Mooring failure rates are still higher than industry targets. Assessing past failures could help industry understanding, say Will Brindley and Andrew Cornley.
 |
|
The Blackford Dolphin semisubmersible mobile offshore drilling unit Images from DNV GL. |
In recent years, a number of high-profile mooring failures have emphasized the high risk nature of this element of a floating structure. The fleet of semisubmersible mobile offshore drilling units (MODU) operating in the harsh North Sea environment has experienced approximately three mooring failures every two years. 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).
An investigation by DNV GL compared UKCS mooring failure rates with industry code targets and Norwegian Continental Shelf (NCS) failure rates to understand how overall reliability is related to the strength capacity of a mooring system. This investigation found that although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the true reliability of said system. Indeed, increasing mooring strength capacity alone may not lead to the increase in reliability that the regulators desire and the industry needs. Instead, focus should be on the less tangible aspects of the entire mooring process, from component selection to operation.
Failure Statistics
 |
|
Top: *MODU Mooring Failures in Heavy Weather: 1996-2005 Bottom: A typical chain link. |
Failure statistics, as summarized in Table 1, suggest that a typical MODU operating in the UKCS will experience a mooring line failure in heavy weather every 24 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. DNV GL’s mooring Offshore Standard E301, for instance, has been calibrated to give a target single line failure rate of once every 10,000 operating years and a target multi-line failure rate of once every 100,000 years. Due to known under-reporting, the actual failure rate is likely to be higher, especially as mooring failures in calm weather and during anchor handling have been excluded from the statistical analysis, as the consequences of such failures are likely to be limited.
Two-thirds of the multi-line failure events resulted in significant consequences including; subsea infrastructure damage, vessel grounding and hydrocarbon release. Although no major incidents have occurred to date, the potential is high for severe subsea asset damage and hydrocarbon release, especially when drilling in an existing development.
MODU mooring component failures are generally not well understood or controlled and have the potential to lead to a catastrophe.
Recent UK HSE mooring regulation
In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UKCS. 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 NCS. The emphasis of the enhanced requirements for MODUs is on increased strength safety factors.
The impact of the HSE’s recommendations on the required mooring equipment for a typical MODU has been evaluated. To achieve this, a mooring analysis model was developed using the time domain analysis package OrcaFlex, illustrated in Fig. 1. A typical North Sea Aker-H3 MODU with an eight-line mooring system composed of 76mm Grade R4 chain was assessed. The strength assessment found that an increase in chain diameter or an increased number of mooring lines would be required to meet the HSE recommendations for the most severe North Sea locations.
For most existing MODUs, this would require prohibitively expensive modifications to the vessel structure. Intermediate options are available for more benign North Sea locations, to meet the enhanced code requirements, without significant changes to vessel equipment.
Table 2 summarizes the code compliance of comparative intact mooring systems. It should be noted that the results presented in this table are representative only, as code compliance will be dependent on water depth, vessel design, mooring system properties, environmental conditions, and the analysis package used.
The comparison of maximum versus allowable mooring line tensions is useful to assess code compliance; however, it still does not answer the question: How does increased strength capacity influence the overall reliability of the mooring system.
 |
|
* Assuming mooring not in close proximity to other installations |
Impact of code requirements on reliability
Mooring code requirements have changed over time, but the strength requirements for the NCS have remained proportionally above those for the UKCS. If the recorded failures are correlated with strength capacity, then the failure rate for the NCS should logically be significantly less than that for the UKCS.
Mooring failure data reported by the Norwegian Petroleum Directorate from 1996 to 2013 indicates that even with the higher strength requirements, the failure rate 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. As a result, designing mooring systems to meet more rigorous HSE requirements may not give the significant increase in mooring system reliability that the industry needs.
Historically, most attention has been focused on strength, perhaps because it is the most tangible failure mechanism. Strength overload though, is but one of a large number of mechanisms that can result in mooring failure, with the statistics suggesting that it these other failure mechanisms that are dominating reliability. Therefore, the industry needs take a wider view of all aspects of moorings to improve reliability.
 |
|
Fig. 1: OrcaFlex mooring analysis model. |
Improving reliability
Work remains to be done to perform a wider and more detailed investigation of historical North Sea failures and their root causes, in order to identify areas where attention can best be focused to improve reliability. This could be more readily performed if all failures were openly reported and investigated by operators.
Conclusions
The failure statistics for MODU mooring systems indicate a mooring line failure would be expected to occur on average every 24 rig years and a multiple-line failure around every 112 rig years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes.
The assumption that increasing safety factors will improve reliability is contradicted by the actual failure rate in the NCS, which does not show a proportional increase in reliability despite the significantly increased mooring system strength requirements.
Work still remains though to find practical ways to improve reliability. An increased focus on improving understanding of past failures is probably the most opportune method to improve mooring system reliability.
Will Brindley joined DNV GL after graduating in 2011 with a Master’s of engineering in naval architecture and ocean engineering from the University of Strathclyde. Brindley’s studies were punctuated by placements at the Samsung Heavy Industries shipyard design office and the Intelligent Engineering composite structures analysis team. Since joining DNV GL he has worked on naval architecture advisory projects including mooring design and updating the Oil and Gas UK Mooring Integrity Guidance.
Andrew Comley is a consultant with DNV GL providing specialized integrity and incident investigation services, recommenced his association with the company in 2008 as the Mooring Integrity Phase 2 JIP project engineer. Previously he has worked for Steel-kit, Kingfisher Marine Services, Noble Denton, UK HSE and Advantica. He gained a Bachelor’s degree in naval architecture and shipbuilding from Newcastle University in 1994 before undertaking Lagrangian computational fluid dynamics research at Glasgow University.
This article is a reduced version of the paper OMAE2014-23395 presented at the Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering, June 8-13, 2014, San Francisco, California, USA.
Subscribe
