The Brenda/Nicol multipurpose field development solution package is poised to take the plunge into 492-ft (150-m) of water in the UKCS Block 15/25a. (Photo courtesy Framo Engineering)

As water depths have increased so has the capital cost of platforms and facilities. The benefits of tying back new fields without having to install new structures have been understood by the industry for decades: massively reduced CAPEX and fast-track development. The technology to overcome these issues has had sporadic growth over the years with the key areas being control systems and flow assurance.

In almost every case, subsea production systems are highly customized for the particular field development and reservoir. In some situations, customization goes down to the individual well level. This has resulted in many different system configurations, which are expensive and difficult to support.
Implementing a complete system might involve contracting with a number of suppliers. Although each company’s equipment performs its task adequately, integration with
the other providers’ equipment is often problematic.

An eye to the future

To bring value to independent operators, both the operators and the service providers have to think beyond today’s problems — they had to develop a solution that would last for the life of the field and meet the operator’s specific project requirements. Designs have to anticipate changing production conditions, and solutions have to be flexible and adaptable. Life-of-the-field solutions must be reliable in the long term to minimize deferred production and downtime.
Experience has shown that planning for the future pays off. The challenge facing an independent operator is to design a cost-effective subsea field development system that can perform today and be prepared for tomorrow.

Multifield control

Brenda and Nicol are two adjacent satellite fields lying about 138 miles (222 km) northeast of Aberdeen. Both fields are operated by OILEXCO North Sea, Ltd. Brenda was developed using four horizontal wells tied back 5.9 miles (9.5 km) to the Balmoral floating production vessel in UKCS Block 15/25a, also operated by OILEXCO. Nicol is produced by a single well tied back 6.2 miles (10 km) to the Brenda manifold (Figure 1).

In planning development of the Brenda/Nicol asset, OILEXCO identified several features that would optimize development and production economics:
• Design the system for six wells, scalable to eight as a hedge against future development;
• Design the system to allow low-cost tie-ins of future adjacent field developments;
• Design-in artificial lift capability to help kick off production and provide sustained lifting assistance over the asset’s life;
• Design a production system capable of high rates at high water cuts;
• Build-in the capability to perform 24 hr/d production metering and monitoring at the manifold;
• Anticipate and prepare for possible sand production at some time during the life of the wells; and
• Design for possible future intelligent well system monitoring and controls as well as chemical injection provisions for the inhibition of hydrates and scale.

In addition to control and monitoring of the production flow at each individual tree and manifold, the system had to provide artificial lift during the field life. The selection process was methodical and thorough. Both the overall technical solution and various artificial lift options were studied including electrical submersible pumps, a seabed multiphase pump, wellbore gas lift and combined gas lift with a multiphase pump. Evaluation criteria addressed topsides footprint, cost, capacity, and reliability, as well as safety and environmental impact with the combination of gas lift with the multiphase pump option offering the best solution meeting all the initial criteria.

OILEXCO wanted to see if this ambitious design could be rendered and implemented by a single supplier. They saw the project as more of a system solution than an aggregation of technology, and seamless integration of all components was a priority. Working on an engineering, procurement and commissioning contract, Framo Engineering delivered an integrated, lightweight, subsea manifold, including pumps, meters and control systems for up to eight wells including remote tie-backs.

Multi-manifold

The single, multi-manifold system delivered was modular, with the following main components (Figure 2):
• Framo multiphase pump;
• Framo multiport selector manifold (MSM);
• PhaseWatcher multiphase flowmeter with Vx technology;
• Schlumberger subC-pod monitoring and control system;
• Power and control umbilical;
• Protection structure and base;
• Sand detector; and
• Chemical injection, chokes and gas lift metering.

All of the equipment and designs used in the solution were field proven and went through extensive reliability testing beyond normal subsea limits. In addition, the final acceptance test included a fully submersed wet test while running the subsea pump in multiphase mode.

Subsea surveillance system

The heart and brains of the system is the Schlumberger subC-pod monitoring and control system by which all communication between the Balmoral control room and the manifold components is routed (Figure 3). Use of this modular plug-and-play system simplifies the installation and reduces the complexity of interfacing the various components.

The flexible monitoring and control system consists of a single topside data hub on the Balmoral platform, which provides the link to a network of subsea electronic modules that monitor and control all the subsea production equipment in the field, including:
• Multiphase pump controls and monitoring sensors;
• Production tree valves;
• Manifold valves;
• Production and gas-lift chokes;
• Multiport selector manifold;
• Multiphase flowmeter and gas lift venturis; and
• Sand detector, pressure and temperature sensors.

In the Brenda field, six dual IWIS-compliant subC-pod modules were mounted in three subsea hydraulic control modules on the manifold to provide fully redundant monitoring and control. Multiplexed controls reduced the number of control and monitoring lines combined in the single 5.9 mile (9.5 km) umbilical. The Nicol control pods daisy chain onto Brenda via a dedicated 6.2 mile (10 km) Nicol umbilical, and the system has the capability to daisy chain further pods for additional potential tie backs and measurements.

Data transmission from the subsea modules to the topside control system is provided in real time by a high-speed (100-MB/s), redundant fiber-optic communications system, which is immune to the effects of electromagnetic interference from the multiphase pump high-voltage power lines in the umbilical.

The system’s open architecture provides transparent communications, independent of protocol, from seabed to surface for multiple sensors such as the multiphase flowmeter and sand detector including switchable 24Vdc power for each device. Since identical subsea hubs can be configured to provide different functions on the manifold, the operator has complete freedom in the selection of control equipment and sensors, eliminating the need for high-cost, project-specific solutions. This configuration flexibility also allows modules to be interchanged, reduces the expense of keeping spares, and enables complete redundancy. This improves overall reliability and availability of the field control system.

Dynamic relevant-time well measurements vital to production optimization are sent through the subC-pod to the host facility and delivered onshore via satellite to a Framo-hosted database in Norway. The operator’s subsea engineers and reservoir management team can view their production data in real time over the Internet from any location in the world.

Integrated manifold

The Framo manifold system encompasses the complete subsea production system for the Brenda and Nicol fields. It combines production flow from the connected wells to the production flow line (Figure 4). Additionally, the MSM has the ability to route flow from any single well through the multiphase flowmeter module in a seamless manner so that the well tests are representative of normal flowing conditions.

The multiphase flowmeter jointly developed by Schlumberger and Framo has the capability to measure accurately the widest range of phase fractions. With its ability to measure flow from each well individually via the MSM, the wells’ adjustable flow controls and gas lift valves can be fine-tuned using commands routed through the subC pod to optimize production from the reservoir.

The downstream, multiphase pump module boosts the combined flow from all wells back to the Balmoral facility. The variable speed pump can operate at any gas/liquid volume fraction, up to 98%. The combination of the pump and gas lift allows the field to operate at high water cuts and high flow rates using minimum lift gas. Dynamic production data acquired by the subC system will be used to optimize operation of the pump and by varying pump speed and gas lift pressures, pump efficiency and reliability can be maximized over the life of the asset.

“Future-proofing” the system

The multipurpose manifold and metering system make the subsea system very compact. CAPEX was considerably reduced through the reduction in tree control systems and centralizing of equipment. Production and total ultimate recovery from the fields is expected to be optimized as a result of continuous monitoring and production control.

Subsea multiphase metering is an excellent tool for production management, troubleshooting and, where applicable, production allocation. Since the Brenda system went online in May 2007, the sub-C-pod modules have had 100% uptime.

Beyond the immediate benefits of gaining full-field control and monitoring from a single system, the operator will realize long-term operational savings. The ability to add new sensors and tie production easily into the manifold effectively “future proofs” the Brenda field subsea system.