CORROSION ASSESSMENT OF ALUMINIUM CONDUCTOR FOR MEDIUM VOLTAGE CABLES FOR SUBSEA UMBILICAL SYSTEM

摘要

High tensile strength aluminum offers great potential as a conductor material for Medium Voltage power cables within subsea umbilicals. Its excellent fatigue performance makes it suitable for dynamic umbilicals and its high tensile strength and light weight make it an ideal candidate for deepwater dynamic umbilicals either as an independent load bearing member or as an electrical conductor taking load share in an armoured or steel tube umbilical. Umbilicals, are the bundles of electrical and hydraulic components that connect and control elements of a subsea Oil and Gas production system. 6000 series aluminum conductors, commonly known as AAAC (All Aluminum Alloy) conductors are widely used on high voltage overhead transmission lines, primarily on long spans due to their increased tensile strength. They have been widely used in various environments and possess an excellent track record. However, the use of AAAC 6000 series in a subsea umbilical system is novel. The cable located inside an umbilical bundle is normally of a wet insulation design, and an area of concern is that the seawater may permeate through the cable insulation allowing the conductor material to be in contact with water throughout its design life. Hence the corrosion resistance of the AAAC 6000 series in a seawater environment is of paramount importance and therefore must be assessed. This paper details the corrosion assessment of the 6000 series aluminum power cable conductor at RINA Consulting Ltd's laboratories together with complimentary field testing. The presented test results are based on long term (6 -12months) laboratory testing. The test programme investigated permeation, impact of temperature, effect of hydrostatic pressure and galvanic corrosion, with the tests being performed on material coupons and cable samples in a simulated seawater environment. Also detailed is the outcome of a full-scale cable field immersion test, 6 months under 1000m depth at seabed temperature of 10°C. The results demonstrated that there was no Cl~ and Na~+ ion permeation through the insulation layer. Also, there is no sign of aluminium conductor corrosion and no drop-in insulation resistance witnessed for all tested samples at temperatures up to 90°C and under a high pressure of 300bar after a one year test period, which is sufficient to qualify use in a subsea application. Based on the results of the presented laboratory testing and field immersion testing, it can be concluded that 6000 series aluminium conductors within 'wet design' Medium Voltage power cables will not be susceptible to corrosion in a subsea environment and could be used in other applications such as renewables.