Optimal construction and transportation method for gravity based foundations for offshore wind farms on commercial scale

Abstract

A new trend to found offshore wind turbines at larger depths or on harder soils is to apply gravity based foundations (GBFs). These foundations are caissons with a set of properties which are different with respect to other caisson types. The number of constructed caissons is large and the timeframe for construction and transportation from land into water is small when these foundations are applied on commercially scaled offshore windfarms. In general, an offshore wind farm is constructed within two years and all GBFs must be constructed and transported into the water in a tight time schedule.

To accomplish the construction and installation of a large number GBFs in a tight timeframe, a new design method is developed. A realistic case study is composed and the target is to construct and install 64 GBFs in front of the Belgium coast in a timeframe of 2 years.
Two designs are considered, a materialefficient design and a design which is optimized regarding the constructability.

When the GBF-design is known, the transportation method from land into water is considered. According to the decision matrix, developed in the literature study, the only feasible method is to apply a semi-submersible vessel. However this solution is quite expensive, whether the storage location is at the construction area or in the sea. Due to the large costs a new alternative method could be feasible. Several new ideas are created and the most promising solution is ‘The immersion structure’.

Because time is the most important factor, the construction planning and construction area layout are considered. Three options are considered and the most optimal construction planning is chosen. On four locations the construction activities are optimized and with the use of a production line it is possible to construct the 64 GBFs in time with minimal use of area and equipment. Because of the small area left, the storage location cannot be designed on the construction area.

The design of the immersion structure consists of a platform which consists of H-beams and two concrete hollow legs. The immersion structure is innovative because it is placed directly on sand and no foundation works are needed. Due to the large capacity the main part of the caisson types can be transported. Because the structure is able to float it can be easily transported.

At last the construction and operational costs of the immersion structure are compared with the cost of renting a semi-submersible vessel. The semi-submersible vessel seems to be the best solution but when the immersion structure is applied at multiple projects it could be a profitable solution. The depreciation and interest costs for the immersion structure are calculated and the immersion structure is profitable when a minimal utilization rate of 22% is achieved during a service lifetime of 20 years. Depending on the market development and the amount of applicable caisson construction projects the immersion structure could be profitable. Comparing with the case study the immersion structure is profitable if three comparable projects are executed in the service lifetime.