Nobody is certain what would happen if a subsea gas pipeline ruptured completely. One challenge would be how to handle the actual break on the seabed.
Another question is the way the plume of gas would behave as it streams up the water column, and data are also needed on how a gas cloud would form and spread at the sea surface. Finding answers to such issues is important for field developers and for responding with the right measures to bring a leak under control.
Facts also have to be obtained about how close vessels and rescue solutions can safely get to the gas cloud – when does it become ignitable and explosive?
No serious gas accidents have occurred on the NCS. But the subsea blowout on Snorre A in November 2004 was very serious, and a gas export line from Jotun A ruptured in the same year.
Technological advances mean that more and more subsea installations are being installed on the NCS, while gas production is set to rise steadily in relation to oil in coming years. These developments underline the importance of learning more about the way gas would behave in the event of an underwater discharge.
Project
Principal engineer Odd J Tveit has been commissioned by the PSA to find the answers to some of the questions involved through a project on risk related to subsea gas discharges.
In an early phase of this work, four companies were asked to calculate a gas cloud on the basis of leak size and specified criteria for wind speed and other meteorological data.
The results deviated widely. Even when the same calculation model was employed, horizontal dispersion of the cloud varied from 180 to 450 metres.
With the differences between vertical heights even greater, the question was who had the right answer – or were all of them incorrect.
New simulations in 2008 showed that deviations between the different models had been reduced and more of the reasons for the variations were identified.
Nevertheless, considerable uncertainty continues to prevail over the consequences of major subsea gas leaks.
Planned
The scientists are looking for planned gas discharges in order to secure empirical data, since only small volumes have been released in trials.
With a pipeline rupture, flows of up to 20 000 kilograms per second are likely. So the possibility, utility and practical execution of a large-scale experiment are under consideration. Work over several years in the risk project has contributed to scientific progress, integrating knowledge and more confidence in the calculation models. This know-how will be beneficial for both design and production.
The PSA believes it is important and necessary for government, the industry and research institutes to continue investigating issues related to subsea gas discharges.
Contact person in the PSA:
Torleif Husebø,
discipline leader, process integrity
E-mail: torleif.husebo@ptil.no
