Mooring System Components

The main components of mooring systems, from the points of attachment of the mooring lines on the FPSO to the seabed, are:

Winches

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Fairleads

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Mooring lines (chains, wire ropes or hybrid materials)

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Surface or submerged buoys

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Clump weights

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Anchor system (drag, embedment, gravity, pile or suction anchors).

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4.5.1 Winches And Fairleads

When reaching the FPSO, the mooring lines are guided through fairleads, which can be either sheaves (pulleys) or bending shoes. The sheaves can handle both chain and wire rope whilst the bending shoe is

designed for wire rope only and is coated with a special high density nylon bearing material to reduce friction.

The payout, haul-in and tensioning of a mooring chain are normally accomplished with a rotary winch (windlass) or a traction winch.

4.5.2 Mooring Lines See Figure 4.10.

Figure 4.10

Mooring Chain and Wire Rope Source – J E & P Associates and SBM.

In ß oating production systems, chain and wire are the most commonly used mooring line materials; chain and wire are also often used in combination.

Chain provides weight and therefore stiffness through the catenary effect whilst the wire rope provides greater elasticity and therefore compliance at high tension levels.

Thus the combination of chain and wire rope provides optimal performance in a wide range of water depths.

In shallow waters (less than 100m), the use of heavy chain through the water column provides the initial high catenary stiffness and the use of wire rope (sheathed in this case) on the seabed provides the compliance at high tensions. In deeper waters, the use of wire rope through the water column helps to reduce the vertical loads and the use of chain at the touchdown point provides the stiffness required. At the fairleads on the FPSO, chains are often preferred to avoid bending loads and also for easier handling.

As offshore applications go into deeper waters (below 1,000m), man-made Þ bres (e.g. Kevlar, polyester and polyethylene) become beneÞ cial because of their superior strength to weight ratios.

Hybrid Systems (Combinations of Chain and Wire Rope) – See Figure 4.11.

Figure 4.11

Typical Shallow Water Catenary Mooring System (North Sea)

Note – for a water depth of 120m the mooring lines step out more than 600m – this takes up a lot of seabed space. The hybrid mooring lines are made up from sections of chain and sections of wire rope.

Chain connects the Þ rst section to the turret. The section on open water is wire rope – cheaper and more efÞ cient. The section at touchdown on the seabed is chain – here there is high erosion of the chain by the seabed

materials. The section along the seabed continues with chain – this has a good friction contact with the seabed.

Lastly the line is Þ nished with the attachment anchor or pile.

Source – J E & P Associates.

In support of cost efÞ ciency, it is common to use a combination of chain and wire rope. A typical arrangement for a shallow water FPSO (i.e. 120m water depth) might be:

1st section from turret = 27.5m chain

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Portion through water column = 100m wire rope

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Touch down section = 300m heavy weight chain

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Final touch down section = 270m light weight chain

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Pile end.

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A semi-taut hybrid mooring line from a spread-moored FPSO is represented in Figure 4.12.

Figure 4.12

Typical Spread Semi-Taut Moored System (West Africa)

In this spread-moored system the chain is attached to the chain stopper porch external to the hull at main deck level. The chain runs down the side of the vessel and through the underwater fairlead at the bottom of the hull.

It then angles out to the seabed. It is a hybrid system of chain and wire rope.

The mooring is a semi-taut system which is like the catenary mooring (Figure 4.11) but with very little line on the seabed. The attachment to the seabed is a suction pile. Above the stopper porch there are deck

sheaves. When the mooring lines are attached to the pulling lines they run through the sheaves and are pulled by a linear winch set on the main deck.

Source – J E & P Associates.

4.5.3 Man-made Fibre Ropes See Figure 4.13.

Figure 4.13

Deepwater Mooring Systems

Use of polyester rope. The taut mooring system design uses less seabed space (line at 45 degrees).

Source – SBM and J E & P Associates.

The technology of man-made Þ bre ropes is at the development stage, although it is advancing rapidly.

The materials mostly used for Þ bre ropes are polyesters, aramids, high modulus polyethylene (HMPE) and polyesters.

The densities of these materials are close to unity (0.98–0.99 for HMPE, 1.38–1.40 for polyester and aramid) and therefore Þ bre ropes are almost neutrally buoyant.

The axial stiffness of Þ bre ropes is a more critical parameter than that of either chain or wire rope, because the stiffness is mostly contributed by axial stretch. The stiffness is not constant and is inß uenced by load level, range, frequency and history. For example there is a factor of two or more between stiffness at installation and stiffness of a worked rope.

The fatigue life of Þ bre ropes is inß uenced by creep, abrasion and external wear. Under constant load conditions the Þ bres will creep (i.e. load backs off). This means the lines must be re-tensioned periodically.

4.5.4 Anchor Systems See Figure 4.14.

Figure 4.14

Mooring Line Attachment to the Sea Bed

The attachment anchor or pile types are: drag embedment anchors, hammered piles or suction piles.

The type is selected by seabed conditions and pulling loads.

Source – J E & P Associates.

Detailed site information is required for anchor system design. This should include geophysical data (bathymetry) and geotechnical borehole data (soil conditions).

Soil conditions at the installation site can greatly affect the selection of an anchor system and soil investigations should include both the nature and the depth of the seabed material.

A large variety of anchor systems is available; they include:

Gravity anchors

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Conventional drag anchors

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Drag embedment anchors

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Suction anchors

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Pile anchors

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Directed Learning: A company that makes anchor systems is Vryhof Anchors. Visit their website www.vryhof.com Go to DOWNLOADS. There you will Þ nd a detailed anchor manual (more than 100 pages). Use it to learn more about anchor systems.