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Project Execution of Multiphase Boosting System in the Gulf of Mexico

Phase 3 layout at the Dalmation field. Source: OTC 29540.

In 2016, the Subsea Integration Alliance (SIA) was awarded the industry’s first deepwater integrated subsea engineering, procurement, construction, installation, and commissioning multiphase boosting-system contract. The scope of the contract called for the supply and installation of a subsea multiphase boosting system in the Dalmatian field in the Gulf of Mexico (GOM). This paper explains the application of the system in the field, and an approach to project delivery using fit-for-purpose solutions that ensured seamless delivery and installation for the operator.

Dalmatian Area Development

An exploration and production company has expanded the Dalmatian field to include a subsea boosting system at Well DC4 for enhanced oil recovery. The phased development project is in Viosca Knoll 786A and is tied back to a platform.

  • Phase 1, completed in 2014, included a 35-km oil-well tieback (DC4-1) and a 39-km gas-well tieback (DC48).

  • Phase 2, completed in 2015, included an 18-km extension to the initial 35-km flowline and catered for the addition of two wells (DC134-1 and DC134-2) and future potential for expansion.

  • Phase 3 included the subsea boosting system and was successfully installed and put into operation in October 2018 (image above).

Dalmatian Boosting System

The Dalmatian project is a brownfield development, extending the plateau production and increasing total recovery using subsea multiphase boosting. The project represents the world’s longest multiphase tieback by boosting at some 35 km; the boosting system is installed at approximately 6,000 ft water depth. The unprocessed wellstream is boosted back to the Petronius field compliant tower installed at a water depth of approximately 1,800 ft.

The system includes a 2.2-MW helicoaxial multiphase pump and is designed to maintain a steady reduced wellhead backpressure, increasing production rates significantly. This is performed through the tailor-made control system adjusting the pump speed with the topside adjustable speed drive (ASD).

System Design

The subsea layout comprises the flow base with the multiphase pump and the subsea umbilical termination assembly (SUTA). The SUTA is near the flow base, and the two are connected through electrical and hydraulic flying leads. The foundation for the Dalmatian boosting system is a mudmat design with skirts. The flow base comprises the inlet valve, outlet valve, and the piggable main bypass header with bypass valve. The retrievable pump module comprises the pump, flow mixer, flow splitter, and subsea control module.

Subsea Pump Description. The pump unit consists of a handling head, three electrical penetrators, a motor with barrier fluid cooling, pump impeller housing, barrier-fluid filter unit, lower barrier-fluid cooler and filter system, and a support plate. The recent development of the motor was a key enabler for the development of two recent long-distance tiebacks including subsea pumping. The projects are located in Australia and the GOM, respectively. In both projects, the motor reduced the hardware scope significantly and simplified associated systems, making them more robust and economically viable.

Umbilical System. The umbilical features an integrated pull-in head and hangoff system for topside installation and a SUTA installed on a separate SUTA mudmat. The multiphase pump (MPP) power and control umbilical is a 35.9-km-long static umbilical. The MPP umbilical consists of 11 steel tube lines combined into eight functional lines, one three-phase medium-voltage system for power to the pump, and one fiber-optic cable and low-voltage quad for signal and power to the subsea control module.

Topside System. The topside system consists of four main subsystems—the barrier-fluid system, the control-fluid system, the power system, and the control system.

The topside part of the barrier-fluid system consists of a hydraulic power unit (HPU), feed accumulators, and tubing. The control of the HPU is incorporated into the overall pump control logic and ensures that sufficient barrier fluid is supplied to the subsea pump motor. The topside part of the control-fluid system consists of an HPU, feed accumulators, and tubing. The control-fluid HPU feeds the subsea control module with the required control fluid to actuate subsea valves when required. The power system consists of an adjustable speed drive, input transformer from the grid power supply, and a step-up transformer. The transformer is required because of the long stepout length. The subsea pump motor is a high-voltage design; hence, a subsea transformer is not required. The control-system cabinets hold the pump-control logic, supplying low-voltage power and communicating with subsea instruments. The topside hangoff equipment connects the topside equipment with the umbilical elements.

Business Case

A key element in making subsea boosting projects like Dalmatian economically viable is reducing the overall supply scope and execution time. For marginal field developments, it is also imperative to have a full understanding of the effects of the boosting system on overall system performance. The system will not only generate increased pressure and flow; it will also affect the performance of the wells, flowlines, risers, and the topside processing system. For the Dalmatian field, a detailed front-end engineering-design study, including integrated asset modeling, was performed in close cooperation between the end user and the technology provider.

Delivery Strategy

Integrated Project Team. Achieving pump startup 23 months from contract award with zero significant incidents related to health, safety, and environment is testament to the success of delivering the project through an integrated alliance model. The alliance partners, working in collaboration with the operator, were able to demonstrate that technical, logistical, and installation risks could be greatly reduced. Risks typically associated with separate installation and equipment-supply contracts from the operator were reduced significantly because the interface is owned and managed by the alliance. Relinquishing this interface requires an element of trust from the operator, but provides an opportunity to improve project efficiencies.

Testing. After initial equipment testing, all integrations and interfaces were subject to verification and a stackup test was performed for subsea equipment. The installation contractor participated in this testing to verify planned subsea installation procedures and to obtain familiar experience with the equipment. During the system operation test, all main equipment was activated to verify system functionality. The client operators joined this session to acquire experience with the control system and human/machine interfaces. The testing phase ended with an angled stackup test between the pump module and the flow base.

Transport and Subsea Installation. The transport and installation of the subsea multiphase boosting system and umbilical tieback to the host facility was the responsibility of the installation contractor (IC). Transportation of the multiphase pump system was managed by the IC with the local support of an equipment provider in Bergen.

The first installation campaign was to install the pump foundation. The foundation was installed to the east of the Dalmatian South producing flowline. The foundation was allowed to settle for a period of 4 months before the second offshore campaign was conducted.

In the second campaign, the flowbase was installed onto the foundation and the umbilical termination mudmat installed adjacent to the foundation. A new messenger wire was installed at the host platform, allowing a dummy umbilical pullhead to be drifted through the J-tube.

The third offshore campaign was the installation of the 34-km-long power umbilical. Following the successful umbilical installation, the operator had to shut in the field to allow the existing rigid jumpers to be removed, after which the new rigid jumpers could be installed to allow production to flow initially through the system bypass and subsequently through the pump unit after installation and commissioning of the pump.

The final subsea scope activity for the IC was to hook up the pump system to the power umbilical through a series of flying leads installed from the pump unit to the umbilical termination assembly.

Installation of Topside Equipment. Installation of the topside equipment was the operator’s responsibility. The power and control building was installed approximately 4 months before the arrival of the main topside equipment. The priority was to have the barrier-fluid and control-fluid HPU operational before the umbilical pull-in. Another goal was to have the control system monitoring and controlling different fluid pressures. Finally, the power system needed to be ready before the pump-unit submersion.

Operational Experience

The performance of the Dalmatian subsea boosting system has exceeded expectations, with a production increase greater than 250% and a high facility uptime for the first months of operation. The MPP quickly reaches start speed, resulting in a suction-pressure drawdown. The control room operator then adjusts the pump setpoint to a new suction pressure. In steps, the desired minimum wellhead pressure is achieved to maximize production rates. The pump speed is automatically regulated to maintain the suction-pressure input, and the pump will maintain a steady optimized wellhead backpressure.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29540, “Dalmatian Subsea Boosting: Project Execution and Early Operational Experience From the First High-Boost Multiphase Boosting System Deployed in the Gulf of Mexico,” by Arill Småland Haglad, SPE, OneSubsea; Ross Cooper, Subsea 7; and Mike Clarke, Murphy, et al., prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed.

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