COSL Innovator: Learning from the accident, making sure it never happens again

October 17, 2016 – COSL Innovator, a deepwater drilling rig built in China to operate in the North Sea was in December 2015, struck by a steep wave, leading to the death of one crewmember, several injuries, and extensive damage to the living quarters.

The rig was working for Statoil on the Troll field in the North Sea, offshore Norway, when the horizontal wave smashed the rig’s two lower decks in the early morning on December 30, 2015.

Initial reports claimed the rig had not been built according to regulations, as it did not have an air gap of 1.5 meters between the underside of the lowest deck and the highest wave crest, and its superstructure was not dimensioned to resist horizontal wave loads.

It would later turn out there were no breaches of regulations, with the Norwegian authorities admitting that the regulations “have been too vague with regard to the applicable calculation methodology for horizontal wave forces on mobile units”.

COSL, the rig’s owner, has said that the practice of “current legislation” did not take into account the horizontal wave loads on external bulkheads, adding that earlier interpretation and practice through the acknowledgment of compliance process also reflects that operators, rig contractors, and rig designers have had the same understanding of these requirements.

Worth noting, the COSL Innovator rig had obtained approvals from the Petroleum Safety Authority and classification body DNV GL, which deemed the rig fit for work on the Norwegian Continental Shelf.

Following the incident and subsequent investigation, DNV GL, has recently issued a new guideline for the rig owners regarding the air gap and horizontal wave loads calculations and requirements, making sure that a similar accident doesn’t happen again.

The Norwegian authorities have demanded from the drillers to comply with the DNV GL guidelines, asking from the companies to deliver proof of compliance by November 1, 2016.


On the back of the release of these new guidelines, Offshore Energy Today spoke to Mr. Ernst Meyer, DNV GL director classification, trying to learn what exactly happened, and how the accident and the new guidelines stemming from it will affect the rig owners.


Meyer: When this wave hit the COSL Innovator, it was the penultimate day of last year, and there was a winter storm. The unit had suspended operation and went into a so-called survival mode, according to procedures. And in survival mode, it’s deballasting, so that the rig is sitting a bit higher in the water, than when it is under operation.

That way, it is more protected against waves.

But still, it was struck by a big and steep wave, and several cabins were damaged severely with windows broken and these forces were entering into the cabin, I mean to the living quarter. Two of the cabins were manned and we had one fatality and four injured. The injury was not serious in the end, but it was a serious event.

What we didn’t like is that so many cabins were damaged. The accident could’ve been worse if it had happened during the night when people were asleep.


OET: COSL, the rig owner, has said that its rigs had successfully withstood harsher weather than the one in December 2015. Can we talk about the wave? How was this one different?


Meyer: That’s the first question. What kind of wave hit? Because we defined that nothing bad shall happen in storms or waves up to one hundred years return period. So, for the first thing to figure out is, was this wave inside or outside of design criteria. And that’s extremely hard to understand because these are statistical measures and it’s not black and white.

So, this was investigated in a model tank, where it was concluded that it’s possible for a wave like this to return within a 100 years period, but at the same time, they could not replicate that wave. And they simulated something like 250 storms.


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