Analysis of Subsea Buried Pipelines and Partially Buried Cables

This research investigation addresses the analysis and numerical simulation of two very important offshore engineering problems. The first deals with the modeling of the steady state thermal field around buried pipelines conveying high temperature wellhead mixtures of oil and gas, and their associated dissolved impurities. These pipelines may be buried using robotic trenching equipment for physical protection or to provide additional thermal insulation. The solution to this complex multi-layer problem is examined using a boundary element model approach. The second challenging problem is that of modeling a partially buried cable on the seafloor that is ensnared by commercial fishing equipment. There are many cables on the seafloor and several obvious systems are oceanic communication cables and the increasing number of subsea power transmission systems associated with the continuing development of offshore wind farms. In this problem an important numerical modeling challenge is to allow the cable to change its length as a result of the entanglement. A different approach is presented, i.e. a meshfree formulation, is specifically developed for simulating this type of subsea cable problem.

A two-dimensional boundary element model was developed specifically to investigate the local steady-state thermal field in the near field of the pipeline. Subsequently, a parametric study was preformed to evaluate the influence of the thermal power loss, burial depth, pipe diameter and soil thermal conductivity on the thermal field. The numerical examples illustrate the significant influence of the backfill thermal property on the temperature at the pipe wall, that the pipe diameter controls the required output thermal power needed to maintain the desired pipe wall temperature, and the importance of pipeline burial depth on seabed temperature distribution above the pipeline.

In order to better address the problem of partially buried subsea cables, a three dimensional meshfree method was formulated and implemented to evaluate the structural response of cables in two dimensional space under accidental loads from trawling activities. The methodology specifically was developed to allow the arbitrary layout of a cable on the seafloor, the lengthening of an ensnared cable length at a boundary, and the inclusion of geometrical nonlinearity due to large deflection. This meshfree method is based upon a slender rod formulation, incorporates radial basis functions (RBF) for shape function construction, and utilizes a Galerkin weak formulation for the discretization of governing equations. The methodology was validated against two benchmark examples which have analytical solutions, and shows good convergence rates to the analytical solutions. Finally, a two dimensional gear-cable example illustrating the adaptive nature of this formulation and implementation to address a sliding length boundary condition is presented.