UK-based Integrated Design & Analysis Consultants (IDAC) was selected by Whittaker Engineering to provide design and analysis services when cracks along the gusset/rail weld connections on the North Sea floating, production, storage and offloading (FPSO) Turret were evident.
The scope of the work included assessing the cracks and suggesting remedial work for the problem.
Whittaker also noticed considerable wear on the top and bottom gripping surfaces.
IDAC along with Whittaker undertook a Finite Element Analysis (FEA) work using engineering simulation software called ANSYS to verify the repair scheme.
The repair work was divided into three phases.
The first phase of the work included studying the feasibility of replacing the old rail and designing a new one.
IDAC and Whittaker studied the feasibility of replacing the old rail with a new one but due to the operational constraints opted for the second proposal which involved bolting a new replacement rail onto the existing rail.
Using the FEA and ANSYS software, the companies evaluated the retrofit design and also removed the old and worn parts.
The second phase of the work involved maintaining the continuity of the segmented replacement rail to avoid any uneven deformation and to aid smooth movement of the gripper.
To ensure smooth movement without any uneven deformation the companies included a tight fit pin which would help transfer the movement to the adjacent rail evenly and smoothly.
The final phase of the repair scheme work involved addressing the cracks at the gusset plate/ rail weld connections.
IDAC suspected the gussets were originally added as positioners to get the existing rail in place with the horizontal plate for welding. The cracks would lead to uncontrolled corrosion if not addressed.
Two solutions were suggested to repair the cracks. The first one involved trimming the gusset plates and re-welding at the joints with the old rail.
The second option included cutting the existing, vertical weld from the top of the gusset plates to the mouse hole and redressing the weld between the gusset and rail to completely remove all the connectivity between the gusset plates and old rail.
The second option was chosen based on the ANSYS.