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Mobile facilities and wave-in-deck loading

The companies are responsible for protecting facilities from wave-in-deck impacts this coming winter. The Petroleum Safety Authority Norway (PSA) has now sent an identical letter to operators of mobile facilities, concerning how they should respond to the accident on the Troll field on 30 December 2015 when the COSLInnovator semi-submersible rig was hit by a wave.

One person died and four were injured. The facility sustained widespread damage to parts of the accommodation quarters.

Investigation report of an incident with fatal consequences on COSLInnovator, 30 December 2015

Investigating in order to learn
Our primary aim in following up this fatal accident was to discover what happened and why, so that both the industry and the authorities could learn from it and prevent a recurrence. Among other things, the investigation detected weaknesses in the methods for calculating horizontal wave impacts on the deck structures of semi-submersible facilities.

Ongoing work
Following the accident, the PSA has worked closely with the Norwegian Maritime Authority (NMA) and DNV GL to systemise the conclusions from previous model tests and incidents. Over the summer, a number of detailed model tests were also performed. The work is very extensive, involving the collation of large amounts of data with complex theoretical models. In the time available, it was not possible to fully investigate every factor. The work is therefore likely to continue for several years.

New guidelines
DNV GL has now issued two new guidelines – one concerning the calculation of potential wave heights for semi-submersible facilities, and one concerning dimensioning in order to withstand associated pressures, depending on the wave height experienced. In these guidelines and its instruction letter to the vessel owners, DNV GL has made recommendations for safe operation for the winters of 2016-2017 and 2017-2018.

Link to OTG 13 and 14: https://www.dnvgl.com/maritime/Offshore/technical-guidelines.html (login required).

Request for documentation by 1 November
The NMA has recently sent a letter to all owners of Norwegian-flagged semi-submersible facilities, calling on them to implement DNV GL's guidelines. The NMA is requesting submission of compliance documentation by 1 November.

Our circular urges the industry to implement the same DNV GL guidelines and the content of the DNV GL and NMA letters to ensure that the facilities withstand the coming autumn and winter storms.

Our letter applies to all facilities on the Norwegian Continental Shelf, regardless of flag state and classification society. It has been agreed that the NMA shall monitor Norwegian-flagged mobile facilities, while the PSA monitors the remainder. Follow-up by both the agencies will be based on the NMA's regulations and DNV GL's guidelines.