Subsea Electrical Power Generation for Localised Subsea Applications
Authors: Stavropoulos Michael , Shepheard Barry , Dixon Mark , Jackson Daniel 
Published: October 2008

This paper introduces and examines the prospect of subsea power generation at seabed level using marine current turbines to facilitate remote subsea tiebacks. Important parameters for the assessment of the feasibility of the technology are identified. The system can use proven ("off the shelf") technology for energy storage, power conditioning and switching, while turbine blade design issues are discussed. Results from a simulation code, built for the assessment of the technical feasibility of the system, are presented and discussed for representative base cases of two single-well production systems (all-electric and electrohydraulic).

Introduction

The substantial depletion of exploitable deposits in shallow waters of the continental shelf has increased the interest of the petroleum industry towards reserves at increasing water depths. As a result, enabling technology for the exploitation of deepwater reserves is developing exponentially. The emerging technology has made the prospect of hydrocarbon production from satellite reserves, which may lie considerable distances from production infrastructure, more attractive, subject to long distance tiebacks becoming more economical. As advances into production technology (e.g. subsea boosting, processing) make the exploitation of marginal fields technically feasible, the principal factors governing the use of subsea tiebacks become predominantly economic.

Localised power generation offers the possibility of reducing the costs associated with the production from subsea tiebacks by:Reducing the required umbilical functionality, with associated reduction in power transmission costs.Reducing or removing power generation systems at the surface platforms, with associated increase of their payload capacity. Umbilicals, who together with flowlines, respresent the largest cost items in the post drilling phase of the production system installation, are used for the transmission of hydraulic and electrical power required for the production system operation, as well as control, data monitoring and chemical injection. From the above duties, the high-pressure power transmission lines (hydraulic power and chemical injection) are the most costly, followed by the electrical power lines, control and data monitoring. The removal of these operations will yield the corresponding cost savings to the system.

Indicative of the potential economic benefits offered by the removal of hydraulic lines is the current trend in the deepwater arena to develop all-electric (hydraulics-free) subsea production systems, which eliminate the need for hydraulics lines in the umbilical.

Furthermore, especially for large stepout distances (&gt;30km), a large proportion (over 50%) of the power supplied from the topsides is lost in umbilical losses. Consequently, as stepout distance increases, the power generation requirements from the surface platform increase accordingly. Reduction or removal of the umbilical power transmission will reduce the power generation requirements from the surface platform, increasing its payload and lowering costs.