Risk-Based Inspection Planning for Mooring Chain

Abstract

The use of risk-based inspection planning for offshore structural components is becoming quite familiar. This paper describes an application of this technique to mooring chain. In many cases, the technique is based on probabilistic modelling of fatigue crack growth in the structural components, and updating of the failure probability on the basis of inspections. The extension of this basis from a single component to very many components is necessary to tackle series systems, such as mooring chain, where a fatigue fracture can arise in any chain link. The theoretical basis for the analysis is described, including details of the model for stochastic dependency between the chain links. Results are shown that compare failure probabilities for a single chain link and a chain segment. The effects of various levels of inspection coverage are illustrated. An example of a cost optimal inspection plan is developed for mooring chain on an FPSO in the northern North Sea.

Risk assessment for structural design criteria of FPSO systems. Part II: Consequence models and applications to determination of target reliabilities

“…The cost of inspection is likely to exceed the expected cost of failure. In fact, inspections of mooring systems are really aimed at preventing simultaneous failures, which is also noted by Mathisen and Larsen [4], even though they do not consider it in their work. The second situation is the one where it cannot be ensured that failures are detected.…”

“…Typically, fatigue failures are defined, somewhat imprecisely, in terms of the existence of visible cracks. In [4] the critical crack size, defined as the crack size that will lead to failure of the mooring line during a one-year extreme event, is observed to be between 12 and 30% of the chain diameter. The problem with this definition is that such failures will only become obvious through regular detailed inspections once the mooring line has lost all or a large part of its capacity.…”

Safety management of deep water station-keeping systems

“…The system reliability crucially depends upon the relative magnitude of component reliability as well as the correlation of load effects and resistances between the components. Mathisen and Larsen [21] and Lardier et al [22] carried out reliability analyses of chain mooring lines, in view of fatigue and fatigue and corrosion, respectively, by recognising the series system nature of the mooring lines. It turns out that some chain links in the splash zone govern the reliability level, especially when corrosion is accounted for.…”

Fracture mechanics analysis for a mooring system subjected to tension and out-of-plane bending

“…These stresses are then used for the FM analysis for predicting the remaining service life of mooring chains at the fairleads. As recommended by Mathisen and Larsen [17], the critical crack depths are set as 12% of the chain diameter at weld sections, 30% of the chain diameter at bend section, and 15% of the chain diameter at crown section. The initial crack depth is assumed to be 0.5 mm.…”