Offshore Wind Resource Assessment and Metocean Data … · Report on OWEN workshop on Offshore Wind...

EPSRC

OFFSHOREWINDENERGYNETWORK

OWEN workshop on

Offshore Wind Resource Assessmentand Metocean Data

DATE: Monday 8 November 1999

VENUE: CLRC Rutherford Appleton Laboratory

FINAL REPORT

Gillian Watson

Report on OWEN workshop on Offshore Wind Resource Assessment and Metocean Data – 8 November 1999

2Gillian Watson November 1999

Table of Contents

1. Background............................................................................................................. 3

2. Research funding opportunities and timetables ................................................... 3

2.1 EPSRC funding.............................................................................................................. 4

2.2 NERC funding............................................................................................................... 5

2.3 DTI funding.................................................................................................................... 6

2.4 EU funding .................................................................................................................... 7

3. A scientific basis..................................................................................................... 9

3.1 General characteristics of the wind field over the sea............................................ 9

3.2 Long and short term variability in wind speed........................................................ 10

3.3 Waves, tides and other coastal processes.............................................................. 12

4. Potential sources of data ..................................................................................... 14

4.1 The UK Meteorological Office .................................................................................. 14

4.2 The British Atmospheric Data Centre (BADC).......................................................... 15

4.3 The British Oceanographic Data Centre (BODC) ................................................... 17

4.4 Wind measurement from space .............................................................................. 19

4.5 Database of wind characteristics ............................................................................ 21

5. Modelling techniques ........................................................................................... 22

5.1 Modelling coastal and offshore winds .................................................................... 22

5.2 Computational modelling of waves, currents and coastal processes ................ 23

5.3 Joint probability of winds, waves, currents and water levels ................................ 25

6. Collecting and analysing site specific data ......................................................... 26

6.1 Danish experiences of taking offshore meteorological measurements .............. 26

7. Discussions ........................................................................................................... 27

8. Feedback comments............................................................................................. 31



1. Background

The Offshore Wind Energy Network (OWEN) hosted a workshop on Offshore Wind ResourceAssessment and Metocean Data on 8 November 1999 at CLRC Rutherford Appleton Laboratory(RAL). The workshop aimed to present a scientific background to the offshore and coastalphysical environment, to highlight possible sources of data, to explore the latest modellingtechniques available and to draw on experiences of collecting data from offshoremeteorological masts. The event acted as a forum for OWEN members from differentbackgrounds to discuss their data requirements and experiences and to consider resourceassessment strategies. Finally, the workshop provided an opportunity for delegates to meetand discuss possible collaborations, research projects etc..Dr Jim Halliday (RAL) began the day by introducing the OWEN network, outlining its objectivesand giving some general administrative details on how OWEN is run. The remainder of theworkshop was divided into five main sessions:

Research funding opportunities and timetables

A scientific basis

Potential sources of data

Modelling techniques

Collecting and analysing site specific data

This report concentrates on only the main points raised during the presentations anddiscussions.Finally, there is a brief summary of the feedback comments made by the delegates.

2. Research funding opportunities and timetables

Speaker: Dr Jim Halliday – Energy Research Unit, CLRC Rutherford AppletonLaboratoryTel: 01235 445559, Email: [email protected]

Dr Halliday drew attention to the four principal funding bodies available to UK researchers andindustry for research in this topic area. These are:

• Engineering and Physical Sciences Research Council (EPSRC)• Natural Environment Research Council (NERC)• UK Government Department of Trade and Industry (DTI)• European Union (EU)

It is important to note that EPSRC and NERC funding is open only to eligible academicinstitutions in the UK. In general, research council funding is restricted to UK universities,together with a limited number of other institutions (contact EPSRC and NERC directly forfurther clarification). However, on many projects, collaboration with industry is seen as highlydesirable. UK Research Council funding is most suited to medium to long-term research(results in 2 – 5 years or more).

By contrast, DTI funding is aimed primarily at commercial organisations in the UK and issuitable for short to medium-term research (results in less than 2 years). The funds can beused for commercial applications on the basis of co-funding with UK industry.



EU R&D funds are awarded by the European Commission to collaborative groups drawn fromat least two EU member states. This source of funding is open to all organisations and is bestsuited to medium to long-term research projects (results in 2 - 5 years or more).

2.1 EPSRC funding

Research on issues related to offshore wind energy is supported by the EPSRC Renewableand New Energy Technologies (RNET) programme. This is a relatively new programmewhich commits up to £3.5 million per annum into research on all related technologies.The first RNET call for outline proposals closed in May 1999. 132 project outlines weresubmitted:

Technology Number of outline proposals (Call 1)PV/Solar 35Fuel cells 15Wind (including offshore wind) 25Marine 10 – 12Biomass 15Flywheels/Storage 15Miscellaneous (including energy inbuildings and energy efficiency)

15 – 18

50 of these outline proposals were short-listed (retaining a similar breakdown of technologytypes) and their authors were invited to develop the outline into full proposals in September1999. The full proposals from RNET Call 1 have now been refereed and the decisions on whichprojects will be funded will be made shortly. EPSRC expects to fund about 20 of the short-listedprojects, accounting for approximately £2 million.The deadline for RNET Call 2 passed in October 1999. 45 additional outline proposals weresubmitted at that time and these are currently being short-listed. Once this selection process iscomplete, short-listed authors will be invited to develop full proposals.A third call for proposals under the RNET programme is expected in the spring of 2000. Thescope of Call 3 has yet to be finalised taking into account the outcomes of Calls 1 and 2.



EPSRC funded researchRenewable and New Energy Technologies Programme (RNET)

Announcements and information relating to this call for proposals, the outline proposal formand, in time, details of awarded projects, are available on the EPSRC web site at:

http://www.epsrc.ac.ukunder the Engineering for Infrastructure, the Environment and Healthcare programmepages.

Budget: £3.5 million per annum

Timetable: Next call for outline research proposals expected Spring 2000

Contact: Dr Alyson ThomasAssociate Programme ManagerEngineering ProgrammeEPSRCPolaris HouseNorth Star AvenueSwindon, Wilts.SN2 1ET

Tel: 01633 892587 or 01793 444441Fax: 01793 444009Email: [email protected]

2.2 NERC funding

NERC currently has no Thematic programme in place for supporting research on offshore windenergy topics. However, a Thematic funding programme for this subject area may beconsidered in the autumn of 2000.

In the meantime, offshore wind related research that falls within NERC’s remit may be fundedin one of two Non-Thematic areas; Marine Science (£5 – 6 million per annum) orAtmospheric Science (£3 – 4 million per annum). Within these Non-Thematic areas there arevarious funding schemes that can be applied to:

• Small Research Grants <£35k, 3 calls p.a.• Standard Research Grants >£35k, up to 3 year project duration, 2 calls p.a.• CONNECT Scheme In collaboration with industry, 2 calls p.a.

A – “proof of concept” studies <£5kB – requires 50% industrial funding >£5k

• Long-Term Support Scheme up to 5 year project duration

NERC funded research

Non-Thematic Mode Research Grants: Marine Science/Atmospheric Science

Announcements and information relating to NERC research funding are available on theNERC web site at:

http://www.nerc.ac.ukunder the Awards and Training, Funding opportunities pages.



2.3 DTI funding

The DTI’s wind energy research programme is managed by the Energy Technology SupportUnit (ETSU) of AEA Technology Environment. However, it should be stressed that policydecisions, as well as final approval for all research projects, originate from the DTI and notETSU.Offshore wind energy related R&D will be funded by the DTI's New and Renewable EnergySupport Programme (NRES), which has been allocated £43.5 million over the next three years.The research projects submitted to ETSU should be focused, rather than general, in nature. Inaddition specific R&D tasks that have been identified will be tendered.Under the NRES programme, it is likely that research on wind energy issues will increase fromthe current level of £1.6 million per annum to approximately £2.0 million per annum for theyears 2000 and 2001. However, it should be noted that these budgets are subject to change.Dr James Craig, head of the Wind Programme at ETSU, is very keen to hear the UKoffshore wind energy community’s views on whether offshore wind resourceconsiderations are considered a major uncertainty and/or a barrier to deploymentof offshore wind farms in UK waters and on the wind industry’s commercialresearch requirements in general. Please contact him directly [email protected] or telephone 01235 433441.

POSTSCRIPT: Since the date of the workshop, James Craig has transferred hisresponsibility for the wind energy R&D portfolio to Ian Fletcher of ETSU – see belowfor contact details

DTI funded research and development

New and Renewable Energy Support Programme (NRES)ETSU co-ordinates New and Renewable Energy Support Programme on behalf of the DTI. Formore information visit the DTI web site at:

http://www.dti.gov.uk/renew/condoc

Budget: £43.5 million to end of March 20021999/2000 - £11.5 million2000/2001 - £14.0 million2001/2002 - £18.0 million

(~£2.0 million per annum for wind energy research and development)Contact: Ian Fletcher

ETSU, AEAT EnvironmentHarwellDidcotOxon.OX11 0RA

Tel: 01235 433266Fax: 01235 433355Email: [email protected]



2.4 EU funding

EU funding is currently made available under the Fifth Framework Programme for Research,Technological Development and Demonstration 1998-2002 (FP5). All funds are made on ashared costs basis.Research on offshore wind energy is likely to come under the Energy, Environment andSustainable Development (EESD) programme. Within EESD the relevant section is Part B:Energy (also known as ENERGIE), which is further subdivided into three sections – CleanerEnergy Systems (including Renewables), Economic and Efficient Energy and a provision forR&D of a generic nature. The emphasis for the FP5 programme is as follows:

⇒ Adopting a problem solving approach. FP5 turns away from the technology pushof previous Framework RTD programmes - there should be quantified objectiveswherever possible, and the programme is concentrated on a limited number ofstrategic issues. Demonstration projects are particularly encouraged.

⇒ The benefits of the research should give European “added value”, with scope forreplication in several parts of the EU.

⇒ Scientific and technological excellence, with effective projectmanagement.

⇒ There is a move towards larger projects and clustering of a number of alliedprojects.



EU funded research

Fifth Framework Programme for Research, Technological Development andDemonstration (FP5)

Announcements and information relating to this call for proposals are available on theCORDIS web site at http://www.cordis.lu under the Energy, Environment and SustainableDevelopment programme pages.

Budget: 1998 – 2002:Energy, Environment and Sustainable Development = 2125 million Euroof which Part B: Energy (also known as ENERGIE) = 1042 million Euroof which Cleaner Energy Systems (inc. Renewables) = 478 million EuroTotal FP5 = 14960 million Euro

Timetable: First call for research proposals closed June 1999 (included offshore wind)Call 2 closed October 1999 (did not included offshore wind)Postscript: It is understood that an extra (unscheduled) call which may

include offshore wind energy, will be issued in February 2000.Call 3 (September 2000) and Call 4 (September 2001) may include offshorewind energy topics.NO CALL IS EXPECTED IN 2002.

Contact: Sarah SidebottomEnergie Helpline UKTelegraphic HouseWaterfront QuaySalford QuaysManchesterM5 2XW

Tel: 0161 874 3636Fax: 0161 874 3644Email: [email protected] site: http://www.dti.gov.uk/ent/energie



3. A scientific basis

Session chair Dr Jim Halliday, CLRC Rutherford Appleton LaboratoryTel: 01235 445559, Email: [email protected]

3.1 General characteristics of the wind field over the sea

Dr Ann-Sofi Smedman, Department of Meteorology, University of Uppsala

Tel: +46 18 471 71 91, Email: [email protected]

Dr Smedman’s presentation concentrated on the wind regime in sea areas that are influencedby land surfaces. There is evidence that in some circumstances this zone of coastal influenceextends a large distance from the shore - perhaps as far as a couple of hundred km from thenearest land. Most of the example data used to illustrate the wind phenomena was measuredin the Baltic Sea.In offshore areas it is generally thought that compared to onshore wind speeds will be higher,there will be a smaller wind gradient and less turbulence. This means that it should be possiblefor offshore wind turbines to have higher energy outputs and suffer less fatigue loadingthereby to have a longer active life. There are, however, various phenomena that may affectoffshore wind conditions including:

Low level jetsSea breeze circulationInternal Boundary Layer (IBL) development

These mesoscale features are driven by differences between the land and sea such as changesin surface roughness, temperature and atmospheric stability.Low level jets are characterised by an increase in mean wind speed accompanied by high windshear, which can in turn spawn high levels of turbulence. Jets may start to develop in coastalwaters as a mass of air is blown off-shore. They occur at heights of approximately 50 to 100mand may increase in intensity over time (6 – 8 hours) as the air mass travels over the sea. Inareas where jets appear, offshore wind turbines will experience higher wind speeds, but willalso encounter more severe loading and fatigue.Sea breezes are driven by convection. As the land heats up during the day, the overlying airwarms and rises. Cooler sea air is drawn inland to replace it and a convection cell develops.The resulting on-shore winds may mask a weak weather pattern and produce very differentwind conditions to those forecast by synoptic models. The convection system may reverseduring the night resulting in an off-shore land breeze.Internal boundary layers form downwind of a step change in surface roughness such as acoastline. In off-shore wind conditions, airflow in the lowest layer is in equilibrium with the seasurface, but there will be an upper layer which retains equilibrium with the air over the land.There will also be an adjustment or blending layer where the two air masses meet but themixed wind profile can extend downstream for a significant distance. In very stableatmospheric conditions (with low mixing rates), it is possible for the hub of a coastal offshorewind turbine to project through the lower layer into the upper airflow. In this scenario, theturbine rotor will experience conditions characteristic of the wind over land rather than the seaand no benefit will have been gained by constructing the turbine in the sea.The wind also interacts with the sea. As the wind passes over the water, energy is transferredfrom the air to form wind-generated waves. This continues until the sea state is fullydeveloped. When the wind drops the sea state also declines, but usually at a slower rate. In



most conditions, the transfer of momentum between the sea and the air mass almost cancelsout, however at low wind speeds a momentum feedback loop may develop that transfersenergy from swell back to the wind.

Questions and comments:

What are the typical height and the height range of low level jets, and how frequent are they?Typical height = ~100mHeight range = 10 – 300mCurrently there is not enough data to estimate the frequency.

Are all the phenomena described above found in areas other than the Baltic Sea?Dr Smedman explained that it is very common for winds in an extensive area of sea tobe influenced by the land. However, such effects may be particularly marked at highlatitudes where water/air temperature differences are large or in enclosed sea areas(e.g. the Baltic).

Will the presence of low level jets affect the optimum tower height for offshore wind turbines?Dr Smedman suggested the towers should be built as high as practicable.

Most UK developers are looking to site offshore wind farms within 10km or so of the coast.What will be the major influences in this inshore region?

In this area Dr Smedman felt that the wind field is likely to be a function of thegeostrophic wind and the temperatures over the land and sea.

3.2 Long and short term variability in wind speed

Dr Jean Palutikof, Climatic Research Unit, University of East Anglia

Tel: 01603 593647, Email: [email protected]

Dr Palutikof used a series of results from Bell Rock Lighthouse and other UK sites to illustratevariation of wind speed with time. Bell Rock is a rocky outcrop off the east coast of Scotland. Itwas selected because it is an offshore site and because of its status as a long-term UKMeteorological Office monitoring site. The results show that wind speeds are seen to vary onall time scales:

Diurnal cyclesSeasonal cyclesDecadal cycles - unforced/internal variations e.g. the North Atlantic Oscillation (NAO)Long-term variations – forced/external variations e.g. greenhouse warming

The scale of these variations is sufficient to have economic implications for onshoreand offshore wind farms.

Diurnal cyclesThe data showed that during the summer months the diurnal pattern of wind speeds at BellRock depends on the wind direction. In westerly conditions (i.e. winds that have passed over aland fetch), Bell Rock experiences a lull in the early morning followed by an increase of windspeed during the day that peaks in the late afternoon. No such variation is seen in easterlyconditions (i.e. winds that have passed over a sea fetch). This shows that the summer diurnalpattern is affected by the type of fetch (land/sea/both) for the site and will vary with location.



During the winter months the Bell Rock data does not show a clear diurnal pattern, even forwesterly winds. This is because the diurnal signature is swamped by large scale atmosphericcirculations associated with east-moving Atlantic depressions that predominate at this time ofyear.Seasonal cyclesA monthly breakdown of the mean annual wind speeds at Wick, a coastal town in north-eastScotland, shows a pattern of high wind speeds during the winter (January maximum) andlower wind speeds during the summer (August minimum). By contrast, a seasonal breakdownof the Bell Rock data indicates a slightly different pattern, with particularly strong wind speedsexperienced during the autumn.Decadal cyclesThere is evidence of decadal variation in wind speeds driven by a phenomenon known as theNorth Atlantic Oscillation (NAO). The NAO index is a measure of the difference in atmosphericpressure across the North Atlantic region, typified by the Azores High and an Icelandic Low:

NAO index Relative status of Azores High Effect in the UK+ve Strong Strong westerly winds-ve Weak Lower wind speeds

An analysis of historical values of the NAO index shows a low frequency oscillation over mostof the 20th century. However, the overall trend since 1970 shows a steady increase in the NAOindex, culminating in a record index maximum in 1996, immediately followed by sharp swingand a record index minimum in 1997. Some climatologists interpret this as a sign of globalclimate change.The NAO index analysis indicates periods (with time scales of 10 to 20 years) with particularlyhigh or low wind speeds. Clearly this will be reflected in the measured wind and gust speedsrecorded during these spells and emphasises the importance of basing wind resourceassessments on analyses of sufficiently long periods of data.There are some historical observations of onshore wind speeds available for the UK, althoughthe records are not continuous and suffer from inaccuracies and inconsistencies as a result ofthe measuring techniques used. In the absence of adequate long-term data, it may be possibleto reconstruct wind data from atmospheric pressure data records, which are generally regardedas being more reliable than wind speed measurements. Historical atmospheric pressure datadates back to 1880 and beyond.Long-term variationsDr Palutikof presented some recent (September 1999) results prepared for the UK ClimateImpacts Programme showing the expected change in mean annual and seasonal wind speedsacross the UK with respect to 1961 to 1990 (the standard baseline period used byclimatologists). These results suggest that the largest increases in wind speed are likely tooccur in the autumn months with up to a 7% increase in mean wind speed predicted for someparts of the UK in 2080. By contrast, the smallest increase (or even a slight decrease) in windspeed is expected during the spring. The model results do not show a distinct trend in thewinter or summer seasons.Predictions of changes in seasonal gale frequencies show a very sharp percentage increase invery severe gales in the 2020’s and 2080’s. Intriguingly, the models indicate a decrease in galefrequencies in the 2050’s.




What level of confidence is there that there will be a decrease of gales in the 2050’s?Dr Palutikof explained that it is difficult to place statistical confidence on the results asthey were not produced by a statistical model. The results are based on the mean offour ensemble models, which means she is unable to estimate the level of confidence inquantifiable terms but she went on to describe the analysis as the current “best guess”of what may happen in the next few decades.

Is there a good correlation between the NAO and onshore wind measurements?In western parts of the UK there is a very high correlation, but there is a decreasingtrend as you go east across the country. Ian Leggett (Shell UK Exploration andProduction) commented that offshore oil and gas industry data sets for the North Seaindicate a very good correlation of wind speed and NAO north of Shetland, but less soin the southern North Sea.Mike Anderson (RES Ltd.) observed that contrary to the NAO index analysissuggestions, wind speeds measured at various operational wind farms between 1970and 1990 indicate that wind speeds have been low during this period. Dr Palutikofexpressed an interest in obtaining this data to add to her analysis.

3.3 Waves, tides and other coastal processes

Dr Alan Brampton, HR Wallingford Ltd


Hydraulics in the marine environment is primarily characterised by waves with a wide range ofamplitudes and periods.

Tidal levels and currentsTides are in fact waves with very long period and wavelength. “Astronomical” tides – the tidalcomponents caused by the influences of the moon, the sun and other astronomical bodies suchas the planet Jupiter - are relatively well understood with predictable periodicity and amplitudesthat can be observed. This allows tidal predictions for particular sites to be drawn up intotables.Unfortunately, there is no simple correlation between a location and the tidal cycle itexperiences. The seas that surround the whole of the UK coastline are relatively shallowcompared to the wavelength of tides. This means that the tidal “wave” detects the seabed andits propagation is affected by the seabed bathymetry as well as the configuration of the landmasses. This results in very complex patterns of tidal levels and tidal currents which can alterradically over short distances. In addition, the tides are significantly influenced by atmosphericeffects which makes them difficult to forecast precisely.Tidal levels are routinely observed by a system of “A-class” gauges around the UK coast andthere is a fair coverage of data on tidal currents held by the BODC (see section 4.3). Numericalmodels are used for “infill” and real-time forecasting.WavesWaves may be generated in several ways (e.g. seismic activity, ships), but it is wind generatedwaves that are of most interest. Wind generated waves may be divided into two categories:

Swell Waves with long periods that were generated by winds at a remote location.These waves often have large wavelengths (?>100m) and may retain alarge amplitude.



Wind sea Waves generated by local winds, often over relatively short fetches. Thesewaves generally have smaller wave heights and shorter wave periods.

There is a reasonable knowledge of offshore waves around the UK. Although there is a limitedamount of measured wave data, this is augmented by synthesised data and models have beendeveloped to provide real-time wave forecasts.Wave heights in UK waters can get very large in exposed areas, but the land offersconsiderable protection to the Irish and North Seas and therefore less wave energy willpenetrate these areas. In general, there is a reduction in wave height as the water depthdecreases, however, waves may be focused by refraction as they pass into shallower water. Invery shallow coastal waters, the effects of seabed friction and wave breaking also act to reducewave heights. Finally, tidal levels and currents can affect wave conditions at a particular site.Sediment transport and morphological changesThe action of large waves with long wavelengths will extend far below the sea surface. Wavescan “feel” the seabed in depths of more than 200m and this may disturb seabed sedimentsthroughout the continental shelf region resulting in movement of soft sediments. In general,sediment transport is dominated by tidal currents in deeper water (>15m) and by windgenerated waves in shallow water (<5m).There is a reasonable knowledge of the UK’s seabed geology (contact the British GeologicalSurvey for more information). The seabed is not always covered by mobile sediments, but insome areas there may be a poor knowledge of surface sediments (due to survey techniquesetc.) or of their movements (e.g. sand waves). In view of this, it is advisable to investigate theseabed as sediment movements can be hostile leading to large short and medium-termchanges in depth.Offshore wind energy installations are more likely to fall victim to rather than causemorphological changes. Long-term trends (over centuries) usually indicate increasing seabeddepths, especially in nearshore areas where increases of the order of a few mm per year maybe expected in waters of 20m depth. In storm conditions, very rapid short-term changes (ofthe order of 10m) may be experienced on sandbanks in shallow water. Historical recordsshowing the positions of mobile sandbanks may indicate some useful information. In addition,local scour and liquefaction problems may be experienced at some sites, particularly in the surfzone.Coastal processesThe effects of coastal processes such as erosion and accretion should be to be taken intoaccount close inshore (within a few km of the coast). Wave breaking effects usually dominatein this area and problems with abrasion and scour are very common making cable routes andlandfalls especially vulnerable. As a general rule, it is prudent to assume long-term coastalerosion, particularly in soft rock areas (Devon to Yorkshire, Irish Sea coasts).In addition, many areas close to the coast have been identified as areas of interest due tobiological, geological or landscape attributes as well as for recreational, amenity andarchaeological reasons.


Does it really matter if cables are buried?Dr Brampton suggested that it would matter greatly if a cable that was intended to beburied was in fact exposed, particularly in areas that are trawled. He also noted thatwave action, scour and abrasion in the surf zone is extremely hostile to cables andshould not be underestimated even though cables are less vulnerable than rigidpipelines.



4. Potential sources of data

Session chair Dr Bill Grainger, Border Wind Ltd.Tel: 01434 601224, Email: [email protected]

4.1 The UK Meteorological Office

Colin Heaton, Marine Consultancy Service, UK Meteorological Office


The UK Met. Office can offer consultancy services for maritime industries in the form of data,analyses (e.g. statistical analyses of weather variables, WAsP runs) and specialist weatheradvice.

Marine data sources available include:Ship reportsMarine Automatic Weather Stations (MAWS)Coastal stationsWave Model Archive

Ship reportsThe database of ship reports contains routine weather observations made by vessel crews onpassage. Over 75 million such reports are stored in the database, dating back over many years(circa 1850). This data can provide an overview of conditions, but does not have specificinformation in the coastal zone. The data should be used with caution as it is made up of singlereports, does not have a continuous record and may have inconsistencies caused by changes inobservation practices.

MAWSThe Met. Office operates a network of offshore weather stations (based on 15 moored buoys, 4lightships and seven offshore platforms or islands) that have been deployed throughout UKwaters and their western approaches in recent years. It should be noted that the MAWSstations are mostly in deep water and as such can provide useful data, but it needs to be usedin conjunction with other data sources (e.g. coastal sites).

Coastal stationsRegular meteorological observations are recorded at over 60 coastal stations around the UK.

Wave Model Archive

The Wave Model Archive contains both wind and wave data from Met. Office numerical models.The forecasts are made using a 2nd generation spectral wave model based on wind input from aseparate atmospheric model. Currently the Met. Office operate a global wave model with a gridresolution of ~90km (~60km from April 1999) and a nested regional wave model of Europewhich has finer grid resolution.

The regional model covers European waters including the North Sea, the Baltic Sea and theMediterranean Sea at a grid resolution of 0.4º longitude by 0.25º latitude (~25km). It includesa representation of bathymetry in coastal waters and models shallow water physics in theseareas. The European model is run with boundary wave spectra taken from the global wavemodel.



Output variables at each grid point in both the global and European wave models have beenarchived at 6 hourly intervals since 1986 and are available 3 hourly from 1988.

In the spring of 2000, the UK Met. Office will introduce a UK regional wave model with a fine(12km) grid. They also plan to upgrade the regional models to include wave currentinteractions and to implement the SWAN (Simulating WAves Nearshore) 3rd generation wavetransformation model for shallow waters.For more information contact the Met. Office Marine Consultancy Service at:

The Met. OfficeMarine ConsultancySutton HouseLondon RoadBracknellBerkshireRG12 2SYUKTelephone: 01344 854 979Fax: 01344 854 254Email: marine&[email protected] URL: http://www.met-office.gov.uk


Are there plans to perform a re-analysis of the wave model data?None at this time.

What verification of the wave model data is made and how does the model perform?In the course of its development, the wave model was validated against measuredvarious sources of nearshore and offshore data. In addition, the output of the wavemodels is routinely verified against offshore observations. The model comparesfavourably to the measured data (given the limitations due to the model resolution) andhas been shown to perform well in the waters off the east coast of the UK. However, itis always advisable to use model output with appropriate caution.

4.2 The British Atmospheric Data Centre (BADC)

Anabelle Ménochet, BADC, CLRC Rutherford Appleton Laboratory


The BADC is the NERC Designated Data Centre for the Atmospheric Sciences and is hosted atCLRC Rutherford Appleton Laboratory. Its mission statement is:

To provide a high-quality archive of atmospheric data to the NERC atmosphericresearch community in a timely and straightforward manner, together withsufficient information about the data to permit its effective use for researchpurposes.

BADC has three main priorities:



1. Support for NERC thematic research programmes and long-term archiving and distributionof NERC datasets.

2. Met. data acquisition e.g. from the UK Met. Office and ECMWF.3. Data brokerage – to help users to find other sources of data, links to other data centres and

caching CD ROMs.Since mid 1996 a formal agreement has been in operation between NERC and the Met. Officethat allows all data intended for NERC-funded atmospheric research to be co-ordinated byBADC. The data is purchased in bulk from the Met. Office archives and distributed free ofcharge to the research community. The data are kept in a secure archive at BADC. Usersare required to complete and sign a “Conditions of Use” form agreeing to use the data for bonafide research purposes only. Similarly, BADC has acquired the European Centre for Medium-Range Weather Forecasts (ECMWF) model re-analysis dataset.There are currently 49 datasets catalogued at BADC. These include datasets produced by NERCfunded programmes, UK Met. Office datasets, ECMWF datasets and various NASA datasets andCD ROMs. All the BADC datasets and documentation are available online at:

http://www.badc.rl.ac.ukBUT SOME DATA HAVE RESTRICTIONS ON THEIR USE AND ACCESS.Other services provided via the web include:

• Conditions of Use form for Restricted datasets

• A searchable catalogue of UKMO stations

• FTP guide for those unfamiliar with FTP

• Links to other sources of information relevant to atmospheric science. The collection canbe searched by keyword or browsed by category.

• BADC help desk – email desk

For more information contact BADC at:The British Atmospheric Data CentreCLRC Rutherford Appleton LaboratoryChilton, DidcotOxfordshireOX11 0QXUKTelephone: 01235 446432Fax: 01235 445848Email: [email protected] URL: http://www.badc.rl.ac.uk


Is any of the BADC data available to commercial organisations?Yes – there is unrestricted access to much of the data online at the BADC web site. Themain exceptions are the UK Met. Office and ECMWF datasets which commercialorganisations must purchase directly from the Met. Office. Information on therestrictions that apply to each dataset is available online.



Is there much marine data available?To date, BADC has acquired mainly land-based data. However, as more marine data isgathered and subsequently transferred to BADC it will become available online (subjectto the same sorts of restrictions as currently exist).

4.3 The British Oceanographic Data Centre (BODC)

Lesley Rickards, BODC, Centre for Coastal and Marine Sciences, ProudmanOceanographic Laboratory

Tel: 0151 653 8633, Email: [email protected]

The British Oceanographic Data Centre (BODC) is hosted by the NERC Centre for Coastal andMarine Sciences (CCMS). In addition to its primary role as NERC's Designated Data Centre foroceanographic data, BODC is also recognised within the Intergovernmental OceanographicCommission (IOC)'s International Oceanographic Data Exchange System as the UK's NationalOceanographic Data Centre. BODC’s mission statement is:

To operate a world class data centre in support of UK marine science by:

• providing data management support for UK marine science projects

• maintaining and developing the UK’s national oceanographic database

• developing innovative marine data products and digital atlases

• collaborating, on behalf of the UK, in the international exchange and management ofoceanographic data

• making high quality data readily available to UK research scientists in academia,government and industry

BODC has extensive data holdings both in its national oceanographic database (e.g. mooredcurrent meter data, wave data, sea level data, temperature/salinity profiles) and in its projectdatabases. Data and information are disseminated through a variety of routes, includingindividual project data set CD-ROMs and the UK Digital Marine Atlas (UKDMAP). Although thedata themselves are not yet available online, searchable catalogues containing full details ofthe data held are available on the BODC web site at:

http://www.bodc.ac.ukData are available from BODC under license. In general, commercial customers are required topay a handling charge for access to the data, but these charges may be waived for bone fideresearch workers. Further details are available from BODC. In the future, BODC is planning tomove towards a web-based access system for some of its data sets.In addition to the national oceanographic database, BODC provides data management supportfor large multidisciplinary oceanographic research projects. The techniques and proceduresinherent in BODC’s approach to project data management were pioneered during NERC’s NorthSea Project and further developed during more recent projects (e.g. Ocean Margin Exchange(OMEX) project, Land Ocean Interaction Study (LOIS)). BODC has taken lead in ensuring theproper data management, working up of some of the data sets, and integrating all the data intocoherent project databases, the aim throughout being to make high quality data available toproject scientists with minimal delay. Project data sets, together with extensive supportingdocumentation and software to explore the data, are available on CD ROM. Further informationis provided on the BODC web pages.BODC also hosts the National Marine Environmental Data Co-ordinator, a post set up by the UKInter-Agency Committee on Marine Science and Technology (IACMST). An integral part of this



work is the collation of information on the UK marine data collecting activity and on the dataheld in UK laboratories, and the dissemination of this information on request.One direct result of this activity is the Directory of Marine Environmental Data Sets held by UKLaboratories, which received some funding from IACMST. The Directory forms the UKcomponent of the European Directory of Marine Environmental Data (EDMED)(http://www.pol.ac.uk/bodc/edmed.html), an EU Marine Science and Technologyprogramme (MAST) project.EDMED aims to provide a comprehensive reference to the marine environmental data heldwithin Europe so as to provide marine scientists, engineers and policy makers with the meansof identifying potentially useful data sets. There are over 2300 data sets from 12 countriescatalogued in EDMED. It includes data collected from the last century through to the present,and covers a wide range of disciplines including marine meteorology; physical, chemical andbiological oceanography; marine biology and fisheries; environmental quality monitoring;marine geology and geophysics, etc.The majority of data sets within EDMED relate to the seas around Europe but entries will foundfor most of the seas and oceans of the world. It should be noted that, although a few data setsinclude remote-sensing data, the Directory does not provide full coverage of satellite senseddata sets as these tend to be documented internationally by the remote-sensing community.Although EDMED is targeted primarily at data sets that can be made accessible toother users, encouragement has also been given to holders of working data sets, ordata of a confidential or restricted availability, to make their data known throughthe Directory. Please contact BODC if you have data that could be added to theDirectory.The United Kingdom Digital Marine Atlas (UKDMAP) has been developed by BODC as awide ranging reference source for information relating to the coasts and seas around theBritish Isles. It contains over 1600 thematic charts which are displayed via a variety ofpresentation methods, including contoured plots of physical, chemical and geologicalparameters; colour coded distribution charts of sea use; biological and fisheries information;oceanographic data catalogues; and geo-referenced directories which present detailedinformation on demand. Textual information which accompanies each chart provides adescriptive overview of the depicted theme, identifies the data source, and informs the user ofany additional data or services available from that source. Further details are available from theBODC Web pages and a brochure is also available.For more information contact BODC at:

BODC Enquiries OfficerBritish Oceanographic Data CentreBidston ObservatoryBidston HillBidstonPrentonCH43 7RAUKTelephone: 0151 653 8633Fax: 0151 652 3950E-mail: [email protected] URL: http://www.bodc.ac.uk



4.4 Wind measurement from space

Dr David Kilham, School of Geographical Sciences, University of Bristol

Tel: 0117 928 8300, Email: [email protected]

Unfortunately, it is not possible to view air movements directly using remote sensingtechniques. To overcome this limitation, it is necessary to make some sort of surrogatemeasurement from which it is possible to infer the associated wind conditions. To date, remotesensing of ocean winds has been achieved in three main ways:

Monitoring cloud motionsActive microwave scatterometry measurementsPassive microwave emissivity measurements

Cloud motion windsRemote-sensed wind measurements began in the mid 1960s following the launch of the firstgeostationary satellites. The original technique was based solely on “cloud-tracing”, whichinvolves observing the vector motions of cloud patterns at certain heights. More recently,development of infra-red (IR) radiometers capable of viewing atmospheric water vapour hasextended the method to allow direct tracking of air mass features.It is important to recognise that the cloud motion method monitors winds at altitude. Inaddition, complex atmospheric circulation (e.g. cloud development) can sometimes result inslightly muddled data. However, wind vectors obtained from geostationary imagery is generallyconsidered to give the best information on high level winds over the ocean.Detection of atmospheric motions through either cloud tracing or water vapour motion hassteadily developed over the last thirty years. Enhancements in the spatial and temporalresolution of the observations have reduced problems associated with cloud development.Furthermore, the increase in imaged wavelengths together with use of ancillary numericalweather prediction (NWP) data has corrected difficulties in assigning cloud heights.Animated sequences (image loops) of Meteosat water vapour images are now used byforecasters as a qualitative tool for routine 4D diagnosis of the atmosphere.

Active microwave scatterometryScatterometry techniques are based on monitoring radar backscatter from ripples on the seasurface. The technique may be used to infer the surface level winds only.Surface winds over the sea generate small cm length “capillary” waves in the water. Thesewavelets have an amplitude related to the wind friction speed. A satellite’s antennae generateradar beams directed towards the sea surface. The microwaves in the beam are scattered bythe capillary wavelets (a phenomenon known as Bragg scattering). The satellite measures theradar backscatter, which is proportional to the capillary wave amplitude. In this way, thesurface wind speed can be inferred.There have been a number of satellites used for scatterometer missions to date including:

• SEASAT-A Fitted with SeaSat Scatterometer (SASS) measuring surface vectorwind stresses and neutral stability wind vectors at height of 19.5m.SEASAT-A had global coverage up to ±79º latitude. Transmissionsceased October 1978.

• ERS The European Space Agency (ESA) European Remote Sensingsatellites ERS-1 and ERS-2 were launched in July 1991 and April 1995respectively). These satellites are fitted with 3-beam scatterometers.



Scatter from the beams are input into a mathematical model thatcalculates surface wind speed and direction - wind speeds 4-24ms-1

(±2ms-1 or 10%), wind directions 0-360º (±20º). The ERS satelliteshave polar orbits, but data can be “quite scant” in some areas.

• ADEOS-1 Fitted with NASA’s NSCAT scatterometer. Between August 1996 andAugust 1997 NSCAT sampled 90% of the ice-free oceans every 2days.

• QuickScat Fitted with the SeaWinds scatterometer. QuickScat was launched in1999 as a follow-on mission from NSCAT

Passive microwave emissivityThe surface of the earth constantly emits microwaves, a phenomenon referred to as passivemicrowave (PM) emissions. The PM emissivity of a section of the earth depends on the surfacetype; land surfaces exhibit high emissivity, however in contrast, calm water has relatively lowemissivity (~0.5).Satellite-based imaging equipment can be used to observe the PM emissions and to measurethe associated microwave brightness temperatures (TBs) of the earth’s surface. Marine windmeasurements may be inferred from this data by monitoring small changes to the oceansurface TBs. The surface of a calm sea has a relatively low PM emissivity, so appears cold inthe microwave TB. The presence of wind stress roughens the water and creates foam andother bubble features. These have the dual effect of increasing the TB and decreasing thepolarisation.PM imagers are usually linearly polarised and therefore this method is capable of inferringsurface level, scalar wind speeds only. This restricts the value of the data. Variousattempts have been made to produce climatological vector wind values using this technique,but the results have proved inconsistent. Furthermore, atmospheric water (precipitation, cloudsand water vapour) can also affect microwave TBs, which means it is not straightforward tointerpret the data. A series of retrieval algorithms have been developed, ranging in complexityfrom simple regressions based on TB versus buoy data, through neural networks to semi-physical models.Traditional PM retrievals cannot be carried out in coastal waters (within ~50 km ofland) because of the influence of the high PM emissivity on the adjacent land. However, atechnique recently developed at the University of Bristol allows wind speed retrievals right upto the land’s edge.Passive microwave measurements have been made continuously from 1978 to present. Duringthis period, passive microwave systems have been installed on a number of satellites including:

• SEASAT-A Fitted with a Scanning Multichannel Microwave Radiometer (SMMR)instrument. Transmissions ceased October 1978.

• Nimbus-7 Fitted with the SMMR instrument. Operated successfully for 8 years.

• SSM/I A series of Special Sensor Microwave/Imager (SSM/I) satellites formpart of the Defense Meteorological Satellite Program (DMSP). Thereare currently 3 operational satellites in orbit with the next launch duein December 1999.

Merged productsThe data from the SSM/I, ERS and QuickScat missions are used as input to the global NWPmodels.



Other wind measurement methodsThree other remote sensing wind measurement systems (two satellite and on surface based)have been used to produce offshore wind data.Active microwave radar altimeters are capable of producing both wind and wave measurementsfrom sub-satellite measurements. However, the altimeter only measures one sub-orbital pointwhich is a severe limitation.A few case studies have been made using visible reflectance changes (optical scattering) in thesurface roughness of the ocean. Successful cases have been limited to localised, “jet-like” windsystems such as katabatic winds.Coastal wind speeds can be measured using VHF “Over the Horizon” radar (Surface RS-OTHradar). The results give similar levels of performance to scatterometry, but the instruments areexpensive and are fixed installations with ranges in the order of a few hundred kilometres. Todate most examples of this type of instrument are experimental installations in the US.

4.5 Database of wind characteristics

Kurt S Hansen, Department of Energy Engineering, Technical University of Denmark

Tel: +45 4593 2711, Email: [email protected]

Kurt drew attention to another online source of wind data at http://www.winddata.com. Itshould be noted that this is not a wind resource database, but instead it is a database ofwind time series intended primarily for wind turbine design purposes. On the web site there areexamples of wind measured under many different conditions at many different sites includingthe Vindeby offshore wind farm. This database contains more than 50,000 hours ofmeasurements.

For more information contact Kurt S Hansen at:Kurt S. HansenDepartment of Energy EngineeringBuilding 404, DTUTechnical University of DenmarkDK-2800, LyngbyDenmarkE-mail: [email protected] URL: http://www.winddata.com



5. Modelling techniques

Session chair Dr Lars Landberg, Risø National LaboratoryTel: +45 46 77 50 24, Email: [email protected]

5.1 Modelling coastal and offshore winds

Dr Ann-Sofi Smedman, Department of Meteorology, University of Uppsala


Dr Smedman categorised wind models as:Simple modelsAdvanced (mesoscale) modelsWeather prediction models

Simple models (such as WAsP) have been available for some time and have been used for awide range of applications. In general, they can be thought of as “wind climate” models, in thatthey predict winds based on large scale “climatic” features such as the pattern of geostrophicwinds. As such, they appear to give a reasonable first estimate of winds over the sea. The mainadvantage of simple wind models is that they are easy to use and can be run on a standard PC.Many simple models include representations of IBL development, however, models of this typedo not include marine mesoscale effects such as low level jets and sea breezes.The next generation of more advanced wind models are currently under development. Windmodels of this type will include marine mesoscale features such as jets, sea breezes and amore sophisticated treatment of atmospheric stability. Mesoscale models are based on a morecomplete representation of atmospheric physics than the simple models, but by their verynature the advanced models are more complex. Unfortunately, this makes advanced windmodels computationally expensive which means they need to be run on a powerful computerand even then the models may have a relatively coarse grid resolution. In addition, the modelsare more difficult to use and require a higher level of expertise to operate. At this stage, noneof the advanced models are readily available in a commercial package in the same way WASP6is.A mesoscale model, developed at the Department of Meteorology in Uppsala, has been appliedto the Baltic Sea. The model results obtained so far clearly show coastal effects and thisdemonstrates that mesoscale models have the potential to be very useful for wind resourceapplications. Although there is relatively little measured data available to verify the model, themodel results compare well with ship observations in the region. The results also indicate thatat a height of 66m, the wind speeds approach geostrophic conditions. However, it has beenshown that wind speeds within low level jets can be higher than their geostrophic equivalents.Dr Smedman suggested that future generations of wind models would need to be 3D models,nested to include several mesoscale models. She also predicted that wind models may extendover increasingly large areas in order to achieve adequate model boundary conditions.


Given that the most important wind resource parameter required is an estimate of the meanwind speed, is it necessary to make such a large investment in developing advanced mesoscalewind models?

Dr Smedman pointed out that most maps of offshore wind resource indicate thatmesoscale processes are of great importance.



Is it more difficult to model offshore winds than onshore winds?Meteorologists have a relatively good understanding of the wind regime over the land.Dr Smedman felt that in the long run, it should be no more difficult to model offshorethan onshore. However, at this time the phenomena that influence offshore winds arenot well understood and there is a scarcity of data on which to base offshore windmodels. This makes modelling offshore winds more difficult, at least in the short term.

Given that developers are unlikely to go ahead with an offshore wind farm project withoutmonitoring the site’s wind regime from an offshore meteorological mast, why do we need windmodels?

Lars Landberg (Risø National Laboratory) commented that wind resource models needto be developed to identify area suitable for offshore wind farms and therefore pin-pointexactly where to site the measuring masts. However, he would not advise a developingan offshore wind farm without measuring the wind conditions at the site.

5.2 Computational modelling of waves, currents and coastal processes

Dr Jane Lawson, HR Wallingford Ltd.


Dr Lawson reminded everyone that the fundamental basis of computational modelling is a setof equations that represent the desired physical processes. Like many other applications, thephysical processes involved in waves, currents and coastal processes are very complex.Therefore, it is usually necessary to apply simplifying assumptions before equations thatadequately describe each process can be derived. The next stage is to use a numerical method,such as a finite difference or finite element technique, to solve the equations. Finally, themodel performs a computational implementation of the numerical methods.It is important to remember that model results will only ever be as good as theequations the model is based on and the quality of the input data used.WavesThe basic strategy for wave modelling is to start with known wave conditions at an offshorelocation, and to use a model to transform those waves inshore.The most common model input is offshore, deep water wave conditions obtained from windwave prediction models, ship observation data or the UK Met. Office wave models. This datamay be in the format of individual wave height, wave period and wave direction combinations,wave climates (tables and roses) or estimates of extreme wave conditions.To transform the offshore wave conditions into nearshore wave conditions, it is generallynecessary to model the processes of wave refraction and shoaling. There are a series of modeltypes that can be applied including (in increasing order of complexity) relatively simple “back-tracking ray” models, “2nd generation” finite difference/finite element models and finally “3rd

generation” models which include additional processes such as wind wave growth, seabedfriction and non-linear wave-wave interaction. It is important to select a model type that isappropriate for the particular site of interest.As waves travel further inshore, it is often appropriate to use more sophisticated models thatinclude not only wave refraction and shoaling effects, but also wave diffraction, wave reflectionand energy dissipation (e.g. wave breaking). There are a range of models to choose fromincluding (in increasing order of complexity) "forward-tracking ray” models and finitedifference/finite element models based on the mild slope equation or the Boussinesqequations. Finally, at sites where many physical processes act together (e.g. very complex



harbour structures) it may be beneficial to construct a scaled physical model. Not only will thephysical model ensure that all the processes are represented and interact properly, but alsoexperience has shown that physical models can be very effective public relations tools.CurrentsCurrent models can represent tidal flows as well as wind-driven and wave-driven flows. Themodels are generally based on shallow water equations solved using finite difference or finiteelement methods and may be classed as either 2D or 3D models.Computational flow models may be used for:

Environmental impact assessments (EIA)Sediment transport modellingDesign of navigation channelsSiltation studies in berths and dredged channelsOutfall design and pollutant dispersion.

The more sophisticated 3D current models are required in situations involving:Stratification (e.g. salinity or temperature)Secondary currents (e.g. river bends)Wind effects (e.g. reservoirs)Horizontal density effects3D flows near structuresBuoyant plumes

Coastal processesModels of coastal processes in nearshore waters will represent the behaviour of cohesive andnon-cohesive sediment, sediment in suspension and can predict erosion and deposition rates.These models require information on the waves, currents and sediment in and around the siteof interest.Close inshore, one-line models are available for long-shore studies of long-term beachevolution. This type of model may be used to assess the impact of structures along a stretch ofcoast. There are also cross-shore models that can predict beach profiles by representingpotential drift rates. These models predict changes in the seabed, identifying areas of erosionand accretion and making this type of model particularly appropriate for design of cablelandfalls.Model selectionWhen selecting any type of model it is vital to consider:

What are the model’s capabilities, limitations and assumptions?What input information is required?Where is the input information required?What will the results be used for?What are the project’s time and financial constraints?

Finally, it is worth noting that users can influence the “performance” of a model. A model’sresults will be affected by the model set up (e.g. the extent of the modelled area, theresolution of the model grid) and by the treatment of the input parameters. As a general rule,as models get more sophisticated they require a greater level of expertise and more experienceto use them effectively.




If a user can influence the performance of a model, how do you know which is correct?All models are calibrated and validated during development - this confirms a model’sability to represent the physical processes concerned. Ultimately, effective modelling ofa particular site comes down to experience. In many applications it is common practiceto use models in a comparative way. The model is first set up to represent the existingsituation and this allows the model set up to be fine-tuned. The tuned model can thenbe used to represent the changed situation and the effect of the scheme underconsideration can be assessed.

How big are the physical models?Physical models must be constructed at a scale that is appropriate to the physicalprocesses under investigation. For example, if fluid viscosity is important the physicalmodel must be scaled to retain appropriate viscous behaviour. Similarly, if sedimenttransport is modelled, the physical model scale may be constrained by the properties ofthe material available to represent the sediment (e.g. coal dust may be used to modelsand).

5.3 Joint probability of winds, waves, currents and water levels

Dr Peter Hawkes, HR Wallingford Ltd.


It is relatively straightforward to assess probabilities of occurrence of variables that are eithertotally independent (e.g. two separate dice) or totally dependent (e.g. two dice that are lashedtogether). Unfortunately, it is more challenging to assess probabilities for variables that arepartially dependent upon each other such as winds, waves, currents and water levels.For the purposes of design of offshore wind farms, winds, waves, currents and water levelsmay be regarded as nuisance variables that will influence the loading and accessibility ofoffshore wind turbine structures. It is possible to assess the level of inter-dependence of two-variable data using a scatter diagram. If the variables are totally independent, entries on thescatter diagram will be random, with no discernible pattern. Similarly, if the variables are totallydependent, all the entries will lie along a particular line indicating the relationship betweenthem. A scatter diagram of two partially dependent variables will show some pattern ofcorrelation, but will also show a degree of scatter in the entries.Clearly, it is necessary to develop a strategy for assessing the probability of occurrence ofpartially dependent variables. It is also worth noting that it is very unlikely that there will beadequate data for design purposes and therefore there will be a need to extrapolate theexisting data to get appropriate design parameters.There are several possible approaches to joint probability assessment. These include (forcommonly occurring events) continuous simulation modelling and scatter diagram assessment,and for extreme events either a manual or analytical approach. The method adopted is likely todepend on the numbers of partially dependent variables concerned, budget, length and qualityof input data and the intended application.Continuous simulation of response/effect makes use of a hindcast model that takes continuousinput data of multiple variables in the form of, say, wave height, wave period, wind speed,current speed and water level, and compares the combined conditions at each time stepagainst a set of threshold functions, e.g. limits of turbine accessibility for each variable or forvarious functions of more than one variable. Such a model could be extended to formulate, for



example, a maintenance assessment by introducing additional limitations such as personnel orplant availability, time of day, cost issues or journey times.In simple circumstances, with few partially dependent variables, a scatter diagram analysis canbe used to predict the proportion of time when conditions lie within an acceptable envelope. Onthe most basic level, areas of a scatter diagram that represent conditions above thresholdlevels for each variable can be discounted allowing the ratio of remaining (acceptable)conditions to discarded (unacceptable) conditions to be assessed.To extrapolate beyond the existing data, distribution functions can be fitted to each “row” and“column” total in the scatter diagram. These variable distribution functions may then be usedto estimate extreme values. In the manual approach, the multiple variables would be combinedusing pre-computed combinations defined in terms of marginal (i.e. single variable) returnperiods. In the analytical approach it is also possible to assess the distribution of variabledependence by transforming the data into a standard statistical model that can be analysed,performing the analysis and then transforming back to the original variables.One problem that will be encountered when estimating joint exceedance extremes is that therewill be multiple answers – several combinations of, for example, significant wave height andwater level that have the same joint return period. Although all of the estimated extremes arepotential “worst cases” for design, in practice relatively few joint probability values need to betested for each design consideration and for each return period to demonstrate the overall jointexceedance pattern.


Ian Leggett (Shell UK Exploration and Production) commented that within offshore oiland gas industry estimates of joint probability appear to vary greatly, with no twoexperts in total agreement. Dr Hawkes noted that in his experience, it is perfectlypossible for the techniques described above to be repeatable and in agreement with theresults from other groups, including those from abroad.

6. Collecting and analysing site specific data

Session chair Dr David Pearce, PowerGen RenewablesTel: 0115 936 2000, Email: [email protected]

6.1 Danish experiences of taking offshore meteorological measurements

Dr Lars Landberg, Risø National Laboratory


Lars talked about some of the lessons learned in the course of the recent Danish programme tomeasure offshore winds specifically for wind energy purposes. He stressed that hispresentation is based on work done by Dr Rebecca Barthelmie.Some key points about offshore wind monitoring are:

• SAFETY IS PARAMOUNT – all personnel must operate in a safety culture, make sure themasts are well lit and are marked on charts etc.

• Test every piece of equipment on land (twice!) before taking it offshore

• Design to minimise the length of time when the offshore masts are inaccessible



• Providing power to the instruments and maintaining communications to download themeasured data is both difficult and expensive. (Note that in the UK there is a special lowmobile phone tariff for data transmissions)

To make a useful analysis of the data measured offshore vertical wind speed profiles areneeded to give information on the atmospheric stability. In very stable off-shore conditionsthere can be a very low IBL (of the order of 10m above sea level). Turbines above this heightexperience “land” conditions. In addition, although turbulence is generally much less intense atoffshore locations, at very low wind speeds there can be high turbulence due to atmosphericstability effects.Risø has tested several methods of extrapolating short-term data including Weibull, WAsP andMeasure/Correlate/Predict (MCP) techniques. All perform tolerably well.The analyses of the offshore wind data have shown:

Roughness adjustments seem to shift the whole wind profile, rather than altering theprofile shape.Mast shadow corrections to anemometer results are generally small (<10% for affectedwind sectors).Stability and IBL effects can be LARGE and are therefore very important.


How well does WAsP perform offshore?WAsP predictions based on coastal mast data have been compared with data measuredat five offshore sites in Danish waters (mostly within ~25km of the land). WAsP givesgood climatological estimates of the offshore wind resource, but some marinemesoscale features (e.g. low level jets and sea breezes) are not included in the WAsPmodel.Colin Heaton (UKMO) commented that WAsP gave encouraging results in their limitedseries of offshore analyses.Since WAsP predicts well offshore (in Danish waters and in the limited UK case) themarine mesoscale effects (less IBL’s which are included in WAsP) do not seem to havean effect on offshore wind climatology.

Has there been a comparative analysis of the Horns Rev (North Sea mast) and Laesø Syd(Kattegat mast) data? If so, does this show the effect of the Jutland peninsula on the prevailingSW winds?

These masts were only deployed in April 1999 so this type of analysis has not yet beenperformed, although there are plans to do so in future.

7. Discussions

Discussion chairs Dr David Pearce, PowerGen RenewablesTel: 0115 936 2000, Email: [email protected]

Dr Lars Landberg, Risø National LaboratoryTel: +45 46 77 50 24, Email: [email protected]

(Note: I have attempted to attribute comments to the correct person and affiliation, however, Iapologise if you made a contribution to the discussion and I either fail to note down yourname, or if in error I attribute it to someone else. Please contact me with any corrections.Thank you. Gillian)



David Hill (CLRC RAL) drew attention to a piece of remote sensing equipment, known asOSCAR, that may prove of use to the offshore wind energy community. In essence, OSCAR is aland-based radar system that is capable of making measurements of wave heights, currentspeeds etc. over a relatively large area (~20km2). Each OSCAR system costs in the region of£0.25 million which is in the same order of magnitude as an offshore met. mast.The need for additional information on extreme wind conditions for design purposes washighlighted, particularly as there is evidence of very large variability in extreme values withinsmall regions. David Kilham (U of Bristol) confirmed that in a recent storm off the Norwegiancoast, the return period of the extreme wind speeds measured varied dramatically over a fewkm. The general requirement for additional data can also be extended to offshore turbulenceintensities which are vital for assessing fatigue loads.It was noted that the mesoscale wind models (discussed in section 5.1) are being applied oververy large sea areas and model mesoscale features that may occur 100km or more offshore.This is not the region being considered by most UK developers, who are planning on sitingturbines 5–10km from the coast, with hub heights of 60-70m.A commercial delegate expressed concerns that wind model results within 2ms-1 of theverification data are described as “very good”. He felt it important to stress that a discrepancyof the order of 2ms-1 in mean wind speed predictions for a site would be critical to the viabilityof an offshore wind farm.There was some debate on the minimum period of wind measurements considered necessaryat a potential offshore wind farm site. Is one year of measurements sufficient (a rule of thumbsometimes used on land)?It was generally agreed that one year of measured data is not long enough to quantify all thevariations in the wind resource at any site (offshore or onshore). Clearly collecting severalyears of site data leads to greater confidence in the derived mean and extreme values. LarsLandberg (Risø) suggested that there is no reason to differentiate between onshore andoffshore sites when choosing the length of time to make wind measurements. However, hewent on to stress that he did not consider a one year monitoring programme sufficient evenonshore. Jean Palutikof (U of East Anglia) emphasised the need to put any period ofmeasurement into the context of the long-term cycles of variability.Bill Grainger (Border Wind Ltd) made the point that it is important to be aware of the overallsituation – MCP methods only work well for sites that are climatologically similar and so it isdebatable whether it is appropriate to use MCP to extend offshore measurements with landmast data.It was thought unrealistic to rely on a solitary source of data. The best strategy may be toconsider a range of data sources and to investigate ways of combining the data into long-termassessment (e.g. using reconstructed historical wind data). However, it is considered unlikelythat all uncertainties associated with wind resource assessment can be eliminated. The keyissue remains how to treat these levels of uncertainty.There was a call for a strategic offshore environmental assessment of offshore wind energy inUK waters. This will be required by politicians and for planning purposes. Who should co-ordinate it? What should it involve? How accurate would the assessment need to be?So… what is needed now? Is it foreseen that an adequate level of accuracy in wind andmetocean predictions can be achieved offshore? If not, what other actions should be taken?During the discussions it was apparent that there is a very high level of concern over the depthof the UK governmental commitment to offshore wind energy. Industrial delegates made itclear that they are waiting for indications that the UK government has fully determined its levelof commitment. Such a decision is keenly awaited in the context of further investment inoffshore wind monitoring and research. Delegates from the wind energy industry also made



the point that unless the industry is certain that electricity from offshore wind farms will besold at commercially viable terms, offshore wind farm developments will not proceed.David Milborrow pointed out that there has already been a basic assessment of offshore windresources in UK waters. This has demonstrated that the UK has a HUGE offshore windresource, and this alone should be sufficient to show that offshore wind energy is worthpursuing in the UK. However, at the moment the UK offshore wind industry is unable to answerquestions on the economics of offshore wind energy because of the uncertainties that stillexist. Clearly, more work is required to refine offshore wind resource predictions.It was noted that the spatial variability of the wind resource within an offshore wind farmwould be less compared to an onshore development. This is because on land a 10-15%variation can be expected due to topographical effects that are not an issue offshore.Therefore, it may be acceptable to have less accurate predictions of offshore wind resourcesthan on land. The argument is as follows: Ultimately, the economic basis for a wind farmdepends to a large degree on the level of uncertainties that exist. Individual turbines within anoffshore wind farm will experience less variation in wind speed than on land. Therefore, theoverall level of uncertainty associated with the wind resource assessment may be similar foroffshore predictions with an accuracy of, say, 0.5ms-1 compared to 0.1ms-1 accuracy onshore.Rebecca Barthelmie (Risø National Laboratory) commented that it is worth noting that due tolower offshore turbulence the wake effects generated by each turbine in the wind farm willlikely propagate over larger distances than onshore, and that the effects of stability on wakepropagation are expected to be larger. The implication is that wind farm design/spacingbetween turbines needs to be carefully considered.Given that MCP techniques are now at the limit of development there is a clear need for furthermodel development. Simple wind models have been shown to give good climatologicalpredictions offshore and there may be scope to develop this type of model further for marineapplications. However, the simple models are unable to reproduce the fine temporal detailassociated with low level jets and sea breezes. To achieve this high degree of accuracy, thesemarine mesoscale features must be included in the models. Several mesoscale wind modelsalready exist (e.g. the Uppsala MIUU model, the Risø KAMM model), although at the momentthey should be regarded as research tools.Clearly models of offshore winds and the other metocean parameters have a central role toplay in exploitation of offshore wind energy and therefore it is important to continue to developsuitable modelling tools for use by the industry. In particular, there are increasing demandsfrom industry for a commercially available, operational mesoscale wind modelling tool. Ascomputers become increasingly powerful, this objective may be feasible on a time scale ofyears, but it is important to recognise that mesoscale wind models will require a step change inthe level of expertise required to operate the tools effectively.Natalie Edwards (ABP Research and Consultancy) revealed that she has been involved with aMAFF funded project that aimed to bridge the knowledge gap in morphological modelling. Thisincluded work to gather data and to develop models. Could a similar approach be used foroffshore wind?Several people suggested it would be useful to create a database of existing offshore wind dataas well as indications of where firms are currently engaged in monitoring projects (withoutnecessarily having to release the data). Peter Clibbon (NEG Micon UK Ltd.) noted that duringthe 1980’s a database of this type was created, although he doubted it had been updated.Overall, delegates expressed an urgent requirement for more measured offshore wind data.Currently there is little transfer of measured data because of its huge commercial value.Although this is understandable in the short-term it may prove to be a very detrimentalstrategy for long-term development. Ian Leggett (Shell UK Exploration and Production)commented that in the early days of offshore hydrocarbon exploration in the North Sea, the oiland gas industry (which is also highly commercial) made the joint commitment to share the



cost of gathering data resulting in joint datasets. This has proved very useful to all partiesconcerned.Many delegates thought it could be vastly beneficial to have a DTI supported national offshorewind measurement programme. David Milborrow pointed out that the UK government did putmoney into onshore wind monitoring before the NFFO initiatives. Could the DTI be persuadedto support something similar in the run-up to OFFO? ETSU have approached DTI withpreliminary plans, but government would welcome the view of industry, about whether aconcerted offshore measurement programme would be beneficial. Mark Thomas (ETSU)encouraged everyone to contact James Craig (for contact details see section 2.3) as this mayhelp to achieve a “critical mass” of support for a particular action. He felt it important that theeffort should be focused into routes where something may be achieved e.g. lobbyinggovernment via ETSU and BWEA. Delegates also stressed the need for a coherent offshorewind resource assessment strategy. (POSTSCRIPT: Since the date of the workshop,James Craig has transferred his responsibility for the wind energy R&D portfolio toIan Fletcher of ETSU – see section 2.3 for contact details.)



OWEN workshop on Offshore Wind Resource Assessment and Metocean Data

MAIN DISCUSSION POINTS

• It has already been demonstrated that the UK has a HUGE offshore wind resource

• The principal knowledge gap is the wind regime in the coastal zone, where windconditions are influenced by both land and sea.

• There appears to be adequate availability of metocean data and metoceanmodelling tools

• Offshore wind resource predictions are not yet accurate enough to answer detailedquestions on the financial viability of offshore wind farms.

• There is a need to investigate ways of combining the data from multiple sources (e.g.wind model data, reconstructed wind data, measured wind data etc.) into accuratelong-term wind resource assessments.

• In the medium-term, it may be acceptable for offshore wind resource predictions to beless accurate than those required on land.

• There is a demand for more sophisticated wind modelling tools. In particular there is aneed for a commercial mesoscale wind modelling tool that can be applied to UKcoastal waters

• There is a severe lack of measured offshore wind data in UK waters, however there areno technical barriers to offshore wind monitoring

• The wind energy industry is waiting for news that the UK government has fullydetermined its level of commitment to offshore wind energy. Such a decision is keenlyawaited in the context of further investment in offshore wind monitoring and research.

• Offshore wind farm development in UK waters will not proceed unless the industry iscertain that electricity produced by offshore wind farms will be sold at commerciallyviable terms.

• There would be a high level of support (from both industry and researchers) for anational wind monitoring programme in UK waters

• There is an IMMEDIATE need for a coherent UK offshore wind resource assessmentstrategy

8. Feedback comments

There were 61 registered delegates for this workshop, representing both research andcommercial organisations.A feedback form was included in the information pack given to each delegate at the workshop.However, the return rate was disappointing with only 11 completed forms to date.How useful was the event?

a) very useful 3 (27%)b) useful 8 (73%)c) not very useful 0d) a complete waste of time 0

Support for a follow-on workshop in this subject area:Yes (5) No (5)

Gillian WatsonOWEN Co-ordinator

November 1999

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Offshore Wind Resource Assessment and Metocean Data … · Report on OWEN workshop on Offshore Wind...

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