PSA cites air gap inconsistencies in rig damage study

Offshore staff

OSLO, NorwayPetroleum Safety Authority (PSA) Norway has completed an investigation into the incident late last year when the topsides of the mobile offshore unit COSLInnovator was struck by a wave.

One person died and four were injured as a result of the impact on the structure. The wave struck the port side of the front bulkhead of the forward box girder and smashed 17 windows, six on the lower deck and 11 on the mezzanine deck.

Water intrusion also caused extensive damage to cabins on these decks, with further damage to part of the living quarters.

Had the incident occurred at a time when more staff were in their cabins, more lives could have been lost, PSA concluded.

PSA’s analysis found that although the wave was steep, weather conditions were probably within the limits for which the unit was designed.

Its investigation revealed two breaches of the regulations:

  • COSLInnovator did not have an air gap of 1.5 m (4.9 ft) between the underside of the lowest deck and the highest wave crest. At the time of the incident, the unit’s superstructure was not dimensioned to resist horizontal wave loads.
  • The system for registering personnel on board did not function satisfactorily.

HSE regulations for the petroleum industry specify requirements for the air gap – the distance between the underside of the lowest deck and the highest wave crest – if the superstructure is not dimensioned to resist wave slamming.

There are various air gap calculations for COSLInnovator, with different results. PSA concluded that inadequate attention was paid to two key analyses during development of the design, and this likely played a part in the extent of the damage sustained.

Topsides must be dimensioned to resist wave loads if a unit has a negative air gap under regulations imposed by both the Norwegian Maritime Authority and DNV GL.

In this case, PSA found, horizontal wave slamming was disregarded, even though a negative air gap was identified in analyses performed by DNV and Grenland Group.

On the basis of its findings the PSA has ordered COSL Drilling Europe to verify through appropriate calculations or model trials that the COSLInnovator, COSLPromoter and COSLPioneer, following modifications implemented after the incident, are in conformity with section 3 of the framework regulations, and that these calculations or model trials must be verified by an independent third party.

Deadline for compliance is Oct. 1.


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