Installation Qualification for Reeled Electrically Trace Heated Pipe-in-Pipe

Abstract

Standard pipe-in-pipe (PiP) technology consists of an inner flowline insulated with high performance dry insulation and an outer carrier pipe that protects the insulation. Electrically Trace Heated Pipe-in-Pipe (ETH-PiP) technology is an adaptation of the standard PiP where electrical trace heating (ETH) and fiber optic (FO) cables are wound around the flowline, under the insulation, to provide active heating and temperature monitoring of the conveyed fluid. ETH-PiP allows operators to optimise field layout and production and minimise the risk of pipeline blockages. Reel-lay is an efficient installation method for subsea pipelines. Reel-lay is well suited to ETH-PiP as critical fabrication activities can take place onshore and before pipeline installation. Reeled PiP flowlines are subjected to plastic bending. During offshore installation, the flowline may see a combination of this plastic bending and high axial tension due to the catenary. The combined loading leads to permanent elongation of the inner pipe which has the potential to transfer tensile loads into the electrical and optical cables. Moreover, the annulus space between the inner flowline and outer carrier pipe is reduced during reeling which may put electrical components at risk of mechanical damage. This paper discusses an extensive qualification programme which was performed to quantify the cable loading during installation, and to demonstrate the subsequent cable and splice integrity and fitness for service. A physical bending trial was performed where several ETH-PiP specimens were subjected to plastic bending on a cantilever bending rig while axial tension was applied to the flowline. This represents an industry first for a practical test which realistically replicates the loading conditions experienced by a PiP system during reeled installation. Throughout the bending trial, the cables were strain-gauged to record axial tension, and the anulus space around the cables/splices was measured to record radial compression. Following bending, the cables and splices were removed from the test specimens and subjected to third party electrical and optical testing to verify integrity. The qualification scope not only verified that cable integrity is maintained throughout reeling, but also quantified the loads involved. Data gathered from the testing was subsequently used to benchmark finite-element (FE) reeling models. These models allow more accurate prediction of cable loading on future reeled ETH-PiP systems, including deep water pipelines.