hull Structure..................................................................................................................................... 56

■ Section 8: Further Analysis Considerations..................................................................................................... 56 8.1 Thermal Effects .................................................................................................................................. 56 8.2 Accounting for Un-Modelled Components ................................................................................... 57 8.3 Added Mass Effects........................................................................................................................... 57

Chapter 3: Dynamic Structural Response........................................................................................................ 61 ■ Section 1: Introduction........................................................................................................................................ 61 ■ Section 2: Direct Dynamic FEA Analysis ......................................................................................................... 62 ■ Section 3: Indirect Dynamic FEA Analysis ...................................................................................................... 64

3.1 Static FEA............................................................................................................................................ 66 3.2 Dynamic FEA ..................................................................................................................................... 66 3.3 Deriving the Dynamic Amplification Factors (DAF).................................................................... 66 3.4 Structural Assessment....................................................................................................................... 67

■ Section 4: Repeated Impact Loads..................................................................................................................... 68

Chapter 4: Material Properties........................................................................................................................... 69 ■ Section 1: Summary............................................................................................................................................. 69 ■ Section 2: Material Properties ............................................................................................................................ 69 ■ Section 3: Plywood .............................................................................................................................................. 69

3.1 General ................................................................................................................................................ 69 3.2 FEA Modelling Requirements ......................................................................................................... 70 3.3 Stiffness Properties ............................................................................................................................ 70 3.4 Strength Properties............................................................................................................................ 71

■ Section 4: Reinforced Polyurethane Foam ....................................................................................................... 71 4.1 General ................................................................................................................................................ 71 4.2 FEA Modelling Requirements ......................................................................................................... 71


iv LLOYD’S REGISTER

4.3 Stiffness Properties ............................................................................................................................ 71 4.4 Strength Properties............................................................................................................................ 72

■ Section 5: Mastic .................................................................................................................................................. 72 5.1 General ................................................................................................................................................ 72 5.2 FEA Modelling Requirements ......................................................................................................... 72 5.3 Stiffness Properties ............................................................................................................................ 72 5.4 Strength Properties............................................................................................................................ 72

■ Section 6: Triplex ................................................................................................................................................. 72 6.1 General ................................................................................................................................................ 73 6.2 FEA Modelling Requirements ......................................................................................................... 73 6.3 Stiffness Properties ............................................................................................................................ 73 6.4 Strength Properties............................................................................................................................ 73

■ Section 7: Metallic Containment System Membranes .................................................................................... 73 7.1 General ................................................................................................................................................ 73 7.2 FEA Modelling Requirements ......................................................................................................... 73 7.3 Stiffness Properties ............................................................................................................................ 73 7.4 Strength Properties............................................................................................................................ 74

Chapter 5: Structural Testing ............................................................................................................................. 75 ■ Section 1: Summary............................................................................................................................................. 75 ■ Section 2: Material Testing ................................................................................................................................. 75 ■ Section 3: Testing of Containment System or Components .......................................................................... 75

Part C: Acceptance Criteria.............................................................................................................. 77

Chapter 1: Acceptance Criteria Overview........................................................................................................ 77 ■ Section 1: Introduction........................................................................................................................................ 77

1.1 Comparative Procedure.................................................................................................................... 77 1.2 Enhanced Comparative Procedure ................................................................................................. 78 1.3 Absolute Procedure........................................................................................................................... 78 1.4 Simplified Absolute Procedure........................................................................................................ 79

■ Section 2: Risk Assessment to Identify Critical Failure Modes..................................................................... 80 ■ Section 3: Derivation of the Capacity of the System....................................................................................... 82 ■ Section 4: Limit State Approach ........................................................................................................................ 82 ■ Section 5: Load/Capacity Utilisation Factor ................................................................................................... 83

Chapter 2: NO96 Containment System ............................................................................................................ 85 ■ Section 1: Failure Modes..................................................................................................................................... 85 ■ Section 2: Buckling of Internal or External Plywood Bulkheads .................................................................. 85 ■ Section 3: Failure of Primary Box Cover Plates ............................................................................................... 85 ■ Section 4: Failure of Bulkheads in way of Primary and Secondary Box Connections ............................... 85 ■ Section 5: Failure of Bulkheads in way of Cleats ............................................................................................ 86

Chapter 3: Mark III Containment System ....................................................................................................... 87


LLOYD’S REGISTER v

■ Section 1: Failure Modes..................................................................................................................................... 87 ■ Section 2: Foam Layers ....................................................................................................................................... 87 ■ Section 3: Failure of Bottom Plywood in way of Mastic Ropes..................................................................... 87 ■ Section 4: Integrity of Secondary Membrane................................................................................................... 88 ■ Section 5: Deformation of the Primary Membrane......................................................................................... 88

Chapter 4: Supporting Hull Structure .............................................................................................................. 90 ■ Section 1: Acceptance Criteria ........................................................................................................................... 90

Appendix 1 ......................................................................................................................................... 91

Chapter 1: Site Specific Environmental Data.................................................................................................. 91 ■ Section 1: Introduction........................................................................................................................................ 91 ■ Section 2: Site Specific Environmental Data .................................................................................................... 91

2.1 Duration of measurement................................................................................................................. 91 2.2 Seastate information.......................................................................................................................... 91 2.3 Wind information .............................................................................................................................. 92 2.4 Current and Tidal data ..................................................................................................................... 92


vi LLOYD’S REGISTER

Nomenclature The following abbreviations are used throughout this document: CCS Cargo Containment System CFD Computational fluid dynamics DAF Dynamic amplification factor FEA Finite element analysis FLNG Floating Liquefied Natural Gas FPSO Floating Production Storage and Offloading FSRU Floating Storage and Regasification Unit GTT Gaztransport and Technigaz IACS International Association of Classification Societies ISSC International Ship and Offshore Structures Congress LNG Liquefied natural gas LPG Liquefied petroleum gas LR Lloyd’s Register PDF Probability density function RBE Rigid body element R-PUF Reinforced polyurethane foam SCF Stress concentration factor SDA Structural Design Assessment SRA Structural Reliability Analysis UF Utilisation factor The main symbols used in this document are as follows: fp Partial factors for load or pressure fc Partial factors for capacity GM Transverse metacentric height

H Internal height of the tank between the primary membrane surfaces

HS Significant wave height LT Internal length of the tank between the primary membrane surfaces

P3hr Probable 3 hour Maximum statistical pressure Pmax Maximum observed pressure TP Peak period TZ Zero up-crossing period V Ship’s full service speed


Introduction

LLOYD’S REGISTER 1

Introduction

Chapter 1: Introduction

Section 1: Guidance Document Section 2: Application

Section 3: Assessment Procedures Section 4: Sloshing Phenomena

Section 5: Documentation

■ Section 1: Guidance Document This Sloshing Assessment Guidance Document provides guidance and recommendations on the assessment of sloshing in membrane LNG tanks. LNG sloshing is a very complex issue as there are many aspects that are difficult to address explicitly by calculation or testing. In addition there are several methods that may be used to assess the various aspects of the problem. This document gives an overview of possible methods with recommended procedures that can be applied. However, this document is to be considered as a guidance document and it is the responsibility of the designer to select appropriate procedures for the assessment of sloshing based on their experience and capabilities. The proposed procedures that the designer intends to adopt are to be discussed and agreed with Lloyd’s Register prior to commencement of the assessment. Detailed reports of the sloshing assessment are to be submitted to Lloyd’s Register for design review.

■ Section 2: Application This Sloshing Assessment Guidance Document is applicable to membrane LNG ships incorporating a NO96, Mark III containment system or similar such containment system and operating worldwide in any sea condition. These guidance notes are primarily intended for ships operating with a barred fill range to avoid sloshing damage, see Figure 1.1.1, where the operating fill range is typically defined as follows:

1. Below a specified maximum allowable low fill height. Typically this is 10%H but may be lower. This value is to be taken as the agreed highest allowable fill height below the barred fill range applicable for the ship. Before 2008, this value was usually 10%LT but since then this value has usually been less, typically of the order of 10%H.

2. Above a specified minimum allowable high fill height. Typically this is 70%H or higher. This value is to be taken as the agreed lowest allowable fill height above the barred fill range applicable for the ship.

This guidance document is also applicable to vessels of any type designed to carry LNG in membrane tanks where sloshing may be an issue. In particular the guidance document is applicable in the following cases:

1. Ships designed to operate with unrestricted fill heights, hence there is no barred fill range. 2. Ships designed to operate with a smaller barred fill range than between 70% and 10%H. 3. Ships designed to operate within a restricted sea area, for example “Coastal service between the Gulf

and the Eastern Mediterranean”. 4. Ships that will load or discharge at offshore terminals which are not in a sheltered environment. In this

case, it is necessary for the ship to be able to operate at any fill height whilst loading or unloading at the offshore terminal. Evaluation of cargo fill heights within the barred fill range in emergency departure conditions is recommended, assuming the vicinity is exposed to higher seastates than the terminal.

5. Ships or vessels designed to operate as an offshore terminal to process oil or gas resources or to store fluids for subsequent discharge to other ships or ashore, e.g. the vessel is a FLNG, FSRU, FPSO.


Introduction

2 LLOYD’S REGISTER

Figure 1.1.1: Membrane tank typical fill range

This guidance document can also form the basis of a sloshing assessment procedure for other ship types that store or transport liquefied or liquid cargoes in large tanks and which are expected to experience significant sloshing loads. This includes:

• Independent tank LNG ships • Moss spherical tank LNG ships • LPG ships • Oil Tankers • Offshore storage or production vessels

Normally such ship types would be assessed using the Lloyd’s Register ShipRight SDA “Sloshing Loads and Scantling Assessment” procedure, however this guidance document may be used instead. In this case the application of this guidance document will need to be specially considered and the proposed assessment procedure is to be agreed with Lloyd's Register’s prior to application.

■ Section 3: Assessment Procedures

3.1 Initial Screening Phase The assessment of the cargo containment system of membrane LNG ships for sloshing loads is very complex and several alternative assessment procedures may be applied. These procedures reflect an increasing complexity of analysis which can provide better design assurance. The selection of assessment procedure needs to take account of the starting point for the containment system design for the new ship and whether there is good service experience with the proposed containment system or with an earlier design variant of this system. It should be noted that any such service experience is only applicable if it is from a containment system from the same design establishment. If this is the case and the service experience history is satisfactory, then one of the comparative procedures may be adopted, otherwise one of the absolute procedures is necessary. This is illustrated in Figure 1.1.2, note the most likely procedures to be used are shown in bold boxes. The assessment procedures are outlined below. The proposed assessment procedure should be discussed and agreed with Lloyd’s Register prior to application.


Introduction


Figure 1.1.2: Guide to the selection of the sloshing assessment procedure

3.2 Comparative Procedure Prior to 2008, the design assessment of the membrane ship containment system was undertaken on the basis of a Comparative Sloshing Assessment Procedure, see Lloyd’s Register’s document, “ShipRight Comparative Sloshing Assessment SDA Procedure, April 2005.” In this Procedure, the calculation of the design sloshing loads was based on an extensive model test programme. Note, this guidance document supersedes the “ShipRight Comparative Sloshing Assessment SDA Procedure”, dated April 2005. The Comparative Procedure is based on the proven good service record of LNG membrane ships to date, see 3.3. It uses the concept of a REFERENCE ship, which has a proven good service record. The design sloshing loads for the REFERENCE ship are derived and the design sloshing loads for the TARGET ship or new ship design are also obtained by the same method. If the design sloshing loads for the TARGET ship are lower than the REFERENCE ship, then the design is acceptable, see Equation 1.


Introduction


)(REFERENCE load sloshing Design(TARGET) load sloshing Design < … Equation 1

Where: The design sloshing load is usually evaluated at model scale by model tests

The Comparative Procedure is generally applicable to membrane-type ships with capacities less than 155,000 m3 and dimensions typical of ships in service before 2007 provided that the lessons learnt from a major damage incident in 2005 are included. The main lesson from this experience is that:

At low fill heights, the highest sloshing pressures may occur in seastate conditions that are not the most extreme. The most extreme seastate is typically defined as the one in 40 year seastate using the environmental data contained in IACS Rec. No. 34, see Part A, Chapter 2, Section 5.

3.3 Proven Good Service Record A proven good service record may be assumed on the basis of:

• No significant or repeated damage incidents to the cargo containment system (CCS) being reported from a series of ships fitted with such a system. Such ships are to be shown to have met the full range of expected environmental conditions for each range of fill height.

• Proven good service record may be accepted on the basis of specified fill heights; for example, a

good service record for high fill levels could be established independently of a good service record for low fill levels .

At the beginning of 2009 the current status of proven good service experience for the main CCS for membrane LNG ships up to 155,000 m3 is as follows:

NO 96 • for high fill heights above 70%H Acceptable • for fill heights between 10%H and 10%L Unacceptable – due to recorded failures of some un-

reinforced primary boxes • for fill heights between 5%H and 10%H Situation under review • for fill heights up to 5%H Acceptable Mark III • for high fill heights above 70%H Acceptable – however some small primary membrane

deformations in the upper regions may be expected • for fill heights between 10%H and 10%L Unacceptable – due to large primary membrane

deformations that have been observed • for fill heights between 5%H and 10%H Situation under review • for fill heights up to 5%H Acceptable – however some small primary membrane

deformations may be expected in the lower regions This status is under continuous review, so consult Lloyd’s Register for the current situation. To establish a proven good service record for a new CCS design, then it is necessary to record information from all voyages. The minimum required information should include dates, fill heights, seastates encountered, ship speed and heading. This information should be used to assess the expected sloshing loads during service and compared against the design sloshing loads. When it can be demonstrated that the in-service loads are comparable to the design sloshing loads and that the CCS has performed as expected, then Lloyd’s Register may accept this CCS design as providing a proven good service record.


Introduction


3.4 Enhanced Comparative Procedure The Enhanced Comparative Procedure allows the strength of the containment system to be included in the assessment. This is suitable for situations where the strength of the containment system has been increased for the TARGET ship compared to the REFERENCE ship. This increase in strength is only applicable to containment systems when the development of the system has been incremental, eg the addition of a second cover plate to the primary box of the NO 96 system. The strength of the containment system on the TARGET ship is considered acceptable if the ratio of the sloshing load over the containment system strength for the TARGET ship is less than the same ratio for the REFERENCE ship, see Equation 2.

)(REFERENCEcapacity systemt Containmen)(REFERENCE load sloshingDesign

(TARGET)capacity systemt Containmen(TARGET) load sloshingDesign

< … Equation 2

Where: The design sloshing load is usually evaluated at model scale by model tests Typically the containment system capacity or strength is evaluated by static testing methods or Finite Element Analysis

The Enhanced Comparative Procedure should be applied to designs where the characteristics of the proposed containment system design are such that the following is applicable:

• the containment system design is similar on both the REFERENCE and TARGET ships • the containment system for both the REFERENCE and TARGET ships is from the same designer • the hull structure configuration and scantlings are similar

Both the Enhanced Comparative and Comparative methods are calibrated against a proven good service record, see 3.3. In the Enhanced Comparative approach many more factors can be evaluated and explicitly defined. However it is still necessary to apply the Enhanced Comparative approach to a known REFERENCE ship in order to ensure adequate design margins. If there is no existing approved design upon which to base the REFERENCE ship, then it will be necessary to use an Absolute procedure.

3.5 Absolute Procedure The “Absolute Procedure” aims to derive the design sloshing load at full scale taking into account scaling effects, compressibility, etc., and compare this with the ultimate capacity based on the application of realistic sloshing impact loads. The ultimate capacity is derived using advanced physical models of the containment system and supporting hull structure and accounting for dynamic effects and non-linear material properties and geometric issues when applicable. The final assessment is made by ensuring that the ratio of the sloshing load to the ultimate capacity is less than a utilisation factor (UF), see Equation 3.

UF<capacity Ultimate

load sloshingDesign … Equation 3

Ideally, the Absolute Procedure should be applied in its entirety; however there are many aspects of the Absolute Procedure for which the industry’s current knowledge or capabilities are not yet sufficient to evaluate fully. Consequently, only parts of the Absolute Procedure can be applied and the procedure effectively becomes a “Simplified Absolute Procedure”.


Introduction


3.6 Simplified Absolute Procedure The “Simplified Absolute Procedure” is the practical application of the “Absolute Procedure”. The design sloshing load at full scale is derived taking into account as many issues as is practically possible. Similarly, the ultimate capacity is derived taking into account as many issues as is practically possible. Partial factors are assigned to those issues that are not explicitly included in the analysis and the design sloshing load and ultimate capacities are then adjusted by the partial factors and the final assessment is made by comparing the load/capacity ratio to a utilisation factor, see Equation 4.

UFffffff

cncc

pnpp <capacityUltimate

loadsloshingDesign

21

21 … Equation 4

Where • fp1, fc1, etc are partial factors for load (pressure) and capacity respectively

Partial factors can be assigned on the basis of:

• comparative analyses. e.g. FEA of static non-linear response versus static linear FEA to derive a partial factor for non-linear issues.

• comparative testing. • best current practice engineering judgements.

The designer will need to assess which parts of the Procedure are practicable and realistically possible to be applied. As knowledge and capabilities improve then more aspects of the Absolute Procedure can be applied and enable an improved measure of the safety of the containment system to be derived.

The Absolute Procedure or Simplified Absolute Procedure should be applied to designs where the characteristics of the proposed containment system design are such that one of the following is applicable:

• The design significantly deviates from previous designs. • It has not been possible to establish equivalence (on a comparative basis) with an existing Lloyd’s

Register approved design. • The design is from a company that does not have any previous containment system design

experience.

■ Section 4: Sloshing Phenomena Sloshing is the violent motion of the free surface of a fluid in a partially filled container or tank. The principal factors that affect the nature of this phenomenon are the tank shape, fill height and the ship motions in a seaway. Typically sloshing is taken to refer to the motion of the free surface of a liquid in a tank. As a consequence of this motion violent impacts can occur on the tank boundaries and this document concerns itself with the design procedures necessary to review the strength of the tank boundary to these sloshing impacts. Sloshing and sloshing impact loads are produced in cargo and ballast tanks of ships as a consequence of the physical movement of the tank due to the wave-induced ship motions arising from the sea states in which the ship operates. Ship motions are stochastic in nature as a result of the many wave amplitudes and frequencies present in a seaway and the ship motions induce fluid motion characteristics having similar frequencies within partially filled tanks. Significant sloshing motions can occur when the ship motion excitation periods, in response to irregular wave excitation, are close to the longitudinal or transverse natural periods of the fluid flow within a tank. Under such conditions the energy in the fluid flow can be large and this can produce


Introduction


significant velocities within the fluid which can result in large fluid impacts on the tank boundaries and supporting structures.

Figure 1.1.3: Internal View of an NO96 Membrane Tank LNG ships with membrane type cargo tanks are particularly susceptible, see Figure 1.1.3; these tanks have the potential to produce strong sloshing phenomena and impacts on the tank boundaries as the tanks have no internal structures which can restrict the fluid motion or dampen resonant fluid motions induced close to tank natural periods. The characteristics of a free surface sloshing wave in a membrane tank are strongly affected by the fill height. The general tendencies with fill height, observed from experimental studies, are illustrated in Figure 1.1.4 and are described below. At fill heights between approximately 70%H to 90%H, the free surface motion due to sloshing typically takes the form of a standing wave. This oscillatory motion of the free surface can produce strong vertical velocity in the fluid which cause impacts on the tank ceiling and corners. As a consequence of the oscillatory fluid motion, significant gas (air or vapour) may become entrapped between the fluid and tank wall causing a cushioning effect. At high fill heights, typically, above about 90%H, the natural gas vapour above the LNG seeks the instantaneous highest point in the tank. Consequently impacts are likely to contain significant entrapped vapour (commonly referred to as air entrapment) especially in the upper tank corners. At intermediate and low-fill heights (typically below 70%H), the free surface motion due to sloshing typically takes the form of a travelling wave or bore wave, characterised by high velocities and low impact angles


Introduction


against the side or end tank walls and low vapour entrapment in the fluid. These bore waves can result in higher impact loads on the tank walls than occur with standing waves.

70%H

~30%H

10%LT10%H

Sloshing pressure Figure 1.1.4: Sloshing Phenomena At Different Tank Fill Heights

■ Section 5: Documentation Detailed reports are to be submitted to Lloyd’s Register for design review. In general, these documents should include:

• the design basis for the subject vessel • drawings of the containment system and supporting hull structure • the sloshing load experimental programme, including ship motion predictions, model test set-up,

details of CFD program (if used), statistical analysis methods used and a detailed summary of the results and design loads calculated

• details of all materials used and structural tests • analysis report for the strength capacity of the containment system and hull structure • report outlining the hazard review and process, the derived failure modes and assessment criteria

More specific documentation requirements are given within the Procedure: Part A for the design sloshing impact load assessment, Part B for the structural capacity assessment and Part C for the assessment of the acceptance criteria.


Introduction


Chapter 2: Sloshing Assessment Procedure

Section 1: Introduction Section 2: Part A: Ship Motions Analysis

Section 3: Part A: Determination of the Design Sloshing Loads Section 4: Part B: Structural Assessment

Section 5: Part C: Acceptance Criteria

■ Section 1: Introduction The process to be followed for the design assessment of a membrane containment system is outlined in Figure 1.2.1. The following sections give an overview of a suggested sloshing assessment process that should be undertaken for the design assessment of the cargo containment system of an LNG membrane tank.

■ Section 2: Part A: Ship Motions Analysis The purpose of this Part is to determine the motions and accelerations at the centroid of the tank being studied for the sloshing analysis. This involves calculating the ship motions for a range of ship loading conditions, speeds, heading angles, wave periods and significant wave heights. This review is to determine the maximum motions and accelerations experienced by the ship. The ship motions are used as input for the tank sloshing analysis. The effect of tank sloshing on the overall motions on the ship is difficult to accurately predict. Several ship motion programs have the capability to include sloshing effects of liquids in tanks. For practical purposes, these are currently limited to the prediction of a linear small amplitude motion of the liquid and do not include impact or non-linear liquid motions. The use of ship motions software that takes into account the effects of sloshing on ship motions should be calibrated with experimental or real data. Under certain conditions, sloshing effects in internal tanks will have a motion dampening effect; in other conditions the phenomena may increase the ship motions. Typically, the sloshing motions of internal tanks will reduce the peak of ship’s roll motion response curve and increase the roll response away from this peak. However, if the tank sloshing motions are close to the natural roll period of this loading condition then it is likely that this coupling effect will enhance the roll motions. In this case, it may be necessary to include this effect in the overall ship motion response. The combined ship motion/tank liquid motion response is dependent on the liquid level in the tanks, size of tanks and the ship’s loading condition as well as the ship speed, heading and seastate. Inclusion of the coupling effect is allowed, but care must be taken to ensure that the selected loading condition represents the worst case. In this case the sensitivity of the sloshing loads to this assumption should be reviewed to ensure that the worst loading condition has been used for the assessment. Due to the possible variability of the loading condition, especially the liquid level in the tank being reviewed and the adjacent tanks, it is considered reasonable to neglect the effects of tank sloshing when predicting the ship motions. When applied to situations where the tank fill heights are much more clearly known e.g. offshore storage and production vessels where operation at all fill heights is required routinely, then the effect of sloshing may be included in the assessment of ship motion responses.


Introduction


Part A: Ship Motions

Loading condition

Speed

Heading

Environmentalanalysis

Ship MotionAnalysis

Analysis ofcoupling effects

(Optional)

Model testsloshing

procedure

Derivation ofsloshing load

Screening Phase

Derivation ofsloshing loadDesign Phase

CFD sloshingprocedure

Part A: Design Sloshing Loads

Design SloshingLoad

Structuralresponse

Part B: Structural Assessment

Finite elementanalysis Physical testing

Part C: Acceptance Criteria

Potential failuremodes

Ultimate capacity

Risk assessmentor Hazard

identificationPhysical testing

Finite elementanalysis

Statisticalprocessing

Acceptance based onLoad/Capacity Utilisation Factor

orComparative Basis

Figure 1.2.1: Flowchart of the Main Elements of the Sloshing Assessment Procedure


Introduction


Critical seastates are to be identified from the ship motions analysis in conjunction with the expected sloshing resonance periods of the internal tanks at various fill heights. The critical seastates are expected to be those where the peak energy content of the ship motions response spectra is close to a resonant frequency of the sloshing LNG liquid in a tank. Such a review process will outline the probable range of ship speeds, headings and wave periods that are likely to produce significant sloshing loads.

■ Section 3: Part A: Determination of the Design Sloshing Loads The design sloshing loads are usually determined from scale model tank tests. Model tests can be used to investigate the sloshing pressures at the tank boundaries for the range of seastates expected during the ships operating life. The testing programme involves physically moving a scale model tank by a time-series sequence of motions corresponding to the predicted ship motions in irregular waves. Generally, the testing model should be no smaller in scale than 1:50. Typically the model testing programme is split into two phases to determine the design sloshing loads:

1. Screening Phase: The critical tank excitation conditions are established along with the critical locations within the tank that result in maximum sloshing pressures. Screening requires the investigation of an appropriate range of tank fill heights, seastates and ship speed/heading combinations. It may be necessary to split the screening process into several stages to establish the critical seastates and ship speed/heading combinations.

2. Design Phase: For each identified critical condition, long duration tests are undertaken to ensure

convergence of the statistical sloshing impact pressures. The sloshing loads are determined at various locations within the tank taking into account the fill height, tank geometry and the containment system’s structural capacity. It is necessary to derive sloshing pressures versus loaded areas to determine the relationship of load variation over a given area.

To date, physical scale model tank tests are the most reliable means for deriving sloshing pressures and loads. Computational Fluid Dynamics (CFD) may also be used to supplement the analysis of fluid motions in a tank. Experience has demonstrated that CFD is a useful tool for initially establishing the scope of a scale model testing programme and later validating experimental results. A CFD analysis may help reduce the number of model tests performed and can provide guidance for the positioning of pressure sensors within the model tank boundaries. Proposals to use CFD to calculate the design sloshing load as the primary or sole method, hence without any physical model tank testing, will be specially considered by Lloyd’s Register.

■ Section 4: Part B: Structural Assessment For the Absolute procedures and the Enhanced Comparative Procedure, a combination of dynamic Finite Element Analysis, full scale testing, component testing, etc. is necessary to derive the structural response and capacity of the containment system and its supporting structure with respect to the design sloshing loads. The structural assessment is to consider all aspects of the containment system and its supporting hull structure. Hence the following containment system details are to be assessed together with the supporting hull structure:

• Flat surface panels, especially above the lower hopper knuckle • Upper corner details (90º dihedral corners, 135º dihedral corners, trihedral corners)


Introduction


The containment system may be reinforced in particular locations throughout the tank. For example the NO96 containment system of a conventional 138,000 m3 LNG ship uses a mixture of standard and reinforced boxes in the cargo tanks. Such structural variations are to be considered when assessing the strength of the system. It is necessary to consider both the stresses and deformations of the containment system and the supporting hull structure as a consequence of the design sloshing loads. The stiffness of the hull structure affects the way loads are transmitted through the containment system into the hull structure, an example is shown in Figure 1.2.2. This effect should be considered in the Absolute procedures, but may be ignored for the comparative procedures.

Figure 1.2.2: Plating Deformation in way of a Single NO96 Box Mounted Over a Stiffener The method adopted for a dynamic FEA may be based on either:

1. Direct Method: A direct calculation of the structural response to design sloshing loads, taking into account dynamic effects due to the sloshing load impulse.

2. Indirect Method: A unit load type approach where Dynamic Amplification Factors (DAF) are

evaluated based on set of unit sloshing loads with a wide range of load impulse rise times and time durations. The DAF are then combined with the results of a static FEA to derive the unit structural responses. Finally the results are factored by the design sloshing load values to obtain the total structural response.

The Direct calculation method provides a less conservative solution, however requires more load cases and lengthier analysis of the results. The Indirect unit load method is more complicated and provides conservative results; however, it is a more generic method and makes the assessment simpler.


Introduction


The effects of temperature dependent material properties should be included in the assessment. Material property non-linearity and strain rate may need also to be considered. The structural assessment of the tank and supporting structure is a complex procedure. Prior to commencement, the assessment methods to be adopted by the designer are to be agreed by Lloyd’s Register. It is likely that the assessment process will need to employ a mixture of structural testing and static and dynamic FEA computational methods. As noted earlier, the complete Absolute Procedure, see Chapter 1, 3.5, may not be practically possible and hence the Simplified Absolute Procedure or the Enhanced Comparative Procedure is to be adopted. For the Simplified Absolute procedure, see Chapter 1, 3.6, a complete list of all design aspects should be documented together with whether these issues are addressed by the use of partial factors or by including these effects into the assessment methods. Suitable partial factors are to be estimated for issues not explicitly addressed and are to be fully validated in supporting documentation. For the Enhanced Comparative Procedure, see Chapter 1, 3.4, a complete list of all design aspects should be documented together with whether these issues are addressed implicitly on a comparative basis or explicitly by including these effects into the assessment methods. A fatigue assessment may also be required. This will typically be the case for new or novel containment systems. When performing a fatigue assessment it is essential to consider the effects of repeated sloshing impacts as well as the response to loads arising from wave actions and LNG related operations (e.g. thermal cycling, loading and unloading). There are two aspects to sloshing related fatigue which are to be considered:

• Cyclic fatigue related to general sloshing motions (wave frequency related) • Fatigue due to sloshing impact loads

Other fatigue aspects that need to be considered but are not covered by this document include: • Cyclic fatigue of the containment system due to flexing of the hull structure due to ship motions

and wave actions • Containment system deformation due to thermal cycling during loading/unloading operations • Hull structure and containment system deformations due loading/unloading operations

Guidance on the required structural assessment analysis is given in Part B of this document. The following issues are not addressed in this guidance document:

• The strength and integrity of the containment system as a consequence of grounding or collision. • The strength and integrity of the containment system as a consequence of accidental damage, such

as impacts following fastenings becoming loose, foreign matter, etc. • The strength and integrity of the containment system due to hull girder loads and other internal

structural loads due to any of the following: wave actions, static actions, thermal actions or loading/unloading operations.

■ Section 5: Part C: Acceptance Criteria The acceptance criteria for each type of containment system will need to be specially considered taking into account the structural arrangements, materials used in the system and the assessment method adopted. The failure modes for existing containment system designs are relatively well known through in-service experience and the design verification process. These failure modes can be used to define the design criteria for new designs provided equivalence can be established.


Introduction


For new containment system designs and for designs being assessed by the one of the Absolute Procedures, the acceptance criteria cannot be solely determined by using previous experience. The potential failure modes are to be determined using a documented risk assessment or hazard identification process. Structural testing and finite element analyses will be necessary to review the potential failure modes, determine the ultimate strength capacities and hence define the acceptance criteria. It is unlikely that the structural testing or finite element analyses will be able to accurately represent the actual in-service conditions experienced by the containment system and hence the ultimate capacities and structural responses will be subject to interpretation. It is therefore recommended that partial factors are introduced to adjust the ultimate capacities, structural response and acceptance criteria to account for unresolved issues and ensure the design appraisal errs to the conservative side. The acceptance criterion for the hull structure is to be in accordance with existing Lloyd’s Register ship design standards. Guidance on the definition of the acceptance criteria is given in Part C of this Procedure.





Chapter 1: Design Sloshing Loads Overview

Section 1: Introduction Section 2: Design Basis

■ Section 1: Introduction In this Part, requirements are given to derive the design sloshing impact loads using sloshing model tank experiments. Alternative methods based on Computational Fluid Dynamic (CFD) analyses, or equivalent, may be acceptable subject to prior approval of the proposed procedure by Lloyd’s Register.

■ Section 2: Design Basis The purpose of the sloshing investigation is to define design sloshing loads for the ship or floating vessel under consideration. The analysis is to consider the range of operational tank fill heights, ship loading conditions and seastates. The design value is usually defined as the probable maximum sloshing load that will occur in a 3 hour period based on a seastate that has a once in a lifetime chance of occurring during the life of the ship or floating vessel. The design sloshing loads are derived using a short term statistical approach. Extreme value analysis statistical approaches or similar will be required for situations where the design load is suspected to occur in non-extreme seastates or where there are restrictions placed on wave heights or similar due to operational restrictions. A long term statistical approach based on the sloshing load that has a probability of occurring once in a lifetime may be used as alternative to a short term statistical approach. However this will require special consideration when applied to ships and floating systems where there are restrictions placed on wave heights or similar due to operation considerations. The application of the long term approach for the Absolute Procedures is not recommended and will require special consideration.




Chapter 2: Ship Motions

Section 1: Introduction Section 2: Seakeeping Analysis

Section 3: Tank Selection Section 4: Loading Conditions Section 5: Wave Environment

Section 6: Ship Speeds and Wave Headings

■ Section 1: Introduction Ship motions in waves are to be calculated for the range of ship loading conditions, environmental conditions and ship operational parameters as defined in this Chapter. Such calculations are necessary to determine the maximum ship motion responses which will excite tank fluid motions that may lead to significant sloshing loads.

■ Section 2: Seakeeping Analysis The seakeeping analysis is to be performed using 3D diffraction ship motion software or similar and will normally be based on frequency domain methods. The software is to be capable of calculating ship motions in irregular waves. More details of the requirements for the ship motions analysis is given in the Introduction, Chapter 2, Section 2.

■ Section 3: Tank Selection All tanks which experience significant sloshing loads are to be considered, however it is acceptable to consider only the tank(s) which are expected to experience the greatest sloshing responses as the basis for deriving the design sloshing loads. When selecting the tank(s) for the sloshing assessment the following factors are to be considered:

• Distance of the tank from the ship’s centre of gravity. Tanks furthest from the centre of gravity of the ship would normally be expected to have the highest ship motion response. Consideration of longitudinal and transverse distance is necessary.

• The size of the tanks. If the furthermost tank from the ship’s centre of gravity is substantially smaller than other tanks, then it may be appropriate to select the larger tank for the assessment. For example No. 1 tank is smaller than No 2 for most conventional LNG ships and hence No 2 tank is usually used. However it may be necessary to consider the smaller tanks as well and demonstrate that these tanks will be subject to lower sloshing loads.

■ Section 4: Loading Conditions Normally, for conventional seagoing LNG ships, loading conditions with the highest transverse metacentric height (GM) which are representative of the operational profile of the ship are to be chosen as the basis for the calculation of the ship motion responses; as high GMs give shorter roll periods which are usually more critical for sloshing. Departure and arrival loading conditions are to be reviewed to determine the condition with the highest GM.




For ships which are required to load or unload at offshore terminals or FLNGs, or similar, the most appropriate loading condition will need special consideration. Similarly the loading conditions for FLNGs, FPSOs etc will need to be specially considered. Typically the following loading conditions for conventional seagoing LNG ships should be reviewed:

• Full loading condition with all cargo tanks filled to 95%H. This condition is to be used for the assessment of tanks with fillings above the upper limit of the barred fill range.

• Ballast loading condition with all cargo tanks at the upper limit of the lower end of the barred fill range, typically 10%H. This condition is to be used for the assessment of nearly empty tanks.

If it is expected that the ship will trade regularly with partial loads, then the following condition is to be additionally checked for the assessment of tanks with fillings above the upper limit of the barred fill range:

• Partially filled loading condition with one tank at 95%H full and all other tanks at the upper limit of the lower end of the barred fill range, typically 10%H.

The loading conditions will need to be specially considered for ships designed to operate with unrestricted fill heights.

■ Section 5: Wave Environment The standard wave environment to be used for the assessment of ships which are operating worldwide is the North Atlantic all-directions scatter diagram as specified in IACS Rec. No. 34.1 This wave environment specifies the probability of occurrence of individual seastates, each seastate being defined by a significant wave height (HS) and zero up-crossing period (TZ). Each seastate is to be characterised using the Pierson Moskowitz wave energy spectrum and is to be assumed to exist for a duration of 3 hours. Normally the sloshing assessment is to be based on short term statistical analysis techniques using seastate envelope curves for the appropriate design life of the ship. For a design life of 40 years, the seastate envelope curve values derived from IACS Rec. No. 34 are given in Table 2.2.1. This data was obtained by interpolating the data in each TZ column for a probability of one occurrence during the design life. For head seas and following seas, the 40-year return envelope HS values are to be applied. See Table 2.2.1. For beam sea conditions, the 1-year return envelope HS values are to be used to decide the extreme seastate HS/TZ conditions. It is assumed that the probability of being in beam seas in the 40 year seastate condition is very remote and will only happen if some other failure occurs, e.g. loss of power. Hence it is considered acceptable to assume extreme seastates on a one year return period for beam seas. See Table 2.2.1. For quartering sea conditions, linear interpolation between the 1-year and 40-year envelope wave heights is acceptable. Table 2.2.2 shows a suggested relationship between heading and HS/TZ condition used for model tests.

1 IACS (1992) Rec No. 34 Standard Wave Data




Table 2.2.1: Seastate Characteristics for 1 and 40 Year Envelopes

TZ (s) 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 13.5 14.5 15.5

HS (m) for 40 years 5.6 8.4 10.9 12.8 14.2 15.1 15.6 15.7 15.5 15.0 14.1

HS (m) for 1 year 3.5 5.9 8.1 9.8 11.1 11.8 12.1 12.0 11.4 10.5 9.3 Table 2.2.2: Heading and Seastate Envelopes for Tank Model Tests Heading (degrees) 0 – 30 30 - 60 60 - 120 120 - 150 150 – 180

Seastate Envelope 40 year Linear Interpolation

between the 40 and 1 year HS values

1 year Linear Interpolation

between the 40 and 1 year HS values

40 year

The Pierson-Moskowitz wave energy spectrum, also called the ISSC spectrum, is to be used to model the wave environment in the North Atlantic or any open ocean areas. The assumption of short crested seas with a cosine squared spreading function may be applied. It should be noted that it is possible for very high sloshing impact pressures to occur in seastate conditions that are not the most severe. This is especially relevant for low filling conditions, typically for the 10%H to 30%H fill heights. For the high fill cases, it is expected that the highest sloshing impact loads will occur from the HS/TZ envelope conditions derived from Table 2.2.1 and Table 2.2.2. However, for the low fill cases, the most onerous sloshing impact loads may occur at wave heights lower than the envelope values derived from Table 2.2.1 and Table 2.2.2. Typically, for low fill cases, the highest sloshing pressures occur when a sloshing bore wave hits the tank wall just as the wave is breaking. In the really severe seastates, the sloshing bore wave becomes too steep and hence unstable and breaks, hence the wave energy is dissipated before it impacts with the tank walls. For lower seastates, the bore waves are less steep and may travel the full width or length of the tank before impacting on the tank wall. This can mean that less severe seastates can produce higher sloshing impacts than more severe seastates on the seastate envelope curve. As a consequence it may be necessary to perform model tests or CFD analyses for a range of seastates below the envelope curves given in Table 2.2.1 and Table 2.2.2. It should be borne in mind during this assessment that the likelihood of such seastates is far higher than the envelope curve values, which effectively means that the ship would expect to see such seastates on a more regular basis. Note the envelope curve seastate values may never be experienced during the life of the ship and hence the acceptance criteria for these events are calibrated taking into account the very low probability of the ship seeing such events. For lower seastates which are expected to be seen, then this is not true and the selection of the design sloshing load needs to be adjusted to reflect the higher probability of seeing such events. For the assessment of ships or other floating systems operating within coastal or restricted waters, it will be necessary to undertake a long term analysis of the site specific wave environment including an assessment of the wind and swell wave directionality issues, derivation of suitable wave spectra, as well as current, water depth and mooring arrangements. The ship motion assessment is to take these effects into account. More guidance is given in Appendix 1. For ships which have LNG tanks which are symmetrical about the centreline, then it is only necessary to consider sloshing due to waves coming from one side of the ship due to the effect of symmetry conditions, hence consideration of ship motions for headings from 0 to 180 degrees only (stern to head seas) will be necessary.




For ships which have tanks which are not symmetrical about the centreline, then it will be necessary to consider sloshing due to waves coming from the port and starboard side of the ship. This is because the ship motion responses are no longer symmetrical for port and starboard wave headings, hence consideration of ship motions for all headings will be necessary.

■ Section 6: Ship Speeds and Wave Headings It is not considered necessary to undertake sloshing analyses at all ship speeds and all wave headings, since some operational restrictions exist and the aim of the analyses is to establish the highest sloshing responses under realistic operational conditions. The assumptions regarding ship speed for different headings and wave heights in Table 2.2.3 may be applied in the assessment of the ship motions for ships operating for worldwide trading. The reduction in ship speed for higher seastates represents a voluntary/involuntary speed reduction.

Table 2.2.3: Assumed Ship Speeds for Different Wave Headings and Wave Heights Ship Speed

Wave Height Head and Stern Seas (for wave headings ≥ 135º and ≤ 45º)

Beam Seas (for wave headings > 45º and < 135º)

HS ≤ 5m Full service speed (knots), V Full service speed (knots), V 5m < HS ≤ 9m 0.5V (knots) 5 knots HS > 9m 5 knots Notes 1 For this document, a wave heading of 0 degrees refers to stern seas, 90 degrees to port

beam seas and 180 degrees to head seas. It is also necessary to consider the following case:

• A ship speed of 0 knots in beam seas (wave heading of 90 degrees) representing an emergency situation where all main engine power or steering is lost.

Alternative documented ship speed versus heading and wave height relationships may be acceptable. For the assessment of ships or other floating systems operating within coastal or restricted water locations, it may be necessary to take account of the following items in determining the appropriate system heading and floating system motions to the wind and wave conditions, see also Appendix 1:

• the co-existence of wind wave and swell wave systems • the effect of mooring systems on the heading of the ship or floating system • the effect of tandem or side by side mooring or other floating bodies




Chapter 3: Model Tests

Section 1: Introduction Section 2: Model Testing Phases

Section 3: Sloshing Model Tank Set-Up Section 4: Results Data Processing

■ Section 1: Introduction Scale model tank tests are currently considered an essential part of deriving the design sloshing load. To date, CFD tools are not considered sufficiently developed to capture the expected impact pressures in a membrane LNG tank taking into account the extent of the analysis required and the associated computing resources required. Proposals to use CFD analysis will be specially considered. The principal quantities to be measured in the model tests are the pressure time series in areas of expected highest sloshing impacts. This data is to be stored and statistically analysed to extract maximum values and other parameters such as impact rise times, etc. There are several Phases involved in a scale model tank test program:

1. Initial Screening Phase: A global distribution of sensors is used to determine the critical combination of seastates, ship speeds, headings and fill heights that lead to high sloshing pressures. Regions within the tank model where these high pressures are experienced are also to be identified. Both high and low fill heights are to be considered.

2. Initial Screening Phase (Wave Height Sensitivity): A review of the effects of wave height on sloshing

pressures for low filling cases only. 3. Refined Screening Phase: Refined testing and investigation of the conditions around the critical

conditions determined during the Initial Screening Phase. 4. Design Phase: Model tests for the identified critical conditions are run for a longer time period with

additional pressure sensors concentrated in areas of highest pressures as determined by the screening phases. The results from the Design Phase provide the design sloshing load scenarios to be used for the structural assessment.

The flowchart in Figure 2.3.1 illustrates the process for the design sloshing load derivation process.




Figure 2.3.1: Suggested Procedure for Sloshing Model Tests

■ Section 2: Model Testing Phases

2.1 Initial Screening Phase The purpose of the Initial Screening Phase is to determine:

• the critical environmental and operational conditions that generate maximum sloshing pressures • the critical fill heights and locations within the tank where maximum sloshing pressures occur

The sloshing model tests should account for the following parameters: fill heights, wave heading, wave period and wave height. Initial heel and trim may also need to be considered. The results from the Initial Screening Phase are further investigated in the Refined Screening Phase and form the basis for subsequent extended model tests in the Design Phase. A series of wave headings and periods are to be tested to establish the critical wave conditions and tank fill heights. The suggested scope of sloshing model tests for the Initial Screening Phase for a Seagoing LNG ship with a barred fill range are given in Table 2.3.1 and Table 2.3.2. Particular attention is to be given to conditions when the peak wave encounter period is close to, or between the tank resonance roll period for a given fill height and the ship’s natural roll period. In such conditions it is likely that critical sloshing loads will be experienced. The same consideration is to be applied for pitching motions.

Sloshingmodel test

Screening Phase(to identify critical scenarios)

Test setup

Model tests

Ship motion (loadingcondition, speed,heading, Hs, Tz)

Filling height

Sensorplacement

Simulationtime

Data processingfor single sensor

Critical scenariosand locations

Design Phase(repeated for each critical senario)

Model tests

Ship motion (loadingcondition, speed,heading, Hs, Tz)

Filling height

Sensorplacement

Simulationtime

Area-based datapost-processing

Time domain Pressurecurves vs locations,

area sizes etc

Test rig

Model tank

Sensors

Calibration

Material

Geometry

Screeningphase

Designphase

Designsloshing load




Table 2.3.1: Usual Initial Screening Phase Testing Range Testing Range Increment Wave Headings 30 to 180 degrees (head seas) 30 degs

Wave Periods 5.5 to 13.5 seconds 2 seconds

Table 2.3.2: Usual Fill Height Testing Range

Fill Height Notes High fill heights 70%H, 80%H, 95%H Assuming 70%H is the value of upper end of the barred fill

range.

Low fill heights 10%H Assuming 10%H is the value of lower end of the barred fill range. Other values may be necessary to ensure lower fill heights are not more severe, see Section 2.3.

Intermediate fill heights

15%H, 20%H, 30%H, 50%H.

Intermediate fill heights are only necessary if the design is intended to operate with fill heights that are inside the conventional barred filled range (usually 10%H to 70%H).

NOTE: 1. For LNG ships with a capacity greater than 155,000 m3 or of unusual design, the fill heights may need to be specially considered. Model tests should run for at least 5 hours full scale to establish the sloshing load characteristics for a scale tank model. An alternative schedule would involve the running of a 6 hour full scale test which would provide two probable 3 hour maximum pressure results. This provides additional scope for comparison and validation of statistical results. The results from the Initial Screening Phase are to be reviewed using 3D surface plots or similar techniques to identify the fill height, seastate and heading combinations which produce the greatest pressures. Such combinations are highlighted as critical cases. It is possible that several critical cases are highlighted for further review in the further screening phases if some combinations give similar results.

2.2 Initial Screening Phase (Wave Height Sensitivity) For the assessment of sloshing loads at low fill heights, an additional phase is required to review the sensitivity of the sloshing pressure to the seastate wave height. As described in Chapter 2, Section 5, the determination of critical conditions for maximum sloshing loads cannot be based simply on the assumption of extreme seastate conditions. Additional tests should be undertaken to review the effect of wave height for all low and intermediate fill heights reviewed so far. It is necessary to review several wave heights for each critical heading and wave period highlighted in the Initial Screening Phase. It is recommended that a wave height step of 2m is used. For example, if the envelope value of wave height is 9.8m, then wave heights of 8m, 6m and 4m should be reviewed. It may be necessary to review this assumption for high fill heights as well by undertaking additional tests to review the sensitivity to wave height. More than one model test run will be required for each heading and seastate combination to provide a sufficient number of impacts and hence ensure good convergence of the statistical pressure results. If the trend




of pressure with wave height is consistently increasing then it may be assumed that extreme wave height is the design condition. However if the trend is nearly flat, then additional test runs should be included in the refined screening phase to review wave height issues more carefully. Each additional model test run should have a unique ship motion time history, hence the phase angles of the wave frequencies used to generate the time history should be changed for each additional test, see also 3.2.

2.3 Refined Screening Phase The purpose of the Refined Screening Phase is to review additional test cases around the critical cases highlighted in the Initial Screening Phase so as to refine the selection of the final critical design cases for the Design Phase. The following test programme is recommended to identify the critical design cases:

• Test additional fill heights around the Initial Screening Phase critical fill heights to ensure that the worst fill height is selected.

• At the resulting worst (critical) fill height, test additional wave headings. • At the critical fill height, test additional wave periods around the critical period. • For low fill heights, additional tests to review the sensitivity to wave height may be necessary

depending on the results of the earlier tests. Some tests in the Refined Screening Phase may be omitted if it is clear that the pressures are decreasing rapidly with a change in heading or wave period. It is expected that more test runs will be necessary to improve the accuracy of the curve fit for the recorded impact pressures.

2.4 Design Phase The purpose of the Design Phase is to derive the design sloshing load scenarios for the structural assessment. The critical sloshing cases and tank regions were identified in the previous Screening Phases. These cases need to be repeated in more detail during the Design Phase to give a sufficiently detailed dataset of pressure measurements. As part of this process, more concentrated clusters of pressure sensors should be placed at the critical locations identified in the Screening Phases in order that load area processing of the impact pressures can be undertaken. The time history results from adjacent sensors in the clusters are combined to give the load area based sloshing results. The results are statistically analysed to enable the design sloshing pressure versus load area curves to be derived for the various tank locations and containment system details. A sufficient number of tests of the critical sloshing cases will need to be run to ensure that statistical convergence of the pressure results occurs. Each additional model test run should have a unique ship motion time history.

■ Section 3: Sloshing Model Tank Set-Up Model tests are to be conducted using rigid plastic, glass or steel vessels fitted with pressure sensors, or similar. The rig is to be capable of being driven through six degrees of motion using time-series signals derived from seakeeping analyses.




The usual medium to represent the LNG is water. The ullage gas is usually represented by air. Froude scaling is to be used to transform the ship motion amplitudes and time scales to model scale. A high sampling rate, at least 20 kHz, is required for the pressure sensors to accurately capture sloshing pressure pulses (peak pressure and rise time). The synchronous collection of data from all sensors subject to the same sloshing event is required so that pressures over larger areas can be calculated by combining the pressure over several individual sensors.

3.1 Tank Model The tank model should be geometrically similar to the inside of the LNG cargo tank under investigation and at a scale of not less than 1:50. For cargo containment systems where the tank primary membrane surface is not smooth (i.e. corrugated or with raised panel connections), consideration of how to include the effect of this “roughness” in deriving the sloshing loads for use with the simplified absolute procedure will be necessary; typically this will be dealt with as a partial factor as described in the Introduction, Chapter 1, Section 3, 3.6. This can be ignored for comparative procedures. The tank model should be sufficiently rigid so that sloshing impacts on the tank boundaries do not significantly influence the measured pressure time history results. The obtained pressure time histories should be reviewed to ensure that unusual resonance frequencies due to vibration of the tank structure are not present. The same applies to the design and construction of the sensor mounting panels.

3.2 Test Rig and Motion Generation The test rig is to be capable of applying the six degrees of freedom of the ship motions generated by the seakeeping analysis to the model tank. The ship motions at the centre of the tank are to be determined by combining the ship motion response amplitude operators with the seastate wave spectrum to produce a time history for the required duration of the test. An Inverse Fast-Fourier transform (IFFT) algorithm may be used to generate the time domain ship motions. Sufficient wave frequencies are to be included in the ship motion frequency domain calculations to ensure that the ship motion time history represents the required statistical values for the required seastate. This time history should not show evidence of repetition of similar patterns over the test duration (usually 5 hours full scale). The selection of phase angles for each wave frequency in the IFFT process is to be randomly selected, note consistent “seed” values may be used to generate consistent phase angles and hence repeatable time histories.

3.3 Video Recording It is recommended that video recordings are taken as part of each test for review purposes. It is suggested that a short period of time stamped video is recorded prior to and following a maximum sloshing impact event.

3.4 Ullage Gas Air is usually used to model the vaporised natural gas ullage. Proposals to use other gases will be specially considered. To improve accuracy it has been suggested that sloshing tests should be performed using an ullage gas which has been chosen to match the quality of the natural gas liquid-vapour mixture at full scale. However, it is necessary to demonstrate that the chosen ullage gas is a better approximation of the full scale condition of boiling liquid LNG and LNG gas vapour.




3.5 Sensors The specification of the pressure sensors is to be suitable for the expected pressure range and pressure rise time. Ideally, sensors should be tested dynamically before they are used or supplied with calibration charts from the manufacturer. In practice, selected testing of some sensors should be undertaken to ensure that the performance is still in accordance with the manufacturer specification. For the dynamic testing of the sensors, drop tests or similar methods may be used. It is recommended that the incident angle of the drop test be greater than 5 degrees. To perform the drop test, a sensor is mounted on a drop wedge and the wedge dropped into water several times from the same height to ensure consistency. Drop tests from several heights are necessary to calibrate the linearity of the sensor. The deviation in the pressure reading and the ratio of averaged pressure versus the Wagner’s solution or similar is to be recorded. Sensors with irregular pressure time series should be eliminated. Sensors with readings over the non-linear limits of the sensors' specifications should be rejected or possibly recalibrated. Changes in temperature can have a significant effect on the sensor readings, especially at the beginning of the test when dry sensors are initially wetted by the sloshing liquid. Precautions are to be taken to ensure that spurious readings due to temperature variations are excluded from the analysis. These precautions can include discarding the first few minutes of the test data which provides time for the temperature of the sensor to stabilise.

3.6 Sensor Placement The positioning and density of sensor placement will depend on the requirements of the various test phases. In general, it is recommended that the sensors are symmetrically placed. For the Initial Screening Phase, many locations are to be instrumented so as an overall pattern of sloshing impacts can be determined. Areas where the highest sloshing impacts are experienced are to be identified and sensors should be consolidated in these areas in the Refined Screening Phase. During the Design Phase, the sensors are to be clustered in the critical areas. The clusters should have tightly packed sensors so that sloshing pressure versus load area graphs can be generated. The locations of maximum sloshing loads are dependent on the fill height and wave heading and the placement and locations of clustered sensors should be selected accordingly. In near beam sea cases, the longitudinal bulkheads may suffer more impacts; whilst in head sea cases, the transverse bulkheads may suffer more impacts. In practice, the corner areas are to be covered with more sensors than the middle flat areas. A general guide for the placement of pressure sensors dependent on fill height is given in Table 2.3.3 and Figure 2.3.2.

3.7 Sensor Density The suggested minimum density of sensors is a distribution of 9 sensors, See Figure 2.3.3, in an area equivalent to 1 m2 at full scale. Ideally, the diameter of the sensors should be small enough to capture impacts over an area that is consistent with full scale geometric details of the containment system structure. This may not be practical, hence it is recommended that the size of sensor allows 9 sensors to be placed close together in an area equivalent to 1 m2 at full scale for the design phase tests. This density and size enables a reasonable representation of the pressure variation over a box of the NO96 containment system and a 3 x 3 array of Mark III corrugation panels to be determined. In addition to the clustered sensors, a less dense coverage of sensors around the tank is necessary to confirm that the selected critical locations continue to produce the highest impact pressures.




Table 2.3.3: Guidance for Pressure Sensor Placement

Fill height Expected Critical Locations Sensor Locations (see Figure 2.3.2)

≤ 20%H Lower part of the side walls Transverse bulkheads near lower part of side walls

E, F, K, L O

30%H ≤ 50%H Side walls Transverse bulkheads. Corner areas of the tank ceiling

E, F, K, L O, N

C, D, I, J 70%H ≤ 85%H Upper chamfer

Upper parts of the transverse bulkhead Corner areas of the tank ceiling

C, D, I, J M, N

A, B, G, H ≥ 90%H Corner areas of the tank ceiling

Upper parts of the transverse bulkhead A, B, G, H

M

A3

G1

A1

A4

M3 M4

G2

A2

B4

C4

I2

B2

C2 H2

D2

E2 J2

D4 K2

E4

N4

O4 F4

L2

F2 N1

O1

A6

M6

A5

Figure 2.3.2: Critical Locations for Sloshing Pressure Sensors

11 1312

21 2322

31 3332

Fore bulkhead

sensors

Upperchamfer

Ceiling

Figure 2.3.3: 9-Sensor Array Example




■ Section 4: Results Data Processing

4.1 Introduction The analysis of tank sloshing data comprises of the following steps:

• Detection of the locations with the highest sloshing impact pressures • Analysis of the pressure sensor data of the relevant locations • Combination of the pressures from adjacent clusters to derive load area results

• Calculation of the design sloshing loads, rise times, based on statistical analysis

These steps are outlined in detail in the following sub-Sections.

4.2 Identification of Sloshing Impact Pressures The Peak-Over Threshold method is to be used to identify the occurrence of sloshing impact events. Recorded pressure peaks that do not exceed the threshold value are to be discarded from the analysis. The threshold value should be selected so that all significant sloshing impacts are captured and that spurious data spikes due to noise and lack of resolution of the recording equipment and sensors are omitted. The pressure results include static pressure components, low (wave) frequency fluid oscillation components and high frequency sloshing impact pressure components. The results of interest are the high frequency sloshing impact pressures. The static pressure and low frequency fluid pressure components are to be removed by applying a high pass frequency filter with a cut-off frequency of approximately 0.17 Hz to the data sample. This is performed for all locations and the results will give an indication of the locations where the highest sloshing pressures occur. The filtered results can only be used to determine the probable locations of high sloshing pressures, they are not to be used for numerical analysis.

4.3 Pressure Curves and Scaling Laws There are two main types of pressure impacts which may be observed during sloshing. These are illustrated in Figure 2.3.4 and Figure 2.3.5. Figure 2.3.4 plots the time history of the sloshing impact pressure predominantly caused by a jet impact or a solid wall of liquid, e.g. a bore wave. Figure 2.3.5 plots the time history of a pressure impact which shows gas entrapment during the sloshing impact event. A combination of these two effects is possible although jet impacts are generally more common and are usually more severe than gas entrapment type impacts. The model test pressure results need to be scaled up to full scale. In general, the Froude pressure scaling law is to be applied for sloshing impacts and for the adjustment of the rise time. Froude scaling is considered to be appropriate for jet type sloshing impacts. When a sloshing impact contains significant gas entrapment, as indicated by large regular oscillations in the pressure pulse, see Figure 2.3.5, then Froude scaling is probably too conservative. In this case, special consideration of the scaling laws may be acceptable as a combination of the Froude and Euler (or Gas) scaling laws may be appropriate. Proposals to use alternative scaling laws to the Froude law are to be agreed with Lloyd’s Register.

Froude scaling law ⎟⎟⎠

⎞⎜⎜⎝

⎛=

msms

fsfsmsfc L

LPP

ρρ




Time scaling law ms

fsmsfc L

Ltt =

Where Pfs and Pms are the pressure at full scale and model scale respectively Lfs and Lms are the length at full scale and model scale respectively ρfs and ρms are the density at full scale and model scale respectively tfs and tms are the time at full scale and model scale respectively

Time

Pres

sure

Time

Pres

sure

Figure 2.3.4: Jet Type Impact Figure 2.3.5: Pressure Impact with Gas Entrapment

4.4 Data Processing of Sloshing Impact Pressures All the sloshing impacts recorded during each test run by each individual sensor and also for each load area, see 4.5, are to be statistically analysed to determine the probable maximum 3 hour pressure values. Different distribution laws should be applied to the statistical analysis in order to determine which method provides the best fit to the recorded impact data. It may be the case that different distribution laws are more suitable for different fill heights. The widely adopted practice is to use either a 3 parameter Weibull, 3 parameter log-normal or General Pareto distribution. Care should be taken for cases where there are few impact events as this lack of data can produce unreliable results. The fitting parameters should be checked to ensure accuracy of the curve fitting. It may be appropriate to review the pressure results with those obtained in similar tank locations as sensors in similar locations are expected to experience similar pressure impacts events. Comparing sensor results in similar locations is a means of checking the consistency of the data and assisting in defining the sensor locations for the Design Phase. The type of pressure impact should also be reviewed to determine the type of impact occurring. It is recommended that summary tables similar to that given in Table 2.3.4 are used to record the data processing during the Screening Phase for each reference area (e.g. area A1, see Figure 2.3.2). A similar table can be established to summarise and compare the results for a region where the results are expected to be similar (e.g. region A, see Figure 2.3.2).




Table 2.3.4: Statistical analysis for fill height, heading, speed and (HS, TZ) combination for each sensor

location

Sensor location Recommended Output

Maximum statistical pressure, P3hr See 1

Sensor No for P3hr See 2

Number of impacts for this sensor

Curve fitting parameters and confidence level See 3

Reference Area (e.g. A1 see Figure 2.3.2)

Maximum observed pressure, Pmax

Sensor No for Pmax Number of impacts for this area

Repeat for each sensor area (e.g. A1, A2… B1… E4…etc) See Figure 2.3.2. Notes 1. Maximum statistical pressure P3hr refers to the most probable 3 hours maximum value predicted for

any sensor within the particular sensor area. 2. Maximum observed pressure Pmax refers to the maximum pressure value recorded during the

model test run based on all sensors within the particular sensor area. 3. Fitting parameters (such as variance, etc) and the percentage confidence level.

4.5 Load Area Processing The structural analysis is to review the effect of sloshing impact loads over the appropriate load area. The Design Phase sloshing data for all sensors in a sensor array is to be post processed in order to derive the design sloshing pressure versus load area curves. The results would normally show that very high pressures are expected at one sensor, but the pressure will be significantly lower when averaged over a larger area. Load area processing may also be applied during the screening phases to support the selection of the critical cases for the Design Phase assessment. Figure 2.3.6 illustrates a possible process for the assessment of the impact pressures within an array of clustered pressure sensors. Sloshing pressure load area shapes are used to determine the average pressure acting on the containment system. Ideally these load area shapes should match the geometrical characteristics of the containment system and the type of impact pressure expected. The load area based processing of the pressure sensor data is to be conducted for all locations within the tank which have been identified as critical regions for sloshing load investigations. For each chosen load area shape, the instantaneous pressure time history is to be calculated by combining the time history pressure results of all sensors within the chosen area. This will give the averaged pressure time history for the chosen area. It is necessary to treat all sloshing impacts in this way for each load area. Hence statistical analysis is to be performed to produce area-averaged statistical pressures and rise times using the same method as for single sensors, see 4.4. The statistical results from area-averaged pressure data will vary with the chosen load area shape. Figure 2.3.7 demonstrates how the pressure results could vary over the 9-sensor array loaded area. It is therefore necessary for several load area shape results to be chosen for consideration in the structural analysis.




Figure 2.3.6: Example of Area-Averaged Based Processing of Pressure Sensor Data

Maximum statistical pressure variation versus area sizes

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

area (m^2)

pres

sure

(bar

)

Vertical line area

Horizontal line area

square area

Figure 2.3.7: Area Size as a Function of Statistical Maximum Pressures




4.6 Statistical Analysis Methods The extreme envelope seastates usually give the most onerous sloshing impact loads for high tank fill heights. For lower fill heights, the highest sloshing impact loads may occur in seastates well below the extreme envelope where the likelihood of encountering these seastates is much higher. In view of this, different statistical post-processing methods are required to derive the design sloshing loads associated with seastates that have very different probabilities of occurrence. For the “once in a lifetime” extreme envelope seastates the mean probable maximum 3 hour value is to be used as the design sloshing load. For seastates below the envelope value, categorised as non extreme seastate events see also Chapter 2, Section 5, the following statistical methods may be used:

• Use extreme value analysis to account for the number of occurrences of the seastate in the design life

• Long term statistical methods to account for the number of occurrences, but note the application of this method to the absolute procedures needs to be confirmed

• A structural reliability analysis approach is also possible, proposal for this method should be discussed with Lloyd’s Register before submission.

Proposals to use other statistical methods should be discussed with Lloyd’s Register before submission for design review.

4.7 Derivation of Statistical Values for Extreme Sea States The design sloshing load for extreme seastates is to be based on the mean probable maximum 3-hour value associated with a seastate that will be encountered once in the design life. Hence the seastate is taken as the envelope seastate which will statistically occur once in the design life, see Chapter 2, Section 5. The standard assumption is that a seastate lasts for a 3 hour period. Each time this seastate occurs there will be a unique probable maximum 3 hour value associated with it. It is common with this kind of situation to treat each 3 hour seastate as an independent event in statistical terms. The design sloshing pressure is then based on a statistical analysis of the responses from each event. For extreme seastates, the design value may be taken as the mean value associated with statistical distribution of the probable maximum 3 hour values. The usual and easier method of deriving the mean probable maximum 3 hour values is to combine the statistical data from the each test run and derive the probable maximum 3 hour value associated with the combined pressure probability distribution function. As noted in 2.4, a sufficient number of tests are to be run for the critical sloshing cases to ensure statistical convergence of the mean probable maximum 3 hour values.

4.8 Derivation of the Statistical Values for Non-Extreme Sea States The design sloshing load for extreme seastates is based on the mean probable maximum 3-hour value associated with seastate that will be encountered once in the design life. This method does not consider the effect of the frequency of occurrence of lower seastates. The number of occurrences of lower seastates is much higher than the extreme seastate. As a consequence the ship is likely to see many of these lower seastates and hence it is necessary to derive the statistical sloshing load based on the number of seastates. For example, using the extreme value method, if the number of occurrences of the critical lower seastate is 100 in the design life, then the design value will be the probable maximum 3 hour associated with the a 1% probability of occurring.




For ships which are required to load or unload at offshore terminals or FLNGs, or similar, and for FLNGs, FPSOs etc, the probability of occurrence of heading and seastate needs to be carefully considered. There are several statistical methods that can be adopted.

1. Use Extreme Value Analysis The design sloshing load for non-extreme seastates is to be based on the probable maximum 3-hour value, P3hr, associated with a probability of occurrence of 1/Nsea where Nsea is number of non-extreme seastates for the critical seastate.

Each seastate is assumed to last for a 3 hour period. Each time this seastate occurs there will be a unique probable maximum 3 hour value associated with it. It is common with this kind of situation to treat each 3 hour seastate as an independent event in statistical terms. The design sloshing pressure is then based on a statistical analysis of the responses from each event.

Each test allows a probable maximum 3 hour value to be derived. These values are statistically independent and hence a probability density function can be fitted to these values, see Figure 2.3.8. Many tests will be required to allow the tail of the probability density function (PDF) to be well defined. A gamma distribution is likely to be appropriate for defining the probability density function of the probable maximum 3 hour values. The design value is then calculated from the probability density function as the value associated with a probability of exceedence of 1/Nsea. In view of the issue of accurately defining the tail of the PDF, an alternative procedure based on combining all the data from each test and fitting a distribution to all the data is likely to be more practical. In this case the design value is obtained by selecting the pressure that has a probability of occurrence of 1/(Nsea Nimp_sea), where Nimp_sea is the number of impacts occurring in one 3hr seastate.




Notes The histogram represents the distribution of P3hr values derived from a series of many model tests. A gamma distribution has been fitted to these results. The mean P3hr value represents the mean value of the P3hr from all tests. The extreme value analysis P3hr represents the P3hr that has a change of 1/Nsea occurring in Nsea seastates. Figure 2.3.8: Example of the use of extreme value analysis to derive the P3hr value associated

with a non-extreme seastate

2. Use a Long term analysis method

The use of a long term statistical method will also account for the number of occurrences of each seastate. In this method, the probability of the sloshing impact occurrence and the probability of the seastate occurrence (from wave scatter data) are combined together with the statistical characteristic pressure values for each seastate. This provides the long term sloshing design value at a certain probability limit. The long term method to be applied is essentially the same as that adopted in IACs Rec. No. 34 for the derivation of the long term vertical wave bending moment, however the equations are modified to account for the number of impact events instead of the number of wave occurrences.




The long term probability Q(P) to exceed an impact pressure value of P is defined as:

( ) ( ) ( )∫ ∫ ∫∞ ∞

=360

0 0 0_,,,,,,|1)( ββββ ddTdHTHpTHNTHPQ

NPQ ZSZSZSimpZS

imptot

Where; ( )β,,| ZS THPQ Probability of exceeding an impact pressure of P in seastate HS, TZ for wave

heading β. The short term probability is derived using the optimum curve fitting method identified for each seastate, see 4.4.

( )β,, ZSimp THN Number of sloshing impacts in seastate HS, TZ for wave heading β

( )β,, ZS THp Probability of observing the seastate HS, TZ for wave heading β. For seagoing ship operation the assumption of uniform probability is normal. For site specific operation, this will need to be specifically derived.

imptotN _ Total number of sloshing impacts the ship will encounter during its design life

The long term pressure is defined as the pressure that is expected to occur once in the design life of the ship, hence if there are Ntot_imp sloshing impacts during the life, then the design pressure is taken as the pressure associate with the highest one of those impacts, see Figure 2.3.9. the probability level is given by:

imptotNPQ

_

1)( =

A Weibull distribution is normally used for fitting the long term sloshing impact pressure distribution. It should be noted that the final long term results are very sensitive to the offset, shape and slope parameters of the distribution function. A parameter sensitivity check should therefore be carried out to review this. In particular, many tests may be necessary to ensure that the statistical distribution for the seastate and heading combinations that give high contributions to the long term value are suitably converged. Typically it is expected that the seastates that contribute most to the long term value are also likely to be the seastates that generate the maximum values using the extreme value analysis method.

Long Term probability plot1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00Pressure

Prob

abili

ty

Probability of 1/Ntot_imp

Long-term Pfor 1/Ntot_impimpacts

Figure 2.3.9: Long term design pressure




3. Use a Structural Reliability Analysis approach.

Using a Structural Reliability Analysis (SRA) approach enables a statistical measure of the probability of failure to be derived. In simple terms, the probability distributions of the sloshing impact load and the structural capacity are derived and compared. The area which is beneath both the load curve and the capacity curve is the area where failure of the system is possible and the size of this area corresponds to the probability of failure of the system.

The design is considered acceptable if the probability of failure is below a target level. This target level should be taken based on equivalent practices in other comparable industries, see Part C, Chapter 1, Section 5. The sloshing impact load probability distribution function needs to be derived taking account of all uncertainties in the load derivation, eg variation in heading, fill height, etc. This process is similar to that used to derive the long term impact pressure. The structural capacity probability distribution function needs to be derived taking account of all uncertainties in the capacity derivation, eg variation in dimensions, thickness, material properties, etc. This SRA approach is illustrated in Figure 2.3.10. This figure illustrates two sloshing load distribution curves and one design capacity distribution curve. For case 2, there is a large area of overlap between the load and the capacity curves, which indicates a high probability of failure. For case 1 the probability of failure is very small as there is virtually no overlap between the load and capacity curves. Hence case 2 represents a much safer design than case 1.

Figure 2.3.10: Assessment using Structural Reliability Analysis

4.9 Design Sloshing Load The structural response is dependent on the magnitude of the sloshing impact and load area and also the duration of the impact pulse, hence it is necessary to determine the typical duration associated with the design sloshing pressure loads. Typically a longer sloshing impact pulse duration will increase the structural response, and it may be the case that it is not always the highest impact pressure that causes the maximum structural response.




As a consequence the design sloshing load is not a unique value, but a matrix of related parameters. The design sloshing load needs to consider tank location, each CCS strength variation, load area shape and size, as well as magnitude and impulse rise/decay time variations. It is usual to assume the pressure impacts can be idealised as a triangular impulse function. The parameters for such a response are the pressure peak, Pmax, the rise time and decay time. The time history of the pressure impulse is usually very complex and it is not always clear how to idealise the pressure pulse. A typical assumption is to assume the rise time as twice the time between a pressure value of 50%Pmax and the peak Pmax, see Figure 2.3.11. A similar assumption is made for the decay time. In some cases this may not result in sensible rise times and different assumptions may be more appropriate. Figure 2.3.12 illustrates the variation of rise time with pressure for all impacts observed during a sloshing model test run. From these values, the most likely maximum rise time associated with various magnitudes of pressure impulses can be determined. The assessment of rise time is necessary for each individual load area appropriate for all the major containment system details, e.g. 90º and 135º dihedral corners, trihedral corners and flat areas. Note that the selection of rise time should also consider the natural resonances of cargo containment system and hull structure and rise times near these resonances may need to be investigated. The ratio of the decay time to the rise time represents the skewness property of the pressure impact. Skewness is defined as follows:

Time RiseTime DecaySkewness =

Previous experimental results have demonstrated that a large percentage of pressure peaks have a skewness of between 1.0 and 2.0. This is to be confirmed from analysis of the test data by plotting the skewness factor. From this an estimate of the applicable skewness factor for the dynamic FEA is to be made. It should be noted that a larger skewness factor, i.e. decay time longer than rise time, should usually be chosen as this will increase the structural response.

50%Pmax

Pmax

Decay timeRisetime

Figure 2.3.11: Example of idealising the impact pressure




Figure 2.3.12: Rise Time For a Design Sloshing Pressure





Chapter 1: Structural Assessment Overview

Section 1: Introduction Section 2:Strength Assessment by Finite Element Analysis Section 3: Strength Assessment by Experimental Methods

Section 4: Review of Major Containment Systems Section 5: Scope of Assessment

Section 6: Documentation

■ Section 1: Introduction This Part outlines the procedure for deriving the strength capacity of the membrane containment system and its supporting hull structure due to sloshing loads. The sloshing impact loads acting on the containment system are transferred to the supporting hull structure and it is therefore necessary to ensure that both the containment system and hull structure have sufficient strength to withstand the sloshing loads. The strength of the containment system can be assessed by calculation using Finite Element Analysis (FEA), by experimental testing or a combination of both methods. The methods proposed in this Part apply to the LNG ships with membrane containment systems. There are two types of containment system that are most often used in membrane tank LNG ships. Both types are designed by GazTransport & Technigaz (GTT). The Mark III system is a layered foam type containment system and the NO96 system is constructed of plywood boxes. Both containment systems provide primary and secondary barriers in compliance with the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code). The systems differ in insulation materials, retaining structures, membrane materials, construction and mounting systems to the surrounding hull structure. As a consequence the structural assessment of each system requires individual consideration. GTT have also designed other containment systems, e.g. CS1, which is similar to the NO96 and Mark III systems in some respects. The principles of the methods described in this Procedure can be applied to the design of any containment system provided due consideration is given to individual design features. These methods need to be used for new containment systems and to improve the knowledge of existing containment systems when it is considered necessary to enhance or review the performance of such systems.

■ Section 2: Strength Assessment by Finite Element Analysis There are basically two finite element approaches which can be used to assess the structural strength of the system and the supporting hull structure, see Figure 3.1.1. Method 1: Direct Dynamic FEA. The structural response of the system to maximum design sloshing impact

load scenarios is calculated using dynamic FEA methods. With this approach, the results are only applicable to the specific design considered as the assessment is only undertaken for the design sloshing impact load scenarios calculated during the evaluation of the design sloshing load for this vessel design.




Method 2: Indirect Dynamic FEA. A combination of static and dynamic FE calculations based on representative sloshing impact load scenarios are used to derive the static response and a Dynamic Amplification Factor (DAF) Envelope curve respectively. The total dynamic structural response is derived by factoring the static response by an appropriate DAF value to determine the “dynamic” response of the system. With this approach the results can also be used for other similar ship designs with the same containment system as the assessment is undertaken on the basis of an envelope approach to the design sloshing load scenarios.

Further discussion of these assessment approaches is given in Chapter 2. Other structural analysis methods such as those based on coupled fluid structural interaction solutions may be applied, the proposed procedure is to agreed with Lloyd’s Register prior to commencement of the assessment. Experimental testing is necessary to verify that the selected FEA models and analysis techniques are adequate.

Dynamicamplification factor

curves

FE model of membranecontainment system and its

supporting hull structure

Direct DynamicFE analysis

Design sloshingload

Acceptancecriteria

Material properties atin-service condition

Failure orpass

Static FE analysis

Method 1 Method 2

Figure 3.1.1: Strength Assessment by Finite Element Methods

■ Section 3: Strength Assessment by Experimental Methods Experimental testing of the membrane system and its components under representative conditions is an alternative method of strength evaluation. The experimental testing needs to be as realistic as possible and should aim to take into consideration the likely sloshing impact load scenarios, cryogenic conditions, flexibility of the supporting hull structure, etc. Such conditions are difficult to replicate in practice and hence a mixture of experimental testing and calculation is necessary to establish the structural capacity of the membrane system.




■ Section 4: Review of Major Containment Systems

4.1 Plywood Box Type Containment System The most commonly used plywood box type containment system is known as the GazTransport NO96 system designed by GTT, see Figure 3.1.2. The system uses a thin sheet of high nickel alloy, Invar, as the primary barrier. The secondary barrier is of the same material and similar thickness to the primary barrier. The insulation system consists of two layers of plywood boxes, which are filled with a granular insulation material, Perlite, or glass wool. The boxes have parallel internal members (bulkheads), which are also made of plywood sheet. Staples are used to fasten the plywood box covers to the external and internal bulkheads. The secondary barrier is located between a primary box and a secondary box. As it is essential that the internal surface of the plywood boxes are flat to support the Invar membrane, mastic “ropes” (also known as resin ropes) are laid on the bottom surface of the secondary boxes adjacent to the hull plating to remove any undulations in the hull plating. The mastic cures against a thin sheet of waxed paper which prevents fixed attachment to the hull. The plywood boxes are held in place by an arrangement of rods, tensioned by spring washers, which are secured via sockets welded to the inner hull. The invar membranes are held in place and made liquid tight by welding to "tongues" which are retained in slots in the plywood boxes..

Figure 3.1.2: Arrangement of GTT NO96 Containment System

4.2 Layered Foam Type Containment System The most commonly used layered foam type containment system is the Technigaz Mark III system designed by GTT, see Figure 3.1.3. This system uses a corrugated membrane of sheet austenitic stainless steel as the primary barrier. The secondary barrier is a layer of Triplex which is a thin aluminium foil with glass fibre cloth glued to each side. The insulation for the primary and secondary barrier consists of polyurethane foam reinforced with glass fibre (R-PUF). A plywood sheet is glued to the top surface of the primary R-PUF layer. The primary membrane is welded to stainless steel "anchoring strips" which are recessed and riveted to the plywood sheet. The Triplex barrier is glued directly to the R-PUF layers. A plywood sheet is glued to the bottom of the secondary R-PUF insulation to which mastic (resin) ropes are applied. Unlike the NO96 system, the mastic "ropes" adhere to both surfaces and serve to glue the containment system directly to the inner hull surface. The complete insulation system including the R-PUF, secondary barrier and upper and lower plywood sheets are manufactured as prefabricated panels. The panels are positioned during installation by a system of studs




welded to the inner hull bolted through the lower plywood, but the greater part of the strength of attachment is provided by the mastic after it cures.

Figure 3.1.3: Arrangement of GTT Mark III Containment System

■ Section 5: Scope of Assessment The purpose of the strength analysis is to ensure that the design of the containment system, it’s components and the hull structure is adequate to resist the design sloshing loads. The strength of the containment system needs to be assessed in the following areas:

• Flat panel areas • Connection areas in way of 90º and 135º dihedral corners (e.g. intersection of the transverse

bulkhead and inner trunk deck) • Connection areas in way of trihedral corners (e.g. intersection of the transverse bulkhead, inner

trunk deck and hopper tank side) It should be noted that the strength of the containment system varies to account for the higher sloshing loads expected in some areas of the tank. Typically, for a conventional LNG ship design with a barred fill range restriction, the containment system used for the upper 30%H of the tank has a greater structural capacity than the lower 70%H. The structural assessment must review the strength of all containments system components taking into account their appropriate strength properties. For all containment systems it is necessary to establish the overall strength of the system and the strength of the individual components. Suitable design criteria, see Part C are to be evaluated taking account of the materials used and the arrangement of these materials. For some corner areas, the structural components can be complex to mathematically model and direct calculations may be therefore very difficult. It may be more appropriate to undertake the assessment of such corner connections using a combination of structural modelling and physical testing.

■ Section 6: Documentation The following documentation is to be provided and submitted to Lloyd's Register as part of the design review:

• List of plans used including dates and versions • Detailed description of structural modelling including all modelling assumptions




• Plots to demonstrate correct structural modelling and assigned properties • Details of material properties used • Details of boundary conditions used in the FE model • Details of applied loadings and confirmation that individual and total applied loads are correct • Plots and results that demonstrate the correct behaviour of the structural model to the applied

loads • Summaries and plots of global and local deflections • Summaries and sufficient plots of von Mises, directional and shear stresses to demonstrate that

the design criteria are not exceeded in any component • Buckling analysis and results • Tabulated results showing compliance, or otherwise, with the design criteria • Proposed amendments to structure where necessary, including revised assessment of stresses and

buckling properties.




Chapter 2: Finite Element Analysis

Section 1: Introduction Section 2: General Finite Element Modelling

Section 3: NO96 Containment System Finite Element Modelling Section 4: Mark III Containment System Finite Element

Modelling Section 5: Hull Structure

Section 6: Materials Section 7: Boundary Conditions

Section 8: Further Analysis Considerations

■ Section 1: Introduction The purpose of carrying out a finite element analysis (FEA) is to investigate the strength of the containment system and hull structure under the expected design sloshing loads. This Chapter outlines the requirements for structural modelling, application of loads and strength assessment methods. Normally linear FEA is to be used for the assessment of NO96 style containment systems which use plywood boxes as the main strength component. A combination of linear and non-linear FEA is likely to be needed for assessment of Mark III containment system types which use structural foam as the primary strength and insulating material. For other containment system designs, the proposed analysis methodology is to be agreed with Lloyd’s Register prior to the commencement of the assessment.

■ Section 2: General Finite Element Modelling For “absolute procedures” it is necessary to assess the structural strength using a finite element analysis of the containment system and the hull structure. For the enhanced comparative method, it is recommended that the hull structure is included in the structural analysis. A sufficient portion of the hull structure and containment system is to be modelled to ensure that the model correctly represents the actual structural response. In general, the hull model should cover three to four web frame spacings. The requirements for the extent of the hull structure to be incorporated into the containment system model are given in Section 5. The minimum extent of the containment system to be modelled is one box for the NO96 type containment system and an area of approximately 1m x 1m for Mark III type containment system. More boxes or a larger area of the containment system may be required depending on the detail being modelled and the type of analysis required. When the containment system is being assessed in a corner region, then the area of containment system modelled should include the whole area around the corner. As there is no similarity between the NO96 and Mark III containment systems, different considerations, in terms of mesh size, element type and boundary conditions are required. The following Sections give guidance on suitable modelling techniques for the NO96 and Mark III type containment systems. These sections are also




applicable for the modelling of containment systems which are similar to these two systems. For other containment system designs, it is recommended that the designer discuss the modelling requirements with Lloyd’s Register at an early stage of the analysis.

■ Section 3: NO96 Containment System Finite Element Modelling The following guidelines are given for the global modelling of a NO96 containment system. It may be necessary to undertake detailed 3D solid models or experimental testing to ensure that the global model is suitably representative. The guidelines focus primarily on the requirements for boxes within flat areas. Special consideration will be necessary for the application to boxes and local details in the corners where the arrangements can be very complex. For the assessment of the NO96 containment system, it is generally adequate to model one complete primary and secondary box. When considering the response of the hull structure it is necessary to model more that one box or to include the stiffness and mass of un-modelled boxes in a simplified manner. Such an extension is necessary as adjacent boxes to the loaded box have a significant effect on the structural response of the hull structure. A reasonable representation of the box fixation arrangements, i.e. connection rods, is usually not important in these analyses. The primary and secondary insulation boxes can be modelled as a continuous structure. It may be assumed that the staples between the cover plates and the box bulkheads provide full bending support and do not need to be modelled. Based on this assumption, a separate assessment of the effectiveness of the staples at these connections may be necessary. The primary and secondary Invar membrane barriers may be ignored. The box cleats may also be ignored from the global membrane system strength assessment. The plywood may be modelled as a linear material with orthotropic properties. Alternatively, multi layer shell elements may be used to model the composite nature of the material. This enables a more accurate representation of the properties of plywood. Each layer is to be defined using single ply laminate properties. Illustrations of the modelling details are provided in Figure 3.2.1 to Figure 3.2.5.

3.1 Box Covers and Bulkheads The box covers and bulkheads are to be modelled using shell elements located at the mid thickness of the plywood sheets. At least 5 elements are to be used for the box cover and base between internal box bulkheads. Typically the expected element size will be in the region of 25 x 25 mm. In some regions the model may have to be refined to allow modelling of the mastic connection and the alignment of the box covers. The box covers should be modelled at the correct locations, this can be achieved by offsetting the cover plates from the top and bottom of the bulkheads or artificially extending the bulkhead depth. In the case of the reinforced primary box with double cover plates, the upper and lower cover plates should be modelled as separate entities and offset the appropriate distance. The cover plates should be connected by stiff spring elements or similar to maintain separation when loaded. Ideally, the two cover plates should be modelled using Gap elements (NASTRAN terminology or the Contact constraint in ABAQUS) or equivalent with friction between them and a no sliding constraint at the staples. Such a model is very complex when




applied to a dynamic FEA. It is therefore acceptable that at the staple locations, stiff springs are used in the direction normal to the plane and in-plane to restrict movement at the staple location. The 6 mm wide gaps in the plywood primary box top cover(s) in which the Invar tongues are located, are to be explicitly modelled. The double thickness internal bulkhead below the cover plate is recessed to take the Invar tongue. The depth of this recess can be included by splitting the thick internal bulkhead into two at this depth. The cover plates are then attached to the top of the split bulkhead elements. This will improve the modelling approximation of the bending support provided to the top cover(s), see Figure 3.2.1.

Figure 3.2.1: Modelling of slot in top cover for invar tongue and connection between upper and lower cover plate

The internal and external bulkheads are to be modelled with at least 7 shell elements over the depth of the box. The air holes in the box may be included by omitting the applicable elements. An element aspect ratio of approximately 1 is to be used wherever possible.

3.2 Invar Primary and Secondary Membranes For most applications it is not necessary to include the primary and secondary membranes in the global FEA model as they provide little contribution to the overall stiffness to the containment system. If modelled they are to be included as thin shell elements, with isotropic material properties. The membranes are typically of the order of 0.7 mm thickness. The membrane is to be offset by the requisite distance so that it is located on top of the plywood. It may be assumed that the membrane is always in full contact with the primary box top cover. The modelling of the secondary membrane is to meet the requirements in 3.4.

3.3 Mastic The mastic or resin ropes should be modelled as solid (3D) elements. At least two solid elements should be used with localised mesh refinement of the lower box cover and hull plating to match. The mastic is bonded to




the bottom plywood cover of the secondary box and is free to slide over the steel hull plating and to separate (in the normal direction) from the plating. Ideally this connection should be modelled with gap elements or equivalent, but this increases the complexity of the analysis significantly. In general, springs may be assumed between these components in the structural model, see 3.5. However it may be necessary to review these assumptions and undertake an analysis to assess the effects of this simplification.

3.4 Connection between the Primary and Secondary Boxes The lower cover of the primary box sits on top of the upper cover of the secondary box. These covers should be modelled as separate entities and offset the requisite distance. The cover plates should be connected by stiff spring elements, see 3.5, or contact constraints to maintain separation when under load. The movement between the two covers is restricted in practice by the presence of the Invar tongues and the connection rod arrangement.

3.5 Spring Elements The stiffness properties of spring elements used in the model should enable a realistic approximation of the correct physical response of the structure to be obtained. Several test analyses may be necessary to derive the appropriate properties. For example studies were undertaken by Lloyd’s Register to determine whether modelling the” free to separate and slide” condition between the mastic and the hull plating using spring elements was acceptable. This was achieved by comparing results using a linear analysis with spring elements with a non-linear analysis using gap elements. The displacement and stress results from the linear and non-linear static analyses were found to be very similar, which indicates that it is reasonable to use linear spring elements to simulate this contact condition. Table 3.2.1 gives values of spring stiffnesses that were found to be appropriate by Lloyd’s Register. Table 3.2.1 Example spring stiffnesses Spring stiffness property Between the double cover plates of the Primary box at the staple locations Axial 300,000 N/mm

Shear 10,000 N/mm Between touching points of primary box bottom cover and secondary box top cover

Axial 300,000 N/mm Shear 0

Between the mastic and the hull plating Axial 300,000 N/mm Shear 0

3.6 Connection Rod to Hull Plating Each NO96 containment box is fixed to the ship’s hull by pre-tensioned coupler rods. Lloyd’s Register investigations suggest that the coupler rods do not have a large influence on the dynamic strength assessment of NO96 boxes coupled to the ship’s hull structure. The coupler rods do have a noticeable influence on hull plating areas in way of the fixation points and locally on the boxes, but this effect is small. If the connection rods are not included in the assessment then it is recommended that a partial factor of 1.05 is applied to all results, see Part C, Chapter 1, Section 1.4




For models that include the connection rod, the rod can be modelled as a beam element attached at its lower end to the hull. The rod has a pin ended fixation at the hull but can be considered as effectively fixed at the connection to the boxes. The connection plate which pulls the boxes onto the hull via the cleats may be represented as a series of stiff beams to transfer the load across the box area below the cleats. Pre-tensioning in the rods can be applied by splitting the rod into two and applying a prescribed displacement between two ends of the rod. Generally, each coupler rod has approximately 3 kN pre-tension force which is split between the 4 connected boxes. The behaviour of the spring washers which are part of the coupling system may be ignored. Figure 3.2.2 shows a typical FEA model demonstrating the arrangement of the coupling between the hull surface, connection rod and insulation boxes.

Stiff beams for cleatson Secondary Box

Stiff beams for cleatson Primary Box

Tension applied byprescribed

displacementbetween these two

nodes

Stiff beams for theConnection Rod

Figure 3.2.2: Example of modelling the Coupler Rod and Rigid Beam Elements to attach the boxes to the hull plating

3.7 Connection between Adjacent Primary and Secondary Boxes Longitudinally there is a 60 mm gap between the primary and secondary box bulkheads which is filled with an insulating material. There is approximately a 10 mm gap between the box covers. Transversely there is a very small gap between adjacent box bulkheads and covers. The upper corners of the adjacent secondary boxes are attached via a plate to a connection rod which also clamps this box to its adjacent secondary insulation box. If only one NO96 box is included in the assessment then it is recommended that a partial factor of 1.05 is applied to all results, see Part C, Chapter 1, Section 1.4 When several boxes are to be included the following method of modelling is suggested:




The insulating material filling the 60 mm gap between the secondary box bulkheads may be modelled using soft springs so as to limit movement between the boxes. The compressive stiffness of the insulating material provides a small amount of support to the lateral bending of the external box bulkheads. Transversely, the secondary box covers are effectively butted against each other and this can be represented by stiff spring elements. A FEA model section illustrating the connection between primary and secondary boxes is given in Figure 3.2.3. Table 3. 2.2: Example spring stiffnesses Spring stiffness property Insulating material in the 60mm gap between boxes Axial 100 N/mm Springs between adjacent secondary box covers in the small gap between boxes Axial 3,000 N/mm

Figure 3.2.3: Example of springs to represent the foam between adjacent boxes




Figure 3.2.4 : Typical Model Including Primary and Secondary Boxes

Figure 3.2.5: Internal View

3.8 Detailed Analysis Finite Element Models For localised stress concentration investigations, 3D solid element Finite Element Analyses will be required. This will be required to assess the combined shear and buckling response and capacity of the cover plate in way of the bulkheads to the sloshing loads. 3D FEA may also be necessary to be assess the response and capacity of local details such as cleats and the methods of fixing the cleats.




Typically detailed 3D solid models using a t x t x t element size, where t is the thickness of a ply layer, will be necessary to quantify the local stress concentration factor (SCF) and derive a suitable acceptance criteria. In some cases the load and displacements from the global model may be suitable for application to the detailed analysis, see Part C, Chapter 2, Figure 4.2.1; in other cases the load cases will need special consideration.

■ Section 4: Mark III Containment System Finite Element Modelling The following guidelines are given for the global modelling of an Mark III containment system. It may be necessary to undertake detailed FEA or experimental testing to ensure that this global model is suitably representative. An example FEA model for the Mark III containment system using higher order 20-noded solid elements is illustrated in Figure 3.2.6. The guidance focuses primarily on the requirements for panels within flat areas. Special consideration will be necessary for the application to boxes and local details in the corners where the arrangements can be very complex.

4.1 Top and Bottom Plywood Covers Each plywood cover is to be modelled using single layer thin shell elements with orthotropic material properties. The size of the elements used for modelling is dependent on the order of the elements selected. For higher order 8-noded shell elements be used, a 30mm x 30mm size is considered sufficient. For 4-noded shell elements, a mesh of 10mm x 10mm, will be required. Generally at least 5 4-noded shell elements or 3 8-noded shell elements are needed between mastic “ropes” in the area where the bending strength of the plywood plate will be checked. The plywood covers are to be offset from the foam by the appropriate distances.

4.2 Primary and Secondary Insulation Layers of R-PUF Each layer of the R-PUF, including the top bridge pad, is to be modelled using a number of solid (3D) elements. The size of the elements used for modelling is dependent on the order of the elements selected. Should higher order 20-noded solid elements be selected, a 30mm x 30mm x 30mm element size is considered sufficient. For 8-noded solid elements, a mesh size of 10mm x 10mm x 10mm, will be required. For a non-linear analysis, elements with non-linear properties which take into account crushable plasticity are to be used. These elements provide a more accurate model but significantly increase the complexity of the analysis.

4.3 Slots in the R-PUF In the primary layer of the R-PUF and the top plywood cover there are narrow slots beneath the membrane corrugations which extend for nearly the full depth of the primary R-PUF layer. The purpose of these slots is to allow expansion and contraction of the membrane without putting high stresses into the R-PUF. These slots are structurally significant and are to be explicitly modelled. The possible contact effects between two sides of the slots under sloshing impact loads may be ignored as it can usually be assumed that thermal contraction will open up the slots more than the impact loads will close them.

4.4 Mastic Ropes




Mastic “ropes” are to be modelled as solid elements with isotropic material properties. The size of the mastic elements needs to be consistent with the element sizes in the bottom plywood.

4.5 Triplex Layer (Secondary Membrane Barrier) The Triplex layer is to be modelled using thin shell elements with isotropic material properties. The Triplex secondary barrier is usually of the order of 0.6 to 0.7 mm thick and glued to the R-PIF or overlapped flexible Triplex cloth. It is usually acceptable to ignore the glue and just model the triplex with a nominal thickness.

Figure 3.2.6: General View of Mark III Containment System Model, (20-Noded Solid Element Model)

4.6 Primary Membrane For most applications it is not considered necessary to include the stainless steel corrugated primary membrane in the global FEA model as it contributes very little stiffness to the containment system. The advantage of including the primary membrane is to simplify the application of the sloshing loads and to simplify the inclusion of the primary membrane mass in the dynamic analysis. Including the membrane with the corrugations modelled allows the sloshing load to be applied to the membrane and this automatically correctly distributes the load into the plywood and R-PUF. If the membrane is included, then it can be modelled using thin shell elements with isotropic material properties. The corrugations may be represented in a simplified manner as shown in Figure 3.2.7 where the corrugation is approximately modelled using 6 shell elements and the corrugation knots are ignored. The membrane is to be offset the appropriate distance above the top ply of the plywood. It may be assumed that the membrane is in fixed contact with the top plywood cover for load application purposes and the offset of the membrane from the mid-thickness of the plywood may be ignored.




Figure 3.2.7: Coarse Mesh Finite Element Model of Primary Membrane (8-Noded Shell Elements)

It is also necessary to undertake a detailed analysis of the membrane to review the plastic deformation of the membrane. This will need to be at least a quasi-dynamic (slow load application) non-linear geometric FEA. The detail of modelling required to undertake such a analysis is illustrated in Figure 3.2.8.

Figure 3.2.8: Detailed Model Required for the Assessment of the Primary Membrane




4.7 Connectivity of the Containment System Components The connections between the various containment system components are to be modelled as follows.

• The top and bottom plywood covers are fully bonded to the R-PUF, hence full connectivity is to be assumed.

• The bottom of the primary R-PUF layer and the top of the secondary R-PUF layer are fully bonded

to a Triplex layer, comprised of a mixture of rigid Triplex, flexible Triplex and glue, and hence full connectivity is to be assumed.

• The mastic ropes are bonded to both the lower cover of plywood and the steel hull structure, full

connectivity is to be assumed.

For global modelling, it may be assumed that the stainless steel membrane is rigidly connected to the top plywood cover. The stainless steel corrugated membrane is supplied in sheets, approximately 1 m wide by 3 m, and is welded to anchor strips. These thin anchor strips are riveted to the plywood and may be neglected. If the thin metal inserts and their screw fixings are to be analysed then a more detailed analysis model will be necessary.

■ Section 5: Hull Structure

5.1 Extent of the Hull Structure to be Modelled It is important to assess the interactions between the containment system and the hull structure and to review the effect this has on the global and local structural responses of both systems. In order to do this it is necessary to include a section of the hull structure within the analysis. The extent of hull structure modelled is to ensure that the static and dynamic response of the secondary stiffeners and primary members in way of the containment system area of interest are not affected by the boundary conditions being placed too close to the area of interest. If the global dynamic response of the bulkhead or inner hull is expected to be significant in modifying the local dynamic structural response, then the extent of hull structural model will need to be specially considered. In general, for a containment system attached to a flat portion of the inner hull or transverse bulkhead, the hull structure model should have the following extents, see Figure 3.2.9:

• Longitudinally or transversely: at least one web frame bay either side of the containment system model. In total the model should extend at least 3 web frame bays.

• Vertically: between adjacent longitudinal stringers.

For a 90º upper corner containment system detail attached to the transverse bulkhead and the inner trunk deck, the hull structural model should have the following extents, see Figure 3.2.10:

• Longitudinally: one web frame bay aft and one web frame bay forward of the containment system detail model.

• Transversely: between adjacent primary girders.




• Vertically: from the trunk deck down to the level of the next primary or hull girder member below the inner trunk deck, typically the main deck at side.

NOTES: 1. Reverse side of bulkhead plating removed to show internal details

Figure 3.2.9: Extent of Hull Model for Attachment to a Transverse Bulkhead

NOTES: 1. Upper deck plating removed to show internal details

Figure 3.2.10: Extent of Hull Model in way of Upper Corner Detail




In general the most critical locations with respect to the induced stresses in the containment system and the hull structure are where the containment system is sited over stiff parts of the hull structure, for example: where the mastic runs over a stringer or stiffener or at the intersection of these components.

5.2 Modelling Details The hull structure is to be modelled using shell (plate bending) elements for the plating and stiffener webs in way of the area of interest. The mesh density is to match the mesh arrangement of the containment system. The hull plating mesh density in way of the mastic must be very fine, but can be rapidly coarsened away from this area. Stiffeners for the frame bay in way of the containment system are to be modelled using at least six elements for the stiffener webs and three shell elements for stiffener flanges. Away from the area of interest, the mesh density may be reduced but at least three elements are to be used to model the webs of stiffeners within the adjacent web frame spacing, see Figure 3.2.10. This ensures the correct dynamic response of the stiffener is represented.

■ Section 6: Materials The material properties used in the structural modelling should take account of the effects of temperature and the appropriate strain rate. The temperature dependent material properties should be selected after undertaking a thermal analysis, see 8.1. For the “absolute procedures” it is necessary to assess material strain rate effects in the dynamic FEA, see also Chapter 4, Section 2. Proposals for procedures to include this are to be agreed with Lloyd’s Register. For the enhanced comparative method, the strain rate effect may be ignored. If material strain rate effects are not included in the assessment then it is recommended that a partial factor of 1.0 is applied to all results, see Part C, Chapter 1, Section 1.4, as ignoring the strain rate effect is assumed to give conservative results. Details regarding material properties are provided in Chapter 4.

■ Section 7: Boundary Conditions

7.1 NO96 Containment System If the hull structure has been modelled, then it is not necessary to apply boundary conditions to the NO96 containment system boxes. For the “absolute procedures”, then the presence of adjacent boxes needs to be considered. Guidance on modelling of adjacent boxes is given in 3.7. If these are not explicitly modelled, then the stiffness of the omitted boxes and the omitted mass must be considered as this will affect the dynamic response of the system, see 8.2.




7.2 Mark III Containment System The Mark III containment system is composed of large blocks, approximately 3x1m. The presence of the top bridge pads and primary membrane effectively bond the adjacent blocks together. Hence the system can be considered as effectively a continuous medium. It may be necessary to apply some constraints to the edge faces of the modelled foam and plywood covers depending on the extent of the Mark III model. These conditions will depend on the model extent and load case being considered:

• If the face is used as a symmetrical boundary due to the extent of modelling and load application, then symmetry conditions should be applied.

• If the face is remote from the loaded area, then it is likely that the influence of the part not

modelled should be minimal and the ends of foam and plywood should be restrained from moving in plane. Vertical deflection (through the foam thickness) should be left free. All rotations should be unrestrained.

7.3 Hull Structure The ends of the hull structure model can be assumed to be fully supported, i.e. ‘built-in’ with all degrees of freedom fixed.

■ Section 8: Further Analysis Considerations

8.1 Thermal Effects The material properties of some components of the containment system are affected by the in-service temperature for the location in the containment system. In particular the temperature through the R-PUF of the Mark III system varies from cryogenic temperature at the primary membrane to near ambient temperature at the bottom plywood cover. For the “absolute procedures” it is necessary to determine the temperature distribution through the depth of the containment system and derive the appropriate stiffness and strength properties. For the enhanced comparative method, the temperature effects may be ignored. An initial thermal analysis is required to allow the appropriate temperature dependant material properties to be selected for the containment system components. The temperature distribution within the containment system can generate stresses due to the different thermal coefficients of expansion of the various components and these may need to be considered in the assessment. For new or novel containment systems, a thermal stress analysis is necessary to review the resulting stresses. This can be considered as a separate analysis and the resulting stresses and deformations added to the dynamic strength analysis.




8.2 Accounting for Un-Modelled Components For the “absolute procedures”, the masses of un-modelled components of the containment system are to be accounted for in the structural model to ensure that the total mass of the containment system is correct for the dynamic analysis. For example the mass due to the Perlite insulation material in the NO96 system needs to be included. This can be achieved by specifying the Perlite as a “non structural mass” and applying this to all the nodes of the boxes or alternatively by adjustment of the density of the plywood. Where the un-modelled components of the containment system provide structural stiffness that will affect the dynamic or static response of the area of interest, this structural stiffness should be included in the model. If the containment system is not modelled over the hull structure frame bays adjacent to the area of interest, then the stiffness of the omitted containment system components should be represented as plate bending elements offset from the hull plating with properties that provide a similar stiffness to that of the combined containment system and hull structure. The mass of the plate elements is to represent the omitted containment system components. For a static analysis then there is no need to include the mass of un-modelled components.

8.3 Added Mass Effects After a sloshing impact event, it is likely that the membrane in way of the impact will remain wetted with LNG for some time after the impact. This will affect the dynamic response of the containment system; in particular the response of the primary box cover plate of the NO96 system and the foam response of the Mark III system. When the natural vibration period of the components of the containment system is less than the rise time of the sloshing impact pressure pulse, then the added mass effect of the liquid will usually increase the natural vibration period. This may result in a larger structural response of the system. Additionally, the resonant periods of the hull structure also need to be included as the containment system is effectively sitting on an elastic foundation. For the “absolute procedures” it is necessary to assess the added mass effect in the dynamic FEA. This may be included using the approach given below, proposals for alternative procedures to include the added mass effect are to be agreed with Lloyd’s Register. For the enhanced comparative method, the added mass effect may be ignored. The added mass is to be calculated based on the assumption of a static pressure distribution using a horizontal waterline at the assumed upper extent of the area of sloshing impact being studied. When the sloshing impact is on the underside of the containment system attached to the inner trunk deck, then a slight static overpressure level is to be applied to ensure that the area of interest is wetted. A static head of 0.15 m is considered suitable, see Figure 3.2.11. To calculate the added mass, it is necessary to model the wetted surface of the primary membrane as a continuous plane. This plane must extend across different tank walls when the location being considered is close to a corner. In theory, the possible wetted surface is the whole of the primary membrane of the containment system. In practice it is not necessary to model the membrane of the whole tank, but it is necessary to include a significant portion of the possible wetted surface of the membrane otherwise the added mass calculation will be incorrect. As only a small area of the primary membrane (or top cover plate if the membrane is not explicitly modelled) of the containment system is modelled then it is necessary to represent the omitted part of the membrane in the added mass calculation.




The added mass may be estimated by the boundary element approach using a fluid element method such as the MFLUID option in MSC NASTRAN. The MFLUID card is used to define the fluid properties and liquid fill height and the ELIST cards are used to define the hydrodynamic wetted panel surface, see Figure 3.2.11. The following paragraphs illustrate how to estimate the added mass using the functions available in the MSC NASTRAN FEA program. For other FEA programs, alternative methods of application should to be considered. Hydrodynamic wetted panels or fluid elements are added to the FEA model which duplicate the primary membrane (or top cover) elements, these must map the primary membrane elements exactly. Fluid elements must then be added to represent the missing portion of the primary membrane not included in the model. Dummy overlay elements are to be used to extend the primary membrane beyond the area of the containment system model. A dummy overlay element is required for each hull plating element. They are to have zero stiffness properties, are to be connected to the hull plating by rigid links or similar and are to be offset from the hull plating by the total depth of the containment system. In practice, the fluid element is just a normal shell or plate element, but with negligible stiffness and negligible material properties. The region that these dummy elements should extend over is defined by;

• Horizontally: approximately 10 m from the edge of the area of interest in both directions. This area is to extend around corners where necessary, in which case the area modelled is to extend a further 10 m or so from the corner, see Figure 3.2.12.

• Vertically: just above the area of impact to approximately 10 m below the area of impact.

The area of these dummy overlay elements may be larger than the portion of the hull structure modelled. When this is the case the dummy overlay elements outside the extents of the modelled hull structure are to be connected to a fixed reference datum plane (or ground) using rigid elements. Figure 3.2.11 illustrates how to define the hydrodynamic wetted panels for the containment system attached to a transverse bulkhead or attached to the inner trunk deck. Figure 3.2.13 illustrates the process applied to an FEA model of an Mark III containment system panel attached to the inner trunk deck. This example shows the “upstand” necessary to achieve a liquid free surface within the range of hydrodynamic wetted panels and the dummy overlay elements which are required to define the upper corner of the tank. If added mass effects are not included in the assessment then it is suggested that the following partial factors are applied to all results, see Part C, Chapter 1, Section 1.4:

• For NO96 systems: partial factor = 1.1

• For Mark III systems: partial factor = 1.0. Studies have shown that including the added mass has little effect.




Dummy overlay hydrodynamicwetted panels elements.To extend for about 10m downthe Bhd

Containment system box

Assumed liquidlevel at impact

Hydrodynamic wetted panelson containment system box

Trunk deck

TransverseBhd

Extent ofhull

structuremodeled

Example showing the assessment of a containment system attached to a transverse bulkhead

Example showing assessment of the containment system attached to the inner trunk deck

Figure 3.2.11: Example Definitions of the Hydrodynamic Wetted Panels for the Added Mass




NOTES: 1. The “upstand” necessary for the liquid to have a free surface within the added mass element list, see also Figure 3.2.13.

Figure 3.2.12 : Example of Hydrodynamic Wetted Panels for Added Mass effects and FEA Model

NOTES: 1. Very fine mesh in way of the modelled containment system boxes, note the meshing on this FEA model allowed 4 panels of Mark III attached to the inner trunk deck to be specified. 2. The “hole” is where the primary membrane sheet of the modelled area of the containment system panel model is located. In this case the hydrodynamic wetted panels were attached to the Mark III primary membrane and hence closed the “hole”.

Figure 3.2.13: Example of Hydrodynamic Wetted Panels for Added Mass Effects




Chapter 3: Dynamic Structural Response

Section 1 : Introduction Section 2: Direct Dynamic FEA Analysis

Section 3: Indirect Dynamic FEA Analysis Section 4: Repeated Impact Loads

■ Section 1: Introduction The purpose of the structural assessment is to derive the dynamic response of the containment system to the applied sloshing loads, taking into account the following aspects:

• the load distributing and dissipating properties of the containment system • the flexibility of the hull structure • the load concentrating effect of the hull structure due to the rigidity of the stiffeners or stringers

supporting the containment system For the “absolute procedures” it is necessary to use dynamic FEA as part of the process to derive the strength and capacity of the containment system and the hull structure. For the enhanced comparative method, then a static FEA may be used as part of the process to derive the strength and capacity of the containment system (and hull structure if included) may be acceptable. The static FEA should be based on the guidelines and principles given in this document. The ultimate capacities of the various components of the containment system and supporting hull structure need to be evaluated. The ratio between the Dynamic Response and the Ultimate Capacity gives the load/capacity utilisation ratio which is compared against the applicable design utilisation factor criteria, see Part C, Chapter 1, Section 5 to verify that the system is acceptable. As discussed in Chapter 1, Section 2, the two suggested methods to assess the structural response to sloshing loads are:

• Direct Dynamic Finite Element Analysis (FEA) • Indirect Dynamic FEA

The Direct Method applies the design sloshing loads scenarios directly to the containment system using a dynamic FEA. This is a potentially more straightforward process than the Indirect Method. The Indirect Method applies representative sloshing loads, defined by a nominal sloshing pressure and a range of rise times, to derive a Dynamic Amplification Factor (DAF) Envelope curve. The results from a static FEA analysis are then factored by the maximum DAF value to derive the dynamic structural response. The advantage of using the indirect method is that the analysis results are effectively independent on the actual design sloshing load scenarios derived from the design load, see Part B. However, it is more likely to provide a conservative estimate of the response. The results from the Indirect Method are also potentially available for use with other similar projects where the hull structure and containment system structure are very similar. The results from the Direct Method may not be transferable as the design sloshing loads scenarios may be quite different.




■ Section 2: Direct Dynamic FEA Analysis The Direct Dynamic FEA is to be performed by applying the impact pulses representative of the design sloshing load scenarios directly to the FEA model and analysing these using dynamic FEA techniques. Each possible variation of sloshing impact pulse needs to be explicitly considered as a FEA load case as the Direct Dynamic FEA method gives the dynamic response of the system to the applied sloshing loads without any further post processing of the results. The sloshing load scenarios that need to be considered for the dynamic FEA have to take account of the following issues:

• The variation in the sloshing load peak magnitude, see Part A, Chapter 3, Section 4.9 • The possible variation in rise time and duration or skewness of the impact pulse, see Part A,

Chapter 3, Section 4.9 • The shape and size of the load area, see Part A, Chapter 3, Section 4.5

The sloshing load scenario cases to be analysed are illustrated in Figure 2.3.12. Assuming this figure is applicable to one particular shape and size of load area, then the sloshing load cases to be considered are indicated by the red circles. It should be noted that pressure pulses with rise times that are near to any natural periods of the containment system, hull structure or combined system should also be investigated. The Direct Dynamic FEA process is illustrated in the flowchart in Figure 3.3.1 and summarised below:

1. For each location of interest (e.g. 90º dihedral, flat area, etc.) the Dynamic Response (DR) is to be assessed for all components of the membrane system and the hull structure taking into account the various possible load area shapes and sizes. A set of load cases covering the possible sloshing pressure load versus load area shape and size is to be selected for each location. This set of load cases is to be selected as described above.

2. The sensitivity of the dynamic structural response due to the predicted variation of the rise time or

skewness of the sloshing pressure pulse is to be reviewed. An appropriate range of rise times and skewness factors can then be selected.

3. A Dynamic FEA is to be undertaken for each load case. 4. The Dynamic Response of each component of the containment system and hull structure is to be

assessed and tabulated. The maximum responses of each component are to be identified and tabulated. 5. The Ultimate Capacity of each component of the system in terms of stress, deformation and buckling

based on the criteria of different ultimate failure conditions is to be derived, see Part C. In some cases the results may need to be post processed to derive the Ultimate Capacity, e.g. buckling of a bulkhead subject to bending and compression.

6. The ratio between response and capacity (Dynamic Response/Ultimate Capacity) is to be derived to

obtain the load/capacity utilisation factor (UF) to check against the acceptance criteria.




Figure 3.3.1: Direct Dynamic FEA Flowchart

DR UF = ------------

UC

Material Properties

Material properties inctemperature andstrain rate effects

Define material propertiesTesting procedurestemperature effectsstrain rate effects

non linear responses

Material propertiesdatabase

Dynamic Response

Dynamic FEA to derivestructural responses

For each1) location

2) load area shape3) area size

Derive dynamic pressureimpulse from design sloshing

pressure P des

Identify the dynamic structuralresponse for each componentof the membrane system and

hull structure (DR)

Design sloshing pressurePdes and impactcharacteristics

for location, load areashape and size

(Part A)

Review sensitivity of dynamicresponse to variations in

pressure impulse, ie, pressurepeak value, rise and decay time,

impulse shape

Post-process DR to derivemaximum or ultimate capacity

(UC)(stress, deformation, buckling)

based onacceptance criteria for

location, load area shape,area size and rise time

versusthe dynamic pressure




■ Section 3: Indirect Dynamic FEA Analysis The Indirect Dynamic FEA is based on factoring the results from a static FEA by an adjustment factor based on an envelope Dynamic Amplification Factor (DAF) curve. The DAF value is the ratio of the dynamic response to the static response for the sloshing pressure rise time being considered. The DAF curve is to be derived using dynamic FEA for a wide range of possible rise times and skewness ratios so that the maximum dynamic response is obtained. The maximum DAF is usually selected for each component of the containment system or hull structure. The first stage is to derive the structural response using a static FEA. The second stage of the analysis process is to derive the envelope DAF curve. A number of load cases are required which encompass the range and variations of predicted pressure pulses that may be encountered. In particular, pressure pulses with rise times that are near to any natural frequencies of the containment system, hull structure or combined system should be investigated. The DAF assessment is to be carried out on the basis of a nominal pressure load being applied to both the dynamic and static FEA of the complete system. It may be appropriate to vary the nominal pressure load to reflect the shape and size of the load area being considered. The load cases are to consider the following:

• The shape and size of the load area, see Part A, Chapter 3, Section 4.5 • The possible variation in rise time and duration or skewness of the pressure pulse, Part A, Chapter

3, Section 4.9 A static response is obtained by applying the design sloshing pressure as a uniform pressure to the appropriate load area and deriving the static structural response. The final structural response is given by the product of the static response multiplied by the DAF. It is expected that there will be several simplifying assumptions included in the derivation of the DAF curves. It is necessary to review the effects of these assumptions in order to ensure that a more conservative final result is obtained as well as to understand the scope and ramifications of such assumptions. It may be difficult to determine the degree of conservatism factored into the Indirect Dynamic FEA method. The Indirect Dynamic FEA process is illustrated in the flowchart provided in Figure 3.3.2 and is summarised in the following sections.




Figure 3.3.2: Indirect Dynamic FEA Flowchart

Derive Structural Response andUltimate Capacity based on

Static Analysis

Post process SR to derivemaximum or ultimate capacity

(UC)(stress, deformation, buckling)based on acceptance criteriafor Location, load area shape

and area sizeverses

Nominal static Pstat

Static FEA to derivestructural responses

Assign Nominal StaticPressure Pstat as nominal peak

dynamic Pdyn

For each1) location


DAF * Pdes * ( SR / Pstat ) UF = ---------------------------------

UC

Material Properties

Material properties inctemperature andstrain rate effects

Define material propertiesTesting procedurestemperature effectsstrain rate effects

non linear responses

Material propertiesdatabase

Derive Dynamic AmplificationFactors

Dynamic FEA to derivestructural responses

For each1) location


Assign nominal peak dynamicpressure Pdyn

Derive the maximum DAF forexpected range of rise times

For each rise time

DAF = Dynamic response (DR) Static response (SR)

For expected range of risetimes

Hence derivethe Envelope DAF curve forlocation and load area shape

versus area size

Identify the static structuralresponse (SR)

for each component of themembrane system and hull

structure

Identify the dynamic structuralresponse (DR)

for each component of themembrane system and hull

structure

DAF

Design sloshing pressurePdes

for each location, load areashape and area size




3.1 Static FEA 1. A static FEA is to be undertaken for a selected nominal static pressure, Pstat, for the following matrix of

sloshing load cases to derive the static structural responses (SR). 2. For each location of interest, e.g. 90º dihedral, flat area, etc., the static response is to be assessed for all

components of the membrane system and the hull structure taking into account the various possible load area shapes and sizes. A matrix of the possible load area shapes and sizes is to be selected for each location.

3. The nominal static pressure is to be based on the design sloshing pressure for this location and

corresponding load area shape and size.

3.2 Dynamic FEA

1. A dynamic FEA is to be undertaken for a selected nominal dynamic pressure, Pdyn, for the same matrix of load cases as the static FEA to derive the dynamic structural responses (DR). The selected nominal dynamic pressure should normally be the same as the static pressure, but there may be cases when selection of a different value is appropriate.

2. The choice of the nominal dynamic pressure affects the dynamic response due to strain rate effect. If

the analysis is non-linear, then the nominal dynamic pressure selected should be close to the design sloshing value.

3. The sensitivity of the Dynamic Response due to the predicted variation of the rise time or skewness of

the sloshing pressure pulse is to be reviewed. Hence the dynamic FEA is to include a range of appropriate rise times and skewness factors for each sloshing load case.

3.3 Deriving the Dynamic Amplification Factors (DAF)

1. The Static Response and Dynamic Response for each component of the containment system and hull structure are to be extracted. The ratio between the corresponding Dynamic Response and Static Response gives the DAF. The DAF is dependent on the rise time of the pressure pulse as well as the location, load area shape and size. For different structural components of the containment system, the DAF curve will differ. Curves of the DAF against rise time for each load area and shape are to be produced.

2. Figure 3.3.3 gives an example of the DAF curve and illustrates the effect rise time has on the dynamic

response. The maximum DAF for any rise time is to be assumed as the rise time is not a singular value. Similarly the dynamic response is affected by the skewness of the pressure pulse and the maximum DAF is to reflect the possible range of skewness that may be expected.

3. From the DAF curves, the maximum DAF for each component is to be identified and tabulated. 4. It may be appropriate to assume a single value of DAF for the whole containment system for ease of

application, in which case the most conservative DAF curve is to be taken, see Figure 3.3.4.




Dynamic amplification factorfor a particular location, area shape and area

and membrane component response

Rise Time

D y n a m i c a m p l i f i c a t i o n f a c t o r

DAF = dynamic response (DR)static response (SR)

Expected range of risetimes

Applicable DAF

Dynamic amplification factor for a particularlocation, load area shape and size

Figure 3.3.3: Derivation of DAF Factor

Dynamic amplification factor envelopefor a particular location, area shape and area size

Rise Time

D y n a mi c A mp l i f i c a t i o n f a c t o r

Each curve representing the differentstructural response of different membranecomponent for the same nominal Pressure

Each curve represents the structural response for adifferent CCS component

1.0

Figure 3.3.4: DAF Envelope Curve for a Particular Location, Load Area Shape and Size

3.4 Structural Assessment

1. The static response of each component of the containment system and hull structure is to be derived by factoring the static response by the ratio of the design sloshing pressure divided by the nominal static pressure (Pdes/Pstat) for each load area shape and size.

2. The dynamic response is obtained by multiplying the static response at the design sloshing load by the

appropriate DAF value. The maximum responses of each component are to be identified and tabulated. This is likely to be higher than those derived using the direct dynamic FEA due to the assumptions when deriving the DAF.




3. The ultimate capacity of each component of the system in terms of stress, deformation or buckling needs to be derived, see Part C. In some cases the ultimate capacity is dependant on the structural response and hence the FEA results need to be post processed to derive the ultimate capacity, e.g. buckling of bulkhead subject to combined bending and compression.

4. The ratio between response and capacity is to be derived to obtain the load/capacity utilisation factor

to check against the acceptance criteria.

Note: The flowchart of the Indirect Dynamic method, see Figure 3.3.2, refers to three different pressure values: nominal static pressure (Pstat), nominal dynamic pressure (Pdyn) and design sloshing pressure (Pdes). The first two are to be taken as convenient nominal values of the sloshing load for the derivation of the DAF and the static response. The value associated with these pressures is only critical for a non-linear analysis when plasticity is possible and hence realistic pressures are to be used in this case.

■ Section 4: Repeated Impact Loads The strength of the containment system with respect to repetitive sloshing impacts with lower magnitudes than the maximum expected should be considered as this is a different phenomenon to the issue of the structural assessment using the maximum sloshing loads. This is effectively a fatigue issue. Fatigue is discussed in the Introduction, Chapter 2, Section 4. The two aspects of sloshing related fatigue which need to be considered:

• Fatigue due to sloshing impact loads • Fatigue due to general sloshing motions

These issues needs further development and proposals to review fatigue due to impact loads should be discussed with Lloyd’s Register. For the “absolute procedures” it is necessary to make an assessment of the effect of repeated impact loads on the containment system. In view of the complexity of these issues, it may be sufficient to provide a reasoned argument as to why the repeated impact issue is not expected to be an issue. For the enhanced comparative method, this issue can usually be dealt with implicitly.




Chapter 4: Material Properties

Section 1: Summary Section 2: Material Properties

Section 3: Plywood Section 4: Reinforced Polyurethane Foam

Section 5: Mastic Section 6: Triplex

Section 7: Metallic Containment System Membranes

■ Section 1: Summary It is the designer’s responsibility to obtain the material property data for the structural analysis and to derive the assessment criteria appropriate to the materials used. This Chapter describes the basis by which the material properties are derived and what properties are required for the structural assessment. The purpose of this Chapter is to provide guidance for the derivation of the material properties. Material properties data may be obtained from the material suppliers, other published sources or by material testing.

■ Section 2: Material Properties Material properties are to be derived for the expected in-service conditions. The properties at ambient and cryogenic temperatures are required for some components depending on their position in the containment system. The thermal coefficients of expansion are to be derived. Sloshing impacts have a very short duration and the effect of strain rate (strain/time or ε/t) on the material properties is to be assessed for the containment system components, see also Chapter 3, Section 6. It is necessary to review which components are likely to be affected by strain rate effects and to what extent. It is necessary to review the magnitude of the sloshing impact load and the associated rise times in order to evaluate the expected strain rate. From this the parameters for strain rate material testing can be set. In addition appropriate structural damping factors need to be evaluated. The following Sections list the material stiffness and strength properties that may be required for the materials normally used within containment systems. Details of the material test requirements are given in Chapter 5.

■ Section 3: Plywood

3.1 General As wood is a natural material, its mechanical properties vary between different batches and even between samples of the same batch. Typical strength and stiffness properties for the structural assessment are to be based on the mean or the combination of mean and standard deviation values from material tests. Plywood material properties also vary with the thickness of each layer and the number of layers.




The individual wood layers are laid up as a composite in orthogonal directions. Plywood has strong and weak directions in terms of membrane and bending stiffness and strength. The strongest configuration for membrane stresses in compression is along the direction of wood grain at the outer ply. The strongest configuration for bending is that with the outer plies aligned in the direction of the bending stresses. Plywood does not exhibit a linear Young’s Modulus for tensile and compressive loads. As the plywood bulkheads are primarily intended to be used to provide compressive support, the compressive Young’s Modulus values may be used. Plywood bending behaviour is also non-linear. For most applications, it is acceptable to assume a linear relationship provided that a suitable equivalent Young’s Modulus is assumed. For detailed analysis of the failure modes of the plywood and for FEA to determine items such as the stress concentration factors for cleats attached to the plywood boxes localised stresses in the box cover plates, etc, the material properties for the individual wood layers will need to be specified. In addition to material data from tests and suppliers, the following references may be helpful;

1. British Standard BS 5268:Part 2:2002, "Structural Use of Timber", Code of Practice for permissible stress design, materials and workmanship

2. Handbook of Finnish Plywood at www.finnforest.com/default.asp?path=1;53;153;1182

3.2 FEA Modelling Requirements In general, plywood can be modelled using orthotropic 2D plate or shell elements. The use of multi-layer shell elements to generate anisotropic properties is also acceptable and may be required for detailed analysis applications. In this case, each plywood layer is to be defined using the single ply laminate properties. The use of multi-layer elements will obviously complicate the calculation process significantly and is not considered obligatory for FEA of the whole box. For the investigation of localised stress concentrations and localised shear and bending response, 3D solid FE models which explicitly represent each layer may be needed.

3.3 Stiffness Properties The following material properties will typically be needed for each type of plywood used in the containment system. Not all data is required for all components as the position of the plywood in the containment system dictates the loads and temperature experienced by the plywood. For convention, the X- and Y-axes are planar to the ply layers of the plywood; the Z-axis is through the thickness of the plywood. For box cover plates:

• Young's Modulus for bending actions in both the strong and weak directions EB-XX and EB-YY • Young’s Modulus for the through thickness direction EZZ • Shear Modulus for membrane actions GXY • Shear Modulus for the two through-thickness planes, GXZ and GYZ • Poisson's Ratio, υ • Density, ρ

For internal and external bulkheads:

• Young's Modulus for membrane compression, both the strong and weak directions EM-XX and EM-YY • Young’s Modulus for the through thickness direction EYY • Shear Modulus for membrane actions GXY




• Poisson's Ratio, υ • Density, ρ

The properties at ambient temperature, cryogenic temperature and a range of temperatures between may be required. Strain-rate influence at different temperatures, ε/t, may also be required.

3.4 Strength Properties The following strength properties are required: For box cover plates:

• Bending strength for both the strong and weak directions, σB-XX and σB-YY • Shear strength for the through thickness direction, τZZ

For internal and external bulkheads:

• Compressive strength in for both the strong and weak directions, σC-XX and σC-YY • Bending strength for both the strong and weak directions, σB-XX and σB-YY

The properties at ambient temperature, cryogenic temperature and in between temperatures may be required.

■ Section 4: Reinforced Polyurethane Foam

4.1 General The stiffness properties of R-PUF are temperature dependent and the temperature through the foam depth varies. The mechanical properties must be determined with the consideration of in-service temperature ranges taking account of the thermal properties of the R-PUF after ageing. The R-PUF response is also significantly affected by strain rate and this needs to be included in the assessment. R-PUF is primarily subjected to compressive and tensile stresses. Tensile stresses occur when the foam rebounds after the impact. Orthogonal axes are labelled X, Y and Z.

4.2 FEA Modelling Requirements R-PUF is to be modelled as 3D solid elements with linear orthogonal material properties or non-linear visco-elastic properties for a non-linear FEA analysis. As the response of the R-PUF is non-linear, the strength analysis of an R-PUF based containment system should ideally be a non-linear dynamic FEA, however this is very complex. An alternative is to use a non-linear static FEA and adjust the results using partial factors derived using a linear dynamic FEA.

4.3 Stiffness Properties The following material properties are required for the analysis:

• Young's Modulus for the orthogonal material directions EXX, EYY, EZZ • Shear Modulus GXY, GXZ, GYZ




• Poisson's Ratio υXY, υXZ, υYZ • Density, ρ

Properties at ambient temperature, cryogenic temperature and a range of temperatures in between will be required. Ideally strain-rate dependent properties at different temperatures, ε/t, should be obtained.

4.4 Strength Properties The following strength properties are required:

• Compressive strength through the depth of the R-PUF. This will probably be in an allowable global and local limit of plastic strain, σC

• Shear strength for the through depth planes, τ The strength properties taking account of strain rate effects and temperature variation may be required.

■ Section 5: Mastic

5.1 General The mastic used in the different types of containment systems is designed to perform different duties. As a consequence the material properties may be different for the different systems.

5.2 FEA Modelling Requirements The mastic is to be modelled as solid (3D) elements with linear isotropic elastic material properties.

5.3 Stiffness Properties The following material properties must be supplied at ambient temperature:

• Young’s Modulus, EC • Shear Modulus, G • Poisson’s Ratio, υ • Density, ρ

The properties of mastic in the Mark III and NO96 containment systems are slightly different.

5.4 Strength Properties The compressive strength properties are required at ambient temperature.

■ Section 6: Triplex




6.1 General A Triplex laminate is used as the secondary membrane in the Mark III containment system. There are two types of Triplex used in the system: a rigid Triplex which is bonded between the primary and secondary R-PUF layers in each of the containment system blocks and flexible Triplex which is bonded to the rigid Triplex to complete the secondary membrane between blocks. The flexible Triplex is also used in the corner details. The material and strength properties are required at the predicted in-service temperature. The X- and Y- axes are planar to the Triplex; the Z-axis is through the thickness.

6.2 FEA Modelling Requirements The Triplex is to be modelled as thin shell elements with isotropic material properties.

6.3 Stiffness Properties The following material properties are required for the in-service temperatures:

• Young’s Modulus, E • Shear Modulus, G • Poisson’s Ratio, υXY • Density, ρ

6.4 Strength Properties The following strength property is to be supplied for the in-service temperatures:

• Yield strength, σY

■ Section 7: Metallic Containment System Membranes

7.1 General The primary and secondary membranes in the NO96 system are Invar. The primary membrane in the Mark III system is stainless steel.

7.2 FEA Modelling Requirements When included, the membranes are to be modelled as thin shell elements with isotropic material properties.

7.3 Stiffness Properties The following material properties must be supplied for the in-service temperature:

• Young’s Modulus, E • Shear Modulus, G




• Poisson’s Ratio, υ • Density, ρ

7.4 Strength Properties The following strength property is to be supplied for the in-service temperatures:

• Yield strength, σY




Chapter 5: Structural Testing

Section 1: Summary Section 2: Material Testing

Section 3: Testing of Containment System or Components

■ Section 1: Summary Physical testing of the complete containment system or components may be necessary to validate the structural analysis. It may also be the preferred option for deriving the structural capacity in situations where structural analysis is not feasible. Experimental testing of the membrane system and its components under representative conditions is an acceptable method for evaluation of the strength of the system. Ideally testing under impact loads and in cryogenic conditions taking due account of the flexibility of the supporting hull structure should be used. In practice these conditions are difficult to replicate and a mixture of experimental testing and calculation will be necessary to fully establish the structural capacity of the membrane system. The test plan procedure is to be submitted and agreed by Lloyd’s Register prior to the commencement of testing.

■ Section 2: Material Testing Physical tests are required to determine the material properties when data from the supplier is incomplete or not available. Whenever possible, the material properties are to be derived using industry standard testing procedures. Sufficient samples need to be tested to ensure that the results are repetitive and representative. Typically a minimum of 5 samples is required. Where an industry standard testing procedure is not available, then the proposed test procedure is to be agreed by Lloyd’s Register.

■ Section 3: Testing of Containment System or Components The tests are to be a representative of the real situation as far as possible. Tests need to consider the following aspects:

• The containment system location, e.g. flat areas or corner details • The load area size and shape • The applied test load including rise times and skewness factors for dynamic load testing • The detailed structural arrangement of the containment system

Special care is to be taken with the application of loads to ensure that the best representation of a sloshing impact load is achieved or if the testing is for static loads that the pressure load is uniformly distributed over a suitable area. Typical testing procedures would include the use of:

• wet and dry drop tests for dynamic testing




• air/water bags, especially for static testing Tests are required for both static and dynamic structural responses caused by the representative sloshing loads. The hull structural support should be included as part of the total test programme. It is likely that the majority of tests will focus on the determination of the static system response but some tests of dynamic response are necessary to review the relationship between the static and dynamic response. It is recommended that the tests are fitted with video recording equipment as well as strain and deflection gauges to record the test results.





Chapter 1: Acceptance Criteria Overview

Section 1: Introduction Section 2: Risk Assessment to Identify Critical Failure Modes

Section 3: Derivation of the Capacity of the System Section 4: Load/Capacity Utilisation Factor

■ Section 1: Introduction The applicable acceptance criteria for each type of containment systems are to be specially considered taking account of the overall and detail design aspects of each containment system. For foam layered type containment systems (e.g. Mark III), the principal failure criteria include foam crushing, plywood failure in way of the mastic ropes and failure of the corrugation. For box type containment systems (e.g. NO96), the principal failure criteria include buckling of the internal box bulkheads, bending/shearing failure of the covers and stability of local design features such as cleats. The principal failure criteria for other containment systems are to be determined by a risk assessment process using HAZID techniques or similar, see Section 2. The method of application of the assessment criteria are dependent on the assessment procedure. The following sections give an overview of the acceptance criteria applicable to each sloshing assessment procedures.

1.1 Comparative Procedure For the Comparative Procedure to be acceptable, then it must be clearly demonstrated that

1. the cargo containment system designs for both the REFERENCE and TARGET ships are virtually identical.

2. the hull structure supporting the CCS is reasonably similar.

3. there has been a proven extensive and good in-service history of the cargo containment system on the REFERENCE ship or other similar vessels

For the Comparative Procedure only the design sloshing loads for the REFERENCE ship and TARGET ships need to be derived. Normally the design sloshing loads are evaluated at model scale by model tests. Note that the same test procedure must be followed at the same testing establishment for both the TARGET and REFERENCE cases. No structural analysis is necessary. The design of the containment system on the TARGET ship is considered acceptable if the design sloshing loads for the TARGET ship are lower than the REFERENCE ship, hence:

referenceett PP <arg Where:

Preference Design sloshing load envelope for the REFERENCE ship

Ptarget Design sloshing load envelope for the TARGET ship

This criteria is to be satisfied for the whole sloshing load versus area curve




1.2 Enhanced Comparative Procedure For the Enhanced Comparative Procedure to be acceptable, then it must be clearly demonstrated that:

1. the cargo containment system designs for both the REFERENCE and TARGET ships are essentially similar and from the same CCS designer. Improvements to the design of the CCS to improve the capability of the CCS can be assessed using the Enhanced Comparative Procedure provided that the improvements are incremental. For example, this procedure is suitable for use with upgrades of NO 96 CCS from Standard to Reinforced or Super Reinforced boxes.

2. the hull structure supporting the CCS is reasonably similar.

3. there has been a proven extensive and good in-service history of the cargo containment system on the REFERENCE ship or other similar vessels

For the Enhanced Comparative Procedure the design sloshing loads for the REFERENCE ship and TARGET ships and the change in strength of the CCS need to be derived. Normally the design sloshing loads are evaluated at model scale by model tests. Note that the same test procedure must be followed at the same testing establishment for both the TARGET and REFERENCE cases. The containment system structural capacity is typically evaluated by static testing methods or Finite Element Analysis or a combination of both. The critical failure modes of the REFERENCE containment system and the TARGET containment systems are to be evaluated and documented. Whilst the improvements in the TARGET containment system will change the critical failure modes, it is to be ensured that the performance is not impaired by the introduction of additional stiffening into the TARGET containment system

The design of the containment system on the TARGET ship is considered acceptable if the ratio of the design sloshing load over the containment system strength for the TARGET ship is less than the same ratio for the REFERENCE ship, hence:

reference

reference

ett

ett

CP

CP

<arg

arg

Where:

Preference Design sloshing load envelope for the REFERENCE ship

Ptarget Design sloshing load envelope for the TARGET ship

Creference Structural capacity of the REFERENCE ship containment system

Ctarget Structural capacity of the TARGET ship containment system

This criterion is to be satisfied for the whole sloshing load versus area curve and for all the different containment system components.

1.3 Absolute Procedure The “Absolute Procedure” is applicable to any design of containment system and supporting structure. The absolute approach aims to derive the design sloshing load at full scale taking into account scaling effects, compressibility, etc., and compare this with the ultimate capacity based on the application of realistic sloshing impact loads. The ultimate capacity is derived using advanced physical models of the containment system and supporting hull structure and accounting for dynamic and non-linear materials and effects when applicable. The final assessment is made by ensuring that the ratio of Load over Capacity is less than a suitable justifiable load/capacity utilisation factor (UF) based on best engineering practice, hence:




UFCP

<

P Design sloshing load envelope, including the load area relationship, the effect of rise time and impact duration and shape, scale issues, etc

C Structural capacity of the containment system and supporting hull structure assessed using non-linear dynamic testing or structural analysis

UF Load/Capacity Utilisation Factor

This criterion is to be satisfied for the whole sloshing load envelope and for all the different containment system components and supporting hull structure. Ideally, the Absolute Procedure should be applied in its entirety; however there are many aspects of the Absolute Procedure for which the industry’s current knowledge or capabilities are not yet sufficient to evaluate fully. Consequently, only parts of the Absolute Procedure can be applied and the procedure effectively becomes a “Simplified Absolute Procedure”.

1.4 Simplified Absolute Procedure The Simplified Absolute Procedure is applicable to any design of containment system and supporting structure. The “Simplified Absolute Procedure” is the practical application of the “Absolute procedure”. In particular the Simplified Absolute Procedure is to be applied to:

• designs which significantly deviate from previous designs

• designs where it has not been possible to establish equivalence (on a comparative basis) with an existing Lloyd’s Register approved design. For example, the Enhanced Comparative Procedure assessment criteria could not be satisfied.

• a design from a company that does not have any previous containment system design experience The design sloshing load at full scale is derived taking into account as many as issues as is practical. Similarly, the ultimate capacity is derived taking into account as many as issues as is practical. Partial factors are assigned to those issues that are not explicitly included in the analysis and the design sloshing load and ultimate capacities are then adjusted by the partial factors and the final assessment is made by comparing the load/capacity ratio to an utilisation factor, hence:

UFfff pmpp <

CP.......21

Where

fp1, fp2,…,fpm are partial factors for load issues 1 to m which have been implicitly included

P Design sloshing load envelope, including the load area relationship, the effect of rise time and impact duration and shape, etc. All issues not explicitly included in the load derivation are to be accounted for using partial factors.

UF Load/Capacity Utilisation Factor

C Structural capacity of the containment system and supporting hull structure assessed using non-linear dynamic testing or structural analysis. All issues not explicitly included in the capacity derivation are to be accounted for using partial factors.

In this equation the capacity is measured as a load and can be derived in a similar manner as follows:




D

FEF

RPC

C =

where

CF is the estimated absolute ultimate capacity including all implicit and explicit issues

( ) UCcnccF CfffC .......21=

CUC is the predicted ultimate capacity derived by analysis or test, hence it excludes issues no explicitly addressed

fc1, fc2,…,fcn are partial factors for capacity issues 1 to n which have been implicitly included

RD is the estimated absolute stress or response including all implicit and explicit issues

( ) FEcnccD RfffR .......21=

PFE is the nominal sloshing pressure used to derive the predicted stress or response RFE

RFE is the predicted stress or response derived by analysis or test, hence it excludes issues no explicitly addressed

fr1, fr2,…,frn are partial factors for structural resistance issues 1 to n which have been implicitly included Table 4.1.1 gives an example of a set of partial factors that might be appropriate for the design assessment of Mark III or NO 96 cargo containments systems. This table is based on Lloyd’s Register’s experience and it is the responsibility of the designer to verify these factors if they are applied in their design assessment. The purpose of the table is to illustrate the process of application of partial factors.

■ Section 2: Risk Assessment to Identify Critical Failure Modes The assessment of the containment system is to review all possible failure modes and address each mode accordingly. It is necessary for the designer to undertake a risk assessment and a hazard review of all possible failure modes, document these and then propose suitable acceptance appraisal methods and acceptance criteria to show that these potential hazards are clearly controlled. It should be noted that the appraisal methods could include a combination of calculation, physical testing and documentation. Documentation may be sufficient by itself to address some hazards. The hazard identification (HAZID), Risk Assessment process, the chosen appraisal methods and proposed acceptance criteria are to be discussed and agreed with Lloyd’s Register. The procedure given in Part B will address many of the expected potential failure modes, but additional analyses may be required to address all the identified hazards. The containment system designers are responsible for the design of the containment system and hence need to identify the possible hazards and failure modes. Lloyd’s Register’s responsibility is to review the Risk Assessment document and assumptions made by the designers and agree with or otherwise their assessment. In the situation where the sloshing assessment is not being undertaken by the containment system designers, then the onus is on the engineer undertaking the sloshing assessment to undertake the Risk Assessment.




Table 4.1.1: Examples of partial factors Load aspects: sloshing load prediction • Hydroelasticity PF=1.0 Exclusion is assumed to be

conservative • Corrugations PF=1.0 Exclusion is assumed to be

conservative for larger load areas. For small load areas, then the corrugation may result in increased loads

• Vapour / air issues • Boiling LNG/ water (RT) issues • Compressibility • Time issues

PF = Froude scaling Froude scaling is assumed to be conservative

Strength Aspects: Structural response (stress, deflection, strain, etc) Hull flexibility Explicit For Mark III a PF of 2.5 would be

required For NO 96 a PF of 1.4 would be required

• Hydroelasticity for Mark III systems

PF=1.0 From FEA including added mass effects

• Hydroelasticity for NO 96 systems

PF=1.1 From FEA including added mass effects

• Strain rate on Young’s modulus PF=1.0 Assumed to be conservative • Linear FEA used instead of non-

linear FEA PF to be derived to be based on comparative analysis

• Temperate effects Explicit • Rise time and Skewness PF = DAF For the indirect

method. Explicit for the direct method

For Mark III a PF of 1.7 would be required For NO 96 a PF of 1.4 would be required

For Mark III a PF of 1.1 would be required For NO 96 a PF of 1.4 would be required

Capacity aspects: Ultimate capacity Mark III systems • Strain rate (foam) Explicit based on test analyses • Temperate effects (foam) Explicit based on test analyses • Primary membrane corrugations Explicit based on test analyses NO 96 systems • Strain rate (plywood buckling)

PF = 1.0 Assumed to be conservative if ignored. Actual factor to be based on test analyses

• Strain rate (plywood bending)

PF = 1.0 Assumed to be conservative if ignored. Actual factor to be based on test analyses

• Temperate effects (plywood) Explicit based on test analyses




The Risk Assessment process is applicable for both the new and existing designs. For the design of a new membrane containment system where there is no previous experience upon which to base the design or to incrementally improve an old design, the requirements need to be clearly specified to ensure that the new design is properly researched and documented. For existing designs, many of the failure modes have been found to be non critical and hence these issues are not included within the development of refinements to an upgraded design. The main issues in the Risk Assessment can be derived using the following techniques: system inspection, hazard identification, component testing and analytical analyses. Chapters 2 and 3 give guidance on the typical failure modes for the NO96 and Mark III type containment systems respectively, but the experience has shown that some of the issues listed are not critical. It is best practice to fully document the design and verification process and results for highly critical systems. The approach applied in this Procedure is intended to allow the designer to provide Lloyd’s Register with the required level of documentation.

■ Section 3: Derivation of the Capacity of the System Each potential hazard or failure mode is to be addressed by one of the following approaches or a combination of these approaches: 1. Determination of the capacity by experimental testing. Due account is to be taken for the difference in

experimental practice and the expected real situation. For example: • Allowances for static ultimate capacity testing versus actual dynamic load application • Allowances for differences in temperature between experimental and actual operating temperatures • Repetition of the tests to ensure statistically consistent data

2. Determination of the capacity by theoretical analysis. For example:

• Comparison against allowable stresses • Empirical buckling equations supported by experimental testing

3. Review against previous proven and justifiable successful operating service – comparative basis. For

example: • Limits on allowable deformations

It may be necessary to derive several capacity functions for the system and its components, such as Ultimate, Fatigue and Serviceability limit state capacities, see Section 4. The derivation of the Ultimate Capacity and, in some cases, the Serviceability limit state is needed for the main strength assessment in Part B, Chapter 3. The assessment of the supporting hull structure is to be based on a review of the stresses and deformations against prescribed allowable values. In addition it will be necessary to check the buckling stability of items such as bulkhead stiffeners and longitudinal stiffeners, supporting brackets and primary member web plating.

■ Section 4: Limit State Approach The limit state approach refers to the assessment of the structural capacity based on the condition of the structure after the load actions have been removed. Hence limit states define whether the structure, or part of a structure, no longer satisfies the design requirements. The structural performance of the containment system and the supporting hull structure can be assessed using the following limit state design definitions:




• ULS - Ultimate Limit State

This limit states corresponds to: • the safety of life • the environment • property (ship and cargo)

under the action of the maximum expected loads effects during the design life of the ship.

Ultimate limit states include attainment of the maximum structural capacity of sections, members or connections by:

• rupture • excessive deformations • instability (buckling)

The effect of exceeding an ultimate limit state is almost always irreversible and causes failure the first time the limit state is exceeded.

• SLS - Serviceability Limit States

This limit state corresponds to: • the functioning of the hull structure or structural members under normal operation • the appearance of the hull structure or part of it • the comfort of people under the action of the specified loads during the design life of the ship. Serviceability limit states include: • unacceptable deformations which affect the efficient use or appearance of structural or non-structural

elements or the functioning of equipment or the comfort of people • local damage which may reduce the working life of the structure or affect the efficiency or appearance

of structural or non-structural elements • excessive vibrations which cause discomfort to people or affect non-structural elements or the

functioning of equipment In the context of serviceability limit state, the term “appearance” is concerned with such criteria as high deformations and extensive cracking, rather than aesthetics.

Additional limit states definitions are sometimes given for: • Fatigue limit states (FLS)

This limit state corresponds to degradation of the structure due to the effect of time varying or cyclic loading. FLS is essentially a special case of SLS.

• Accidental limit states (ALS)

This limit states corresponds to the ability of the structure to resist accident situations. The structural assessment is for ALS is usually performed using SLS irreversible or ULS principles.

■ Section 5: Load/Capacity Utilisation Factor For the Simplified Absolute Procedure and the Absolute Procedure, the target value for the Load/Capacity Utilisation Factor should generally be less than 0.65 for an assessment made on the basis of ultimate limit states




(ULS). The value of 0.65 is an approximation based on 1/(1.1x1.5), where 1.5 is the Euro code2 factor most commonly used for "live" loads or dynamic loads, which sloshing most closely resembles, and 1.1 is the Eurocode factor commonly used for the capacity function. The application of this factor will be dependant on the kind of assessment performed, and whether the CCS is of a “proven” design or otherwise. For a new or novel CCS, not dissimilar to Mark III or NO96, then a load/capacity utilisation factor should generally be less than 0.5 for assessment modes based on ultimate limit states. For assessment of design criteria based on irreversible serviceability limit states (SLS), the utilisation factor will need to be specially considered. For the Simplified Absolute Procedure, some aspects of the design problem will not have been explicitly accounted for and in addition many assumptions would have been made during the assessment. Consequently, a complete list of all design aspects addressed explicitly and all those addressed implicitly are to be documented. In addition all limitations and assumptions used are to be fully documented together with an assessment on the consequences to the results. The derivation of the attained Utilisation Factor will need to include allowances for approximations, limitations and implicitly addressed issues by one of the following methods: 1. the application of partial factors to the capacity and load values to represent the uncertainties in the

calculations 2. use of a combined enhanced comparative basis and simplified absolute basis. For this assessment: as many

elements of the absolute approach are adopted as far as is practicably possible for both a REFERENCE ship and the TARGET ship. In this case the value of the required UF will be based on the result from application of the procedure to the REFERENCE ship. The same procedure is then applied to the new design and the derived UF compared with that from the reference case.

2 EN 1990 'Eurocode: Basis of structural design', www.eurocodes.co.uk




Chapter 2: NO96 Containment System

Section 1: Failure Modes Section 2: Buckling of Internal or External Plywood Bulkheads

Section 3: Failure of Primary Box Cover Plates Section 4:Failure of Bulkheads in way of Primary and Secondary Box

Connections Section 5: Failure of Bulkheads in way of Cleats

■ Section 1: Failure Modes The strength assessment of the NO96 containment system is to address the critical failure modes identified below. Other failure modes may also need to be considered and the designer is to identify and document these modes, see Chapter 1, Section 2.

■ Section 2: Buckling of Internal or External Plywood Bulkheads For internal bulkheads, the buckling failure is mainly caused by compressive forces. For the external bulkheads, the buckling is a result of the combined effect of the compressive and bending actions. The presence of holes in the bulkheads needs to be taken into account in the assessment. Standard classical buckling equations may be used which account for bending as well as compressive loads. The stresses are to be taken from the FEA mean stresses over a breadth of the bulkhead equivalent to the bulkhead depth to be considered.

■ Section 3: Failure of Primary Box Cover Plates The top cover plate(s) of the primary box are directly subjected to the sloshing impact loads. This induces bending stresses and shear stresses in the top cover plate. These stresses should be reviewed to ensure that they are within acceptable limits for the plywood material. It should be noted that the effect of the support provided by the plywood bulkheads to the box covers needs to be specially considered. If the plywood bulkhead has very sharp corners, then this will induce a large increase in the local bending and shear stresses in the bottom plies of the box cover in contact with the bulkhead (see Figure 4.2.1). The acceptance criteria must take this into account. A very detailed 3D solid model using a t x t x t element size, where t is the thickness of a ply layer, may be necessary to quantify the local stress concentration factor (SCF) and derive a suitable acceptance criteria. This SCF may be applied to the FEA results obtained using the standard analysis to derive the total stress to be used for the assessment.

■ Section 4: Failure of Bulkheads in way of Primary and Secondary Box Connections Crushing failure of the bulkheads in way of the cruciform connection between the primary and secondary boxes and between the box cover plates is to be reviewed. Here the stiffness of the cruciform connection attracts load as it passes through the primary and secondary boxes, this results in high localised compressive




stresses in the bulkheads and high through thickness compressive stresses and shear stress in the primary box bottom cover plate and the secondary box top cover plate. The crushing strength of the plywood bulkheads will need to be determined through material testing. If it can be shown that this aspect is not an issue, again through material testing, then no analysis will be necessary. The through thickness compressive stresses are to be derived using 3D solid FEA model or via material testing to show that the capacity is more than adequate.

Figure 4.2.1: Bending Stress Concentration at the Bottom Piles of Top Cover Plate

■ Section 5: Failure of Bulkheads in way of Cleats The detailed construction of the boxes include many cleats (local small plywood supports to allow either securing of the hull connection rods or cover plates in the 90 and 135 degree dihedral corner areas). The loads applied to these cleats can result in high localised compressive and bending loads being introduced into the main box structure which can reduce the ultimate capacity of the bulkhead to which the cleat is attached. This needs to be explicitly addressed. If this issue is being addressed by FEA, then a detailed 3D FEA using each ply layer modelled as a solid element will be necessary to review the local behaviour. Historically this issue has been addressed by box testing. For FEA assessment, a local detailed 3D solid element analysis will be necessary to allow the load paths through the cleats and adjacent plywood to be derived and the resultant stresses in the system determined.




Chapter 3: Mark III Containment

System Section 1: Failure Modes

Section 2: Foam Layers Section 3: Failure of Bottom Plywood in way of Mastic Ropes

Section 4: Integrity of Secondary Membrane Section 5: Deformation of the Primary Membrane

■ Section 1: Failure Modes The strength assessment of Mark III containment system is to address the critical failure modes identified below. Other failure modes may also need to be considered and the designer is to identify and document these modes, see Chapter 1, Section 2.

■ Section 2: Foam Layers The foam used for the insulation is a structural component which will be subject to compressive and shear loads. After the pressure impact, the foam will rebound and generate tensile stresses in the foam and at the interfaces between the foam and plywood and between the foam and triplex. The most likely critical locations where crushing of the foam might be an issue is in way of structural discontinuities as follows: • Above the mastic ropes - as the majority of the impact load transferred to the mastic rope is via the area of

foam above the mastic rope. This needs to be reviewed especially above mastic ropes where these pass over a stiffener web or stringer web plate

• In way of plywood keys or plywood supporting the secondary membrane (Triplex) in the corner locations. Here the change in stiffness and/or depth of the foam leads to structural discontinuities and possible local stress concentrations

It is to be confirmed that the local deformation (crushing) is not such that it will lead to failure of the foam as a structural entity. Issues such as laminar tearing of the foam under high crushing loads followed by rebound are to be assessed and documented. The foam needs to be structurally adequate to absorb repeated sloshing impacts as well as continuous compression and expansion due to sloshing loads that do not result in impact events (disintegration failure mode). The bonding between the foam and the plywood top and bottom covers and also to the Triplex secondary membrane needs to be adequate to withstand the continuous compressive and tensile loads induced by the sloshing impacts and the consequent rebounds.

■ Section 3: Failure of Bottom Plywood in way of Mastic Ropes The compression of the foam results in bending and shearing of the bottom plywood due to the support offered by the mastic ropes. Bending stresses and through thickness shear stresses in the plywood are to be




assessed to ensure these are acceptable. It is important to ensure that the correct bending strength for the orientation of the plywood is selected. The critical locations are expected to be above the mastic ropes where these pass over a stiffener web or stringer web plate. It may also be necessary to review the compressive strength of the mastic. It will be necessary to consider local stress concentrations if the construction of the mastic rope gives a sharp edge support for the plywood. In this case, a very detailed 3D solid model using t x t x t element size, where t is the thickness of a ply layer, will be necessary to quantify the local stress concentration factor (SCF) and hence derive a suitable acceptance criteria. This SCF may be applied to the FEA results obtained using the standard analysis to derive the total stress to be used for the assessment.

■ Section 4: Integrity of Secondary Membrane It is to be ensured that any membrane stresses induced into the secondary membrane are acceptable. Local bending of the Triplex due to local discontinuities in the stiffness of the containment system is to be reviewed. Typically this will occur near the corner locations when the triplex is attached to the plywood sheet the primary key and the secondary foam. The flexing of the rigid and flexible Triplex layers is to be reviewed in areas where there are structural discontinuities to ensure that the flexing stresses do not induce premature fatigue failure or glue failure.

■ Section 5: Deformation of the Primary Membrane The functionality of the corrugated membrane is dependent on the corrugations maintaining their shape. It is to be confirmed that the corrugations will not deform plastically under the applied sloshing loads. If this criteria is considered to be too restrictive, then a design criteria may be set using SLS irreversible principles. Hence some permanent deformations would be expected in-service and the design criteria sets out to define the maximum expected and allowable amount of deformation. The design assessment process should also review the expected number of deformations up to the design criteria. The degree and extent of allowable permanent deformations must be agreed with Lloyd’s Register and the owners prior to acceptance of this design criteria. It must be demonstrated that the acceptance of permanent deformation does not reduce the life of any other structural item or effect the capacity of any other failure mode. The use of a design criteria based on allowable deformations must also be accompanied by an in-service monitoring or inspection procedure to ensure that the expected degree of deformation is not exceeded. This procedure should also address a repair procedure. Figure 4.3.1 illustrates the application of the design assessment process to an allowable permanent deformation criteria. It show a permanent deflection versus pressure load curve. The allowable deformation has been set at the SLS deformation limit and hence the SLS capacity is given by the P(SLS) value. For this kind of material and structural configuration the ULS capacity is somewhat arbitrary as it can be hard to define a definitive failure point as the material is so ductile. The capacity value at which no permanent deformation of the membrane occurs is also shown. In the region between the SLS value and the no deformation value, the number of deformations is given by the summation of the probability of sloshing loads occurring with values between P(SLS) and P (no deformation), This is illustrated in Figure 4.3.2, hence the probability is given by Prob(SLS) – Prob(no deformation).




Figure 4.3.1: Example of Approach to Allowable Deformations of the Primary Membrane,

Pressure Versus Permanent Deformation Graph

Figure 4.3.2: Example of Approach to Allowable Deformations of the Primary Membrane,

Pressures Versus Probability Graph




Chapter 4: Supporting Hull Structure

Section 1: Acceptance Criteria

■ Section 1: Acceptance Criteria The following items of the supporting hull structure are to be reviewed:

• membrane plate stresses • localised bending stresses in the plating in way of the mastic and over stiffeners or stringer web plate • stiffener web shear stresses • stresses in the stiffener flange due to stiffener bending • buckling of stiffener and stringer web plate and supporting brackets.

The acceptance criteria for the hull structure are given in Table 4.4.1 on the basis that only the sloshing loads are applied in the analysis.

Table 4.4.1: Acceptance criteria for hull structure in way of sloshing impact loads Location Acceptance

criteria Comments

Membrane plate stresses σvm < 0.75 σy Over an area of s x s

σvm < 1.0 σy

Over an area of 3t x 3t Localised bending stresses in the plating in way of the mastic and in way of stiffeners or stringer web plate

σvm < 1.5 σy Over an area of t x t

Stresses in stiffener flange σa < 0.75 σy τ < 0.75 τy Mean shear stress over the web depth Stiffener web

Shear buckling λ = 1.0

Using mean shear stress over the stiffener depth. Shear buckling capacity based on a plate panel of hw x hw

Stiffener end brackets σvm < 0.95 σy Buckling λ = 1.0

Stringer web plating σvm < 0.95 σy Buckling λ = 1.0

Using mean shear stress over the attached stiffener depth Shear buckling capacity based on a plate panel equivalent to the stiffener web depth as follows of hw x hw

Symbols σy minimum specified yield stress τy minimum specified shear yield stress σvm calculated von Mises stress σa calculated axial stress τ calculated shear stress s stiffener spacing t plating thickness hw stiffener web height λ buckling factor of safety. buckling capacity to be derived using standard IACs buckling formulae


Appendix 1


Appendix 1

Chapter 1: Site Specific Environmental Data

Section 1: Introduction Section 2: Site Specific Environmental Data

■ Section 1: Introduction This appendix gives brief guidance on the requirements for the environmental data required to assess the design sloshing loads for membrane LNG ships operating at offshore terminals or for application to FPSO’s, FSRU, FLNG’s etc. The main tasks required for the derivation of the environmental data to assess the design sloshing loads are as follows: 1. Detailed assessment of environmental information. Basically the required environmental data is the same as that

required for siting any offshore floating production platform or similar. 2. Derivation of the proposed operational envelope of the ship including:

• mooring systems and ship heading control • loading conditions (tank fill heights) including frequency and when each loading condition will be used. • environmental operational envelope • emergency procedures for severe weather conditions • potential operating conditions in the event of an accident, eg loss of anchor, loss of propulsive power, etc

3. Ship motion analysis

• possibly considering mooring arrangements, water depth effects and fluid motions inside partially filled tanks • consideration of the appropriate wave energy spectrum is necessary, this includes the combined wind sea and

swell sea systems if appropriate.

■ Section 2: Site Specific Environmental Data The following environmental data is required. Note that some of this information may not be explicitly required for a sloshing assessment, but would be necessary for other approval issues. For site monitored data, the longer the monitoring period the higher the confidence in the validity of the data. Whether the data is derived from hindcasts or actual site monitoring, the key areas of interest are:

2.1 Duration of measurement Long term studies are required to ensure all trends are established. In addition very severe and infrequent extreme events are to be documented including hurricane/typhoon type conditions.

2.2 Seastate information

Wave height (HS), wave periods (TP, TZ), dominant wave direction. Seasonal data is to be derived.


Appendix 1


1. The existence of separate wind and swell systems is to be documented. 2. The relationship between wind speed and wind wave data is to be recorded. 3. The relationship between swell wave systems and wind wave systems is to be recorded. 4. The spreading of the wind sea and swell sea systems is to be recorded, spreading information for each wave

direction is to be recorded. 5. The wave energy content of all seastates is to be recorded and the most suitable wave energy spectra identified for

the wind and swell wave systems, if appropriate. It may be appropriate to take account of wave direction in identifying the most suitable wave energy spectra.

2.3 Wind information Wind speed versus direction and season. Need to include mean values, gust values and gust frequency and duration.

2.4 Current and Tidal data Variations with tide, season and water depth.

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