Concept of a Pipeline Protection against Ice Ridges and Vortex-Induced Vibrations

Master's Thesis in Offshore TechnologyThe development of offshore oil and gas fields involves a few challenges concerning the pipeline on-bottom protection, particularly in the Arctic zone. In the shallow waters the ice ridges may make dangerous gouges, hence, damaging pipelines. In the deep waters when a pipeline has free spans, it is subjected to vortex-induced vibrations (VIV). This thesis is focused in these challenges having in mind that other aspects like damages due to falling objects, trawling and interaction with anchors also are of concern for a full study of the protection of pipelines. The research is provided by the example of the Sakhalin offshore. It is considered to be a sub-arctic zone which includes a few important oil and gas fields. The thesis describes the pipeline interaction with the first-year ice ridges. A study of probable sizes, mechanical properties and geometrical parameters of the ridges is performed in order to calculate the ice scouring depth. Further, in order to analyze the motion of cylindrical structures undergoing VIV numerical simulations are used as an example. In the numerical simulations, the behavior of the near-bottom VIV of a single cylinder and two closely spaced cylinders is investigated at high Reynolds number. The study is performed using the computational fluid dynamics code OpenFOAM. 2D models are simulated at a Reynolds number, Re=3.6x10e6, gap ratio e/D=1 and 2 for the single cylinder and the two cylinders, respectively, and at range of distance between cylinders L=4D to 6D. These values are considered realistic for tandem on-bottom pipelines. In both the ice ridge and the VIV cases the protection of the pipeline may be required. The protection of a pipeline due to ice scouring is investigated by means of trench construction. To investigate the effect of reducing the effect of VIV, the elimination of free spans due to rock dumping is considered. Finally, a discussion of the environmental impact of trenching a pipeline or rock dumping is given