Stressed skin design in offshore modules

Abstract

Offshore wind power provides an alternative to fossil fuels while negating the effect of visual and auditive distortion of onshore wind turbines. Located several kilometres offshore, the transmission cable resistance becomes substantial. To limit the losses, the voltage in the cables is increased in a substation. As this structure is often the largest in a wind farm, profits can be expected when optimizing this topside structure. Topside structures for the oil \& gas industry generally require to be ventilated in order to prevent gas stacking. The equipment in the substation module, however, is vulnerable for the corrosive sea environment. Therefore, the structure needs to be sheltered. Hereby, the question rises if a weight reduction can be obtained by incorporating the shelter in the structural design. The thesis is carried out at Iv-Oil \& Gas, who provided the basis of a comparative case study. For the research to be generally applicable, the case study design is simplified. Then a frame form the simplified model is chosen as a basis for the comparison. For a single governing load combination, two designs are constructed: a beam-column design complying to the conventional oil \& gas design practice and a stressed skin design that incorporates the sheltering wall as a structural element. Both of the designs are then compared on steel mass and weld labour. The stressed skin design is found to be considerably lighter than the beam-column design. However, in terms of welds, the stressed skin design consists of a higher weld volume and a higher weld pass length. These aspects contribute to the total manufacturing costs of the design. Concluding, both of the designs have their advantages and disadvantages. It is made clear that applying the dividing wall as a structural element in the format of a stressed skin design provides a competitive design in terms of structural mass and weld labour. The final conclusions based on the comparison may vary dependent on the resources available at the manufacturer.