Offshore drilling contractor COSL Drilling has said that the Petroleum Safety Authority’s investigation report into a fatal incident on board the COSLInnovator rig will have consequences for many of the rigs on the Norwegian continental shelf.

The offshore safety authority published its investigation report on Friday, April 8, 2016.

Separate from this, an internal investigation group, with members from COSL Drilling Europe, the owner of the rig, Statoil and Aker Solutions, conducted an independent investigation and shared the conclusions with the Petroleum Safety Authority in the beginning of March 2016.

The rig’s owner said that the findings from the PSA’s report mainly correspond with the findings from the internal investigation.

The offshore driller explained that, since it was not given sufficient time and opportunity to perform a detailed review of the PSA report prior to it becoming public, there were certain facts which need to be clarified and addressed.

As with other rigs on the Norwegian shelf, the COSLInnovator is designed and constructed in accordance with applicable regulatory requirements and the enforcement of these. The rig has been subject to approval by DNV GL, and holds an Acknowledgement of Compliance (AoC) issued by the Petroleum Safety Authority with the assistance of, among others, the Norwegian Maritime Authority.

The Petroleum Safety Authority points out in its investigation report that the COSLInnovator deck box, at the time of the incident, was not dimensioned to withstand horizontal wave loads of the magnitude that occurred. COSL Drilling said that the practice of current legislation did not take into account the horizontal wave loads on external bulkheads. This is also noted in Petroleum Safety Authority’s investigation report.

Earlier interpretation and practice through the AoC process also reflects that operators, rig contractors and rig designers have had the same understanding of these requirements.

Demand for new regulatory practice?

COSL Drilling noted that the report therefore represented a demand for a new regulatory practice from Petroleum Safety Authority’s side.

“It is likely that such a change will have consequences for many of the rigs on the Norwegian continental shelf, which will also have to document compliance in a similar manner,” said COSL Drilling.

COSL Drilling Europe has three drilling rigs of the same design on the Norwegian continental shelf: COSLInnovator, COSLPromoter and COSLPioneer. These have a combined operating time of about 11 years in the North Sea.

According to COSL Drilling, all rigs have on several previous occasions experienced significantly worse weather conditions than that which occurred on December 30, 2015, without experiencing horizontal wave loads (slamming) on the deck box.

The drilling contractor said it was unclear whether the wave that hit COSLInnovator was of an extreme nature or whether it lies within the current design criteria set by DNV GL. Both investigations discuss uncertainties relating to the analysis of the decks distance to the water surface (“air gap”) and previous model tests. New calculations or model tests are therefore indicated to verify these conditions, the company said.

The rig’s owner stated that substantial improvements were made to the COSLInnovator already prior to the investigations; these included blinding of the windows in the deck box. The reinforcements will be further verified through calculations with input from model tests, and the size of the horizontal forces which may arise in the design area of the rig may be calculated. Results of the model tests will also give useful learning to DNV GL relating to possible changes in requirements for design loads for horizontal wave forces, COSL Drilling added.

Model tests are planned in cooperation with rig designer and with input from DNV GL and Statoil’s specialist group. COSL Drilling Europe said it was making good progress with the preparation of these tests which will be conducted by Marintek (Norwegian Marine Technology Research Institute) at SINTEF in Trondheim.

The offshore driller explained that the tests were expected to answer the questions related to whether the rig design experienced negative air gap within the expected wave spectrum at time of the incident, or it experienced an abnormal, steep and breaking wave outside DNV GL’s design requirements for floating rigs.

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