single anchor leg mooring (SALM)

SINGLE ANCHOR LEG MOORING

DEFINITION

-A mooring facility dedicated to the offshore petroleum discharge system. Once installed, it permits a tanker to remain on station and pump in much higher sea states than is possible with a spread moor.

COMPONENTS

·         A SALM system consists of the following main components

·         Buoy Body

·         Universal joint

·         Chain with chain swivel or tubular column

·         Flexible pipe

·         Base (ballasted and/ or piled)

FUNCTIONING

-The Single Anchor Leg Mooring or SALM: Prevents collision damage to the swivels by placing them underwater and below the keel level of the tanker. Any damage should then only affect the simple surface buoy and be relatively cheap to repair. The underwater swivels do however have maintenance disadvantages. To prevent the flexible loading pipe clashing with the mooring chains the catenary is replaced by a single, nominally vertical, tensioned chain mooring leg. In shallow water the fluid swivels are on the base. In deep water the fluid swivels could be attached part way up the mooring leg. This would ease maintenance of the swivels and the flexible pipes from the swivels to the tanker.

 The buoy body provides buoyancy and place to accommodate the other components.

The floating buoy is anchored to the seabed by one single anchor leg, connected to a base type anchor point (ballasted and/ or piled). The buoy can be attached to the base by either one single chain or by a chain or tubular column, this can be seen in the pictures below.

The connection between the buoy and the vessel, and between the buoy and the base, can be established in various ways. These are described below.

 Fluids flow either through a flexible pipe from the base on the seabed directly to the ship or flow through the base and the tubular column, via a swivel to the ship. The submarine hoses are long enough to adapt to all the motions of the buoy. The fluids are transferred between the buoy and the FPSO through one or more floating hoses. The flow through the buoy goes via a swivel, which allows flow between the geostatic parts and the rotating parts of the buoy.

Auxiliary components can be things such as navigational aids for maritime visibility, fenders to protect the buoy, boat landing, power provisions, etc.

 The SALS system (Single Anchor Leg Storage) is a specially designed principle for offloading purposes. The system consists of the following components: a slim riser, which is connected to a mooring base below, and a rigid mooring yoke through a universal joint above. In the rigid mooring yoke, a buoyancy tank is incorporated, which keeps the riser under pretension by way of its positive buoyancy. The rigid mooring yoke is connected to the storage tanker by a rigid arm.

The cargo line is constructed either of pipes and jumper hoses to bridge the universal joints or of pipes and swivels without hoses.

The fluid swivel is mounted on a platform, which is an integral part of the mooring yoke. This means that the swivel is not a stressed member. Besides, as the swivel remains almost permanently out of water, there results a smaller and simpler construction requiring less maintenance.

Because of the special anchoring system, the behaviour of the SALS is almost completely independent of the water depth.

IMPLEMENTATION

The buoy can be attached to the base by either one single chain or by a tubular column.

SALM system, with a single chain.

SALM system, with a tubular column.

There are two types of buoy bodies:  

Turntable buoys

 The body that provides the buoyancy is geostatically fixed with the mooring lines. The upper part can rotate freely, allowing the FPSO to weathervane.

 Turret buoys

 The turret is geostatically fixed with mooring lines. The body that provides the buoyancy can rotate around the turret and thus allows the FPSO to weathervane.

The FPSO can be moored to the buoy in two ways:

 Hawsers 

These are polyester or nylon mooring lines. Usually, a chain section is attached to both sides of the elastic section, to prevent wear and abrasion at the fairleads. The elastic part of the hawsers damp the loads and tanker movements.

Yoke

A rigid arm is used to connect the FPSO to the buoy. This takes away the risk of the FPSO bumping into the buoy. However, this system can only be used for permanently moored FPSO’s and is more costly than a hawser arrangement.

Climate	:All weather functionality.
Number of risers	:Only one.
Depth	:Applicable in a great range of water depths.
Construction/ Installation	:Construction and installation are relatively fast and cheap.
Costs	:Cost-effective system.
Reliability	:The past has shown that the system is very reliable.
Offloading	:In tandem or via an offloading bouy. QATAR: Construction contract award for planned $500,000,000 offshore gas field expansion project, J. RAY MCDERMOTT S.A. [USA] Order #: 101805.: An article ... & Plant Operations in the Developing World