Two serious incidents occurred on the Norwegian continental shelf (NCS) in 2004. One took place on Snorre A on 28 November, and the other involved a fracture in the export pipeline from Jotun A to Statpipe A on 20 August 2004. Both incidents were investigated by the Petroleum Safety Authority Norway (PSA).

The PSA pursued projects from 2005 directed at subsea gas releases and the risk associated with such incidents. These efforts aimed both to establish state-of-the-art knowledge on the subject and to identify possible challenges which the industry needed to overcome.

This work eventually concentrated on the actual release scenario – from the leak/fracture in a pressurised pipeline on the seabed, via flow/spread of the gas up through the water column, to its subsequent dispersion in the atmosphere above the sea surface.

Having a good technical grasp of this scenario is crucial for understanding the risk picture and opportunities for risk mitigation associated with subsea gas releases. The growing number of seabed facilities on the NCS also increases the probability for incidents of this type.

 Picture of West Vanguard and the gas blowout at the Halten Bank in the Norwegian Sea in 1985.
A gas blowout occurred on West Vanguard on 6 October 1985 while the rig was drilling an exploration well on the Halten Bank in the Norwegian Sea. Gas ignited after 20 minutes, producing an explosion which killed one person and caused great material damage. Photo: Stavanger Aftenblad

A 2006 seminar at the PSA, attended by the industry and consultancies working on the issue, concluded that knowledge about subsea gas releases was deficient. Four companies were asked to calculate a gas cloud based on the size of the release with specified criteria such as wind speed and other meteorological data. Their results varied greatly, even when the same simulation model was used. The questions then were who was right, or was everyone wrong.

Sintef initiated the Sure project on an advanced modelling tool for investigation of subsea gas release in 2013. This work recently ended with the completion of three sub-projects. See the description of the project in Sintef’s summary report on subsea gas release (below).

Information and reporting 

The PSA had provided information about the work on the risk associated with gas release on its website. Identical letters have also been sent to the industry on the issue.

Norwegian and international companies have participated in this activity, and the knowledge acquired can be applied both on the NCS and abroad. Sintef has reported on its work with the simulation model in international research arenas.

The model which has been developed ranks as a world leader for subsea gas release. In addition to financial support, the industry has contributed to acquiring valuable data related to large-scale trials. This has been important in validating the gas release model.

Using the model

The model describes how much gas reaches the surface. That information is then used to calculate gas dispersion in the atmosphere over the release site. Earlier models were limited both in their simulation of physical laws in a release and their application in relation to release depth and size, for example. The present model can be used from shallow depths (30 metres) right down to 1 000 metres, and simulates releases as large as 300 kilograms per second.

Work on the risk associated with subsea gas releases has contributed over many years to advancing and coordinating knowledge and to improving confidence in the calculation models. The lessons learnt will be beneficial in both design and operational phases.

In the PSA’s view, it will be important for the industry to utilise the knowledge gained.

More information is available on the Sintef website:

Process Safety – Subsea gas release

or see the YouTube video