Will Brindley and Andrew P Comley, DNV GL, investigate the reliability of North Sea mooring systems.
In recent years, a number of high profile mooring failures have emphasised the high-risk nature of the mooring capabilities of a floating structure. Semi-submersible mobile offshore drilling units (MODUs) operating in the harsh North Sea environment have experienced approximately three mooring failures every two years; this is based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK continental shelf (UKCS). Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system.In this article, mooring failure rates are compared with the Norwegian continental shelf (NCS), the UKCS, and with industry code targets, to understand how overall reliability is related to the strength capacity of a mooring system.
Figure 1. Orcaflex mooring analysis model.
Failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing mooring systems to meet more rigorous HSE requirements, would require extensive upgrades to existing units, and may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.
Moorings for floating offshore units are generally acknowledged to be failing at a high rate. MODUs operating in the North Sea in particular appear to have a particularly high failure rate.
Table 1 summarises the MODU failure statistics during heavy weather for the most recent decade of available data from 1996 to 2005. The annual failure rate per MODU is based on an average of 34 MODUs operating in the UKCS during this time. The actual rate of mooring failure is likely to be higher than Table 1 suggests, as single line mooring failures are known to be under reported.
The single line failure events resulted in drilling downtime and mooring line reconnection costs. The three recorded multi-line failure events resulted in consequences including subsea infrastructure damage, personnel evacuation, shutdown of nearby production facilities and hydrocarbon release. No events were catastrophic, but it is the author’s opinion that MODU mooring failure has the potential to lead to a high consequence event. Mooring failures in calm weather and during anchor handling have been excluded, as the consequences of such failures are likely to be limited.
Target reliability levels
DNV GL’s mooring Offshore Standard E301 has been calibrated to give a target probability for single line failure of 1x10-4 and a target probability for multi-line failure of 1x10-5.
There is an argument in the industry over whether single or multi-line failure should be used to define a mooring system failure. The authors of this article consider that a single line failure represents a system failure, given that the failures are not well understood, and have the potential to lead to unacceptable consequences.
It is clear from Table 1 that the actual failure rates are several orders of magnitude greater than those assumed in the code calibration process. The statistics suggest that after 10 000 rig years, only one failure is predicted by the code, whereas the actual failure rate is more than 400 times greater.The reliability assessments used to calibrate the probability of mooring line failure typically employ a large number of statistical distributions to define the uncertain strength resistance and loading parameters. For a mooring system, some of these uncertainties are difficult, or nigh on impossible, to quantify with any degree of confidence, such as gross manufacturing defects and human design error. Structural reliability analyses are typically not intended to take into account such unknowns, which are generally assumed to be controlled by practical mitigation measures. It is possible that the discrepancy between the failure rates presented in Table 1 could be a result of:
- Strength failures due to these unquantified uncertainties.
- Unrelated failure mechanisms such as fatigue.
To get to the root of this problem, the industry needs to understand whether overall failure rates are in fact related to strength capacity – the failure mechanism that is the focus of most MODU mooring code requirements.
MODU mooring systems are typically assessed to ensure code requirements for a specified mooring line tension safety factor are met. This is where the safety factor is defined as the component break load capacity divided by the maximum analysed mooring line tension. Modern codes applicable to MODU mooring design requirements in the UKCS include; API-RP-2SK, ISO 19901-7 and DNV-OS-E301. Although the codes specify different combinations of environmental return period and safety factor, they tend to result in similar strength requirements for MODU mooring systems.
Figure 2. Semi-submersible mobile offshore drilling unit.
In light of recent high-profile mooring failures in the UKCS, the HSE has recognised that existing mooring design codes could be enhanced. As a result, the HSE has introduced recommendations for more onerous requirements for mooring systems. Although the authority’s recommendations are subject to interpretation, it is understood that the spirit is for alignment with the Norwegian Maritime Directorate (NMD) requirements for operation on the NCS.
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, which accounts for both dynamic vessel and mooring system response, as illustrated in Figure 1. A typical North Sea Aker-H3 MODU with an 8 line mooring system composed of 76 mm Grade R4 chain was assessed. Figure 2 illustrates a typical semi-submersible MODU.
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, Figure 3.
Mooring chain diameter requirements to meet typical existing ISO 19901-7 and enhanced NMD No. 998 code requirements to meet environmental conditions in the Northern North Sea are presented in Table 2. For MODUs, intact safety factor requirements generally govern the mooring system design for both the ISO and NMD compliant mooring systems.
For most existing MODUs, a prohibitively expensive retrofit of the vessel structure would be required to accommodate an increased mooring chain diameter. If the vessel structure and equipment has sufficient reserve load capacity, the chain grade could be increased from R4 to R5 to increase the mooring load capacity. Alternatively, the mooring loads could be decreased through modifying the mooring line make-up. Two readily available mooring components – polyester fibre rope or heavy 120 mm diameter chain – can be employed in the active catenary section of the mooring line to reduce the maximum loads. Fibre rope reduces wave frequency dynamic tensions due to its low weight and stiffness properties. Heavy chain increases mooring system stiffness and damping, thereby reducing resonant low frequency motions.
Figure 3. Typical chain link.
The code compliance of the intact mooring systems tends to be governing is summarised in Table 3. It should be noted that the results presented in Table 3 are representative only. Mooring code compliance will differ depending on the 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 code compliance influence the overall reliability of the mooring system?
Impact of code requirements on reliability
Table 3 represents typical mooring equipment requirements between the UKCS (ISO compliant) and the NCS (NMD compliant). Historically, mooring code requirements have altered 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.As indicated by available statistics for the NCS between 1996 and 2005, the failure frequencies (as reported) are higher in Norway than in the UK for MODU mooring systems.
Limited data between 2010 and 2011 indicates that even with the introduction of additional code strength requirements, the failure rate remains at a rate similar to that of the UKCS. More detailed investigation of the statistics would be required to ensure factors such as failure reporting rate has not biased the results.
The comparison of the available failure statistics between the UKCS and NCS indicates that overall, the failure rate is not correlated with strength capacity of the mooring equipment alone. Some measures to increase strength may actually decrease overall reliability. Grade R5 chain, although having a higher break load has been known to have lower fracture and crack tolerance. Similarly, as the chain diameter increases so problems with consistent heat treatment and weld quality can occur, thus potentially reducing reliability. Fibre rope is sensitive to failure due to external damage, as evidenced by a high proportion of fibre rope failures recently experienced in the Norwegian sector.
Increased strength requirements may lead to the selection of larger MODUs with the addition of high thrust capacity. Larger displacement vessels have been known to attract high loads in excess of what was predicted from conventional analysis. Thrusters, if not correctly tuned, can give significantly less benefit than is assumed in mooring analyses, and in some circumstances could actually increase mooring loads. This cannot be captured in a safety factor focused calibration of code, and is one example of where increased strength criteria may be counterproductive in terms of overall reliability.
The topic is being paid greater attention after several line breakage incidents in storms in recent years, and there is a clear need for improvement in the industry. Overload has been identified as one critical factor, and the proper modelling of extreme wave-induced forces needs to be investigated. This phenomenon is currently the subject of the EXWAVE JIP, which is applicable to drilling vessels (MODUs) as well as floating production platforms such as FPSOs and semisubmersibles, moored and/or DP controlled.
It is the author’s opinion that in general there is too much focus in the industry on strength capacity alone. Strength is clearly not a strong indicator of reliability and the industry needs a more robust way to assess mooring systems that takes a global view of reliability.
Mooring integrity risks could be reduced by addressing the more practical operational issues, including:
Improved failure incident reporting and investigation so that the underlying causes of station keeping incidents can be understood and prevented.
Closer attention to the manufacture, selection, traceability and loading history of equipment, both owner and rental, such as chain, Kenters and winch load cells.
Enhanced installation, inspection and maintenance requirements.
Verifying actual vessel response against theory, including the impact of any modifications.
A safer future
The failure statistics for MODU mooring systems indicate a mooring line failure would be expected to occur on average every 20 rig years and a multi-line failure around every 100 rig years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes.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. An increase in MODU mooring system chain diameter from 76 mm to 87 mm would be required to meet the HSE recommendations for the most severe North Sea locations. Intermediate options are available, including using heavy chain inserts to reduce mooring loads, which would meet the HSE requirements for more benign locations.
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. This finding indicates that the overall failure rate is not correlated with strength capacity of the mooring equipment alone. Therefore, increasing mooring line safety factors in isolation is unlikely to increase the overall reliability of the mooring system, and some measures to increase strength may actually increase overall risk.
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.
To bridge this gap DNV GL is launching the Mobile Mooring Reliability 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 Q1, 2015.