Assessment of lowering strategies for monopile installations from a dynamically positioned vessel

Abstract

The rapid rate in technological advancements leaves little to none options for our environment to react. The excessive use of "conventional" energy is bringing planet Earth to its knees. However, people started recognizing the value of renewable energy sources.
One of the major renewable sources is the wind energy. The wind energy can be divided in two categories; onshore and offshore wind energy. Offshore wind energy has many advantages in comparison with onshore but comes with a major disadvantage, the installation cost. Since installing an offshore wind turbine is far more complex than installing the same wind turbine onshore, different ways needs to be developed to compensate for the excess costs.
Even though the cost is a significant aspect, it is not the most critical one. Since
more power is required, bigger offshore wind turbines are needed, thus larger foundation structures and bigger water depths. The conventional Jack-Up Barge that was being used for such operations so far is driving to a saturation on its operability due to limitations on maximum crane capacity and maximum water depth. For the aforementioned reasons, a floating vessel is considered to install bigger foundations but this leads to the loss of fixed ground that the Jack-Up Barge provided. This creates a significant problem that motions are generated and disturb the installation process. For this reason, a compensating strategy should be developed to allow such installations.
Such solution comes from TWD with the Motion Compensating Pile Gripper. In order to reduce the installation costs and to allow installation of wind turbines in higher depths of water, a floating vessel has to be deployed instead of the conventional Jack-Up Barges. The use of a floating vessel though, generates motions that disturb the installation procedure. This is the reason that TWD came up with a compensating gripper. This gripper uses hydraulic cylinders to generate forces for the monopile and vessel in order to counteract unwanted motions and to keep the monopile in the required position.
In this thesis, initially a functional design is conducted. This analysis concludes with the possible design strategies that can restrict the monopile through the lowering operation. After the possible design strategies are established, a simulation model is generated that involves environmental loads, a vessel model, a monopile model and a gripper model.
Through that simulation model, the different alternatives are tested and imulated
in order to evaluate the performance of each and observe their responses. Finally, after all the alternatives are simulated, a Multi-Criteria Analysis takes place that evaluates each one of the possible strategies based on various criteria in order to conclude to the winning strategy that is then suggested to be implemented in the lowering operation of the monopile erection process.