For reasons of economy, this document is printed in a limited number. Delegates are kindly asked to bring their copies to meetings and not to request additional copies.

I:\SLF\51\13-1.doc

INTERNATIONAL MARITIME ORGANIZATION

IMO

E

SUB-COMMITTEE ON STABILITY AND LOAD LINES AND ON FISHING VESSELS SAFETY 51st session Agenda item 13

SLF 51/13/1

10 April 2008 Original: ENGLISH

GUIDELINES FOR VERIFICATION OF DAMAGE STABILITY REQUIREMENTS

FOR TANKERS AND BULK CARRIERS

Development of guidelines for the verification of damage stability on oil, chemical and gas tankers

Submitted by Denmark, Finland, Germany, Norway, Sweden, the United Kingdom

and Intertanko

SUMMARY

Executive summary:

This document proposes draft guidelines for the verification of damage stability requirements contained in existing instruments for tank vessels, and invites other Administrations to consider these in developing a new work programme item to ensure consistent verification of damage stability on such vessels prior to departure.

Strategic direction:

2

High-level action:

2.1.1

Planned output:

2.1.1.2

Action to be taken:

Paragraph 3

Related documents:

MSC 83/25/14, MSC 83/22/2, MSC 82/18/3, MSC 82/18/4 and MSC 82/18/5

Introduction 1 Further to submission of document MSC 83/25/14 concerning the lack of damage stability verification on tank vessels prior to departure, inviting the Committee to include a new work programme item to develop guidelines for verification of damage stability for tankers prior to departure to sea, the United Kingdom and co-sponsors welcome the inclusion of a new work programme item for the SLF Sub-Committee to consider this issue in co-operation with the DE and STW Sub-Committees as necessary and when requested by the SLF Sub-Committee.

SLF 51/13/1 - 2 -

I:\SLF\51\13-1.doc

2 The annex to this document contains draft guidelines for verification of damage stability for tankers which are submitted for information and for comment by other Administrations. It is proposed that these draft guidelines and any comments upon them are considered as a basis for discussion by any working group established to consider this new work programme item. Action requested of the Sub-Committee 3 The Sub-Committee is invited to note the information provided and to take action as appropriate.

***

SLF 51/13/1

I:\SLF\51\13-1.doc

ANNEX

DRAFT GUIDELINES FOR VERIFICATION OF DAMAGE STABILITY FOR TANKERS

1 Introduction 1.1 Stability approval for tanks ships requires verification that loading conditions comply with intact and damage stability criteria defined within the instruments applicable to each vessel type. 1.2 Approval is commonly established by verifying that standard intact loading conditions demonstrating a range of possible loading patterns each comply with the appropriate intact and damage stability criteria. Following verification and approval of the stability information, vessels are restricted to operations which only employ the standard intact cond itions from the approved stability information, or conditions which do not deviate significantly from these. 1.3 Many tank vessels have been found to operate in loading conditions which are substantially different from the standard intact conditions, noting that relatively small changes in any of the following variables can severely degrade damage stability margins determined during initial verification and that many vessels exhibit margins at or close to zero.

.1 condition draught ;

.2 condition trim;

.3 condition heel;

.4 cargo tank filling levels;

.5 cargo SG value;

.6 ballast tank filling levels;

.7 global free surface; and

.8 condition intact GM. 1.4 In considering what constitutes a loading condition that deviates significantly from an approved condition, one interpretation is that no individual tank should vary by more than 2% of the mass assumed in the approved condition, and that condition intact GMf should lie within 20 mm of that indicated in the approved condition. 1.5 Where a loading condition is planned which lies outside these limits, intact and damage stability compliance must be verified prior to departure, by on-board checks or by submitting the loading condition to the flag State (or their delegated certifying authority) for approval. 1.6 PSC inspection of tank vessels indicates that these safeguards are not being met in practice on substantial numbers of ships, particularly where on-board strength computer programs required by classification societies to verify longitudinal strength also give a “green light” for intact stability compliance. 1.7 The lack of adequate verification of damage stability compliance prior to departure adds significantly to the potential severity of the outcome following damage, particularly where intact condition GMf has been reduced artificially below that of the approved intact condition to improve seakeeping behaviour for reasons of comfort. Such conditions can still meet intact stability but may have severely reduced damage stability compliance.

SLF 51/13/1 ANNEX Page 2

I:\SLF\51\13-1.doc

2 Application 2.1 The issues described in section 1 affect vessels which are certified under the provisions of the following instruments:

.1 Annex I of MARPOL; .2 International Code for the Construction and Equipment of Ships Carrying

Dangerous Chemicals in Bulk (IBC Code); and .3 International Code for the Construction and Equipment of Ships Carrying

Liquefied Gases in Bulk (IGC Code). 3 Proposal for rectification 3.1 Noting that obtaining damage stability approval of individual planned loading conditions by submission to the flag State (or their delegated certifying authority) prior to departure is not a practical proposition, having regard to the number of cases involved and the need for fast response, it is considered that on-board verification using a combined intact and damage stability program offers the best solution. 3.2 The United Kingdom, in conjunction with its co-sponsors, have considered the issues involved in conducting such damage verifications and consider that a standardized calculation method should be used for this purpose, to ensure that such verifications are conducted in a consistent fashion. 3.3 It is anticipated that this standardized calculation method would be adopted by flag States and IACS classification societies to ensure consistent damage stability verification for the purposes of:

.1 validation of damage calculations supporting initial stability information approval; .2 approval of critical KGf or GMf data for damage stability verification to be

applied within an IACS URL5∗ approved Type 2 stability program; and .3 approval of calculation method to be employed by damage stability programs

submitted for approval as IACS URL5 Type 3. 3.4 The United Kingdom, in conjunction with its co-sponsors, welcomes the introduction of a new SLF work programme item to consider the introduction of a standardized calculation methodology for tank ship damage stability approval and submits the following guidelines for this purpose, inviting other delegations to consider and comment upon these as a basis document at any working group established to implement the new work programme item.

∗ A full text of this unified requirement may be found at the following link

http://www.iacs.org.uk/document/public/Publications/Unified_requirements.

SLF 51/13/1 ANNEX

Page 3

I:\SLF\51\13-1.doc

[DRAFT] GUIDELINES ON STANDARDIZED CALCULATION METHOD FOR APPROVAL OF TANK VESSEL DAMAGE STABILITY

1 These guidelines describe a standard systematic process for damage stability analysis as this applies to tank vessels. Adoption of such a standard method of analysis is expected to reduce or eliminate any scope for differences of interpretation within existing IMO instruments. 2 Damage stability analysis is required by Annex I of MARPOL, the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code) and the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code). 3 Compliance is demonstrated using a deterministic method of analysis for each of these classes of vessel. The deterministic calculation method is based upon fixed damage assumptions (damage length, transverse extent, vertical extent). 4 Depending on the ship type, size and the potential risks to the environment posed by the cargo carried, compliance with a specific compartmentation standard is required to be demonstrated. 5 In the case of tank ships analysis of substantial numbers of service conditions does not ensure compliance in any condition of loading, due to the potential variation in cargoes carried and their distribution within the ship, and the fact that operating conditions which diverge significantly from approved conditions are not permitted. 6 Where tank vessels do not strictly adhere to approved loading conditions, experience has shown that reliable verification of damage stability is only possible prior to departure if an approved computer is available on board, using an approved IACS URL5 damage stability calculation program of type 2 or 3.

Part A – Data checks Section A 1 − Reference points All weathertight openings, unprotected openings and the deck line required to verify stability compliance should be identified and their correct representation in stability information, stability calculations or stability program confirmed. Section A 2 − Ship geometry This needs to be examined against the requirements of the applicable instrument for damage extent, and all cases of side and bottom damage should be determined for the vessel having regard to separation of bulkheads and the presence of any steps or recesses. Damages cases should be determined for both sides. All cases of lesser damage should also be determined and this will include single compartment damages for two compartment ships, damages of lesser length, damages of lesser height (including omission of double bottoms or deck tanks only), and should also include omission of single double bottom tank from a pair, or single deck tank from a pair, where applicable.

SLF 51/13/1 ANNEX Page 4

I:\SLF\51\13-1.doc

Consideration should also be given to the presence of cross-levelling ducts of substantial cross-section and other pipes, ducts or tunnels positioned within the damage extent considered in a particular case, which may lead to flooding of additional compartments beyond this extent unless suitable means are provided to prevent this eventuality. Evaluation of all damage cases (including lesser cases) should be included in the stability calculations or stability program submitted for approval.

Part B – Damage stability Section B 1 − General All damage calculation should be determined on the basis of free trim. Evaluation of damage stability criteria shall be determined from data calculated over a range of angles, in the order of at least 0, 5, 10, 15, 20, 25, 30, 40, 45, 50 and 60 degrees. All calculations should be conducted on the basis of sea water SG to be input or confirmed by the user. Determination of final equilibrium position and stability evaluation Damage calculations for evaluation of the final equilibrium position should be made on the basis of lost buoyancy. In determining the final equilibrium position after damage and the residual stability characteristics of the vessel, the intact condition should first be modified to deduct the liquid content of any compartment included within the damage case under consideration. The lost buoyancy calculation should then be calculated from this modified initial condition and the residual stability curve determined at the associated displacement. For evaluation of residual stability at equilibrium the guidance given in MSC/Circ.406/Rev.1, regarding consideration of the applicable residual range, should be interpreted to mean that all parts of the positive range between the angle of equilibrium and the upper limiting angle used for evaluation may be taken into account when evaluating range, GZ and area requirements. Intermediate stages of flooding and stability evaluation Intermediate stages of flooding should be evaluated for damage stability compliance in addition to the final equilibrium condition. For this purpose six flooding stages should be evaluated, with stage six equating to the final equilibrium position. All compartments should be considered individually (separately) flooded. Intermediate stages of flooding should be evaluated using the added mass method, and the filling of all spaces subject to damage should be varied in equal stages between the initial filling (or zero) and the final filling determined at equilibrium.

SLF 51/13/1 ANNEX

Page 5

I:\SLF\51\13-1.doc

Cross-levelling arrangements Cross flooding arrangements should only be considered via ducts of a large cross-sectional area. Filling of a cross-flooded compartment should be assumed to take place concurrently with flooding of the associa ted compartment on the damaged side of the vessel. That is, the compartments should be considered common. In this respect, ducts should be considered to be of large cross-sectional area if cross-flooding is completed within 60 seconds as calculated in accordance with resolution MSC.245(83). Where cross-flooding completes in more than 60 seconds, when calculated in accordance with resolution MSC.245(83), then no account should be taken of the arrangement for reducing heel or improving stability characteristics of the residual GZ curve. In determining the adequacy of cross-levelling arrangements, the cross-sectional area of air pipes serving such cross- levelled compartments should be confirmed as providing a minimum free cross-sectional area of 10% of the cross- levelling duct. Progressive flooding During the consideration of damage stability requirements, permanent immersion of any weathertight opening, such as air-pipes, through which progressive flooding may take place in the equilibrium condition should be considered to terminate the GZ curve. Section B 2.1 – MARPOL Annex I, regulation 28 Damage stability compliance is required in all cargo operating conditions for all vessels to which Annex I applies. It should be confirmed that all damage cases for the vessel size, type and date of build have been considered in submitted stability calculations or within a submitted stability program. Compliance with damage requirements may be demonstrated through stability calculations considering each damage case applied in turn to every standard loading condition included in the stability information. In applying free surface corrections during direct damage calculations, the actual moment of fluid transfer should be applied to each compartment for each angle of heel calculated. Compliance may also be shown through direct calculation of planned loading conditions within an IACS URL5 Type 3 stability computer or by planned loading conditions showing a margin on critical KGf or GMf data stored for this purpose within an IACS URL5 Type 2 stability computer. Where critical KGf or GMf data are used for this purpose, their derivation should take account of the guidance given in Part C.45 Section B 2.2 – IBC and IGC Codes, regulations 2.4, 2.5, 2.6, 2.7, 2.8 and 2.9 Damage stability of all standard cargo loading conditions should comply with the requirements of regulation 2.9 of the Code following application of the damages specified in regulations 2.5, 2.7 and 2.8.

SLF 51/13/1 ANNEX Page 6

I:\SLF\51\13-1.doc

It should be confirmed that all damage cases for the vessel size, type and date of build have been considered in submitted stability calculations or within a submitted stability program. For the purpose of determining loading conditions, due regard shall be taken of the requirements of regulation 2.2.3 for minimum free surfaces to be allowed for consumable liquids. In applying free surface corrections during direct damage calculations, the actual moment of fluid transfer should be applied to each compartment for each angle of heel calculated. Compliance with damage requirements may be demonstrated through stability calculations considering each damage case applied in turn to every standard loading condition included in the stability information. Compliance may also be shown through direct calculation of planned loading conditions within an IACS URL5 Type 3 stability computer or by planned loading conditions showing a margin on critical KGf or GMf data stored for this purpose within an IACS URL5 Type 2 stability computer. Where critical KGf or GMf data are used for this purpose, their derivation should take account of the guidance given in Part C.45

Part C – Critical KGf or GMf data Section C 1 − General The required scope of damage stability analysis is determined from the applicable damage stability standard, and aims to provide the ships master with clear intact stability limits which ensure compliance with damage stability requirements. This may be achieved by determining limiting KGf (or respective GMf) data, containing the admissible intact stability values which ensure damage stability compliance for the draught range covered. When combined with limiting KGf (or GMf) data for intact stability compliance, these can be employed to ensure safe loading. Taken in combination with the need to provide an auditable record for PSCI purposes, confirming that verification has been undertaken for all locations in the periphery of the vessel, it is further considered that such verifications should be made using an IACS URL5 type 2 loading instrument. Treatment of condition heel Intact loading conditions used as an input to damage stability evaluation using critical KG/GM data, should be shown upright with heel zero where critical KG/GM data have been derived on this basis. For the purpose of developing such loading conditions it should be accepted that conditions which exhibit a heel of less than 0.25 degrees may be considered as upright, and heel may be set to zero, as it is considered that movement of small items of deadweight unaccounted for in the loading condition may be used to bring such a condition upright in accordance with good seamanship practice.

SLF 51/13/1 ANNEX

Page 7

I:\SLF\51\13-1.doc

Section C 2 − Limit curve calculation Limiting KGf or GMf data may be derived for individual damage cases, for a group of damage cases (typically for a combination of all damage cases relating to a particular single compartment – including cases of lesser extent) or for all damage cases. Alternatively, limiting data may be derived for one or more cases of damage evaluated over a range of draught and trim. Whatever method is used, each set of limiting data should be derived for application to an identifiable intact floating position (defined by intact draught and trim) over a range of potential tank fillings. Where several tanks are included in the damage case(s) considered, guidance should be given on how these fillings are to be controlled if restrictions on tank filling have been considered. It may be noted that damage compliance can vary significantly for differences in cargo SG or cargo levels, and that tank restrictions should be defined by mass of their content to reduce such potential discrepancies to a minimum. Initial conditions for derivation of each limiting curve should be specified with regard to: Condition draught Condition trim Cargo tank filling levels Cargo SG value Ballast tank filling levels This description should identify which variables have been fixed (if any), which are subject to particular assumptions (if any), such as minimum/maximum filling, and which variables have been employed as axes for the plotting of the data. Where limiting KGf (or GMf) data have been derived by taking the envelope of several cases of damage, damage locations, draughts or tanks fillings, etc., these assumptions should also be stated.

___________