Development and Qualification of a High Differential Pressure Subsea Pump
Authors: Cunha Leonardo Salino , Felix Thomas , Meuter Paul , Bourne Matt , Fletcher Nicholas , Vasconcellos J. Henrique , Hollingsaeter Terje Fernando 
Published: June 2014

Growing global demand for hydrocarbons has forced key operators to expand field developments towards deeper waters and remoter areas with increasing step out distances. At the same time, brownfields already in production are facing high water cut and low reservoir pressure; these scenarios have created a demand for more powerful subsea pump systems.

FMC Technologies and Sulzer Pumps have jointly collaborated to develop a new multiphase pump system. This new solution is a helico-axial pump that is driven by an innovative 3.2 MW permanent magnet motor (PMM). This subsea pump is distinguished by its high power and efficiency, with a design optimized for the seabed environment. PMM technology has been applied and proven for topside and onshore applications but now is also qualified for subsea use. The key advantage of using PMM is the high power density, the ability to operate at higher speeds and improved efficiency in a more compact design compared to traditional induction motors, these key differentiators are of significant benefit to subsea processing production schemes.

The pump unit proven during the qualification program was designed for water depths of up to 2000 m [6562 ft], internal pressure rating of 345bar [5000 psi], and a design temperature of 80°C [176°F] and furnished with helico-axial multiphase hydraulics and a highly tolerant water-based barrier fluid (coolant/lubricant/motor pressurization fluid). Radial hydraulics are also available, as well as a combination of helico-axial and radial (hybrid hydraulic) depending on gas volume fraction (GVF).

A full size subsea pump system, complete with motor, pump cartridge, fluid conditioning system, process control and additional systems was engineered and constructed and installed at a purpose-built pump test facility able to simulate multiphase field operational conditions. The initial qualification test program of the 3.2 MW, 5000 psi multiphase pump module is now complete and complementary system endurance testing is reaching conclusion.

This paper presents an overview of the key features of the pump and motor system, and the qualification program to which this boosting system was subjected. The paper also will describe case studies and the pump selection criteria for these scenarios.

Introduction

Electric submersible pumps (ESP) have been broadly applied in, or adjacent to, subsea wells in order to provide additional energy to the reservoirs. The early adoption of ESPs was a natural choice as they were already suitable for submerged environments, however as this type of pump was designed for downhole installation its geometry featured small diameters and long lengths. Short mean time between maintenance (MTBM), high costs of intervention and lost production during maintenance led oil companies to look for alternative pump solutions.

Despite multiple design iterations in both vertical and horizontal configurations for mudline pump motor units using ESPs, the systems continued to suffer from short MTBM and other limitations due to the geometrical constraints resulting in a very low power density pump system.

Since the pump installation location has evolved away from being downhole, the diameter limitations no longer apply and thus a shorter motor-pump unit with higher power density can be realized, facilitating installation and intervention activities. Furthermore, the consequent reduction of the number of pump stages improves rotordynamic behavior and allows the development of more powerful and reliable machines.