Coastal Refinery Limited
Single buoy mooring (SBM) or Single point mooring (SPM) is a loading buoy anchored offshore that serves as a mooring point and interconnect for tankers loading or offloading gas or fluid products. SBMs are the link between the geostatic subsea manifold connections and the weathervaning tanker. They are capable of handling any size ship, even very large crude carriers (VLCC) where no alternative facility is available.
Parts of Single Buoy Mooring
There are four parts in the total buoying system: the body of the buoy, mooring and anchoring elements, product transfer system and other components.
The buoy body usually is supported on static legs attached to the seabed, with a rotating part above water level connected to the (off)loading tanker. The two sections are linked by a roller bearing, referred to as the "main bearing". The moored tanker can freely weathervane around the buoy and find a stable position due to this arrangement.
Mooring and Anchoring Parts
Moorings fix the buoy to the seabed. Buoy design must account for the behaviour of the buoy given applicable wind, wave and current conditions and tanker sizes. This determines the optimum mooring arrangement and size of the various mooring leg components. Anchoring points are greatly dependent on local soil condition.
* Anchors or piles to connect the mooring to the seabed.
* Anchor chain
* Chainstoppers to connect the chains to the buoy
A tanker is moored to a buoy by means of a hawser arrangement. Oil Companies International Marine Forum (OCIMF) standards are available for mooring systems.
The hawser arrangement usually consists of nylon rope, which is shackled to an integrated mooring uni-joint on the buoy deck. At the tanker end of the hawser, a chafe chain is connected to prevent damage from the tanker fairlead. A load pin can be applied to the mooring uni-joint on the buoy deck to measure hawser loads.
Hawser systems use either one or two ropes depending on the largest size of vessel which would be moored to the buoy. The ropes would either be single leg or grommet leg type ropes. These are usually connected to an OCIMF chafe chain on the export tanker side (either type A or B depending on the maximum size of the tanker and the mooring loads). This chafe chain would then be held in the chain stopper on board the export tanker.
A basic hawser system would consist of the following (working from the buoy outwards):
Buoy Side Shackle and bridle assembly for connection to the padeye on the buoy; Mooring Hawser Shackle; Mooring Hawser; Chafe Chain Assembly; Support Buoy; Pick-up / Messenger Lines; Marker Buoy for retrieval from the water.
Under OCIMF recommendations, the Hawser Arrangement would normally be purchased as a full assembly from a manufacturer.
Product Transfer System
The heart of each buoy is the product transfer system. From a geostatic location, e.g. a pipeline end manifold (PLEM) which is located on the seabed, this system transfers products to the offtake tanker.
The Basic product transfer system components are:
* Flexible subsea hoses, generally referred to as “Risers”
* Floating hose string(s)
* Product swivel, valves and piping
The risers are flexible hoses and connect the subsea piping to the buoy. Configuration of these risers can vary depending on water-depth, seastate, buoy motions, etc.
The product swivel is the connection between the piping in the geostatic parts and the rotating parts of the buoy. Product swivels range in size depending on the size of attached piping and risers. Product swivels can provide one or several independent paths for fluids, gases, electrical signals or power. Swivels are equipped with a multiple seal arrangement to minimise the possibility of leakage of product into the environment.
Floating hose string
Floating hose string(s) connect the buoy to the offloading tanker. The hose string can be equipped with a breakaway coupling to prevent rupture of hoses and subsequent oil spills.
Other Components of SBMs are:
* Boat landing, providing access to the buoy deck
* Fendering to protect the buoy
* Lifting and handling equipment to aid materials handling
* Navigational aids for maritime visibility
* An electrical subsystem to enable valve operation and to power navigation aids or other equipment.