The two governing fluid transfer systems between a turret and an FPSO are the swivel system and the drag chain. The swivel system consists normally of a vertically stacked layer of swivel elements positioned in the turret center, which allows the vessel to rotate freely around the turret without limitations. By contrast, the drag chain system employs flexible hoses to impose a limited rotation of the vessel.
Maritime Tentech's new transfer system consists of flexible hoses - the illustration shows the system with 11, 12-in. ID hoses (as per Kristin requirement).
Maritime Tentech has now introduced an alternative technique called the Tentech drum transfer system (TDTS - patent applied), which uses flexible hoses to transfer hydrocarbons between the turret and the vessel. These hoses are connected to hard pipes on the turret and also on an adjacent, surrounding platform on the vessel's deck.
Several hoses can be bundled together of equal or varying diameters. The bundle is built into a supporting rack where it is arranged vertically. This rack is flexible, and can easily be curved horizontally to the hose's minimum bending radius. It takes all the tension in the system, allowing the hoses to elongate within the rack. The hose rack slides onto the platform and turret decks by means of low-friction pads.
A curved guide drum on the turret rolls up the rack with flexible hoses as the vessel rotates in one direction around the turret. When the vessel rotates in the opposite direction (back towards neutral), the winch pulling the rack will assist roll-off from the guide drum. Winch tension will be adjusted in order to maintain the rack curvature around the turret guide drum by tensioning the flexible rack.
The guide drum can roll up the rack in both rotational directions from its initial neutral heading/position. Drums will be designed to fit the hose's diameter, either as a single or twin-transfer system. Either can be designed also for drilling operations. The guide drum is located on the turret's edge such that when the rack curves around it, the main part of the rack rests on the platform. Weight on the turret is then kept to a minimum.
Same configuration as per Kristin requirement, but in 225deg position.
In an emergency, the hoses can be disconnected from the turret. Flexible hoses and umbilicals will be disconnected on the turret side of the hose rack, transferred off the turret, and supported on the surrounding platform. Each hose may also be replaced without interfering with production through other hoses.
Compared with a traditional drag chain system, the main difference of the TDTS is that the hose spooling direction from the termination point on the turret can be both clockwise and counter-clockwise, owing to the guiding drum arrangement. Also, the platform deck mainly takes the weight of the rack with hoses on the vessel, not on the turret, as is the case with drag chains. Advantages claimed for the TDTS over drag chain systems are:
- Up to 60% reduced hose lengths
- Up to 50% reduction in turret transfer deck diameter
- Increased relative rotation limitation between vessel and turret - >400° for a single system, and >260° for a twin system - improving operability
- Very low sensitivity to minimum bending radius of the hose
- Very low sensitivity to relative rotation between turret and vessel
- Low sensitivity to numbers of hoses due to possibility of installing the TDTS twin system
- Minimum disconnect/re-connect time.
Compared with a swivel system, the TDTS has the advantages that:
- Hoses may be replaced during production
- Allows sand production
- Unlimited capacity of power and signal transfer
- Less flow manifolding on the turret
- Allows reliable high pressure and temperature loops since no dynamic seals are required.
If extended radius bends are to be employed, allowing for pigging through the transfer system, the TDTS design lends itself to this feature due to the central location of the hose termination point relative to the turret center and riser locations. This allows for minimal directional and elevation changes in the pipe routing to the risers, thereby leaving a central internal area free of piping for riser pull-ins.
On the vessel side of the TDTS, pigging is undertaken either through temporary or permanently installed extended radius spool pieces placed downstream of the branch connections to the manifold. The area requirement for the hose termination and hard pipe interface at the vessel side is what defines the overall boundary of the TDTS. Manifolds and flowlines can be accommodated in the area below the transfer deck.
A qualification program for the TDTS has recently been performed in cooperation with Saga Petroleum, with a study report already delivered. The base case was to apply a TDTS onto the planned Kristin Field FPU (a Tentech vessel) in the mid-Norwegian Halten Bank. Comprehensive analysis of the TDTS system was performed in this study.