Abstract
Offshore umbilical cables often include both power cables and elements such as metallic tubes and signal cables. It was concluded from a recent failure of stainless tubes that ac corrosion was the cause of the failure. This paper describes how ac corrosion can lead to tube failure when a puncture exists in the insulating tube coating. The variation in power cable charging currents along the umbilical cable causes a nonzero voltage to exist in the tubes, even though they are grounded at both ends. If the current density associated with the puncture current is sufficiently high, ac corrosion will result. The current density is calculated as a function of the cable length and the variable speed drive operating frequency for three alternative power cable designs. Comparison with measured corrosion rates shows that ac corrosion can corrode a hole in the tube in weeks or months, depending on the cable design and length, and also on the electrical operating conditions. Analytical expressions and numerical procedures are presented, which can be used to determine whether a given umbilical cable installation involves the risk of ac corrosion. It is also shown that substantial heating effects in the puncture area can result in case of excessive harmonics in the supply voltage.
Highlights
POWER umbilical cables are widely used in offshore oil and gas applications for providing electric power to subsea installations, e.g. subsea pumps and compressors
In this work we show that in the case of umbilical cables of substantial length, a non-negligible voltage will develop on insulated metallic tubes and armors even when they are solidly grounded at both ends
The longitudinal variation in the flow of charging currents in submarine umbilical power cables gives rise to a non-zero voltage on metallic tubes via magnetic induction, even when they are solidly grounded at both ends
Summary
POWER umbilical cables are widely used in offshore oil and gas applications for providing electric power to subsea installations, e.g. subsea pumps and compressors. In addition to supplying electrical power at medium voltage, they usually include other elements such as signal cables, fiber optic cables, and tubes which carry chemical fluids. The length of such cables can reach up to a few tens of kilometers. In this work we show that in the case of umbilical cables of substantial length, a non-negligible voltage will develop on insulated metallic tubes and armors even when they are solidly grounded at both ends. The possibility of local heating problems in a tube puncture is evaluated
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