By Andreas Fabricius, Senior Risk Control Consultant, AON Global Risk Consulting, Canada
In this article we look at some of the issues related to the use of monopiles for deep-water (x60m) offshore wind farms. The experience from use of jacket foundations at these water depths can partially be
translated from the Oil and Gas industry. The unique loading pattern is, however, significantly different.
Wind, waves, and current will affect the substructure. The topside load on a platform is static, however
the loading pattern that comes from the interaction of the WTG rotor creates a very different kind of lateral effect on the whole structure, adding complexity and challenges as wind farms move further out into deeper (x60m) waters. Uncertainties in design are difficult to address at this stage with an industry
learning curve for deep-water applications expected. The risk of failures can however be reduced, by an
increased effort to monitor quality of workmanship and fabrication, construction, and installation. This is
in both owner and insurer interest, and focus should be on knowledge transfer in the industry.
Current global trends are moving towards WTGs of 15 MW+ in capacity by 2024, with wind farms beginning to be developed further offshore [1] in increasing water depths. Fixed foundations still have a significant advantage at this time, in terms of much lower LCOE compared to floating and will likely be the foundation of choice until the price for floating reduces to similar levels. Currently, only 16% of the global offshore wind project pipeline is floating wind [2].
Total Insured Values (TIVs) and loss estimations are increasing inline with growing project sizes. A single
loss of a WTG or structure 120km offshore could lead to a property loss of anywhere from 15-50M$ (About $11 million – $37 million), together with a business interruption in the range of $1.6M to $5M, pending actual lead time (6-18 months) and weather and other factors influencing the replacement / repair work. As an example, a loss 10 years ago of 1-2M$ (About $745,000 – $1.5 million), could now be 10-15 times higher, with the same (or potentially higher) loss frequency.
Monopile foundations account for 63% of the worldwide installed capacity. These have typically been
a maximum of 6-7m in diameter and 60m in length. With the planning of wind farms moving from
transitional waters (30-60m) to deep waters (x60m) and with WTGs getting larger (rotor diameter x200m),
the size of these monopiles will significantly increase, and the structural behavior will significantly change
[3]. How will these giant structures respond to the effects of wind, waves and current over time?
With increased diameters and lengths, the risk of monopile buckling both during the installation and
operational phase is currently being studied in a joint industry initiative called VERBATIM [4], with results
expected to be available to the industry by Q1 2023.
Further potential issues are related to local scouring effects of the monopile. The change in flow pattern
around the monopile near the seabed creates a situation where the bed is “worn out” with time, changing
the dynamics of the monopile itself. This can, in a worst case scenario, lead to premature failures of the
structure. Scour protection is part of the design process and has been studied since the 1950s, but its
effectiveness in deep waters for monopiles is yet to be proven.
Given all these uncertainties, the use of monopiles in deep waters could to some extent be viewed as an
unproven technology [5] (e.g., for a new gas turbine model this would be eight thousand hours of service
without any issues). In translation, these deep-water fixed foundations have little real-world experience
for this kind of application and will be met with doubt until they are proven to be stable and able to resist
all sorts of degradation.
Offshore wind claims experiences
Offshore wind related claims are growing in both frequency and severity, which was to some extent
expected as the market grows in both size and complexity.
Contractor errors and equipment defects have been reported by one of the leading insurers, GCube, as
the largest root cause of claims up to date [6], while claims related to subsea cables are most frequent
(30%), followed by gearbox damage (15%) and damage to the transformer or foundation (jacket or
monopile) both at roughly 10%. The subsea cables are responsible for approximately 55% of the claimed
losses while foundation damage accounts for a further 23% [6].
When wind farms are moving from transitional to deep waters, another element of uncertainty will be
added to the equation, namely the structural response over time of the monopile foundations. The load
transferred from the topside structure, in this case a tower and nacelle/rotor, will generate lateral loading
from the WTG movement. This is significantly different from the static the load coming from e.g., an
offshore process platform or a substation.
How will the loading effects from both deep waters (current, waves) and the now potentially very large
(15 MW) turbines play out over time?
Measures to reduce risk
The largest cause of claims between 2010 and 2020 was as previously mentioned related to contractor
errors and equipment defects [6]. In the quest for reduction of LCOE, quality control and assurance could
suffer, leading to potential immediate and long term problems, higher premiums and potentially
increasing number of claims.
One way to address these issues is to apply stricter quality control measures through all stages of the
project (fabrication, construction/installation and operation). Quality control engineers should have full
access to the fabrication site at all times. Spot checks should be completed by the Lenders and Owners
representatives, as well as representatives from the insurance industry. It is in both the insurers, and the
insured’s interest to keep the cost of risk transfer as low as possible. This can only be accomplished with
full technical transparency over the entire project lifecycle.
The majority of claims for the largest reported category; subsea cables, can be attributed to contractor
errors [6]. This is worrying, given the exponential growth of the offshore wind industry and the likely
addition of new and less experienced contractors entering the market. It becomes even more important
for the project owners that the quality of the works if being followed and monitored. Industry claims of
experience and knowledge needs to be translated into development of methodologies for avoiding
recurrent issues.
Real-time monitoring of movement (vibration, displacement) and structural stresses (strain) will become
even more important as the projects move further offshore. Data is needed for maintenance and
inspection planning, as well as to prove to the insurance industry that these kinds of foundations can be
reliable over time.
So, what now?
What can be done to reduce the risk, and how should project owners position themselves to best tackle these challenges? Industry experience has shown that measures need to be taken in order break the current trend and tackle the upcoming deep-water challenges. There are several key actions should be considered:
- Involve insurance carriers or brokers risk engineering in early stages of the project.
- Don’t let overall cost drive all decisions – robust engineering and a proven and reliable track record should be on top of the list.
- In the absence of track record – increase the scope of the Quality Control program starting at the fabrication phase, with involvement of Brokers Risk Engineering/Insurance carriers.
References
[1] H. Diaz and C. G. Soares, “Review of the current status, technology and future trends of offshore
wind farms,” Ocean Engineering, vol. 209, 2020.
[2] National Renewable Energy Laboratory (NREL), “Offshore Wind Market Report: 2022 Edition,” U.S
Department of Energy , 2022.
[3] C. Moore, “Warning that increasingly large offshore wind turbines risk getting too big for their
foundations,” New Civil Engineer, 10 11 2021. [Online]. Available:
https://www.newcivilengineer.com/latest/warning-that-increasingly-large-offshore-wind-turbinesrisk-getting-too-big-for-their-foundations-10-11-2021/. [Accessed 11 10 2022].
[4] Offshore Staff, “Study assessing buckling risks for larger wind turbine monopiles,” Offshore
Magazine , 24 05 2021. [Online]. [Accessed 11 10 2022].
[5] J. Rasaad, “Renewable energy insurers face increased exposure from uproven technology,” 25 03 2019 [Online]. Available:
https://insuranceday.maritimeintelligence.informa.com/ID1125560/Renewable-energy-insurersface-increased-exposure-from-unproven-technology. [Accessed 17 10 2022].
[6] GCube enewable Energy Insurance , “Unchartered Waters, Navigating emerging risks and rising
claims in global offshore wind,” GCube , 2021
