Dynamic Simulation of a Hydraulic Cylinder for Heave Compensation of Deepwater Drilling Risers
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- Institutt for marin teknikk 
This thesis presents global finite element simulations of a hydro-pneumatic riser tension cylinder used for tensioning and heave compensating of marine drilling risers. The global finite element models are made for use on a personal computer, which give some limits in available computational capacity, but convenient for use of the models. The main goal for the simulations is to find the contact forces acting between the cylinder rod surface layer and the bearing rings inside the stuffing box of the cylinder. This information is needed to understand the reasons for the excessive wear of surface layers that has been experienced and lead to costly damages of heave compensation systems. To establish the models a literature study of riser analysis, heave compensation and friction between surfaces modelling is done and described. Measurement of friction forces in order to identify the Stribeck curve for the bearing material has been done on a test cylinder, lubricated with a fire resistant water glycol hydraulic fluid. The friction force is measured as a function of contact pressure and speed at different temperature, which are the parameters needed to define the Stribeck curve for the actual system. The size of the bearing and the difference in friction force between boundary lubrication friction and mixed lubrication friction described by the Stribeck curve, bring in modelling issues for the time domain analysis. The size of the bearing requires small finite elements, which in turn requires very short time increments in order to obtain stable time integration when using an explicit method. The fast variation in friction force gives convergence issues also for the implicit integration method. Computational costs are therefore high for the applied type of simulation. The results show that metal to metal contact between cylinder rod and stuffing box may occur in certain situations even within the design limit of cylinder, due to high contact pressure and bending of the rod. The main contributing factors to this high contact pressure and bending are: • Bending moments in the cylinder and rod occur due to weight of the structure when it has a spatial inclination relative to the vertical. • Lateral accelerations of the cylinder ends due to vessel and drilling riser motions. • Low bending stiffness of the cylinder rod when the riser tension is low.