Following an incident involving COSLInnovator on the Norwegian Continental Shelf in December last year, around 100 semi-submersible rigs approved by DNV GL will be reviewed, with some requiring modifications.
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, with one person being killed.
Preliminary assessments indicate that a limited number of rigs will be subjected to modifications or operational limitations, said DNV GL in a company statement.
“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,” said Ernst Meyer, DNV GL Director for Offshore Classification.
“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,” he added.
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 a 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,” said Ernst Meyer.
“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,” he added.
Rigs that are unable to prove a positive air gap in all sea conditions, including the hundred-year wave, will be required to remove windows in exposed zones. If strength calculations show that further structural modifications are necessary, such modifications will be required as part of a 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,” Meyer said.
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