The trends in risk level project aims to present the level of risk in petroleum operations on the NCS and at land-based plants.
This work identifies specific areas with the highest probability for major accidents, and one of its principal findings is that drilling and well-related aspects make a substantial contribution to overall major accident risk.
Loss of well control and the failure of barriers intended to prevent such incidents can have disastrous consequences.
The combination of high pressure in parts of a well and low formation strength in other sections – often combined with high temperature – creates opportunities for loss of well control during drilling.
So a set of barriers must be put in place for such operations. During drilling, this can comprise a blow-out preventer (BOP) and a homogenous column of drilling fluid.
The BOP incorporates valves which can shut off the drill string and, in extreme cases, cut through the string and seal off the borehole.
During production, one or more downhole valves installed in the well can be closed in the event of problems. A set of valves – known as the Christmas tree – is also found on surface or subsea installations to shut down the wellstream.
Kicks and blow-outs
A kick is an instability in the well following the intrusion of gas, oil or water. If barriers fail, a blow-out can occur. Kicks are defined as a sudden, powerful and uncontrolled escape of gas, oil, mud and water from the well.
A blow-out occurs when control of well pressure is lost. This can happen through technical equipment failure, procedural errors, inadequate mud weight or stuck pipe (swabbing). The usual cause is a combination of several factors.
The risk potential of a blow-out will vary, partly in relation to well construction, the type of fluid flowing and formation properties.
Responses to a blow-out will vary with the circumstances and include activating the BOP, connecting a relief well or bullheading.
Blow-outs on the NCS
Only one serious oil blow-out has occurred on a Norwegian offshore installation in the production phase. This Ekofisk Bravo accident in 1997 happened when a downhole valve failed in a production well during a workover.
Nobody was killed, but 9 000 tonnes of oil were spilt to the sea over roughly a week until the well could be brought back under control.
An underwater gas blowout took place on Snorre A on 28 November 2004. Had conditions been just slightly different, a major accident could have occurred with large loss of life. The PSA’s investigation report identified a total of 28 failures to comply with the regulations.