Reducing Uncertainty Through the Use of Mooring Line Monitoring
Authors: Elman Priscilla , Bramande Joseph , Elletson Edward , Pinheiro Karina 
Published: June 2014

The offshore oil and gas industry is continuing to push into deeper water and more onerous environments, using increasingly bigger vessels and equipment. This, combined with more erratic and unpredictable global weather patterns has increased uncertainty in offshore production operations.

Monitoring mooring lines can help reduce this uncertainty by providing tools to calculate fatigue accumulation, based on tension measurement, during major storm events. This greater understanding helps to optimise inspection and maintenance schedules and assess the likelihood of future mooring line failures. Added to this, most FPSOs are only designed to cope with the failure of a single line. If this is not detected then increasing loads on the remaining lines may result in additional failures. This is regarded as a system failure and could lead to the FPSO breaking away from its moorings and drifting off station. This can have consequences for both well control and riser integrity, resulting in huge costs for operators. Unless the operator inspects on a regular basis or monitors in real time it is impossible to know for certain whether all mooring lines are in place.

A popular technique for monitoring mooring systems is to measure mooring line angle (using accelerometer based inclinometers), and use this measurement to infer theoretical mooring line tension. Whilst these systems are effective at alerting operators to a line failure, the fact that tension must be inferred requires a certain amount of uncertainty in the calculation. This uncertainty is difficult to quantify and thus has been little understood. Also, the use of shackle load cells can give varying results depending on where the chain is sitting on the load shackle. These often fail in service due to the dynamic nature of the mooring line and the typical shackle location within it. This paper documents the sea trial of a new mooring line technology capable of measuring both mooring line angle and direct line tension. The improved accuracy associated with direct monitoring of line tension can help reduce levels of uncertainty in offshore operations and thus reduce future levels of conservatism in design and analysis models. This can help save costs and increase efficiency for future operations, whilst also helping support safety strategies.

INTRODUCTION

The number of Floating Production Systems (FPS) in operation has increased rapidly over recent decades, and with exploration and production (E&P) activities moving into deeper and more isolated locations this growth is expected to continue over the coming years. The past decade saw the number of permanently moored FPSs (including FPSO, FSO, Semi-submersible, Spar and offloading buoys) double to around 400 installed facilities [1]. Over the next 5 years this number is expected to grow by a further 50% [2].

Because floating installations are moored to the seabed and usually cannot move off station, they are subject to whatever weather comes their way. Environmental conditions offshore can lead to deterioration of mooring lines over time, increasing the likelihood of failures. There are various sources of potential breakages in mooring lines, a selection of which are listed in Table 1. Although failures can occur at any point along a mooring line the majority of failures occur at an interface or discontinuity [1]. These include:Between the mooring line and vessel- either at the fairlead or in the hawse pipe;At connections between two types of line- including shackles and H-links;Where buoys, clump weights or tri-plates are attached to the line;In the thrash zone- where the line dynamically contacts the seabed;Where the line descends into the seabed to connect with the anchor pile.

The past decade has seen 21 mooring issues reported on FSUs. 8 of these can be classed as system failures (with multiple line failures sustained), with 4 of these incidents (Gryphon Alpha, Nan Hai Fa Xian, Hai Yang Shi You and Liuhua) leading to vessel drift and rupture of the risers. However, even incidents with a single line breakage led to damage being sustained on additional lines which may have led to further premature failures if undetected [1].

3 mooring incidents involving two vessels (Fluminese FPSO and Jubarte FPSO) were reported in Brazil between 2001 and 2011. The details of these incidents are found in Figure 1 and summarized below:P-34 FPSO: Parted in the lower chain segmentsFluminense FSPO: Damage to top chain and connector