The Power Buoy was designed for North Sea operations with a water depth of 482 ft (147 m) and a survival wave of 75.5 ft (23 m), but the system is suitable for greater depths and has been validated for wave heights up to 98 ft (30 m). (Images courtesy of Ocean Resource)

The Power Buoy, which was developed for a specific early production application, can be used for subsea field control, remote pigging, multiphase pumping, chemical injection, subsea production support, and for the provision or distribution of large-scale power. It provides medium voltage electrical power (3.3 kV) to operate up to eight subsea electric submersible pumps (ESPs) simultaneously in eight discrete subsea wells, transmitting produced oil back to an existing platform facility approximately 8.7 miles (14 km) from the subsea development itself. The power buoy can be used to boost fluid pressure in depleting fields either by powering and controlling remote electric submersible pumps (ESPs) or subsea water injection equipment.

This buoy can generate and distribute up to 4.1 MW of electrical power with five units running (with one unit rated spare). This power can be used to provide a standalone continuous power supply to existing offshore facilities (including existing offshore platforms or semisubmersible units lacking sufficient power for upgraded applications) or directly to provide controlled power to subsea equipment such as ESPs or other production or separation equipment. As configured, it can also be used for storage system heating, monitoring and conditioning and remote water injection.

The power buoy system comprises a vertically aligned cylindrical floating buoyant structure anchored under buoyant tension to a reinforced concrete gravity base structure (GBS). The buoyant structure is designed to provide high stability with a secure, spacious, and maintenance-friendly environment in which to locate and maintain various pieces of equipment below sea level.

The buoy shape forms a column or spar that is narrow at the surface with a 32.8-ft (10-m) wide multi-compartmented buoyant chamber below sea level.

A central access shaft (CAS) runs from the boat landing deck approximately 19.7 ft (6 m) above lowest astronomical tide (LAT) to a multi-compartmented equipment chamber located between -29.5 ft to -82 ft (-9 to -25 m) LAT. Access to the surface structure is by purpose-designed rigid inflatable boat or dynamically compensated gangway. Experience has shown that boarding can generally be achieved in conditions equivalent to an 8.2-ft (2.5 m) significant sea.

Boarding can be achieved at three positions, accommodating all weather directions. The CAS provides access to and throughout the multi-compartmented equipment chamber. A watertight door at the boat deck level provides entry to the CAS.

A tubular mast section projects from the boat landing deck to a height of 95 ft (29 m) LAT and provides support for antenna, air intakes/exhausts, fluid tank vents, and navigation equipment well above the maximum wave crest height.

Layout

The multi-compartmented equipment chamber is split into five discrete levels. Deck 1 is the control and low-voltage electrical distribution deck. This level houses the main control and communications systems along with low-voltage electrical distribution and bulk consumables storage. The deck is divided into three watertight compartments.

Deck 2 is the first combined generator deck and houses three 820 kW diesel-driven generators and ancillary systems (cooling, batteries, silencers, filtration etc.). The third deck is outfitted much like Deck 2 and houses additional generators.

The variable speed drives (VSDs) are on Deck 4. This level houses eight working and one spare VSD for powering and controlling the eight subsea ESPs. A medium voltage switchgear is also on this level.

Deck 5 is the diesel storage deck. It houses diesel transfer pumps and coalescers as well as an emergency diesel fuel supply suitable for 10 days of operation.
Entry to each deck level and between the compartmented areas within each deck is via watertight doors from the CAS.
Power and control umbilical lines, together with fuel resupply, are routed into the power buoy via an integrated I-tube that leads to a termination area above the highest mean water level.

GBS foundation

The buoy is anchored under buoyant tension by eight galvanized spiral steel strand tethers connected to a 3,000-metric-ton GBS on the seabed. The base comprises a robust reinforced concrete cellular structure designed as a self-floating unit. The structure is divided into five discrete cell groups that can be independently ballasted to control the descent of the base to the seabed. The base can also be deballasted to enable it to be removed and recovered at the end of its operational life. The GBS is fitted with tether connecting boxes to allow the dual redundant tether system to be automatically connected. And a steel skirt can be fitted to accommodate poor soil conditions.

The GBS can also be used to mount umbilical termination units to provide for the distribution of power or control systems at the seabed.

Operating mode

The power buoy operates autonomously, requiring only regular maintenance visits at intervals of three or four months. It is expected that operational experience will enable this maintenance period to be extended.

Onboard processing enables a degree of executive action to be locally implemented to maintain the correct functioning of all on-board systems. Remote monitoring and data acquisition are routed via both line-of-site and satellite links to provide fail-safe communications. All main control systems are designed to Safety Integrity Level 3.

Autonomous processing enables pre-determined operational patterns to be incorporated. For example, continued operation and power production can be set to operate for any designated period subject to the maintenance of key systems within preset parameters.

Vital parameters are continually monitored and reported to a control panel at the host control station either on an offshore platform or onshore to ensure trouble-free operation. Operational availability is estimated at 99.97%.

Offshore installation and design

The GBS is a self-floating unit and is towed to site then water ballasted and positioned by four anchor handlers. The buoy is towed to site as a self-floating unit and is installed to the GBS using a ballastable sinker and pull-down system. The tether system is designed to be remotely installed using ROV-operated fittings.

The power buoy is capable of operating in any sea conditions. Although designed for a North Sea application with a water depth of 482 ft (147 m) and a survival wave of 75.5 ft (23 m), it is suitable for greater depths and has been validated for wave heights up to 98 ft (30 m). Studies have been implemented for a major oil company for water depths to 6,562 ft (2,000 m).

Extensive model testing has been carried out for a variety of conditions at Qinetic’s Large Wave Flume at Haslar in Portsmouth, UK. Testing has included in-place operation, towing, transportation, and installation. The results of these tests were used to validate and correlate the computer analysis conducted to estimate wave loadings and motions.