Dutch engineering company Iv-Offshore & Energy and naval architecture and marine engineering company Nevesbu jointly developed a floating concept for an offshore substation that offers an economic solution for offshore wind farms that are built far offshore.
At water depths beyond 150 metres, the costs of the renowned ‘bottom- founded’ structures increase exponentially, making floating solutions an interesting option. lv-Offshore & Energy and Nevesbu designed a new concept that would provide an efficient, economic and reliable alternative to current standard substation designs.
Lennart Buitendijk, Naval Architect at Nevesbu, and one of the initiators of the concept: “Nevesbu began investigating which unique floating applications could be devised to provide a solution. There are already many concepts for floating turbines, but not for a floating substation. We formed a joint initiative as Iv-Offshore & Energy has already designed many offshore wind substations, and Nevesbu has specialist knowledge of floating structures. The challenges presented with this type of floating solution include the lifespan of the dynamic power cables and the allowable motions of the transformers, rectifiers, and associated systems. Floating an offshore converter station weighing roughly 10,000 tonnes with minimal motions is a complex challenge, but our studies show that it is possible!”
Suitable deepwater areas
The concept that Nevesbu and Iv-Offshore & Energy have developed is designed for the harsh weather conditions that are characteristic of the deepwaters of the Mediterranean, North Sea, Bay of Biscay, and the Aegean Sea, in particular. The concept also minimises the impact on marine life.
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Some essential principles for the design of the floating transformer platform are: the concept must conform to the set requirements for wind energy at sea, it must not be too heavy in steel weight and must not be complex in terms of fabrication, safety, and reliability must also be guaranteed, it must be possible to guarantee a very high availability, and the platform must have a lifespan of at least 30 years. In addition, the social costs of supplying sustainable electricity must remain affordable, and the solution itself should therefore not be too expensive in terms of costs.
From static to dynamic
The most significant challenge is the transition from a static bottom-founded platform to a dynamic floating platform. Wind turbines must be capable of operating in conditions up to Beaufort 8, which means that the floating platform must continue to function when contending with waves of 8 to 12 metres high. Existing high voltage (HV) equipment, however, is not suitable for high accelerations. To guarantee motions are kept to a minimum, motion analyses were carried out to calculate how different models behave at sea in strong winds with high waves. These analyses are based on the roughest seas in the world, such as the North Atlantic Gulf Region, the Bay of Biscay and the sea near Santa Barbara, California. If the concept is suitable for these waters, it can certainly also be applied in calmer waters.
The concept of the floating platform is designed in such a way that it can be applied in water depths in excess of 150 metres and with minimal vertical motions in sea conditions. The HVDC platform has a deck area of 85 by 85 metres and is positioned approximately 20 metres above the water’s surface. When the platform is installed at sea, it will be held in position with the help of so-called ‘tendons’ that are vertically anchored to the seabed. The floating structure is designed to minimise the vertical motions, which also reduces the loads on the export cables suspended below the platform. Tuning the motion behaviour of the platform for different water depths and wave conditions is also relatively simple.
The concept has now been thoroughly worked out and appears to be feasible. Various studies have been carried out that have provided insight into the possibilities and most appropriate application for use in deepwater areas. Currently a design check is being performed to see how the design can be further improved and made more (cost) efficient. The companies are hoping to find a developer that will see the potential in this concept and thus develop the concept further together and put it into practice. Source: lv-Offshore & Energy