Offshore mooring systems guideline

A variety of mooring systems have been developed for use on deep water offshore oil and gasfields, and in the vicinity of certain ports, to allow the loading of large vessels and the permanent mooring of floating storage vessels or units.

These offshore systems include large mooring buoys, manned floating structures of over 60000 tons designed for mooring vessels up to 500000 tons, and platforms on structures fixed at their lower end to the seabed.

They allow a vessel to moor forward or aft to them, and to swing to the wind or stream (weather vane). They are termed Single Point Moorings (SPMs), or those which are a form of mooring buoy are termed Single Buoy Moorings (SBMs). SBM is the generic term accepted throughout the offshore oil industry for tanker loading buoys. One leading manufacturer of these buoys is S.B.M. Inc. of Monaco, although alternative manufacturers offering differing designs are strongly represented in this important market.

Like production platforms, SPMs are normally marked by lights and a fog signal is sounded from them. On charts, an offshore mooring is shown by the symbol for a tanker mooring of superbuoy size.

If the mooring is connected to the bottom by a rigid, pivoted or articulated structure, it is shown by the symbol for an offshore platform.

The mariner should give all offshore moorings a wide berth if not intending to use them.

Types of Single Point Moorings (SPMs)

There are two main types of SPMs: Catenary Anchor Leg Moorings (CALMs) and Single Anchor Leg Moorings (SALMs). Each type has developed a number of variations.

Catenary Anchor Leg Moorings (CALMs) incorporate a large buoy (SBM) which remains on the surface at all times and is moored by 4 or more anchors which may lie up to 400m from the buoy.

Mooring hawsers and cargo hoses lead from a turntable on the top of the buoy, so that the buoy does not turn as the ship swings to wind and stream.

Exposed Location Single Buoy Mooring (ELSBM) (a development of CALM) is designed for use in deep water where bad weather is common. With this type of SPM the buoy is replaced by a large cylindrical floating structure. The structure is surmounted by a helicopter platform, has reels for lifting hawsers and hoses clear of the water, and is fitted with emergency accommodation. Its anchors may lie up to half a mile from the structure.

SPAR mooring is similar to an ELSBM, but the floating structure is larger and incorporates storage facilities so that in adverse weather production can continue. It is permanently manned.

Single Anchor Leg Mooring (SALM) consists of a rigid frame or tube with a buoyancy device at its upper end, secured at its lower end to a universal joint on a large steel or concrete base resting on the seabed, and at its upper end to a mooring buoy by a chain or wire span. Oil flows into the frame through the universal joint at its lower end and out of the frame through a cargo hose connected to a fluid swivel-assembly at its upper end. When the pull of a vessel is taken by the mooring buoy, the frame inclines towards the vessel and the buoy may dip. When the vessel swings to wind or stream, the frame swings with her on the articulated joint at its foot.

This type of mooring is particularly suited to loading from deep water sub-sea wellheads.

Articulated Loading Column (ALC) is a development of the SALM with the anchor span and buoyant frame or tube replaced by a metal lattice tower, buoyant at one end and attached at the other by a universal joint to a concrete-filled base on the seabed. Some are surmounted by a platform which may carry a helicopter deck and a turntable with reels for lifting hawsers and hoses clear of the water, and have emergency accommodation. These are termed Articulated Loading Platforms (ALPs).

In bad weather, a tower may be inclined at angles up to 20° to the vertical.

Single Anchor Leg Storage (SALS), consists of a SALM type of mooring system that is permanently attached to the stem or stern of a storage vessel through a yoke supported by a buoyancy tank. Tankers secure to the storage vessel to load.

Other loading systems :
Mooring towers are secured to the seabed, and surmounted by a turntable to which ships moor. At some mooring towers, a floating hose connects a fluid swivel-assembly in the turntable to the vessel, at others an underwater loading arm carries a pipe from the turntable to the vessel’s midship manifold

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