DNV GL Review After COSLInnovator Death
DNV GL is reviewing its approval of around 100 semi-submersible rigs as a result of the incident involving COSLInnovator in December 2015. Preliminary assessments indicate that a limited number of rigs will be subject to modifications or operational limitations.
The semi-submersible rig COSLInnovator was drilling for Statoil in the Troll field when it was hit by a large, steep wave. Several windows on the rig's two lower decks were shattered. One person was killed.
The incident investigation report presented by the Norwegian Petroleum Safety Authority in April 2016 concluded that the incident involving COSLInnovator has provided new knowledge that must be utilized in order to prevent similar incidents in the future. DNV GL therefore published a new technical guideline (OTG-13 – Prediction of air gap for column-stabilized units) as early as in June 2016. This gives a consistent and updated approach for calculating the air gap - the clearance between the highest wave crest and the bottom of the deck box in all relevant sea conditions.
Last week, DNV GL asked all owners of DNV GL-classed semi-submersible rigs to provide updated documentation of each rig's air gap.
Rigs that, based on the new technical guideline (OTG-13), can confirm a positive air gap, will be able to operate as before without reinforcement or operational limitations. This is expected to apply to most of the semi-submersible rigs operating on the Norwegian shelf.
“I can't indicate how many rigs have negative or positive air gaps before each rig's calculations have been performed,” says Ernst Meyer, DNV GL Director for Offshore Classification.
“A limited number of rigs may not have a positive air gap, but most of these will be able to avoid changes. The prerequisite is that they are able to document a positive air gap for a specific location, or that they simply do not have windows that may be exposed to waves.”
Those that are unable to prove a positive air gap in all sea conditions – including the hundred-year wave may be required to remove windows in exposed zones. If the strength calculations show that further structural modifications are necessary, such modifications will be required as part of the permanent solution.
Rigs that are certified for worldwide operation must be documented according to North Atlantic wave data. Most rigs operate in milder areas, such as the North Sea, and can postpone modifications that may be necessary in the Norwegian Sea or Barents Sea.
DNV GL is the classification body that certifies the largest number of semi-submersible rigs, and these rigs operate under the most extreme weather conditions globally.
The work behind the new guideline includes the use of updated statistical weather data and knowledge acquired from several independent model tests conducted in light of the COSLInnovator incident.