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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

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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.

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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

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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’.

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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.

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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.

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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

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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

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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.

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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.

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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

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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.

http://www.marine.ie/home/

15

Figure 4-1: Annual Average Wave Power

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Figure 4-2: Annual Average Wave Height

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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 …