Gulf of Mexico Wet Tree Deepwater Riser Concepts with Sour Service

Abstract

This paper details the methodology and knowledge obtained from a recent riser concept study for a deepwater development in the Gulf of Mexico. The proposed development is a wet tree development tied back to a floating facility with sour production service. Semi submersibles, Spars and Tensioned Leg Platforms are considered in combination with steel catenary riser, lazy wave riser and single vertical import riser (SVIR). The riser design is challenging due to the requirement of artificial lift, resistance to high production pressures, accommodation of sour service upon onset of water injection and the need for large diameter export lines. It is prudent to be conservative in the initial phase of design in order to account for the possibility of detrimental design changes in the later phases of the project. A review of the conservatism involved in the preliminary riser concepts study is conducted in this paper. The demands made by such conservatism for exotic and novel strength and fatigue mitigation concepts such as lumped masses on the SCRs, titanium touch down zones and light weight coating is discussed. It is observed that fatigue knockdown due to sour service on steel, titanium or clad pipes is the most contributing factor in driving production riser design towards the requirement for novel technology. The need for knowledge of accurate sour service knockdown is highlighted. In this paper, the relative performances of the aforementioned vessel riser combinations are presented. The effectiveness and previous track record of the fatigue mitigation technologies for sour service are reviewed. Finally, the benefits and limitations of each vessel and riser system are compiled and the factors considered by the operator in selecting one particular system are discussed.