Improved Method of Estimating Cable Pull-In Resistance by Factoring Variable Seabed Friction as a Function of Cable Displacement

Abstract The aim of this technical paper is to present an enhanced methodology for improving the estimation of subsea cable resistance (pulling loads) during cable pull-in. The methodology has been proposed based on a validated case study demonstrating the cable second end pull-in by considering the seabed friction in terms of the changes in cable displacement. The product under installation in this case study was a power cable being pulled into a J-tube. Accurate estimation of friction coefficients are essential for the reliable estimation of pull- in loads. Hence, a detailed geotechnical study has been performed and presented. During the pull-in, the power cable must overcome the seabed friction, which includes lateral and axial components, both of which depend on the soil embedment and resistance. Unlike the conventional industry approach, in this methodology, the pull-in tension and loads are estimated by factoring the impact of breakout friction and residual friction explicitly (discrete friction values for every cable displacement value). Variable soil friction coefficients based on cable displacement have been defined during the study. Improved reliability in predicting the pull-in load is of benefit for accurately designing the product under installation, the pull-in head, selection of the platform winch, winch wire and other accessories. The design parameters for the pull-in analysis include maximum allowable pull-in load of the cable, seabed topography and cable surface roughness. The results of the case study which are operationally significant are the maximum pull-in load at the platform winch and maximum load on the cable pull-in head. In general, the pull-in loads at platform winch and cable pull-in head are observed to be maximum and equivalent when the cable pull-in head reaches the platform topside. However, for specific cases when the breakout friction is significant due to the cable embedment in the seabed soil, the peak pull-in loads are observed at the commencement of pull-in. This peak load developed on the cable pull-in head is amplified at the platform winch due to the capstan effect arising from the J-tube curvature and friction between the cable and J-tube. It is evident from this case study that the definition of breakout friction and residual friction as a function of cable displacement is relevant, when the effects of soil embedment (breakout friction) dominate the resistance.