Published on 22 May 2018 by Brysen Lin
The description of Spread Moored versus Turret Moored Systems are highlighted in terms of design and performance.
|Spread Moored||Turret Moored|
|Vessel Orientation||Fixed heading||3600 weathervaning|
|Field Layout||Prefer flowline arrangement||Fairly adaptable, distributed flowline arrangement|
|Environment||Mild to moderate||Mild to extreme|
|Riser System||Adapt to various riser systems||Location of turret requires special design|
|Riser Number and Arrangement||Can be flexible with additional tie-ins||Expansion is limited and required commitment|
|Vessel Motion||Depend on environment/vessel directionality||Weathervaning reduces motions|
|Vessel Arrangement||Components spread out and requires extensive interface||Compact design and FTL|
|Offloading Capability||Depend on environment/vessel orientation||Flexible/adapatable with changing environment|
|Station Keeping||Required larger number of anchor legs||Smaller number of anchor legs needed|
A spread moored system is moored in a fixed orientation and more sensitive to global environment intensity and direction. The spread moored system is typically installed with its bow towards the prevailing environment. This makes the vessel susceptible to waves incident at large relative wave angles, increasing the probability for substantial vessel motions, especially roll. The spread moored system typically has a larger number of legs with increased component size than an equivalent turret moored FPSO. This results in typically four groups of anchor legs, and coupled with the installation equipment and requirements leading to extensive interfaces between mooring, vessel and topsides engineers. The spread moored FPSO is easily expandable as it is more flexible to riser addition during the life of the field, but prefers a riser arrangement that has the flowlines approaching beam-on to the vessel.
An important aspect to consider with spread moored vessels is the offloading performance of the system over the life of the field. As the vessel orientation is fixed, a tandem moored export tanker has a limited range of relative heading with respect to the FPSO when tandem offloading from the vessel, especially in inclement weather, not directly aligned with the FPSO orientation. In order to maintain the shuttle within this allowable zone additional tug assistance may be required as compared to a similar turret moored system, or a second offloading station may be required at the other end of the FPSO to improve the offloading uptime. The environment mis-alignment with the FPSO orientation can also lead to difficulties in approaching and leaving the FPSO before and after the offloading of product.
In extreme cases where the use of a spread moored system and multi-directional environmental conditions does not provide the desired uptime for tandem offloading, a satellite offloading station may be installed approximately one nautical mile from the FPSO. In deepwater, flowlines are suspended between the FPSO and the remote offloading station (typically a large CALM buoy). This alternative provides a very high uptime due to its single point mooring ability (similar to a turret moored system) but results in much higher CAPEX costs and the risk associated with the reliability of the flowlines.
Turret moored systems orient themselves to the prevailing environment direction that allows its use in harsh, multi-directional environments with minimized loads and vessel motions. A not so obvious advantage of a weathervaning turret mooring system is that as it adjusts itself to the prevailing environment it is not as sensitive to poor design environmental criteria which is common in areas where new development takes place. The turret system provides a compact load and fluid-transfer system with a minimum number of anchor legs required. The weathervaning ability helps provide more constant offloading conditions for export tankers, helicopter operations, and discharges from flares. However, the passive weathervaning ability of the turret mooring system requires the location of the turret at the vessel bow that is the location of the maximum vessel motions and therefore requires a riser system that is robust enough to withstand the motions at the turret. A turret mooring system is not readily adapted to the addition/modification of riser systems (needs to be designed in to the system), and its design and fabrication requires specialized engineering and manufacturing techniques and knowledge.
FPSO units have grown in number, size and sophistication, and the type of mooring system selected for each unit is a key design consideration. Technically, a spread mooring system is a single FPSO sub-system whereas a turret system is an integrated grouping of several FPSO sub-systems. A comparative cost evaluation between the two systems should normalize the inherent differences between a turret and spread mooring system, and also consider how each selection impacts the overall cost of the FPSO, the field layout design, and the development and operations cost and activities.