Metocean Procedures Guide for Offshore Renewables

Produced and Compiled by OFFSHORE RENEWABLES SPECIAL INTEREST GROUP Issue 2 – November 2018 CONTENTS

1. INTRODUCTION 2

2. METOCEAN CONSIDERATIONS ...... 2 2.1 Cost Implications ...... 3 2.2 An Integrated Approach ...... 3 2.3 Focal Point ...... 3 2.4 Contractors ...... 3 2.5 Relevant Standards ...... 4

3. METOCEAN THROUGH PROJECT LIFECYCLE ...... 5 3.1 Site Selection and Feasibility ...... 5 3.2 Development and Consents ...... 5 3.3 Design and Certification ...... 6 3.4 Installation ...... 6 3.5 Operations and Maintenance (O+M) ...... 7 3.6 Decommissioning ...... 7 3.7 Strategy Overview ...... 7

4. METOCEAN DATA ...... 9 4.1 Data Sources ...... 9 4.2 Metocean Processes ...... 9 4.3 Data Management ...... 10 4.4 Data Sharing ...... 10

5. METOCEAN ANALYSES ...... 12 5.1 Operational Climate ...... 12 5.2 Extreme Climate ...... 13 5.3 Weather Windows and Adverse Weather Downtime ...... 13 5.4 Input to Design Loads Cases ...... 14

APPENDIX 1 – STANDARDS ...... 15 A1.1 ISO ...... 15 A1.2 DNVGL ...... 15 A1.3 IEC ...... 15 A1.4 Bureau Veritas ...... 15 A1.5 EMEC ...... 15 A1.6 Carbon Trust ...... 15 A1.7 IOGP ...... 15 A1.6 UK HSE ...... 15

APPENDIX 2 - GLOSSARY ...... 16 A2.1 General ...... 16 A2.2 Wind ...... 17 A2.3 Wave ...... 17 A2.4 Current ...... 18 A2.5 Water Levels ...... 18 A2.6 Units and Convention ...... 18

APPENDIX 3 – IN-SITU DATA COLLECTION ...... 19 A3.1 Scope of Works ...... 19 A3.2 Schedule ...... 20 A3.3 Data and Equipment Loss ...... 20 A3.4 Data Specification ...... 20 A3.5 Health, Safety and Environment (HSE) ...... 20 A3.6 Measurement Campaign Specification Sheet ...... 20

APPENDIX 4 – MODELLING ...... 23 A4.1 Inputs ...... 23 A4.2 Methodology ...... 23 A4.3 Deliverables ...... 24

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 1 the two disciplines should be considered, where ACKNOWLEDGEMENTS appropriate, to enable cost savings.’ The Offshore Renewables Special Interest Group Metocean in the context of this document excludes (ORSIG) has produced and compiled this proce- estimation of the energy resource that can be extracted dure guide, with support from the Operational from the atmosphere or ocean. However, both resource Oceanography Special Interest Group (OOSIG). It assessment and metocean engineering ultimately rely is based on a draft document originally produced on the same types of data and similar analysis methods. for SSE Renewables by Ian Leggett of OceanExpert It is useful, but not essential, that both engineering Limited in 2012. and resource assessments are derived from the same data sources. Effective coordination between these IMarEST members who have contributed are: applications has the potential to minimise costs. For Name Company example, floating LiDAR buoys utilized in wind resource Alice Goward Brown Bangor University assessments could also provide wind, wave and current Mark Calverley Blue Ocean Consulting data to support subsequent engineering design for Doug Creswell HR Wallingford minimal additional cost, enabling greater consistency Laure Grignon LGS between metocean and resource assessment. Jamie Hernon MetOceanWorks Gus Jeans Oceanalysis Whilst this document relies heavily upon experience Ian Leggett OceanExpert Garry Mardell Consult-on-sea gained from offshore wind developments on fixed struc- Adam Nicholls Gardline tures, the majority is also applicable to emerging technol- Micheal O’Cathain SSE Renewables ogies, including floating wind, wave and tidal. It draws pri- James Parker Cefas marily on UK experience, but procedures may be applied Zoe Roberts Vattenfall Wind Power in other parts of the world taking into account relevant Alastair Stagg Fugro GEOS local metocean conditions and engineering standards. Richard Wakefield NIRAS This guide applies to the OR site area, cable routes, maintenance base(s) and access routes throughout a project’s life, from early development through to decom- 1. INTRODUCTION missioning, and can also be used to support Environ- This procedures guide outlines the metocean activities mental Impact Assessment (EIA) during the consenting required to support all phases of an offshore renewable process. The majority of the associated metocean activ- (OR) energy project, through development and con- ities take place during the development and pre-con- struction to ongoing operations and maintenance (O+M). struction stages of the project, however continued data It is intended as a tool for all those involved in offshore acquisition during O+M can add significant value. It must renewables, including project managers, structural engi- be emphasised that this is a generic guide illustrating neers, surveyors and logistics personnel, based within OR typical approaches to be adopted. The implementation companies or support organisations and contractors. of the metocean strategy may vary from the approach suggested here, as individual projects may have differ- Metocean is a technical engineering discipline that ent ways of defining stages within their governance pro- addresses meteorological and physical oceanographic cess. However, the principles will be the same. matters. It originated in the oil and gas sector in the late 1970s and is equally applicable to the OR sector. A concise list of references, applicable standards and a Metocean is primarily concerned with quantifying the glossary of terms are included in this guide. effects of weather and sea conditions (specifically wind, wave, current and water level) and more broadly the physical aspects of the water column (such as conductiv- 2. METOCEAN CONSIDERATIONS ity and temperature). All offshore activities are affected by prevailing metocean conditions. There is a requirement for As metocean is specifically concerned with physical metocean information and support at all stages of a oceanography, it does not address chemical, biological project, and throughout its operation after installation. or environmental considerations. However, there is often A recommended strategy for ensuring this information overlap between the metocean and environmental and support is provided appropriately is outlined in Sec- requirements of OR projects, so collaboration between tion 3.7, and presented in terms of key deliverables.

2 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES Because many of the metocean requirements are 2. Ensuring data collection starts early in the project dependent on one another, for example, reliable statis- lifecycle, enabling measured data to be collected tics need quality long-term datasets, it makes sense to over a sufficient length of time. view and conduct metocean activities in a coordinated 3. Selecting technically competent metocean person- and integrated way. nel or contractors to: a. Perform in-situ measurement. 2.1 Cost Implications b. Independently quality control the data. One of the key drivers to growth within the OR market is c. Conduct data analysis studies. the requirement to reduce the cost of energy, especially d. Provide weather forecasts. in the context of zero-subsidy offshore wind projects. 4. Encouraging metocean contractors to work There is considerable scope within the metocean com- together where appropriate to maximise data ponent to drive cost savings throughout the project life- returns, ensure data quality and improve forecasts. cycle. In particular, investment in metocean at an early 5. Enabling good communication between providers stage of the project will result in significant cost sav- and end-users of metocean information, and the ings in later phases. Changes in metocean criteria dur- timely and appropriate dissemination of resulting ing the project lifecycle can introduce large engineering data and information. Early dialogue between design variations and result in programme delays, leading to engineers and metocean practitioners is fundamen- increased capital and operational expenditure. Reduc- tal to ensuring requirements are fully understood. ing uncertainty (and hence risk) in engineering design loads, understanding the impact of adverse metocean 2.3 Focal Point conditions on construction schedules and time to first To further enable the pursuance of metocean activities revenue as well as optimising weather windows, are all in a coordinated and integrated fashion, it may be ben- key to minimising capital expenditure. Subsequently, eficial that a Metocean Focal Point is established in sup- optimising weather windows to support maintenance port of projects. This can be filled either by an in-house scheduling and conducting early intervention when position or an external consultant and may not neces- metocean impacts on structural integrity occur, can sarily need to be on a full-time basis depending on the have significant influence on operational expenditure. size of company or project.

Early engagement of metocean expertise is one of 2.4 Contractors the key steps in reducing overall project risk and miti- Metocean activities can be broadly classified into five gating future cost variation within the project budget. categories with respect to services offered by metocean The inclusion of a specific project or company role of contractors: Metocean Focal Point should be considered. 1. In-situ measurement. 2. Numerical hindcast modelling. 2.2 An Integrated Approach 3. Numerical forecasting. There are three key elements to consider with respect to 4. Satellite observations. an integrated metocean approach: 5. Data analysis and desktop studies. 1. Data Collection Metocean data are acquired (or purchased if such Some contractors offer a comprehensive service cover- data already exists), either in-situ, and/or by run- ing all categories whereas others occupy specific niches. ning numerical models. Recent developments have involved progress towards 2. Data Management greater digitalisation of deliverables, with many com- Measured or numerical data is quality controlled and panies now offering online metocean databases and stored in order to allow ready access and search capa- analysis services. It is important to ensure that, in choos- bility or offered as data services in a digital environment. ing contractors, due account is taken of their relevant 3. Data Analysis competency, experience, reputation, health and safety Stored and real-time data is analysed to produce record and quality assurance system. key statistics that will be used in project work pack- ages or to support live operations. As part of the contracting process it is imperative to have a clearly defined, fit-for-purpose scope of work. This should Issues to consider include: be developed by the Metocean Focal Point in conjunction 1. The metocean process is iterative in nature, allowing with the project team. Further discussion regarding the for the evolution of data and data products in order scope of work for observational and modelling campaigns to support the different stages of the project. can be found in Appendix 3 and 4 respectively.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 3 Photo credit: Vattenfall

2.5 Relevant Standards design. The main difference arises because metocean is There are a wide range of offshore energy industry stand- now also the resource itself. A diverse range of recent ards and guidance notes released to inform offshore standards consider some specific requirements for off- Developers of the importance of metocean data, suitable shore wind, tidal and wave energy. acquisition methods, and analysis to achieve development requirements. Some also address resource assessment Some differences exist between the various guidelines, requirements, design, survivability and environmental for example in relation to wind profile relationships. impacts. In many cases, metocean information is con- Reliable results may not always be produced by sim- tained within guidelines with a much broader remit. ply following one guideline, especially if it is out of date or not relevant to the region or application. A notable Standards and guidelines have been issued by organisa- example is provided by UK HSE OTR 2001/010 (details tions such as the International Organization for Stand- in Appendix 1). This was published in 2002, but is only ardization (ISO), the International Electrotechnical a reissue of Department of Energy Guidance Notes from Commission (IEC), the International Association of Oil 1990. This is very much out of date and all more recent and Gas Producers (IOGP) and DNV GL. A summary of standards take precedence, especially those highlighted some widely used key documents is given below, with a below. Despite this, OTR 2001/010 is still cited in some more extensive list provided in Appendix 1. offshore renewable energy projects.

Following the historical development of offshore energy, A suitably experienced metocean practitioner must the first standards originate from the oil and gas indus- ensure appropriate data and methods are used to try. Most of the metocean content is equally applicable to quantify the key metocean processes of critical engi- offshore renewables from the perspective of engineering neering impact.

STANDARD FULL REFERENCE

ISO 19901-1:2015 International Organization for Standardization (2015). International Standard, Petroleum and natural gas industries - Specific requirements for offshore structures - Part 1: Metocean design and operating considerations. Second edition, 2015-10-15, ISO 19901-1:2015.

DNVGL-RP-C205 DNVGL (2017). Recommended Practice, Environmental Conditions and Environmental Loads, August 2017, DNVGL-RP-C205.

IEC 61400-3:2009 International Electrotechnical Commission (2009). International Standard, Wind turbines - Part 3: Design requirements for offshore wind turbines, Edition 1.0, 2009-02, IEC 61400-3:2009.

IEC TS 62600-2:2016 International Electrotechnical Commission (2016). Technical Specification IEC TS 62600-2:2016. Marine energy - Wave, tidal and other water current converters - Part 2: Design Requirements for marine energy systems.

IOGP Report 447 International Association of Oil and Gas Producers, (2011). Health, Safety and Environmental guidelines for metocean and Arctic surveys, Report No. 477.

4 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 3. METOCEAN THROUGH PROJECT LIFECYCLE Metocean plays a key role in all aspects of the OR project lifecycle:

SITE SELECTION DEVELOPMENT DESIGN AND AND FEASIBILITY AND CONSENTS CERTIFICATION INSTALLATION O+M DECOMMISSIONING

Typically, an OR project will be broken down into smaller The briefing note should identify the conditions that components (or work packages), each requiring special- may prevent the technical feasibility and economic ist metocean inputs. The metocean process is iterative, viability of a potential project. If a specific risk is identi- improving in accuracy and quality at every phase of the fied, it may be that further investigation is required at an project lifecycle, with prior metocean information being early stage of the project. added to, or superseded, as the project progresses. 3.2 Development and Consents Due to the importance of metocean input in all aspects Early in the development and consents phase, defini- of the project lifecycle, it is recommended that any tion of the preliminary metocean dataset is required, metocean expertise be embedded within, or directly fulfilling the requirements of the project for the pur- available to, a project development team and close rela- poses of consenting and concept design. This data- tionships developed with work packages throughout set provides a best estimate of metocean conditions every phase. A strategy, to be adopted to ensure that based on available measured data or coarse, uncali- metocean decisions, requirements and products appro- brated, unvalidated model data if insufficient measured priate to an OR project are delivered throughout its life- data is available, obtained for little or no cost. Online cycle, is provided in Section 3.7. metocean databases based upon regional metocean models may be utilised. It should be noted that the metocean process may be entwined with energy resource assessment. It is therefore Preliminary data and analyses will generally include: necessary to consider requirements in each phase from • Conservative, quantified description of conditions at the resource perspective, as well as that of engineering. a representative location or locations. • Quantification of potential hazards and assessment 3.1 Site Selection and Feasibility of associated risk. Within this phase, it is necessary to understand the • Concurrent time series of wind, wave, current and general metocean conditions across the geographi- water level cal region of interest, and to identify any conditions or • Derivation of operational, extreme and weather processes that may pose a significant risk to the project downtime statistics and preliminary loads cases. (examined further in Section 4). Preliminary metocean data and analyses will also con- It is pertinent to conduct a literature review, identify any tribute to other aspects of the project, such as defini- available data sources in the region, and to summarise tion of the environmental baseline and impact stud- the information obtained in a short briefing note. The ies regarding foundation impact on metocean regime, amount of work that is required at this phase depends scour, and sediment dispersion due to piling, seabed on the lease award process. In some European coun- preparation or cable burial. Estimates of weather down- tries, much of this work is carried out by the awarding time based on preliminary data will be used to inform authority. the contractual basis for site characterisation surveys (such as geophysical and geotechnical). A briefing note would typically include: • General description of the proposed site. Requirements for this dataset will be project specific • Summary of available data sources and data gap as a combined consequence of site conditions, project analysis. definition, project team and the methodologies used • Identification of potential hazards. for preliminary design. It is therefore recommended • Estimates of wind, wave, current and water level that the detailed content of the preliminary engineer- conditions. ing data and analyses be defined in coordination with • Recommendations for the intended metocean strategy. the project team.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 5 At this early stage in the project the metocean criteria • Quantified description of conditions at a number of should be conservative, so that the impacts become less locations depending on spatial variability. onerous as more reliable data become available. This • Associated long time series. can lead to natural conflicts between the interpretation • Presentation of validation methodology and results. of ‘onerous’ in resource assessment and engineering • Statistical analyses, including operational and that must be carefully managed. extreme criteria, fatigue and weather downtime. • Additional specialist analyses as required by project It is within this phase that an appropriate in-situ meas- teams. urement campaign (see Appendix 3) should be initi- ated, balancing scale and cost, to enable: It may be beneficial to continue refining the metocean 1. Definition of the environmental baseline. data to reduce their uncertainty, ensuring that conserv- 2. Validation of future numerical modelling. atism is reduced rather than increased. 3. Development of a detailed design basis. Typical statistical analyses undertaken and presented The timescales of the measured dataset required for con- within a metocean study are described further in Section sent applications and design purposes may differ, with 5. Data users are moving towards the direct use of time observational datasets usually required to cover several series data, reducing the need to produce detailed statis- years for operability assessments and engineering design so tics, as computing power becomes more greatly available as to capture inter-annual variability. However, in accelerated via cloud services and automation is developed. projects the overall length of the measurement phase might be (necessarily) shortened. It is worthwhile considering the The full dataset and analyses will be required as soon deployment of a continuous long-term in-situ measurement as is practicably possible within the design phase and campaign throughout the development and design phases will form the basis of engineering design and installation and onwards into construction and O+M. planning. Risks and any potential shortcomings should be identified, and outcomes presented in summary Towards the end of the development and consents form to the relevant stakeholders, ensuring they are phase, execution of well-validated, detailed metocean fully understood. It is vital to ensure that project teams modelling (see Appendix 4) should be underway, using are made aware of newly available data and analyses industry best practice techniques and utilising all avail- and their associated uncertainties, and that use of previ- able observational data. ously supplied preliminary data and information ceases, in order to reduce risk and prevent inaccuracy being car- It is important to consider synergy and coordination ried into the design phase. with other aspects of the project, such as EIA and resource assessment. This will ensure cost-saving, pro- It is recommended that the final metocean modelling study mote consistent approaches and maximise the value of is certified by an appropriate standardisation authority, to integrated observational campaigns. ensure completeness, plausibility and verifiability. Cer- tification should be undertaken as soon as is practicably 3.3 Design and Certification possible to enable a smooth design process and avoid By the time of transition into the design phase, a suitably delays. Depending on the available timescales, the final reliable metocean dataset (representing all key processes) design basis and final metocean study may be certified should be finalised, based upon the combined outcomes concurrently. When seeking certification, the structure of of detailed numerical modelling and site specific data the design basis should follow that of the standard against acquisition. This dataset will inform the engineering which it is being certified. Attention should be paid to the design basis, which will be used consistently across the required nomenclature to avoid confusion. project for engineering design and decision-making, as well as programme definition and installation planning 3.4 Installation taking into account expected weather downtime. Prior to an installation campaign, it is necessary for Devel- opers and their Contractors to have a clear idea of the To ensure it meets their needs, requirements for the final range and likelihood of weather downtime affecting con- metocean dataset should be agreed in advance with struction and installation. This will enable negotiation of stakeholders. The full dataset will comprise a high quality, contractual terms and allocation of appropriate cost and accurate account of metocean conditions from detailed programme contingency. Risks related to weather down- hindcast modelling validated against in-situ measure- time may be shared by both parties, or assigned to one ments, taking account of spatial variability, including: party depending on the preferences of those concerned.

6 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES The risk of weather downtime must be calculated by anal- ysis and/or simulation of the proposed installation sched- ule using the best available metocean data (for example, the final metocean dataset once available). Developers should enable a fair bidding process by ensuring that a consistent approach to weather downtime risk estima- tion is adopted by bidders. This could include the use of the same metocean dataset (supplied by the Developer), and a consistent method of analysing weather downtime. Indeed, the fairest method of assessing bids may be for Contractors to supply their proposed working schedules and day and downtime rates, and for the Developer to calculate the price of each Contractor independently.

In the lead up to the installation phase of the project, it is necessary to procure weather and ocean forecasting ser-

vices for the duration of the offshore installation activities. Fugro credit: Photo This should be undertaken in conjunction with real-time monitoring, which may be an extension of the previous further understanding of the impact of localised events metocean measurement campaign. A real-time monitor- so as to revise and adapt the O+M strategy as necessary. ing approach combined with improved forecasting and Furthermore, metocean measurement and modelling modelling capability from assimilation of measurements requirements should be agreed with asset management is of paramount importance in ensuring the safety of stakeholders to complement structural monitoring cam- teams working offshore. It is also likely to increase instal- paigns and understand structural performance. The lat- lation efficiency by providing marine personnel with reli- ter includes all elements of the structure including depth able information from which they can make decisions, of pile burial, and scouring events that may impact the minimising downtime due to adverse weather and reduc- structural integrity of the pile. ing the risk of compensation claims from Contractors. The ability to correlate structural performance to actual Prior to installation, the Marine Warranty Surveyor and metocean conditions helps to improve: engineering mod- the Developer and/or Contractor will agree the metocean els; the safety factors applied in the engineering process; limits and their applicable durations for the intended the management of risk of catastrophic failure during the operations. Appropriate standards regarding marine asset’s life; and the asset’s End of Life expectancy based operations should be consulted. Further conservatism on actual fatigue experienced. Cost benefits will likely will be applied when deriving operational limits, to take arise from such an approach, particularly as offshore into account the level of accuracy possible when assess- wind moves into regions impacted by tropical storms. ing the actual weather conditions under which an opera- tion will take place. The level of additional conservatism 3.6 Decommissioning can be reduced by using real-time measurements, con- The metocean requirements during decommissioning are sidering more than one forecast, or by having a weather very similar to those during installation. These include forecaster on site. Additional conservatism may occur provision of hindcast data for programme planning, and as a result of human decision making, in particular, if the weather forecasting with real-time data for work planning. forecast is not trusted by the parties involved. It is there- fore recommended that 24/7 access to an experienced 3.7 Strategy Overview weather forecaster is arranged, possibly remotely, and A recommended metocean strategy is summarised in a used unreservedly during installation. Gantt chart shown in Figure 1. Activities have been cat- egorised at a high level: in-situ data collection, numeri- 3.5 Operations and Maintenance (O+M) cal modelling and desktop studies. Emphasis within the The O+M phase of a project has similar requirements to strategy has been placed upon the production of a high that of the installation phase, that is, provision of weather quality database of metocean data in time series for- forecasting and real-time data for work planning. In addi- mat, from which subsequent metocean analyses may be tion, operations will also require large scale monitoring, conducted. Consultation with project teams throughout including the continuation of metocean measurements. each phase is necessary to ensure that analyses con- This enables performance to be analysed and supports ducted fulfil the specific needs of the project.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 7 Figure 1: Gantt chart describing the metocean strategy in terms of key deliverables and required activities throughout the project lifecycle.

DESIGN DELIVERABLE ACTIVITIES SITE SELECTION DEVELOPMENT AND INSTALLATION O&M DECOMMISSIONING AND FEASIBILITY AND CONSENTS CERTIFICATION Metocean Literature reviewD briefing note Data collation from free sources and gap analysisD

Preliminary Purchase of dataD engineering dataset and ModellingM analyses (calibrated with publicly available data or uncalibrated)

Data analysisD

Environmental impact analysisDMS

Full In-situ engineering measurementS dataset and analyses Fully calibrated modellingM

Data analysisD

Other Weather forecast modellingM

Real-time in-situ measurementS

Strategy optimisation and condition monitoringDMS

End of life assessment

D Data analysis and desktop studies, including satellite observations M Numerical hindcast and forecast modelling S In-situ measurement

8 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES Photo credit: Wello credit: Photo 4. METOCEAN DATA The required level of accuracy, and therefore cost, of metocean data evolves with the project. Public data are often available, and can be used to provide a prelimi- nary characterisation of metocean conditions on site. At this early stage, inherent uncertainties and limitations of the data should be acknowledged, and, where possible, the potential implications understood and interpreted if relevant. As the project progresses and the design is refined, more accurate statistics are required. This ena- bles increased certainty in the design basis. As outlined

in Section 3, preliminary metocean criteria should be Photo credit: Minesto conservative, so that impacts become less onerous dur- ing later stages. Examples of key processes affecting extreme and oper- 4.1 Data Sources ating conditions within the extensively studied North There are 3 fundamental sources of metocean data, Sea include: obtained by: • Winds forced by atmospheric disturbances, notably 1. In-situ measurement. mid-latitude storms. 2. Numerical modelling. • Wind-sea waves generated by the local near surface 3. Satellite observations. wind field. • Swell waves generated by the remote near surface These are subjected to various analyses to produce a range wind field. of statistics and deliverables suitable for engineering appli- • Tidal currents and water level driven by predictable cations. Within the context of metocean data gathering, a astronomical forcing. fourth effective type of metocean data may be added: • Surge currents and water level driven by less pre- 4. Statistics from pre-existing proprietary reports or dictable atmospheric disturbances. publications. The spatial and temporal resolution of selected datasets Some of these data can be obtained from public domain must be sufficient to resolve the dominant metocean sources, or collaboration with other operators with inter- processes. For the examples given for the North Sea, time ests in the region. However, an offshore development intervals between 3 hours and 10 minutes will generally usually requires, and directly benefits from, a dedicated suffice. However, more frequent sampling is required to site-specific data collection effort. A suitable metocean capture the small scale, rapidly varying processes that database usually includes all of the above. In-situ meas- dominate in other regions (for example, turbulence, soli- urement provides reliable quantification of conditions at tons and squalls). In regions where high impact events a specific site during the instrumentation deployment such as tropical cyclones occur, analysis of spatial data period, but are usually of much too short a duration for such as storm tracks are required. Such events are not reliable quantification of long-term trends and statistics. well represented in large-scale numerical model datasets.

Numerical models provide the required longer term con- Where effects of climate change on site conditions are text, but require validation and calibration using in-situ understood, those effects should be incorporated into measurement or satellite observations. Numerical models the metocean analysis, for example, the poleward migra-

and satellite observations both provide valuable spatial tion of tropical Wello credit: Photo cyclones, or changes in sea-ice cover. coverage, well beyond a limited set of in-situ measure- ment sites, but have limitations in terms of temporal and Although OR projects need site-specific (and possibly spatial resolution. As such, a good metocean database is technology-specific) information, a general understand- the result of careful combination of different data sources. ing of the broader metocean conditions in a region is Further details are provided in Appendices 3 and 4. useful. These can be found in the regional annexes of ISO 19901-1 (2015). Metocean processes control all key 4.2 Key Processes metocean parameters, including those relating to wind, Selected data sources must represent the key metocean wave, current and water level. A summary of specific processes that impact the development site. These pro- parameters under each of these categories is given in cesses vary considerably from region to region. the Glossary (Appendix 2).

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 9 Photo credit: Wello credit: Photo 4.3 Data Management ers are focussed on enabling better use of data through Metocean data is a valuable resource which needs to be the development of applications and machine learning managed. Its value is dependent on the data being: in order to derive and exploit information. • Reliable (appropriate level of quality control applied for the intended application); 4.4 Data Sharing • Fully described (i.e. metadata); In addition to the value metocean data offers to indi- • Discoverable; and vidual companies, contributing data to the wider com- • Accessible. munity provides benefits in terms of improved under- standing of the marine environment, and other broader The level of quality control applied to data should be societal benefits. To enable this, a number of marine included in the metadata to permit assessment of its data management schemes are offered at national, reliability for a particular purpose. Reports relating to regional and global scales. the deployment of the instruments (for example, instru- ment calibration) should be considered as part of the The International Council for the Exploration of the data process and digitalisation of this information ena- Sea (ICES) is a global collaborative effort with a bles a robust audit trail regarding the quality of the data. well-established data centre managing large marine environmental datasets. Regional initiatives include The meteorological and oceanographic communities the European Global Ocean Observing System have undertaken significant work to create common (EuroGOOS) and the European Marine Observation parameter naming and metadata standards. Common and Data Network (EMODnet), where EMODnet’s parameter naming is important to ensure data users physics portal has been developed to provide access understand the parameter being utilized and to enable to physical data sets made available from partner discovery. Data suppliers often use different naming countries in addition to the array of data available conventions which can lead to confusion, therefore it is via EuroGOOS. recommended to adopt a naming convention. Perhaps the mostly commonly utilized convention is the Climate Wind Farm Developers are encouraged to contribute and Forecasting (hiip://cfconventions.org/index.html) to local or regional GOOS initiatives such as the Baltic which also offers metadata standards for NetCDF files Operational Oceanographic System (BOOS), North (frequently used for modelling outputs). West European Shelf Operational Oceanographic System (NOOS) and Integrated Ocean Observing How data are stored will be dependent on the resources System (IOOS) in the US. These connect directly to of a company, and its degree of digitalisation. The size of the Copernicus Marine Environment Monitoring Ser- metocean datasets can be relatively small, but they are vice (CMEMS) which provides access to both global increasing rapidly with changing technology. Metocean in-situ data and model data, enabling global collabo- data also has a strong temporal element, which is often ration and cooperation. Copernicus is a core opera- not considered in wider geoscience datasets held by tional information service funded by the European Developers. Some may choose to undertake this inter- Union and as such is a sustainable and reliable ser- nally whilst for others it may be appropriate to out- vice. The CMEMS provides regular and systematic source to suitably skilled contractors. Consideration core reference information on the state of the physi- should also be given to other types of data that the cal oceans and regional seas. The observations and stakeholders may be accessing to determine whether forecasts produced by the service support all marine the metocean data can leverage existing digitalisation applications. within an organization. To promote the sharing of, and improved access to, col- Stakeholders may require the data in various formats for lected marine environmental data at a national level, use in their applications, as such the company will need and specifically for the UK, the Marine Environmental to determine how to enable data users to interact with Data and Information Network (MEDIN) has been cre- the data. An application programming interface (API) ated. Associated with MEDIN, individual Data Archive would allow access in single or multiple formats directly Centres (DACs) have been established for discrete sci- by another software application, however a database entific areas to provide secure, long-term storage and administrator could also deliver data requirements. The provide access to marine data. The use of MEDIN also availability of database and associated software from complies with the INSPIRE Directive for consenting pur- cloud providers offers relatively low cost solutions as poses (hiips://inspire.ec.europa.eu). Specifically related software code requirements are reduced. Cloud provid- to the metocean technical area, the UK Met Office is

10 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES the DAC responsible for marine meteorological data, ted to the Marine Data Exchange (MDE). Other equiva- making it available via a number of separate facilities. lent organisations may apply in other parts of the world, The British Oceanographic Data Centre (BODC) is the not specified in this document. DAC responsible for hosting marine biological, chemi- cal, physical, sound and geophysical data. In the UK, the Website URL’s for the various data initiatives are given Crown Estate requires offshore wind data to be submit- in Figure 2 .

ORGANISATION URL

BOOS hiip://eurogoos.eu/roos/baltic-operational-oceanographic-system-boos

CMEMS hiip://marine.copernicus.eu

EMODnet hiip://www.emodnet.eu

EuroGOOS hiip://eurogoos.eu

ICES hiip://www.ices.dk

IOOS hiips://ioos.noaa.gov

MEDIN hiip://www.oceannet.org

MDE hiip://www.marinedataexchange.co.uk

NOOS hiip://eurogoos.eu/roos/north-west-european-shelf-operational-oceanographic-system-noos

iMarDIS hiips://www.imardis.org

Figure 2: Website links for respective observational systems and collaborative initiatives

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 11 5. METOCEAN ANALYSES The type of analyses undertaken in the process of an OR project will generally incorporate assessment of opera- tional climate, extreme climate and weather downtime using statistical techniques, and preparation of inputs for structural design load calculations, either by the Developer or by an external consultant. 5.1 Operational Climate The operational climate is a statistical description of the normal metocean conditions experienced at the site. It is these conditions which drive fatigue and govern day- to-day operations. Typically, the operational metocean climate is described using scatter or bivariate tables of metocean parameters (see Figure 3), and rose plots (see Figure 4). This may include, but is not limited to, the following: • Significant wave height by month Figure 4: Example rose plot (courtesy of MetOceanWorks Toolbox) • Significant wave height vs. wave direction • Significant wave height vs. mean wave period • Significant wave height vs. peak wave period Generally, data will be presented as a percentage fre- • Significant wave height vs. wind speed quency of occurrence for all or part of the dataset. • Significant wave height vs. wind direction Number of records or frequency of occurrence scaled • Wind speed by month to parts per hundred thousand or average number of • Wind speed vs. wind direction waves per year may also be presented. • Total surface current vs. surface current direction • Total depth averaged current speed vs. depth aver- Directional sectors will typically be a maximum of 30 aged current direction degrees, centred on North. Often, tables and roses will • Vertical current profile and current shear be presented for annual, seasonal or monthly partitions • Tidal water levels. of the data, by directional sector, or by intensity bin.

Figure 3: Joint frequency table depicting percentage occurrence of significant wave height versus peak wave period (courtesy of MetOceanWorks Toolbox)

12 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES Figure 5: Example extreme value analysis for significant wave height (courtesy of MetOceanWorks Toolbox)

This allows closer scrutiny of the metocean regime at of two variables, in order to reduce the conservatism the site, taking into account sheltering from the coast or inherent in combining parameters assumed to be exposure from the open sea. independent of one another, which is often the case. As such, it is highly recommended that extreme value In the earlier phases of the project, operational climate analysis is conducted by persons possessing demon- may be summarised in less detail. However, as the pro- strable competence. ject progresses, and particularly in approach to final design, the full suite of operational climate descriptions The outcomes of extreme value analysis may include, will be required, including consideration of parameter but are not limited to, the 1, 10, 50, 100, 1000 and 10000 relationships by intensity bins and directionality. It is year return period of relevant metocean parameters, typical for information to be presented in more than one omni-directional and for each directional sector, all year way, for example, significant wave height direction vs. and by month. Directional sectors should be 30 degrees wind direction by wind and wave intensity bins. or lower. Consideration should be given to the pres- ence of wave breaking, which may limit wave growth for 5.2 Extreme Climate selected sites. The extreme climate is a statistical description of the abnormal metocean conditions experienced at the site. 5.3 Weather Windows and Adverse Weather Downtime Weather window persistence statistics (for example, Exact values of metocean parameters for a selection Figure 6) may be required by any work package result- of return periods are derived from statistical meth- ing in offshore works, including survey, civil and struc- ods of extreme value analysis (see Figure 5). Extreme tural engineering, turbine engineering and cabling. value analysis theory is complex and evolving, and it is necessary to understand the drivers of fluctuations Persistence statistics may be requested as part of a in metocean parameters, such as large scale atmos- metocean study, however, specific combinations of pheric circulation patterns. To account for inter- operational limits for particular or combined series of annual variability, it is fundamental that time series operations are difficult to pre-empt early in the pro- of data to be analysed are both long enough (>30 ject lifecycle. It is therefore recommended that more years) and have the temporal resolution to capture complex assessments of operability are conducted in peaks. Extreme value analysis should, where possible, close partnership with logistics and construction plan- take into account the joint probability of occurrence ning teams. Figure 3: Joint frequency table depicting percentage occurrence of significant wave height versus peak wave period (courtesy of MetOceanWorks Toolbox)

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 13 Condition Duration (h) Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Average Hs<1.5 3 47% 57% 64% 78% 83% 84% 85% 81% 70% 61% 52% 45% 67% Hs<1.5 6 46% 55% 61% 74% 79% 81% 82% 77% 67% 58% 51% 45% 65% Hs<1.5 12 42% 52% 59% 74% 79% 79% 80% 76% 66% 56% 47% 39% 62% Hs<1.5 24 35% 44% 52% 67% 72% 74% 76% 70% 58% 48% 39% 31% 56% Hs<1.5 48 26% 33% 42% 56% 62% 63% 66% 59% 47% 37% 30% 22% 45% Hs<1.5 72 20% 26% 35% 47% 54% 54% 59% 50% 39% 29% 24% 16% 38%

Dedicated studies with regards to persistence may also Figure 6: Percentage probability of the condition significant be contracted, once the operational limits and weather wave height (Hs) <1.5m existing for varying weather window window durations are fully understood. Alternatively, durations. simulation of the installation programme against time series may be conducted by a competent practitioner ture). Such derivations should be conducted by special- to build up probability distributions regarding the likely ist practitioners and may not be within the capability of time taken to complete the installation sequence. the metocean contractor.

5.4 Input to Design Loads Cases As noted in the introduction, this procedures guide is In fixed offshore wind, numerous loads cases are con- heavily focused on offshore wind developments involv- ducted according to the key standards (Section 2.5). The ing a fixed structure, due to relative technical maturity. majority of loads cases will incorporate descriptions of Many of the general principles will also apply to floating the combined hydrodynamic and aerodynamic condi- wind, wave and tidal, however, some specific details will tions impacting upon the structure with varying severity differ. The project team should be consulted at an early referred to as reference sea states and reference wave stage in order to correctly scope the metocean require- heights, for example: ments for wave, tidal and floating wind developments, in conjunction with the available guidance and stand- • Normal Sea State ards (see Section 2.5). Characterised by a significant wave height, a peak period and a wave direction and associated with a concurrent For floating wind energy and wave energy, a major dif- mean wind speed, such that the significant wave height ference from a metocean perspective will be the charac- is the expected value for the associated wind speed. terisation of metocean conditions required to quantify impact upon a floating structure, which reacts dynami- • Severe Sea State cally to its environment. For example, it is necessary to Characterised by a significant wave height, a peak characterise details of the full directional wave spectra. period and a wave direction and associated with a con- A major challenge is designing for platform motion with current mean wind speed, such that the combined wave corresponding motion of the wind turbine blades, or height and wind speed has a return period of 50 years. electrical power take-off in the case of wave energy.

• Extreme Sea State With regard to tidal energy, one major difference will be Characterised by a significant wave height, a peak period the need to characterise ocean turbulence. This is analo- and a wave direction, wherein the significant wave height gous to the need to characterise atmospheric turbulence has a specified return period of 1 and 50 years. for wind turbine engineering, but may bring specific new challenges to the data collection strategy. Potential The derivation of these sea state scenarios requires tidal energy development sites are situated in regions of detailed knowledge of structural design in order to strong tidal flow, which are often highly turbulent. Exten- ensure that the most appropriate combinations of sive guidance for characterisation of ocean turbulence for parameters are applied (for example, taking into current turbine engineering are available from the Car- account the eigen-frequency sensitivity of the struc- bon Trust-funded (TiME) project (see Appendix 1).

14 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES APPENDIX 1 – STANDARDS A1.1 ISO A1.4 Bureau Veritas • International Organization for Standardization (2015). • Bureau Veritas (2015). Guidance Note NI 572 DT R01 International Standard, Petroleum and natural gas E. Classification and Certification of Floating -Off industries - Specific requirements for offshore struc- shore Wind Turbines. tures - Part 1: Metocean design and operating consid- • Bureau Veritas (2015). Guidance Note NI 603 DT R01 erations. Second edition, 2015-10-15, ISO 19901-1:2015. E. Current and Tidal Turbines. • Bureau Veritas (2016). Guidance Note NI 631 DT A1.2 DNVGL R00 E. Certification Scheme for Marine Renewable • DNV (2018). Offshore Standard DNV-OS-C101. Design Energy Technologies. of offshore steel structures, general (LRFD method). • DNV (2018). Standard DNVGL-ST-0119. Floating A1.5 EMEC wind turbine structures. • European Marine Energy Centre (2009). Draft guide- • DNV (2018). Standard DNVGL-ST-0126. Support lines and standards for the marine renewable energy structures for wind turbines. industry. hiip://www.emec.org.uk/standards/ • DNV (2018). Full Standard DNVGL-ST-N001 Marine operations and marine warranty. A1.6 Carbon Trust • DNVGL (2017). Recommended Practice DNVGL-RP- • Carbon Trust (2015). Turbulence: Best Practices for C205. Environmental conditions and environmental the Tidal Power Industry. Part 1: Measurement of tur- loads. bulent flows. MRCF-TiME-KS9a. • DNV (2016). Standard DNVGL-ST-0437. Loads and • Carbon Trust (2015). Turbulence: Best Practices for site conditions for wind turbines. the Tidal Power Industry. Part 2: Data processing and characterisation of turbulent flows. MRCF-TiME- A1.3 IEC KS9b. • International Electrotechnical Commission (2005). • Carbon Trust (2015). Turbulence: Best Practices for International Standard, Wind turbines - Part 1: Design the Tidal Power Industry. Part 3: Turbulence and requirements, Third edition 2005-08, IEC 61400- turbulent effects in turbine and array engineering. 1:2005. MRCF-TiME-KS10. • International Electrotechnical Commission (2009). • Carbon Trust (2016). Offshore Wind Accelerator. International Standard, Wind turbines - Part 3: Design Recommended Practices for Floating LiDAR Sys- requirements for offshore wind turbines, Edition 1.0, tems. Issue 1.0, 25 October 2016. 2009-02, IEC 61400-3:2009. • International Electrotechnical Commission (2015). A1.7 IOGP Technical Specification IEC TS 62600-101:2015. Marine • International Association of Oil and Gas Producers, energy - Wave, tidal and other water current convert- (2011). Health, Safety and Environmental guidelines ers - Part 101: Wave energy resource assessment and for metocean and Arctic surveys, Report No. 477. characterization • International Electrotechnical Commission (2015). A1.8 UK HSE Technical Specification IEC TS 62600-201:2015. Marine • UK Health and Safety Executive (1995). OTH 94 426: energy - Wave, tidal and other water current convert- Energy industry metocean data around the UK. ers - Part 201: Tidal energy resource assessment and • UK Health and Safety Executive (2001). Offshore characterization Technology Report 2001/022: Weather-sensitive off- • International Electrotechnical Commission (2016). shore operations and metocean data. Technical Specification IEC TS 62600-2:2016. • UK Health and Safety Executive (2002). Offshore Marine energy - Wave, tidal and other water current Technology Report 2001/010: Environmental Con- converters - Part 2: Design Requirements for marine siderations. OTR 2001/010. energy systems. • UK Health and Safety Executive (2005). Research • International Electrotechnical Commission (2018). Tech- Report 330: A coordinated approach to metocean nical Specification. Wind energy generation systems - activities on the UK continental shelf. Part 3-2: Design requirements for floating offshore wind • UK Health and Safety Executive (2005). Research turbines, Edition 1.0, 2018-07, IEC TS 61400-3-2:2018. Report 392: Wave Mapping in UK Waters.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 15 APPENDIX 2 - GLOSSARY but offer relatively poor reliability. The reliability of the A2.1 General forecast is dependent on the forecast horizon and syn- Adverse weather downtime optic situation. Periods when offshore operations must be suspended due to adverse weather conditions, due to the exceed- Hindcast ance of operational limits. Numerical modelling of a historical period. Hindcast techniques allow a numerical wave, current or wind Analyses model to be verified against available observations, Statistical analysis of selected datasets to produce thereby improving accuracy and reliability and allowing information or products, for example, design criteria or the production of long-term time series of data. weather downtime. In-situ measurement Data Measurement or observation of real metocean condi- Individual measurements or model outputs at discrete tions at a specific location. timesteps, for example, hourly. Loads Dataset (or time series) The forces, deformations, or accelerations applied to a A record of parameters at discrete timesteps for a given structure or its components, resulting in stresses, defor- time period, for example, 2 years of observational wave mations, and displacements in structures. Excess load or data or 40 years of hindcast wave data. This time series overloading may cause structural failure. of data is the basis for all statistical analyses. Metadata Design criteria Data or information that provides information about A statement of the worst-case (or extreme) metocean other data. Metadata may be descriptive, structural, conditions that a structure or vessel must be designed administrative, reference and/or statistical. to withstand, including information on the cyclic loading which drives fatigue damage to the structure. Metocean A technical engineering discipline that addresses mete- Extremes orological and physical oceanographic matters, origi- The metocean conditions associated with specific nating from the oil and gas sector in the late 1970’s and low probabilities of exceedance in any year. Usually equally applicable to the offshore renewables sector. expressed in terms of return periods and derived from statistical extreme value analysis, often taking into Metocean processes account the interdependence between variables. All-encompassing term for the physical processes gov- erning the metocean regime that vary from region to Fatigue region, such as tides, ocean circulation or atmospheric The progressive and localized structural damage that phenomena. occurs when a material is subjected to cyclic loading. i.e. the weakening of material that occurs when a material is Mid-latitude storm subjected to repeated loading and unloading. Synoptic scale low pressure weather systems occurring at mid-latitudes. Forecast Prediction of future metocean conditions. Determinis- Numerical modelling tic forecasts may be model driven (direct output from Computer simulation of the known dynamics of the real a numerical model) or forecaster driven (output from a world. numerical model informed by skilled forecasters using a variety of guidance data such as in-situ or satellite Operational limits measurements). Ensemble forecasts utilize a range of The metocean conditions up to which an activity can be initial conditions or physical perturbations in order to performed, for example, significant wave height < 1.5m. generate a number of possible outcomes, permitting a probabilistic forecast to be generated. Due to the cha- Return period otic nature of atmospheric and oceanographic condi- The duration (in years) within which an extreme value is tions, forecasts are generally constrained to a 7-10 day expected to be equalled or exceeded once, for example, period ahead. Seasonal forecasts are sometimes used once in 1, 10, 50 or 100 years.

16 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES Satellite Observation A2.3 Wave Data derived from satellite missions supporting Bi-modal (or multi-modal) sea metocean sensors including altimetry (water level, wind A Sea State in which the Wave Spectrum is comprised speed and wave height), Synthetic Aperture Radar of two (or more) independent wave systems. These imagery (wave periods), visible and infrared imagery are relatively common and of interest for engineering (sea surface temperature, ocean colour, etc), microwave and operations. Work in multi-modal seas can require radiometers (salinity) and reflectometers (wave/wind). separate estimates of wave parameters for each com- ponent. Scour The removal of sediment such as sand and silt from Directional spreading around an object (resulting in a local depression), due A measure of the breadth of the wave spectrum with to an increase of flow velocity around that object. direction.

Solitons Frequency spreading Nonlinear wave motions in the water column causing A measure of the breadth of the wave spectrum with potentially high impact short duration current events. frequency.

Spatial resolution Maximum individual wave height The spacing of model grid points or observations, for The highest individual wave crest to trough excursion in example, 1m, 100m, 3km or 1 degree. a wave sample.

Squall Mean zero-crossing period A sudden sharp increase in wind speed resulting from The average period between successive (up or down) localised events such as thunderstorms. crossings of waves in a sea state.

Temporal resolution Peak period The frequency of measurements or model output in time, The period corresponding to the frequency where the for example, 1 second, 1 minute, 10 minutes or 1 hour. spectral density (see below) reaches its maximum.

Turbulence Sea (or wind-sea) Disturbed, semi-random motion of air or water particles, Component of the sea state related to locally generated occurring where flow interacts with a structure. wind.

Weather window Sea state The minimum time period within an operational limit Condition of the sea during a period of time in which its required to perform a given installation or maintenance statistics remain approximately stationary. In a statistical activity, for example, significant wave height <1.5m for a sense the sea state does not change markedly within the weather window of 6 hours. period. The time period during which this condition exists is often assumed to be 3 hours, although it depends on A2.2 Wind the particular weather situation at any given time. Wind speed and direction The sustained wind speed over a given period at a given Significant wave height height above the sea surface (typically 10 metres) and A measure of wave energy, defined either as the mean associated direction, usually expressed as follows: wave height of the third largest crest to trough wave

• 1 hour mean – mean wind speed measured over 1 hour heights (H1/3) or as a spectral measure (Hm0). • 10 minute mean – mean wind speed measured over 10 minutes Swell • 1 minute mean – mean wind speed measured over 1 Component of the sea state in which waves generated minute by wind remote from the site have travelled to the site, • 3 second gust – maximum three second average rather than being locally generated. wind speed measured within the associated mean wind speed averaging period (e.g. the 10min average Wave crest elevation wind speed was 7.5ms-1 with an associated 3s gust of The elevation of an individual wave crest above the still 8.2ms-1). water level.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 17 Wave direction effects of tides and storm surge to the water depth, but The direction from which a wave propagates. excluding variations due to waves. It can be above or below mean sea level. Wave spectrum The wave spectrum or spectral density describes how Surge the variance of the sea surface elevation is distributed The change in Still Water Level (either positive or nega- over frequency and propagation direction, from which tive) that is due to meteorological (rather than tidal) all wave parameters may be estimated. forcing.

A2.4 Current Tidal levels Residual current A range of different levels related to the various daily, Residual currents are caused by a variety of physical monthly, annual and decadal astronomical cycles. Semi- mechanisms and comprise a large range of natural diurnal tides are described as: frequencies and magnitudes in different parts of the • HAT – the highest level of tide which occurs once world. The residual current is the part of the total every 18.6 years current that is not constituted from harmonic tidal • MHWS – mean high water springs components. • MHWN – mean high water neaps • MSL – mean sea level Tidal current • MLWN – mean low water neaps Tidal current is the part of the total current related to • MLWS – mean low water springs the astronomical forcing alone. As a result, tidal currents • LAT – the lowest level of tide which occurs once are predictable, with accuracy increased by analysis of every 18.6 years the total current against increased numbers of harmonic constituents. Sites where diurnal tides dominate may alternatively use: • MHHW – mean high high water Total current • MLHW – mean low high water The combination of tidal and residual currents makes up • MHLW – mean high low water the total current which is the resulting (primarily) hori- • MLLW – mean low low water zontal flow of water experienced. Note that some regions also experience seasonal varia- tions in water level. A2.5 Water Levels Amphidromic point Tidal range A location where the tidal range is zero. Either the vertical difference between a high tide and the succeeding low tide, or HAT and LAT. Chart datum (CD) The level of water that charted depths displayed on a Water depth nautical chart are measured from. A chart datum is gen- In offshore engineering, taken to be the vertical distance erally a tidal datum, typically mean sea level or lowest between the sea floor and a designated datum, typically astronomical tide. LAT or MSL

Extreme water level (EWL) A2.6 Units and Convention An engineering abstraction calculated by combining • Units are expressed using the Standard Interna- extreme wave crest elevation, tide and surge. EWL is tional (SI) convention: important in determining interface levels above LAT or - Distance (wave height, surface elevation and MSL for offshore structures. Other factors such as water water depth) in metres depth uncertainty and seabed subsidence should be - Time (wave periods) in seconds considered when determining interface heights. - Velocity (wind or current speed) in metres per second Ordnance Datum (OD) • Wave and wind direction is expressed as ‘coming A vertical datum used by an ordnance survey as the from’ in nautical degrees, i.e. degrees relative to true basis for deriving altitudes on maps. north positive clockwise. • Current direction is expressed as ‘going to’ in nauti- Still water level (SWL) cal degrees, i.e. degrees relative to true north posi- An engineering abstraction calculated by adding the tive clockwise.

18 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES APPENDIX 3 – IN-SITU DATA COLLECTION

The start of any metocean data collection is deter- to be engineered, and the potential risk of failure to mined on a project-specific basis, but will generally be evaluated. be initiated at the beginning of the development and • New techniques using machine learning will enable consent phase, with consultation between the project better understanding of the structural integrity and teams. When making a decision with regards to the scour behaviour, which may allow reduced inspec- commencement of a measurement campaign, strong tion programmes. Such techniques will require consideration should be given to the entire project training data sets to be effective, and also to allow programme, taking into account the desire for long- exploration of the techniques. term on-site measurements. • Use of a weather forecast service to assist with deployment and recovery phases. A3.1 Scope of Works A long-term metocean measurement campaign is The metocean measurement campaign should com- encouraged, covering the development and design prise, as a minimum, measurement of wave, water level phase of the project, and maintained throughout con- and current conditions, unless long-term in-situ data are struction and O+M phases in order to support forecast- already available at the site. ing with real-time data. It is usually cost-effective to simultaneously collect The following aspects should be considered with respect additional environmental monitoring data, including but to a data collection programme: not limited to: • Data collection must be initiated early enough to • Seabed samples, water samples and a time series collect sufficient data (at least 1-2 years for devel- of suspended sediment concentrations to support opment needs, preferably longer to ensure valida- impact studies. tion of design criteria, depending on availability of • Current profiling transects using a vessel mounted other data). ADCP, particularly where sandbanks and mobile • Benefits of real-time access to data for monitoring sandwaves are present and may result in complex and quality control purposes. current fields and spatial variation across the site. • Potential for collaborative data collection efforts, • High resolution bathymetric profiling across the for example, in conjunction with resource, using site, particularly in regions on complex seafloor multi-use measurement platforms such as floating topography. laboratories. • Data quality control frequency, e.g. monthly or It may be prescient for a Developer to buy and maintain quarterly or as and when data becomes available. their own measurement equipment for use through- • Data archiving options, i.e. in-house or with a con- out the project lifecycle, which can be relocated around tractor. site once the project moves into construction and O+M • Continuation of data collection into the operational phases, in order to obtain real-time observations for fore- phase of any development to provide: information casting and work planning. The cost of maintaining this for maintenance activities, to improve and/or verify has to be weighed against the cost of a comprehensive forecasts and future statistical analyses, and to sup- service, and the reliability gained from using an experi- port maintenance activities. enced contractor. Alternatively, equipment can be hired • Site specific data available to forecast services will which may provide better value for short term campaigns. likely improve and enable the verification of fore- casts, which allows for inclusion of performance cri- The number of locations where measurements should teria to maximise commercial benefit. be made will depend on the size of the site in question, • Extension of data collection into the operational the complexity of the oceanography in the region, and phase will provide valuable data for understand- the existence and availability of prior metocean meas- ing structural performance. The fatigue life of urement data. It is likely that potential contractors will the structures can be verified using the actual be able to provide recommendations as to the optimum loading rather than the loading derived from campaign design, therefore the Developer is encour- hindcast data. aged to discuss in depth with potential contractors their • Detailed knowledge of the metocean conditions requirements and eventual intentions for the use of driving scour events will help mitigation measures observed data.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 19 A3.2 Schedule technology failure, so it is important to ensure that It is pertinent to consider the scheduling of a metocean any contract incorporates risk management and emer- data collection campaign in order to ensure the best gency response in order to rectify issues and minimise opportunity for gathering information on extreme loss of data. events, however, deployment of metocean equipment mid-winter is ill-advised. Developers should initiate A3.4 Data Specification procurement of contracted services in good time to It is recommended that raw, as well as processed data, ensure deployment by, for example, the end of sum- be obtained from the contractor. A metocean measure- mer for 1-year campaigns, taking into consideration the ment campaign will generally result in large amounts likelihood of vessel schedules having been set well in of raw data. It is necessary to plan in advance how and advance. Lead-times for the contracting and deploy- where data will be stored and archived, how it will be ment of metocean equipment will vary from company accessed, and to ensure adequate quality control meth- to company, and Developers should initiate contract- odologies. Refer to Section 4.3 for further information. ing in a timely manner to allow for the full tendering process, acquisition of necessary licenses and future A3.5 Health, Safety and Environment (HSE) availability of potential contractors. During the planning of a metocean measurement campaign, a detailed, site-specific method statement The possibility for alignment with other measurement should be developed and a risk assessment performed. campaigns (for example, those requiring installation of It is recommended that the relevant HSE guidelines be meteorological masts or fixed or floating LiDAR) within consulted (see Section 2.5 and Appendix 1), including the project should also be considered. Where possible, the Developer’s Employer’s Requirements and local it is recommended that metocean data is collected procedural documentation (of particular relevance on alongside other measurement campaigns, in order Construction and Operational sites). Personnel to be that the full dynamics of a site can be captured con- used in offshore operations must possess the correct currently. However, factors such as lead time and cost experience and certification (for example, offshore sur- of concurrent deployment must also be allowed for. vival training and medical). Vessels and other equip- ment (such as cranes) should be third party certified to A3.3 Data and Equipment Loss ensure their suitability for the task and compliance with Consideration must be given to the level of data loss requirements. deemed acceptable by the developer. Data loss can be mitigated by real-time transmission of data, redundancy Given the nature of metocean measurements, of equipment in the measurement strategy, and servic- weather is a key variable which will affect data col- ing of deployed equipment at appropriate intervals. lection procedures. It is important to have awareness of the weather windows required for safe operations. It should be noted that the deployment of metocean In general, it is recommended that, when possible, equipment may require licensing from relevant national moorings are deployed at neap slack water and in low bodies and is likely to include, for example, the issue of wind conditions. Monitoring of the weather forecast a Notice to Mariners. In some locations, it may be neces- will be required in order to identify a suitable win- sary to request an addition to navigational charts and, for dow in which to conduct deployments or retrieval of busy shipping areas, the use of higher grade guard buoys equipment, with the vessel master having the final should be considered. Additionally, in areas of intense say in commencement of the works from a vessel fishing activity, it may be beneficial to contract a local handling and deck safety perspective. Fisheries Liaison Organisation to assist in the engage- ment of the industry and distribution of information. A3.6 Measurement Campaign Specification Sheet When designing a metocean measurement campaign, Assessing the risk to third parties of installed equip- a number of elements need to be defined within the ment is extremely important. In addition, assessing the Scope of Works. In order to aid in the definition of an risk to the equipment from shipping and other activities appropriate campaign and assist in deliberations with such as trawling, is key to the execution of a successful potential contractors, a Measurement Campaign Spec- measurement campaign. Analysis of shipping risk (for ification Sheet is provided in Figure 7. A completed example, using available AIS data), and the presence example for the design of a current measurement of unexploded ordnance (UXO) should be undertaken. campaign is provided in Figure 8. The sheet should be Metocean measurement campaigns often experience modified to reflect the specific project requirements in issues such as collisions with vessels and mooring or any given case.

20 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES MEASUREMENT CAMPAIGN SPECIFICATION SHEET: [PROJECT NAME] DATA COLLECTION TYPE: [DATA TYPE] REVISION NUMBER DATE APPROVED

HSE Plan A full HSE plan is required for all offshore works

Parameters required Define parameters required

Intended use of data Define onward use of data (design criteria, operational support, structural integrity)

Deployment duration Define how many months data should be collected and start date

Device type and number required Define preferred device of known, and number of locations

Locations Define locations for deployment

Sampling depths Define levels (relative to seabed OR surface) that data is required.

Sampling interval Define sampling frequency

Data specification Define the specification to be used

Servicing interval If required define servicing interval

Final report Define report requirements in detail

Interim reports Is interim reporting required? When?

Deployment reports Are deployment reports required?

Emergency response reports Are emergency response reports required?

Data Delivery: Real Time Yes/no? If yes then define where require.

Data Delivery: Raw and processed Raw and/or processed data archive archive required and spec

Figure 7: Measurement Campaign Specification Sheet – blank

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 21 MEASUREMENT CAMPAIGN SPECIFICATION SHEET: OFFSHORE WIND FARM DATA COLLECTION TYPE: CURRENTS REVISION NUMBER DATE APPROVED

HSE Plan A full HSE plan is required for all Detailed HSE plan to be provided offshore works ahead of each offshore activity.

Parameters required Define parameters required Depth averaged current speed and direction

Intended use of data Define onward use of data (design criteria, Construction support operational support, structural integrity)

Deployment duration Define how many months data should 2 years starting March 2015 be collected and start date

Device type and number required Define preferred device of known, and Bottom mounted ADCP x1 number of locations

Locations Define locations for deployment 52.5N, 3.2E (WGS84)

Sampling depths Define levels (relative to seabed OR Entire water column at 1m intervals surface) that data is required. with first data 2m from surface

Sampling interval Define sampling frequency Data to be collected every 20 minutes

Data specification Define the specification to be used Company standard as defined in Appendix X*

Servicing interval If required define servicing interval Equipment to be serviced at least once every six months. Contractor to define required servicing interval

Final report Define report requirements in detail Full report to company standard as defined in Appendix Y* required within 4 weeks of equipment demobilisation.

Interim reports Is interim reporting required? When? Interim data reports to be supplied within 1 month of service visits.

Deployment reports Are deployment reports required? Deployment reports detailing final location, time of deployment and vessel schedule, conditions at time of deployment.

Emergency response reports Are emergency response reports In the event of an emergency, such required? as missing equipment, reporting detailing all aspects of recovery and redeployment.

Data Delivery: Real Time Yes/no? If yes then define where Real time data to be displayed on require. WWW within 15 minute of collection. Data also required via direct radio link on Vessel “Big Crane”

Data Delivery: Raw and processed Raw and/or processed data archive Archived data should be delivered in archive required and spec .ascii format.

* Refers to potential appendices included in measurement Figure 8: Measurement Campaign Specification Sheet – Currents example campaign Scope of Work or similar

22 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES APPENDIX 4 – MODELLING A4.2 Methodology The data collection campaign often provides data at a When reviewing proposed methodologies, attention limited number of points, for a limited duration. In order should be given to: to extrapolate these results to a wider area, for a dura- tion long enough to be used in design (preferably 20-40 1. Accuracy. The more accurate the model, the more years), or for a scenario different from the present con- costly. For example, in the very early stages of develop- ditions (i.e. with additional structures present in the ment, it may be sufficient to have a qualitative model. water), a modelling exercise is required. 2. Numerical model(s). Modelling of the key parame- Wind, wave, current, water level and sediment condi- ters usually requires the use of dedicated wave, wind tions are generally modelled using dedicated numeri- and hydrodynamic models. It is necessary to ensure cal hindcast models. Because these parameters are so the model(s) appropriately represent the physics of critical to the design of the project, it is cost efficient to the site. In order to capture the complexities of the invest in a comprehensive modelling study. Such stud- region, coupled modelling may be required. For EIA ies can be carried out internally or externally; however purposes, it is important to ascertain that the model any modelling that also feeds into the Environmental would be accepted by the regulator. The choice of Impact Assessment may be better accepted if carried numerical model may be dictated by the stage of out externally, by a consultant well known to the regula- the project lifecycle. For example, during site selec- tor. In some countries, the choice of a local consultant tion, depth-averaged models are sufficient for tidal may also contribute to the acceptability of the study by stream site selection, however during the design the regulator/stakeholders. phase a 3-D model is necessary for the vertical pro- file of the site to be captured (IEC 62600-201 TS). A4.1 Inputs Figure 9 summarises the inputs required for the model- 3. Calibration. Model calibration should be undertaken ling of wave, wind, current, water level and sediment. to ensure an accurate outcome. Typically, this is The resolution of bathymetry data should be consistent done by running the model(s) under various setups with the model resolution, which should be chosen to for short periods and comparing their results with appropriately represent the physical processes to be observations, in order to tune the model setup for the modelled. location before initiating the full model run. Note,

WAVE WIND CURRENT WATER LEVEL SEDIMENT USUAL PROVIDER

Bathymetry data Client

Boundary conditions Contractor

Sediment Client distribution

Atmospheric forcing Contractor

Calibration data Client

Physics modules Contractor

Figure 9: Inputs required by a modelling study.

METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES 23 however, that in some instances, models may be models and used for preliminary design; however, inadvertently calibrated against data that has been the models should be thoroughly calibrated and assimilated. validated against in-situ site measurements prior to detailed design or resource assessment. 4. Validation. The model results should be extensively validated and the validation exercise documented 2. An estimate of the impact of the project on wind, in detail within the final deliverable. Typical presen- wave, current and sediment regimes during con- tations include plots of modelled versus measured struction and operations. If the impacts on these data alongside statistical comparative measures, parameters can be argued to be negligible based for example correlation coefficient, root-mean- upon present knowledge and theoretical considera- square-error and scatter index. Again, it is important tions, they may not need to be modelled. to ensure models are not inadvertently validated against data that has been assimilated. Traditionally, time series at a number of points are pro- vided as part of the deliverables, along with analyses 5. Resolution. The model resolution, both spatial and based on these time series (see Section 5), and a cali- temporal, should be sufficient to appropriately repre- bration and validation report demonstrating the accuracy sent the physics of the site. The higher the resolution, of the data. However, the data may be available to the the more expensive it is to run the model, therefore modellers at all points in the model domain, in which case unnecessarily fine resolution would not be cost effec- it may be cost-efficient to request the complete data set. tive. Many modelling consultancies are able to provide the full dataset in the form of a database, accessed online or via a A4.3 Deliverables hard drive. The delivery method should consider the digi- Two key deliverables are expected from the modelling: talisation path being adopted by the Developer, and how the data are made available to those using it directly and 1. A comprehensive, long-term data set of wind, wave, via software packages. Many cloud providers now offer current and water level. Early in the development solutions that readily deal directly with numerical model of the project, these may be based on uncalibrated outputs such as NetCDF files or GRIB files. Photo credit: Marine Power Systems (MPS) Systems Marine Power credit: Photo

24 METOCEAN PROCEDURES GUIDE FOR OFFSHORE RENEWABLES The Institute of Marine Engineering, Science and Technology 1 Birdcage Walk, London, SW1H 9JJ, UK t +44 (0)20 7382 2600 www.imarest.org

Photo credit: Vattenfall