SUBSEA PROTECTION: Protection solutions against scouring

In the previous article we spoke about the importance of accurate wind measurement for an optimum wind farm design. But also underwater there are elements that need to be taken into consideration when designing the foundation and cable structures for each wind turbine. Waves and currents can cause erosion of the seabed around a fixed structure, a phenomena called scour.

Most of the North Sea seabed is made up of sediment of varying sizes, which allows continuous movement of seabed particles caused by waves and currents. This movement changes around fixed structures washing the sand away from one side until a hollow pit is formed on this side of the structure. This does not necessarily have to be too detrimental to the strength of the structure depending on its design; a multi-piled jacket foundation suffers less than a single monopile foundation.

As a consequence a monopile foundation scour should be prevented as much as possible. In contrast to jacket structures, with monopile foundations the load is transferred laterally from the foundation in the upper part of the seabed which is therefore affected by the scour effect. The total structure of the turbine, with a large load concentration at the top, makes this type of foundation more susceptible to fatigue caused by the flexing of the tower at a similar frequency to the wave variations. The effect of this is another factor that is increased by scour. Therefore scour is a major concern when designing offshore wind turbine foundations and should be properly dealt with to minimise the effect. There are, however, also several methods to minimise scour around the foundation itself. We have listed below some of the different methods offered by the industry to prevent scouring at monopile foundations.

Rock dumping

The most common strategy to date to prevent damage caused by scouring is to place stones on the seabed around foundations. In addition protective rock layers are also often placed on array and/or export cables with the same purpose. Boskalis Offshore provides, besides transport, installation and cabling works for offshore wind farms, these specialist rock dumping services.

Before installation of the wind turbine foundations an initial scour protection filter layer can be placed on the seabed at each foundation location. Following the installation of the foundations further erosion protection can be provided by placing larger stones, a so-called armour layer, around the foundations. This will provide optimal protection against erosion from waves and/or current. The rocks are selected so that the increased current around the structure will not be able to wash them away. The scour protection work is mostly executed by DP fallpipe vessels, of which Boskalis owns and operates two. These vessels are able to transport and accurately place large quantities of rock on or around subsea structures at great water depths. The pre-installation filter layer can be placed through the vessel fallpipe.

The same vessel is used to place postinstallation armour layer using the side dump unit which enables the use of larger rock sizes, and also to install the layer close to the monopile foundation.

Rock as protection against scouring is a proven method which has been applied in multiple offshore projects. Once placed, the rock will be able to adapt to changing seabed levels and occurring depressions will be filled in.

As a result, progressive failure will not occur. This is considered to be a major advantage for using rock. Furthermore, the rock will not degrade; it is eco-friendly and has a long lifetime.

Some examples of offshore wind projects where Boskalis executed the scour protection installation:

• West of Duddon Sands Offshore Wind Farm

• Placement of scour protection filter layers at 108 locations before monopile installation. Rock material was shipped from Norway by bulk carrier to Holyhead and discharged into the DP fallpipe vessel directly from the first vessel in a ship to ship transfer.

• The second phase of rock installation was executed when the foundations and turbines were in place. The DP fallpipe vessel installed 8-18″ (20-45cm) armour rock around the 108 turbines via the purpose built rock side dump unit.

• Export cable protection at crossing locations by means of installing filter and armour layers.

• Meerwind Offshore Wind Park

• Pre-installation phase: placement of 1-5″ filter material for 80 monopile locations.

• Post-installation phase: armour rock (grading 100-700kg) was installed at 80 wind turbine locations, using the side dump unit of the DP fallpipe vessel.

• DanTysk Offshore Wind

Post-lay rock installation on three locations where the array cables cross a telecommunication cable. In addition, rock was installed to protect unburied cables at the scour protection near the wind turbine generators.

• Anholt Wind Farm

Installation of scour protection at 42 WTG foundations. Each foundation consisted of a 1-4″ filter layer with an armour layer of 2-8″ rock on top. Change the current flow or change the shape of the foundation at the sea bed or change the material matter of the seabed.

Packed rocks

Rock dumping is not the only way to prevent scouring. Other solutions include rocks in flexible containers, net bags or sacks placed to mould themselves around the base of the turbine tower. The sacks can be filled at low cost onshore with any, preferably eco friendly, heavy material that will withstand corrosion and erosion over the years. After preparation onshore they can be taken offshore in any vessel or pontoon with deck gear available to lower it to or drop it on to the seabed. Maintenance to the containers or sacks may be required.

German NAUE GmbH & Co. KG is an example of a company that developed a scour prevention solution by means of geotextile sand containers. Their Secutex® Soft Rock Sand Container is made of highly robust, staple fibre, specially needle-punched nonwoven Secutex® geotextile and are filled with sand. The container is flexible and can adapt to variable seabeds. They can be placed before piling. Their solution has been used on the German Amrumbank West offshore wind farm.

Belfast-based Ridgeway Marine offers the concept of a filter unit, consisting of a polyester filter bag, made from 61% recycled bottles, and filled with stones, for scour protection. Their filter bags are produced by Japanese Sumisho Kyowa Limited and has been used by MSS Engineering at the EDF Teesside Offshore Wind Farm. The filter unit is a robust bespoke net which has a lifetime of 50 years in saline water and a one point lifting ring for fast and accurate deployment.

However if they do break and spill their contents in time, the material provides almost the same protection as would be afforded by rock dumping, with the exception that the rocks would be smaller and more easily moved by strong currents.

Rubber derivatives

Another method is currently undergoing tests but ‘kills two birds with one stone’. Vast amounts of used truck and car tyres are to be found in dumps on almost all city limits. Usual methods of disposal produce some form of pollution or require expensive machinery. But by connecting them to form chains of mats and laying them on the seabed they are proving to be a long term and stable form of protection at low cost. The tyre materials have the added advantage that they do not degrade easily and therefore do not pollute the sea.

Ironically the Continental Group, a company making new truck and car tyres, has an alternative method developed from a product that requires hardly any modification. A group member company, the ContiTech Conveyer Belt Group, has tested mats made up of a conveyer belt type material, 12mm to 15mm thick, at the Deltares Institute in the Netherlands with simulated extreme storms and streams. The anchored mats with a diameter of up to 52m have proven to keep the seabed under the mats intact to provide a stable long term solution which, with styled surfaces to promote sand deposit, encourages new life forms to establish.

Dick Hill