Internal cutting of offshore wind turbine monopiles

Abstract

Offshore wind energy is considered one of the most promising solutions for sustainably meeting the society's growing energy needs. This is why there are major construction projects and massive expansion plans for new offshore wind farms worldwide. In the North Sea in particular, the technology has been used to generate electricity for several years. Looking at the oldest wind farms still in operation, it will soon be time to decommission them once the turbines have reached their service life of 20 to 25 years. However, especially the removal of the foundations is considered challenging because not many comparable projects have already been realised. The most commonly used type of foundation is the monopile due to its simplicity. During installation, these long steel piles are driven deep into the seabed to provide a secure foundation over the entire service life. One option for decommissioning is to cut the piles from the inside a few metres below the seabed and then pull them out. So far, this procedure has mainly been used for individual small piles. Scaling up to several large monopiles of a wind farm involves uncertainties. On the one hand, it is unclear how larger monopiles will behave during the cutting process, and on the other hand, the operational performance of several piles that are removed one after the other is uncertain.

When cutting monopiles, theoretically no complete cutting progress can be achieved because the pile breaks off beforehand. In this work, a calculation model was developed that predicts the cutting progress at which a monopile fails due to the hydrodynamic forces acting on it. Based on an existing project in which the internal cutting technology was used, a minimum progress was defined that must be reached before failure is allowed to occur. This makes it possible to determine which sea states are permissible in order to achieve the specified cutting progress. With the calculation model, a parameter study was carried out to find out which structural and environmental parameters have the greatest influence on failure. Additionally, two real wind farms were considered as case studies to investigate realistic parameter sets. One of these wind farms has relatively small monopiles as foundations, while the other has much larger ones. An operability analysis was also carried out for these case studies in order to determine the duration of the foundation decommissioning operation and to be able to analyse the weather downtime. In addition to calculating when a monopile fails during the cutting process, the forces required to pull the pile out of the seabed afterwards were calculated. This allows to determine the required crane capacity of the working vessels used for the operation.

The results of the calculations show that larger monopiles fail at an earlier cutting process than smaller monopiles. However, this difference is not large, which is due to the fact that together with the hydrodynamic forces, which increase with the pile size and length, also the wall thicknesses increase. This means that larger forces can generally be withstood. The operability is good for both wind farms from the case studies. Furthermore, it was found that it is important for the operability how the connection between monopile and transition piece was realised and whether ROVs have to be used. The crane capacity of the vessels used can be lower than that of the vessels used during installation. The additional force required to pull the monopiles out of the seabed does not make up for the weight lost by leaving a large part of the foundation in the soil.