The rack plate of the CJ70-X150 extends 100 mm beyond the chord scales, i.e. only the rack plate will be in contact with the dock bottom.
The rack plate of GustoMSC CJ70-X150 extends 100 mm beyond the chord scales, i.e. only the rack plate will be in contact with the dock bottom.
Due to this leg design, we could see that the jack-up could be suitable for dry docking at Hanøytangen. All CJ70 design units have the same footprint with three rack areas of 1050 mm x 210 mm under each leg. The centre rack steel plate has a yield strength of 690 MPa.
We made some preliminary calculations of yield and buckling loads including typical weights of hull, leg and variable load. The calculations came out with the result that margins were satisfactory. Sliding and overturning calculations all proved a very low utilisation load and were found acceptable.
Our initial intention was to place the CJ70 directly on the bottom of the dry dock. However, having checked the capacity of the bedrock it was obvious that the bedrock would not be able to withstand the load from the rack plate. We therefore installed some steel plates of various thicknesses below the rack plates and calculated the effect.
However, the preparatory phase was much more difficult than expected. The pressure at the rack contact area exceeded 30,000 tons per m2. Even with steel plates up to a height of 1,600 mm, we had difficulties transferring the rack load into the bedrock. It proved to be almost impossible to obtain a satisfactory low pressure that would be tolerated by the bedrock.
Another issue was the high steel plinths for the CJ70, which would make it very difficult to perform a normal docking operation. We therefore decided to submerge the steel plinths into the bedrock. However, we still had difficulty spreading out the forces.
The solution was to cut a circular 1-metre hole in one of the upper plates and then force the pressure outwards. We also widened the lower plates to 3m x 3m followed by 2.5m x 2.5m plates and 2m x 2m plates. Finally top plates of 1m x 2m are installed.
A 400 mm strong reinforced concrete slab was casted to transfer the forces from the steel plinths and legs into the bedrock. A 1-metre thick reinforced concrete wall was casted around the plinths. The steel used for the plinths has a yield strength of 350 MPa except for the top plate, which has a value of 690 MPa. Subsequently, DNV-GL in Oslo evaluated and verified the conceptual design, and we received the final acceptance letter on September 20, 2017.
