Offshore wind

Offshore Wind Farm Integrated Into Subsea Field Development

A system was designed to extend the offshore wind-energy concept from the power grid to a subsea field-development application. The system integrates a floating foundation with a wind-turbine generator, with all the required utilities hosted directly onboard the same floater.

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Typical floater arrangement with basket, tendons, and substructure. Credit: OTC 30721.

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.

Introduction

Saipem launched an initiative aimed at finding a solution for the 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 that supports wind-turbine tower and provides necessary floatability. The substructure is connected by tendons to a basket counterweight filled with solid ballast that provides 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.

Solution Description

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, accounting for up to 80% of the cost of a subsea control umbilical, 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. Therefore, wind persistence affects only diesel storage size or tank-refilling frequency.

Field 1: Small-Power Production. The maximum power demand is 1.2 MW, 40 km from the floating production facility. For this small-power-production case, 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, based on a preliminary mooring-design analysis.

Floater-Function Configuration

The following two configurations have been retained for further study:

  • All functions on a single floater—When more than one floater is needed, this means that 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—For this configuration, 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 if wind is insufficient.

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. Depending on power demand, 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.

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Fig. 1—System application on Field 1. SDU = subsea distribution unit; WI = water injector.
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Fig. 2—System application on Field 2. SDU = subsea distribution unit; WI = water injector; GI = gas injector.

System Electrical-Distribution Architecture

The wind turbines generate power at 690 V, and the current is typically transformed to either 33 or 66 kV in order to perform an economic 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. Fiber-optic cables embedded in the interarray power cables between each pair of floaters are used to connect all turbines in an ethernet network with the power-management system in the main FWT.

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 standalone capability, the system must allow the remote supervision of the wind farm and of the subsea-production-control system by the operator in the main production control room. For that purpose, the system design includes a radio datalink to implement a wireless network between the main offshore floating wind turbine and the production facility.


This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30721, Floating Offshore Wind Farm Integrated Into Subsea Field Development: Case Study of the Saipem Windstream Concept, by B. Mauries, G. Arcangeletti, SPE, and C. Colmard, Saipem, et al., prepared for the 2020 Offshore Technology Conference, originally scheduled to be held in Houston, 4–7 May. The paper has not been peer reviewed.