01 jbp rules

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IACS JOINT BULKER PROJECT - Technical Backgrounds A3 (CCS/KR/NK) UNITAS (BV/GL/RINA) - RS

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IACS COMMON STRUCTURAL RULESJoint Bulker Project

General introduction to Technical Background documentation

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Table of contents

1.

INTRODUCTION 3

1.1. General 3

1.2. Purpose of the document 3

2. IMPLEMENTATION OF SAFETY OBJECTIVES 3

2.1. Basis of structural design 3

2.2. Goal-based Standards 4

2.3. Functional requirements 5

3. EQUIVALENT WAVE CONCEPT AND RULE LOAD CASES 6

4. STRENGTH CRITERIA 7

4.1. Characteristic values of loads 7

4.2. Ultimate strength of hull girder 7

5. LOAD CASES FOR PRIMARY STRUCTURE CHECKING OF BULKCARRIERS 8

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1. INTRODUCTION

1.1. GeneralThe present document introduces the main principles underlying the Joint Bulker Project setof structural rules for single side skin and double side skin bulk carriers of more than 90 m inlength.

The main aspects considered in these Rules are:- Compliance with the proposals of the common submission of Goal based standards by

Bahamas, Greece and IACS provided in MSC document MSC 78/6/2

- Conformity with the requirements and technical justifications of IACS UnifiedRequirements applicable to bulk carriers (e.g. UR S1A, S12, S17, S18, S20, S21, S25,S26, S27, S31, Z10.2, Z11.2.2)

- Explicit consideration of operational aspects (e.g. loading conditions in UR S25,loading/unloading operations, use of DSS spaces for ballast or void)

- Means of access for inspection and maintenance in to chapter 2 section 3 of the Rulesaccording to the amendments to chapter II-1 and to the Technical provisions for means of access for inspections reviewed by MSC 78.

- Explicit requirements for coating at the newbuilding stage of ballast tanks, cargo holdsand double side skin spaces whatever their use, to be completed by the

- Technical requirements for condition assessment of the structure of a ship already inservice during the classification periodic surveys provided in chapter 12 of the Rules.

1.2. Purpose of the document

The purpose of this document is to explain the way JBP implemented classificationrequirements into a consistent set of class rules, based of the state of the art concepts of safety and reliability standards.

The document is divided as follows:

Section 2 relates the Joint Bulker Project class requirements with the relevant parts of theBahamas, Greece and IACS submission for the Goal based standards

Section 3 introduces the equivalent wave concept and defines the basis for definition of the sea loads in the JBP Rules.

Section 4 presents the strength criteria used in the structural rules.

Section 5 discusses the loading conditions and standard load cases to be taken intoaccount for finite element calculations of DSS bulk carriers, taking UR S25 into account.

2. IMPLEMENTATION OF SAFETY OBJECTIVES

2.1. Basis of structural design

The class rules are to be based on simplified approaches for load formulation, corrosionprocess and structural models, like elastic finite element analysis, in order to allow a

reasonably effective and practical assessment of the ships hull structure.

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Figure 1 illustrates the logical structure of the class rules. The general safety objectives atthe top are linked to the reliability and risk-management approach of the modern structuralconstruction standards and are declined in the structural class rules at the lower level.

Fig. 1. Structure of the class rules

2.2. Goal-based Standards

The development of the bulk carrier rules has been undertaken following the generalprinciples which have been defined in the Bahamas, Greece and IACS proposal to IMO for the Goal based construction standards (document MSC 78/6/2) , illustrated in figure 1herebelow.

Figure 2: General organisation of Goal Based Standards according to document MSC 78/ 6/2

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JBP members mainly took into consideration the Safety objectives (tier I) and Functionalrequirements (tier II) of the proposal to IMO, considering that the Common Structural Rulesfor bulk carriers themselves are part of tier IV.

The safety objectives defined in the in the document MSC/78/6/2 are recalled here:

1. Design life of 25 years

2. North Atlantic environmental conditions

3. Structural safety

4. Structural accessibility for overall and close up inspection

5. Quality of construction

2.3. Functional requirements

The functional requirements defined apply to the Common structural rules for bulk carriers as

follows: Design life: the design working life, defined as the assumed period for which the ships

hull is to be operated with anticipated maintenance but without major repairs is 25 years.This applies both to ultimate strength of the hull girder (Ch 5, Sec 2) and to fatigue checkof the structural details (Ch 8). For sea induced loads in the North Atlantic environmentalconditions described in IACS Recommendation 34 (Nov. 2001), this correspondsapproximately to 0.8x10 -8 probability of exceedance per cycle. In practice, thecharacteristic value of the loads have been used at a probability level of 10 -8 probability of exceedance per cycle, with a multiplicative coefficient of 1.012 to take into account a 25years return period.

Protection by coating is required for all ballast tanks and cargo holds.

Corrosion is taken into account by considering suitable corrosion additions on top of netdesign scantlings.

The design, scantlings and protection of the structural elements are such that nosignificant deterioration is likely to occur within the period between successive detailedinspections (2.5 years).

All limit states, including buckling, are to be expressed as explicit rule combinationformulas in order to be easily programmed by designers.

The ultimate limit states of hull girder and secondary stiffeners are to be checked for acharacteristic value of actions corresponding to the design working life, i.e. 25 years.

The strength of the primary structure is to be assessed through elastic domain analysis(finite element models) for a characteristic value of loads corresponding to 10 -8 probabilityof exceedance.

Residual strength: Flooding of dry spaces is one of the accidental design situationsexplicitly dealt with. Contact damages are indirectly considered through general designrequirements (double sides) or robustness requirements like the minimum rulesthicknesses of structural elements.

Means of access: The need for critical parts of the structure to be available for inspectionwithout exposing the surveyor to unreasonable level of danger is to be considered in thedesign.

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3. EQUIVALENT WAVE CONCEPT AND RULE LOAD CASES

The sea induced loads are defined for a return period of 25 years. For loads in the North Atlantic environmental conditions described in IACS Recommendation 34 (Nov. 2001), thiscorresponds approximately to 0.8x10 -8 probability of exceedance per cycle. In practice, the

characteristic value of the loads have been used at a probability level of 10-8

probability of exceedance per cycle, with a multiplicative coefficient of 1.012 to take into account a 25years return period.

The principle of the equivalent wave approach is the following:

Given a long term value R R,LT of a load parameter (e.g. the bottom pressure or the transverseacceleration at a given location having an assigned probability of exceedance) and based onthe assumption that the maximum long term value of the parameter is well approximated byits maximum short term value in worst short term conditions, an equivalent wave is definedas the sinusoidal wave (in time and space) which, applied to a hydrodynamic model of thehull, gives a sinusoidal load parameter having an amplitude equal to R R,LT .

Common Structural Rules for bulk carriers use the Equivalent Wave Concept , asexperience shows that this concept is operational and well adapted to the loading of a finiteelement model of the ship and is clearly defined and understood by both thehydrodynamicists and structural engineers. The scope of this simple loading model issufficiently general to be applied to local scantling calculations as well as direct loading of afinite element model of a complete ship at sea.

Design load cases are derived from the results of the equivalent wave approach assummarised below.

For each equivalent wave (i.e. for each wave maximising a specific load parameter) thevalues of the other parameters induced by that wave are calculated.

The value of each load parameter for the equivalent wave is expressed as a fraction of itslong term value, so that, for each wave, a combination is obtained in which themaximised parameter appeared with a factor equal to 1,0 and the other with lower combination values.

Each combination identifies a load case.

Analysing the load cases so obtained for all the considered ships, it is possible to identifysome recurrences in the combinations between the load parameters. Finally, the designload cases are identified.

This process allows identification of four basic load cases defined in the loads TechnicalBackground document.

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4. STRENGTH CRITERIA

4.1. Characteristic values of loadsThe characteristic values of loads used in the Rules are provided in Table 1 as follows:

Yielding Buckling Ultimatestrength Fatigue

GlobalStrength Hull Girder

9 10 -8 With

UltimateStrength

9 10 -8 NA

Plating 9 10 -8 9 10 -8 9 10 -8 NAOrdinary Stiffeners 9 10 -8 9 10 -8 9 10 -8 NA

L o c a

l

S t r e n g

t h

Structural details connection(stiffeners and primarysupporting members Reference load only)

NA NA NA 9 10 -4

Primary supporting members 9 10 -8 9 10 -8 9 10 -8 NA

Table 1: characteristic values of loads

4.2. Ultimate strength of hull girder

The principle of the analysis is as follows:

The hull girder is loaded by the combination of vertical still water bending moment and waveinduced vertical and horizontal bending moments, according to the following process: whenthe bending moment increases, the deck and bottom are subject to compression and tensionstresses. When theses stresses exceed the local strength of a particular structural element(stiffener and associated plating), partial buckling or yielding of this element occurs, and thisfailure further propagates to adjacent elements, ultimately leading to collapse of the wholecross-section and breaking of the hull girder.

Compared to the existing buckling criteria of UR S11, the main advantage of the ultimatestrength approach is clearly to obtain an overall picture of the hull girder condition takingexplicitly into account the containment of failure effect produced by hard corners or reinforced components like longitudinal girders.

The ultimate hull girder background document provides detailed explanation about theincremental iterative procedure implementing the Smiths method and nonlinear finiteelement method taken into account in the rules.

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5. LOAD CASES FOR PRIMARY STRUCTURE CHECKING OFBULK CARRIERS

Ch 4 Sec 7 of the Rules provides the list of standard loading conditions proposed for thefinite element analysis of DSS bulk carriers in order to define an allowable domain of internalloading large enough to contain the majority of the loading conditions commonly expected inservice.

The loading conditions described in UR S25.5 of IACS are the basis for the describedloading conditions. They have been completed by additional hypotheses on hull girder andlocal loading combination of effects in order to design a set of ship design loading conditionscovering the main design conditions for the finite element analysis of the structure.

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