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fibre optics, or cable-mounted equipment, the limitations inherent in these outdated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.
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.
