Fairlead Mechanism Applications
Aside from acting as an overboarding and towing device,
the Fairlead Mechanism has also returned to its roots, serving in level-wind systems. This
is particularly useful when a winch is located in a confined space or at an angle to the
cable path, and space limitations do not allow incorporation of a full-sized sheave in the
level-wind device, or when the fleet angle from winch to overboarding hardware is greater
than can be accommodated by a sheave.
A demanding application
for the Fairlead Mechanism is deployment and recovery of ocean bottom seismic cables. As
it traverses the stern of the vessel and travels straight down to the seafloor, the cable
may undergo a change of direction of 90 degrees or more. In deep water with a large
diameter cable, the loads wrapped over such a large angle lead to enormous loads at the
sliding bearing interface. These overboarding units must be routinely tested to more than
70 tonnes at the factory. (photo above: A Cable Overboarding Unit
with 105 degree wrap angle undergoing 70 tonne static load testing.)
A typical Fairlead Mechanism installation for laying
ocean bottom seismic cable.
Another challenging task is the handling of cables with
ODIM Spectrum’s hydrodynamic drag-reducing fairings installed. Flexible TufLine™
fairing systems—now commonly used for drag reduction—travel relatively easily
through a handling system, and wrap onto a winch without difficulty. However, the rigid
TufNose™ fairing system, popular for its maximum drag reduction, requires special
care. ODIM Spectrum addressed the demand for Fairleads capable of handling fairings by
designing a Fairing Uprighter. As its name implies, the Uprighter guides each fairing into
a tail-up position as it approaches the handling system, preventing it from snagging.
Future Growth
In 1974, when ODIM supplied its first towed streamer
winch system to Geco-Prakla, a typical seismic survey vessel towed a single streamer
cable. The number of streamers towed from today’s larger 3-D/4-D seismic vessels is
typically between eight and 12, with companies planning in the near future to achieve 16
to 20 array configurations, each about eight kilometres long with spreads approaching two
kilometres. The backdeck systems required to handle these sensitive electro-optical tow
cables and streamers must be sophisticated and extremely reliable.
