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A blueprint for MOU mooring reliability

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William Brindley William Brindley
Engineer
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Within the harsh marine environment of the UK Continental Shelf, there are more than 25 moored semi-submersible drilling and accommodation units in operation. These facilities were built to standards relevant at the time of their construction, with the keel of the eldest laid in the early 1970s, before any international standards for the assessment of mooring systems existed

In the wake of recent mobile operating unit (MOU) mooring failures, DNV GL recently gathered industry stakeholders at a technical seminar on mooring performance and reliability in Aberdeen. Mooring failure rates are higher than industry targets, with approximately three failures reported every two years, emphasising both the high risk nature of this element of a floating structure and the need for progress in this area.

Attendees from a range of operators, contractors and consultants - including Shell, Total and Transocean - discussed current practices and challenges. The group called for a common performance standard for moorings that is aligned with the current realities of the UK Continental Shelf operating environment.

Creating one clear best practice

Attendees to DNV GL’s mooring seminar commented on the confusion in the sector around the number of standards in practice surrounding the safe and reliable mooring of MOUs. They called for the industry to come together to align best practice for the UK Continental Shelf so as to ensure standards are fair, justifiable and consistent for all operators.

“In recognition of the high mooring failure rates, the UK’s Health and Safety Executive (HSE) has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS),” said Will Brindley, naval architect at DNV GL.

“Although the HSE’s recommendations are subject to interpretation, it is understood that the spirit is for alignment with the Norwegian Maritime Directorate requirements for operation on the Norwegian Continental Shelf (NCS). Here, the strength requirements for MOU moorings are proportionally higher than those for the UKCS.”
 
Despite these higher requirements, mooring failure data reported by the Norwegian Petroleum Directorate shows that even with the higher strength requirements, the failure rate on the NCS remains at a rate similar to that of the UKCS, for both single- and multiple-line failure.

“This suggests that reliability does not correlate well with mooring system strength capacity. It is clear that the industry needs take a wider view of all aspects of moorings to improve reliability under a common set of best practice,” added Brindley.

The importance of greater attention on operational aspects was reinforced by the underwriters present at the seminar, who provide insurance cover for MOUs.
 
“Raising an awareness that design code compliance with regards to mooring integrity alone is not enough, is also a message worthy of a mantra, given loss experience is often driven by operational shortcomings” said John Munnings-Tomes, chief risk engineer at Navigators Technical Risk.

The power of collaboration

Knowledge sharing was also a key topic for discussion at DNV GL’s seminar. Participants agreed that valuable experience and technical knowledge about past mooring failure incidents has been lost due to a lack of clear guidelines and little information sharing between industry players.

“Without insight into mooring failures, it is difficult to know where to focus efforts and analysis,” one speaker commented. “For example, are the recent mooring failures a result of failures in the design code or of the failure to address the issue of fatigue in the operation and maintenance of the heritage mooring systems?”
 
In order to focus future efforts, participants at the seminar identified the need to gather and share failure data in all circumstances that have resulted in a mooring failure.

“The firm recommendation from those attending our event is that operators should follow a consistent approach to investigate failures and ascertain the reasons for them. This includes reporting all station-keeping incidents, such as mooring line failure and anchor drag. This information could easily be kept anonymous, but it should be disseminated into wider industry,” said Brindley.

“To drive progress, the industry should establish a forum in which to discuss causes of failures and to identify corrective measures for the benefit of the sector at large,” he added.

To address the key issues identified during the seminar DNV GL is launching a mobile mooring reliability joint industry project (JIP), with a focus on improving reporting of mooring incidents and operational practices in the UK sector. Partners are currently being engaged for the JIP and the project is expected to kick-off during the first quarter of 2015.

Other recommendations from the seminar included the need to:

  • Review current practices on inspection, operation and maintenance of winches, load cells and fairleads.
  • Provide a clearer definition of anchor test tensioning requirements based on a consequence approach that should be carried out to minimise risk to assets/personnel.
  • Review the adequacy of chain inspection frequency and techniques to detect fatigue failures, qualifications of chain inspectors and inspection procedures.
  • Implement operational procedures to improve traceability and loading history of mooring equipment.