Extending Life in Offshore Wind

Earlier this year, trade body Wind Europe said it expects more than 20 GW of onshore wind farms will be repowered in the next ten years. But what of offshore wind?

With the first offshore wind farms starting to reach their initial design life, and some already being decommissioned (the world’s first offshore wind farm, Vindeby, was commissioned in 1991 and decommissioned in 2017), it’s a topic that is being looked at.

In the UK, the first commercial wind farm was North Hoyle, commissioned off the North Wales coast in 2003. It consists of 30, 2MW Vestas turbines on monopile foundations in about 12m water, about 8km from shore. Others soon followed, including Scroby Sands, commissioned in 2004 and consisting of 30, V80 turbines totalling 60MW.

“When they were being designed, the standard design life was 20 years,” says Huw Traylor, Principal Consultant and Business Manager, Offshore Technology Department, DNV. “More recently (for new wind farms), that’s changed to 25 and we’re seeing 30 years as a standard now,” he says, reflecting licensing regimes, such as the UK’s Round 4, which now offer 60-year licenses, up from 50, allowing for two full project life cycles (based on 30-year life span).

Asset life extension

However, as well as building new wind farms to last longer, operators are looking at how to squeeze more life out of their existing farms as they near the end of their 20-year life. Scroby, for example, was recently the subject of a DNV study to assess how much longer RWE could continue to squeeze energy out of it, beyond its 20-year expected life.

Through an assessment, looking at the design assumptions, such as soil stiffness, which impacts the frequency of the structure and fatigue cycles, and then actual data, actual frequency, to calculate loading from wind and waves, etc., as well as corrosion survey data, condition reports, and SCADA data, an additional five years of life was added to the farm. That’s 25% extra time to keep earning cash.

DNV makes these assessments using its Bladed software, a simulation tool used to optimize turbines at every phase of its design, in this case using a time history analysis, with an aero-elastic model developed by DNV.

It’s worthwhile work, says Traylor. It’s harder to predict conditions offshore, compared with onshore, so designs can be conservative, which means they can have more life in them, he says. As-built conditions might be better than assumed; the operating conditions might be different. At Scroby, for example, the turbines had been well designed, the ground conditions were generally better than assumed, and it was found that updated ambient turbulence was found to be lower than the original design calculations.

What about repowering

It can also be a more realistic option than repowering, he suggests. But asset owners are still mulling their options, says Traylor. “We were recently asked to look at what the best options are for an asset; sweat it until the operating costs of enhanced maintenance exceed the cost of keeping it running or take a refurbishment strategy for the blades and certain other parts to extend the lifetime. It’s all about balancing the levelized cost of energy (LCOE).”

Repowering does have challenges, he says. For one, today’s turbines are much, much larger than they were 20 years ago.

Indeed, repowering hasn’t been tackled to any significant degree yet in offshore wind. Five 550 kW WindWorld turbines, installed in 1998, off the coast of the island of Gotland, Sweden were re-commissioned in 2018, after undergoing an extensive technological and mechanical upgrade by Momentum Gruppen. The project included the replacement of nacelles, blades, and control systems using newly refurbished parts from five Vestas V47-600 kW. The towers, the foundations, and the subsea cables all passed an extensive durability test. The result was that the turbine’s lifetime was extended by 15 years, and the expected yearly output was doubled from ca. 5,000 MWh to ca. 11,000 MWh.

Wind Europe has also pointed to the Windplan Groen project in the Netherlands, where it says 98 turbines totaling 168 MW capacity are being replaced by 90 more powerful turbines with a total 500MW capacity. It has also noted that Belgium was considering repowering one of its existing offshore wind capacity in order to significantly boost its 2030 target for wind at sea as part of Europe’s efforts to become less dependent on Russian energy imports.

Challenges

But fewer than 10% of end-of-life wind turbines (to date mostly onshore) are repowered, and operators are discouraged by slow and complex permitting procedures and changing legislation, says Wind Europe.

Another challenge is around the size of newer equipment – turbines have grown massively since 2003, from 2MW units to 14MW and now 15MW units today. Monopiles built in the early 2000s – at ❮4m diameter – wouldn’t be able to take these units that today require 15m diameter monopiles, says Traylor. What’s more, the turbines would need different spacing to manage wake effects. That then runs into consenting issues – having to get a new consent for what’s effectively a new design and new tip heights. For the earlier wind farms, such as Scroby, which are close to shore, increasing the tip height might not be so easy as they’re so much more visible.

That’s not stopping people from looking at the idea. In order to use existing foundation structures for repowering, German institute Fraunhofer Institute for Wind Energy Systems is developing a foundation concept which involves the strengthening of existing monopiles to enable offshore wind turbines to be repowered, as part of the InGROW project.

A Dutch project, DecomTools, says repowering existing sites could help achieve ambitious wind energy targets in Europe. Refurbishment would also be better, environmentally, but life extension would be cheaper, it found. It also says the case for repowering offshore wind is complicated, with the harsher environments accelerating wear and tear, and corrosion and erosion of components (blades, foundations etc.).

Due to harsh conditions and high costs, frequent site visits to analyze the structural health are difficult while electrical infrastructure is difficult to change, without bearing high costs, says DecomTools, which aims to develop a sustainable end-of-life approach for offshore wind turbines, concluding in January 2023.

The case for 20MW turbines

This could change in the future, however. In another study, DNV looked at what the maximum of a turbine could get to. “As the size of turbines has got bigger, the LCOE has got lower,” says Traylor. “But the benefit starts to tail off at 20MW, which tends to suggest the increase in size will stop.” At that point, manufacturers could start to optimize what they have got, which would then lend itself to repowering of those systems, he says. But that’s still a long way off.

Until then, asset owners are focusing more on life extension, says Traylor. “Most of the big developers have got an approach or are putting an approach in place. A lot of the big developers are building extra life time into their structures in anticipation of going for longer lives and that’s coming from the owners and lenders who have been asking if they can assume 35-year lives.”

That includes better upfront analysis, such as soil condition assessment, which could give greater certainty around design life. This is being done by the Carbon Trust-led Pile Soil Analysis (PISA) project, which delivered a new design method, displacing the existing methodology which was based on oil and gas platform design methods, where piles are smaller in diameter and longer.

Taking a life management approach

But there are also learnings offshore wind could take from oil and gas, particularly around maintaining offshore structures, tackling corrosion, designing structures for longer life and structural integrity management, says Traylor. “Oil and gas has developed structural integrity management expertise over 30-50 years and this knowledge could be adapted for offshore wind,” he says. That could be more focused inspection and testing and using digital twins to continuously monitor the health of a structure. “We’re right at the start of this approach at the moment in offshore wind,” he says, particularly for assessing the impact on the structure and the life left in it.

A life management approach would need a twin for every location, because each location is different, but also networked, so that load is spread throughout a wind farm, instead of leading turbines bearing the brunt. This is all about software, says John King, Wind Turbine Loads & Control business lead at DNV. “So it’s very easy to change long term and you can bank on there being some improvements over the next 20 years.”

The benefits could be huge if engagement is early, which could even be before it’s known what turbines will be used, says Traylor. Indeed, for some new build developments, owners are increasingly looking at how long they can manage their long-term investments for. Investors, especially, are interested in how long their investments could keep paying out – even before they invest and wind farms are built.