On-bottom Stability of a Subsea pipeline in Irregular Waves

Pipelines in a subsea environment are exposed to a very complex system of hydrodynamic reaction. The on-bottom stability has long been a important study for offshore practice. The soil types, wave conditions as well as the structure itself are considered to be the main focus of the on-bottom stability analysis for a subsea pipeline. The objective of this thesis is to study the on-bottom stability in a series of complex environmental conditions through the dynamic analysis, as well as the generalized stability method and absolute lateral static stability method. The effect of different parameters regarding stability is investigated through the sensitivity analysis. Models will be built using PONDUS, a software developed by MARINTEK for the dynamic on-bottom stability analysis. The linear wave theory, the JONSWAP spectrum and the energy-based soil model will be considered for irregular waves and soil-pipe interactions respectively. Based on the different case set-ups, the results will be compared and discussions will be made on the methodology of pipeline stability analysis, considering the sensitive variables as follows. Both regular waves and irregular waves have been studied in the past. Behaviours of the pipe in irregular waves are studied further in this thesis based on the previous research. Important development patterns and characteristic values such as displacement, forces and velocity will be compared between cases with regular and irregular waves. The mechanisms are further discussed based on simulation results. Same values taken for the wave height and wave period of regular waves, the significant wave height and the peak wave period of irregular waves. Results indicate that the maximum wave-induced water particle velocity is larger in irregular waves. It suggests that when considering the environmental condition, 10-year return currents and 100-year return irregular waves is the most critical load combination. These recommendations are important to obtain a more realistic and reliable model among all the cases in the present study for the on-bottom stability analysis. Furthermore, the stair-like development pattern of dynamic displacement is quite distinguishable when the pipeline is exposed to underwater environment with irregular waves. The pipe-soil interaction is also investigated in this thesis. Past studies took only one single type of soil for the investigation of pipe-soil interactions. However, the seabed soil condition is actually more