Selection of Instrumentation for Monitoring the

Sea State at an Offshore Wind Farm Site(1974 Words from Executive Summary to Conclusion)

April 2011LIST OF CONTENT

List of content

2Executive Summary

3

1. Introduction

4

2. Description of Site Condition

4

3. Why Monitor Sea State at Robin Riggs?

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4. Instrumentation for Monitoring Sea State

5

4.1 Accelerometer buoy

74.2 Wave radar

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4.3 Acoustic Doppler system (ADCP)

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5. Recommendation for Robin Riggs

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6. Conclusion

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References

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EXECUTIVE SUMMARYThis paper examines various instruments used to monitor sea states, with the intention of selecting the most appropriate for Robin Riggs offshore wind farm located in Solway Firth.

The site condition of the location of the wind farm was examined for information that would guide on choice of instrument. The merits and limitations of several instruments considered are noted, and compared with the site conditions and alternative instruments. Factors that guided decision include suitability for site condition, accuracy and reliability of data from instrument, cost, maintenance frequency, and adaptability for real-time monitoring.

Waverider buoy was recommended for the offshore wind farm. Apart from being suitable for the site, data from waverider buoys buoys are reliable, the instrument can be adapted for real-time monitoring, and maintenance frequency would be low if mooring is properly done and vessel movement in the area prohibited. While the instrument may be relatively expensive, the benefit during the wind farm operational life would justify the cost. 1. Introduction

A typical offshore wind structure consists of turbines above the water surface, and a support structure comprising of the foundation below water and a partially submerged tower. Environmental factors like waves, wind, current etc typically impact on construction, operations and maintenance of offshore facilities. The Robin Riggs offshore wind farm situated in the Solway firth is one of such wind farms in the UK. Construction of the facility was completed in April 2010. Periodic maintenance of the wind farm would however continue.This write-up looks at the challenge of accessing the turbines using marine vessels. Sea State varies continuously with time as wind condition changes. In subsequent sections, we would discuss the site condition at Solway Firth (the site of Robin Riggs Offshore wind farm), how sea state would affect accessibility of the turbines, instruments that can be used to monitor the continuously changing sea state. A particular instrument would be recommended with reasons.2. Description of the Site Condition

Robin Riggs is an estuary located in the Solway Firth, a sandbank midway between Galloway and Cambrian Coast. Mudflats and sandflats dominate the Firth. The distance of the Robin Riggs wind farm from shore is about 11km from Galloway Coastline and 13.5km from the Cumbrian coastline, and the wind farm occupies an area of 18km2. Most of the Solway Firth is less than 10m (30 ft) deep (Graduate School of Oceanography, 2011). The maximum water depth in the location around Robin Rigg Wind farm is 12m, while the minimum water depth could approach zero. An extremely dynamic environment lies below the surface. The Solway's funnel-like shape and shallow depth affect the tidal ebb and flow, creating strong currents. Occasionally, a tidal bore can develop at the head of the Firth, which sculpt the soft sediments into sand banks, sand bars, and deep channels (Graduate School of Oceanography, 2011). This could have impact on instruments moored to the sea-bed.Commercial shipping does not pass through the Robin Rigg area of the Solway. While small pleasure craft and fishing vessels may pass closer to the wind farm, there is nevertheless an exclusion zone of 50m around all of the wind farm structures. This assures of minimal obstruction of moored instruments by passing vessels.3. Why Monitor Sea State at Robin Riggs?In a bid to carry out maintenance works on the turbines, workboats are used to navigate the area. The water depth is very important as the vessel can run aground (leading to damage/wreckage) where the draft exceeds the water depth. Also, the elevation of the water might be important for technicians to be able to reach the turbine height. Sea state information is required to aid effective planning and to avoid negative situation that could be unsafe. What really is sea state, and what do we really need to measure?

Sea State is the appearance of a body of water at a certain location and instant. It is the state of waves formed mostly due to the action of wind and swell. Important parameters that describe the wave include wave height, wave period, wavelength and direction of the waves. However, the most important parameter to mariners is the significant wave height which is obtained from measured wave data.4. Instrumentation for monitoring sea stateMonitoring sea state is essentially regular observation and recording of wave parameters over a period of time. Data obtained would be useful for making decision on the feasibility of undertaking tasks at each instant in a marine environment. For Robin Riggs, it would aid decision making when planning turbine maintenance activities.Typical ways of obtaining data include:1. Visual Observation

2. Direct Measurement with instruments that are in contact with water. Examples include wave staff, pressure gauge, shipborne wave recorders, accelerometer buoys, Acoustic Doppler devices etc.

3. Remote sensed measuring using wave radar- it could be platform-mounted, satellite-mounted or aircraft-borne

4. Use of modelled data Data obtained from visual observation is subjective and yields data that is irregularly spaced in both space and time. It also cannot provide real-time data and the accuracy of measured data is low (American Society of Civil Engineers 1996). Modeled data also has its shortcoming. Wyatt (2009) notes that most modeled data err in underestimation of model wave height in swell dominated seas, and overestimation of model wave growth in fetch limited conditions. These are major inadequacies that make them inappropriate for use in Robin Riggs wind farm. Table 1 discusses other instruments we did not consider for Robin Riggs with reasons.

Table 1 Sea State Monitoring Techniques not Considered for Robin Riggs

Technique/InstrumentReason for not recommending

Visual Observation Accuracy is low

Not adaptable to real-time monitoring

Measurement not objective

Wave Staff Subject to marine fouling. A large quantity of mud and sand move with each tidal cycle; this would make marine fouling a major issue Strong current in the firth could damage or distort wave staff orientation

Much effort and time would be spent on maintenance

Pressure Gauge Subject to marine fouling. Huge quantity of mud and sand would frequently foul instrument Diver required to install and maintain the equipment

Much effort and time would be spent on maintenance

Ship-Bourne Wave Recorder Ships dont navigate the Robin Riggs site as there is a 50m exclusion zone around it.

Cannot provide real-time monitoring for the entire operational project life

Thus, to reliable data, we need to use instruments. Some important wave measuring instruments we considered for the Robin Rigg offshore wind farm include are discussed below.

4.1 Accelerometer BuoyHere, sea state observations are obtained with a floating buoy which is usually spherical in shape. Buoys are usually moored in water depths of less than 200m. The buoy tracks the wave motion (i.e. it moves up and down with the wave), and is thereby accelerated vertically. This acceleration is integrated twice to give the vertical displacement. The data is either stored in the buoy, or it is transmitted to a shore station either directly or via satellite link (Bendfeld et al 2009). Figure 1 illustrates the operation of a wave buoy.

Fig. 1 - Functional principles of the measuring buoy (Bendfeld et al 2009) Buoys are relatively simple to install and adaptable for real-time monitoring. Bendfeld, Krieger, and Splett (2010) note that a buoy is a very accurate device, though expensive in comparison to some other instruments. In view of the benefit obtained, it is a cost effective means of collecting data (Pandian et al 2010). Buoys have the disadvantages of measuring the waves indirectly, being prone to damage and/or loss due to collision, mooring failure, heavy storm surges etc.

4.2 Wave Radar

Radar sensing techniques are all remote sensing techniques and so are not in direct contact with the waters surface. They can either be platform mounted, airborne or satellite mounted. Wave Radar has advantages in terms of potential wide area coverage, and the data can be processed in near real-time. HF Radar was considered for the wind farm. It is a shore based remote sensing system that has the ability to remotely sense environmental parameters over the sea beyond the horizon limit associated with conventional radar systems. Aucott (2002) notes that HF Radar installations are expensive to set up compared to wave rider buoys. After this initial cost however, the radar requires little maintenance.

4.3 Acoustic Doppler Systems (ADCP)Acoustic Doppler systems have the advantage of measuring current velocity profile as well as waves, thus obviating the need to deploy two instrument systems for simultaneous wave and current monitoring. The instrument is usually mounted in a frame on the bottom, and is upward-looking. It has the advantage of being located below the surface where risk of loss by collision, vandalism etc. is reduced. A disadvantage of ADCP is the absorption of the acoustic signal energy. It is also disturbed by noise like raindrops on the water surface, breaking waves etc (Bendfeld et al 2009). 5. Recommendation for the Robin Riggs Project

The most appropriate wave monitoring instrument for Robin Riggs wind farm need to be: Suitable for the site condition

Capable of providing reliable and accurate data Cost not excessive Capable of providing real-time monitoring, low maintenance frequency and costWhile each instrument has their merits and limitations, I suggest a waverider buoy for the Robin Riggs Project. Reason for its selection over other options is discussed thus:Suitability of Site Condition The maximum water depth of Solway firth is 12m. This is within the 200m limit for the use of wave buoys.Accuracy and reliability of data Waverider buoys have proved to be a very reliable instrument for the collection of wave data. Most other instruments (including Radar and ADCP) are usually compared with to ascertain their accuracy. Waverider buoys are a proven technology for monitoring wave height and period. The Datawell Waverider buoy is today regarded as the international standard for ocean wave measurement (Wyllie and Kulmar 1995). Real-time Monitoring Waverider Buoys (like many other instruments in use) are adaptable to real-time monitoringCost While a Waverider buoy is relatively more expensive than some alternatives, the benefit derived could justify the cost. As the Robin Rigg Offshore wind farm has an operational life of 20 years, the cost could be spread over the period (E.ON UK 2007). It is however much cheaper than the HF radar. Met Office (2011) note the cost of HF radar is four times more expensive than buoy.Maintenance Frequency and Cost Unlike remote sensed instruments (like HF Radar), wave buoys are prone suffer damage due to collision. Such occurrences dictate the frequency of maintenance. However, as no commercial shipping pass through the area, and with the 50m exclusion zone around the wind farm structure, the likelihood of collision with buoys is reducedPrecautions to note The mooring system must be appropriate and properly anchored to the sea-bed. Pandian et al (2010) notes that most problem with waverider buoys can be eliminated through the use of good mooring system.

Enforcement of the 50m exclusion zone around the wind farm to reduce chances of collision with the buoys

6. ConclusionIn view of the effect of sea state on maintenance works in Robin Riggs Offshore wind farm, the most appropriate instrumentation required to monitor sea states has been considered. Among the various alternatives considered for use, the waverider buoy was selected based on suitability for the site condition, cost and adaptability for real-time monitoring. It would provide a cost-effective means of obtaining accurate and reliable data over the operational life of the wind farm. Its major disadvantage is the risk of damage due to collision with vessels, loss of mooring integrity etc. These can however be mitigated with proper design of the mooring system and enforcement of the 50m exclusion zone around the wind farm structure.REFERENCES

AMERICAN SOCIETY OF CIVIL ENGINEERS, 1996. Hydrology Handbook. 2nd ed. [online]. United States of America: ASCE Publications. Available from: http://books.google.com/books?id=P-244g77npUC&printsec=frontcover&dq=Hydrology+handbook+By+American+Society+of+Civil+Engineers [Accessed 10 April 2011].Bendfeld, J., Ditscherlein, R., Splett, M. & Voss, J., 2009. ADCP and Waverider Measurements for O&M at offshore windfarm locations. [online]. Paderborn. Available from: http://www.ontario-sea.org/Storage/27/1877_ADCP_and_Waverider_Measurements_for_O&M_at_offshore_windfarm_locations.pdf [Accessed 10 April 2011].Bendfeld, J., Krieger, J., and Splett, M., 2010. Wave and Current measurements for Offshore Windfarms. [Online]. Germany. Available from: http://www.ewec2010proceedings.info/allfiles2/285_EWEC2010presentation.pdf [Accessed 19 April 2011].E.ON UK, 2007. Project Update. Robin Rigg Newsletter, Spring 2007 Issue 1, p.4. MET OFFICE, 2011. Wavenet and HF radar [online]. Exeter. Available from: http://research.metoffice.gov.uk/research/ocean/wavenet/index.html#whatis [Accessed 19 April 2011].

GRADUATE SCHOOL OF OCEANOGRAPHY, 2011. Solway Firth. [online]. Rhode Island, RI: Office of Marine Programs. Available from: http://omp.gso.uri.edu/ompweb/doee/science/descript/solway.htm. [Accessed 10 April 2011].

Pandian P. K., Emmanuel, O., Ruscoe, J. P., Side, J. C., Harris, R.E., Kerr, S. A. & Bullen, C. R., 2010. An overview of recent technologies on wave and current measurement in coastal and marine applications. Journal of Oceanography and Marine Science. [online]. Vol. 1(1). pp. 001-010. Available from: http://www.academicjournals.org/joms/pdf/pdf2010/jan/pandian%20et%20al.pdf [Accessed 18 April 2011].Aucott, L., 2002. Development of a National Wave Recording Network for England and Wales. In: J. McKee Smith Coastal engineering 2002: solving coastal conundrums. Cardiff: World Scientific. pp. 134 -152.WYATT, L.R., 2009. Using HF radar to monitor waves, currents and winds for marine. [online]. Energy Research Seminar, University of Sheffield. Available from: http://www.sheffield.ac.uk/content/1/c6/07/18/96/15%20Jan%202009%20Radar%20for%20wave%20and%20wind%20measurement.pdf [Accessed 18 April 2011]

WYLLIE, S.J. and KULMAR, M.A., 1995. Coastal Wave Monitoring. [online]. Hobart: Manly Hydraulics Laboratory. Available from: http://mhl.nsw.gov.au/www/wyllie.htmlx [Accessed 18 April 2011]

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