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  1. IDAC
25 March 2009

Case Study: FPSO Turret Rail Gripper Analysis

Whittaker Engineering, engaged to design and manufacture a new set of grippers for the anchor turret of a floating, production, storage and offloading vessel (FPSO), required the consultancy support services of IDAC to assist with the modelling and analysis of the existing anchor turret rail and new gripper assembly.

The FPSO anchor turret project

The FPSO anchor turret, which is rotated to prevent wind-up of the anchor wires as the vessel weather vanes, incorporates a 22m diameter, 50mm thick circular steel rail with four grippers positioned around its circumference. The grippers, when energised with a clamping force of up to 1500Te each, transfer the 300Te jacking cylinder tangential loads into the rail to rotate the turret.

Other similar rail designs have shown evidence of in-service weld cracking; consequently, this FPSO turret rail had been fabricated and stiffened with radial and circumferentially aligned plates.

Whittaker Engineering needed to know how the existing rail, distorted slightly during fabrication, would operate with the new gripper design and what stress levels could be expected within the rail and its stiffening under various loading conditions.

A 22m diameter steel rail is not suited to easy prototyping so finite element analysis offered a cost-effective and accurate method of analysing different model types and load configurations.

CAD models

A series of rail models were created 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.

Further models were created in each case to determine the effect of the radial and circumferential gussets and worst-case scenarios where the gripper was positioned over the most severe warping. The gripper was fully modelled and attached to the rail using a series of contacts, including details of bonding, friction and sliding behaviour. The defined loads of self-weight, clamping and tangential jacking loads were then applied, individually and in combination.

Results indicated that the tangential side loads from the jacking cylinders significantly raised the rail stresses. As expected, placing the gripper over the most extreme warping produced the highest stresses of all load orientations. While the rail warping associated with the fitting of the added plate work was contributing to high localised material stresses, a clean rail without intermediate stiffening was also shown to suffer higher stresses and deflections in known problem areas.

Cost benefits

Whittaker Engineering had to report back to their client with final recommendations for the existing turret rail design and future in-service monitoring. This required knowledge of how the rail was responding to load application in differing gripper orientations and the effect of practical plate-work changes.

The cost and time-scale of prototyping these load scenarios would have been prohibitive. Using IDAC and ANSYS finite element analysis, Whittaker Engineering were able to gain these insights in just a few weeks and at a fraction of the cost. These results provided an excellent overall picture of the behaviour of the system.

ANSYS DesignSpace

Whittaker Engineering are already users of the ANSYS DesignSpace program. This advanced design and verification software allows quick and accurate analyses of component and assembly structural performance. The rail analysis however required more complicated contact and constraint issues so the services of IDAC and ANSYS Mechanical were a logical choice. IDAC supports, trains and offers consulting services with the full range of ANSYS products so understands the best approach to different analyses.

Established in 1983, Whittaker Engineering provides quality engineering design and build services to the offshore oil and gas industry.

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