Some of the world's largest reserves are heavy oil reservoirs, defined as liquid petroleum of less than 20°API gravity or more than 0.2 Pa.s (200 cP) at reservoir conditions, [4]. Production of heavy oil in combination with increasing water cut (WC) brings a potential for very high emulsion viscosities. In combination with gas, the high viscosity provides operating conditions highly challenging to multiphase subsea pumps.

In 2013, OneSubsea was awarded the Engineering, Procurement, and Construction (EPC) contract for Total's Moho 1bis development in the Republic of the Congo in West Africa. The contract included a subsea pump station with two 3.5 MW HighBoost pumps (helico-axial multiphase pump with balance piston) capable of handling high viscosities and gas volume fractions (GVFs).

As part of the Moho project, and to qualify the HighBoost technology for high viscosities, a full-scale test loop was built to verify pump performance at liquid viscosities up to 0.8 Pa.s (800cP). To cover the complete Moho operating range 0.001-0.8 Pa.s (1-800cP) and 0-75% GVF, the first article pump was tested on nitrogen and three different liquids: water, hydraulic oil, and gear oil. An extended analysis on the performance of helico-axial pumps in this unexplored domain of laminar and transition flow regimes was carried out.

Extensive amounts of test data were gathered during the 2-year qualification period. After testing and design optimization, the pump performance was significantly higher than predicted. The knowledge gained also served as valuable input to the pump protection logics customized for high-viscosity pumping. During the program the pump has proven its ability to perform startups on viscosities up to 30 Pa.s (30,000cP).

Qualification of this technology for high viscosity has widely extended the domain of high power, high flow and high differential pressure (dP) helico-axial multiphase pumps. Along with their proven track record in deep water and long step-out distances, the ability to pump high-viscosity fluid will enable future development of other heavy oil reserves going forward. The pump system described in this paper was successfully installed, commissioned, and started during the spring of 2017 to boost the viscous production. As of January 2018, it has been running with 100% availability following the startup.

This content is only available via PDF.
You can access this article if you purchase or spend a download.