Pumps for Subsea Processing: Integration Issues
Authors: de Salis Jacques 
Published: October 2008

Offshore exploration and processing are targeting at fields in deep and ultra deep waters (down to 3,000 m or 10,000 ft below surface). The high (hydrostatic and friction) pressure losses in flow lines, associated with deeper water developments, tends to be a constrain in actual production rates and in the overall field recovery. Reducing the high back-pressure on the wells is the driver for seabed processing and artificial lift technologies.

The potential economic benefits of subsea boosting increases with increased water depths. However the deeper the equipment the more critical becomes its reliability due to the high intervention costs. Tools such as quantitative risk assessment help in reviewing the overall system reliability.

Several critical issues related to equipment choice and pump module design are raised.

A subsea injection pump design suitable for deep waters is described.

Subsea Processing

The dream of subsea processing is to take processing of hydrocarbons to the sea bed and producing straight to shore instead of depending on expensive surface hardware. Subsea processing encompasses today subsea separation with water re-injection subsea and/or oil, gas or multiphase effluent boosting.

This is made possible today by many advances in development and deployment of key elements, such as multiphase pumps, multiphase meters, flow lines instrumentation, electric connectors and also in mastering flow assurance issues.

Subsea processing requires rotating equipment to be located subsea and also some means of accommodating possible load changes due to process fluctuations.

In the case of one pump with an electric drive this is generally solved by speed regulation with a variable speed drive system. The drive system comprises frequency converters and step-up transformer located on surface. The use of a step-down transformer subsea is dictated primarily by the step-out distance.

For several subsea electric driven machines at distance over a few kilometers a power distribution system is no longer economical and a subsea power distribution system would be required.

Deepwater issues

Several issues have a major impact on equipment selection and critical for the actual pump (multiphase, water injection, oil boosting) module design. These issues play an increased role directly linked to the greater water depths (1,000m or 3,300 ft) as compared to existing solution for lesser water depth.

Process data.

Due to the long flowlines relatively high discharge pressures are required. Furthermore generally high flows have to be handled. Both flow and pressure requirements result generally in installed powers in the range of 1 to 2 MW (say 1,500 to 3,000 HP) and sometimes higher.

Number of installed units.

The choice lays in the number of units and their respective size to be installed at sea bottom. The decision will be influenced by redundancy issues, drivers availability, power distribution, intervention costs and overall economics.

Intervention.

The installation and retrieval operationswill have to be conducted guidelineless. Assumptions of the interventions frequency is a crucial factor in determining the overall economics of a subsea project.

Pressure.

External pressure is the static pressure of the sea water surrounding the pump module.