Described by the European Commission in 2008 as ‘the energy of the future’, the resulting growth in the offshore wind industry helps to ensure security in Europe’s future energy and a transition into a low carbon economy. Weather however remains a major risk to offshore operations and knowledge of site conditions assists in determining the optimum times during which certain procedures can be performed safely. While advances in technology and industry maturity will make offshore wind an increasingly attractive investment, the industry is always in need for solutions which reduce costs and increase operational efficiency. Since 1996, RADAC has therefore been developing the WaveGuide: a very accurate radar based system to measure the sea state, vital in planning operational and development activities in the offshore environments.
Wave and tide data are of great importance during all stages of offshore wind farm development and operation as well as many other sectors. Vessels, such as those used for jacking operations, crane operations and towing, rely on real-time wave data. Their operations are typically limited by a maxi- mum wave height of two meters or less. Therefore reliable and real-time data are essential for maximized exploitation of weather windows and improved safety during these offshore activities.
From the development phase onwards, wave data is required to calculate the wave load in order to define the requirements of the platform design. In addition, during its operational life, the actual wave climate is monitored to validate the expected endurance models and justify the lengthening or shortening for the platforms lifetime. Accurate analysis of the possible exploitation period can make an enormous difference in cost.
Over the past 40 years the standard method for obtaining wave information is by the use of wave directional buoys. The main advantage of using a buoy is its independence from sup- port structures. However the disadvantages of this method include the high maintenance costs and the risk of long periods without measurements when a buoy breaks from its mooring or runs out of power. Buy based system are also extremely expensive to deploy and recover for servicing.
Down-looking Frequency-Modulated Continuous Wave (FMCW) radars for wave measurements have been used for 25 years. The popularity of these systems have grown as prices for FMCW radars have fallen and the increased use in the field have proven them to be a very robust and low maintenance alternative to buoy systems. As such, they are slowly becoming a preferred method for collecting environmental data, and are now more widely applied for water level, tide, harbor oscillations and wave direction and height at sea.
Image courtesy: Planet Ocean
The WaveGuide
RADAC supplies systems for remote monitoring waves and tides, including wind farms and installation vessels. The RADAC WaveGuide is an accurate radar system introduced in 1996, incorporating advanced technologies that make it an easy to use, reliable and robust device for measurement of level, tide and waves up to the most extreme conditions.
The WaveGuide radar measures the distance to the water sur- face several times per second. Mounted high above the water with no moving parts, preventive maintenance or cleaning is not required. The system is compact and easy to install, suited to accurately measure wave direction, heights and tides in all weather conditions. Recalibration is also never required due to the long-term stable zero reference.
The WaveGuide control system (data server) is a small low powered module that can be located distant from the sensor head and which facilitates commissioning, remote servicing, data collection, data processing, presentation and logging. It incorporates an internal web-server so that any web enabled device connected to the same network can access the wave data without the requirement for special software or hard- ware.
The WaveGuide comes as standard in four different models, differing in functionality from simple water level to a system which measures all directional wave parameters.
Wave Direction
RADAC’s showpiece is the WaveGuide Direction. The direction- al system consists of three downward-pointing radars. With an array of three radars, the elevations of the sea surface is measured at three positions. Knowing the slopes and the phase relations, the directional spectrum can be accurately calculated. RADAC is the only company that brings this technology to the market.
RADAC has conducted field trials at Prinses Amaliawindpark, 25 kilometers off the Dutch coast. Two versions of the Directional WaveGuide were compared to a Directional Waverider buoy over a period spanning several months. The field trials successfully indicated no statistical difference in the direction- al information from the buoy and from the WaveGuide radars.
Therefore, the Directional WaveGuide can not only accurately and reliably measure wave direction, but the high quality data is achieved without the hassle of service operations and break away buoys.
Onboard
The same technology can be applied directly to vessels to aid in their operations. The WaveGuide Onboard installation compensates for the vertical motions of the ship, so it measures the waves the ship actually has to endure. A significant ad- vantage of the WaveGuide Onboard is the continuous availability of wave data, especially at night, in rough seas and under heavy weather conditions with limited visibility.
Additionally, the WaveGuide Onboard measures the waves as they are encountered by the ship itself. This data is ideally used to determine the operational limitations of the vessel in harsh conditions. Therefore, a vessel in transit can adjust its speed and course to avoid critical headings. This not only helps to identify safe heading and speed, but also monitoring the actual wave load to determine the lifetime of the vessel.
As the distances measured have to compensate for the vertical motions of the radar itself, a motion sensor is incorporated into the sensor unit. The WaveGuide Data Server takes care of synchronization, collection and compensation of the data collected. The compensated radar signal is processed in the same way as the standard WaveGuide mounted on a fixed platform. Using the available “Wave and Tide Processing Software” raw data, spectra and parameters can be requested and presented on the web browser.
It has been and will be important to warn for flood risk due to storm surge, especially as a result of the rise in sea levels. Additionally, it is of great importance for coastal defense and harbors to monitor actual stress levels on dikes, dunes and break waters, not only to determine construction requirements but also to monitor changes in wave climate. Real-time measurements are also useful for safe vessel guidance and (un)loading of ships.
Additionally, collecting data provides insight into natural phenomena and patterns. This aids in predicting water levels in the future, increasing peace of mind. Especially where the river flows into the sea, extremely high water levels during storm conditions can cause flooding.
Besides the danger of flooding, the lack of water is equally important. In those situations water level information is crucial to water distribution planning. River water level data is essential for shipping as it influences safe passage of vessels (sea gauge/river clearance).
In areas such as coastlines, harbors and deltas, various natural phenomena occur including waves, tides, storm surge, tsunami and harbor oscillations. Despite this diversity, all are accurately measured by the WaveGuide, without interfering with harbor traffic.
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