The single point mooring (SPM) buoy consists of a cylindrical shaped hull buoy that is permanently connected to the seabed by means of multiple mooring chains. The buoy contains a bearing system or turntable which allows a part of it to rotate around the moored static part. When moored to this rotating part of the buoy with a mooring connection, the vessel is able to freely weathervane around the static part of the buoy.
As the moored vessel rotates itself into the environment direction, the system will minimise the loads on the mooring system of the buoy. This capability of the SPM system is also used during the approach of a offloading vessel. The vessel will approach the buoy with its bow into the dominant environment, maximising control while minimising the need for tug assistance.
The mooring system (the lines and anchor points) is always specifically designed to match the vessel’s requirements and local environmental conditions.
The mooring system can also be combined with a fluid transfer system that enables connection of (subsea) pipelines to the tanker. The fluid transfer system includes submarine hoses between the pipeline end manifold (PLEM) at the seabed and the buoy, and hoses between the buoy and the tanker. In the buoy a swivel provides the fluid transfer path between the geostatic part and the rotating part of the buoy.
The type of bearing used and the split between the rotating and geostatic parts determine the concept of the buoy. The following concepts are distinguished: bogey wheel buoy, turntable buoy and turret buoy.
The turntable buoy is a further development of the bogey wheel buoy (first design for an SPM developed around 1958) to eliminate the horizontal loading of the rotating frame on the swivel bearing. This was achieved by supporting the rotating frame by its own large-diameter 3-race roller bearing.
The turret buoy is a further development of the turntable buoy with some major adaptations. In the turret buoy design the geostatic part is limited to a turret assembly consisting of a turret shaft and a chain table fitted in the centre line of the buoy body. The buoy body can rotate around this turret via a 3-race roller bearing.
This concept allows the bearing and all other mechanical equipment to be located inside the buoy body where they are protected against the salty environment and seawater. The space inside the buoy can easily be increased by a spacious deckhouse fixed on top of the buoy body.
SPM systems are typically designed for nearshore applications with water depths starting from 15 metres. The mooring connection between the buoy and the vessel depends on local environmental conditions and requirements for the permanent mooring of the vessel. The mooring connections use the following systems: hawser, rigid arm and soft yoke system.
The single point mooring buoy with a hawser system includes a large-diameter polypropylene line between the single point mooring buoy and the vessel. This system requires no modifications to the vessel hull structures and can use the standard mooring strong points at the bow of the vessel.
The highly flexible hawser system is suitable for vessels of different sizes and displacements, and for short duration mooring as well as for semi-permanent mooring. In the case of semi-permanent mooring, redundancy in the hawser system is provided by dual lines. To avoid buoy-kissing between the vessel and the buoy, thrust astern is required by the vessel or by tug assistance. This is especially urgent when the environmental conditions suddenly change in direction or when these are very mild allowing the vessel to drift towards the buoy.
Typically the hawser line length varies from 40 to 100 metres, depending on local environmental conditions. On the vessel side, the hawser includes a chafe chain arrangement to protect it against excessive damage from contact with the vessel bow. On the buoy side, the hawser is connected to a load pin by means of special shackles. The load pin measures the load in the hawser during the mooring. This enables the operators to record load history and the remaining lifetime of the hawser in order to ensure its timely replacement.
The hawser mooring system can also be combined with a fluid transfer system that enables connection of (subsea) pipelines to the midship manifold of a conventional tanker. The fluid path between the buoy and the tanker is realised by means of one or more floating hoses to the tanker standard midship manifold. To protect the hoses, they are provided with a single or double acting marine break-away coupling.
The single point mooring buoy with rigid arm system includes a rigid steel frame that is connected between vessel and buoy by means of large hinges.
This requires a dedicated vessel as the hull structure is modified to support these hinges. The hinges reduce the forces on the rigid arm as both the vessel motions and the buoy motions are not fully coupled. The rigid steel frame keeps the distance between the vessel and buoy fixed, allowing permanent mooring of the vessel without the need of thrust astern.
The rigid arm system can also be combined with a fluid transfer system. The fluid path between the buoy and the dedicated tanker is realised by means of jumper hoses.
The single point mooring buoy with a soft yoke system includes a rigid steel frame that is connected to the buoy by means of hinges and to the vessel by means of a pendulum structure.
This requires a dedicated vessel as its hull is modified to support the pendulum structure. Typically the pendulum structure includes two pendulums that provide a restoring force when the vessel moves away from the neutral position relative to the buoy. This restoring force maximises the range of local environmental conditions in which the SPM system can moor a vessel.
Typically the soft yoke consists of a ballast weight attached to a chain or a rigid steel member with uni-joints on both ends.
The soft yoke system can also be combined with a fluid transfer system. The fluid path between the buoy and the dedicated tanker is realised by means of jumper hoses.
The SPM buoy hull is cylindrical in shape suitable for mooring offloading tankers, FSO, FPSO or any production assets during exploration phases. The hull is divided into identical watertight compartments separated by a partition wall to act as cofferdam.
A fender skirt surrounds the lower part of the SPM Buoy hull. This circular fender, made of heavy duty pipes, is designed to protect the hull against minor accidental collisions with work vessels and the bow of offloading tanker.
The skirt contains the six chain stoppers housings. Self-aligning chain stoppers for chains are designed to allow for the chain stopper to align itself according to the chain angle and therefore reduce the risk of Out-of-Plane Bending (OPB) failure of the mooring chain. Self-lubricating plain bearings are inserted at the chain stoppers trunions to ease the rotation and reduce the friction. Chain stoppers are of ratchet type providing easier hook-up of the SPM Buoy to the chains legs during offshore installation.
Bollards are located on top of the hull to facilitate the installation of the SPM Buoy by providing strong and reliable points. Towing padeyes are fitted at the bottom circular fender level to allow for wet-tow of the SPM Buoy to the installation site.
The turntable center structure is designed to transmit the load created by the moored vessel through the mooring hawser arrangement. The turntable is connected to the mooring hull via plain bearings designed against the hawser loads induced during the maximum operating environment. The moored vessel is able to safely weathervane around the SPM Buoy and align herself with the prevailing environment while remaining moored to seabed with suitable mooring chain legs. This solution enables the limitation of mooring forces on the SPM Buoy to their minimum. A locking mechanism is provided on the turntable to facilitate installation and maintenance operations. An air winch with a hook installed on the ballast arm of the turntable will be provided to retension the mooring chains if required.
The SPM turntable is connected to the main hull with low friction plain bearings guaranteeing optimal life expectancy.
A rotation test shall be performed and validated demonstrating the integrity of the main bearing.
A system of seals will prevent water ingress, and will contain the lubricant grease inside the MSRB. Holes will be available in order to take samples of used greased in order to analyze them to detect any abnormal situation. The MSRB is mounted on top of the stationary - rotating system to allow easy access for regular/planned visual inspections. The MSRB is encapsulated inside a tight cover to avoid contact with sea water. In addition, a system of water barriers is added. The vertical face of the inner ring must be protected against corrosion by a special coating to be proposed by the VENDOR.
VENDOR shall provide details regarding the MSRB lubrication during initial startup and maintenance in operation.