The Petroleum Safety Authority Norway (PSA) has completed its investigation into the incident on 30 December 2015 when the mobile offshore unit COSLInnovator was struck by a wave. One person was killed and four injured, while the unit suffered extensive damage to part of its living quarters.
At about 16.38 local time on 30 December 2015, part of the topside on COSLInnovator was struck by a wave. The unit was disconnected from the well at the time and raised to its survival condition.
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.
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 30 December were probably within the limits which the unit was designed for.
Findings
The 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.
- The system for registering personnel on board did not function satisfactorily.
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 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.
Requirements for dimensioning the topside
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.
Documentation
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. It is 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).
