Fairleads are used to guide a wire rope or chain from the vessel towards the anchor. They are usually attached to the turret structure to control the mooring lines.
During a routine inspection on the Chevron operated Captain FPSO (floating production, storage and offloading vessel) the company identified an integrity issue with one of the vessels eight fairleads – noting that one of the bolts connecting the gypsy wheel shaft to the skirt of the fairlead was missing. This identification reinforces the importance of Chevron’s rigorous maintenance, reliability and integrity procedures.
All fairleads on the Captain FPSO have since been been retrofitted with an axle pin retaining shaft, designed and supplied by Whittaker Engineering Ltd, which improve the overall integrity of the design.
Whittaker Engineering Ltd required a number of structural analyses to be conducted on the Fairlead design, to investigate the cause of a failure in the bolts and to evaluate the work that would be required to prolong the life of the Captain FPSO Fairleads. Finite Element (FE) analyses were performed on the original and retrofitted designs.
An FE model was created to investigate the cause of a failure in the bolts that hold the gypsy wheel axle to the fairlead skirts. A static nonlinear (frictional contact and large deflection) solution was set up to evaluate the behaviour of the bolts and the resistance of the fairlead spindle to rotate.
The geometry created in Autodesk Inventor, was imported into ANSYS DesignModeler for defeaturing and mesh preparation. The geometry featured the original design and the retrofitted gypsy wheel pin. During the course of the project the chain was also added into the FE model. The chain was modelled from its connection at the tensioner winch to the fairlead (about 40 links), and down through the spindle to the gypsy wheel, finishing at the last chain link that would possibly contact the fairlead skirt.
The models were meshed with high order 3D solid elements and nonlinear frictional contacts were used to simulate the contact relationship between the individual chain links, the chain links and gypsy wheel and finally chain links and fairlead skirts. The welded connections were modelled as contact interfaces using a bonded behaviour.
Two loading scenarios were analysed:
1.Out of Plane Loading: The effect of an out of plane mooring chain force on the bolts and the moments needed to initiate the fairlead realignment (rotation of the fairlead to align itself with the mooring line direction).
2.Chain Twist: The effects on the bolts of a 45deg rotation/twist of the mooring chain.
The original and retrofitted models were both analysed under the same two loading conditions with the aim of comparing the behaviour before and after the repair work.
The chain loadings were associated with a nominal condition in calm sea state determined by Chevron and provided for the analyses. The models were loaded in three stages with the first two stages being the same for both loading scenarios and the last stage distinguishing between the out of plane load and the chain twist, as follows:
1.Initial tension of 95 tonnes was applied to the end half link of the chain in its straight configuration, and the bolts at the gypsy shaft were pre-stressed with a load of 75kN each.
2.The chain was forced to acquire its final in-plane configuration wrapping around the gypsy wheel with an angle to vertical of 30 degrees
3.Out of Plane: the free end link of the chain was displaced out of plane, contacting and pushing sideways at the fairlead’s skirt.
Chain Twist: the top end link of the chain was twisted 45 degrees while the bottom end of the chain was fixed sideways
The chain was fully fixed at the top link and the fairlead was constrained at the ORKOT bearings, the external rings of the bearings were fully constrained and a frictional contact was defined between the spindle and the internal faces of the bearings, a 0.15 friction coefficient was used.
The mooring chain itself was also investigated in order to understand the torsional and bending stiffness behaviour of the chain. Models were created (see graphic top right) to evaluate the influence of the shape of the links and number of links in the chain. Both the torsional and bending stiffness investigations demonstrated nonlinear behaviour that was very dependent on the geometry and number of links, the graphical representation of this can be seen to the right.