Inertia regime loading on a submarine pipeline in random waves

Abstract

An experimental investigation has been carried out on random wave-induced forces on a smooth horizontal submarine pipeline held fixed at various gaps from a plane boundary. The pipeline was subjected to random waves with Pierson-Moskowitz spectrum (P-M spectrum) at various energy levels to achieve different significant wave conditions. Based on the significant wave concept, the pipeline is considered to lie in the predominantly inertia regime of wave loading. The analysis of in-line and transverse forces are carried out in the frequency domain using spectral density method. While the in-line force is analysed considering only the inertia term of the classical Morison equation, a new spectral density model is developed for the transverse forces on a pipeline in random waves. The in-line hydrodynamic coefficient of inertia and the transverse hydrodynamic coefficients of lift and vertical inertia are evaluated in the frequency domain and through the use of least-squares technique. The in-line and transverse hydrodynamic coefficients are found to be a function of gap ratio of the pipeline from the plane boundary and correlate very well with the potential flow results at least within the range of significant Keulegan-Carpenter number or period parameter encountered in this study. Further, the results of the random wave force tests are compared with those of regular waves under similar pipeline conditions.