SS: Advances in Deepwater Riser Technology: Gulf of Mexico Wet Tree Deepwater Riser Concepts with Sour Service
Authors: Deka Dhyanjyoti , Campbell Mike , Kakar Karan , Hays Paul Robert 
Published: July 2010

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.

Introduction

Subsea riser design is one of the most challenging engineering aspects of a deepwater field development. Risers constitute the conduit that connects the floaters at the surface to the subsea wellhead. The primary challenge in riser design emanates from the fact that these are dynamic structures highly susceptible to environmental and operational loads. As the global demand for hydrocarbons has increased, offshore projects have moved deeper and deeper and riser design has become more challenging than ever before, involving novel technologies and materials. The latest field developments in the GoM are often in the order of 5,000 ft water depth or more; requiring extensive engineering in order to come up with an optimum riser design.

This paper describes a case study of a GoM deepwater riser design in the pre-FEED stage. The water depth at this location is greater than 5,000 ft and a variety of riser-vessel combinations are assessed. The technical performances of the range of riser designs are presented with a summary of benefits and limitations of each type. Design data and constraints such as sour production are highlighted and the demand for unconventional technologies driven by the harsh environment and pre-FEED robustness requirements is demonstrated. The commercial feasibility and track record of the different riser solutions are also discussed. Finally, the recommendations and learnings from this study are summarized.

The material presented in this paper provides ample insight on how deepwater risers behave in the harsh GoM environment. It is understood that not many project analysis findings are presented in the public domain. In that light, the sections herein are indeed valuable guidance to a designer in the preliminary stages of project development. Certain riser technologies presented herein are considered novel and to date have not been implemented in the field. It is expected that offshore projects will utilize more of these technologies in the coming years. The work conducted in this paper is a stepping stone for such technologies to be realized.