Car tyres protecting turbines from scour
Mon, 17 March 2014
A UK based company has introduced a patented, innovative and cost-effective solution for scour protection of marine structures, including wind turbines, also launching a new career for of old car tyres.
Scouring of the seabed around fixed marine structures is a well-known phenomenon that can affect offshore oil and gas platform installations, coastal structures and outfalls, bridge abutments and subsea cables and pipelines. It is a problem that also has to be addressed by the offshore wind and wave and tidal energy device industries. Lowestoft based Scour Prevention Systems has identified the renewables sector as an area of interest, particularly with the pressure to reduce costs in the battle to gain credibility as a viable alternative cost-effective form of energy generation. An indication of the potential market is the company highlighting that an estimated 9,300km of subsea oil and gas pipelines are projected to be installed between 2012 and 2017, along with 11,000km of inter-array and export cable required at offshore windfarms over the next ten years.
Traditional monopile foundations can be particularly susceptible to scour, especially at sandy locations, in areas of strong tidal and wave produced currents. Wave and tidal flows are disrupted as they pass the structure causing vortices and seabed erosion which in turn forms a scour hole around the base of the monopile. This can affect structural stability and vibration frequency, potentially accelerating fatigue damage to components. Scour can be a particular problem for inter-array cabling running between turbines, unplanned exposure of the J-tube cable outlet leading to stress being put on the cable suspended there from. Cables can become exposed as they exit the foundation, allowing them to hang freely across the scour hole creating a ‘washing line’ effect. A traditional and effective method of combating scour is by precision placement of rock and stone armour or concrete mattresses around the structure and Scour Prevention Systems follows a similar principle but using used car tyres connected together to form mats.
Placing the mats flat on the seabed is key to their ability to entrap sediment within their centres. The trapped sediment makes the mats self-stabilising and prevents the lowering of the seabed by scouring. As the high energy tidal flow transports sediment over the mat, the flow is disturbed and speed reduced. Sediment caught up in this disruption becomes entrapped within the mat’s void spaces. The ideal shape and form of a car tyre is clearly an important feature of the principle. The density of tyres is perfectly suited for stabilising the seabed, and being less dense than the seabed sediment, the tyres do not sink into the seabed like denser materials such as rock and concrete. The tyres are roped closely together into mats and simply craned onto the seabed, an operation that can be carried out from relatively small installation vessels. They can be used to repair existing scour by being placed on top of infill material such as aggregate, geotextile sandbags, gravel filter units or nets of tyres. The mats stabilise the infill material, preventing secondary scour. Removal of the mats is obviously an important consideration, not only for decommissioning but should access to cables be required. The forces acting on the mats over time are not insignificant and with removal in mind, polypropylene lifting rope is provided at one end. Lifting one end of the mat allows the mat to be ‘peeled’ away from the seabed, the tyres losing their infill.
Investment and innovation grant support from the Technology Strategy Board (TSB) allowed the mats to be demonstrated for cable protection at E.ON’s Scroby Sands offshore windfarm in September 2012. Five scour prevention mats successfully stabilised the seabed for over a year, despite continuous migration of sand waves across the site. Speaking after the trial, Mike Lewis, COO Wind Power said: “After a lengthy two stage trial at the Scroby Sands Wind Farm, the mattress system has proven, beyond doubt, that it stabilises the immediate seabed area on which it has been installed, thereby eliminating the onset of scour.”