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1. Introduction

1.1 Project Overview

Whilst the offshore wind energy industry is still very young when compared to traditional forms of energy such as oil and gas, the expected end of life for some of the initial wind farms is rapidly approaching. Early round 1 wind farms such as Middelgrunden commissioned in 2001 are planned for decommissioning in 2021. The decisions facing wind farm owner/operators is to extend the lifetime of their assets, repowering their turbines or whether to decommission sites completely.

As assets age, the questions surrounding the assessment of remaining useful life become ever more prominent for the key stakeholders (owners, operators, insurers, legislators to name a few). Life extension is also a topic of interest for the sector to maximise the potential from existing assets. Life extension will be dependent on the availability of robust and reliable methods for the assessment of remaining useful life.

The decision on lifetime extension is complex and experiences to date are limited. We need to learn from the lessons of historical industries to establish standards in monitoring which will help to support these decisions on an individual location basis. Data, both operational and from the design phase for comparison, is the key to providing answers to the decisions facing owners and operators. Gathering this data may be an investment but should provide insights and cost efficiencies for wind farms reaching expected end of life.

1.2 Headlines

• The design operating life of an offshore wind turbine is typically 20-25 years. By 2030, approximately 14 windfarms are approaching this milestone. These accounts for 1.2GW of power currently generated through offshore energy annually.

• Experience from oil and gas industries on the lifecycle calculations for monopile foundations do have some application to offshore wind, but the learning should focus on consistent data collection for ongoing fatigue analysis. Developing a methodology for data collection and analysis for monopiles will demonstrate value in supporting the life extension decision.

• Industry standards for data monitoring of offshore wind turbines should be developed. Recognised standards would ensure consistency in safety levels and quality control across the entire industry.

• Operational data is now readily available for collection and needs comparison analysis against the design data to establish the best parameters for setting those industry level standards.

• Data modelling with the latest technologies can validate the results from the design period. The modelling software and sensor technology currently available has evolved and improved since the initial monopile design. The possibilities of artificial intelligence would allow further development in analysis and modelling.

• The business case for extending the lifetime will always be driven primarily by financial viability as well as the feasibility of structural integrity.

Monopile Remaining Useful Life Assessment Scoping Study SummaryTony Fong | September 2018

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2. Monopile Life Assessment

2.1 Remaining Useful Life

Assessment of Remaining Useful Life (RUL) of a structure is a typical requirement as assets age. For offshore wind foundation structures RUL is typically dictated by fatigue damage. Key drivers for RUL assessment can include:

• Regulatory requirements;

• Certification requirements;

• Insurer requirements;

• Asset integrity management;

• Life extension justification.

This list highlights some key drivers for RUL assessment, however it is far from exhaustive. Currently within offshore wind, the need to assess asset integrity is focused around designer / manufacturer warranty requirements or for operationally reactive reasons such as the occurrence of accidental or extreme events. The proactive assessment of integrity of monopiles for RUL is currently less common, however this anticipated to become a growing focus as offshore wind assets age toward their design lives. A primary reason for this is a current lack of consistent and accepted approach for RUL assessment within the industry, this was also highlighted during ORE Catapult industry engagement in the Offshore Wind Farm Substructure Monitoring and Inspection Report, PN000205-LRT-001.

Fundamentally the evaluation of RUL is a verification of the validity of design assumptions with respect to real site conditions and asset response. Different approaches to RUL assessment exist and are captured in industry standards (such as DN-VGL-ST-0262). These however are high level and do not provide guidance on the methodology of how these approaches can or should be performed. Life assessment can be achieved by practical, analytical or structural monitoring (or a combination of) methods. These are summarised below:

Practical

• Physical assessment of structural integrity through inspection and evaluation of maintainence/operational history

• Calculation and justification of remaining useful life based on practically obtained evidence.

Analytical

• Measurement of turbine response and local site conditions

• Calculation of fatigue loading using generic state of the art load simulation models

• Compare measured site/asset data with original design conditions.

Structural Monitoring

• Measurement of turbine response and local site conditions

• Direct structural monitoring of load history on the structure

• Extrapolate monitoring history across the structure (where no sensors are fitted)

• Compare again design values and evaluate reamining life.

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2.2 Methodology and Data

The importance of data capture and monitoring of assets is immediately apparent as a fundamental requirement for any RUL assessment methodology. The data requirements (data type, frequency and period) will differ for each method of assessment.

No industry standard currently prescribes set exact methodologies for carrying out the evaluation of remaining useful life of an offshore wind monopile foundation, hence there are no standard monitoring or data requirements across the sector. A standard on lifetime extension of wind turbines was published by the DNVGL in 2016 (DNVGL-ST-0262) which prescribes a high level process, however it does not provide detailed strategy for monitoring and inspection.

The analytical methodology for life assessment and extension is foreseeably the most likely process to be applied for the foundation structure as solely practical or monitoring approaches are likely to be unfeasible due to the number of assets and difficulty of direct inspection of portions of the subsea structure. A summary flow diagram of the process for analytical approach is illustrated in Figure 1.

Figure 1: Summary of DNVGL-ST-0262 Analytical Part for Life Extension

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The analytical approach requires environmental and type (generic turbine and foundation) and site (specific turbine and foun-dation) specific analysis to be performed, both of which will require monitoring and analysis of in-service structures in order to evaluate the foundation performance.

Industry working groups are forming to discuss the topic of foundation integrity and possible life extension of assets, howev-er it is unlikely that a detailed standard will be available in the near term to assist the offshore wind industry with making vital key decisions on what data collection and RUL assessment methodologies.

2.3 Industry Feedback

ORE Catapult have performed extensive stakeholder engagement with owner-operators and O&M service providers in rela-tion to monopile integrity and life assessment. A summary of the key findings are presented below.

General Findings

• There are currently no industry standards available for inspection of sub structures/monopiles.

• Inspections are typically occurring at the owner/operators discretion and by local teams.

• Often the inspection and monitoring requirements are driven by experience of the O&M engineering team and are reactive in nature, typically driven by issues encountered (e.g. unplanned water exchange within the monopile or manufacturing concessions).

• Typically the evaluation of monopile life is performed by a mix of analytical approach (checking site conditions against design assumptions) and Structural Monitoring (checking physical structure response against design assumptions).

• It is believed that the life of the monopile will likely extend past the designed life of 20-25 years.

• Improved guidance on inspection and code on remaining useful life assessment is extremely sought after within the industry.

• Industry working groups are currently forming discussions and concepts for generating guidance and standards in the area of RUL with a focus for onshore wind. However these are unlikely to be available in the short/medium term and high level.

Design Standards

• Design standards are being used thoroughly by industry in the design of offshore wind foundations.

• Design standards and data received from instrumented sub structures are being used to determine/calculate the global structural behaviour of monopiles

• If global bending behaviour is acceptable and below the design requirements then the structure will last its lifetime. Possibly even past its design life.

• Conservative design assumptions exist.

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Instrumentation and Monitoring

• Typically only a small portion of sub structures fleet have been instrumented within a windfarm

• Higher levels of monitoring and instrumentation are applied to the transition piece in comparison with the monopile, however the area around the mudline of the monopile is generally more critical.

• Instrumentation and analysis of data are generally carried out by and O&M service provider/contractor.

• There are challenges associated with data acquisition from O&M service providers or contractors especially as the need to carry out in-house analysis rises.

The general finding following engagement with offshore wind owner-operators is that the industry are seeking technical guidance on methodologies for the assessment of monopile life. This includes guidance on monitoring and modelling require-ments which will meet the likely future issue of industry standards surrounding life extensions and structural integrity man-agement.

There are currently no standardised approaches or clear guidance on monopile life assessment, although much of the industry is performing, to varying degree, structural assessments of their assets often in a reactive approach to justify integrity.

The industry is welcoming the creation of standards in this area, however also feel that the generation of a standard will take a long time and that guidance and best practice is required in the near term to ensure that instrumentation and monitoring techniques currently being developed are done so appropriately in order to comply with future requirements.

It was highlighted that the majority of existing assets have limited instrumentation and monitoring, with much of the structur-al integrity justification requiring extrapolation from limited data. Advancements in sensor technology and modelling simula-tions can aid this, however guidance is welcomed on methodologies for the treatments of older assets with limited data when considering life assessment.

2.4 Learning from Offshore Oil and Gas The Oil and Gas industry has been established for a significant number of years and one of the main comparisons to be made with offshore wind is that of the integrity of offshore structures and the fixed production platforms. There are lessons learned which can be transferred to offshore wind however several differences mean that they are not directly applicable; specifically, the cost structures in the industry. Oil and Gas generally have one large asset made up of many component parts, however, within offshore there are many assets within a single farm/plant. Therefore, while extensive monitoring of a single platform was cost efficient for O&G, the investment to monitor all monopiles in a wind farm to the same level of accuracy may not be feasible.

What can be adopted from the O& G industry is the methodical approach used in the assessment of monopile lifespan. The rigor of reviewing the assets against appropriate regulations and standards and assessing the risk, cost effectiveness and calculations of fatigue/strain on the monopile is an approach transferable to the offshore wind industry. With advancing tech-nology and the availability of operational data, the industry should look to create a new data model which can be configured for the specific site, taking into account the uncertainty of the conditions (e.g. marine life, meteorological data) and remodel-ling the design data to review the outcomes.

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2.5 Recommendations

A number of recommendations for further work have been identified following this preliminary industry engagement carried out by ORE Catapult.

• Bring together industry and foster a working approach to RUL in the near term. High level principles are already available for RUL from within the offshore wind industry and other industries. Standards however are unlikely to be available in the near term, yet for effective justification of structural integrity data capturing is anticipated to be necessary. In lieu of existing guidance or standards in this area, ORE Catapult propose the formation of industry collaboration (i.e. in the form of a joint industry project) to identify and capture the current best practice within the industry.

• Ensure industry best practice and future standards development are compatible. With current development of standards working groups to address RUL assessment for onshore wind, industry best practice and development of future standards must be aligned. Any JIP’s developing industry best practice must also ensure harmony with future standards (and vice versa). ORE Catapult propose the facilitation of such groups to ensure an unbiased and transparent exchange of knowledge between best practice development and the generation of standards.

• Support technology development, demonstration and validation. As best practice and standard development progress, the implementation of current and new technologies to support RUL are likely. Monitoring of structures and site conditions will be key in the justification of long term integrity. Supporting the development of technology, demonstration and validation shall be a fundamental requirement to ensure cost effective and robust monitoring is possible. For example the identification and validation of a standardised monitoring framework for RUL justification is anticipated to include measurement and data acquisition technologies which require development and validation.

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