Definition
Turret is the major component of a vessel or any floating production unit (FPSO, FPO) to which the risers coming from the seabed are connected.
Turret name refers the rotating function of this large equipment to facilitate the rotation of the vessel according to winds, waves and dominating current while the risers remain connected to a fix part on the seabed.
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In relatively calm waters, such as in West Africa, turrets can be located externally to the ship structure, hanging off the bow of the FPSO.
For harsher environments like the North Sea, the turret is generally located internally.
The turrets, mooring lines and risers can be designed to be:
- Permanently connected in order to remain on location for long periods of time
- Disconnectable, capable of quick disconnection through a procedure lasting just a few hours.
Most ship-shaped FPSOs in the North Sea are purpose-built and are permanently moored.
In areas of the world subject to cyclones (northwestern Australia) or icebergs (Canada), some FPSOs are able to release their mooring/risers turret and steam away to safety in an emergency.
The turret sinks beneath the waves and can be reconnected later.
The turret fulfils more functions than weathervaning. It also up lift the oil and gas and associated water and condensates production risers as well as it supports piping for water or gas injection when used.
The turret supports the (multi pass) fluid swivel and forms the interface between sub-sea facilities and topsides.
The electrical power supply of submerged equipment, the control system, all the information and communication systems pass also through the turret.
As all risers pass through the inside of the turret bearing, it is considered as the bottleneck to the number of risers and therefore potential further field production enhancement.
With a turret the offloading can be executed in tandem to the FPSO with shuttle tankers.
This provides a more operational uptime and less risk of tanker collision compared with spread-moored FPSOs.
An FPSO turret requires a highly specialized fabrication.
If the turret is attached to the floating unit, the turret is part of the mooring system and called Turret Mooring system.
Depending on the type of vessel its operations location internal or external turret may be used.
The internal turret is located in the front end of a vessel. Sometimes it can be found in the middle.
As the internal turret is appropriate for a large number of risers, it has good fluid transfer capabilities.
The largest internal systems can accommodate up to 100 or more risers.
Internal turret mooring systems also allow vessels to remain on location permanently.
This holds even in the most harsh environments, as can be seen with the FPSOs in the North Sea.
An advantage of the internal turret in comparison to the external turret is that mooring forces can be transferred more easily into the hull.
The external turret is located outside the ship’s hull.
It comprises a steal box located at the bow or stern of the vessel, which provides a foundation for the bearing and turret.
The external turret is mostly set-up above the waterline, but sometimes also under.
A swivel provides product and utility connections between the tanker facilities and the seabed.
Being positioned outside of the hull the external turret can bear less risers and production equipment than internal turret.
But the design of the external turret make it easier to be mounted on tankers converted into FPSO.
So most of the new-built or purpose-built FPSO for large oil and gas fields will use internal turret while converted FPSO shall be preferred for medium or smaller developments.
