Hydraulics in use
Ship lifting from dry dock - Stena Line
How to lift a special shaped ship from and place it back into a dry dock without damaging it ?
That is the question Stena Line was faced with regarding its ferryboats from the HSS 1500 series. These ships are massive catamarans, made of aluminium and epoxy. They weigh 3.140 tons, have a length of 110 meters, a width of 40 meters and a draught of 4.5 meters. Stena Line developed a special method which involves a hydraulic lifting system designed by Holmatro.
Inside the dry dock Stena Line built 10 concrete towers (height 10 m, ø 2 m), in 2 rows of 5. On top of each tower: a steel carrying frame with a wooden covering mounted on 4 lock nut cylinders. The ship is manoeuvred very precisely above the towers after which the dock is slowly emptied. The moment the ship’s bottom, in between both floats, nearly touches the towers, the plungers of all 40 hydraulic cylinders are pushed out. The cylinders are operated from the quay by means of 3 Vari pumps. Dependent the reading of the pressure gauge each cylinder plunger is pumped out just until the carrying frame touches the bottom of the ship. The remaining water is pumped out of the dock and the ship is resting on the cylinders. The load is mechanically locked with the lock nuts on the plungers. Secured and in an empty dock the ship is now ready for major repairs. And after being serviced the ship is launched smoothly back into the water in reverse order.