ABSTRACT

Hydraulic, Electrohydraulic and Multifunction umbilicals form a critical and key component in Subsea Oil and Gas production systems. A recent Government report has documented evidence which supports the longstanding concern of operators, contractors and regulators over their reliability.

Among recommendations made in that report was the need for improvements in overload protection to prevent expensive damage caused by trawler and ship anchor snagging.

This report documents the development of a new generation of ‘weak link’ connector, its use in current and future subsea systems and the associated savings both in capital and operational terms.

INTRODUCTION

With the increasing need to develop marginal fields, the use of subsea oil and gas production systems have increased over the last 20 years quite dramatically. There are currently over 150 subsea completions in the North Sea alone, with some predictions estimating that by the year 2000 this will rise to 400. The North Sea is in the forefront of this development, with its relatively shallow waters making satellite well tie back to existing production platforms that much easier. The past decade has seen the trend for the use of more satellite wells located at greater step out distances. In addition, the complexity of subsea control systems has increased, demanding higher reliability of critical components. One of these critical components is the umbilical.

FIG.1 UMBILICAL TYPICAL CROSS SECTION (Available in full paper)

FIG.2 ERC FIELD SAMPLE AREA (Available in full paper)

The subsea umbilical performs a multiplicity of functions which can be summarised as follows:-

  • Control of subsea-completed production or injection wells.

  • Control of pipeline safety valves.

  • Injection of chemicals in subsea wells.

  • The Distribution of electrical power.

Fig.1 shows a typical umbilical cross section.

Operators, contractors and regulators are therefore demanding umbilicals which are longer, more sophisticated and more reliable than ever before. The umbilical is however, a very vulnerable key component, which is exposed to the high risk of significant external mechanical damage.

A recent survey by the Engineering Research Council for the Department of Energy showed that from a total selection of 180 umbilicals, 54% had suffered from various problems. Fig2 shows the areas from which these samples were taken. Out of the samples analysed, just over 15% of umbilicals which suffered from mechanical damage during service received that damage as a result of trawler and anchor chain snagging. Whilst in overall numbers, the actual amount of recorded incidents is relatively low, the possible effects can be catastrophic.

A typical armoured umbilical could easily sustain an axial tension of 60 tonnes without breakage. This could inflict severe damage on its interface and associated equipment, such as a Tree or Manifold for example.

To protect critical equipment and the umbilical itself, some operators incorporate a ‘weak link’ in the umbilical line. This weak link usually takes the form of a mechanical device designed to separate in a controlled manner at a pre-set axial overload force.

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