Protection of subsea pipelines against ice ridge gouging in conditions of substantial surface ice
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The development of Arctic offshore hydrocarbon fields involves transportation systems for oil and gas, which are represented either by tankers shipping or by pipeline systems. The later have sustained behavior with respect to hydrocarbons delivery and relatively non-sophisticated operational requirements. However, some important challenges regarding Arctic conditions have to be carried out before the pipeline is constructed. Attention is given to the conditions of a specific hydrocarbon field of the Sakhalin offshore and a design of the 28 km offshore pipeline. Hydraulic assessment determines the size and number of pipelines as also temperature and pressure profiles, while mechanical estimations provide the wall thickness. As a result the main pipeline design aspects regarding dimensions and stresses occur are obtained, which is required for the next stage of the thesis. In the second part the issue of the pipeline interaction with first year ice ridges is described. A study of probable sizes of ice ridges, their peculiarities and morphology is performed in order to evaluate the design ridge geometry, physical properties and to understand how a ridge interacts with the soil. Consequently, there introduced two models for estimation of the maximum gouge depth, caused by the ice ridge scouring the seabed. At the same time the research shows that even below the gouge the forces transmitted through the soil could be adverse, such that the proper protection of a pipeline is required. A beam model of a pipeline exposed to bending and tension in terms of combined transverse and lateral loadings is proposed and analyzed. Based on the limit state design criteria the required trench features and mainly soil conditions for the safety reasons of the pipeline are proposed. Simultaneously the pipeline failure probability is assessed. As a result it was proven that the pipeline might be buried just below the probable scour depth if the certain conditions of the “sandwich” backfilling with a weak soil layer on the bottom are met. After gaining an understanding of the physical processes related to gouging, numerical modeling is established in the last section. A finite-element analysis in ANSYS 13.0 software is carried out. The results obtained provide both: justification of the proposed theoretical models, and more precise assessment of some parameters when necessary, namely the soil behavior subscour, which is almost impossible to describe theoretically. Conclusions summarize the acquired findings, provide reasonable recommendations for the offshore pipelines design in the Arctic regions and give the scope for future works and studies.
Master thesis in Offshore technology