Page 58: of Offshore Engineer Magazine (Nov/Dec 2014)

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Vessels

Fig. 1: OrcaFlex mooring analysis model.

Recent UK HSE mooring regulation Impact of code the HSE recommendations for the most requirements on reliability

In recognition of the high mooring failure severe North Sea locations. rates, the HSE has introduced recom- For most existing MODUs, this Mooring code requirements have changed mendations for more stringent mooring would require prohibitively expensive over time, but the strength requirements strength requirements for units operating modifcations to the vessel structure. for the NCS have remained proportion- on the UKCS. Although the HSE’s recom- Intermediate options are available for ally above those for the UKCS. If the mendations are subject to interpretation, more benign North Sea locations, to recorded failures are correlated with it is understood that the spirit is for meet the enhanced code requirements, strength capacity, then the failure rate for alignment with the Norwegian Maritime without signifcant changes to vessel the NCS should logically be signifcantly

Directorate requirements for operation on equipment. less than that for the UKCS.

the NCS. The emphasis of the enhanced Table 2 summarizes the code compli- Mooring failure data reported by the requirements for MODUs is on increased ance of comparative intact mooring sys- Norwegian Petroleum Directorate from strength safety factors. tems. It should be noted that the results 1996 to 2013 indicates that even with the

The impact of the HSE’s recom- presented in this table are representa- higher strength requirements, the failure mendations on the required mooring tive only, as code compliance will be rate remains at a rate similar to that of the equipment for a typical MODU has been dependent on water depth, vessel design, UKCS, for both single- and multiple-line evaluated. To achieve this, a mooring mooring system properties, environmen- failure. This suggests that reliability does analysis model was developed using the tal conditions, and the analysis package not correlate well with mooring system time domain analysis package OrcaFlex, used. strength capacity. As a result, designing illustrated in Fig. 1. A typical North Sea The comparison of maximum versus mooring systems to meet more rigor-

Aker-H3 MODU with an eight-line moor- allowable mooring line tensions is useful ous HSE requirements may not give the ing system composed of 76mm Grade to assess code compliance; however, it signifcant increase in mooring system

R4 chain was assessed. The strength still does not answer the question: How reliability that the industry needs. assessment found that an increase in does increased strength capacity infu- Historically, most attention has been chain diameter or an increased number of ence the overall reliability of the moor- focused on strength, perhaps because it mooring lines would be required to meet ing system. is the most tangible failure mechanism.

Strength overload though, is but one of

Table 2: Intact Mooring System Code Compliance a large number of mechanisms that can result in mooring failure, with the statis-

ISO 19901-7 NMD No 998 tics suggesting that it these other failure

Mooring system

Central Northern Central Northern mechanisms that are dominating reli-

North Sea North Sea North Sea North Sea ability. Therefore, the industry needs take * 76mm grade R4 chain ??

??

a wider view of all aspects of moorings to improve reliability.

76mm grade R5 chain ??

??

Improving reliability

Fibre rope insert ???

?

Current understanding of past failures indicates closer attention to the follow-

Heavy chain insert ???

?

ing practical aspects may signifcantly improve MODU mooring reliability: 12 line 76mm R4 chain ????

•

Optimizing mooring system design by selecting components with proven 87mm grade R4 chain ????

reliability and ensuring components are compatible; * Assuming mooring not in close proximity to other installations

November 2014 | OE oedigital.com 60 000_OE1114_Vessels_DNV.indd 60 10/21/14 7:53 PM