Abstract

Over the last two years Noble Denton has been undertaking a Joint Industry Project (JIP) to investigate how to improve the integrity of the moorings used by Floating Production Systems (FPSs). The JIP has surveyed the world wide performance of all types of FPS mooring systems including FPSOs, semi submersible production units and Spars. Wide ranging support from 23 sponsoring organizations including operators, floating production contractors, regulatory authorities, equipment suppliers and inspection companies has enabled access to a significant pool of data.

This paper utilizes the JIP data to discuss the following:

  • Causes of system degradation

  • Consequences of mooring failure

  • Key areas to check on a mooring system

  • Fatigue implications of friction induced bending

  • Options for in-water inspection

  • The importance of connector design

  • Methods to detect line failure

  • Contingency planning

A few pioneering floating production units have now been on station for many years. Review of inspection data from these units shows that selective repair may be needed to maintain the design specification right up to the end of the operational life. It has been found that wear can be faster on leeside, as opposed to windward lines and that certain weighted chain designs are susceptible to damage.

The likelihood of line failure and the implications need to be better appreciated. Following failure, it may well take several months to implement a full repair, due to a lack of spares/procedures and possible non-availability of suitable vessels. However, it has been found that carefully planned and coordinated inspection operations can detect potential issues early on before more serious deterioration takes place. In general, mooring monitoring/instrumentation and access for in-water inspection seem not to be as advanced as might be expected for a system which is safety critical. Hence good practice recommendations are included which can be applied to both existing and planned future units.

Introduction

Unlike trading ships, Floating Production Systems (FPS's), stay at fixed positions year after year without regular dry docking for inspection and repair. Since they cannot move off station, they must withstand whatever weather is thrown at them. Hence at times, depending on their location, their mooring systems need to withstand high storm loadings. Typically during design, mooring systems for harsh environments do not have much reserve capacity above what is required to withstand survival conditions. Therefore deterioration of the lines over time can increase the likelihood of single or multiple line failures. Multiple line failure could conceivably result in a FPS breaking away from the moorings and freely drifting in the middle of an oil field.

The Mooring Integrity JIP has been concerned with assessing how mooring systems have performed in the field to identify the level of degradation which has taken place. Hence the project has looked at FPSOs, Semi submersible production units and Spars through out the world. The key objectives have been:

  • To feedback operational and inspection experience to the industry and to mooring designers

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