A High-Voltage System for Subsea Electrical Submersible Pumping
Authors: Duncan Neil , Scott P.A. , Schwelm E.R. 
Published: October 2008

Electrical Submersible Pumps (ES.P.'s) have for many years been used to maintain and increase production from oil wells. One of the major contributors to ES.P. failures is recognized as the electrical system. These problems compound when this technology is applied to a subsea developed well. This paper describes a system by which, when used with a subsea tree designed for ease of pump replacement, these problems are reduced.

The aim of the Subsea Submersible Pumping (S.S.P.) Project was to develop a system where ES.P.'s could be safely and economically replaced from a floating vessel. The electrical feed-through of the high power for the ES.P. was a key element in the design process. A novel design of subsea tree evolved which permits the recovery of the E.S.P. without the need to retrieve the tree. An integral part of this design required the use of high power, wet mateable electrical connectors. As these connectors were identified as requiring development, a prototype "design, build, and test" program was commissioned.

INTRODUCTION

Early North Seadevelopments centered on surface production trees located on floating or gravity structures. The production fluid flowed naturally under well bore pressure. As production rates fell, several different types of systems, such as gas lift and gas or water injection, were utilized to boost production. One technique that became popular was the ES.P. (an electrical pump suspended from the bottom of the production tubing and submerged in reservoir fluid). Offshore to date, these pumps have been fitted on platformbased wells only.

Due to the requirement to cut production costs and exploit marginal fields, the cost of field development is reduced by the increasing use of subsea developments tied back to existing surface production facilities. The need to assist, or boost, production from these completions, is perceived as the next step in exploiting subsea resources. Therefore, the S.S.P. Project was conceived to engineer a system where an electrical submersible pump could be deployed in a subsea completion. Wherever possible. field-proven equipment and techniques have been used to reduce technical risk and effect an early acceptance.

The basic project specification is based on a completion up to 20 Km from the host facility, a pump depth up to 15,000 ft. and a production rate up to 20 thousand barrels per day (Figure 1). This results in a pump power requirement of 350–400 KW.

In addressing the reliability of existing systems, the pump and electrical system emerged as the elements exhibiting the greatest maintenance requirement. To this end, the project determined that these elements should be recoverable for workover/repair with minimum unnecessarydisruption to the components such as the tree. Existing subsea equipment does not allow this. For any system to be successful, it must be financially viable. A major financial consideration with subsea developments is the relatively high cost of workover when compared with a surface completion.

The above considerations resulted in the design of a tree that allows the recovery of the tubing hanger, production tubing, and pump string without the need to remove the tree.