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Feature Article

Developing a Buoy-Based Offshore Wind Resource Assessment System

AUTHOR:

Graham Howe



WindSentinel annotated diagram.

The cost of building an offshore wind farm is substantial, and the industry is currently focused on ways to reduce it. One area that is presently receiving a great deal of attention is wind resource assessment. Offshore wind farm developers need to gather the wind resource data required to support the financing and development of their projects. The current standard approach onshore is to build a meteorological mast equipped with anemometers; however, offshore this solution faces numerous challenges: The current cost of an offshore met mast for a U.K. Round 3 project is estimated at around €10 million—approximately $15 million; the time taken to receive permitting can range from months to years; and a met mast can only be redeployed to another location at a significant cost, if at all.

The solution to these challenges is to build a movable and reusable remote sensing platform capable of collecting wind data remotely in the harshest ocean environments on a 24/7 basis at heights up to 200 meters. In order to achieve this, the current solution is to deploy a floating lidar. However, these sensors offer their own technical challenges.


Lidar
Lidar operates by emitting timed sequences of laser beams that hit airborne particles (aerosols). The sensor reads the back scattered light from these impacts and deduces wind speed and direction from a combination of the Doppler shift in the laser and the relation between the readings from the sequence of lasers.

Onshore, there are two major commercial lidar on the market: the Zephyr and the Windcube.

The Zephyr is a conical sweep sensor, with each volume of air measured by a sweep that takes 50 readings in a second. In order to take the next height measurement, the lidar is moved up, so it can take 10 seconds to retrieve a complete set of data over 10 range gates.

The Windcube is a sequential pulse system that collects data by a series of pulses over four vectors. Each series of pulses takes 4.5 seconds to gather the data for a single volume of air over up to 10 range gates.

Both of these approaches offer challenges when the lidar is deployed on a buoy, given the angular motion of the platform that will impact the quality of the data gathered.


WindSentinel
In 2008, AXYS Technologies (Sidney, Canada) set out to develop the WindSentinel, a system that would address these challenges.

Three major components make up the WindSentinel. The first is the Vindicator III Lidar manufactured by Optical Air Data Systems (OADS) of Manassas, Virginia. Developed from technology originally applied to emergency landing for helicopters in low-visibility desert conditions, the Vindicator pulses three lasers simultaneously at 1,000 times a second, averaged down to one second; no other commercially available lidar can do this. Each one-second reading is an engineering unit, representing a single volume of air at six range gate heights. This simultaneous pulse feature enables the Vindicator to correct algorithmically for motion based on a single motion calculation compared to conical sweep or sequentially pulsed lidar.

The second component is the NOMAD Buoy Platform. The NOMAD (Navy Oceanographic Meteorological Automatic Device) has been successfully deployed worldwide for more than 50 years. It withstands extreme marine weather, with low operational costs. The NOMAD 6-meter hull was originally designed for the U.S. Navy’s offshore data collection program. The AXYS NOMAD, used as the platform for the WindSentinel, is a modified version of this proven design. To date, NOAA reports no capsizing of NOMAD hulls.

Lidar requires a significant power supply, typically around 100 to 120 watts in standard operation and up to 400 watts in extreme cold. The WindSentinel power supply has been specifically engineered to support long-term remote deployments of up to a year at this level, with minimal service trips.

Sealed lead acid rechargeable batteries power the WindSentinel. These batteries are charged by a wind generator and two 210-watt solar panels backed up by a high-efficiency diesel generator for times of extended lulls in the wind. In this configuration, the WindSentinel can support the typical system power draw of 135 watts and, in extremes, can support up to 500 watts. With this capacity, the WindSentinel is capable of meeting its goal of long-term remote deployments. To continue this article please click here.


Graham Howe is accountable for international business development, renewable energy for AXYS Technologies. His primary focus is the introduction of the WindSentinel offshore wind resource assessment system to the commercial market. To date, Howe has worked on the construction, delivery and deployment of four WindSentinel projects and has an in-depth understanding of both client requirements from systems of this type and the details of system implementation and validation. Howe represents AXYS at an international level in the development of standards for floating lidar devices, most notably with the IEA Task 32 project team.



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