Subsea Processing and Boosting - Technical Challenges and Opportunities
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18261, "Subsea Processing and Boosting - Technical Challenges and Opportunities," by R.M. Bass, Shell Intl. E&P, prepared for the 2006 Offshore Technology Conference, Houston, 1-4 May.

Subsea oil-pressure boosting often enhances subsea deepwater oil developments. Currently available subsea processing and boosting solutions, global portfolio needs for subsea oil boosting, and the limitations of existing technologies in meeting these needs are described.

Introduction

Fig. 1 shows how subsea-processing applications have increased in recent years. Subsea processing can include the following.Subsea oil-pressure-boosting systems.Subsea oil processing.Bulk water separation.Sales-quality oil polishing.Subsea raw-seawater injection for reservoir-pressure support.Subsea gas compression.Subsea gas dewpointing and dehydration.Flow assurance.Sales-quality processing.

Oil Systems

Subsea oil-pressure boosting can enable ultradeepwater light-oil production, can enable deepwater heavy-oil production, can increase ultimate recovery for deepwater light oil, and may be used to reduce flow-assurance costs. In all cases, subsea boosting reduces or eliminates the back-pressure on the wells resulting from the riser hydrostatic head and the riser and flowline pressure drop caused by high viscosity. Although many reservoirs have sufficient pressure to produce to the seafloor without the use of downhole artificial lift, many ultradeepwater light-oil and deepwater heavy-oil reservoirs have pressure that is near hydrostatic pressure and cannot produce to the sea surface for more than a short period of time.

Moderate-depth light-oil wells frequently are economical without artificial lift. However, as pressure declines in late life, reducing the backpressure can add significantly to ultimate recovery.

Subsea oil-pressure boosting can add value to deepwater oil projects. At the time this paper was written, 10 applications were in place, and an additional five had been announced. Subsea oil-pressure boosting has been proved in water depths to approximately 3,000 ft, depending on desired boost pressure, gas content, and viscosity. Significant cost or technical challenges remain for light and heavy oil in deeper water.

Bulk subsea water separation can be used to debottleneck water-processing capacity where topside debottlenecking cannot be accomplished, thus enabling additional production to an existing host. Because topside debottlenecking is possible for many situations, subsea water separation has selected applicability. Bulk subsea water separation has been used without solids-handling equipment for relatively shallow water depths and easily separated oils. Solids-handling components have been tested onshore and will be incorporated into an announced project. Water-depth limits can be extended with moderate changes in separator-vessel design. Electrostatic coalescers are being tested in topside facilities to increase separation efficiency for difficult oils. Separators for deep water remain a technical challenge.

Subsea raw-seawater injection can be used to debottleneck topside facilities when additional topside injection capacity cannot be accommodated. Because topside debottlenecking is not needed for many situations, subsea raw-seawater injection has selected applicability, with two projects announced. These systems will use proven subsea injection pumps and redesigned versions of solids-filtering components that have been proved in topside application.