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Addressing the Challenges of Offshore Wind

Introduction

The need to decarbonise the global energy supply requires a renewable energy mix to replace more traditional power generation services. With regard to the UK, offshore wind is an important component of this drive towards renewable energy.

While offshore wind power is more difficult and expensive than land-based engineering, the greater abundance and consistency of offshore wind offsets these difficulties. With a relatively low water depth around the coast of Britain and no significant occurrence of hurricanes, offshore wind provides a good solution for UK renewable energy.

The UK currently leads the world for offshore wind capacity, benefiting from experience of offshore engineering gained from the oil and gas industry.i It is therefore understandable that offshore wind is projected to provide 10% of the UK’s national electricity by 2020.ii

There are also additional market drivers that make offshore wind an attractive proposition, including decreasing costs (including for inspection and maintenance), longer operational lifetimes, and lower investment per MW. Indeed, there has been a 30% reduction in offshore wind costs per MWh in the last two years.iii

However, there are still a number of challenges related to the use of offshore wind.

Offshore Wind Challenges

The design, manufacture and operation of offshore wind assets have their own set of challenges including corrosion, fatigue, erosion, lightning strikes and biofouling. Addressing these challenges and maintaining the operational availability of offshore wind turbines will become increasingly important as the reliance on offshore wind energy grows. Here some of the materials challenges that affect foundations, transition pieces and turbine blades are discussed.

Foundations and Transition Piece Challenges

As offshore wind farms move to increasing water depths and aim to operate with larger turbines, foundation designs have needed to adapt accordingly. This has led to both increased monopile sizes and an increased interest in the use of jacket structures, both of which present new manufacturing challenges. There is a drive for cost reduction through high throughput fabrication and advanced manufacturing methods are required.

The aggressive marine environment means that the monopile foundations are subject to both internal and external corrosion. Internal corrosion can be exacerbated by a limited exchange of trapped seawater, while intermittent electrolytic contact can cause extensive corrosion at the splash and tidal zone. Added to this are concerns surrounding microbial induced corrosion and biofouling. There is also a requirement for high visibility coatings at the transition piece. However, conventional paint systems can suffer from both damage and UV degradation, requiring expensive maintenance.

There are challenges relating to fatigue, including the effect of loading during the initial piling operations and the cyclic loading of the structure from wind and waves. These fatigue difficulties can be intensified by the composition of the seabed and any developing biofouling, which increases hydrodynamic load as well as creating challenges in routine inspection and maintenance.

Turbine Blade Challenges

Significant improvements in efficiency and reductions in cost have been achieved through the use of larger turbine blades, with the next generation of composite blade structures expected to be over 100m in length.iv However, the move to ever larger blades can also create logistical barriers. Manufacturers face challenges with transporting the blades to installation sites and are considering segmented blade designs which can be bonded on-site before final installation. The proposed increased size of turbine blades is limited by weight, meaning that lighter materials such as thermoplastic foams and alternative composites are being considered. Lighter blades allow for easier installation and repair as well as improving performance. However, there are inherent difficulties with composite manufacture, such as the misalignment of fibres and inconsistent resin distribution, which can lead to lowered fatigue strength.

The impact of fatigue on turbine blades is an ongoing challenge, with each blade being subjected to more than 100 million loading cycles over the course of its lifetime.v The cyclic loading of the blades is also worsened by leading edge erosion and ice build-up.

Leading edge erosion is caused by the repeated impact of rain, ice and particulate matter which leads to a loss of aerodynamic efficiency and can compromise the structural integrity of the blades, leading to water ingress and UV damage. Even a small amount of leading edge erosion can result in a ~5% drop in annual energy production.vi

The increased height of turbines and the span of blades both raise the risk of lightning strikes and the cost of repair. Lightning strikes can result in the loss of turbine blades and damage to electrical systems. Although there are existing lightning strike protection systems, failure can still occur due to moisture ingress, the detachment of diverter strips and the erosion of blade surfaces, among other factors.

The Future of Offshore Wind

Although the UK currently leads the world for offshore wind capacity (and plans to double its capacity by 2030), China is expected to lead the world in offshore wind capacity from 2021.vii

The Chinese market has its own particular set of challenges to address related to foundation design for local seabed conditions and corrosion issues caused by the regional environment. China has seen damage to existing offshore structures due to typhoons. With limited windows for conducting repairs, due to the availability of transfer vessels, there is a greater need for decreased inspection and maintenance requirements.

As shallow locations close to shore become full to capacity, fixed bottom turbines are being installed at greater depths and further from shore. This increases the cost of the foundation construction and installation, while the distance from shore can create problems with electrical transmission. Floating wind turbines allow access to even deeper waters where wind speed is typically higher and more consistent.

There is currently one floating wind farm in operation, Hywind off the coast of Scotland which comprises five turbines.viii Another is currently being installed, WindFloat, with three turbines off the coast of Portugal.ix

However, cost is a limiting factor when compared to fixed bottom offshore solutions. A floating offshore wind solution costs approximately £160 per MWh while a fixed bottom offshore farm costs just £40 per MWh.x/xi Currently the largest cost of floating offshore wind is the foundation. In order to reduce the cost and improve the viability of floating wind farms there is a need for a greater understanding of the effect of structural loading on cables and other structures. More research also needs to be undertaken with regard to the ageing of components under cycling loads, while maintenance cost needs to be improved with through new biofouling solutions.

 

What Can TWI Offer?

TWI can assist with the current and future challenges of offshore wind exploitation using our expertise in materials, welding, coatings, modelling, sensors and composites. We have already been instrumental in exploring solutions to many of the challenges faced by the offshore wind industry, such as through the Hiperwind, MIMRee and CROWN project, is addressing corrosion issues through the use of thermally-sprayed aluminium coatings.

Our materials expertise includes resolving issues related to corrosion, fatigue and fracture.

As experts in all aspects of welding and joining, TWI can assist with developing high throughput/low cost manufacturing methods.

TWI can assist with issues including lightning strike protection, leading edge erosion protection, corrosion protection, antifouling and anti-icing.

  • Modelling

TWI’s modelling expertise and structural integrity knowledge can be utilised to ensure your assets remain in service for as long as possible.

Sensors can be used for the remote monitoring of bolts, blades, mooring chains and other key components.

  • Inspection

Our extensive knowledge of non-destructive testing methods continues to be used to monitor offshore wind turbines, providing vital information for future designs.

  • Composites

TWI continues to progress composites research across the business, meaning that we can help with matters related to blade joint design, materials selection and development and composite joining.

Please contact us for more details on how TWI can help with your offshore wind project.

 

References

i  Offshore wind is first renewable technology to agree Sector Deal with Government, https://www.renewableuk.com/news/440922/Offshore-wind-is-first-renewable-technology-to-agree-Sector-Deal-with-Government.htm

ii  OFFSHORE WIND INDUSTRY PROSPECTUS, OCTOBER 2018,

iii  Offshore wind energy price plunges 30 per cent to a new record low, https://www.independent.co.uk/news/business/news/offshore-wind-power-energy-price-falls-record-low-renewables-a9113876.html

iv  https://www.ge.com/reports/extreme-measures-107-meters-worlds-largest-wind-turbine-blade-longer-football-field-heres-looks-like/

v  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5706232/

vi  https://www.sciencedirect.com/science/article/pii/S1364032119305908

vii  https://www.economist.com/britain/2019/09/21/lessons-from-britain-the-worlds-biggest-offshore-wind-market

viii  https://www.equinor.com/en/what-we-do/hywind-where-the-wind-takes-us.html

ix  https://www.euractiv.com/section/energy/news/worlds-second-floating-wind-farm-sets-sail-for-portugal/

x  https://www.independent.co.uk/news/business/news/offshore-wind-power-energy-price-falls-record-low-renewables-a9113876.html

xi  https://windeurope.org/wp-content/uploads/files/policy/position-papers/Floating-offshore-wind-energy-a-policy-blueprint-for-Europe.pdf

 

 

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