Conceptual development of a wedge connection for offshore jacket foundations

Abstract

More offshore wind projects are being developed. Shallow locations are more occupied and a tendency to deeper water and to larger wind turbines is seen. These create more bending moment which can be more easily resisted by jacket substructures. The state of the art installation process of a jacket is as follows. The foundation piles are driven into the seabed. The jacket leg is installed by sticking a stabbing cone into the stick-up length of the foundation pile and fill the space between the cone and the pile with grout. In this situation a lot of grout, steel and installation time are used. An alternative connection could be a wedge connection which is considered in this thesis. A wedge connection consists of four parts: jacket leg, connector element, wedges and foundation pile. The challenge to apply this connection to jackets is to overcome the installation tolerances. Due to the inaccuracy of the driving of the foundation piles, the holes in the jacket legs and the foundation piles do not align and the wedges cannot be installed. The installation tolerances consist mainly of center-to-center distance, pile verticality and a vertical tolerance. The maximum translational tolerances are 75 millimeter while the maximum rotational tolerance is maximum 0.83 degrees. 23 different concepts have been developed to deal with the installation tolerances. All the concepts have some drawbacks and do not fully solve the problem without downsides. The magnitude of the installation tolerances is therefore reduced and from a multi-criteria analysis a screw thread connection is proposed. The advantages of the screw thread connection are: a reduced amount of material, no grout is required and the installation time of the jacket to foundation pile connection can be reduced up to 75 percent. The screw thread connection overcomes the vertical tolerance and the tolerance around the longitudinal axis of the foundation pile by rotation of the connector element. In case of an pile inclination, the connector element and the foundation pile do not fully align and a gap remains in between them. By applying a preloading force to the connection it is still not possible to close this gap. The space between the connector element and the foundation pile has to be filled with an epoxy resin. The screw thread connection does fulfill the requirements for the ultimate limit state based upon hand calculations for net stresses, bending stresses and contact stresses. The fatigue limit state of the wedge connection meets the requirements. Only the fatigue limit state of the screw thread does not fulfill the requirements based upon a hand calculation. The three largest issues of the screw thread connection are the fabrication of the screw thread profile, the fatigue in the screw thread and the installation of the epoxy resin between the connector and the foundation pile. The first step to develop the screw thread connection further is an more extensive investigation to these three issues.