Installation and handling of steel permanent mooring cables

Extending steel technology into greater depths

Sara L Sefton, Kevin Firth, Stuart Hallam
Bridon International

Catenary (left) and taut-leg moored floating vessels. [8,398 bytes]
Inverted catenary mooring array. [11,115 bytes]
Semi-taut leg mooring array. [8,992 bytes]
Physical properties of cable contructions [39,452 bytes]
Product Design Life Recommendation [42,689 bytes]
Current demands on exploration and production have seen installation of facilities in ever-increasing water depths. Conventional catenary moored floating production facilities are currently utilized in water depths up to 860 meters.

At deepwater locations, the extended mooring line lengths result in greater self weight of the cable. This increased weight influences not only the operational loads, but also installation loads, which must be carefully considered on deployment of cables.

For floating systems there are two basic mooring configurations - catenary mooring and taut mooring. In catenary systems, the floating structure moves laterally in response to environmental loads. The overall compliance of this system, and hence it's stationkeeping performance, is determined by the water depth, the weight of the line, and the mean tension. In comparison, a taut-moored system depends on the elasticity of the mooring rope to provide its restoring force.

Conventional composite chain and steel wire rope catenary mooring systems are being used successfully on floating systems in water depths up to 860 meters. However, in depths beyond 1,000 meters, the vertical load component can become significant due to the fundamental feature of the catenary mooring system - its weight.

The semi-taut and inverted catenary mooring arrangements introduce buoyancy into the mooring line combination. The result being a potential 40% reduction in resultant turret force, and 25% reduction in resultant anchor leg force, when compared to an equivalent conventional catenary arrangement. Sorrel et al (1997) suggest that through utilizing the semitaut arrangement, the operational depth of a steel system is extended to 2,000 meters.

It is apparent that the continued use of steel mooring line components is still possible in the increasing water depth, maintaining a confidence provided by the proven technology of steel based systems.

Design life

The two cable constructions utilized for permanent mooring systems are six strand wire rope and spiral strand. Spiral strand is the key product for permanent mooring applications. It's high strength to surface area offers increased corrosion resistance. The torsional balance of the spiral strand also lends itself to the addition of a sheathed coating, extending the applicable design life still further.

Spiral strand is recommended for non-inspectable systems of design lives in excess of 10 years. The sheathed product is used where a design life of 20 years and above is required. The lower corrosion resistance offered by the six strand product makes it suitable for design lives of a maximum 10 years.

This recommendation does not preclude the use of ropes in fields with design lives in excess of the recommendation. DNV Certification Note 2.5 "Certification of Offshore Mooring Steel Wire Ropes" (May 1995) states that both rope types may be considered, provided the system is inspected and the possibility of replacement is available. Bridon's design recommendation is based on the assumption that replacement of cables is neither desirable nor economical on a life time cost basis.

The weight of steel products becomes a disadvantage in deeper waters. The need to keep loads to a minimum, both in terms of deployment and systems loads, has led to the development of mooring cable products with a higher strength-to-weight ratio. Reduced cable self weight has been achieved through improved use of materials, manufacturing techniques, and engineering design.

High strength mooring cables Considering the metallurgical properties required of steel for rope making, the options are limited to developments in the high carbon steel. Hence, increased tensile strength carbon steel wires required for wire rope and spiral strand constructions used in the application have been developed.

For example, Bridons Diamond Blue six strand wire ropes have been developed for drilling rig operations, available in bright and drawn galvanized condition, which have achieved breaking loads in excess of 30% higher than the API 9A stated minimum figure. If an increased breaking load is not required, the same breaking load can be achieved with a smaller diameter, approximately 30% lighter rope.

With longer term permanently moored production systems, such increases in breaking load are not attainable in six strand ropes, due to the requirement for larger diameter wires and heavier zinc coatings required to achieve the extended design lives. However, the breaking loads offered for the Bridon DWM six strand range of products, for example, with heavy galvanized outer wires and zinc anode wires, are still in excess of 15% higher than the API 9A requirement.

There are no long established specifications which offer detailed minimum breaking load and weight figures for the spiral strand construction. The way has been led by the rope making industry and data has published within DNV certification note 2.5. Developments in wire manufacture of have enabled the production of high tensile grade heavy galvanized wire.

Improved strand breaking load figures are achieved by using an increased number of smaller diameter higher tensile wires, the result being a range of spiral strand products (Bridon SPR2, for example), which exhibit a 10% increase in the previously published strand breaking load figures.

Alternatively, this can be translated as a 10% weight saving - the previously stated breaking loads being achieved with smaller diameter strands. The large diameter wires which offer greater corrosion resistance are retained in the outer two wire layers.

Cable terminations

It is essential that the socket termination strength at least matches that of the cable. Similar advances have been made in the design of cable terminations. Higher strength materials allow higher strength products to be used with no weight penalty or alternatively lighter weight moorings to be used with no strength loss.

Further to the improvements made by materials selection, a closed socket design has been developed, facilitating the direct connection between cable and chain, without need for additional connection links or triplates.

Negating the need for additional connection hardware not only offers a substantial weight and cost saving, but also simplifies assembly at site reducing the number of components and need for installation spares.

The lighter weight products will allow designers to increase the water depth in which steel mooring systems are economically viable. Conversely, the system loads and, as a consequence, the installation loads experienced will be reduced for comparable systems.

For example, the typical overall weight (in air) saving achievable for a 1,775-meter length mooring cable of 1,000 tons minimum breaking load, typically for a spread mooring type floating system for offshore West Africa is detailed in an accompanying table.

Installation parameters

The bending stiffness of each generic construction gives an indication of the minimum allowable bending radius. The more compact construction of the spiral strand product results in a higher bending stiffness and hence the allowable minimum bending radius is greater than that required for the more flexible six strand product.

For design lives of 20 years and above, spiral strand products can be supplied with a poly ethylene sheathed coating. The physical shield provided by the sheathing is essential to maintain the extended life time, as this provides the barrier against water ingress and subsequent corrosion.

Hence, minimizing the potential for permanent damage to the sheathing is a priority during installation for a sheathed spiral strand. The limiting factors are defined by the properties of the sheathing which, although being susceptible to damage, is durable and not unduly problematic to handle.

Radial thickness of the sheathed coating is specified considering the following key issues:

An allowance to ensure potential in service abrasion does not penetrate the sheath allowing water ingress
Maintaining acceptable flexibility of the strand for handling, as the addition of the sheathing results in an increased bending stiffness value.

Further to the specified constructional constraints on bending, the effect on the sheathed coating of bending under load must be considered. The compressive load on the sheathed coating is the governing factor on the allowable bending of the sheathed product. The maximum allowable bearing pressure is 21 N/sq mm. This equates to the compressive yield strength of the extruded polyethylene, preventing permanent deformation of the sheath.

There are two crucial scenarios for consideration of handling of sheathed product:

Flat drums, such as a stern roller, chute or barrel
Multi-wound drums, where sheathing on sheathing point contact occurs.

As a sheathed cable passes over a curved surface such as a flat drum, the sheathing flattens and the applied load is supported by this flattened area. The amount by which we allow the cable to flatten governs the minimum bending.

Experience suggests that if a the plastic is allowed to deform by 15%, it will recover to an acceptable final dimension for operation, after the load is removed. Through consideration of the available supporting area and the applied load, the following equation is derived:

D=4W/ps_b(dX 0.15t)^0.5D = bending diameter (mm) W = Applied line load (N)s_b = Sheathing bearing pressure (21 N/sq mm) d = sheathed cable diameter (mm) t = sheathing radial thickness (mm)

Multi-layer drum

For deployment from a rotary type winch system, the situation occurs where loaded sheathed spiral strand is supported by the sheathing of the lower wraps of strand.

On each wrap, the cable must move from the valley between two wraps on the layer below into its neighboring valley, in order to traverse across the reel "cross over." Hence, there is an instantaneous situation where sheathing to sheathing point contact occurs.

It is on the final wrap of the penultimate layer when this loading scenario becomes critical, that is the smallest diameter and potentially the greatest applied load, the majority of the cable having been deployed. However, as the peak load scenario is at the point of cross over between valleys and is purely an instantaneous occurrence the applied pressure is permitted at 30 N/sq mm, which is tending towards the sheathing ultimate strength.

To ensure the line load (W) at penultimate layer does not exceed the maximum allowable "peak" pressure at cross over of 30 N/sq mm:

W=D_penps_p(d X 0.15t)^0.5/4D_pen = diameter over 1st strand layer (mm) d = overall sheathed cable diameter (mm) t = sheathing thickness (mm)sp = maximum sheathing bearing pressure (30 N/sq mm)

Gripping sheathed spiral strand

The overboarding load must be restrained. This can be achieved by either the available braking on a rotary winch, with the loads maintained as dictated by the above equations or by gripping the cable directly.

Two factors must be considered when gripping a sheathed cable - the allowable bearing pressure and the potential for slip between the inner surface of the sheathing and the strand:

Maximum allowable bearing pressure is, as stated previously, 21 N/sq mm. Hence, we ensure no permanent deformation of the sheathed coating.

Investigations into the potential for slip between strand and sheath suggests that, when using a factor of safety of 2:1 on the applied load, a coefficient of friction between strand and sheath (M) of 0.1 in the axial direction can be assumed.

A high proportion of this expected friction is due to the keying of the sheathed coating into the interstices of the wires in the outer layer of the strand. Therefore, the coefficient of friction along the helix of the strand is significantly lower than that stated above. Hence, any gripping mechanism must be prevented from rotation.

Conclusions

Advances in mooring system design have developed mooring arrangements, using proven technology steel components, which are both technically and economically suitable for 2,000 meters water depth.

In support of mooring in these depths, Bridon has developed lighter weight steel wire ropes and strands for all design life requirements, advances in termination design for lighter weight and simplified connection, and know ledge of handling and deployment requirements. Proven products are available for long-term durability, without undue constraint on usage.

References

Blair, Etheridge, Hall & Poranski (1995) "Composite Catenary Anchor Leg Systems for Multiple Riser Floating Production Vessels," DOT Proceedings, 1995

Firth, K.M., Sefton, S.L., "Technological Advances in Steel Mooring Cables: Finding solutions for even greater water depths," FPS '98.

Sorrel, Fulton & Librino, "Installation of Deep Water Moorings," ASME Energy Week '97.

Lohr, "Auger's Lateral Mooring System: Key Learnings," 28th OTC Conference 1996.