Subsea processing is a key aspect of ExxonMobil’s subsea technology qualification efforts. Consequently, the reliable delivery of high voltage power to subsea equipment offshore, such as multi-phase pumps, is essential. The base case involves power delivery up to 30 kV via copper cables embedded in umbilicals to a water depth of 1,500m over a design life of 30 years. Since umbilicals are often suspended from a floating facility in deepwater, the power cables are subjected to static load due to the umbilical weight and to cyclic loads in response to the floating facility motions. Therefore, sufficient mechanical resistance to preclude electrical failure is needed to achieve safe and cost-effective designs.
To that end, an experimental program was completed aimed at developing a qualification methodology and collecting data on the effect of static and cyclic loads on the mechanical and electrical performance of copper power cables. This paper presents the experimental approaches and results obtained from creep, fatigue, and electrical tests performed on 18/30kV, 95mm2 copper conductor cable specimens.
Specifically, the program included (a) five monotonic tension tests to failure to characterize the cable, (b) 11 creep tests to develop a load-time performance curve, and (c) 59 fatigue tests under pulsating tension, of which 50 used short-cable samples to evaluate the effect of mean load and nine used long samples for electrical testing following fatigue. A fatigue curve is proposed that includes the effect of mean load and temperature. However, fatigue results can be sensitive to the cable material and mechanical characteristics of a particular cable.