Over the years, DNV has been engaged in a significant amount of failure investigations for the offshore industry and in particular the subsea industry. This paper will describe experience form the subsea control system failure investigation without going into the disclosing details of manufacturer. Through these practical examples it will further be described the principles of a new methodology developed for ‘Qualification for New Technology’ and the related benefits. The need for such a systematic approach has been recognised by the offshore industry, which has sponsored DNV during the preparation of a recommended practice for "Qualification of New Technology" /I/
DNV has been engaged in study, testing, verification and trouble shooting of subsea production systems and their control systems since they first were installed in the North Sea
Malfunction of subsea systems may become extremely costly Therefore failure investigation are important both in conjunction with insurance and to avoid the occurrence of similar failures in the future. The experience DNV is gathering during failure investigation is used in cooperation with the industry for developing methods that reduces the probability for failure Examples form subsea control systems are explained in this paper. These illustrate how a failure analysis is typically conducted at a DNV laboratory, and how the likelihood of these failures occuring could have been reduced by following the principles outlined in the recently published DNV RP /1/
The examples quoted are all associated with subsea umbilicals cables and jumpers
An umbilica1 is used in the control and monitor of subsea production units. These are multipurpose links between the topside and the subsea facilities of a large design variety. Typically, these consist of small bore pipes, electrical conduct and fibre optical conducts as illustrated in Figure 1 (please note that all figures are printed at the end of this paper) Therefore their design is complex and contains a large numbers of different materials with various properties.
The failure cases described below are limited to an electrical failure and a control fluid tubing Failure. Such failures, if redundant systems are not installed may cause production loss with significant financial consequences not only through the loss of production but also in respect of a repair and perhaps replacement Electrical short circuit caused by salt water penetration and a consequent reduction the insulation resistance has been experience in many electro/hydraulic control systems used in the North Sea over the last two decades The causes of these failures appear to be different in each case, but always result from an underestimation of the difficulties of maintaining electrical insulation in submerged saltwater conditions
During a marine operation a chain accidentally bit the #x0022;dynamic#x0022; part of the umbilical, i e the part extending from the surface to the sea bottom. This caused damage to the umbilical resulting in control fluid leakage. The umbilical was therefore retrieved and brought to the manufacturer for repair.
The repair work was completed, and the umbilical was ready for final flushing before reinstallation During this flushing/cleaning operation a rather heavy leakage was observed in an original part of the umbilical at a location for from the original damage.