Guidelines for the Alleviation of Excessive Surge Pressures on ESD for Liquefied Gas Transfer Systems

This publication provides advice at every level of liquefied gas carrier operation. It encourages a mutual understanding of safe transfer procedures between designers, engineers and operators of both liquefied gas carriers and terminal loading and unloading systems. Readers are also made aware of the factors contributing to surge pressure hazards and the benefits of a linked ship/shore ESD system in mitigating these.

Liquefied gas shipping has a very impressive, in fact unprecedented, safety record for the loading, carriage and delivery of liquid hydrocarbons by sea in bulk. Surge events occur very infrequently due to the consideration given to the liquefied gas transfer system, using guidance and standards such as this publication. Specialists skilled and experienced in the analysis of these transfer systems are required to analyse the systems during the design phase of a project. System operators should also be aware of the recommendations and guidelines contained within to protect against surge events.

Surge pressures can lead to very high stressing of pipelines and equipment, with possible outcomes of physical damage and/or loss of containment. The purpose of this document is to provide familiarisation with the concepts of surge pressure and to provide practical guidance and recommendations to the designers, engineers and operators of both liquefied gas carriers and terminal loading and unloading systems. This knowledge and familiarity are intended to assist them to:

• Recognise the potential hazards of surge pressure
• understand the factors that affect or create surge pressures
• review the engineering and operating procedures of the cargo transfer system
• understand aspects of surge pressure control on the other side of the ship/shore interface to enable mutual understanding of safe transfer procedures
• understand the potential benefits of a linked ship/shore ESD system in mitigating surge pressures.

These guidelines are not intended as a design or operational code of practice, nor is guidance given on the accurate prediction of maximum surge pressures. Further, the guidelines do not offer guidance on the accurate prediction of maximum surge pressures. They must not be used in isolation to examine the safety of any system, but they may be of assistance in the recognition and assessment of surge pressure potential when reviewing liquid cargo transfer systems in existing and new bulk liquefied gas carriers and shore marine terminals. Reference should be made to the SIGTTO publication, ‘ESD Arrangements & Linked Ship/Shore Systems for Liquefied Gas Carriers’.

These guidelines have been prepared to enable the operating management of terminals and liquefied gas carriers to review the design and engineering as well as the operating procedures of their cargo transfer systems. Such reviews may assist in avoiding the generation of excessive surge pressures and momentum change, on activation of ESD1, ESD2 or other events.

The primary principle of surge control is that the maximum and minimum pressures that can be developed under worst case surge conditions must not exceed the design pressure of the ship and terminal cargo transfer systems. The majority of known surge incidents on LNG and LPG systems have been found to result in pipe movement or flange separation, due to inadequately supported piping, rather than pipe bursts due to over pressurisation. Due consideration of the worst case conditions during the design stage would influence the selection of cargo transfer rate, pipeline diameter, valve closure speed, pipeline length and piping support arrangements. A computer based mathematical model surge pressure and pipeline force analysis should be performed to determine the maximum surge pressures and the piping loadings. It is important to consider the system as a whole, including all onshore and offshore elements.

1. Introduction and Scope

2. Surge

2.1 What is Surge?

2.2 Liquefied Gas Cargo Transfer Systems

2.3 Secondary Surges

2.4 Line Pack Effect

3. Surge Hazards and Risk Management

3.1 Hazards and Risk Management

3.2 Risk Assessment

3.3 Bowtie

3.4 Bowtie Hazards

3.5 Causal Factors

3.6 Barriers

3.7 Surge Bowtie

4. Recommendations and Guidelines

4.1 Principal Design Recommendations

4.2 Principal Operational Recommendations

Annexes

Surge Technical Background

Surge Pressures in a Typical Loading System

Surge Pressures in a Typical Unloading System

Valves

Design Approach to Alleviate Surge Pressures

Surge Pressure Relief

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.

https://www.sigtto.org/about-us/