UK-based Integrated Design and Analysis Consultants (IDAC) undertook an analysis on FPSO turret rail gripper to find out how the existing rail will operate with the new gripper design.

The FPSO anchor turret includes a 22m diameter, 50mm thick circular steel rail with four grippers positioned around its circumference and is rotated to prevent wind-up of the anchor wires as the vessel weather vanes.

The grippers can transfer the 300Te jacking cylinder tangential loads into the rail to rotate the turret, once it is energised with a clamping force of up to 1500Te each.

During the test, IDAC examined how the existing rail, distorted slightly during fabrication will operate with the new gripper design.

The test also showed the amount of stress levels could be expected within the rail and its stiffening under various loading conditions.

IDAC created a series of rail models to cover a range of design possibilities.

Measurements of the rail had shown warping in two planes due to weld shrinkage around the gusset plates.

IDAC created solid CAD models to represent a perfectly circular rail as well as a warped one.

The test revealed that the tangential side loads from the jacking cylinders significantly raised the rail stresses.

It also showed that placing the gripper over the most extreme warping produced the highest stresses of all load orientations.