Buoy delivery - not as easy as it looks…
Mv Happy Sky was contracted to load and discharge the heaviest STP buoy ever constructed, to which ultimately an FPSO will be coupled. The deciding factor for the client to choose Happy Sky was the fact that she is capable of lifting a 1300 mt buoy without additional aids like stability pontoons.
The buoy itself weighed 1260 mt, 1305 mt including grillage, which made it the largest we have handled to date. For safe handling, a number of extra actions were taken, increasing the deck strength capacity of the supporting tweendecks by placing ship’s pillars, and designing and installing a special guiding system.
Besides, in order to determine the limiting environmental conditions for the discharge operation, BigLift used the advanced hydrodynamic software tool MOSES. In a multi-body analysis to assess the feasibility of the operation, the coupled motions of Happy Sky, its cranes, the rigging, and the buoy were calculated.
Happy Sky loaded the buoy in Singapore, after accessories had been taken on board in Batam and the special guiding frame was mounted. This bumper system had been designed in-house in order to be able to discharge the buoy damage-free in offshore conditions, as there was only 20 cm to spare between the grillage and the protruding tubes underneath the buoy. So, with this special guide fitted to the vessel’s coaming and keeping a slight portside heel, it was ensured the cargo leaned against the guide system in the right direction. When unloading the buoy out at sea, in the Beagle Gulf offshore Darwin, it was lifted off the grillage and placed overboard in the water. This required the assistance of the full crew, not least since a great number of lines and hoses, attached to the buoy, should not become entangled. Once in the water, the buoy needed to be ballasted to its correct buoyancy to reach a stable condition. So, during the next 24 hours, the buoy was hanging in the vessel’s cranes. The whole operation required 24 to 30 hours smooth seas while unloading, plus a three-day favourable weather window to enable it to be towed into its designated spot. After all was said and done, we could look back on a very complicated, but satisfactorily executed project.