Single Point Mooring
In the configuration of single point mooring the ship is moored on only one side of the vessel. This has the main advantage that the is able to weathervane around this point (usually at the bow) and always is in the direction in which the vessel endures the least amount of environmental forces acting on the vessel.
There are multiple SPM systems. The most commonly used systems are the Single Point Mooring (SPM) Buoy System, Tower System and the Turret Mooring Systems. These systems will be explained briefly below.
Single Point Buoy Mooring
The buoy mooring systems consists out of a floating buoy which is moored to the seabed permanently. It can be moored with a single anchor line or with a catenary layout. The buoy has a bearing ring which can rotate around the position of the buoy. When a vessel is connected to this ring it has the ability to turn (weathervane) around this ring. The buoy can be equipped with an fluid transfer system. In this case the buoy is connected to a pipeline end manifold (PLEM) on the seabed and the vessel can connect to this system. There are multiple ways to connect the vessel to the buoy. For instance a rigid arm, which is a rigid steel frame connected to the vessel and buoy with large hinges, a soft yoke, which is a rigid frame connected to the buoy with hinges and to the vessel with a pendulum structure, or a hawser system, which is a large diameter polypropylene line connecting the vessel and buoy.
Single Point Tower Mooring
In the tower mooring system the vessel is connected to a tower which is founded to the seabed by means of gravity or piles. The tower has a bearing system which allows the vessel to weathervane around the geostatic location of the tower. Usually this system is used near shore or at locations with limited water depth. Due to the rigid structure the tower mooring system is well equipped for harsh environments. Typically the vessel is connected to the tower with a hawser system or a soft yoke. In the tower a swivel provides the ability to transport fluids from the subsea to the vessel.
Turret Mooring Systems
A turret mooring system, is a system connected to the vessel. This can be done internally or externally. The systems consists out of a turret connected to the vessel by means of a bearing. In this way the vessel can rotate around the turret to endure the least amount of environmental forces.
Internal Turret Mooring System
The internal turret is integrated in the hull of the vessel and is located more to the center of the vessel compared to the external turret. This way it can withstand higher forces compared to the external turret and can have more risers and more mooring lines.
External Turret Mooring System
The external turret is usually mounted on the bow of the vessel and because of this it will be cheaper to construct. In the design phase no internal space needs to be reserved for the construction and it could even be mounted in a later stage of development.
A downside of this is that the external turret can accomodate fewer risers and umbillicals. In addition to this it can withstand lower environmental forces.
Disconnectable Internal Turret
The internal turrent can be design is such a way that it can be disconnected. This is convenient especially in arctic conditions. Multiple months a year there is a lot of drift ice which can destroy the vessel and risers. In these months the risers and can be shut off and the turret can be lowers to a specific depth at which the ice can't damage the equipment. When the ice has melten again the vessel can return to it's position, the turret can be brought back to the surface and reconnected to the vessel.
