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Reporting of mooring failures



To remind inspectors of the legal requirements for reporting mooring failures.

Reporting requirements

1  Component failures, e.g. single chains / wires, connector links, fairleads etc., are reportable under DCR Regulation 9 Reporting of danger to an installation.

2  System failures, where more than one component fails e.g. two or more mooring lines, are reportable as a dangerous occurrence under RIDDOR.


3  It has become apparent that there is confusion as to whether a mooring failure should be reported under RIDDOR or DCR or if it should be reported at all. Analysis of existing DCR reports indicates significant under reporting of mooring failures.

4  The rate of mooring line failures has reduced since the early 1980's. This is due to a number of factors including better manufacturing processes, the use of stronger chain, advances in design codes and a better understanding of line failure mechanisms including corrosion, abrasion and fatigue. However, failures still occur in both relatively new and old mooring lines. Failure rates for semi-submersible MODU mooring lines are around 0.5 per annum and for FPSO mooring systems a single line failure may occur approximately every 5 years.

5  The hazards arising from a single mooring line failure may not always appear obvious, especially when the total mooring system is designed to withstand the failure of a single mooring line.

6  When a single mooring line fails the remaining intact lines take greater tension to counteract the environmental forces. However, if the remaining intact lines are also weakened to a similar extent then progressive failure of other lines may occur. The loss of two, or more, lines is generally regarded as a system failure that could result in uncontrolled loss of station, loss of stability, loss of well control or riser integrity.


7  IMT inspectors should ensure duty holders are aware that all mooring failures are reportable under DCR, or RIDDOR, as appropriate.

8  All incidents should be reported immediately to OSD4.3 as required by the OSD Inspection Manual Section 9.2.27 (c) - Notification guidance. OSD4.3 will investigate, in conjunction with the duty holder, to ascertain underlying causes and the potential for further mooring system or component failures. During the time to reinstate the mooring line the duty holder may also have to take pre-emptive action to reduce the likelihood and consequences of a system failure by measures such as optimising tensions in the remaining lines, reducing operating limits, suspending drilling operations or shutting in wells.

Further information

Further information can be obtained from, OSD4.3, Ground Floor, Rose Court, London

Updated 2012-03-27