Company Integrates Offshore Windfarm Into Subsea Field Development
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Operators are moving active production and injection equipment onto the seabed with the aim of reducing capital expenditures (CAPEX) or topside space requirements. Moreover, they want to minimize new production floating facilities. Given this scenario, overall electric power needs may become an issue because of the extra power demand caused by the increasing number of electric consumers placed subsea. The complete paper discusses a floating wind-turbine solution that is particularly cost-competitive for deepwater locations and that can unlock the possibility of deploying large wind-powered generators far from the coastline in deep water.
Saipem launched an initiative aimed at finding a solution for management of subsea field power demand bearing in mind two primary considerations:
- Minimize CAPEX by reducing the distance between the subsea-field production location and the topside equipment supporting this production
- Decarbonize the field by adopting a renewable energy source
Concept Background and Potential Application
The operator has developed a floating substructure technology for offshore wind farms known as HexaFloat. This concept uses a minimal floating hexagonal tubular substructure supporting wind-turbine tower and providing necessary floatability. The substructure is connected by tendons to a basket counterweight filled with solid ballast providing stability with pendulum-restoring forces. The assembly of the basket and the substructure behaves as a rigid body if all tendons are loaded.
This assembly provides flotation with excellent stability thanks to the distance between the center of gravity and the center of buoyancy. Because this stability is provided by weight, large hydrostatic stiffness is unnecessary. As a result, only the central cylinder is exposed to the wave energy. The whole floating system can be anchored with three to six low-tension mooring lines, depending on environmental conditions.
The Windstream system that uses the previously described technology is a proprietary solution under development. It is designed to extend the offshore wind-energy concept from the power grid to subsea field-development application. The system consists of the integration of the floating foundation with a properly sized wind-turbine generator, with all the required utilities hosted directly onboard the same floater.
A floating power-generation facility based on a wind turbine installed in the proximity of subsea production facilities is a way to develop the following advantages for long-tieback field developments:
- Reduction of required power generation capability and footprint onboard the floating production, storage, and offloading (FPSO) vessel or platform
- Reduction of umbilical and cable length for subsea production and processing facilities
- Reduction or elimination, in certain specific long-tieback scenarios, of subsea power distribution equipment
Because hydraulic and chemical lines are among the most-burdensome components of subsea control-system distribution CAPEX, a further benefit can be achieved by equipping the floating power-generation facility with chemical-injection skids, thus reducing the length of these lines.
System Application to Subsea Field Development
In an operator study, the system has been applied to an oil field that is a long tieback to existing facilities (Field 1), and an oil field combined with subsea processing with high power demand (Field 2). To ensure continuous power availability, power storage or backup power generation can be considered. For short-term applications, backup diesel power generators have been considered in the study.
Field 1: Small Power Production. The maximum power demand is 1.2 MW, 40 km from the floating production facility. A single wind turbine of 1- to 2-MW production capacity is considered to power the subsea equipment. In the event that only control systems are in operation, solar panels all around the platform can be used to provide the needed power for such low-power-consumption control-system loads.
Field 2: Large Power Production. The maximum power demand is 16 MW, located 10 km from the floating production facility, with many umbilicals attached to the FPSO. For the large-power-production case, a minimum of two floaters, with wind-turbine size up to 12-MW production capacity each, are considered to match the required power demand. The distance between each floater to be considered for the study is 3.5 km.
The following two configurations have been retained for further study:
- All functions on a single floater—When more than one floater is needed, the other floaters will be standard Hexafloat units, hosting only the wind turbine generator. This configuration limits design modifications to a single unit.
- Functions split on various floaters—One floater will gather the chemical-storage and injection skid and subsea power and control equipment. Backup power production and associated diesel storage will be split between each floater of the configuration so that each floater is able to provide its own backup power.
After evaluation of these configurations, field schematics for Fields 1 and 2 were determined and are shown in Figs. 1 and 2, respectively.
Chemical products are stored at the bottom of the central shaft and are segregated from the electrical rooms located at the top of the shaft. An upper platform is dedicated to the laydown area and to backup-power production. Diesel storage can be located either on the platform or at the bottom of the shaft below the other chemical products. A crane is located on the platform to ensure lifting operations from supply boats for refilling and maintenance operations.
System Electrical-Distribution Architecture
The wind turbines generate power at 690 V, and the current is typically transformed to either 33 or 66 kV to perform a power transfer through the interarray cable. All emergency-shutdown functions related to safety aspects are assumed to be performed by safety systems coordinated with that of the wind turbine generator.
The architecture of the power-distribution system accords with the following key drivers:
- Unmanned floater
- Minimize amount of equipment and its weight and dimensions
- Minimize heat generation
- Maximize independent operability of subsystems
- Maximize independent retrievability of subsystems
- Minimize subsea equipment
- Segregation of all equipment affected by the risk of explosion
The buoyancy modules, properly distributed along the cables in a W shape, reduce the dynamic coupling of the cable with the floaters and result in a better dynamic response and reduced loads and fatigue.
System Control Architecture
The unit may be located very far from the main production facility and, therefore, is designed as a standalone system not relying on a control cable connection with a production facility. Thus, the floater incorporates a power-management system, an electrical-network-management system, and a programmable logic controller for supervision of the entire power plant and the master control station (MCS) that monitors the subsea-production-control system.
When the power demand requires more than one floating wind turbine (FWT), one will be designated as the main FWT and will host, in addition to other control equipment, the switchboards to collect, receive, and isolate the electrical power from other FWTs.
The MCS is connected to the subsea hub by means of a dual redundant link in the dynamic umbilical that provides the power and chemicals to subsea users.
System Remote Monitoring and Control
In addition to its stand-alone capability, the system must allow the remote supervision of the windfarm and of the subsea-production-control system by the operator in the main production control room. The system design includes a radio data link to implement a wireless network between the main offshore floating wind turbine and the production facility.
Company Integrates Offshore Windfarm Into Subsea Field Development
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