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Seminar and report: Risks associated with gas emissions under water

Do we know enough about the behaviour of a subsea gas release up through the water column and above sea level? Four companies have simulated the same test case - with somewhat different results.


 

Summary report May 2007: Gas discharges under water

This report is a summary of a project conducted by the Petroleum Safety Authority Norway (PSA) in 2006 dealing with gas discharges under water, and the risk associated with such incidents.

The report contains a concise description and discussion of three incidents involving gas discharges under water on the Norwegian shelf in recent years. These incidents were selected with the intention of illuminating the characteristic and important features of discharges under water, and the problems related to these incidents.

Seminar (published November 2006):

On Wednesday 15 November, the Petroleum Safety Authority Norway (PSA) presented parts of the project "Risks associated with gas emissions under water".

During the seminar with the same title, the companies presented their methods, models and the results of a predefined assignment/test case.

Over the course of the day, examples were also provided of how installations and personnel on board are exposed to gas in connection with various types of releases. How far the industry has come within this field was also discussed as well as which areas require further clarification or improvement of methodology or models. The companies already have a good and open dialogue regarding these challenges.

A need for more knowledge
The basic task was to calculate the dispersion of gas on the surface following a pipeline fracture. The companies described how the cloud would behave (height and length) on the basis of characteristics such as the size of the emission, water depth and wind speed. The summary shows that the results vary, and that there is room for improvement as regards methods and calculation models.

Several incidents over the last few years highlight the need for a better insight into how a hydrocarbon emission under water behaves. The fracture in the gas export pipeline from Jotun A in August 2004, and the underwater gas blowout from a well on Snorre A in November of the same year exemplify this.

Reducing uncertainty
Some of the goals of the project were to reduce the uncertainty and learn from each others' calculation methods. Another goal is to gain an overview of methods developed to describe the progress of a gas emission, and the dispersion of the gas in air.

This is the first time several companies simulate the same test case independently of each other - thereby testing out different calculation methods.

Examples were also given during the day of how installations and the people on board are exposed to gas in different types of emissions. One also discussed how far the industry had come in mastering this issue, and which situations in particular require special knowledge.

The companies taking part in the test case were DNV, Scandpower, Safetec and Lilleaker. In addition, SINTEF described a model for calculating flows and dispersion of gas in the water column and in the transition sea/air.

The full project report will be completed by the end of 2006, and will also include the topics:

  • Leaks/detection and localisation of leaks.
  • Impact on vessels or other objects in the gas flow
  • Ignition
  • Emergency preparedness
  • When vessels or other facilities are exposed to gas

Contact in the PSA:
Odd Tjelta, project leader
E-mail: odd.tjelta@ptil.no