OneSubsea was awarded an engineering, procurement and construction contract for Total’s Moho 1bis development. The contract included a subsea pump station with two 3.5-MW high-boost multiphase pumps to meet the forecasted production profile (Figure 1). Also, OneSubsea committed to a qualification program verifying the boosting capabilities at viscosities that had never been tested with subsea multiphase pumps before.
The maximum differential pressure requirement was set to 110 bar. The pumping system was designed to operate one pump at a time according to wet spare philosophy as well as parallel operation of the pumps. The substantial head, capacity and viscosity requirements resulted in the highest power rating ever supplied for a subsea multiphase boosting system.
In addition to the standard factory acceptance test and endurance test, OneSubsea committed to demonstrating the high-boost pump’s ability to operate continuously on 800 cp at various GVF while monitoring rotor dynamic and hydraulic performance. An additional performance mapping on viscosities ranging from 50 cp to 800 cp also was executed.
A high-viscosity multiphase test loop was designed and built for full-scale testing of the pump. This setup comprises a 40-sq-m (430-sq-ft), two-phase separator, chokes, process coolers and advanced measurements to secure viscosity control and conditioning of the multiphase fluids for precise performance characterization. The test loop enabled operation through the full capacity range of the pump: a liquid flow rate of up to 600 cu. m/hr (21,189 cf/hr), or 90,000 bbl/d, and 0% to 75% GVF.
Throughout the test campaign, the engineers in the pump control room learned many valuable lessons. The combination of high-viscosity and multiphase conditions proved to have a surprising and severe impact on the rotor dynamics of the pump. A large number of research-related operating hours laid the foundation to update design principles, performance models and protection logic. Following this, the upgraded pump was run through the full test matrix of varying viscosities and GVF. Its performance was dynamically stable under all conditions. In this case, the investment in a comprehensive test program was instrumental for the reduction of project risk. The gained competence and test facilities developed during this project will undoubtedly be leveraged to provide guidance and ability to execute future high-viscosity pump projects (Figure 2).