New kid on the block

September 1, 2017

Elaine Maslin reports on a new pump under development that could be the next step in subsea multiphase boosting.

Aslaug Melbø, engineering manager, subsea systems, BHGE, speaking at UTC Bergen. Photo from UTC.

Testing started this summer on a new subsea pump being developed by Baker Hughes, a GE company (BHGE). It’s no ordinary pump. It has no barrier fluid, for a start, reducing the complexity of umbilicals and reducing topside support requirements. The impellers rotate around a static central shaft, avoiding rotor dynamic issues associated with conventional pumps.

It could be the next step in subsea multiphase boosting, a space in which new technology is needed, according to Pierre-Jean Bibet, rotating machinery department, expert in pumping systems, Total E&P. Speaking at the Underwater Technology Conference (UTC) in Bergen this June, he said the uses for multiphase pumps have been expanding, including transportation of fluids to a floating production system, to increase production from reserves by lowering wellhead pressure and producing viscous fluids.

The future potential is significant, but for new projects to come on, the biggest challenge is the Delta P (differential pressure), he says. “The issue is the higher power requirement to generate more head (pressure), which means higher speed and more head per stage. With existing designs, you can generate Delta P of 210bar at 60% gas volume fraction (GVF), which is almost the starting point for long tiebacks and deepwater projects. We need cost effective multiphase systems for ultra deepwater and long tiebacks.”

But, barrier fluid is a weak point, resulting in complex umbilicals and complex barrier fluid control, Bibet says. Dealing with pressure gradients during transient periods is also a challenge.

BHGE hopes to have tackled these issues with its Modular Compact Pump (MCP). It presented the concept at UTC. The pump is being developed under a joint industry project (JIP) with Statoil, Chevron, Shell and Total, plus US$2 million (NOK17 million) funding from the Norwegian Demo 2000 fund for the JIP’s Phases 2 and 3, running from this autumn.

Because of how it’s been designed, it will be smaller than a standard subsea pump, by about half, says Alisdair McDonald, business leader, Subsea Power & Processing at BHGE.

BHGE is looking to develop modules of four independently controllable stages, with each impeller equipped with its own variable speed drive (VSD), enabling a greater flexibility in operating envelope over field life. The modules could then be stacked depending on process needs and built up into larger systems.

“In a conventional life of field scenario, you have to rebundle a pump system to accommodate life of field conditions,” McDonald says. “With ours, you just change the speed of the individual modules, which you can do on the fly.”

“It’s about performance and maximizing head,” Aslaug Melbø, engineering manager, subsea systems, BHGE, told UTC. “It can be placed inside an existing structure, like a four-slot template, or in a separate structure. It is scalable. Because we add the same modules several times you can build the pump and get a very high differential pressure or reduce the number of stages and get a compact pump. It opens up a whole new operating range than what we have today.”

Melbø says that system cost could be 20-37% less than current pumps and, with no hydraulics to deal with, it would fit well into the all-electric strategy. McDonald says that a version of the MCP, the same size as a 3-4MW pump on the market today, would be able to pack 6 MW – to give an idea of the higher power density. The smallest version envisioned would be a 1-2 MW pump, which could be small enough to put on a Xmas tree, McDonald says, where it could also have the potential to act as a choke.

A three-stage prototype has been built and is being tested, with a variety of gas volume fractions, at BHGE’s facility in Bari, Italy, over summer. This is Phase 1 of the JIP. Phase 2 will start in the fall and run for about 18 months.

The next phase will focus on qualifying some of the key components, including the axial and radial process lubricated bearings, made from polycrystalline diamond bearings, a form of synthetic diamond. A process loop has already been built and testing of the bearings has started, including throwing “tonnes of sand” at them, and running them almost dry, to see how they perform. Phase 3 will see a full-scale pump built and qualified to TRL4. A complete unit is expected to be ready by 2020.

BHGE is also involved in DNV GL’s subsea boosting JIP, which is looking to standardize subsea boosting, in the hope this will reduce costs and increase adoption.

BHGE’s MCP. Illustration from Baker Hughes, a GE Company.

Gullfaks umbilicals challenge

Statoil’s Gullfaks subsea compression project came under the microscope at the Underwater Technology Conference (UTC) in Bergen, Norway – namely, the challenge it faced with a leak in a combined power and control umbilical.

The Gullfaks subsea compression project came onstream in October 2015. The project comprises two, 5 MW subsea wet gas compressors.

Fiber optic communications, compressor power, low voltage power, barrier fluid, and MEG are supplied from one, 16km-long common power and control umbilical from the Gullfaks C platform and the compressor station.

Shortly after start-up, pressure was lost on one of the fluid lines in the combined power and control umbilical. The compressors were removed and the umbilical was retrieved and was found to have leakages on three of the seven super duplex steel tubes in the umbilical.

Analysis carried out by manufacturer Nexans found that the failure mechanism was corrosion related to the influence from the power circuits (AC corrosion). This had been because parts of the steel tubes were unsheathed.

UTC was told that the power core, one of 20 elements in the umbilical, generates heat and can induce currents in the other metallic elements in the core. Those grounded at both ends of the cable will have a voltage that builds up. Further, harmonics in the system increase the voltage.

Where there is bare (unsheathed) steel, corrosion can start, as the sheathing acts as both corrosion protection but also as electrical insulation. The higher the voltage, the quicker the corrosion could start.

While testing helped show the reason for the fault – the bare steel corroding – the presenters said more work needs to be done to understand the mechanism and so to be able to define requirements for tubing with sheaths, including joints. Asked how the steel had been exposed in order that it could corrode, the presenters gave no explanation, however.

Two new umbilicals have now been produced and were installed in May. The compressors were due to be installed this summer with compressor re-start slated for August, as OE went to press.

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