Investigation finds flaws on COSL rig after fatal incident

COSLInnovator; Image: PSA

Norway’s offshore safety watchdog, the Petroleum Safety Authority (PSA), has revealed the results of its investigation of a fatal incident on board the COSLInnovator rig that occurred in December 2015, offshore Norway. 

At the time of the incident, the COSL Innovator, built in 2011 and owned by COSL Drilling, was working for the Norwegian oil company Statoil on the Troll field in the North Sea.

The semi-submersible offshore drilling rig’s contract was subsequently terminated. As a result of the cancellation, the owner of the semi-sub slashed 230 jobs.

To remind, after the rig was struck by a wave, on December 30, 2015, one person was killed and four injured, while the unit suffered extensive damage to part of its living quarters. The unit was disconnected from the well at the time and raised to its survival condition.

According to the PSA, the wave struck the unit on 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 caused extensive damage to cabins on these two decks. One person was killed and four others suffered light injuries from the damage which followed the wave’s impact with the unit. The wave also caused deformation to the forward bulkhead on the box girder.

Had the incident occurred at a time when more of the people on board were in their cabins, more lives could have been lost, the safety authority said.


Wave characteristics


Wave characteristics have been calculated after the incident on the basis of weather observations, the unit’s probable position when the wave struck and assessments of the damage suffered. The PSA has concluded that the wave was steep, but that the weather conditions on December 30 were probably within the limits for which the unit was designed.



The safety agency’s investigation has revealed two breaches of the regulations: COSLInnovator did not have an air gap of 1.5 metres 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.

Furthermore, the system for registering personnel on board did not function satisfactorily.

The safety agency also revealed that the windows on the unit were not designed to resist wave loads. It was found that bolts with a lower strength grade than the supplier’s specification had been extensively used to attach windows.

Observations have revealed that the attachments (bolts) had roughly the same breaking strength as the glass. The PSA has therefore concluded that the use of bolts with a lower strength grade was not determinative for the extent of the damage.


Requirements for air gap


The 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 its superstructure is not dimensioned to resist wave slamming. A significant part of the investigation has sought to establish on how wave slamming could reach the forward bulkhead of the topside. Analyses carried out in the design phase to clarify the air gap have therefore been prioritised in this investigation.

Various air gap calculations exist for COSLInnovator, with different results, as well as a model test. The PSA said it considers that inadequate attention was paid to two key analyses during development of the design. This has probably been significant for the extent of the damage sustained in the incident, said the agency.


Requirements for dimensioning the topside


The agency emphasized that topsides must be dimensioned to resist wave loads if a unit has a negative air gap. Set by both the Norwegian Maritime Authority (NMA) and DNV GL, this requirement does not distinguish specifically between vertical and horizontal wave slamming (related to the box girder’s underside and forward bulkhead respectively).

The PSA has observed that horizontal wave slamming was disregarded, even though a negative air gap was identified in analyses performed by both DNV and Grenland Group. The work carried out during the design period was largely in line with experience and industry practice for mobile offshore units at the time the project was pursued.




Various owner constellations and different engineering companies were involved during development of the design from 2005 to 2008. Disagreements and inadequate information existed between the players, particularly when weight increases occurred during the construction process. Not all documentation was transferred at once when the engineering company changed. The agency stated it was therefore unclear who had access to what documents at given times.


System for registering personnel when mustering


Activities in the acute phase were largely conducted in accordance with procedures, but it took about 40 minutes to establish a full overview of personnel on board (POB), the PSA informed.

On the basis of the findings made during the investigation, the PSA has given notification of the following order:

Pursuant to section 69 of the framework regulations on administrative decisions, see also section 11 of the management regulations on the basis for making decisions and decision criteria and section 19 of the framework regulations on verifications, we hereby order COSL Drilling Europe AS to verify through appropriate calculations or model trials that the COSLInnovator, COSLPromoter and COSLPioneer units, following the modifications implemented after the incident, are in conformity with section 3 of the framework regulations, see section 10, sub-section 1.1.3 of the Norwegian Maritime Authority’s regulations on construction of mobile offshore units (FOR 87/856), see chapter 9 of the report. Furthermore, COSL Drilling Europe AS is ordered to have these calculations or model trials verified by an independent third party.

The agency noted that this order must be complied with by October 1, 2016. The PSA must be notified when the order has been complied with. A plan for the work must be submitted to the PSA by May 13, 2016.

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