CONDITION MONITORING TRIPODS
INSPECTION, REPAIR & MAINTENANCE FOR OFFSHORE WIND FARMS
Most offshore wind farms have a design life lasting 20-25 years. Whilst ongoing maintenance is important, one-off repairs due to damage will become a reality as structures begin to age.
The three most common foundation types in wind farms installed to date are monopiles, jackets, and tripods.
MONOPILES
Monopiles consist of a hollow steel pile driven into the seabed. Because the verticality of the pile cannot be guaranteed, a transition piece is placed over the steel pile and provides the levelling mechanism for the turbine and its tower. Grout is injected into the annulus between the pile and transition piece to form the connection. Monopiles have been used at wind farms such as Gwynt y Môr and West of Duddon Sands.
JACKET FOUNDATIONS Jacket foundations, similar to the lattice towers used in offshore oil and gas projects, are increasingly being used as turbines exceed 5 MW, and as wind farms move further offshore into deeper water. Jacket-technology has been deployed for many decades and in extreme water depths, lending itself flawlessly to offshore wind as the installation process becomes more industrialised, as demonstrated during the successful foundation installations at Ormonde and Thornton Bank offshore wind farms.
The tripod is a hybrid of the two, broadening the footprint of a monopile by adding three piled connections close to the seabed. This is to increase load distribution in deeper waters. Trianel wind Farm Borkum, formally known as Borkum West II, has made use of this design.
SCOUR PREVENTION
The most common issue facing the substructures for offshore wind installations is scour around the base of the foundation.
Current velocity increases locally around a mass that lies on, or protrudes from, the seabed. This causes localised seabed erosion adjacent to the structure and is known as scour. Scour causes major stability-related problems for underwater structures.
Surveying the seabed topography and local currents can help engineers to plan if scour may occur, and to what degree. This can influence the design of the structure and enable operations teams to plan scour rectification measures into the installation phase, or incorporate closer inspection regimes during the asset’s operational lifetime to aid future inspection, repair, and maintenance requirements.
Ambico Foundocean grouting Services, the joint venture set up by Foundocean and Ambico Group in 2010, designs, fabricates, and installs specialist grout bags that can prevent scour, and actively re- instate the seabed around both monopile and jacket-based foundations. These can be individual bags or ‘donut’ shaped formworks. They are manufactured from a flexible, high strength synthetic woven fabric that is permeable to water but not to grout, and have been deployed in water as deep as 1,400m.
Such scour prevention bags have already been used offshore in the oil and gas industry for freespan and scour rectification projects. The formworks have built-in anti-scour fronds that open out to form a fan-like array which slows down the local current and causes suspended particulate matter to settle. Over time, the fronds become engorged with sand and sediment, re-instating the seabed with natural components. This fibre-reinforced bank will then resist further erosion.
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The formworks can be fitted by divers or remotely operated vehicle (ROV). They are transported empty and filled with grout in situ, making them easy to transport and incredibly accurate to place.
Rock dumping is the alternative to formwork-based solutions. A vessel transports rocks of various sizes to the destination. There are several ways to place the rocks at the base of the structure, including craning or using a flexible fallpipe over the side of the vessel. An ROV observes the procedure to check the accuracy of the rocks being dumped.
CABLE PROTECTION AND SUPPORT Like the turbine and the foundations, the cables play a critical part in a wind farm system’s operation. The likelihood of the cables being overstressed or damaged can be reduced significantly by supporting and protecting them. This can be achieved by installing a pyramid-shaped grout bag where the cable touches down to the seabed. Further protection is then provided by placing formworks, rocks, or burial in a trench, along its length.
STRUCTURAL REPAIRS De-commissioning foundations and installing new ones can be extremely expensive and time consuming. Structural reinforcement operations to lengthen design-life expectancy offers a cost- effective solution to offshore wind farms when considering life-extension or re- powering and can also be considered when reviewing options to repair following structural damage.
Grouted member strengthening is the process of filling platform jacket members with a grout mix to reinforce the existing member. Once completed, the strengthened members can extend the structure’s service life beyond its original design life by providing improved stiffness and strength. Member strengthening can be performed above and below sea level on vertical, horizontal, and diagonal members, as well as jacket legs. The concept can also be extended to the stiffening of very large tubulars like those used in monopile foundations.
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www.windenergynetwork.co.uk
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