The subsea pump system has added a new level of technical sophistication to King and the Gulf of Mexico as a whole. (Image courtesy of BP King)

In December 2007, BP began operating its record-breaking subsea system at its King field development in Mississippi Canyon Block 84 in the US Gulf of Mexico. This significant development holds the potential to increase recovery of oil from deepwater fields.

King field consists of three wells, King D5 and D6, producing since 2001 and King West, D3, producing since 2003. All three wells are wholly owned and operated by BP. Located in water depths ranging from 5,000 to 5,600 ft (1,525 to 1,708 m), these wells are produced back to the Marlin tension-leg platform.

Previous to the King field, the deepest subsea pump installation was in 3,000 ft of water. The farthest installation from its host facility was previously six miles. Overcoming a sizable technology gap that challenged the delivery of the project, subsea pumps designed to operate in more than 5,500 ft of water (qualified to 9,850 ft) were installed 16 miles (25 km) away from their host facility — the longest ever step-out for a variable speed drive.

In 2006, prior to its recent subsea installation, the operator installed a new wellhead-mounted array creating a junction for enhanced well servicing. Known as the Multiple Application Re-injection System (MARS), the system is comparable to a “USB port” for the wellhead. It allows production optimization systems to be mounted directly onto Christmas trees.

The MARS system eliminates the need for de-oiling the King pipeline, removes the need for retrieving existing jumpers and reduces the risk of hydrocarbon leakage. The system also reduces the well shut-in period during installation and commissioning.

Multiphase pumping

As a field is produced, the reservoir pressure naturally drops, causing production to progressively fall. Subsea boosting can reduce the backpressure on the wells, potentially increasing the flow and improving recovery.

According to the operator, the recently installed pumps were designed at a time when King’s wells were still flowing under their own pressure. It was known from the outset that they would decline each year and eventually require some form of boosting. Subsea pumping was chosen to accomplish this after a rigorous selection process.

Pumping also increases the distance over which the well stream can be transported, which extends potential step-out distances. Although the technology is applicable in all water depths, it offers special benefits in deep water where alternative solutions are limited.

Production handles three phases — oil, water and gas (with little or no water production). The multiphase pumps were designed to handle all three fluids and also to be sand tolerant. They are designed to pump 75,000 b/d with a 725 psi differential pressure. Inlet pressures at the King wells range from 696 to 1,841.5 psi and should show a significant improvement of both early flow rates and reserve recovery.

The system’s manifold is set on a suction pile. Both multiphase twin screw pump modules, pump station subsea control module and all flying leads are separately retrievable. The system is pressure rated to 5,000 psi, and can withstand temperatures from -20 to 180ºF (-28? to 84?C). The equipment weighs 88.5 tons, with a footprint of 31 ft (9 m) long by 12 ft (3.5 m) width at a height of 23 ft (8 m).

Powering up

With a pump throughput of 330 cf/hr the system requires 1.3 MW per pump delivered at 6.6 kV. The operator admits that of all the physical constraints on the platform, the most notable is delivering power.

This project required the development of several new technologies and techniques including a long distance, high-voltage distribution system capable of controlling multiple pumps at different speeds using advanced variable-speed drive systems. This was accomplished by designing umbilicals that avoided “cross talk” or interference. It also required the use of simplified 12 kV remote operated vehicle (ROV) connectors to increase reliability.

The umbilicals used to link the pumps to the distant control platform were developed as self-contained delivery lines for hydraulic fluids, chemical feeds and lubrication oil to reduce weight and cost. They were designed to house a new fiber-optic communications system to ensure high speed pump response in respect to speed and lubricant oil pressure.

The 16-mile long, single-service umbilical is unique to King field. The concept was conceived when the operator encountered cost and platform tie-in limitations with only one basket available. The umbilical was designed to overcome several challenges including improved communication speed and workable high-voltage connections suitable for ROVs.

Like many of the industry’s subsea umbilicals, it contains standard elements like super duplex stainless steel tubes for transporting hydraulic fluid for the operation of valves and other equipment on the pump modules, lubricating oil top-up and chemical lines, low voltage electrical cables to power the subsea control system and fiber optic cables for transmitting control signals to the pumps and receiving information.

The modules contain an advanced condition-monitoring system to check continuously on the behavior and status of numerous components. The umbilical also contains six high voltage electrical cores, rated to 24 kV, which provide three-phase supplies to the pumps — when the umbilical reaches the first pump it “splits” and reduces in size to carry the services to the second pump, some 2.5 miles (4 km) away.

In 2008, the operator plans to drill a new well 1.5 miles (2.4 km) from the pumps (King south) bringing it on stream late 2009. Other tie-ins are likely.

Next steps

The subsea pump system has added a new level of technical sophistication to King and the Gulf of Mexico as a whole. The operator is working to advance technology in areas such as imaging and reservoir interpretation. New techniques will continue to increase industry knowledge of the subsurface.

The company also wants to develop improvements for deep wells, which require more complex drilling, completion and well designs. Other needs include host facility modifications that can deal with ever increasing riser weights and the need to handle increasingly high pressure/high temperatures at the topside and, in areas applicable to the King Subsea system, using technology to overcome pressure reduction and problems with flow assurance.