Protection of subsea infrastructure in ice environments

Pike, Kenton P. (Kenton Patrick) (2008) Protection of subsea infrastructure in ice environments. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis begins with an outlook of the offshore Newfoundland oil and gas industry. While hydrocarbon resources are plentiful, adverse operating conditions and risk of impact from encroaching icebergs leads to challenges in design, project execution and operation. Acceptable risk levels regarding hydrocarbon release to the environment have to be met by providing sufficient protection for vulnerable assets. A discussion on the parameters involved in determining contact risk between the keel of an iceberg and a subsea structure is provided. -- For subsea structures, many protection concepts have been considered for application in ice environments such as the Beaufort Sea and Grand Banks. Glory holes have been the selected concept for major field development and production schemes on the Grand Banks, however, this method may not be feasible in the development of marginal fields that only require limited subsea infrastructure. Other protection concepts which have been considered are discussed as well. -- The protection of subsea installations required for subsea tie-back developments via tubular frame protection structures is proposed in the present study. Three different geometric configurations are analyzed. The first configuration consists of a rectangular framework similar to overtrawlable structures used in the North Sea. The second structure has a large circular base and a smaller circular top portion, with the top and base connected using straight inclined members, to give the appearance of a truncated conical skirt. The third is a modification of the second, incorporating curved instead of straight inclined members. Structural based finite element models were employed to predict the structural response of the frames subjected to ice loading. Primary failure mechanisms during ice-subsea structure interaction are assessed using an energy approach. Design loads are estimated using a simple ice load model accounting for crushing failure of the ice keel over the contact area. The rectangular frame model behaved well in comparison to the other configurations. This concept also offers relative ease of design and construction. For the circular base models, the benefit of introducing curved members was evident. -- Progress in this research area should involve simulation of a wider range of ice contact events. It is suggested that the finite element model be improved toward continuum interaction of a deformable tubular protection frame structure with an iceberg keel substructure modeled using kinematic constraints representing iceberg size and stability.

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