The extended lift operation to deliver the Wellbay module (M5) combined with the Flare Tower (M8) from the Miller Platform in the North Sea to the shore using the Semi-Submersible Crane Vessel S7000 was sensitive to the wave environment. Weather and response forecasts were used to assist go or no go decision. Responses of interest such as main hook loads, crane tip motions and clearances between the M5/M8 and vessel crane booms were forecasted and monitored. The results of the weather and response forecasts and the data from the monitored parameters are compared to identify some uncertainties in the weather and response forecasts. To reduce these uncertainties, a method using the sea states measured from the wave rider buoy (WRB) deployed in the Miller Platform Removal campaign has been developed.
Marine operations to install jackets/towers and topsides for new fixed offshore platforms or to remove them after the end of their service life are performed within allowable weather windows. These windows are conventionally based on the comparison of weather forecasts and limiting sea states. The limiting sea states for a particular operation such as topside removal from an offshore platform are found from a dynamic analysis using standard wave spectra (typically JONSWAP spectra), during an engineering phase. The uncertainty in wave spectra used in the engineering analysis can be taken into account using a risk parameter (γ) in the calculation of most probable maximum/minimum (MPM) responses to define the limiting sea states. However, the γ factor is not considered, as the inaccuracy in wave spectra is addressed through an alpha factor (α) to account for uncertainty in weather forecasts. The value of the α factor, recommended by the DNV-GL classification society may be used to de-rate the operational limits (DNV-GL AS, 2016). This approach is generally conservative.
With the increase in computational power, and improvement in numerical weather prediction (NWP) models, the general pattern of the forecast weather is quite accurate up to about three days. For 48 hour weather forecasts wind speed is within 5 m/s 94% of the time and wave height is within 0.5 m 78% of the time (Galvin, 2014). Combining forecast two-dimensional (2D) wave energy spectra output from the NWP models with vessel response amplitude operators (RAOs) has made vessel response forecasts possible (Lai, et al, 2006).
