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A Unique Fairlead Mechanism for Cable Handling

This space-saving cable guidance device has fast become the standard on multi-streamer seismic vessels, with applications on surface and submarine towed array systems and for specialized offshore, cable-laying, and ROV requirements.

Proper handling of a marine cable is often underrated. Yet vessel downtime and cable replacement resulting from damage can be extremely expensive. Whether it is tough wire rope, complex electro-optical cable, or delicate oil-filled streamer, the service life of a cable depends directly on the guidance components of its overboarding system. If these do not address the cable�s physical characteristics and limitations, and the nature of its application, rapid deterioration and failure of the cable can result.

The importance of the handling system and the high cost of cable failure is not new. Pulleys of various sizes were used in ancient Roman derricks. The cables of more recent sailing ships were reeled off large capstans and paid-out through flared hawse holes to prevent damage to the manila fibres. There are few fundamental differences between these beginnings and some contemporary solutions to cable overboarding, such as the bellmouth (a fixed, trumpet-shaped guide) and the sheave (basically a pulley).
  

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The Overboarding Problem

Overboarding equipment handles and protects a cable where it crosses the side of a vessel and enters the water. An inadequately designed overboarding system can contribute to premature cable failure in several ways.

Bend radius violation is perhaps the most common problem. For example, a large sheave that meets the minimum bend radius of the cable during routine towing may nonetheless allow it to bend excessively when the tow-off angle increases. Abrasion and weakening of the cable surface can occur anywhere there is relative motion between the cable and the handling system components it contacts, particularly when loads are high. Point loading, such as that caused by small fixed rollers, may locally deform the cable structure, weakening it and damaging internal conductors.

The well-designed sheave is a good overboarding solution if it swings to accommodate changing tow-off angles. Without the ability to swing, the cable will be pulled across the sheave flange, resulting in fatigue and abrasion. In an application with a large-diameter cable and a correspondingly large sheave, the space required for the sheave to swing becomes problematic. And on vessels towing multiple cables, space is a primary concern.

Bellmouths address the problem of tow-off angles by providing radial support in any direction. However, they allow the cable to be dragged across a fixed surface which�regardless of how well-greased it is�can cause severe abrasion and frictional heating, especially when under high tension.

A hybrid approach combines the characteristics of the bellmouth and the sheave, employing multiple small rollers mounted on a fixed curved surface. This may be the worst possible solution, because each roller applies localized high compressive stresses and cyclical bending to the cable, and the resulting wear is comparable to that caused by running the cable over a similar number of full-size sheaves. As a result, cable life can be greatly abbreviated.

When cables include plastic sheathing, fine conductors, fibre optics, or cable-mounted equipment, the limitations inherent in these outdated techniques become prohibitive.

Improved Fairlead Mechanism

In 1991, the Canadian Navy commissioned the Ocean Systems Group of Spectrum Engineering, Inc. (now ODIM Spectrum Ltd.) to develop an entirely new approach to overboarding for the CANTASS towed array system. The result of this project was the Improved Fairlead Mechanism, a device used for both level-winding (uniform wrapping of a cable on a winch drum) and overboarding. The unique Fairlead Mechanism was soon in demand by the offshore seismic oil and gas exploration industry, and is now standard equipment across the backdecks on all the latest seismic survey vessels. More than 300 Fairlead Mechanisms of various types are installed.

Military applications also expanded, and the Fairlead Mechanism has been employed to handle towed sonar systems on naval surface ships, including the Thomson Marconi LFAPS system for the Royal Australian Navy�s FFG-7 and ANZAC frigates. It is currently being developed for towed array guidance aboard submarines, where its space savings will be particularly welcome.

The most important innovation of the Fairlead Mechanism is the flight chain concept. A segmented belt of �flights� cradles the cable through an arc meeting its minimum bend radius requirement. At the end of the maximum arc of contact with the cable�determined by the actual application�the flights return directly under the unit to the beginning of the arc. Space reduction compared to a sheave with the same effective radius typically approaches 75 percent. This savings equates to room for more equipment on the back deck.

the bellmouth, sheave, and Fairlead Mechanism.
A comparison of different overboarding techniques; the bellmouth, sheave, and Fairlead Mechanism.

The improvement in unit size leads to the considerable tow-off capabilities of the Fairlead Mechanism. When mounted to swing freely, the most extreme angles can be accommodated without violating the minimum cable bend radius. While similar mounting of a sheave is possible, its larger size means it will swing through a much larger space.

The flights slide over a smooth track surface on proprietary low-friction bearing pads. Each flight supports the cable on a wear-resistant urethane cushion that is profiled to match the cable and maintain control at the largest fleet angles (the cable angle between the winch and the Fairlead Mechanism). By conforming to the surface of the cable, the urethane distributes axial loads more uniformly over the cable surface and minimizes point loading. Varying diameters on a single cable (due to instrumentation, connectors, the combination of different cable sizes, and so forth) are handled successfully. Large diameter items can ride on the tops of the flights.

The effectiveness of the Fairlead Mechanism is maximized when it is mounted with three degrees of freedom. This allows it to support and protect the cable while responding to sea conditions, vessel motions, and tow-off angles. A ball joint mounting rated at 17 tonnes vertical safe working load (SWL) was developed to provide free movement in a compact space. For applications where this amount of freedom is not required, hanging brackets with one or two discrete rotating joints are used.

Hanging brackets also work as �keepers�, preventing the cable from lifting away from the Fairlead Mechanism as a result of ship movement in high sea-states. In these dynamic conditions, the cable may �hunt� within a zone limited entirely by the flights and keeper.

The simplicity of the basic Fairlead Mechanism design leads to its high reliability and low maintenance.

ODIM Fairleads - Challenges, Applications and Future Growth

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ODIM Spectrum Ltd. 
597 The Queensway, Peterborough, Ontario, Canada  K9J 7J6
Tel (705) 743-9249 | Fax (705) 743-8003
  Email: General Information and Sales: info@odimspectrum.com     Website:
www.odimspectrum.com
  

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