LR rule change

Common Structural Rules forDouble Hull Oil Tankers

Rules

aft for comment

Prepared by

June 2004

Summary of Technical Rule Changesfor the June 2004 Rules

6 January 2005

© American Bureau of Shipping, Det Norske Veritas and Lloyd's Register 2005

Summary of significant Rule changes 6th January 2005

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CONTENTS INTRODUCTION SUMMARY OF RULE CHANGES

• Section 7/2.1.1 Still water bending moment • Section 7/2.1.2 Still water shear force • Section 7/3.5.4.2 and 3 Internal Dynamic Pressure • Section 8/1.1.2.4 Loading conditions required to be included in the loading manual • Section 9/1.2 and 1.4 Table 9.1 Hull Girder Ultimate Strength • Appendix B/Section 2.3/

o Table B2.3 - FE load cases for tankers with two oil-tight longitudinal bulkheads o Table B2.4 - FE load cases for tankers with one centerline oil-tight longitudinal bulkhead

• Appendix B/Section 2.3 o Table B.2.5 - FE Dynamic Load Conditions

• Appendix B/Section 2.4 o Table B.2.6 - Ship Parameters for Calculation of Loads and Accelerations

• Appendix B/Section 2.4 o Table B.2.7 - Required Target Hull Girder Vertical Bending moments and Shear Forces.

(Renamed as: Applicable Rule dynamic pressure, cargo accelerations, shear force and bending moments)

• Appendix B/Section 2.4/2.4.7.1 Internal pressure in cargo and ballast tanks • Appendix D

o Table D.5.1 - Advanced Buckling Assessment for longitudinal strength o Table D.6.1 - Structural Elements for Buckling Assessment


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Introduction

The draft Common Tanker Rules (CTR) were printed in June 2004. One of the core objectives of the CTR development has been to make the development process and resulting Rules as transparent as possible to facilitate understanding and support. This document is issued in response to this objective. While the draft Rules became public in June 2004 their genesis has continued and two parallel tracks have been followed for making changes. The first is the completion of an extensive series of calibration and development tasks where items were found in need of revision. The second is the Technical Committee (TC) hearings. The latter commenced, initially with August 31st

as the deadline for submissions, then, September 30th

, and now after consultation with industry and with agreement from our colleagues at IACS this period is extended up to 31st

December. At the time of writing this document there have been just under 2600 questions submitted including many very detailed comments and calculations. All the questions and approved answers can be reviewed using the JTP Knowledge Centre on the JTP website www.jtprules.com. The results of the consequence assessment previously published 23rd

July 2004 have now been superceded and a brief slide presentation showing the preliminary results for the revised Rules, reflecting the above tasks, have been published on the JTP website. A new consequence assessment has commenced using the Classification Societies’ proprietary software that have been updated for the new Rules and the results of which harmonised to ensure consistency. Publication of the results is planned to coincide with the next version of the Rules scheduled to be published at the end of March 2005. The changes shown in this document are those that may have a significant impact on the scantling requirements and consequently on steelweight. A number of additional changes have been made based on comments from Technical Committee members and industry. Further documents1 detail JTP’s response to questions from Technical Committee members. It is not intended to provide the actual Rule changes within this document, but rather to indicate the scope, reason and impact for the change planned.

FOOTNOTE 1 The first summary of comments and responses to Technical Committee members – dated 30th Oct. 2004 The second summary of comments and responses to Technical Committee members – dated 2nd Nov. 2004 The third summary of comments and responses to Technical Committee members – dated 1st Dec. 2004 The fourth summary of comments and responses to Technical Committee members – dated 22nd Dec. 2004


Summary Technical Rule Change to the June 2004 JTP Rules S R

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This document summarises proposals to change the technical aspects of the June 2004 version of the Common Structural Rules for Double Hull Oil Tankers and is made available solely to provide advance general information on enhancements under consideration by the JTP project team. Readers should note that the technical changes listed in this document are yet to be reviewed and approved by the applicable JTP Governances, and as a consequence are subject to amendment or withdrawal at any time from the date of this document to the date of formal adoption.

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ection 7/2.1.1 Still water bending moment

ule change description: A rule minimum hull girder still water bending moment for seagoing and harbour/sheltered waters respectively has been included in the JTP Rules.

eason: The minimum rule hull girder still water bending moment is included to ensure that the vessel is given permissible limits that allow for reasonable operational flexibility regardless of conditions specified in the loading manual.

mpact: The impact of the rule change will be limited for tankers provided with loading manuals which include a reasonable variety of loading conditions. For tankers provided with loading manuals with very few conditions and where the conditions are tuned to reduce the hull girder loads, there will be some impact in the form of increased scantlings. Increases will mainly be related to the need for increased buckling capacity of the deck structure.


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Summary Technical Rule Change to the June 2004 JTP Rules Section 7/2.1.2 Still water shear force Rule change description:

A rule minimum hull girder still water shear force for harbour/sheltered waters has been included in the JTP Rules. The rule minimum value is set in relation to the permissible seagoing hull girder shear force limits.

Reason: The minimum rule hull girder still water shear force is included to ensure that the vessel has reasonable operational flexibility during loading and unloading operations based on loading conditions given in the loading manual.

Impact: The impact of the rule change will be limited in terms of required scantlings but will ensure that the rule limits are clearly defined.



Summary Technical Rule Change to the June 2004 JTP Rules S R

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ection 7/3.5.4.2 and 3 Internal Dynamic Pressure

ule change description: Factors have been added to reduce the cargo tank internal pressure formula to account for ullage and dynamic affects due to less than 100% full cargo tanks. The factors affect the pressure due to ship motion and acceleration in the transverse and longitudinal directions and are representative adjustment of the dynamic component of the internal pressure. The factors do not affect the static components. The ullage effect and dynamic component adjustments are not applied to the pressures of ballast tanks.

eason: The inclusion of the factor was made in order to improve the rule cargo pressure formulations for strength considerations based on comparison with direct calculation results. In addition it was found that the combined load / acceptance criteria for local cargo tank members resulted in excessive requirements when compared to the existing classification rules.

Impact: The inclusion of the factor results in a reduction to the dynamic component of the cargo tank internal pressure from that of the June 2004 version of the JTP Rules. This in-turn results in scantling reductions in locations where the dynamic load components govern the determination of cargo tank scantlings, such as the upper portions of the tanks and transverse bulkheads. This will also affect the primary support members.


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Summary Technical Rule Change to the June 2004 JTP Rules Section 8/1.1.2.4 Loading conditions required to be included in the loading manual Rule change description:

Conditions required to be included in the loading manual have been expanded. • The JTP Rules will require that both a “light” and “heavy” ballast condition is included in the

manual. The “light” condition is defined as a condition where only clean ballast tanks in the cargo area are used while the “heavy” condition is a ballast condition in which the fore peak tank is also full.

• The JTP Rules will require that a propeller survey/removal afloat condition is included in the manual.

Reason:

• The inclusion of the “heavy” ballast condition is included in order to allow that the master can fill the fore peak tank and increase the forward draught if heavy weather is encountered, without being limited by the permissible still water hogging moment.

• The inclusion of the propeller survey/removal condition is included in order to ensure that the permissible hull girder bending moment limits of the ship allow for inspections/repair of the propeller in case of damage.

Impact:

The inclusion of the additional loading conditions may somewhat increase the design still water hogging moments for the vessel.



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Summary Technical Rule Change to the June 2004 JTP Rules Section 9/1.2 and 1.4 Table 9.1 Hull Girder Ultimate Strength Rule change description:

The still water bending moment to be used in the hull girder ultimate strength criteria has been constrained not to be less than 90 % of the permissible still water bending moment. The partial safety factor for the wave bending moment has been reduced from 1.35 to 1.3.

Reason: The change has been made in order to meet the concern that the ship may operate at higher still water bending moments than the maximum sagging value from any seagoing condition given in the loading manual. The hull girder ultimate strength criteria has been derived using a structural reliability analysis approach, and the reason why 90% has been applied as opposed to 100% relates to the fact that it is unlikely that the ship operates at the permissible value in combination with the most extreme wave bending moment. The reduction in the partial safety factor for the wave bending moment can be justified on the basis of a slightly more strict definition of the still water bending moment, while maintaining the overall safety level.

Impact: On average, the change is not expected to give any changes in the required scantlings. However, in cases when the permissible still water bending moment is equal or very close to the maximum seagoing sagging value in the loading manual, the change gives a criterion that is slightly less strict. In cases where there is a significant gap between the permissible and the seagoing sagging value, the change gives a slightly stricter criterion.



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Summary Technical Rule Change to the June 2004 JTP Rules Appendix B/Section 2.3/ Table B2.3 FE load cases for tankers with two oil-tight longitudinal bulkheads Table B2.4 FE load cases for tankers with one centerline oil-tight longitudinal

bulkhead Rule change description:

(1) Revised assignment of still water bending moment (SWBM) and shear force (SWSF) to be used for each loading pattern.

(2) Introduction of filled ballast wing tanks in checker-board loading patterns. Introduction of new loading patterns for harbour load-cases. Removal of the normal ballast condition from the set of standard seagoing load cases

(3) Revised assignment of dynamic load conditions to loading patterns.

(4) Definition of load cases required for the assessment of midship region scantlings and load cases required for the assessment of strengthening in way of individual transverse bulkheads due to hull girder shear loads.

(5) Revised ship draught for loading patterns; The minimum draught used for sea-going load cases reduced from 2/3 Tsc to 0.6 Tsc. The maximum draught used for sea-going load cases with one or more wing cargo tank

empty reduced from Tsc. to 0.9 Tsc.

(6) For tankers with two oil-tight longitudinal bulkheads, the Rule minimum sea-going load case with all cargo tanks abreast empty is to be assessed at a reduced draught of 0.6 Tsc (previously 0.9 Tsc). A corresponding loading case in harbour is added based on a ship draught of 0.8 Tsc. A deeper draught for these loading patterns is optional and will be assessed if included in the loading manual.



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Reason:

(1), (2) and (3) In the first draft of the JTP Rules (version: June 2004), a conservative approach was adopted in assigning hull girder shear force and bending moment to be used with the FE load cases. This approach resulted in the need to apply large adjustment forces to achieve the required shear force in some load cases. This is due to the assigned shear force not reflecting the value that can actually be achieved by the loading patterns considered. Further, large adjustment forces can introduce unrealistic deformation of the structure resulting in abnormal scantlings in those areas affected.

The assignment of still water loads to each loading pattern has been revised and new harbour loading patterns introduced to ensure that the still water bending moment and shear force values used in the FE load cases represent the worst values that can be realistically achieved in operation. Filling of wing ballast tanks is introduced for checker board loading patterns to give the worst still water shear force scenario. Assignment of dynamic load conditions to loading patterns is revised to maximize combined still water and wave hull girder loads. The normal ballast loading pattern has been removed since this loading pattern does not result in stresses that govern the scantling requirements for primary supporting members or for buckling assessment.

(4) The application of different load cases to different longitudinal regions of the hull is introduced to increase the flexibility of the JTP Rules. Further, it is introduced to eliminate the over conservative scantling requirements reflected in the first draft of the JTP Rules for the strengthening of structural members subjected to hull girder shear loads. Load-cases for assessing the hull shear capability are introduced to enable strengthening to be carried out in way of individual transverse bulkheads in accordance with the permissible shear force at the position considered. In the first draft of the JTP Rules, a conservative approach was adopted where the maximum shear forces (positive and negative) are used for the assessment of strengthening requirement for hull girder shear loads.

(5) The minimum draught used for sea-going load-cases is reduced to provide more operational flexibility. Full scantling draught in association with the full dynamic wave pressure in beam sea conditions for loading patterns with empty wing cargo tank(s) results in a substantial increase in scantlings of side and transverse structures. It is considered that the mandatory requirement of full scantling draught with these loading patterns is not feasible since in most cases when the wing cargo tank(s) are empty, the full scantling draught cannot normally be achieved. Where a draught deeper than 0.9Tsc is required, the associated loading pattern is to be specified as an optional condition.

(6)

It is not considered necessary to include a deep draught with all cargo tanks empty abreast, condition as a mandatory requirement for tankers with two oil-tight longitudinal bulkheads (typical for VLCC designs). The deep draught loading condition with all cargo tanks abreast empty is not adopted for all designs. This condition will result in a higher still water shear force than other conditions and therefore will require additional strengthening of side shell, inner hull and longitudinal bulkhead. However, in order to provide adequate flexibility in operation, it is mandatory that this condition is assessed at a standard draught of 0.6 Tsc (sea-going condition) and 0.8 Tsc (harbour condition).


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Impact:

Rule changes (1), (2) and (3) have eliminated the need for large adjustment forces to achieve the assigned hull girder shear forces in all load cases, resulting in correct deformation of the structure. The required scantling increases computed from application of the first draft of the JTP Rules are now less severe. Rule change (4) results in a less significant scantling increase at side shell, inner hull and longitudinal bulkhead compared to the first draft of the JTP Rules. A strengthening assessment for hull girder shear force can be applied to individual transverse bulkhead positions to optimize the strengthening where required. Rule change (5): Reducing draught for loading patterns with cargo wing tank(s) empty results in a less significant scantling increase for side and transverse structures than that required by the first draft of the JTP Rules. A lower minimum draught for sea-going loading patterns will not give any significant change to the scantling requirement but will give an increase in operational flexibility. Rule change (6) reduces the strengthening requirement for the hull girder shear carrying members and avoids penalising designs which do not require such a loading pattern.


Summary Technical Rule Change to the June 2004 JTP Rules AT R

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ppendix B/Section 2.3 able B.2.5 FE Dynamic Load Conditions

ule change description: Revision of load combination factors for hull girder loads for beam sea and oblique sea conditions. For beam sea conditions, the load combination factors for vertical wave bending moment, horizontal wave bending moment and vertical wave shear force are set to zero (previously ranging from 0.1 to 0.3, 0.1 to 0.2 and 0.1 respectively). For oblique sea conditions, the load combination factor for vertical wave shear force is set to zero (previously 0.1).

eason: The change is necessary to simplify the application of the procedure and to enable a more logical combination of the beam sea dynamic load conditions when the loading pattern is subjected to either hogging or sagging still water bending moments (negative or positive still water shear force).

mpact: The minor changes to the load combination factors have no impact on the scantling requirement as computed from the first draft of the JTP Rules. .


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Summary Technical Rule Change to the June 2004 JTP Rules Appendix B/Section 2.4 Table B.2.6 Ship Parameters for Calculation of Loads and Accelerations Rule change description:

The contents of the table have been revised in accordance with the changes in the loading patterns and ship draughts contained in Appendix B/Section 2.3/Tables B.2.3 and B.2.4. The GM for standard load cases is to be calculated using the rule formulae. Formulae have been added for calculating GM and roll radius of gyration for partially loaded conditions. A procedure has been introduced for calculating GM and roll radius of gyration for optional loading conditions.

Reason:

Revision is made to reflect the changes in loading patterns and ship draughts in Appendix B/Section 2.3/Tables B.2.3 and B.2.4. The introduction of additional formulae and a procedure for calculating GM and roll radius of gyration is to provide a closer estimation to the actual values resulting in a more accurate roll acceleration induced cargo inertia pressure and to achieve greater consistency in assessment.

Impact:

For standard part-loaded conditions at 0.6 Tsc, the new GM and roll radius of gyration formulae give slightly higher values than those calculated from the first draft of the JTP Rules. The change results in a slightly higher cargo inertia pressure due to increased accelerations. However, the resulting increase in scantlings of the tank boundary structure is not expected to be significant.



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Summary Technical Rule Change to the June 2004 JTP Rules Appendix B/Section 2.4 Table B.2.7 Required Target Hull Girder Vertical Bending moments and Shear Forces. (Renamed as: Applicable Rule dynamic pressure, cargo accelerations, shear force and bending moments) Rule change description:

A complete revision of Table B.2.7. The table is revised to support the rule changes to Appendix B/Section 2.3/Tables B.2.3 and B.2.4, item (4) by defining the load-cases required for the assessment of the midship region scantlings and load-cases required for the assessment of strengthening in way of an individual transverse bulkhead due to hull girder shear loads.

The table describes the procedure for determining the external wave pressure, accelerations, hull girder bending moments and vertical wave shear forces to be used in the load-cases for different assessments.

Reason:

The approach adopted in the first draft of the JTP Rules is considered to be over-conservative resulting in inappropriate scantling increases.

In the first draft of the JTP Rules, the strengthening requirement for structural members subjected to hull girder shear loads is based on the combination of the maximum of all permissible still water shear forces at the transverse bulkheads and the maximum wave induced hull girder shear force. No provision was made in the first draft of the JTP Rules for assessing the strengthening requirement in way of an individual transverse bulkhead considering the permissible still water shear force and wave induced hull girder shear force at the given bulkhead position.

Since the permissible shear force is not normally the same value for every bulkhead position, load-cases for assessing the hull shear capability are introduced to enable strengthening to be carried out in way of individual transverse bulkheads.

Load-cases are introduced to enable optimization of strengthening for hull girder shear loads in way of individual transverse bulkheads in accordance with the permissible shear force at the position considered.

Impact:

By removing the conservative requirement of applying maximum shear forces, the Rule change results in less significant increases at the side shell, inner hull and longitudinal bulkhead compared to the first draft of the JTP Rules. Strengthening assessment for hull girder shear force can be applied to an individual transverse bulkhead position to optimize the strengthening where required.



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Summary Technical Rule Change to the June 2004 JTP Rules Appendix B/Section 2.4/2.4.7.1 Internal pressure in cargo and ballast tanks Rule change description:

Factors have been introduced in the formula for the cargo tank internal pressure calculation to account for the reduction in pressure due to ullage effects.

Reason:

The changes are needed to reflect the amendment made to the internal cargo pressure calculation in Section 7/3.5.4.2 and 3.5.4.3.

Impact: The inclusion of the factors results in a reduction of the dynamic component of the cargo tank internal pressure compared with the draft CTR, June 2004. The changes reduce the loads applied on cargo tank boundary structure, such as transverse bulkheads, resulting in a lower scantling requirement.



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Summary Technical Rule Change to the June 2004 JTP Rules Appendix D Table D.5.1 Advanced Buckling Assessment for longitudinal strength Table D.6.1 Structural Elements for Buckling Assessment Rule change description:

Assessment method for web of double bottom longitudinal girders connected to hopper tank side, and for web of horizontal girders in double side tank connected to hopper tank side, has been changed from buckling strength (BS) to ultimate capacity (UC).

Reason: The change is done to obtain consistency with the assessment method for double bottom longitudinal girders in line with longitudinal bulkheads. It was also found that the buckling strength (BS) assessment resulted in high scantling increase compared to other structural members, for the test vessels considered. To make this strength assessment less conservative, as well as give allowance for the redundancy with respect to compressive strength in the hopper area, ultimate capacity (UC) was found to be appropriate.

Impact:

This modification will reduce the required thickness of these members somewhat, compared with the previously proposed rule criteria.


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