Application of Amendments to Gas Carrier Codes Concerning Type C Tank Loading Limits

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Published Date

January 2012


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Application of Amendments to Gas Carrier Codes Concerning Type C Tank Loading Limits

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This booklet is designed to serve as a reminder to ship owners and terminal operators of the improvements in safety codes for cargo tank loading limits provided by the 1998 amendments.

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Changes to the IGC Code have made compliance less precise than conforming to the original code because the criteria for determining the lowest cargo density anticipated becomes the highest operational cargo temperature encountered. Predicting this temperature requires judgement on the part of the shipowner and the ability to convince Administrations that the selection is reasonable.

 

At its 14th session in December 1984, the IMO Bulk Chemicals Sub-Committee expressed its willingness to consider amending the codes governing cargo tank loading limits.

 

SIGTTO and IACS believed that for Type C tanks the codes reduced protective measures available under fire conditions or increased the risk of unwanted venting of cargo. By updating the codes, protection could be improved.

 

The IMO’s Marine Safety Committee, at its 61st session (MSC61), agreed to change the IGC Code and made that protection available.

 

These new amendments mean that Type C tanks, under fire conditions, can become filled with liquid. In this situation, the Code requires that the tank’s internal pressure does not exceed 20% above the Maximum Allowable Relief Valve Setting (MARVS). IACS/SIGTTO were able to demonstrate that Pressure Relief Valves (PRVs) could prevent the tank pressure exceeding 1.2 x MARVS.

 

To be certain that PRVs will perform as designed, inlet pressure losses and built-up back pressures must be within the PRV manufacturer’s limits.

 

IMO delegated the production of these guidelines to IACS/ SIGTTO who submitted the document to the 65th session of the Marine Safety Committee (MSC65). MSC65 recommended that the guidelines should be accepted, and they came into force on 1st July 1998.

 

The object of this booklet is to remind shipowners and terminal operators of the improvements in safety that these amendments provide.

1 Introduction

 

2 Changes to the Codes - What they are and what they Mean

2.1 Loading Limits - Unamended Procedures

2.2 Code Changes to Increase Protection

2.3 The New Amendments

2.4 The Effects of the New Amendments

 

3 Advantages Realised - A Guide to Administration Recognition

 

4 Recommendations

 

5 Summary

 

Appendix 1 Confirmation of Vent System Adequacy

Appendix 2 Highest Temperatures

Appendix 3 Loading Limit Assessment Method

At its 14th session in December 1984, the IMO Bulk Chemicals Sub-Committee expressed a willingness to consider amending the Codes governing cargo tank loading limits.

 

SIGTTO and IACS believed that for Type C tanks the Codes, in providing for certain safety features, reduced protective measures available under fire conditions or increased the risk of unwanted venting of cargo. By updating the Codes, protection could be improved.

 

IMO’s Marine Safety Committee at its 61st session (MSC61) agreed to change the IGC Codes and made available that protection.

 

The new amendments mean that Type C tanks, under fire conditions, can become liquid filled. In this situation the Code requires tank internal pressures not to exceed 20% above the Maximum Allowable Relief Valve Setting (MARVS). IACS/SIGTTO were able to demonstrate that Pressure Relief Valves (PRVs), whose capacity is calculated using methods set down in the IGC Code, were able to prevent tank pressures exceeding 1.2 × MARVS.

 

To be assured, PRVs will, at all times, perform as designed, inlet pressure losses and built up back pressures must be within the PRV manufacturers limits.

 

This latter requirement caused MSC61 to qualify acceptance of the Code changes. They stated that the Organisation would produce Guidelines as to how these pressure losses could be assessed with both liquid and vapours passing through the vent system

simultaneously.

 

IMO delegated the task of producing the Guidelines to IACS/SIGTTO who completed the work submitting a finished document to the 65th session of the Marine Safety Committee (MSC65). This publication bore the rather daunting title of: “Guidelines for Evaluating the Adequacy of Type C Tank Vent Systems for the Assignment of Amended Loading Limits Under Chapters 8 and 15 of the Gas Carrier Codes”.

 

MSC65 recommended to IMO’s 19th Assembly that the Guidelines be accepted. The Assembly acceded to this request clearing the way for those amendments, agreed at MSC61, to come into force on 1st July 1998.

 

In the meantime. IMO has, by means of a Circular Letter to Administrations, made provision for Shipowners to take immediate advantage of the loading limit procedures. If Owners can

demonstrate their ships have adequate vent systems, under the terms of the Guidelines. Administrations may then permit loading to the new arrangements.

 

IGC Code changes have made compliance a less precise process than conforming to the original Code. This is because the criteria for determining the lowest cargo density anticipated becomes the highest operational cargo temperature encountered. To predict the “highest operational cargo temperature” requires some judgement on the part of the Shipowner and an ability to convince Administrations that the selection is reasonable.

 

The object of this booklet is to remind Shipowners and Terminal Operators of the improvements in safety which these new amendments provide. It also shows how the advantages can be taken in full.

SIGTTO

The Society of International Gas Tanker and Terminal Operators (SIGTTO) is an international body established for the exchange of technical information and experience, between members of the industry, to enhance the safety and operational reliability of gas tankers and terminals. Learn more: https://www.sigtto.org/about-us/

Title: Application of Amendments to Gas Carrier Codes Concerning Type C Tank Loading Limits
Number of Pages: 52
Product Code: WS1363K
ISBN: ISBN 13: 978-1-85609-125-1 (9781856091251), ISBN 10: 1-85609-125-2 (1856091252)
Published Date: January 2012
Binding Format: Paperback
Book Height: 180 mm
Book Width: 120 mm
Book Spine: 5 mm
Weight: 0.10 kg
Author: SIGTTO

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