|Title: Calculation of the Seakeeping Behaviour of a Jack-Up Vessel During the Jacking Sequence in Time Domain.
|Written by: Maximilian Liebert
|in: <em>ISOPE, Shanghai, China</em>. (2020).
Abstract: Offshore wind farms play a decisive role in the exit from fossil-fuel energy in Germany. These offshore wind turbines are oftentimes installed with jack-up vessels. At the site of operation these vessels lift their hull above the water surface by extending their jacking legs. The vessel´s seaway-induced motions at the moment of initial contact between leg and seabed define the critical dynamic structural loads onto the legs and the jacking mechanism, which have to be minimised. Therefore, an analysis of the seakeeping behaviour during the sensitive jacking sequence is required in the early design stage in order to define the vessel´s operational limits of sea state. The ship design environment E4 is an open software framework, being developed by the Institute of Ship Design and Ship Safety, which provides various methods for the dynamic analysis of a digital twin of the actual ship. However, these methods are restricted to conventional ship-type structures whereas the analysis of the seakeeping behaviour during the jack-up process further requires a consideration of the jacking legs´ hydrodynamic influence. For a holistic analysis of the seakeeping behaviour, the forces onto the legs were thus accounted for by a Morison approach in the calculation of the required response amplitude operators. This paper presents the calculation of the vessel´s seakeeping behaviour in irregular waves in time domain on the basis of these modified RAOs. The applied method calculates the vessel´s response motions in six degrees of freedom. Since the roll motion tends to show the largest response amplitudes in a seaway this degree of freedom is calculated in a non-linear way with respect to the wave amplitude and the roll angle by solving its equation of motion. The motions in the residual degrees of freedom are calculated from the underlying linear response amplitude operators. The achieved results are validated by model tests of a jack-up vessel with extended legs in irregular long-crested waves. This extended method represents a quick computational tool for early design applications in order to determine limiting sea states for the jacking sequence, which is a major design aspect of such ships.