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 TB 55-1900-232-10
(Text continued from page 6-9.)
5-4.5
seen by a non-elastic towing system Appendix N, supported by experience, quantitatively demonstrates that short-term
peaks can be much greater than steady-state loads in heavy seas. There are several factors or actions that can reduce
the magnitude of these peak loads. These are listed in the following paragraphs.
5-4.5.1 Towing Ship Actions. The tow should have been planned in full consideration of resistance of the tow, tow
hawser and tug selection, towing connection and selected towing speeds appropriate to the expected weather. In some
cases, the forecast will be inaccurate or unavailable, or the tow may be carried out on an emergency basis by the first
available tug. In addition to the use of the automatic feature of its towing machine, there are four actions the towing ship
can take to reduce peak towline forces:
a.
Increase towline scope
b. Reduce power and speed
c.
Change course
d. Bring tug and tow in step.
Increasing the towline scope is usually the first action taken by the tow ship under deteriorating sea conditions However,
water depth may limit allowable catenary. Furthermore, present knowledge suggests that the catenary is not as effective
a "spring" as was formerly thought. Nonetheless, increasing towline scope, if possible, is appropriate under worsening
sea conditions.
A reduction in tow speed reduces peak towline tensions in two ways. First, steady tow resistance decreases, allowing the
catenary to increase, providing an increased ability to absorb the relative motions of the tug and tow. Second, the
reduction in steady tension reduces the base to which the dynamic tensions are added, thereby reducing peak tensions.
The towing course has a significant effect on dynamic motions of the tow ship and the tow. In general, the dynamic
effects on the towline are minimized when the seas are on or near the beam However, in heavy weather, resulting rolling
motions probably are more dangerous from a stability stand-point than the advantage of reduced peak towline tensions.
Dynamic seakeeping effects on the towline generally are most severe with following seas. Heavy sea conditions,
therefore, usually dictate steaming into the seas at reduced speed. In the heaviest storms, it may be appropriate to let
the tow pull the tug downwind, with the tug maintaining only enough power to assure steerageway.
Results of seakeeping studies described in Appendix O suggest that, in some cases, dynamic towline loads may be
significantly reduced by steering a course 30 to 60 degrees relative to the sea direction. This assumes an acceptable
degree of tug and tow rolling, as well as the ability to maintain course control.
Sometimes adjusting the towline length will bring the tug and tow in step This applies to tugs and tows that are small
relative to the length of the waves, and assumes a regular, unconfused wave system such as is sometimes found off the
U.S. West Coast. In most cases, however, the ocean surface is composed of many different wave components coming
from random directions Therefore, a particular towline scope seldom will keep a tug and tow in step for very long.
5-4.5.2 Changing Towline Composition. Several aspects of the towline design can significantly affect the peak towline
tensions Some are adjustable during the tow; some are not, but nonetheless should be considered during the tow
planning phase. These measures include;
a. Increasing towline scope
5-19
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