Single buoy mooring

Single buoy mooring (SBM) or Single point mooring (SPM) are loading buoys anchored offshore and serve as a mooring point for tankers to (off)load gas or fluid products. They are the link between the geostatic subsea manifold connections and the weathervaning tanker. The main purpose of the buoy is to transfer fluids between onshore or offshore facilities and the moored tanker. They are capable of handling any size ships, even very large crude carriers (VLCC) where no facility is avaiable.

Parts of single buoy mooring

There are mainly four parts for the total buoying system, body of the buoy, mooring and anchoring parts, product transfer system and other components.

Buoy body

Buoy body is mainly static legs connected to the seabed, and a rotating part above water level connected to an (off)loading tanker. These are connected to each other by roller bearing, referred as "main bearing". The moored tanker can freely weathervane around the buoy to a stable position due to this arrangement.

Mooring and anchoring parts

Mooring arrangement is to fix the buoy to the seabed. This requires predicted behaviour of the buoy for various wind, wave and current conditions and various tanker sizes. This determine the optimum mooring arrangement, and size of the various mooring leg components. Anchoring points are greatly dependent on local soil condition.

Mooring components

*Anchors or piles to connect the mooring to the seabed.
*Anchor chain
*Chainstoppers to connect the chains to the buoy

Hawser arrangement

A tanker is moored to a buoy by means of a hawser arrangement. International Oil Companies International Marine Forum (OCIMF) standards are available for mooring systems.

The hawser arrangement usually consists 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 would usually be connected to an OCIMF chafe chain on the export tanker side [either type A or B depending on the maximum size of 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;Mooring Hawser Shackle;Chafe Chain Assembly;Support Buoy;Pick-up / Messenger Lines;Marker Buoy for retrieval from the water.

Under OCIMF recommendations, the Hawser Arrangement would be normally purchased as a full assembly from the Rope 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

Risers

The risers are flexible hoses and connect the subsea piping to the buoy. The configuration of these risers can vary, depending on water-depth, seastate, buoy motions, etc.

Product swivel

The product swivel is the connection between the piping in the geostatic part and the rotating part of the buoy. Product swivels range in size depending on the size of the attached piping and risers. Product swivels can provide one or multiple independent paths for fluids, gasses and electrical signals or power. The swivels are equipped with a multiple seal arrangement to minimise the possibility of leakage of product to the environment.

Floating hose string

Floating hose string(s) connect the buoy to the offloading tanker. The hose string can be equipped with a break away coupling, to prevent rupture of hoses and subsequent oil spills.

Other components

Other Components of SBM are:
*Boat landing, providing access to buoy deck
*Fendering to protect buoy
*Lifting and handling equipment to aid materials handling
*Navigational aids for maritime visibility
*Power provisions to enable valve operation, to power navigation aids and for other equipments.

Major agencies connected with SBM

# Single Buoy Moorings Inc. [http://www.sbmoffshore.com/]
# SOFEC Marine Terminal. [http://www.sofec.com/MarineTerminals.aspx/]
# Bluewater Offshore. [http://www.bluewater-offshore.com/]
# Subsea Oil and Gas Direcory. [http://www.subsea.org/]