Late in December 2015, a semi-submersible drilling rig was struck by an enormous wave, leading to the death of a worker aboard the platform – first fatality in Norway since 2009.

The rig, COSL Innovator, was working for Statoil at the Troll field in the North Sea, offshore Norway. Four more workers were injured.

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.

According to the Norwegian authorities’ investigation, 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 investigation also revealed that the sea state at the time of the accident was less severe than the wave condition for which the unit was designed.

The safety authority’s report further concludes that the drilling rig did not comply with the requirement to have minimum 1.5-metre positive air gap between the bottom of the lower deck and the wave crest, ref. NMD-regulations for Mobile Offshore Units 2013.

According to DNV GL, a classification society, the accident has revealed that horizontal wave loads to the deckbox structure of column-stabilized units can occur from large and steep waves developed from storms which recur more frequently than 100 years.

DNV GL’s investigation

In a statement on Thursday, June 9, 2016, DNV GL said that that horizontal wave loads have not been adequately considered in current industry standards, analytical tools and practices, and model tests.

The accident is the first of its kind to any column-stabilized unit that DNV GL is aware of, the organizations said, adding that designers, yards, regulators and class societies have previously focused on structural risks posed from vertical loads caused by wave slamming and run-ups as well as horizontal forces from static water pressure in the event of damage stability conditions.

This accident has provided new insight which need to be considered, DNV said.

As a consequence, and in order to evaluate the adequacy of own rule requirements and pertinent industrial design practices, DNV GL has investigated and analyzed the technical circumstances surrounding the accident.

The conclusion from this work is that horizontal wave loads must be considered for the deckbox structure if a column-stabilized unit is designed to operate with a negative air gap in storms recurring every 100 years or less.

DNV GL is currently developing two new Offshore Technology Guidelines (OTG) that can be used to document compliance with classification rules stipulated in DNV GL–OS–C103. These rules require that the annual probability of negative air gap is less than 10-2 or require that loads are adequately accounted for in the design:

Offshore Technology Guideline for calculation of the air gap for column-stabilized units and

Offshore Technology Guideline for calculation of the slamming loads to be applied for design/reinforcement of structures exposed to direct horizontal slamming loads from waves.

Recommendations to owners

According to DNV GL, this accident has brought a deeper understanding with respect to air gap and wave slamming on column-stabilized units. The impact of this learning is relevant for all owners of column-stabilized units, as the original design documentation may be insufficient to prove the air gap that can occur during heavy storms and, consequently, the capacity to resist slamming loads to the deckbox structure.

“All owners should therefore revisit the design documentation of their own rigs with particular focus on the air gap analysis. The DNV GL Guidance Notes, which will be available in June 2016, can be applied to substantiate that the unit has sufficient strength to withstand horizontal wave loads in accordance with existing rules,” DNV GL says.