An interview with LR’s Will Hodshon and Nigel Carey

Editor’s Note: The offshore wind market is beginning to show signs of going global.  In spite of the COVID-19 pandemic, the industry remains active and is developing in a number of regions outside of its biggest market -- Northern Europe.  But offshore wind cables are still a cause for concern.  We have all heard the statistic that 80% of the insurance claims for offshore wind losses come from cable problems, but new cable issues are still presenting themselves. 

SubCableWorld recently had the pleasure to speak with Lloyd’s Register’s (LR) Will Hodshon and Nigel Carey about a developing issue that is just being realized – the unburying of cable due to sediment mobility.  Their comments are below.

Will: Sediment mobility is very topical in the offshore wind cable industry right now.  It’s an issue that is being focused on in the United Kingdom as it’s something that we need to understand in order to find out why some cables are becoming unburied.  There are guidelines for lots of different aspects of installing cables, but one thing that there isn’t is a guideline for sediment mobility. The beauty of technology and innovation is that remote presence and remote engineering principles can be applied to address and monitor this issue, anytime, anywhere.

The issue is how to cost effectively monitor sediment mobility through the whole year to properly understand the seasonal sediment transport system in an area.  It’s only with a more detailed understanding that we may be able to determine exactly how deep to install a cable.  It really is an interpretive / subjective process – if there are strong currents, granular sediments and bedforms present there is probably significant sediment movement and try to bury the cable deeper. 

What’s happening now is that everything is being done to account for sediment mobility, but cables are still being exposed.  It’s evident that there are no real guidelines for people to follow and interestingly not any physical survey or geotechnical techniques to actually measure sediment stability / movement with depth in the soil profile, which is fundamental to calculating how deep the cable needs to be buried. 

Currently, the technique that uses high-resolution bathymetry survey is used where you can measure bedforms.  This is repeated the next year so you can match the ridges of the bedforms from one year to the next and effectively spot the difference both vertically and laterally.  Normally you can recognize a lot of the larger bedforms again and you can map the ridges and see if those forms have moved at all.  This then shows if sediment is migrating that way or this way with an increase or decrease in aspects such as height. 

But what’s coming out is an understanding that often these surveys are done at the same time of the year, usually in the summer.  A bedform is mapped in June of one year and then in June or July of the next year and it appears that there is not much sediment movement; that things are pretty much in the same place.  The best method is is to do the mapping at very different times of the year, such as one in the middle of winter and one in the middle of summer and compare those two over the period.  You can map the spring and neap tides and perhaps monitor at low tide and high tide.  From conducted studies you can see that the bedforms are changing between the tide cycles.  When you average it all out there’s actually quite a lot of movement in there. 

In many cases sediment is being redeposited as fast as it’s winnowing away.  There are huge volumes of sediment completely replacing itself but the sediment transportation system is set up so perfectly that  bedforms are being removed at the same speed as they’re being replaced and retaining the same shape, so if you were to map it over time it looks like there’s no change.

In the surveys alone, you wouldn’t see that.  It is something that is quite difficult to monitor. Currently, there isn’t a technique where you can look at the quantity of sand grains moving across and site or route which brings us to the next issue – what is the actual depth of the sediment mobility.

This is something that has become really important.  The depth of the mobility varies, It’s only through the exposure of cables that were buried at significant depth that we know that it’s happening.  It’s key to excavate through a bedform and bury cables below the trough level of the bedform, however that’s seemingly not enough and cables are still being exposed. 

Continuously monitoring sediment mobility within the soil profile offshore provides the solution; in the absences of this technology, we need to try to stagger surveys more and acquire high resolution bathymetry data in a range of scenarios like: at different times of the year, at neaps and springs, during dry and flood periods near shore, after long calm periods and after storms offshore.  I would say at the moment this would be one of the better methods we’d have of understanding the sediment mobility variation of a site or route with the current technology.

Nigel: Recently, a body of professionals in the UK have gotten together under the Society of Underwater Technology (SUT) and started a committee to study this problem.  There needs to be someone doing this; to create a community and to create guidelines to come up with ideas on how we can deal with this because these cables are getting exposed and we need to understand why. 

Export cables are very restricted in the landfall locations and the route they can take.  You can only re-route them slightly before it becomes a major issue.  That’s why there’s a lot more focus on the ground conditions of where they’re coming in, a lot more focus on things like UXOs and boulders and an understanding on how the cables can be protected for their lifecycle. 

I work with an offshoot of SUT called Offshore Site Investigation and Geotechnics (OSIG), which is voluntary.  We’ve just completed a paper on guidelines for the renewable industry in general that also covers cable routing. 

This is not just an issue in the United Kingdom.  If you’re in an area with a temperate climate and you have strong currents and granular soils, you’ll probably need to deal with the issue of sediment mobility and how cables can become unburied.  In the United States, you especially see these conditions as you move down the east coast.  Groups in the U.S. and other countries that are looking at guidelines for cable laying will have to consider these conditions in light of what we’re learning about sediment mobility.  Sharing information will help everyone to understand this issue. 

Lloyd’s Register supports some of the most ambitious clean energy projects around the globe, every day. From wind power generation, to grid connection, to carbon capture storage – LR provides clients with independent perspectives, deep domain expertise and engineering excellence along the full project cycle, ensuring project risks are reduced and asset performance is optimised.