Subsea Direct Electrical Heating (DEH) of pipelines has been in operation for more than 10 years and is therefore considered by most Operators as proven technology. Aside from the electrical aspects, there are mechanical design challenges with installing a cable piggyback fashion onto a HPHT flowline that expands and contracts with the temperature of the flowing wellstream. The cable may also need protection against fishing gear which becomes an integral part of the DEH pipeline system.
Depending on the pipeline operating temperature, the DEH cable may experience excessive axial tensile strain due to thermal expansion and global buckling unless sufficient cable overlength is incorporated during pipeline installation. The cable will be subjected to additional strains at a thermal buckle location when subject to trawl gear pullover. During DEH cable operation, the flowline will initially be cold and contracted but the cable expands due to heating of the cable itself, adding to any installed compression to provide overlength. The following study has been performed to investigate strains in the cable and pipeline due to their responses to thermal and trawlboard loads and to determine the required overlength to keep the cable within strain limits.
Finite Element models of the flowline with piggybacked DEH cable was developed with both ANSYS and ABAQUS in order to analyse their interaction during installation and operation on the seabed. The influence of friction between cable and protection system, and position of the cable on the flowline was simulated in order to quantify the required overlength to be installed during pipelay to avoid overstraining the cable in tension as well as compression.
