TOWER MOORING

 

Definition

A rigid tower which is fixed to the seabed and serves as a permanent mooring point for tankers and FPSOs.

 

Components

  • Tower

  • Mooring connection

  • Product transfer system

 

Functioning

The tower is a rigid structure that is connected to the seabed. A turntable is fastened to the tower with a roller bearing, allowing the vessel to weathervane freely around the tower. The connection between the turntable and the vessel can be established in various ways. These are described below.

Product is transferred from a geostatic location on the seabed, e.g. a PipeLine End Manifold (PLEM) through risers, which are connected to the tower. From the risers, the fluid flows through a swivel to the jumper hoses, which connect to tower piping to the FPSO. The fluid swivel allows flow between the geostatic parts and the rotating parts.

On top of the tower there is ample deck space available for manifolding, pigging and auxiliary equipment.

 

Implementation

 There are three different systems to connect the offloading vessel to the tower:

         Yoke system

A yoke is the connected to the turntable with special joints to allow the vessel to pitch and roll. The yoke includes a large ballast tank filled with water to provide the necessary restoring force to minimize vessel motions. The two mooring links suspend the tank from a support structure mounted on the vessel.

Submerged yoke system

This system is similar to the regular yoke system, however the yoke is completely submerged, minimizing the effect of waves and/or ice forces. Also, because the yoke connection is not at the top of the jacket structure, but close to the bottom, there is a large reduction of the overturning moment. This allows a much simpler solution for the foundation and the tower design.

Disadvantage is that there is no easy access from the vessel to the tower structure anymore.

 

 

 

 

 

 

 

 

 

 

 

Hawser system

This system is also similar to the regular yoke system, but the yoke is replaced by a hawser arrangement. This makes the system much less complicated and thus cheaper. This system can easily be disconnected and therefore it is often used for the mooring of shuttle tankers. It is not very often used for FPSOs. Fluid transfer from the tower to the vessel cannot take place through jumper hoses anymore, but submarine hoses are needed.

 

 

 

 

 

 

 

 

 

Advantages

  • Only minor modifications to the vessel are needed

  • All mechanical equipment is located above sea level (not with submerged yoke system)

  • The system is applicable in very small water depths, where it is not possible to use other systems

  • No submarine hoses are needed, but fluid can be transferred through jumper hoses from the tower to the tanker (not with hawser system)

  • For the yoke system, direct access is provided from the vessel to the tower structure

 

 

Characteristics

 

Climate

Tower mooring systems are suitable for applications in shallow and medium range water depths with high currents.

Number of risers

Suitable for many risers.

Depth

Applicable in very shallow up to medium range water depths

Construction / Installation

Tower yoke mooring systems can be mounted on the bow or the stern of converted tankers or new-built vessels relatively easily.

Costs

Largely dependent on the size of the jacket structure. Very cost-effective in shallow waters.

 

Offloading

 

Tower mooring systems can accommodate multiple gas or fluid paths together with electrical connections to the sub sea piping.

Conversion

This system very well suitable for conversion.