spread mooring systems

GENERAL FEATURES:

· Uses traditional shipboard mooring equipment

· Turret structure and bearing are not needed

· Fluid and gas swivels are not needed

· Electrical power and control swivels are not needed

· Easily accommodates a large number of risers and umbilicals

 

A typical mooring system utilizes a set of anchor legs, normally arranged in a symmetrical pattern, attached to the bow and stern of the vessel. This style of mooring maintains the vessel on location with a fixed heading. Thus, its application is dependent on a site where the prevailing severe weather is highly directional.

 

The spread mooring system seems to be the simplest way of mooring an FPSO. The system consists of mooring lines attached somewhere to vessel. These connections can be relatively simple because the overall system does not allow the floater to “weathervane”. To weathervane means that the ship can rotate in the horizontal plane (yaw) into the direction where environmental loading due to wind, waves and currents is minimal.  

Conversions to the vessel do not need docking, which saves a lot of time and money.

The lack of weathervane capabilities means an increase of environmental loads, and thus an increasing number of mooring lines and / or line tension. This may result in the fact that alterations to the hull of the vessel for attaching the lines can become a very cumbersome operation.

Spread moorings can be used in applications requiring long service life, in any water depth, and on any size of vessel. Since the vessel is held essentially at a constant heading, the requirement for a turret structure with a large capacity slewing bearing, as well as the associated swivel stack for fluid, gas, chemical, hydraulic power, electrical power and control transfer is not needed. Virtually all types of anchor leg configurations can be used with this system. Typically, risers and control umbilicals are located amidships on both sides of the vessel. This arrangement provides ample room to accommodate a large number of risers and umbilicals. In applications with highly directional prevailing weather, the shuttle tanker offloading facilities are typically located at the stern or the bow; other deepwater applications may require a dedicated buoy terminal for cargo transfer.

 

In the mooring systems site the different kinds of moorings have been explained. Read more

 

Riser and mooring requirements in deepwater

 

In deepwater applications, the selection of both the riser and mooring system represents a significant factor in the overall feasibility and cost of the system. Therefore, methods for reducing the riser and mooring design requirements and minimizing the cost of these systems become increasingly important.

The use of spread moored floating, production, storage, and offloading vessels (FPSOs) with steel catenary risers (SCRs) are being favoured for deepwater applications with mild environmental conditions such as West of Africa. For these proposed applications, SCRs are hung from a spread moored FPSO along the side of the vessel’s hull or at the stern of the vessel with the first and second order motions of the vessel governing

the design and configuration of the risers system.

 

The spread moored FPSOs with SCRs have a number of drawbacks, some of which are outlined below:

• The dynamic motions of the FPSO are transferred directly to the SCR at the hang-off positions. This leads to fatigue life issues in the riser touchdown point on the  seabed, which govern the cross-sectional design of the pipes.

• With increasing water depth, the length of mooring line required and the cost of the mooring system also increase; hence there are significant savings to be made by optimization of the mooring system.

• The potential for interference between the SCRs, mooring lines of the FPU, and mooring lines of neighboring platforms. A typical FPSO is the centerpiece of a deepwater field with wellhead platform and offloading buoy mooring lines in close

proximity.

• The minimal ability for a spread moored FPU to weather vane into the direction of the environment if the dominant environmental conditions are on the side of the vessel.

• Long installation times for SCRs, resulting in a large delay between the arrival of the FPU on site and the first oil date.

(Journal of Offshore Mechanics and Arctic Engineering NOVEMBER 2004, Vol. 126 / 273)

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