In April 2012 Heerema Marine Contractors (HMC) successfully performed the float-over of the North Rankin B topsides. With a weight of 24,250mT it is the heaviest deck installed in a single piece in the harsh offshore environment of Australia’s North West Shelf to date. During years of preparations extensive hydrodynamic analyses have been undertaken to achieve design loads, optimize the float-over equipment and ultimately determine operability limits for the installation.

As the float-over analysis contains various components with non-linear characteristics (e.g. fenders, leg mating units, mooring lines), a non-linear problem solving approach was used. The selected analysis tools use an efficient time step integration enabling a large number of simulations to be run. The strong non-linearity of the system requires a Monte Carlo simulation to obtain statistically reliable results. The numerical model included purpose built modules for the float-over equipment (such as LMUs) taking into account relevant details and project specific geometries. Available data from model tests was used in calibration and validation of the model.

Towards the offshore execution phase, the limiting combinations of wind sea and swell waves were identified. Forthese combinations the system response remained (just) within its ‘Not To Exceed’ values. During the offshore operation, prior to the actual float-over, the spectral wave conditions, wind and current were constantly monitored to validate the forecasts. Also the barge motions were measured and compared against results from the numerical models. This was used in support of the final decision to proceed with the float-over operation.

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