Dynamic Analysis on Failure Modes of Tub Mounted Cranes

The trend in Offshore Engineering is towards exploring in deeper waters and more harsh environments. As a consequence, topsides become larger and heavier. In order to keep up with the demand for more lifting capacity, Heerema announced a New Semi-submersible Crane Vessel (NSCV). This vessel will contain two 10 000mt Tub Mounted Cranes (TMC), which will be constructed by Huisman. In order to ensure safety, some level of redundancy has been implemented in the crane’s hoisting systems. However, it was found that there was insufficient knowledge about the consequences of a wire failure in one of the hoisting systems. In this thesis three possible failure cases were investigated: boom hoist cable failure, main hoist cable failure and a drop of the load. In addition, this thesis also looks at possible ways to reduce the dynamic effects of a wire failure. Lagrange’s equations were used to derive the equations of motion of the crane and the main hoist lower block. Using these equations of motion a dynamic model was created in MATLAB, using an Ordinary Differential Equation (ODE)-solver to solve the equations of motion. For all three cases animations were created in order to provide a visual validation of the models. Additional validation was performed by a comparison with results obtained using simpler models with only one or two degree(s) of freedom. Parameter studies were performed on all three cases and different scenarios that could occur. For the boom hoist failure case the conclusion is drawn that none of the investigated parameters have a significant influence on the dynamic overshoot that occurs when one of the two wires fails. The overshoot is governed by the inertia of the load and boom. This is not the case for main hoist failure, where the geometry of the main hoist block had a large influence on the resulting force in the wire. Other parameters that influenced the results of the analysis were the initial length of the main hoist system and the stiffness of the rigging between the hook and the load. With the current design the risk exists that when wire failure happens, the other wires will not be able to cope with the dynamic overshoot and the system will fail. However, it is unlikely that wire failure will happen due to overload in the normal operating case, as a safety factor of three is applied. The third case, a drop of the load, proved to be the least severe case for the wires. Stress waves were witnessed in the results; however the effect of these were not significant. Even with the effects of stress waves taken into account the force in the wires remained below the initial value with the load still suspended from the crane. Further research on this case should focus on the bending of the boom. ?? ?Lastly, the influence of implementing a shock absorber in either the boom or the main hoist system was analyzed. For the boom hoist system the improvements were minimal, and the constant interaction of a damping system is undesired, which leads to the conclusion that it has no further potential. Implementing a shock absorber in the main hoist system resembles much of a Passive Heave Compensator (PHC) and could potentially improve the system. However, current PHC’s do not have the right parameters to have a significant influence. The main reason for this is that the influence of the compensator is divided over many falls, which suppresses the influence. Further research on this subject should focus on the behavior of the sheaves and falls in the system for two reasons: first, for determining the time it takes for a failing wire to un-reeve and lose its carrying capacity; second, in order to determine the effects of the reeving in a system with a shock absorber.