The Jack-up frame

Abstract

The offshore wind industry is entering a new level of maturity. Announcements of bigger offshore wind turbines, the interest in new locations for offshore wind farms in harsher environments and the appearance of zero subsidy bids are proof of a rapid development. The next generation turbines are expected to be significantly larger and heavier compared with the current operating turbines. This poses new requirements for safe and efficient installation, requirements that go beyond the capabilities of existing jack-up installation vessels and equipment. Therefore, to avoid bottlenecks for future development, new installation equipment is needed. The goal of this thesis is to find an efficient way for installation of future offshore wind turbines with a rated power of up to 20MW.

The characteristics of these large size turbines were studied by examining the relation between the rated power and the rotor diameter of operating offshore wind turbines. The derived dependencies between the desired power and the required area of the rotor were validated with data from announced turbines. Extrapolating these dependencies has resulted in a prediction for the 20MW turbine of a rotor diameter of 250 metres, a hub height of 160 metres above sea level and a nacelle with a mass of around 1100 tonnes.

To be able to develop new concepts for the installation of these turbines, interviews were conducted with industry experts and criteria were derived. Next, upscaling of the equipment of the current jack-up vessel was investigated, already existing concepts were reviewed and new concepts were developed. Based on the set criteria, a suitable installation concept was chosen.

The chosen concept eliminates the need for lifting the heaviest component (the nacelle) to the highest height (hub height) by dividing the tower of the turbine into several segments. It consists of a temporary installation frame that can be placed from a jack-up vessel on top of the foundation of an offshore wind turbine. While installing the frame on the foundation with a crane, the nacelle, hub and blades are mounted together on the deck of the jack-up vessel, forming the rotor nacelle assembly (RNA). Then, the RNA together with the first segment of the tower is placed in the frame, skidded sideways and brought up by a built-in jacking mechanism in the frame. It needs to be brought up 45 metres, so the following segment of 40 metres can be skidded underneath. While jacking the first segment, the following segment is placed next to the lift frame and prepared to be skidded. After the skidding of the next segment is finished, the previous segment is lowered on top of the other segment and they are mounted together. While fastening the connection, the jacking mechanism is lowered by recycling the strokes so it can start lifting the next segment. This is repeated until the complete turbine has been installed. When all the tower segments are installed, the turbine can be commissioned and the frame is retrieved.

Optimisation of the concept has been performed by highlighting the logistical process regarding placement of the frame, lifting of the turbine and retrieval of the frame. A concept design is presented that can install the future offshore wind turbines with a rated power of up to 20MW. It is able to install the large size turbines faster compared to upscaling the existing installation equipment, it can be used for several turbine sizes and it only requires small modifications on the design of an offshore wind turbine.

The concept consists of a jack-up vessel from where the installation is performed offshore. This was preferred over a floating vessel, since movement of the jack-up vessel is reduced significantly when lifted out of the water. For wider applicability of the developed concept, for example on a free floating vessel, (non jack-up), further research is required to reduce motions between the turbine and the foundation.