Conceptual Design of a Pile Installation Frame with Adjustable Footprint

Abstract

An offshore wind turbine is placed on a so called “foundation” to gain height above the sea surface level. One of the used foundations is the jacket structure, which is kept in place by pre-piled supports. The piles are hammered into the seabed first, after which the jacket legs are stabbed into the piles. A template is used to guide the piles during installation and to secure the relative distances as well as the straightness and depth of the piles with respect to each other. This template is called the Pile Installation Frame (PIF). Seaway Heavy Lifting has designed a PIF with a fixed footprint for the Beatrice offshore wind farm project. However, there is a need to make the PIF adjustable in order to use it in different situations (e.g. environmental conditions, pile designs and jacket configurations). Therefore, this thesis investigates the following two questions: “How can the current PIF be altered in order to make the footprint adjustable for various footprints?”, and: “What are the influences of variations in pile designs and conditions?”. A trade-off method is used to find the most feasible conceptual design. Resulted in interchangeable frames connected between the original pile supports and the center base frame (with equipment and a lifting point). The integrity of the concept is checked for one of the operational scenarios, namely, “the in-place scenario”, i.e., the PIF is at the seabed and the piles are stabbed into the sleeves with a hydraulic hammer on top of one of the piles. A model of the concept design is made with the structural simulation software SACS, that does not account for the second order bending effect of the pile. Therefore, a new calculation model is developed. This model calculates the reaction forces from the piles onto the PIF, and it is based on the linear wave theory, a linearized approached current profile, and the Morrison equation. The bending of the pile is computed using the Euler-Bernoulli beam theory, which is iteratively solved to take into account the second order bending effect. The PIF is checked for the minimum square footprint of 20 m, 24 m and the maximum of 32 m for a representative reference condition (Beatrice offshore wind farm). From the check, the maximum stress level in the members and braces is below half of the maximum allowable stress. Furthermore, the deformations at the pile supports in the sleeve are in the same order of magnitude than the current PIF. Hence, these are not critical for the installation tolerances in the same conditions. To show the influences of the variations per project in pile design and conditions, an analysis is executed. From the analysis, it is concluded that the forces on the frame are maximum when the hammer is at the seawater level. Moreover, a pile diameter of 2.2 meter is the optimum for the reference condition. It is also observed that, the influence of the pile thickness is not considerable with respect to the forces onto the frame. The forces in the members and braces of the frame are mainly caused by the pile reaction forces transferred to the PIF. Therefore, the calculation model can be used for a first estimation of the PIF integrity. When the reaction forces are lower than for the reference conditions, the PIF can be used; when they are higher, an additional analysis with SACS is required.