Over recent years, umbilicals have been increasingly used in many off-shore applications world-wide for subsea control, chemical injection etc. The failure of these components in a remote subsea installation can be extremely expensive both in lost production and rectification.
Unfortunately, due to the nature of these products and their working environment, failures often occur during manufacture, installation and service. These failures are in certain instances avoidable, but in other instances failures result from a lack of appreciation of the limitations of the product.
This paper describes many of the mechanisms which govern umbilical failure and explains how they may be practically avoided. The subject is treated in a manner which will allow both suppliers and users to identify the factors which are most relevant to their own circumstances. Topics such as bending failure, fatigue and chemical resistance are covered.
The scope of interest in umbilicals is wide and ranges from umbilical manufacturers, control system suppliers through installation companies to oil companies. All of these companies have, to a greater or lesser extent, a requirement to understand the product in greater depth in order to mitigate their risk.
Umbilicals are vital links to subsea equipment, ROV's and other subsea systems. They carry power, signals, chemicals and are used extensively for condition monitoring and therefore it is important to understand their construction and limitations in order to appreciate how the product may fail. Umbilicals are generally robust and their design will vary with application requirement, although under certain loading regimes they can easily be damaged. Time restraints during installation, sea states and the inaccessibility of the product one installed necessitate a fault free installation and three is therefore a need to get the job completed correctly and efficiently first time.
This paper explains some of the more common failure mechanisms of umbilicals and is intended to assists designers, installers and users of umbilicals to avoid many of the pitfalls associated with their use. Terminations are not covered to any great extent in this paper however their effect on stress levels in umbilicals should be carefully considered. It is important to ensure that terminations are considered at the concept stage and not treated as an ‘add-on’.
Potential failure, or indeed actual failure of an umbilical can be initiated at any stage in the life of the product; from concept through to installation. It is useful to consider each of these stages as follows and it is recommended that a failure audit is performed for all umbilical designs through its life using the following broad headings:-
Concept
Specification
Design & Review
Production & Testing
Installation
In-Service Use & Maintenance
Recovery
Elements within an umbilical such as electrical conductors, fibre optics and armour wires, are generally ‘laid-up’ or twisted together during production in a helical or oscillatory fashion as shown in Figure 1 below. These construction techniques have been used for many years and are employed in order to reduce the strain experienced.
