Investigation into crawling behaviour of a friction based reverse float-over operation

Heerema Marine Contractors (HMC) is a world leading marine contractor in the international offshore oil and gas industry. HMC specializes, among other activities, in the decommissioning of offshore located oil platforms. Current methods exist for safely removing topsides and substructures, yet research into new and cheaper ways is continuously done at HMC. Common practice now is to remove the topside with the use of a Semi-Submersible Crane Vessel (SSCV) and lift the topside onto a barge. With a new method, a reverse float-over operation, the SSCV would not be needed at all for this procedure.This operation has not been performed before. The set-up for this research is a barge which will connect to the topside due to friction forces on the interface between these two bodies. The use of stabbing cones or clamps would increase the horizontal station keeping of the barge beneath the topside during this operation. As this would require a lot of work being done on the topside before this operation can take place they are not used in this research. Therefore it was chosen to have the barge stick to the topside due to friction. In computer simulations a model of the H-851, a HMC owned barge, is located beneath an earth fixed topside. The deballasting of the barge is simulated by applying an external lift force on the centre of gravity of the barge. External wave forces are acting on the barge. When the barge is being pushed against the topside, friction forces allow the barge to crawl beneath the topside due to a stick slip mechanism. This thesis looks into the crawling behaviour by making a new friction model. A simplified model was created to improve the understanding of the friction forces between 2 bodies. A sensitivity study has been performed to analyse the crawling behaviour of the barge as a function of five parameters: Deballast velocity, vertical interface stiffness, horizontal interface stiffness, horizontal interface damping and coulomb friction coefficients. This is done by defining a base case and changing parameters to check the influence on the crawling behaviour.Crawling behaviour is observed during two phases: The initial contact phase, where the barge alternately makes contact with the topside, and the compressed interface phase, where the barge is in constant contact with the topside. As waves travel in positive x direction, the barge shows crawling behaviour in negative x direction in the initial contact phase. In the constant compressed phase the barge shows crawling both in negative and positive x direction. During the initial contact phase more friction is applied when the barge moves in positive x direction.Optimisation runs have shown the influence of the 5 parameters on the crawling behaviour of the barge. Increasing the horizontal damping reduces the crawling distance of the barge. When the friction coefficients are increased the barge reaches a stick condition sooner. With higher coefficients there is also more crawling behaviour in negative direction. Increasing the deballast velocity reduces the crawling distance of the barge. Varying both the horizontal and vertical interface stiffness changes the dynamical behaviour of the barge, and thus also the crawling behaviour.The case used in this research is a simplified case. Further research into the crawling behaviour, and different behaviour, is needed to model a reverse float-over operation.