Calculation of the Seakeeping Behaviour of a Jack-Up Vessel During the Jacking Sequence in Time Domain

摘要

Oshore wind farms play a decisive role in the exit from fossil-fuel energyin Germany. These oshore wind turbines are oftentimes installedwith jack-up vessels. At the site of operation these vessels lift their hullabove the water surface by extending their jacking legs. The vessel’sseaway-induced motions at the moment of initial contact between legand seabed define the critical dynamic structural loads onto the legs andthe jacking mechanism, which have to be minimised. Therefore, an analysisof the seakeeping behaviour during the sensitive jacking sequenceis required in the early design stage in order to define the vessel’s operationallimits of sea state. The ship design environment E4 is an opensoftware framework, being developed by the Institute of Ship Design andShip Safety, which provides various methods for the dynamic analysis ofa digital twin of the actual ship. However, these methods are restricted toconventional ship-type structures whereas the analysis of the seakeepingbehaviour during the jack-up process further requires a consideration ofthe jacking legs’ hydrodynamic influence. For a holistic analysis of theseakeeping behaviour, the forces onto the legs were thus accounted for bya Morison approach in the calculation of the required response amplitudeoperators. This paper presents the calculation of the vessel’s seakeepingbehaviour in irregular waves in time domain on the basis of these modifiedRAOs. The applied method calculates the vessel’s response motionsin six degrees of freedom. Since the roll motion tends to show the largestresponse amplitudes in a seaway this degree of freedom is calculated ina non-linear way with respect to the wave amplitude and the roll angleby solving its equation of motion. The motions in the residual degreesof freedom are calculated from the underlying linear response amplitudeoperators. The achieved results are validated by model tests of a jack-upvessel with extended legs in irregular long-crested waves. This extendedmethod represents a quick computational tool for early design applicationsin order to determine limiting sea states for the jacking sequence,which is a major design aspect of such ships.