Extending the limits of steel mooring cables for ultra-deepwater

Cautions in deploying cable systems

Sara L Sefton
Kevin M Firth,

Stuart Hallam
Bridon International Ltd,
UK

As offshore activity moves into ultra-deepwater, concerns are raised about the perceived deployment limits for steel cables. Bridon International, a supplier of specialized cable solutions, is continuing research and development to enable it to deal with prospective limits. Bridon has deployed steel cables for 30 projects to water depths exceeding 2,800 ft.

At ultra-deepwater locations, extended mooring line lengths result in a greater weights of cable. This increased weight influences installation line loads, which must be carefully considered when deploying cables. Safe and cost effective deployment in deepwater locations is feasible and Bridon explains the key parameters for steel cables and its impact upon connecting mooring components.

Two cable constructions are primarily utilized - six strand and spiral strand.

• Spiral strand is the key product for long term permanent mooring applications. Spiral strand is recommended for non-inspectable systems with design lives in excess of 10 years, with sheathing utilized where design lives exceed 20 years.
• While six-strand rope is primarily a general work rope, it has been used in mooring applications where long-term life expectancy is not required. The lower corrosion resistance offered by the six strand product makes it suitable for a maximum 10 year design life.

Installation guides

When installing both products, a number of key aspects require consideration. Cable bending is the first. Spiral strand exhibits a higher bending stiffness and hence requires a greater minimum bending radius than the more flexible six strand cable. The required bending radius depends upon the applied axial load. Working from zero load to one equal to 40% of the cable breaking load, a D/d ratio of 15-26 is recommended for six strand cable, while the spiral strand requires a D/d ratio of 24-60.

Further to these constructional constraints, the effect of bending under load on the polyethylene coating must be considered for sheathed cable. While the polyethylene sheath increases bending stiffness, it also helps protect construction integrity, holding the outer wires in position.

The compressive load on the sheathing governs allowable bending. The maximum recommended bearing pressure is 21 N/mm² (3,045 psi), which equates to the compressive yield strength of the extruded polyethylene, thus preventing permanent deformation. There are two crucial installation scenarios to consider.

1. Flat Drums (stern roller, chute or barrel): As sheathed cable passes over a flat drum, the sheathing flattens with the applied load supported by the flattened area. Allowable flattening governs the minimum bending radius. Expe rience suggests 15% deformation will recover to an acceptable level once the load is removed.

   Consideration of the allowable pressure and supporting area enables calculation of the minimum allowable drum diameter for a given axial line load:Fig 1 [40,082 bytes]

2. Multi-layer drums (rotary drums) at "cross over:" For deployment from a rotary drum system, the sheathed spiral strand is supported by the lower strand layers. To traverse the reel, the cable must "cross over" lower layers when moving between valleys, creating an problem where point contact occurs.

   As "cross over" is an instantaneous occurrence, the applied pressure is permitted to increase to 30 N/mm² (4,300 psi), tending towards the ultimate strength of the polyethylene.

   The worst case scenario is the final wrap of the penultimate cable layer on the drum, where the applied load is potentially at its greatest, with bending diameter at its lowest. The line load at penultimate layer "cross over" can also be calculated:Fig 2 [33,653 bytes]

In reality, the flat drum scenario will be the limiting factor. Should the available installation equipment possess insufficiently dimensioned drums to accommodate required line loads, the solution is to lessen the line load at the overboarding point or increase the drum diameter, whichever is most feasible.

Overboard

Once the cable, whether six strand or spiral, has been overboarded, careful handling is required to ensure safe touchdown at the sea bed. The cable must be laid under a constant nominal tension with the deployment vessel maintaining constant forward movement with the touch down radius respecting allowable bending. This ensures the cable is laid without the formation of loops. Should a loop occur, the cable self weight will prevent unwinding and an applied axial load will tighten the loop, resulting in a permanently kinked cable.

It has been suggested that induced torque driven into the cable could result in a helical path towards the seabed, exacerbating the risk of loop creation. To overcome this scenario, a rotation resistant cable should be used as a lowering wire and indeed as a mooring component.

This seriously questions the use of six strand cable due to its propensity for rotation under load, thus inducing torque into other mooring line components. The torsionally balanced spiral strand does not exhibit this characteristic, but risks being a victim of induced torque from an unbalanced lowering line. As in ultra-deepwater, the induced torque may be sufficient to overcome the spiral strand's torsional stiffness. The use of non-rotating steel cable constructions or synthetic fibre ropes is therefore recommended when lowering spiral strand in ultra deepwater applications.

Dealing with torque

The torque generated by six strand cables under load and its impact both on the cable itself and other components connected in series, is currently causing concern. The initial findings of ongoing research suggest greatly reduced fatigue life in the six strand cable itself, with a perception that connected components could similarly be affected. Eight strand ropes will be similarly affected.

Bridon is assisting in researching this issue, and shares concern that the use of six and eight strand cables could lead to increased risk of damage to mooring components, whether chain, synthetic fiber, or spiral strand, during installation or in service.

In conclusion, ultra-deepwater installation does not present the problems perceived. Through consideration of cable properties criteria for evaluating their safe deployment has been established.

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