CFD of lifting a submarine (DSRV) out of moonpool.
This blogpost shows how computational fluid dynamics (CFD) is used to reduce the risk during a complex lifting operation by simulating the transient hydrodynamic forces.
Which crane and vessel do you use?
An innovative way to lift heavy objects out of the water is to use a moonpool, which essentially is a vertical tunnel in the middle of the ship. The advantage of a moonpool lift relative to a conventional lift is that the vertical force acts close to the centre of gravity, and thus generates much smaller pitch or roll moments to the vessel. Consequently, one can use a smaller vessel. The requirement on the crane will depend on the maximum forces that acts on the object during the lifting operation.
Computing hydrodynamic forces during a moonpool lift
There is a challenge with pulling a complex geometry out of a narrow water column, especially when the whole ship is moving with the waves. The challenge is that there is no theory that can analytically estimate the forces. In this particular case it was necessary to combine Orcaflex simulations, where the sea state and vessel motions were computed, with Star-CCM+, where the water and air flows around the complex geometry was computed. The result was a transient simulation of the submarine moving through the moonpool and out of the water. Three main phenomena contributes to the hydrodynamic force :
- The “vacuum effect” that occurs as the submarine moves faster than the water is able to fill the room behind it.
- The “lifting effect” that occurs as the submarine is pushing water in front of it when it moves relative to the water surface.
- The viscous force that arise from the water flowing down the sides of the submarine.
Point 1 and 2 are what is usually refered to as added mass, while point 1 are usually termed drag. All of these effects will be present while the submarine is submerged, but it is especially at the critical stage where the submarine passes through the water surface that they really become important. Anyone who have lifted something out of the water can recognize the challenge of estimating the forces that occur at that point. Had the water surface been still, the dynamics could have been avoided by reducing the lifting speed sufficiently that the process became quasi static. However, due to the ocean waves, the water surface is in continuous vertical motion relative to the moonpool.
The plot below shows the vertical force acting on the cranes as the submarine is pulled out of the moonpool during the worst acceptable sea state. Note that the peak force (at time=1s) is about 4 times higher than the static weight (at time=30s). The video below shows how the water flows in the same case.