8.1 Introduction 548.1.1 The RAVE Service Project 548.1.2 Environmental conditions in the North Sea 54
8.2 Methods 558.2.1 Study design: Oceanography 558.2.2 Study design: Geology 57
8.3 Results and discussion 608.3.1 Currents 608.3.2 Scouring effects 61
8.4 Perspectives 64
8.5 Acknowledgements 64
Literature 65
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Oceanographic and geological research at alpha ventus : Instruments for predicting environmental conditions and interactionsBettina Kühn & Anja Schneehorst
The last decade has seen growing demand for off-shore wind power. The weather conditions in the North Sea guarantee large wind energy yields. Yet strong winds lead to high and rough waves, in-creasing the costs and risks of windfarm construc-tion and operation. Outside of the oil industry, there is little prior experience in building large structures in offshore conditions. What kinds of forces act on wind turbines? How often can ves-sels transfer people or material to and from wind-farms? How can structural integrity be ensured in rough offshore conditions and do foundation structures cause changes in sediment structure? To answer these questions, the alpha ventus test site was dedicated as a playground for ongoing offshore research. Reliable information on the prevailing sea state – wave heights, wave direc-tions and wave periods – is essential for planning offshore logistics and cost-intensive shipping and work operations. Such information is also essential for safety purposes. The transfer of people from a vessel to a wind turbine is one of the most critical situations for offshore engineers. At alpha ventus, the windfarm operators only allow access to wind turbines at wave heights less than 1.5 m (alpha ven-tus Health & Safety Executive, DOTI 2009). But even simple maintenance work is heavily affected by wind and waves (. Fig. 8.1).
The RAVE Service Project is coordinated by the Federal Maritime and Hydrographic Agency (BSH) and divided into several work packages covering a wide range of technical and scientific measure-ments. A central feature of the test site research was that measurements were taken directly from off-shore structures. As many sensors as possible were mounted prior to erection of the turbines at sea to reduce the amount of difficult offshore installation work. Sensors under the water surface in particu-lar had to be installed in advance onshore by the scientists. Technical measurements on the two dif-ferent foundation types were conducted by the Ger-man Wind Energy Institute (DEWI) and GL Gar-rad Hassan Deutschland (GL). GL also conducted measurements at transmission level. BSH collected
geological and oceanographic data to obtain reliable information about interactions between offshore facilities and the marine environment. The FINO1 research platform in the direct vicinity of alpha ven-tus was also part of BSH’s oceanographic research programme. Meteorological, oceanographic and ecological data has been collected there since 2003, when the German government first decided to in-vestigate the offshore environment and in particular the prevailing wind conditions (▶ Information box: FINO1 research platform). BSH also coordinated and supported all measurements for the RAVE research initiative in the test site area.
The focus of the geological investigation pro-gramme was on interactions between the piled foundations, the seabed and the oceanographic pa-rameters (e.g. wave, tide and current), and sediment shifting in the windfarm. The depth and extent of scour can influence the stability of a structure. In the absence of in-situ measurements, a combination of fixed single-beam echosounders and full-coverage multibeam echosounding was applied to acquire a spatio-temporal dataset of scour dynamics at alpha ventus.
8.1.2 Environmental conditions in the North Sea
The prevailing weather conditions in the North Sea are ideal for the exploitation of renewable energy. Winds tend to be stronger and more sustained off-shore than onshore. Atmospheric circulation over the German Bight is dominated by the Westerlies, hence the annual average wind direction is west or southwest (. Fig. 8.2).
The North Sea is situated on the European con-tinental shelf. Water depths in the German Bight are around 40 m. The North Sea is open to the At-lantic Ocean and therefore dominated by a tidal signal entering from the northwest. The tidal wave circulates anticlockwise along the coasts. Its centre is the amphidromic point where no water elevation takes place and where there is no tidal range at all. Leaving the deep water and entering the continen-tal shelf, the tidal currents increase and turbulent mixing occurs. Solar heating causes stratification of the water column, resulting in warm water in up-
Chapter 8 • Oceanographic and geological research at alpha ventus 54
per layers and colder water below. These two drivers compete unequally as the tidal force is dominant. Stratification of the water column like that found in the Baltic Sea is therefore rare. Wind conditions modulate, increase or decrease the ocean currents driven by the tides.
The seabed of the North Sea was shaped during the last ice age more than 10,000 years ago. Today’s seabed was part of the land masses and sea level was much lower than at present. After the melt-ing of the glaciers, the world sea level increased, flooding the land masses and creating the North Sea. The sediments close to the alpha ventus test site are homogenous, their main components be-ing fine sands that differ only in their content of fine material.
The geological model of the upper subsurface (. Fig. 8.3) of the alpha ventus site is based on the results of a high-resolution sub-bottom profiler sur-vey performed by the BSH vessel WEGA in 2008. The survey was carried out before construction at al-pha ventus started. The seismo-stratigraphic model was set in relation to the sediment characteristics found from drillings taken during research for alpha ventus. It displays the geological features in the up-per 5 m of the seabed, such as palaeochannels and trenches which are of relevance for understanding the scouring process.
8.2 Methods
8.2.1 Study design: Oceanography
A range of sensors and measurement devices were mounted on wind turbines and in the test site for detailed investigation of oceanographic conditions. Temperature sensors, directional wave riders, video cameras and current sensors captured data to anal-yse interactions between the windfarm and the ocean. The sensors had to be replaced every half a year, making it necessary to exploit spells of good weather. Different sensor types were used to compare different measurement techniques in order to obtain high-resolution, reliable and redundant datasets and to develop the best possible monitoring approach.
A directional wave rider collected sea state data and transmitted it immediately to a receiver so that the information would be directly available to wind-farm operators. This information was invaluable as it meant that the decision on whether to send a vessel to a windfarm could be based on data updated every half hour. The buoy was attached to an anchor by an elastic rope. Passing waves lifted the buoy up and down, en-abling the parameters wave height and length, period and direction to be registered and transmitted.
A radar gauge and video cameras were mounted on two wind turbines to capture selected high wave events. The cameras were triggered by the informa-tion collected by the wave rider. If the wave rider re-corded waves over 2 m, a signal was sent to the cam-
55 88.2 • Methods
. Fig. 8.1 (a) Transfer from zodiac to boat landing. (b) Engineer climbing down the boat landing (photo: Marine Measurement Network / BSH).
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. Fig. 8.2 Scheme of the marine current model of the German Bight. Blue arrows indicate the tidal wave entering the North Sea and circulating anticlockwise. Green arrows indicate mean wind conditions.
Chapter 8 • Oceanographic and geological research at alpha ventus 56
era, which then started recording. Independently, the camera captured waves of extreme heights using an event-driven mode. This worked by choosing an event window at a specific height (15 m above sea level); if there was any movement in the window, the camera started recording. The investigation of extreme wave events was important for safety ques-tions and load calculations.
Acoustic Doppler current profilers (ADCP) col-lected current and sea state data and were placed in a frame on the seafloor close to the wind turbines. They were battery driven and had to be replaced periodi-cally. An ADCP sends out a signal that is reflected back by small particles floating in different layers of the water column (such as plankton and floating sediment particles). A sensor calculates the current direction from the time it takes for the outgoing sig-nal to be reflected back to the sensor. The sea state parameters are captured using a pressure sensor in the ADCP. When a wave passes, the pressure on the sensor rises or falls, allowing wave heights and peri-ods to be calculated from the pressure differences.. Figure 8.4 gives an overview of the sea state
sensor positions in the test site and . Fig. 8.5 shows a typical measurement setup on a wind turbine. Some sensors were battery driven, while others were cable-connected. The project started out with mainly cable-connected sensors but loss of cable ca-pacity led to more battery-driven setups. This was a lesson that had to be learned – collecting data 75 km off the coast is not easy.
8.2.2 Study design: Geology
To monitor scour depths and processes, 33 single-beam echosounders were attached to a tripod (AV7) and a jacket foundation (AV4) before the turbines were erected at sea. At the beginning of the project, there were no single-beam echosounders for depths of 30 m on the market. It was therefore necessary to improvise using chandlery sensors. These were mounted in advance at the dockyards, so recording started very early, in 2009. Unfortunately, pile driv-ing while erecting the wind turbines destroyed most
sensors on one of the wind turbines. Apart from the losses from pile driving, the sensors worked well in 30 m of water even though they were not designed for such depths.
From 2009, single-beam echosounders began re-cording data at high temporal resolution. A single-beam echosounder works by sending an acoustic signal vertically downwards to the sea floor and mea-suring the time that passes until its reflection comes back. The instrument than calculates the depth using the sound velocity based on temperature and salinity. A value was determined from five simultaneous mea-surements every 10 minutes, making it possible to record 120 values per day. The results were time-se-ries for the several years since installation. They were corrected by a median filter and made it possible to determine both scour depth and scour dynamics. The echosounders at the tripod started recording in 2009 and those on the jacket foundations in 2010, the jacket foundations having been taken offshore later. As just mentioned, pile driving caused a lot of dam-age to the sensors, and nowadays only four sensors on the tripod foundations collect data. There are ten active sensors on the jacket foundations.
In addition to the single-beam echosounders, which only provide local measurements, we also acquired hydroacoustic measurements with multi-beam sonar (. Fig. 8.6). The aim of these measure-ments was to scan the seabed from a vessel mov-ing at a distance of 40 m around the structure. The transmitter simultaneously sent out multiple beams in a fan-like formation transverse to the ship’s mo-tion. Coverage of the scanned area depended on the aperture angle and water depth. Scans were done in four transects until the bathymetric image of the seabed was complete. To obtain the vertical depths, the vessel’s motion additionally had to be recorded. A motion sensor was therefore integrated into the measuring system. The sensor captured the vessel’s drift, roll and pitch. A gyro compass was also in-corporated to determine the fan orientation. Mor-phodynamic snapshots have been taken in this way at selected wind turbine locations every spring and autumn between 2009 and 2013 to record the scour situation before and after winter storms.
57 88.2 • Methods
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. Fig. 8.4 Environmental measuring points in the alpha ventus test site, showing the four sensor types used to collect sea state data (during 2009–2013).
Chapter 8 • Oceanographic and geological research at alpha ventus 58
Information box: How does scour develop?The phenomenon of scour can occur in the sur-roundings of a structure located in a flowing me-dium such as the sea or a river. Changes in current patterns can result in increased erosion of the seabed: A flow pattern is disturbed by an obstacle (e.g. a rock or a pile foundation), passes around it and develops a horseshoe vortex (. Fig. 8.7b). This may cause depressions around the structure which are then called scour (. Fig. 8.7a).Scour investigations are crucial in assessing the structural integrity of an offshore construction. Measurements of scour depths and dynamics are key parameters in periodic inspections by wind-farm operators. The scope of measurements done during the RAVE Service Project went beyond the efforts of windfarm operators.With regard to site investigations and construc-tion of offshore wind turbines, initial experience showed that, as with environmental investigations, lack of experience and standardisation posed a challenge to licenses. To improve legal and invest-ment certainty, BSH has published a standard on the subject. Compiled in cooperation with a group of experts, the standard lays down detailed mini-mum requirements for mandatory geological/geophysical and geotechnical site investigations at planned windfarm sites (Standard Ground In-vestigations for Offshore Windfarms, BSH 2008).A further standard has been drawn up that speci-fies the requirements for offshore wind turbine
design and ensures that all installations and struc-tural components are certified (Standard Design of Offshore Wind Turbines, BSH 2007).
. Fig. 8.7 Schematic diagram of scouring: (a) decrease in sediment, (b) correlation between current parameters and grain size distribution (after Hamill & Lucas 1999).
. Fig. 8.6 Diagram of a multibeam echosounder measure-ment system.
59 88.2 • Methods
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8.3 Results and discussion
8.3.1 Currents
The marine current profiles were taken by bottom-mounted ADCP sensors. The western and south-eastern ADCP were cable-connected (. Fig. 8.5), while the ADCP close to the wind turbines were battery-driven.
The main currents at the alpha ventus wind-farm were heavily influenced in speed and direc-tion by the tidal signal. . Figure 8.8 shows the mean current directions and velocities in the dif-ferent water layers measured at the FINO1 plat-form between 2010 and 2011. The water column can be classified for illustration purposes into three main layers: In the upper layer (10 m below the sea surface), current flow was mainly driven by the wind and varied in direction and intensity. Down towards to the bottom, the water layers became heavily influenced in current direction and veloc-ity by the tidal signal. Closest to the seafloor (5 m above the bottom), the current showed a strong east-west trend following the tidal rhythm. Close
to the bottom, friction played an important role and affected current velocity.. Figure 8.9 shows the bottom currents 5 m
above the sea floor. The currents are split into their vector components with each subplot showing a dif-ferent component. The two upper subplots show the horizontal current components. U represents the east (positive)-west (negative) current; v represents the north (positive)-south (negative) component. The third subplot shows the vertical velocity w, with positive values indicating movement to the surface and negative values indicating downward movement. The vertical velocities captured in alpha ventus were small but mainly negative. The tidal signal can be identified in the horizontal currents. A balanced cur-rent pattern can be identified in the upper subplot; the currents show the same intensity in the eastward and westward directions. The pattern in the subplot below is not as balanced, but the tidal rhythm can still be identified. The three current components varied strongly in intensity, but the identified cur-rent signal at alpha ventus matched with the overall current vector modelled for the North Sea, turning north-eastwards in the German Bight (. Fig. 8.2).
. Fig. 8.8 Mean current velocities and directions, divided in 4 m layers, at FINO1 platform between 2010 and 2011 (graphs: Jens-Georg Fischer / BSH).
Chapter 8 • Oceanographic and geological research at alpha ventus 60
8.3.2 Scouring effects
An impression of the scour depths and dynam-ics was gained by evaluating the time series from the single-beam echosounders. The diagrams (. Fig. 8.10a,b) show the maximum values from the measurement series. The individual time series for the different echosounders are partially comparable, but in principle each time series stands on its own. This is not surprising as scouring did not take place simultaneously at all locations.
In the case of AV7 (. Fig. 8.10a), one time se-ries was included in the analysis for each pile. As can be seen, the NE and SE pile show similar re-sults. The scour measurements for the west pile did not reach the same depths. The measurements for the central segment (C) correlated with the trends at the piles but not with the depths. At the end of 2012, the sensor broke down. All curves show a rapid increase in scour depth in the first half year. On average, scour increased by 40 cm per month
during the first half year. After that initial period, the scouring process slowed down. Since 2010 the average has been 5 cm per month. From 2009, lo-cal scours developed separately around each pile of the AV7 tripod structure. These evolved later on into one large scour beneath the whole tripod struc-ture. From March 2010, the individual scours could hardly be distinguished.
The same analysis was done for the jacket foun-dation on AV4. The graph (. Fig. 8.10b) shows the time series for the north (N), east (E) and south (S) single-beam echosounders. Sonar at the western pile was unfortunately not working. Data were recorded at AV4 from September 2010. As with AV7, a sig-nificant scour developed in the first half year, with sediment removal at 30 cm per month. This process slowed down from 2011 with an average reduction of 2 cm per month. A look at the S curve in spring 2013 shows a sudden increase in scour depth of around 30 cm. It has not yet been possible to clarify why this happened. The scour hole is still subject
. Fig. 8.9 ADCP time series for the bottom layer (5 m) over two days in March 2014 at 140 m distance to AV4.
61 88.3 • Results and discussion
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Chapter 8 • Oceanographic and geological research at alpha ventus 62
to ongoing monitoring. All curves show short-term scour reductions with a maximum duration of two months (. Fig. 8.10a, grey dotted box), but such reductions did not occur at all piles at the same time. It has not been possible to link these peaks to weather events, but it seems likely that there have been local sediment slips. The scour around the
piles for the AV4 jacket structure (. Fig. 8.11b) de-veloped locally around each pile and has not so far evolved into a single scour.
The eroded sediment accumulated mainly to the east of both structures, although a slight ac-cumulation of sediment to the west could also be observed.
. Fig. 8.11 Diagram of the seabed topography and scour holing at AV7 (a) and AV4 (b) from boat-based multibeam echo-sounding surveys (March 2012).
63 88.3 • Results and discussion
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The main outcome of the curves was to deter-mine trends. The method developed here provides insights on scour development over time in a way that was not possible before. The scouring process is still in progress and its monitoring and interpreta-tion is a task for future research projects.
The multibeam measurements made it pos-sible to compile high-resolution elevation models. The survey results confirmed the data from the fixed single-beam echosounders (. Fig. 8.11a,b) and provided a broader view of the area of inter-est. Although the analysis was not continuous and only represented a snapshot of the scour situation, it contributed to a better understanding of the time series from the stationary single-beam echosound-ers. The method enabled the extent of the scour to be determined as well as the depth.
The results from both foundation locations demonstrated the different influence of foundation design on the effect of scouring. Based on the data from the geological subsurface model of alpha ven-tus (. Fig. 8.3), the deeper scour at AV7 could not be attributed to subsurface conditions within the test site area. It is notable that the scour at AV7 and AV4 is still increasing, even though at a very low rate. In the case of AV7, the results showed the development of a local and a global scour. At AV4, the local scour was more pronounced than the global scour.
Scour holing can be increased or reduced ac-cording to situation with regard to marine currents at the site in question. At alpha ventus, the tidal signal has a strong influence on current speed and direction, with a strong promoting effect on sedi-ment transport. It can be concluded that different types of foundations will show different scouring behaviour depending on sediment, water depth and currents.
8.4 Perspectives
Many windfarms have been built in the North Sea in recent years and there are many more to come. The experience gained in the alpha ventus test site marked only the beginning, since many questions remain unanswered and new questions have arisen. The collected data has led to a better understand-ing of the marine environment. A need has been
demonstrated for monitoring systems and online sea state data, as the weather changes very quickly offshore. The knowledge obtained about scour dynamic may be used in planning future founda-tions, and small changes in base designs may re-duce material fatigue and hence cost. Correlations between sediment transport and current speed and direction were a focus of geological and oceano-graphic research in the RAVE Service Project and an insight was gained for the first time into the de-velopment of scours at offshore wind turbines. Fur-ther investigation needs to be done to identify the parameters responsible for scour development. As this process is highly complex, it needs to remain a focus of ongoing research, but the interaction of single-beam and multibeam sonars in combination with sea state and current sensors seems to be a good approach.
Future projects that investigate other wind tur-bines and their scouring behaviour are in the plan-ning process. The sensors available on the market have improved and still need to be tested. To mini-mise the risk of being surprised by the weather and losing cost-intensive construction time, it is crucial to know about prevailing conditions in the wind-farm area when planning offshore work. It is essen-tial that the acquired experience and information be shared, not least because cost reduction remains a key factor for industry. Mistakes have to be made as part of the learning process but need to be kept to a minimum. Private-sector competitors should therefore work more closely together and communi-cate very openly on the problems they encounter in order to benefit from each other’s knowledge.
8.5 Acknowledgements
We would like to thank our colleagues at the BSH Marine Measurement Network, the BSH Geology Sub-Division and the crews of the VWFS ATAIR and the VWFS WEGA. They all do a great job and without their capable help the project would not have collected so much data and been so successful.
Chapter 8 • Oceanographic and geological research at alpha ventus 64
Information box: FINO1 research platformIn 2002, the German government decided to erect dedicated research platforms in the North Sea and the Baltic Sea to improve knowledge of offshore conditions with a view to planned and approved offshore windfarm projects. Two research plat-forms were built in the North Sea: FINO1 next to alpha ventus, 45 km north of the island of Borkum (. Fig. 8.12), and FINO3, 80 km off the island of Sylt. The FINO2 research platform was erected in the southwestern part of the Baltic Sea. The FINO project is funded by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU). The FINO1 research platform started data acquisition in summer 2003 and a huge amount of environmental, meteorological, ocean-ographic and ecological data has been collected since. The platform is equipped with a wind met mast and physical oceanographic sensors under the water surface that capture temperature, con-ductivity, pressure and oxygen. Sea state data is collected with a sea state buoy and a radar gauge. Video cameras are additionally mounted on the platform to capture extreme wave events. The research focus is on optimising offshore logis-tics, developing safe shipping systems, detecting changes in ocean dynamics and developing me-teorological and oceanographic model forecasts. Ecological research focuses on long-term moni-toring of bird migration and artificial reef develop-ment on offshore structures. BSH operates a large database of comprehensive meteorological and oceanographic measure-
ments generated at the three research platforms. Use of the FINO data for research purposes is free of charge. For further information or data access, see ▶ www.bsh.de. For further information about the FINO project, see ▶ www.fino-offshore.de.
BSH (2008). Standard Ground Investigations for Offshore Wind-farms. Bundesamt für Seeschifffahrt und Hydrographie, Hamburg and Rostock, 40 p.
BSH (2007). Standard Design of Offshore Wind Turbines. Bundesamt für Seeschifffahrt und Hydrographie, Hamburg and Rostock, 48 p.
DOTI (2009). Arbeitsschutz und Sicherheitskonzept zum Off-shore Windpark alpha ventus der Betreibergesellschaft DOTI GmbH & CO KG, German Version, 72 p.
Hamill I & Lucas T (1999). Computational fluid dynamics mod-elling of tundishes and continuous casting moulds. Fluid Flow Phenomena in Metals Processing, pp. 279–286.
