When bulk liquid fuel tankers transport products (refined or unrefined), they need to make use of a short-term mooring system when loading and offloading the product. The mooring system is either a single point mooring system (SPM) or a conventional buoy mooring system (CBM). 

An SPM is anchored, serving its purpose as a single mooring point, but it also acts as a means of connection between loading and offloading tankers. A CBM, however, consists of multiple mooring buoys that can hold a vessel or tanker perfectly in place. 

The buoys

The buoy used in SPMs is a Singebuoy body. Anchor legs extend to the sea bed below to hold it in place and independently moors the tanker or vessel with chains. The chains are relatively flexible, allowing the tanker to weathervane freely, giving it stability in most weather conditions. Above water, the buoy body rotates and is connected to the loading/offloading vessel via a roller bearing. The size of the buoy used will be determined by how much counter-buoyancy is needed and the chains used will depend on environmental conditions and the size of the moored tanker. 

CBMs simply act as mooring vessels with no connection to any loading or unloading components and, therefore, do not assist with transferring refined or unrefined fuel. The buoy’s size is determined by the counter-buoyancy needed. Unlike an SPM, the chains mooring the vessel hold it fast and do not allow for any weathervaning. 

Mooring components

In order to hold the vessels in place, buoys also need to stay put, and this is achieved by mooring them to the seabed. For SPM buoy bodies, two mooring options are available. The first is Catenary Anchor Leg Mooring (CALM). When CALM is used, anchor points are set up at a distance from the buoy. From there, chains are extended and attached to hold the buoy in place. Alternatively, SPMs can make use of Single Anchor Leg Mooring (SALM). This is where the buoy is moored with a single leg anchored in the seabed. CALM is easier to maintain, so most SPMs make use of this mooring system these days. 

Mooring buoys and mooring legs work together to get the job done for CBMs. Depending on seabed soil conditions, clump weights, piles or high-capacity anchors may also be used. Mooring assembly is done through the centre of a CBM for quickness of ease. And unlike SPMs, which have six to eight anchors, CMBs only have one mooring leg as the system only needs to work in one direction. 

Product transfer components

The most important component of an SPM is its product transfer system, which carries fuel between a tanker and the PLEM on the seabed. This system is made up of flexible subsea hoses, floating hose strings, swivels, valves and piping and a marine breakaway coupling for emergencies to prevent oil spills. 

The product transfer system in a CBM is much simpler and consists only of offloading hoses and the same kind of marine breakaway coupling as in an SPM. When the hoses are not in use, they are laid on the seabed out of the way. 

Ancillary components

SPMs also have a few extra components that aren’t present in CBMs. These include a boat landing, buoy-protecting fenders, handling, lifting and navigation aids, and optional components for a hydraulic system. 

Our oil & gas services

OSC Marine has been providing specialist underwater support to the Oil & Gas industry since 1962. Our services include:

– Inspection, repair & maintenance to most Oil and Gas Marine structures

– SBM Support Contracts with Terminal maintenance

– Specialised repair services

For a full list of our Oil & Gas services, click here