How Deep can the Monopile go in Offshore Wind?

By Karl Davis. 

Yesterday I came across an excellent article written by Cameron Dunn from Arup titled “Windfarms: has the death of the monopile been greatly exaggerated?”. It got me thinking about the journey the humble monopile has taken, and more importantly how far do we think it can go…

Seven years ago when we started looking at the Rampion offshore windfarm site, with 20-40m water depth, everyone expected this would need to be a mix of monopiles in the shallow areas, with Jackets in the 35m+ regions. At the time I was working for LICenergy, and we felt pretty good about ourselves managing to make monopiles work across the whole site – that was working with the MHI Vestas 3.45MW and maximum 6.5m diameter piles.

Jump forward to today, where we’ve got 10+MW turbines, and projects in 60m+ water depth. The question we often ponder in the office (maybe we should be running a pool?), is how deep will the monopile go. But its not a simple question to answer. The key limitations can be broken down into these categories: Fabrication, Transport, Installation, Soils, Metocean, Tower and Turbine Specification.

Fabrication

To make your monopile work in increasing water depth, with larger and heavier turbines, we need longer and larger diameter monopiles. Length is not typically a limitation, but maximum diameter can be. We’re now seeing the capability of some fabricators to roll 12m! diameter cans, so we’ve got a little headroom for now.

Transport

How do you transport a 12m diameter, 100m long, 2000Te monopile? Very carefully! But we don’t see this as a limitation.

Installation – Lifting 

Lifting and driving become difficult when you get really big. The existing fleet of jack-up installation vessels start to see limitations above 1000Te monopiles, although we see vessel cranes being upgraded to push these limits, the next step will be using floating (not jacked up) Heavy Lift vessels which is being proposed by some of the installers.

Installation – Driving

This is where it gets tricky! IHC and Menck keeping building bigger hammers, but development and construction of these are long lead items, and there might be some technical size limitations. Also, the risks of pile-tip buckling increase as the D/t ratios go up which they tend to do with larger monopiles.

Soils

This is where things become site specific. To extend the range of the monopile, we want a nice stiff soil to help with increasing the natural frequency. But we don’t want it too stiff to make drivability an issue, which might force us to use Jackets with drilled and grouted piles. 

Metocean

Depth doesn’t really matter if we don’t have any wind or wave loads on the structure. Give me a perfectly calm sea and no wind, and we can probably put a Monopile in 100m water depth. The nuance is in the predominant wave frequency and peak wind velocities. Site specific assessments are critical to understanding this properly.

Turbine Tower

Keeping the structures natural frequency sufficiently high in increasing water depth is the challenge. As towers get taller (to reach the higher wind at altitude) this becomes more difficult. The one tool we have to work with here, is designing stiffer towers – this typically requires increased diameter. Reducing fatigue loads via tuned mass dampers can help too. The key here is for WTG suppliers and foundations designers to work collaboratively and design an integrated structure from pile tip up to the WTG nacelle. Advances continue to be made in this area, although no one is doing this fully just yet!

Turbine Specification

This is where it gets truly interesting, and perhaps a little more complex. WTG manufacturers will specify an allowable frequency range for the combined foundation-tower-turbine structure. This effectively puts a hard limit on how far we can push the monopile in terms of water depth, for a given turbine. The willingness of WTG manufacturers to relax these requirements can have a big impact. The simple mass of the nacelle is also really important, as that has a big impact of the structure’s natural frequency – light nacelles make your life easier. Then comes the sorcery that is WTG control algorithms. Clever controls can widen the frequency band the foundations can be designed for, and even reduce the loads on the foundation. This all comes at a cost to energy production, so there is (again!) a trade-off to be had between maintaining an economical foundation design and extracting maximum energy from the wind. 

The Upshot

So what does this all boil down to? Our engineering intuition (gut-feel!) is that on a good site (favourable soils and metocean), working with a cooperative WTG supplier, putting a 10MW turbine on a monopile in 55m-65m of water depth is possible, provided you can find a way to handle and install a 10-12m diameter monopile.

The real question is, given your specific site, and your choice of turbine, what foundation type will yield the lowest installed cost, at the lowest risk to your project. These are the questions that Empire Engineering specialise in answering, using our concept design and analysis skills and our wealth of experience, to identify the right foundation choice for your site.

To find out more, please get in touch with the team at Empire Engineering.

Article update : July 2020

In the 12 months since posting this article, it has proved by far and away the most popular piece we have published in this series. A year on and there are still hundreds of readers every month. Clearly this is a hot topic that requires more discussion. In July 2020 we are hosted a special webinar as part of our tech sessions series. This is now available to watch in your own time through the link below.