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  1. IDAC
21 January 2010

IDAC Conducts Design and Analysis of FPSO Turret Rail Repair Scheme

Founded in 1983, the last 25 years has seen Whittaker Engineering develop into a respected and reliable contributor to the North Sea oil industry, specialising in the design, manufacture, installation and maintenance of FPSO turret turning systems.

As one of the few specialists in this area, Whittaker Engineering’s solutions are the result of an evolution in design from existing systems. From original concept through to offshore maintenance, their designs are engineered to fit the onboard working practices and procedures, delivering tough, reliable and cost-effective solutions.

North Sea FPSO turret project background

A floating production, storage and offloading vessel (FPSO) is a type of floating tank system used by the offshore oil and gas industry, and designed to take all of the oil and gas produced from a nearby platform, process it, and store it until the oil and gas can be offloaded onto waiting tankers or sent through a pipeline.

The FPSOs are not fixed permanently to the seabed but are designed to be moored and remain on station for long periods of time. Highly sophisticated mooring systems have been developed, which enable FPSOs to operate safely and reliably.

In areas where weather conditions can be extreme, vessels have a central mooring arrangement located within the hull in a turret. The turret is controlled by the ship through a system of 4 x 300t jacks and 4 x 1,000t grippers. The grippers grip onto a rail, which runs round the turret, and the jacks turn the turret relative to the ship.

In extreme circumstances, the grippers must be capable of turning the ship around the turret, although for normal duties they are used to overcome friction in the turret bearings and prevent mooring chain and riser wind-up.

Whittaker Engineering was required to carry out an assessment and subsequently some remedial work on the North Sea FPSO turret. Cracks were seen propagating along the gusset / rail weld connections and there were concerns over how long it would be before the crack travelled further. In addition to this, some considerable wear had developed on the top and bottom gripping surfaces.

Finite element analysis

IDAC was required to provide FEA work in order to verify the repair scheme. Three different phases to this work were considered as follows:

  • Design and analysis of a new replacement rail
  • Maintaining the continuity of the segmented replacement rail
  • Addressing the cracks at the gusset plate / rail weld connections

Design and analysis of a new replacement rail

Replacing the old rail was not an option due to operational constraints. Whittaker Engineering instead proposed that a new replacement rail be bolted onto the existing rail. A finite element analysis (FEA) using ANSYS was conducted to evaluate the retrofit design of the bolting arrangement between the existing and new rails in the FPSO turret rail.

Worn out parts of the rail were removed, and the new rail was added in small segments and bolted onto the old rail. The number, size and location of the bolts were designed and verified by the use of FEA. An optimum design of eight bolts per gusset plate sector was derived. The location of the bolts was also important because of the peel-back action due to the large moments on the rail introduced by the weight and jacking force of the gripper.

Maintaining the continuity of the segmented replacement rail

As the replacement rails are made up of segments, any uneven deformation of the adjacent rail segments due to the varying gripper loads would hinder the movement of gripper over the segmented replacement rail.

The solution was to include a tight-fit pin, which would transfer the deflection / movement to the adjacent rail. This was modelled in ANSYS and found to be a very acceptable solution. The figure to the right shows the pin being positioned between the two segments of the replacement rail during a test.

Addressing the cracks at the gusset plate / rail weld connections

It was suspected that the cause of these cracks was due to the peeling of the rail, and it further raised the question as to why the gussets were there in the first place. It was suspected that they were originally added as positioners to get the existing rail in place with the horizontal plate for welding. The weld between the horizontal plate and the rail is the most important connectivity.

A strategy was required to address the cracking problem, which gives rise to uncontrolled corrosion if left unattended. As such two options were suggested and analysed for.

Option 1 involved trimming back the gusset plates to the dimensions indicated by Whittaker Engineering and to re-weld at the joints with the old rail. Option 2 was to cut through the existing, vertical weld from the top of the gusset plates to the mouse hole and to redress the weld between the gusset and rail. This would remove all connectivity between the gusset plates and the old rail.

Both options were analysed using FEA and from the results it was recommended that the existing vertical welds between the gusset plates and the old rail be cut through and the exposed materials be treated for corrosion protection.

FEA design benefit

IDAC was able to carry out many design iterations using FEA in order to establish the repair strategy for the gryphon turret rail. Whittaker Engineering, having worked closely with IDAC, was able to present the recommendations to their client with confidence having gained an insight into the behaviour of the turret rail and adjacent components under various loading situations.

Ken Whittaker, the proprietor of Whittaker Engineering, commented on the results of the study. “We rely on IDAC to produce practical and robust engineering solutions when our designs go beyond the realms of hand calculations. They produce an excellent finished engineering package which gives credibility to our designs and products. Whittaker Engineering have been working with IDAC for many years and they are a pleasure to work with.”

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