DNV GL issues guidelines on drilling rig air gaps
Around 100 drilling semisubmersibles approved by DNV GL will undergo further reviews following the accident offshore Norway late last year involving the COSLInnovator.
It appears that a limited number of rigs will be subjected to modifications or operational limitations.
TheCOSLInnovator was drilling for Statoil in the Troll field when it was struck by a large, steep wave, which shattered several windows on the rig’s two lower decks. One person was killed.
Ernst Meyer, DNV GL director for Offshore Classification, said: “Since the incident, we have made great efforts to identify what happened, understand how this could happen and, most importantly, implement actions to prevent similar incidents from occurring again.
“We have been working with rig owners, designers, operators, and authorities towards a common goal; to ensure the safety of all those working on board the rigs.”
Petroleum Safety Authority Norway’s investigation concluded earlier this year that its findings must be used to prevent similar incidents occurring in the future.
In response, DNV GL published a new technical guideline (OTG-13 – Prediction of air gap for column-stabilised units), said to deliver 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 semis that it had classed to provide updated documentation of each rig’s air gap.
Those that can, based on the new guideline, provide a positive air gap will be able to operate as before without reinforcements or operational limitations. This should be the case for most semis operating on the Norwegian shelf.
“A limited number of rigs may not have a positive air gap,” Meyer said, “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.”
As for those rigs where the owners cannot demonstrate a positive air gap in all sea conditions – including the hundred-year wave – Meyer added: “Initially (for the next winter), these rigs will 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.
“The most important thing is that the windows are removed before the coming winter. This action eliminates the largest risk elements if a similar incident occurs.”
Rigs certified for worldwide operation must be documented according to North Atlantic wave data. Most rigs operate in milder areas, such as theNorth Sea, and can postpone modifications that might be needed for the Norwegian Sea or Barents Sea.
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