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ORESSuMReport.pdf
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Offshore Renewable Energy Site Suitability Mapping (ORESSuM)
Research · September 2015
DOI: 10.13140/RG.2.1.3988.5284
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Irish Sea Suitability Mapping for Novel Offshore Foundations (ISSMaNOF) View project
Scour Potential Evaluation of the Western Irish Sea Mud Belt (SCOPE) View project
Mark Coughlan
Irish Centre for Research in Applied Geosciences
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ORESSuM Offshore Renewable Energy Site Suitability
Mapping
Mark Coughlan,Dr. Andrew Wheeler and Dr. Boris Dorschel
2
Executive Summary
High wind speeds coupled with energetic tidal regimes and strong waves exist off the coast of
Ireland. It forms a large, relatively untapped renewable energy source that could help Ireland reach
its green energy targets in the future. So far offshore wind energy has only been harnessed at
Arklow Bank on the east coast whereas tidal and wave energy devices are still largely at the research
and development stage.
In order to expand the offshore renewable energy sector Ireland needs to be able to provide
detailed information and data from a variety of disciplines when it comes to site selection for
offshore installations. In this regard a large amount of data pertinent to the offshore renewable
energy sector has been collected over the decades, particularly as part of the INFOMAR program.
This data forms a sound basis from which to create geological models for areas identified as being
potential sites for development. However, there is scope for additional data collection, especially
with reference to studying seabed dynamics. Practices such as the deployment of ADCP’s and other
such current velocity monitoring equipment during the course of site surveys would greatly enhance
the comprehensiveness of the survey and its ability to provide the necessary data for not only the
initial geological model but also subsequent studies such as scour modeling and sediment transport
which, over time, need to be monitored throughout an installations lifetime.
Similarly, the importance of adequate sample collection is highlighted in the case studies mentioned
within with particular reference made to the OSIG guidelines which serve as a useful reference with
regard to site survey best practice.
By assessing the INFOMAR dataset and its ability to meet industry requirements it is possible to
tailor future INFOMAR data collection and, in doing so, address one of Ireland’s key policy objectives
of enhancing the knowledge economy and renewable energy sector, which are the current
government’s perceived key economic drivers.
3
Contents
Contents............................................................................................................................................ 3
1 Project Background.................................................................................................................... 5
1.1 Renewable Energy.............................................................................................................. 5
1.2 Sites Around Ireland ........................................................................................................... 5
1.3 Current State of Irish Offshore Renewable Sector ............................................................... 5
1.4 Scope of the Study ............................................................................................................. 7
2 Methods .................................................................................................................................... 7
2.1 Industry Appraisal .............................................................................................................. 7
2.2 Data Collection and Integration .......................................................................................... 8
3 Appraisal of Industry Needs ....................................................................................................... 9
3.1 Introduction ....................................................................................................................... 9
3.2 Geological Features ............................................................................................................ 9
3.2.1 Seafloor Morphology ................................................................................................ 10
3.2.2 Quaternary Geology ................................................................................................. 10
3.2.3 Solid Geology ........................................................................................................... 11
4 General Physical Features ........................................................................................................ 13
4.1.1 Seabed Bathymetry .................................................................................................. 13
4.1.2 Wind Characteristics ................................................................................................. 14
4.1.3 Wave Characteristics ................................................................................................ 14
4.1.4 Tidal Regime ............................................................................................................. 17
4.1.5 Distance to Shore ..................................................................................................... 18
4.2 Ecological Factors ............................................................................................................. 19
4.2.1 Special Areas of Conservation (SAC’s) ....................................................................... 19
4.2.2 Special Protection Areas (SPA’s) ............................................................................... 19
4.3 Socio-economic Factors.................................................................................................... 22
4
4.3.1 Commercial Navigation ............................................................................................ 23
4.3.2 Fisheries ................................................................................................................... 23
4.3.3 Cables ...................................................................................................................... 23
4.3.4 Shipwrecks ............................................................................................................... 28
4.3.5 Military Installations and Firing Ranges ..................................................................... 28
5 Assessment of INFOMAR Dataset Compatibility ....................................................................... 31
6 Site Survey Methodology ......................................................................................................... 34
6.1 Current Information ......................................................................................................... 34
6.2 Desktop Study .................................................................................................................. 36
6.3 Geophysical Survey .......................................................................................................... 37
6.4 Geotechnical Survey ......................................................................................................... 41
7 Case Studies ............................................................................................................................ 45
7.1.1 Sheringham Shoals ................................................................................................... 45
7.1.2 Scroby Sands ............................................................................................................ 48
8 Conclusions ............................................................................................................................. 53
8.1 Data Gap Analysis............................................................................................................. 53
8.2 General Conclusions ......................................................................................................... 54
9 Recommendations ................................................................................................................... 56
10 References ........................................................................................................................... 61
1 Project Background
1.1 Renewable Energy
Renewable energy comes from inexhaustible sources which are continually replaced (e.g. wind,
hydro power, direct solar power, wave, biomass, geothermal and tidal). The utilization of these
sources produces little or no carbon dioxide as well as other such greenhouse gases identified as the
main drivers of global climate change today, widely considered the most pressing environmental
issue of the modern age.
The Irish State is committed to achieving a target of 16% of all its energy needs (heat, transport,
electricity) to come from renewable sources by 2020 under Directive 2009/28/EC as part of the EU’s
commitment to the Framework Convention on Climate Change signed at Kyoto in 1997. Ireland has
at its disposal ample potential from renewable sources to achieve this goal, particularly in its
offshore sector (i.e. offshore wind, wave and tidal energy)
1.2 Sites Around Ireland
The greatest potential for offshore wind energy in particular lies off the western coast. However,
water depth increases too rapidly and so installations have to be located close to land where wave
exposure is high and connection to the grid presents a problematic issue. Similarly, wave energy
potential is greatest off the west coast and is a more feasible option here than wind. The south coast
is generally unfeasible due to a lack of shallow water close to shore and the close proximity of
bedrock or rock exposure to the seabed. The east coast provides the necessary shallow water
conditions to make wind energy viable as well as depths with high energy hydrodynamic regimes
which make tidal energy a possibility. However, this high energy also makes scour a destructive and
limiting factor.
1.3 Current State of Irish Offshore Renewable Sector
In recent years tidal and ,in particular, wave energy have become the main focus of offshore
renewable energy in Ireland whereas wind has become the most viable and hence most developed
with installations constructed at Skerd Rocks, Codling Bank, and Arklow Bank, with further sites set
to be developed at Dundalk Bay, Bray and Kish Banks. The Irish Government has set a target to have
500MW of wave and tidal capacity in operation by 2020, transforming the island of Ireland into
‘Europe’s Battery’.
6
Research into tidal energy has been largely conducted in estuaries most notably Strangford Lough
which has the worlds first commercial scale turbine commissioned in 2007. Issues identified with the
installation of these turbines focus mostly on socio-economic factors and their operational layout.
Socio-economic factors relate to navigation paths for commercial and recreational vessels being
blocked. Usually, turbines are constructed in banks with lines of turbines one behind the other
resulting in energy being dissipated once it passes the first line contributing to lower energy yields
for subsequent lines of turbines. Areas within the Irish Sea have been identified where tidal energy is
high and that are sufficiently distanced from shore so that the above constraints are negated.
However, the high energy hydrodynamic regimes of these areas also have increased seabed scour
which is a limiting factor in constructing foundations.
Ireland has a first-rate ocean energy research base represented by both academic and commercial
interests with world class levels of expertise in project design, testing and mooring design. The
Marine Institute, in association with Sustainable Energy Ireland, has established an Ocean Energy
Test Site for scaled prototypes of wave energy devices in Galway Bay where, most notably, Ocean
Energy Ltd. and Wavebob Ltd. are currently testing prototypes. The Sustainable Energy Authority of
Ireland also plans to develop a National Wave Energy Test Site to be located off Annagh Head, west
of Belmullet in County Mayo. This test site will provide a location for the temporary mooring and
deployment of wave energy machines in order to monitor their ability to generate electricity and
survive open ocean conditions.
Wind currently remains the most viable of all offshore renewable energy sources because of
Ireland’s large wind resource (up to 9m/sec in some places). The majority of these projects have
targeted the banks located offshore of the eastern coast. This includes most notably the
construction of seven turbines at Arklow Bank located roughly 11.7km offshore in an average water
depth of 20m. Each capable of generating up to 3.6MW, totaling 25MW altogether. Phase 2 of the
Arklow Bank project is currently dormant but environmental impact assessments (EIA’s) and site
surveys are currently being carried out at Codling Bank, Dundalk Bay and the Kish and Bray Banks in
the Irish Sea as well as Skerd Rocks on the west coast for the potential construction of turbines. With
increasing technology in this sector the construction of windfarms in water depths between 30-40m
is becoming more and more feasible. This pushes locations for potential windfarms further offshore
away from banks where sedimentary environments are less mobile and therefore scour is less of a
problem.
7
1.4 Scope of the Study
We will assess the current state of the offshore renewable energy sector in Ireland identifying the
key datasets as identified by industry as being of vital importance in the construction of offshore
renewable energy installations, investigating the availability of these datasets and assessing
INFOMAR’s ability to provide such datasets highlighting gaps
The chief data requirements referred to consider seabed geology and sedimentary dynamics related
data. However, the scope of the study also considers socio-economic, ecological, metocean and
general physical factors.
A number of case studies regarding data use in successful offshore projects are also presented.
2 Methods
2.1 Industry Appraisal
Telephone interviews and email correspondence were carried out with people identified as having
an interest and currently working in the offshore renewable sector in Ireland as well as abroad.
Similarly the Irish Renewable Energy Summit 2010 was attended in order to network and speak first
hand with industry players as well as gather information regarding the current state of the sector.
The information sought through these correspondences was;
The company’s current interest in Irelands offshore renewable energy sector;
The perceived datasets required in offshore installation construction;
Which of these datasets were sourced from INFOMAR or other state bodies;
Which of these datasets were acquired by third party investigations and surveys;
The level of satisfaction regarding dataset availability and usability;
Any potential areas that company had earmarked for future site investigation.
8
2.2 Data Collection and Integration
The basic data source for this desktop study report comes from phone interviews, email
correspondence and data exchange with key people involved in the offshore renewable energy
sector and government bodies undertaken during early 2010. These correspondences provide
technical and non-technical information as well as data pertinent to the industry. Available
information and data, where possible, were summarised in excel spreadsheets, geo-referenced,
incorporated into a GIS. The table below gives an overview of the available data sets and their
sources utilized.
Data Type Data Source
Seabed Sediment Classification GSI (INFOMAR)
Bedforms and Topograhy GSI (INFOMAR)
Faults GSI (INFOMAR)
Water Depth (Bathymetry) GSI (INFOMAR)
Wind Speed, Direction, Frequency Marine Institute
Wave Height, Direction, Period and Power Marine Institute
Tidal Range and Period Marine Institute
Offshore Weather Reports Marine Institute and Met Eireann
Special Areas of Conservation National Parks and Wildlife Service
Special Protected Areas National Parks and Wildlife Service
Commercial Navigation Irish Maritime Development Office
Fisheries Marine Institute
Pipelines and Cables Kingfisher Information Service, Department of Petroleum Affairs
Shipwrecks Underwater Archeological Unit
Military Exclusion Zones Department of Defence
9
3 Appraisal of Industry Needs
3.1 Introduction
Site suitability mapping is an integral part of the construction of offshore installations and requires
data acquisition from across a variety of disciplines. INFOMAR offers a variety of mapping products
for different offshore areas. However, very often companies are forced to carry out in-house surveys
or hire private specialized groups to supplement publically provided data or gather data that is
missing or outstanding.
By documenting industry data needs and subsequently assessing INFOMAR’s ability to satisfy these
needs it is possible in the future to tailor INFOMAR site surveying to maximize data collection
potential hence increasing its data output helping make offshore renewable energy a more
economically viable option in Ireland.
The various datasets and requirements we identified in consultation with industry can subsequently
be grouped into the following broad headings:
Geological Features (Chapter 3.2)
General Physical Features (Chapter 3.3)
Ecological Factors (Chapter 3.4)
Socio-economic Factors (Chapter 3.5)
3.2 Geological Features
An in-depth knowledge of the seabed geology is particularly important to offshore renewable energy
resource development as it heavily influences anchoring and foundation construction for offshore
renewable energy installations which in turn affects cost, one of the main drivers in such projects.
Generally speaking, Geological Features can be divided into three distinct headings:
1) Seafloor Morphology, including:
Seabed slopes and gradients
Bedforms
Seabed Dynamics
2) Quaternary Geology, including:
Sediment type and classification
Depth of transition to bedrock
10
3) Solid Geology, including:
Bedrock description
Faults
Oil/gas accumulations
3.2.1 Seafloor Morphology
3.2.1.1 Seabed Dynamics
Seabed dynamics is primarily concerned with mobile sediment and the hydrodynamic regime which
drives them. Essentially the prime concern associated with seabed dynamics is scour which
destructively affects foundations of offshore installations. Negating scour is a costly issue in
construction and where possible areas with high scour potential are excluded from site mapping.
Predicting scour is largely done by modeling based on a series of geotechnical parameters measured
in the sediment from the area under investigation.
3.2.1.2 Seabed Slopes and Gradients
Seabed gradients and slopes are also important in the site identification process as installation
foundations are generally not constructed on slopes greater than 5 o
3.2.1.3 Bedforms
Bedforms are the result of sediment mobility and hence are important to identify when establishing
the hydrodynamic regimes, hence scour potential, of potential sites.
3.2.2 Quaternary Geology
Quaternary geology and associated structures heavily affect foundation design. Classification of
these sediments can be carried out using acoustic techniques. Sub-bottom profiling can identify
sedimentary strata continuity and thickness as well as sedimentary mega-structures. However,
acoustic data must be supported by groundtruthing by way of coring and sampling.
3.2.2.1 Sediment Type and Classification
Information regarding seabed sediments is an important factor in offshore installation construction
as analysis of sediment samples can influence foundation design. They may be used in geotechnical
investigations to assess the various properties of the sediment including strength, cohesion,
liquefaction potential and scour potential as well as the extent of sediment mobility across an area.
As a rule, dense sands and silts are preferred, although gravel is often acceptable.
11
Seabed sediment studies have been carried out most successfully utilizing a combination of broad
scale remote sensing (i.e. acoustic backscatter) and small scale ground-truthing. Information
regarding seabed sediments should be ideally consist of a shapefile containing polygons showing the
dominant type of sediment. This data should be referenced to WGS84 and interpolated from the
best available data for the area. Similarly, for developers who do not have access to adequate
mapping software, a Google Earth kmz file available to download should be provided.
3.2.2.2 Depth of Transition to Bedrock
One of the prime concerns for offshore renewable energy installation construction is the depth to
bedrock and subsequently its composition and competency. Again, this is primarily to assess
suitability for foundation construction. Monopile foundations are preferred for windfarm
development, usually reaching a depth of roughly 35m. Therefore it is important to carry out seismic
surveys which image the sub-seabed to at least 50m. A potential error arising from sub-bottom
profiling is the misinterpreting of glacial till as bedrock so groundtruthing becomes all the more
important in that respect. Coring has the added advantage of providing physical samples for lab
based geotechnical and physical property analysis.
3.2.3 Solid Geology
3.2.3.1 Bedrock Description
As mentioned the depth from seabed to bedrock is of importance in relation to foundation design
and construction. In addition, the subsequent composition and competency of this bedrock is also
important information.
Information regarding bedrock description should be delivered using a shapefile which uses
polygons to show the predominant bedrock with information regarding type, age and formation
name. The additional option of a Google Earth kmz file would be of use to developers who didn’t
have access to adequate mapping software.
3.2.3.2 Faults
Sub-seabed faults are naturally planes of weakness and knowledge of their locations is vital in siting
installation construction. Although Ireland is seismically a quiet area, within the Irish Sea low
magnitude events are often recorded. Similarly, faults can act as conduits allowing for the
accumulation off shallow gas. Therefore the mapping of scarps and other features is important and
can be done using multibeam and side scan sonar techniques.
12
3.2.3.3 Oil/Gas Accumulations
Gas, which may accumulate in the subsurface, proves problematic for site investigations and
foundation installations. Some of the potential risks resulting from shallow gas accumulations
include:
Loss of vessel buoyancy
Blowouts
Gas kicks and minor flows
Loss of drill/installation jack up
Uncontrolled environmental emissions
Technogenic hydrate formation
13
4 General Physical Features
Only certain general physical features are applicable to each of the three main types of offshore
renewable energy. In addition, the limits or constraints the data associated with these features
places on siting potential installations is less strict as the technology can often be tailored to meet
varying conditions in the wind, wave or tidal regime at a certain site.
The main general physical features are:
1) Water depth, namely:
Seabed Bathymetry
2) Wind characteristics, including:
Wind speed
Frequency
Direction
3) Wave characteristics, including:
Average period
Wave height
Direction
Significant wave height exceeded 10% and 50% of the time
4) Tidal regime, including:
Maximum current amplitude
Spring tidal ranges
Tidal Period
5) Distance to shore
6) Landfall description
7) Offshore weather reports
4.1.1 Seabed Bathymetry
Water depth is an important factor in site identification for installations and so detailed and
widespread bathymetric maps are a necessity. Construction costs of offshore installations increases
with water depth and so generally greater than 35m water depth is not exceeded. In saying so,
evolving technology is pushing installations to potentially greater depths.
Bathymetry should be delivered in an ASCII file as gridded data referenced to WGS84, interpolated
from the best available data for the area and corrected to mean sea level as well as lowest
astronomical tide. The gridded data should be prepared up to the high water line. The gridded data
14
should be available in resolutions of 1 minute, down to 3 seconds. The additional option of a Google
Earth kmz file for download would help those developers who do not have access to mapping
software.
4.1.2 Wind Characteristics
Wind characteristics are an obvious important consideration for installing wind energy installations.
Generally a wind speed of 9 ms -1
is preferred. Wind data recorded should consist of wind speed,
direction, frequency and maximum gust. In order to assess the wind generating potential of an area
there must be a continual dataset recording hourly for the past decade.
The Marine Institute has 6 weather buoys located around the Irish coast which constantly record
wind speed and direction as well as the maximum gust. This data is available from the Marine
Institute in an excel spreadsheet form (http://www.marine.ie/home/).
4.1.3 Wave Characteristics
The 6 Marine Institute buoys also collect wave data in the form of wave height, period, peak period,
mean wave spread and mean direction. These data sets are provided by the Marine Institute as excel
spreadsheets. Also available are shapefiles for the average practicable power (see Figure. 3-1) and
the average wave height (see Figure 3-2). The data from these attributes were derived from the
Marine Institute’s Accessible Wave Energy Resource Atlas published in 2005.
15
Figure 4-1: Annual Average Wave Power
16
Figure 4-2: Annual Average Wave Height
17
4.1.4 Tidal Regime
In order to harness tidal energy an understanding of the hydrodynamic regimes is needed. ADCP
deployments provide crucial hydrodynamic data that are necessary to calibrate tidal models. Tidal
models can then input into understanding sediment dynamics in terms of migration and pathways.
Additional tidal data are provided by 13 tidal gauges located around the coast of Ireland. These data
sets are online available from the Marine Institute at http://www.marine.ie/home/services/
operational/oceanography/ TideGauge.htm
Figure 3-3 Marine Institute Tidal Gauges
18
4.1.5 Distance to Shore
Similar to water depth, the distance from an installation to shore (and subsequently grid connection)
influences the cost of construction. Generally the distance from shore to these installations is
between 10-20km. In The Netherlands and the U.K, the 12 nautical mile zone is excluded from
offshore windfarm development.
19
4.2 Ecological Factors
Ecological factors refer to marine mammals, birds and fish habitats, nursing grounds and migratory
routes. As part of an Environmental Impact Assessment concerning the construction of offshore
installations the primary areas of focus are the various species that either reside or pass through the
area under investigation, their numbers and concentrations and seasonal variability.
4.2.1 Special Areas of Conservation (SAC’s)
The National Parks and Wildlife Service is the national body charged with the conservation of a range
of habitats and species in Ireland. Subsequently they identify and assess Special Areas of
Conservation or SAC’s which are areas in Ireland of upmost concern regarding wildlife conservation
and are considered important not just in Ireland but also in Europe (see Figure 3-3). SACs have a
legal grounding in the EU Habitats Directive and cover an area of roughly 13,500 km² with 47% of
this area concerning the marine and large lakes. In addition to the in and near shore SAC’s, also
offshore four SACs exist in water depth deeper than 500m.
4.2.2 Special Protection Areas (SPA’s)
The National Parks and Wildlife Service also identifies Special Protection Areas (SPAs) that cover
breeding, feeding, roosting and wintering areas of birds under the EU Birds Directive (see Figure 3-
4).
Shapefiles for SACs and SPAs are available from the National Parks and Wildlife Service.
20
Figure 4-4: Special Areas of Conservation
21
Figure 4-5: Special Protection Areas
22
4.3 Socio-economic Factors
Socio-economic factors represent constraints that usually discount an area as a potential site and
include:
1) Commercial navigation, including:
Shipping lanes
Ferry routes
2) Fisheries
3) Dredging for aggregates
4) Recreational leisure
5) Distance to port
6) Oil/gas exploration
7) Pipelines and cables
8) Wrecks and other archeological sites
9) Military installations and firing ranges
10) Aviation
23
4.3.1 Commercial Navigation
A potential danger in the construction of offshore installations is the possibly of an accidental
collision with vessels. Around the Irish coast, it is the Irish Sea which sees the most activity in terms
of commercial navigation with ferries running from Ireland to the UK and France along the Dublin-
Holyhead, Rosslare- Pembroke, Rosslare- Cherbourg and Rosslare- Roscoff routes (see Figure 3-5).
These routes see up to 10,000 crossings a year and represent an important source of national
income through tourism. Figure 3-5 shows the main commercial routes fro and to Ireland. Additional
information regarding shipping channels, along with other merchant routes, can be obtained from
The Irish Maritime Development Office (http://www.imdo.ie/imdo/).
4.3.2 Fisheries
In 2009 the Marine Institute published the Atlas of Commercial Fisheries around Ireland which
assessed and reviewed Ireland’s exploitation of the 75 various fish species in Irish waters under the
EU’s Common Fisheries Policy (CFP). Data regarding this publication is available from the Marine
Institute as GIS shapefile and include such data as target fish, fishing methods and habitats (see
Figure 3-6 and Fig. 3-7).
4.3.3 Cables
Underwater cables, pipelines and gas interconnectors are constraints for offshore installations. No
offshore construction is allowed within a buffer-zone of generally 100m these features.
Most underwater cables, pipelines and gas interconnectors are located on the east coast of Ireland
transporting oil and gas between Britain and Ireland (see Figure 3-8) Information regarding these
pipelines can be acquired from the UK Department of Energy and Climate Change
(http://www.decc.gov.uk/) and the Petroleum Affairs Division (http://www.dcenr.gov.ie/Natural/
Petroleum+Affairs+Division/).
Information on underwater cables can is provided by the Kingfisher Information Service
(http://www.kisca.org.uk/). This information is available to download in a number of different
formats.
24
Figure 4-6: Ferry Routes
25
Figure 4-7: Target Fish Type
26
Figure 4-8: Fishing Methods
27
Figure 4-9: Cables
28
4.3.4 Shipwrecks
Shipwrecks and other archeological sites encountered during INFOMAR seabed mapping are duly
logged and the locations are recorded in a GIS database. These data are available for download as
shapefiles (see Fig. 3-9). Many shipwrecks are however not detected on the standard resolution
bathymetry used and go unrecorded.
The shipwreck inventory pictured (Figure 3-9) contains roughly 250 wrecks. According to the
Underwater Archeology Unit of The National Monuments Service there are between 1,500 to 2,000
wrecks located in Irish waters. These data come from a variety of sources including divers and fishing
trawler reports, hydrocarbon exploration and admiralty charts amongst others. All shipwrecks
reported are available for viewing from the National Monuments Service in the form of physical
maps. Ongoing work by the National Monuments Service will make this data accessible in digital
format early 2011.
As such there is no set exclusion zone set around shipwrecks. It depends on the nature of the wreck
and extent of its debris. Usually the exclusion zone is in the region of 100 to 300m in diameter.
4.3.5 Military Installations and Firing Ranges
Military installations and in particular firing ranges pose an obvious constraint to offshore
installations. Information regarding the location and extent of these ranges can be procured from
the Department of Defence (www.defence.ie). See Figure 3-10.
29
Figure 4-10: Shipwreck Inventory (GSI)
30
Figure 4-11: Military Exclusion Zones
31
5 Assessment of INFOMAR Dataset Compatibility
The main focus of INFOMAR programme is to provide high resolution bathymetric maps. These
products directly address offshore renewable industry needs. Water depth is a major constraint for
offshore installations and high resolution bathymetric information therefore crucial for planning and
costing offshore devices.
Another part of the INFOMAR programme is to deliver a seabed sediment classification scheme. To
date for the areas of Galway Bay, Bantry and Dunmanus Harbour, the Waterford coast, Cork
Harbour, Sligo and Donegal coast as well as part of the East coast the sediments have been
classified. However, these are unsupervised classifications and are based solely on acoustic
properties and statistical analysis. Where groundtruthing data is available INFOMAR intend to apply
this to the classification maps in order to improve their value with Bantry and Dunmanus identified
as starting points. Similar to its sediment classification coverage, INFOMAR provides backscatter and
multibeam bathymetry data for a limited area comprising of Cork Harbour, the Waterford and
Wexford Coasts, much of the East Coast around Dublin and Louth, Galway Bay, the Donegal and Sligo
coasts as well as much of the Kerry and Cork Coast from the Shannon Estuary as far south as
Dunmanus. These data are available as gridded datasets with a 10m resolution.
These INFOMAR datasets provide essential information for hydrographic models necessary to
predict current directions and velocities. In addition, a sediment classification is required to establish
a more robust, up to date, dynamic geological model.
Geological factors such as seabed dynamics change over time and so it is vital to have up to date and
accurate data. In this respect INFOMAR plays an integral role in developing offshore renewable
energy. The datasets it provides form the basis for any modelling and geotechnical analysis
essentially influencing site selection and foundation design as well as cable route selection.
Many of the datasets mentioned already pertaining to offshore renewable energy are utilised in
constraint mapping exercises to identify sites with the best potential. The geological factors not only
aid in this site identification process but also heavily influence anchoring and foundation design of
offshore installations which in turn affects cost, one of the primary drivers for such projects. The
INFOMAR programme already provides bathymetric, sedimentological and geological data sets.
32
Additional geological (seismic) data sets would however support the site selection process for
offshore installations. A summary gap analysis of the INFOMAR dataset can be found below in
Table 1.
Data Recommended INFOMAR Suggestions
Multi- beam
Echo Sounder
Frequency of 100 kHz or higher.
Delivered in an ascii file as gridded
data referenced to WGS84,
interpolated from best available data
corrected to MSL and LAT. Option of
Google Earth .kmz file
95 kHz EM1002 or 200 kHz EM3002
system. Data corrected to LAT and
available in grid (.ers), ascii (.dat),
map (.png), Fledermaus (.scene),
Google (.kmz), zipped ESRI ArcGIS
GRIDS.
Adequate
Side-scan
Sonar
Frequency of 100 kHz or higher Edge tech side scan sonar Adequate
Sub-bottom
profiler
Boomer or alternative systems of
comparable or better performance
and sufficient signal penetration.
Near surface resolution min. 1m
Possibly supplemented by sub
bottom profiler or chirp sonar in
surface sediment layer vertical
resolution min. 0.5m.
3.5 kHz SES Probe 500 pinger and
500 – 2000 Hz Geo-Spark 200
sparker with data available in .jpeg
or .segy format
Adequate
Shallow
Geotecnical
Survey
Vibrocore and Cone Penetration Test
(CPT) to depths of 6 -8m
Geo Resources 3000 + 6000
vibrocore with depths of 3-6m
which are scanned, logged and
stored.
Possibly
deeper core.
Provision of
CPT data.
Video and
Grab Survey
Video reconnaissance by way of drop
down camera and seabed samples of
<1m depth to ground truth and use in
grain-size analysis.
Day, Shipek, Van Veen and Box
Corer. Samples undergo particle
size analysis.
Possible
provision of
seabed
imagery.
33
Hydrodynamics
Study
Deployment of acoustic wave and
tidal monitor (ADCP or MiniLander).
Deployment of wave buoy.
Deployment of passive sediment
traps. Gathering of CTD data.
Tidal and Wave data available from
Marine Institute. CTD data also
available.
Introduce site
specific wave
and current
monitoring by
way of ADCP
data or
equivalent.
Make
provision for
deployment
of sediment
traps for
hydrodynamic
surveys
Table 1- Data Gap Analysis
34
6 Site Survey Methodology
6.1 Current Information
The importance of site investigation cannot be overstated as it is a critical step in the construction of
offshore renewable energy installations. A study by the Delft University Wind Energy Research
Institute found that 25% of total project capital expenditure is linked with the foundation design.
Foundation design and cable route design in turn is ultimately decided by factors such as the sub-
seabed geology and seabed dynamics.
As such, there are very few documents regarding guidelines or recommended best practice
protocols when it comes to site investigation for offshore renewable energy installations. There are
a few existing documents which are related to the subject and much information is drawn from site
investigation methods for the offshore hydrocarbon exploration sector. These documents refer
mostly to the geotechnical analysis aspect of site investigation and subsequently foundation
planning and design and apply to different types of sites, structures and budgets.
A number of nations, most prominently Germany, Denmark and the Netherlands, have developed
regulations regarding the construction of windfarms (and hence offshore windfarms by default)
which are set down in law. As a result these regulations take precedence over other guidelines or
recommended practice.
Other multidisciplinary groups (such as the Society for Underwater Technology) have looked at the
issue of site investigation in relation to foundation behaviour and as a result of a series of
conferences have produced a collection of publications on the matter, most notably “Offshore Site
Investigation and Geotechnics, Confronting New Challenges and Sharing Knowledge” and “Offshore
site Investigation and Foundation Behaviours: New Frontiers”.
Additional relevant documents give guidelines with respect to laboratory testing of geotechnical
samples, including:
Norsk Standards 8000 to 8017
EN1997, Eurocode 7, Geotechnical Design.
International Standards Organisation; Geotechnical Investigation and Testing, Identification
and Classification of Soil, Part 1: Identification and Description ISO 14688-1:2002
35
American Society of Testing and Materials, (2005) Volume 04.08, Soil and Rock (1) D420-
D5779. Annual Book of ASTM Standards
American Petroleum Institute
British Standards Institution (1999) BS5930: Code of Practice for Site Investigation
British Standards Institution 91990) BS1377:Methods of Tests for Soil for Civil Engineering
Purposes
The basic components of a site investigation are the preliminary site assessment and geophysical
and geotechnical studies. In addition, environmental baseline surveys and environmental impact
assessments are required for offshore construction projects. These may also involve gathering
seabed samples for chemical, biological and physical analyses. Notable overlap between such
surveys and standard geophysical and geotechnical surveys require strategic survey planning in
order to achieve a maximum efficiency of overall data collection.
36
6.2 Desktop Study
The first phase of a site investigation involves a desktop study collating all available data and
information regarding the area from across a variety of disciplines to establish a base model. Based
on the outcomes of this desktop study, it is possible to identify areas of information conflict or
deficiency. Required data sets include:
Geological databases
Bathymetric information
Geophysical databases
Geotechnical databases
Metocean data (tides, currents, wind, wave etc.)
Seismic data
Data on human activities (e.g. pipelines, wrecks, cables, aggregate dredging, navigation
routes etc.)
Resulting from this desktop study the key outputs are usually:
A constraint map for the area of interest
An outline geological model for the area of interest
Preliminary definition of key geological processes and their status (e.g. active, dormant etc.)
Estimated metocean conditions
Identification of major geotechnical risks
These outputs would form a baseline against which to identify changes in seabed process since the
last data was recorded and establish an outline for possible additional survey and monitoring
requirements as well as areas of insufficient information.
37
6.3 Geophysical Survey
The geophysical forms the basis of any geological model of an area. It provides the initial seabed and
sub-seabed model which can then be groundtruthed by the geotechnical survey.
The objectives of the geophysical survey should be to:
Provide an accurate bathymetric chart of the area of interest
Chart natural seabed features and any obstructions, debris or wrecks
Produce isopach charts for sedimentary units
Map the depth to the rockhead. Seabed penetration to 50m or so is typical for geotechnical
purposes
Locate any structural complexities or geohazards within the shallow geological succession
such as faulting, accumulations of shallow gas, buried channels etc.
Provide detailed geological interpretation to show facies variations and structural feature
changes via appropriate maps and sections
Design a geotechnical sampling and testing programme following the completion of the
geophysical survey
Produce a comprehensive interpretation report on the survey results obtained to assist
design of the offshore foundations/structure and cable burial.
The design of the survey should be based on the survey needs such as water depths and other
physical requirements. The minimum requirements for each survey are outlined below.
38
Geological Survey Monitoring
Targets Survey of bathymetric conditions Recording of local depth changes (scouring)
Scope Each turbine site to be covered at least once
Along offshore wind farm components (longitudinal lines on both sides) At least 200m to either side
Time-scale Once In the years after completion, once a year in spring
Method Single-beam echosounder if the seafloor is relatively level, or multibeam system if it’s rough. Positioning better than 5m + 5% of the water depth. Accuracy for reduced depths according to IHO Standards for Hydrographic Surveys
Multi-beam echosounder. Positioning better than 5m + 5% of water depth Accuracy of reduced according to IHO Standards for Hydrographic Surveys
Presentation of results Bathymetric map of surveyed areas Reported water depths must be sound velocity corrected and related to chart datum (tidal correction) Data must also be provided in digital form
Bathymetric map of surveyed areas Reported water depths must be sound velocity corrected and related to chart datum (tidal correction) Data must be provided in digital form
Table 2 Requirements for echosounder surveys
Geological Survey Monitoring
Targets Survey of sediment types and structures Verification or calibration of interpretation by means of grab samples
Recording of erosion areas, scouring and obstructions Verification and/or calibration of interpretation by means of grab samples
Scope Each energy installation component to be surveyed at least once Complete coverage of the area if the sea floor is heterogeneous
At least 200m to either side
Time-scale Once In the first years after completion, once a year in spring
Method Frequency 100kHz or higher Coverage max 2x100m Recognition of cubic features > 1m Digital recording Cruise speed max. 4 knots Equipment positioning better than 10m
Frequency 100kHz or higher Coverage max. 2 x 75m Recognition of cubic features >1m Digital recording Cruise speed max. 4 knots Equipment positioning better than 10m
Presentation of results Digital SSS mosaic of profiles (horizontal resolution 0.5m) Map with interpretation of the side scan sonar profiles Data have to be additionally provided in analogue form
Digital SSS mosaic of profiles (horizontal resolution 0.5m) Map with interpretation of the side scan sonar profiles
Table 3 Requirements for side scan sonar (SSS) surveys
39
Geological Survey
Targets Determination of type and location of geological units
Scope Each offshore installation component shall be covered by at least one longitudinal and one cross section Max. spacing of cross sections 2000m It is recommended that longitudinal sections comprise of 10m spacing along total foundation breadth and one cross section
Time-scale Once
Method Boomer or alternative systems of comparable or better performance and sufficient signal penetration. Near surface resolution min. 1m Possibly supplemented by sub bottom profiler or chirp sonar in surface sediment layer (e.g. along planned cable routes), vertical resolution min. 0.5m Cruise speed max. 4 knots Deployment up to sea state max. 4
Presentation of results Profiles and profile interpretation such as geological longitudinal and cross sections Map showing spatial position of boundaries between geological units and structural elements (e.g. isolines map)
Table 4 Requirements for seismic surveys
The above surveys are geared towards acquiring information with regard to foundation siting and
design. A different set of requirements apply to surveying relating to routes for infield cables and
power export cables. The main issues are concerned with horizontal cover and penetration in the
investigation area as well as seabed hardness or trenchability. The technical requirements are
outlined below.
Route survey Monitoring
Targets Bathymetric and morphological
investigation of the planned cable
route
Mapping of wrecks, other obstructions
and ammunition
Investigation into sediment
composition, geological stratification
and geotechnical properties of the
upper sediment layer
Mapping of existing cables and
pipelines
Determination of cable route and
length
Detection of possible free spanning of
cable
Checking of rockfills or comparable
cable safety features
Measurement of cable depth
40
Table 5 Requirements for geological survey of cable routes
Route survey Monitoring
Scope Complete coverage of 200m wide
corridor
Monitoring of complete cable routes in
the first years
After a sufficient database is available,
modified monitoring intervals may be
applied for within the framework of
the periodical inspections
Time-scale Once In the first years completion, once a
year in spring
Methods Multi-beam; positioning better than
5m + 5% of water depth and accuracy
for reduced depths according to IHO
Standards for Hydrographic Surveys
Sidescan sonar; frequency 100 kHz or
higher; measuring range max. 2 x 100
m; recognition of cubic features >1m;
digital recording; cruise speed max.
4kn; sonar positioning better than 10m
Sub-bottom profiler, chirp sonar or
alternative systems of comparable or
better performance
Vibrocorer or CPT down to the
planned burial depth, spacing to be
determined on the basis of seismic
data
Magnetometer or active metal
detection system
Cable tracking system or suitable other
method for cables buried in sediment
Multi-beam; positioning better than
5m + 5% of water depth and accuracy
for reduced depths according to IHO
Standards for Hydrographic Surveys
Side scan sonar if necessary; frequency
100 kHz or higher; measuring range
max. 2 x 100m; recognition of cubic
features > 1m; digital recording; cruise
speed max. 4 kn; sonar positioning
better than 10m
Presentation of results Map (horizontal scale 1:5000, vertical
scale 1:200) showing all survey results
Map (horizontal scale 1:5000, vertical
scale 1:200 showing all survey results)
41
6.4 Geotechnical Survey
In essence geotechnical surveys groundtruth the data seen in the geophysical survey to confirm the
geological/geophysical model and should provide all the necessary seabed data to supplement the
design project for foundations as well as cable burial and protection.
The programmes for the geotechnical surveys usually comprise samples for further lab analysis and
in situ tests such as cone penetration tests. The choice of sampling and testing locations will depend
on the lateral and vertical variability of ground conditions as revealed by the geophysical survey and
the preliminary site assessment. Project specific factors that can influence sample site selections are:
Size, location and foundation type of any seabed structures
Complexity of geological model
Presence and distribution of geotechnical hazards
Variability and uncertainty in geotechnical terms
In terms of foundation design for offshore installations, the main issues include:
Bearing capacity. Regardless of the structure, the sediment must have sufficient capacity to
carry the static and cyclic loads of the foundation with an acceptable margin for excessive
displacements and failures.
Permanent displacements. (e.g. settlement). The static load will cause initial displacements
in addition to displacements brought on by consolidation and creep in the sediment beneath
and outside the foundation. Cyclic loading from wave action will increase shear strain and
dissipation of cyclically induced pore pressure causing additional permanent displacements.
Cyclic displacements. Cyclic loads cause cyclic displacements of the sediment.
Foundation stiffness. Necessary for structural dynamic analyses from wave and/or
earthquake loading.
Soil reaction stresses. Results from static and cyclic loads depending on the density of the
sediment. Soil reaction stresses may redistribute with time due to creep and cyclic
degradation of the soil modulus.
Penetration of skirts.
Pile drivability. Sufficient depth for foundation piles is essential.
Liquefaction potential analyses. This is important in areas where sands and silts are present
in areas which are seismically active or where heavy wave loading is expected.
42
Scour and erosion. Waves and currents can cause scour and erosion around the base of
foundations. It is most prominent in areas dominated by sands and typically increases as
water depth decreases.
Punch through. Rapid, uncontrolled penetration may occur in sediment where there is
limited soil strength increase with depth, and in sediment where there is a strong layer of
limited thickness overlies a weaker layer.
In order to address these issues, additional basic sediment and rock parameters have to be analysed.
Clay Sand,Silt or Gravel Rock
General description
Layering
Grain size distribution
Organic material
content
Mineralogy
Total unit weight
Atterberg limits
Water content
Remoulded shear
strength
Sensitivity
Undrained shear
strength
Carbonate content
Over consolidation
ratio
Indicative shear
strength
Sediment stress history
General description
Layering
Grain size distribution
Maximum and
minimum densities
Relative density
Water content
Organic material
content
Angularity
Sediment stress history
and over-consolidation
ratio
Angle of shearing
resistance
Drained angle of
resistance
General description
Rock quality
designation
Water absorption
Total unit weight
Unit weight of solid
blocks
Unconfined
compression strength
Mineralogy
Carbonate content
Table 6 Basic sediment and rock parameters
43
Design Issue Parameter
Bearing capacity Monotonic shear strengths under different stress paths
Cyclic shear strength under combined average and cyclic shear
stresses for triaxial and simple shear stress paths
Permanent displacements Compressibility
Permeability
Permanent shear strain and pore pressure under combined
average and cyclic shear shear stresses for triaxial and simple
shear stress paths
Compressibility after cyclic loading
Cyclic displacements Cyclic shear strain as function of cyclic shear stress under
combined average and cyclic shear stresses for triaxial and
simple shear stress paths
Initial shear modulus
Foundation stiffness Cyclic shear strain as function of cyclic shear stress under
combined average and cyclic shear stresses for triaxial and
simple shear stress paths
Initial shear modulus
Damping
Soil reaction stresses Monotonic and cyclic shear strengths
Compressibility under virgin loading and reloading
Cyclic and permanent shear strains and permanent pore
pressure under combined average and cyclic shear stresses for
triaxial and simple shear stress paths
Seabed topography, objects on the seafloor
Liquefaction potential Initial shear modulus
Cyclic shear modulus degradation curves
Damping
Coefficient of reconsolidation
44
Design Issue Parameter
(For all the above) Cyclic shear strain and permanent pore pressure contour
diagrams for at least one representative average shear stress
(e.g. simple shear tests with (τa=0)
Coefficient of reconsolidation
Skirt penetration Undrained aniostrpic monotonic shear strengths
Remoulded shear strength (or sensitivity)
Drained angle of shearing resistance
Residual interface angle of shearing resistance
CPT resistance
Seabed topography and objects on the seafloor
Boulders in the soil within the skirt penetration depth
Pile capacity and
drivability
Axial and lateral response
Shear strength
Sediment modulus or strain at 50% ultimate strength
CPT cone resistance
Scour/erosion Permeability
Water depth
Significant wave height
Wave peak period
Mean current velocity
Current direction
Grain size
Surface roughness
Density of sediment
Table 7 Additional parameters for specific design issues
45
7 Case Studies
The case studies outlined below represent pioneering work where techniques and practices were
applied and subsequently lessons learned. In the case studies, different problems were encountered
and the experience gained offer some solution to similar problems arise in future projects.
7.1.1 Sheringham Shoals
The Sheringham Shoal windfarm is located between 8 and 12 nautical miles (17 to 20km) off the
coast of North Norfolk covering an area of 36km 2 approximately. Water depth ranges from 15 to
22m at lowest astronomical tide (LAT) with a tidal range of 5m roughly. A number of
geophysical/geotechnical surveys were carried out on the area prior to construction between 2004
and 2007 as listed below.
Survey Description Data Application Date Hydrodynamics and sediment Study
Deployment of acoustic wave and tidal monitor; deployment of wave buoy; 5 grab samples
EIA – hydrodynamics and geomorphology Determination of metocean Conditions
Winter 2004 - 2005
Bird/marine mammal surveys
29 boat-based visual surveys; 7 aerial visual surveys
EIA – ornithology EIA – marine mammals
2004 - 2007
Acoustic, video and grab survey
Broad scale swathe bathymetry; side-scan; acoustic ground definition system; magnetometry with some seabed sample grabs
EIA – marine ecology Geophysical site characterisiation Provision of bathymetry data for site characterisation
Summer 2005
Benthic and epibenthic faunal surveys
Including 2, 7, 11m depth beam trawls
EIA – natural fisheries EIA – marine ecology
Summer 2005
Geophysical survey Single channel analogue surface tow boomer survey of offshore wind farm site and cable route, comprising 133km of survey lines
Provision of geological data for site characterisation
Summer 2005
Shallow geotechnical survey 30 vibrocore samples and 19 CPTs up to depths of around 6 – 8m
Provision of geological/geotechnical data for site characterisation EIA – marine archaeology
Summer 2006
Drop-down video survey Clear water box video of seabed EIA – marine ecology EIA – hydrodynamics and geomorphology Fulfil ecological survey consent
Summer 2006
Magnetometer survey Magnetometer survey of borehole locations
EIA – marine archaeology Fulfil health and safety requirements for geotechnical survey
Summer 2006
Geotechnical Survey 5 boreholes, 4 x 50m and 1 x 70m depth
Provision of geological/geotechnical data for site characterisation
Summer 2006
Table 8 Surveys during development stages of Sheringham Shoal offshore wind farm
46
The data from these various surveys fed into the development of the EIA and facilitated front-end
design (FEED) studies. Table 7 also shows the large degree of overlap between surveys meeting
environmental and engineering requirements.
The structure of the surveys and their timescales largely followed recommendations suggested by
the OSIG guidance notes. The first stage was a desktop study to determine the geological makeup of
the site from publicly available information. For the Sheringham Shoals, this was carried out by an
independent geological specialist mostly from British Geological Survey (BGS) data. Based on this
study, a geophysical survey, a shallow geotechnical survey and an acoustic, video and grab survey
were planned.
After the completion of these surveys and analyses of the gathered data, isopach charts for the
predicted levels of bedrock and overlying formations were produced. However, the geophysical
survey had been hampered by bad weather and inadequate equipment; hence the raw data
contained a significant amount of noise. Therefore, a number of assumptions had to be made to
predict the deeper geology of the site, in particular, below 20m beneath the seabed. Similarly, strata
predicted by the desktop study varied from those interpreted by the geophysical survey in terms of
depth and structure. The successive geotechnical survey was planned in order to calibrate available
geophysical data and gain a better understanding of deeper site geology. The key aims of the
geotechnical survey were:
Calibration of geophysical data to assist in the development of more accurate geological
mapping of the site
To obtain geotechnical properties for preliminary foundation design
During the course of the survey four 50m boreholes and one 70m borehole were completed in a 10
day period. Boreholes were drilled utilising a composite borehole system consisting of a repeated
process of a push sampling of 1m intervals followed by a CPT of 3m.
An environmental statement was submitted by the operating company Scira after all necessary
environmental surveys were successfully completed. However, following the completion of the
geotechnical site investigation programme and subsequent analysis of results, the project was found
to be unable to model costs for the whole wind farm with complete accuracy. Two key reasons for
this were:
47
Uncertainty over the distribution and depth of soil types across the site
Uncertainty regarding bedrock properties.
However, data gathered allowed for ‘best’ and ‘worst’ case scenarios to be formulated, allowing the
risks to be quantified.
The uncertainty regarding soil types was subsequently traced back to inaccuracies in both the BGS
survey data and the geophysical survey data and the key flaws regarding the geotechnical survey
appeared to have emanated from the lack of penetrative depth of the equipment used and its
inability to distinguish bedrock (in this case chalk) from clay. Poor data quality was blamed on bad
weather at the time of collection.
Although poor weather conditions cannot always be anticipated, the use of a secondary data
collection system would have mitigated against this effect. Subsequent advice to Scira
recommended the use of air-gun survey equipment in addition to boomer as it gives deeper seabed
penetration despite being comparatively expensive.
With regards to the geotechnical survey, samples proved to be important in identifying various
geological layers that were correlated with CPT data in order to calibrate the geophysical data sets.
In general, the push sampling system that was proved to be successful in sampling the clays and
sands encountered which accounted for 70% of the total soils met. However, the structureless chalk
which made up the bedrock proved mush harder to obtain undisturbed samples from. In retrospect
the piggyback coring system they used was unsuited to this type of ground conditions.
Lessons were also learned from the selection of borehole location for the Sheringham Shoals
project. Following expert advice, drilling sites were selected for intersection of survey lines from the
site geophysical survey done in 2005 and available BGS data (see Figure 11). They were also designed
to target specific geological features that were expected to exist and calibrate both sets of
geophysical data. Subsequently, areas within the corners of the site were left without boreholes. It
was later found that borehole findings did not match geophysical survey data or data available from
the BGS database with regard to bedrock depth making it difficult to map the stratigraphy of the site
area. It was admitted that the poor quality of the original geophysical data reduced significantly the
usefulness of the geotechnical survey results. More equally spaced borehole pattern would have
potentially resulted in a better understanding of geological features across the site.
48
Figure 12 Borehole locations at Sheringham Shoal
7.1.2 Scroby Sands
The Scroby Sands offshore wind farm is located 2.5km off Great Yarmouth in the North Sea
consisting of 30 wind turbines each piled 30m into the seabed. Construction took place in just under
a year with three export cables bringing power ashore.
Work carried out prior to the construction consisted of an intense programme of research and
monitoring. The scientific objectives were:
To collect datasets of waves and currents over a spring/neap cycle on the sandbanks for use
in calibration and validation of numerical models for potential impacts of wind farms.
To assess gross changes in sediment transport during winter and summer seasons pre- and
post- construction to compare any effects due to wind farm construction.
To undertake suspended sediment monitoring during wind farm construction using a
combination of optical backscatter sensor (OBS) profiles and water samples in order to
monitor potential effects of piling and ship movements.
To produce a GIS showing the sedimentological and hydrodynamic distributions for use in
interpretation. Maps of individual bedfroms were created from interpreted side-scan sonar
records from the Southern North Sea Sediment Transport Study (www.sns2.org). Five
snapshots of the bedforms enabled the detection of any gross changes of sediment
transport regime and allowed for a comparison of seasonal and inter-annual variations. This
49
information also supported the zone management of the wind farm by monitoring and
assessing the impact of scour protection around each monopole.
To liaise with numerical modellers such as HR, ABPMer, Posfords and Halcrow to test their
models against gathered datasets.
These objectives were achieved through a series of seabed surveys (namely side-scan sonar and
swathe bathymetry) as well as the deployment of seabed landers (Cefas ‘MiniLanders’) before,
during and after construction of the wind farm. The data sets were used to assess changes in seabed
bathymetry, bedforms, currents, waves and suspended sediment concentrations and consequently
disturbance of sedimentary environments and sediment transport paths.
As part of the EIA process, developers had to understand and quantify the relevant processes
operating at the site. This was done by the following of investigations:
A time-series of swathe bathymetric surveys over the entire site and the export cables
routes. This was placed into a historic context based on analysis of historical charts;
Shear-stress exceedance diagrams for key locations within the area along with the export
cable route;
Particle-size information for sediments from representative locations within the area and
along export cables or alternatively, particle settling velocities;
Estimation of the size and shape of scour pits and wakes and the nature of any emplaced
scour protection;
Estimation of the disturbance caused by the construction of the wind farm, e.g. jetting,
ploughing, ‘grouting in’ or from seabed levelling for gravity-based structures;
Assessment of sub-bottom geophysical acoustic data, to identify historic directions of
sediment transport.
Prior to construction of the wind farm four MiniLanders were deployed around the site area
collecting a series of datasets including water temperature, salinity, depth, tidal current speed and
direction, significant wave height, wave period and turbidity throughout the water column. On each
MiniLander, passive sediment traps were mounted (‘Booner tubes’) to collect ambient suspended
sediment.
During construction and subsequent monitoring of the wind farm, a number of monitoring strategies
were proposed in keeping with best practice:
In locations of expected sediment transport for significant periods of time, a comprehensive swathe bathymetric survey was undertaken, ideally a time-series of surveys, allowing an analysis of sediment transport processes over the bank. Bi-annual seasonally linked surveys
50
permit the quantification of key aspects of the sediment transport budget, the identification of net sediment transport pathways and any potential areas of net erosion or accumulation;
In regions where sediment transport was expected to be weak, a selection of representative scour pits were monitored. If the scouring exceeded the predictions in the EIA, a more systematic swathe survey was undertaken across the area and repeated at appropriate time intervals;
High-resolution swathe bathymetry surveys of scour pits and associated scour protection measures were undertaken to identify the extent, volume and integrity of any scour protection used. This allowed for the monitoring of any secondary scour pits caused by scour protection;
Regular swathe bathymetric surveys of the export cable route checked for any cable free- spans (compromise of the cable), exposure (risk to shipping/fishing) or movement from the desired location;
During pile-driving, grouting or cabling operations, suspended-sediment monitoring was carried out, especially if the surface sediments or the immediate subsurface sediments had a high proportion of easily re-suspendible components, had elevated levels of contaminants or if the operations took place near a conservation site or within a Special Area of Conservation.
A time line depicting process monitoring in relation to stages of construction can be seen below.
51
2003 2003 2003 2003 2004 2004 2004 2004 2005 2005 2005 2005
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Survey
Technique
Seabed Landers
Side-scan Sonar
Swathe
Bathymetry
Construction
Activity
Piling
Turbines
Cables
Table 9 Timeline of monitoring at Scroby Sands
Despite the large success of the monitoring programme in establishing a good knowledge base of
seabed processes within the area, several oversights resulted in data gaps. Most notably;
Swathe surveys were only undertaken during sufficiently calm weather.
The aim of the survey was the broad-scale mapping of the entire sand bank upon which the
wind farm was located and associated bedforms. As a result, accuracy close to monopiles (
roughly up to 5m away) was most likely impaired due to backscattering from strong
reflectors and shadow effects of the monopiles on the differential global positioning system
(DGPS);
The scope of the hydrodynamic and sediment transport programme was to cover physical
scales which reflected the large size of the wind farm area resulting in smaller impacts (e.g.
0-10m scale) not being included;
During the construction and cable-burial operations turbidity measurements were not
recorded;
Wave statistics were only taken during the pre-construction phase.
The lack of long-term wave statistics for the site and export cable routes made it difficult to assess
extreme wave events. Similarly, long-term (>annual) changes and variability of sediment particle-size
distributions across the site and cable route were necessary to establish sediment transport regimes
and pathways.
52
Current and wave height/period time-series data recoded by the MiniLanders was subsequently
converted into bed shear stress. Coupled with the critical erosion thresholds for various particles
sizes (see Figure 12)
Figure 13 Time-series of bed shear stress and particle critical erosion sizes (Soulsby, 1997)
From swathe bathymetry data, Digital Elevation Models (DTM) were generated. Based on the DTMs,
volumetric analyses were performed for the entire area or from selected zones to identify scour pits,
scour pans and scour wakes. In order to quantify these changes, the differential volumes between
older and younger DEMs were calculated.
Successive swathe surveys at various stages of construction also allowed for an assessment of the
impact of monopiles on migrating sandwaves by comparing the position of sandwave crest at the
various stages. By combining the results from swathe bathymetry and the MiniLander deployment, it
was possible to generate a sediment dynamic model for the Scroby Bank. Data collect on waves,
currents, tidal elevation and suspended sediment concentrations made it also possible to:
Assess change in sediment transport magnitudes and pathways and;
Assist verification and validation of wave and tidal numerical models in shallow water
associated with sandbanks
53
8 Conclusions
8.1 Data Gap Analysis
Data Recommended INFOMAR Suggestions
Multi- beam Echo
Sounder
Frequency of 100 kHz or higher.
Delivered in an ascii file as gridded
data referenced to WGS84,
interpolated from best available data
corrected to MSL and LAT. Option of
Google Earth .kmz file
95 kHz EM1002 or 200
kHz EM3002 system.
Data corrected to LAT
and available in
grid(.ers), ascii (.dat),
map(.png), Fledermaus
(.scene), Google (.kmz),
zipped ESRI ArcGIS
GRIDS.
Adequate
Side-scan Sonar Frequency of 100 kHz or higher Edge tech side scan sonar Adequate
Sub-bottom
profiler
Boomer or alternative systems of
comparable or better performance
and sufficient signal penetration. Near
surface resolution min. 1m
Possibly supplemented by sub bottom
profiler or chirp sonar in surface
sediment layer vertical resolution min.
0.5m.
3.5 kHz SES Probe 500
pinger and 500 – 2000 Hz
Geo-Spark 200 sparker
with data available in
.jpeg or .segy format
Adequate
Shallow
Geotecnical
Survey
Vibrocore and Cone Penetration Test
(CPT) to depths of 6 -8m
Geo Resources 3000 +
6000 vibrocore with
depths of 3-6m which are
scanned, logged and
stored.
Possibly deeper
core.
Provision of CPT
data.
Video and Grab
Survey
Video reconnaissance by way of drop
down camera and seabed samples of
<1m depth to ground truth and use in
grain-size analysis.
Day, Shipek, Van Veen
and Box Corer. Samples
undergo particle size
analysis.
Possible provision
of seabed imagery.
54
Hydrodynamics
Study
Deployment of acoustic wave and tidal
monitor (ADCP or MiniLander).
Deployment of wave buoy.
Deployment of passive sediment traps.
Gathering of CTD data.
Tidal and Wave data
available from Marine
Institute. CTD data also
available.
Introduce site
specific wave and
current monitoring
by way of ADCP
data or equivalent.
Make provision for
deployment of
sediment traps for
hydrodynamic
surveys
8.2 General Conclusions
General feedback from industry regarding the quality of information provided by INFOMAR was
positive. As part of the various correspondences with industry, it was envisioned that potential sites
of investigation and development should be considered for INFOMAR surveys in the near future.
However, with the imminent publication of Irelands Strategic Environmental Assessment draft
released in January, industry was awaiting its result and conclusions before deciding on future
projects.
The existing datasets cover a variety of aspects relevant to the offshore renewable industry and form
a solid basis on which to prepare a desktop survey prior to any geophysical or geotechnical survey.
A good desktop survey is a baseline against which subsequent data collected can be measured.
Identifying data gaps and missing information allows for tailored data collection to be planned. This
makes the data acquisition more efficient and allows for developing robust and reliable geological
models. The importance of the desktop study was highlighted in the Sheringham Shoal case study.
In relation to data gaps, the main issues arose with regard to geotechnical data. Geotechnical data
are of central importance for foundation constructions, in particular for the monopile foundations
that are commonly used for wind turbines.
Information on seabed dynamics was also identified as a vital for offshore installation development.
Currently, this information is largely unappreciated or lacking in data. Scouring is the primary
concern in relation to seabed dynamics. This has been realised at the Arklow Bank windfarm project
55
where, following Phase 1, strong currents coupled with geotechnical sediment properties around the
bank have caused severe scour problems at the foundations.
Data acquisition on seabed dynamics can be assisted by the use of multibeam water column
imaging, a technique largely developed by the Canadian Hydrographic Survey. Patterns within water
column scattering profiles give good indication of watermass distribution. Similarly, individual
watermasses can be defined by variations in density (by measuring suspended particles) and sound
speed as well as zones of turbulence and the presence of zooplankton species. The issues associated
with this technique, however, are distinguishing genuine water column features from anthropogenic
ones such as third party sonar interference, vessel engine and propeller noise and bubble wash
down. Nonetheless, the data that is recorded proves useful in establishing a model for the water
column which can subsequently feed into hydrodynamic and sediment transport studies.
General guidelines for site surveying for offshore renewable energy installations come from related
documents of the offshore hydrocarbon exploration sector. However, the Offshore Site Investigation
and Geotechnics Group (OSIG) of the Society of Underwater Technology (SUT) have developed a
competent set of guidelines for data acquisition and equipment selection with regard to site
selection for offshore renewable energy installations based on shared experience and knowledge of
its authors and members. It is largely focused on data acquisition with regard to engineering and
construction aspects of offshore project and neglects survey requirements for EIA purposes. These
guidelines have been widely adopted for a number of offshore wind farm site investigations across
England.
Although the OSIG outlines guidelines with regard to site investigation there is little scope for
geotechnical surveys. The DNV (2004) is referenced to covers sediment investigation for offshore
wind turbines and provides all necessary data for detailed design.
56
9 Recommendations
For future site investigations and surveys, it is clear that a multidisplinary approach is required with
seabed dynamics and geotechnical parameters a key consideration. As suggested by OSIG guidelines,
this will involve a multi-stage investigation with the first phase comprising of a desktop survey
detailing all previously available data and information about the area. The second phase is to map
the seafloor acoustically using a multibeam echosounder system (MBES) and side scan sonar (SSS). In
conjunction with this survey, seismic data may also be acquired. It is suggested that seismic data
should be recorded along survey lines spaced at 50m intervals with cross lines every 250m. It is
highly suggested that the use of air-gun equipment and a secondary data collection system should
be used during the geophysical survey as highlighted in the Sheringham Shoal case study. Swathe
data should be recorded along survey lines spaced no more than 3 times water depth. Magnetic,
resistivity and electromagnetic surveys, data should be recorded along lines as closely spaced as
possible. Considering that the majority of potential new sites for offshore renewable energy lie
within the 20m water depth zone, ideally, 100% covered with MBES should be obtained. If possible,
current velocity meters (e.g. Acoustic Doppler Current Profiler (ADCP)) should be deployed. Ideally,
ADCPs should be left on the seabed for a period of more than two tidal cycles, preferably covering
the time interval of highest energy conditions (strong tidal events such as the spring and equinox
tides). Information on predicted tides for forthcoming strong tidal events can be obtained from the
meteorological office.
Following on from information gathered in the first leg of the survey, a detailed second survey can
be planned to gather samples to groundtruth acoustic data sets. Biological sampling supports habitat
mapping. In addition, a series of drop down video surveys can be carried out to groundtruth
backscatter data, aid in habitat mapping and assist in facies classification. Additional survey for more
extensive location sampling (e.g. coring) may also be carried out. Sampling and borehole location are
selected based on the outcomes of the previous surveys, geotechnical expertise and the desktop
study. The OSIG provides a good evaluation of the use of various seabed sampling and collecting
equipment and their suitability for various sediment types (see Table 9 – 12).
57
Table 10 Conventional Test Methods (after OSIG, 2005)
Suitability Scale
1. Poor or inappropriate 2. Acceptable for non-critical analyses 3. Moderately good 4. Good 5. Very good
58
Table 11 Special Testing Methods (after OSIG, 2005)
Suitability Scale
1. Poor or inappropriate 2. Acceptable for non-critical analyses 3. Moderately good 4. Good 5. Very good
59
Table 12 Seabed Sampling Equipment (after OSIG, 2005)
Table 13 Downhole Sampling Equipment (after OSIG, 2005)
Suitability Scale
1. Poor or inappropriate 2. Acceptable for non-critical analyses 3. Moderately good 4. Good 5. Very good
Information with regard to geotechnical data required for offshore installation foundations is
comprehensively outlined in the DNV (2007) and should be used as a reference.
The work carried out by INFOMAR provides a variety of datasets which are widely used by the
offshore renewable energy industry to build good geological models. There is, however, more scope
for collecting data that can contribute to developing further dynamic models (particularly sediment
dynamics and scour) and subsequent site monitoring. Better information on geotechnical
parameters of the sediments is also crucial for foundation design, the main financial concern of
offshore renewable energy projects. By extending the time-scales of site surveys, it is possible to
monitor seabed dynamics.
60
By following the guidelines set out by OSIG and DNV it is possible to develop a phased site survey
methodology which is both efficient and productive in acquiring the relevant data necessary to
develop an offshore installation.
61
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