Large Floating Monopiles

Abstract

The offshore wind market experiences large growth over the last decade. There is a clear trend to installation in deeper waters and to larger wind turbines, to maximize the energy output. The majority of offshore windmills use a monopile foundation, which due to the current trends are getting larger. Seaway Heavy Lifting (SHL) is a company that installs these monopile foundations, with their Heavy Lift Vessels (HLV). The largest HLV of Seaway can transport and install three monopiles at a time. However, with larger monopiles it is expected to be reduced to two monopiles or even less. In order to be competitive in the monopile installation market, SHL is looking for a method to transport the monopiles to the HLV, instead of using the HLV for transportation. A previous comparative study showed that floating transport of monopiles to the HLV is the most favorable method. However, SHL is unexperienced with this method and therefore requires a study on developing the best method to install the monopiles within the workability of the HLV. Therefore, the thesis goal is:
"Design of a method for installation of large floating monopiles in heavy weather conditions".
To design such a method, it will first be determined what steps are necessary and which options can used to fulfill these steps. With a multi criteria analysis will be determined what combination will lead to the best method. The best two methods will be subjected to a sensitivity analysis to determine its feasibility.
The method consists of five steps: floatation, towing, mooring, hook-in and lifting. A multicriteria analysis shows that using airbags is the most favorable method of making the monopile float, while using a flange clamp for towing and lifting. In order to reduce motions, the mooring can be either on the side of the vessel in a gripper frame, or by sinking one end of the monopile to the seabed. When the monopile is in moored position the pre-rigged rigging can be taken over by the main hook of the crane.
Both methods are modelled in the simulation software package Ansys AQWA. A soil model has been made to assess the influence of the grounding on the motion behaviour of the monopile. Due to the soil properties it is faster and relatively accurate to model the connection between the soil and the monopile as a hinge instead of applying the soil model. The system is subjected to different waveheights and wavelengths, while varying the direction of the environmental forces, the trim of the monopile on the seabed and the orientation of the monopile to the vessel.
The installation of a grounded monopile in heavy weather is a feasible option. The simulations have shown that installation is possible up till a significant waveheigth of two meters with a peak period of 8s. The least trimmed position has the best performance over different environmental forces directions. The best option for installation is in a configuration where the vessel and the monopile are facing the environmental conditions head-on, with a 42 degrees monopile trim. However, it has to be noted that other monopile orientations with respect to the HLV are performing good as well, as long as the trim of the monopile is 42 degrees. Using other trim options is possible, but limited to specific orientations.
The installation of monopiles with a gripper attached to the vessel seems to be a viable option, but only once the soil model is improved. The simplification of the soil model by replacing it with a hinge gives uncertainties in the real force acting on the interface between the vessel and the grounded monopile. However, when considering the RMS force of 30 minute simulations without taking lateral forces into account, the results are well within the set boundaries.
Thus can be concluded that installation of a floating monopile is feasible, when using the grounding method. It is recommended to improve the soil model for both methods, to improve the accuracy of the results. To ensure a safe installation and a safe operation during its lifetime, the structural integrity of the monopile should be investigated. Small deformations can drastically reduce the fatigue life of the structure. An end-plug to lower the monopile to the seabed and reinforce the structure could be an option to ensure the structural integrity. Furthermore, it should be investigated if a gripperframe is a realistic option due to its size. The dimension could turn out impossible for catching and releasing extra large monopiles in heavy weather conditions. Lastly, it turns out that grounding a monopile is more time consuming and difficult than anticipated. Therefore it is recommended to research the installation of a free floating monopile or mooring a floating monopile alongside the HLV.