Subsea booster pumps experience, confidence developing

Eleven booster pumps delivered, three to go

• This illustration shows the Machar Field layout. [20,857 bytes]

A contract has been signed for the delivery of two multiphase subsea booster pump units (SMUBS) for BP's Eastern Trough Area Project (ETAP) in the UK sector of the North Sea. The Framo Engineering pumps will boost the unprocessed production from the Machar Field through a 35.2-km pipeline to the Marnock Central Processing Facility (CPF) platform for further processing. This contract follows the delivery of Framo subsea multiphase flow meters for ETAP.

The development of a multiphase booster pump system started in 1987 and multiphase meters in 1990. Framo Engineering has gained experience with more than 30,000 running hours from several field pump installations and a number of applications are underway or planned:

• Work has started on two ELSMUBS for Mobil's Zafiro Field offshore Africa, where the pumps will operate at 550 meter depth.
• A 2-MW subsea electric water injection pump for Norsk Hydro's Troll C Field will be ready for testing and installation in 1999.
• This month, commissioning starts on six subsea booster pumps to the Lufeng field offshore China.

The first subsea multiphase meters in operation worldwide, installed in the East Spar Project offshore Australia, have operated successfully for over one year. The subsea meters for BP's ETAP Project will begin operation in the spring of 1999. A single multiphase meter to Marathon's West Brae Project has already gained three months of successful operation time.

Multiphase boosting

The Machar Field will deliver 30% of ETAP's oil revenue. BP's options were to install a separate wellhead platform or a subsea multiphase booster pump system.

Multiphase boosting is simply the ability to add energy or pressure to an unprocessed well flow. The two subsea booster pumps will increase the production from the Machar Field by minimum 4,000 b/d of oil by adding 21.5 bar (312 psi) pressure to the unprocessed wellstream. This will mean a total of 4,000 b/d of oil extra production. The recoverable reserves for the field will increase 65-120 million bbl.

In the early days of the Machar field's life, the reservoir will be sufficiently energetic to push the product all the way back to the CPF without any assistance. Water injection, soon after first oil, will maintain the reservoir pressure at, or close to, the virgin reservoir pressure and hence increase the recoverable reserves.

However, significant water breakthrough is expected within two years of first oil. Therefore, to help maintain plateau oil production, some of the energy in the injection water will be used to drive two parallel turbine driven multiphase booster pumps in the main production line to push the production 35.2 km back to the CPF.

This combination of pumps and turbines is integrated into a booster unit, which will be built and performance tested by Framo Engineering.

The two multiphase booster pumps will be installed 30 meters downstream of Machar's production manifold at 84.5 meters depth. They will be installed in a separate structure, which is a common base structure with the water injection valve module. A common homogenizer or flow mixer will be installed immediately upstream of the two pumps in order to smooth out the effects of any slugging. The homogenizer will also provide optimum conditions for dividing the production into two equal flow streams, one for each parallel booster pump.

High-pressure injection water from the CPF gives the necessary power to each multiphase pump. Under normal operation, the full injection water flow rate of 65,000 b/d is routed through the two parallel turbine drivers (32,500 b/d for each) with a pressure drop of 2,175 psi over each pump package at nominal conditions.

The two turbine-driven pumps will be arranged in a bypass on the existing flowline. The piping and valving required for the by-pass is an integral part of the pump module.

Isolation valves have been specified to accommodate full isolation of the booster stations in the case of equipment failures, scheduled maintenance, or for general safety reasons during production. The production can then be routed through the main production lines to the platform (to the extent that the wells can produce on natural drive).

Pump system

The ETAP pump design is based on the Poseidon hydraulic technology, which utilizes the dynamic pump principle with helico-axial impellers. A roto-dynamic machine with a helico-axial design is a combination of a centrifugal pump and an axial compressor.

The mechanical design is based on Framo technology, applied to heavy duty rotating equipment operating at high speeds, high pressures, and high power ratings. The retrievable pump package consists of, from the top:

• Running tool interface device
• Flow mixer
• Multi-stage pump
• Hydraulic turbine drive
• Four Framo hydraulically actuated collet connectors (inlet and outlet for production and water injection drive fluid)

The Framo helico-axial multiphase pump offers the following main features:

• Self-contained system with a minimum of parameters to be monitored during operation (only flow-rates for the turbine drive water and pump discharge pressures).
• Stiff shaft/bearing system has been specified to accommodate for variable loads and possible pure gas or pure liquid operation.
• Duplex stainless steel/solid stellite are specified for crude wetted parts to withstand sand production and possible harsh corrosive environments.
• In the case of abnormal operating conditions exceeding the specification, the pump will be shut down automatically and if required, isolated from the production by the closing of the dedicated platform installed isolation valves.
• An upstream flow mixer has been included in the pump design to provide homogeneous inlet flow conditions.
• Stringent safety requirements have been applied throughout. The inlet mixer and pump housing will be designed to accommodate the pressure rating of the inlet and outlet pipework.
• Operational flexibility has been provided by the use of a self-regulating pump principle in combination with speed regulation, and the built-in possibility to replace the internal hydraulic elements (impeller/diffusers) to meet new booster specifications.
• Simple and controllable procedures will be adopted during all expected modes of installation and operation. Simplicity will meet the requirements of easy running and retrieval for low cost maintenance using a light intervention vessel.
• Efficient operation during field life with flexibility to cope with expected changing well conditions: Unattended/remotely operated.
• Flexible operation range. The system can operate on off-design duty and can operate efficiently and remain stable in transient conditions.
• Can withstand expected corrosion, erosion and abrasion with minimum performance losses (less sensitive to sand production).

The ETAP multiphase booster pump contract follows Framo's delivery and operation of the world's first subsea multiphase booster pump for Shell's Draugen Field (SMUBS) at 270 meters depth. This unit boosted a single well's oil production by more than 5,000 b/d. The system has not experienced shutdowns or malfunctions, and shows coorelation between expected and achieved production enhancement.

Installation, maintenance

The water injection valves, piping, and supports, together with the booster pumps, production valving, and piping, are integrated within a common skid base. This skid base is further integrated within the water flood manifold.

The basic maintenance philosophy for ETAP is "maintenance-through-replacement." Pump installation and maintenance will be carried out from a light intervention vessel. A complete spare booster pump is available for replacement of either of the two 50% units in operation.

Change-out of the retrievable pump unit is very simple and is carried out with a running tool. Both booster pump units are individually diverless retrievable by a remote operated running tool guided by guide wires and guide posts. The installation and retrieval procedure is here outlined:

1. Install guidepost with guide-wires on pump module.
2. Load the retrievable package (pump) into the running tool on the light intervention vessel.
3. Check all hydraulic interface connections on running tool.
4. Deploy running tool with retrievable pump.
5. Land out pump unit on pump support structure.
6. Activate hydraulic actuated lock of all collet connectors.
7. Perform leakage test of metal seals for the collet connectors.
8. Unlock running tool from interface connector on top of the pump.
9. Pull the empty running tool.

The water injection (WI) valve module is locked to the skid base such that all valves and chokes are retrievable should maintenance be required. Retrieval is performed by divers.

Valves and monitoring equipment are controlled from the main SCM located on the water flood manifold. The interface is in a common electric/hydraulic stab plate.

Multiphase meters

BP ETAP will rely for both Machar and Monan Field on subsea multiphase flow meters for reservoir management, instead of the more traditional dedicated testlines. The BP ETAP multiphase flow meters will be installed at 91.4 meters water depth on the Monan field and at 84.5 meters water depth on Machar field.

Oil, gas and water flowrates in addition to pressure and temperature for the unprocessed well fluid will be measured before it flows to the Marnock platform. Subsea metering was found by BP to be the method of the optimum well testing.

The meter's adaptability to varying flow regimes and gas volume fractions is ensured by the mixer, which homogenizes the multiphase flow, thus simplifying the metering operation.

BP, Marathon, and Western Mining Corp. selected the subsea multiphase meter design which utilizes a barrel-styled meter with an insert cartridge incorporating all active instruments. The insert is locked into the barrel and equipped with metal seals for full pressure integrity.

The receiver barrel with the integrated mixer is permanently installed on the subsea structure and includes no active elements. There are no requirements for straight pipe lengths upstream or downstream the meter. The receiver barrel also serves as the inlet and outlet housing and functions as a guide and support for installation of the insert cartridge.

Data transmission to the surface installation is via a RS-422 Modbus RTU link through the production control system pods.

Operating conditions for the flow meters are as follows:

• WC at actual condition - 0-90%
• GVF at actual condition - 41-99%
• Total flow rate - up to 1500 cu meters/hour
• Design pressure - 230 bar
• Flow regimes - all (the mixer or homogenizer makes the meter independent of any flow regime).

The ETAP multiphase flow meters are provided with facilities for taking subsea liquid samples. The liquid sample is taken out from the bottom of the flow mixer compartment, which always will be dominated by liquid. A combination of slop and sampling bottles are connected to a ROV.

The sampling valves are mounted on the panel of the insert cartridge. The bottle is filled by opening the ROV operated sampling valve. When the valve is closed, the bottle is released and brought to the surface by the ROV, where complete liquid samples (oil and produced water) can be analyzed with standard laboratory equipment.

The multiphase meter is field-proven and fully commercialized. The first three subsea meters in the world are operating successfully: one in Marathon's West Brae Field and two in the East Spar Field off Australia. The ETAP meters will be in operation by spring next year.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.