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Report on calculation of subsea gas discharges

In 2006 the Petroleum Safety Authority Norway (PSA) completed a project involving subsea gas discharges and risk related to such incidents. The results of the work have now been processed and discussed in the report ”Risk related to subsea gas discharges – dispersion above the sea surface”.


The work done in 2006 concentrated on the actual discharge scenario; from leakage in/rupture of a pressurised pipeline on the seabed, flow/dispersion of gas up through the water column and subsequent dispersion in the air above the sea surface. Ample technical knowledge of this scenario is crucial to understanding the risk picture and the possibility of impacting the risk related to subsea gas discharges.

On the basis of a defined task, a test case, the four companies DNV, Scandpower, Safetec and Lilleaker calculated a number of different discharge scenarios with their models, independently of each other. By comparing the results a picture would emerge showing to what extent the industry masters this field. This would also be a good basis for an improvement process, should this prove necessary.

The project was concluded with a seminar on 15 November 2006, where the results were presented and discussed. The conclusion following the seminar was that deviations among the companies were greater than desired, and that there was considerable room for improvement. The results of this project are described in the report: Risk related to subsea gas discharges - 2006/2007 (see link box)

A continuation of the work with this in mind was initiated following discussions between the interested parties in the spring of 2007. The results of this continuation are presented and discussed in this report. The work has mainly concentrated on gas dispersion above the sea surface, but the PSA has also commissioned a report by Fanneløp and Bettilini. This report is based on the existing models for subsea discharges with an assessment of the validity of substantial discharge rates.

There are still considerable differences in the results of the calculations from the participating companies. It is recommended that the industry continues to work on reducing the uncertainties related to the dispersion calculations by continuing the work completed so far. This report calls attention to a number of specific factors that are assumed to be the source of the differing results. 

Contact person in the PSA:

Odd Tjelta
Principal Engineer
Telephone: (+47) 51 87 64 11
odd.tjelta@ptil.no