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At 9 p.m. on 19th November 2001 the towline (a 2¾"
335-ton steel cable) between the tug DE DA and the
900-foot oil tanker ATIGUN PASS parted in a storm 96
miles off Newport, Oregon. Six days and a
broken emergency tow line later the tanker was
eventually under control before it was grounded. |
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19th November 1997 M/V GREEN LILY was lost off the East
Coast of Bressay, Shetland in force 7 increasing to
force 10 winds. Three tugs, two connected towlines
(which parted after 51 and 5 minutes respectively) and
one successfully connected anchor cable (which also parted) were not
able to save the vessel. |
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Ships will continue to need towing and rescue in the
future... |
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...but
the towline will not need to fail. |
Towing
a ship, an oilrig or a barge in bad weather or in strong currents is one
of the most difficult and hazardous operations to undertake. The kinetic energy generated
when two heavy vessels suddenly are forced by the waves in opposite
directions is often enough to part the towline regardless of its thickness. The
consequences can be catastrophic, as can be seen on the news from time to
time. Disasters will happen again.
The
so-called "constant tension winches" used onboard tugs sound
good in theory but these only measure and react to the force in the tow
line
and will attempt to adjust the pull force accordingly. However, it is too late
to react when the tow line tension exceeds the limit. A fraction of a
second later - before the "constant tension winch " has had time
to react - the line will have snapped.
DFD®Instruments
have developed a hydraulic technology which ensures that the towline tension truly
is
constant regardless of the pull force variation pattern.
The system actively
heaves in and pays out tow line instantly in such a manner that e.g. two large
vessels can tow each other successfully with a tow line much thinner than
normally required for towing, should that be needed.
Innovative aspects
of dynamic tension control:
Instead of reacting
to tension stresses in a towline, this system ensures in a dynamic way that the
towline tension never exceeds, or, if required, never goes below certain
pull force limits, even if the tow line stress changes are sudden and
unpredictable.
 Main
Towing Advantages:
- Safe towing can
take place in bad weather.
- Increased maneuverability
for tanker escorting
- Considerably
more tug flexibility with regards to the ratio of Breaking
Load/Effective Bollard Pull, minimum towline length and other,
traditionally, restricting towing limitations.
- Sea rescue
operations will be able to take place in currently impossible weather
conditions to avoid environmental disasters
- Significantly
reduced consumption of tow line!
- As an emergency
towing system onboard commercial ships the technology will improve the
ability of these ships to rescue each other, or at least hold each
other
without parting the towline until rescue tugs arrive. Currently two
large vessels cannot successfully tow each without the towline parting
due to the very high kinetic energy generated by the movement
of each of the ships.
- As a mooring
system this technology will prevent oilrigs, ferries and other vessels
from pulling loose in bad weather.
Current stage of
dynamic towing technology
Small scale modeling
of the hydraulic principle has been carried out.
The concept is ready
for full scale prototype development. The prototype should ideally be placed on
board a tug but any vessel will be able to demonstrate its
capabilities.
Target Markets
Immediate primary
market
The various maritime
towing
operations currently undertaken every day around the world. Vessels like
tugs, supply vessels, anchor-handling vessels, rescue vessels, etc. are
potential users of this dynamic towline tension control technology.
Secondary markets
- As emergency
towing system onboard commercial ships
- Mooring systems
with dynamic tension control for vessels
- Rescue
and evacuation systems for oilrigs, ships or onshore uses
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