Offshore Pile Drilling From a Floating Vessel

Abstract

To fulfil the ever-increasing need for wind energy, European offshore wind farm sites are selected in deeper waters with seabed conditions which can consist of hard consolidated sediments or even rock. The deeper sites require the use of floating wind turbine foundations that are moored off to anchor piles in the seabed. For rock seabed sites, the anchor piles must be drilled. As the water depth of these sites increases, commercially available jack-up vessels are no longer able to operate. Therefore, the anchor pile drilling operation must be performed from the deck of a floating vessel. An extensive techno-economic analysis has led to the finding that a topside-operated drilling rig mounted on a large construction support vessel with heave compensation (HC) added is the most cost-effective configuration. The performed research focuses primarily on the determination of which HC method is most effective at changing water depths of 50 m up to 200 m. Leading to the understanding which site requires the use of active HC, limiting the resources required to construct future floating wind farms. The configurations are tested for relevant wave conditions, determined by assessing potential European floating wind farm sites.

Firstly, the research assesses the maximum allowable topside displacements before the drilling column reaches either the operational limits of plastic failure or bottom hole assembly lift-off. Secondly, the operational vessel motions are determined for the relevant environmental conditions. By comparing the results, the need for HC in the drilling configuration is determined. Third and finally, the passive and active HC methods are assessed for a 3-hourly time simulation under the before-mentioned environmental conditions. The assessment is performed using two performance criteria; weight on bit variation and the occurring drill-string stresses.

The performed analyses and simulations show that the vertical upward vessel motion is the limiting factor for the operation’s workability. Also, HC is required in every considered environmental condition. Further, the system operating with passive compensation shows a decreased stiffness with respect to the active system, most noticeable at 50 m water depth. This leads to higher frequency vibrations and stress variations being present in the drill-string of the active system. This effect is no longer noticeable for water depths larger than 50 m.

For locations with a water depth of 50 m, the active system shows favourable workability results. The active system shows a larger sensitivity to wave conditions with larger wave heights, as the stiffness is larger and more stress variations occur as a response. However, the results remain more favourable in comparison to the passive system as the lift-off percentage is significantly smaller. The passive and active systems show similar results when considering short waves in 50 m water depth, this is best witnessed in the weight on bit and lift-off percentages.

For locations with a water depth of 100 m and 200 m, the active and passive HC systems show comparable results for the performance criteria, for all considered wave conditions. The stresses remain within the ultimate limit state, the fatigue damage is negligible in comparison to the time required to perform the drilling operation, and the lift-off percentage for both configurations are in the same order. Therefore, as the workability of the two systems are so comparable for a water depth of 100 m and 200 m the availability, day-rate, and mobilisation complexity of the equipment will determine which HC system is most effective per project.