Due to the increasing demand for sustainable wind energy, larger wind farms with more giant turbines are required. Therefore, more is expected of the existing installation vessels for wind turbines. These changes make it increasingly difficult to find capable jack-up vessels to transport and install these new "mega turbines". Another trend in the wind industry is that the demand for wind farms is increasing outside of Europe, where protective laws often apply, such as the Jones Act. The Tetrahedron crane can lift higher without increasing the weight or size of the crane. Installing wind turbines with a Tetrahedron crane whilst supplying the wind turbine components with a feeder vessel would mitigate most problems with the current trends in the offshore wind industry. However, no research is done on performing lifts with a Tetrahedron crane from a moving vessel. This research aims to determine whether it is feasible to perform lifts from a feeder vessel subjected to heave motion with a Tetrahedron crane and analyse this feasibility for large quantities of data points in a relatively short period.
A case study is used to assess the feasibility of executing feeder lifts. The case study is used to provide constraints and parameters for two models that simulate a lifting operation. Both models describe the location of the load from the start of the lifting process till it is safely suspended far above the deck of the feeder vessel. The first model, the 1D model, is based on a free body diagram that provides a simplified representation of the Tetrahedron crane. This model can assess the feasibility of executing feeder lifts for entire data sets. The second model, the 2D model, represents the Tetrahedron crane as a system of elements and nodes. This model is used to validate the single lifting operations of the first model. Both models generate output data that provides insight into the dynamic forces that the crane and components endure during a lifting operation and if the load that is lifted experiences a re-hit or not. From this, it can be concluded whether a lifting operation is successful or not. Successful in this sense means no re-hit takes place, and the dynamic forces remain within limits set by the turbine and crane manufacturer.
While comparing the results of the two models, the 2D model approximates reality better because the dead weight of the crane construction is included where the 1D model does not. By adjusting the 1D model to account for the dead weight, results can be determined for entire data sets. A Tetrahedron crane has a higher lifting speed and a lower crane stiffness than a commonly used luffing boom crane. When both cranes have a comparable re-hit percentage, the results showed that the dynamic forces in the lift operations with the Tetrahedron crane are lower than with a luffing boom crane. In addition, analyses were made regarding the re-hit percentage for varying wave heights, wave periods and masses.
The outlined case study represents a realistic situation, and re-hit percentages within this case study are low. The re-hits that occur can be indicated, and it is also evident in which period lifting operations can be started safely. However, measures must be taken to ensure that the dynamic forces remain within the limits of the crane and turbine components. For the crane, this means that a greater lifting capacity is required. For the vulnerable turbine components, the dynamic forces must be reduced by, for example, a passive heave compensator.