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TB 55-1900-232-10
APPENDIX L
ESTIMATION OF DYNAMIC
TOWLINE TENSIONS
L-1 INTRODUCTION
This appendix addresses recent developments in quantifying the impact of relative motion between a tug and
its tow on the towline connecting the ships. Data are provided that will be useful to the tow planner, given the
sea conditions expected during the tow. More importantly, the data will be of immediate use to the U.S Navy
towing ship in predicting an acceptable risk of extreme dynamic towline loadings, rather than assuming that
sufficient margin exists in the traditional factors of safety applied to steady state tensions.
L-2 BACKGROUND
This section describes the problem of towline dynamics.
L-2.1 SHIP DYNAMICS . All seamen are well aware of ship motions in a seaway, particularly rolling, pitching
and yawing. But there are three additional types of ship motion that are less apparent - heave, sway, and surge,
for a total of six independent ship motions In a towing scenario, the tug and tow both experience their own
motion, for a total of twelve degrees of freedom acting on the towline connecting them. The towline also acts on
both ships. These dynamic effects can cause the failure of the towline at unexpected times, when average
tensions are well within acceptable limits. Traditionally, the complex problem has been unquantifiable, and still
is addressed through the traditional method where not all variables are quantified-the use of a factor of safety.
In towing, the factors of safety are applied to the steady-state towline tensions, as described in Chapter 5, and
the new strength of the towline. The factors of safety used are those determined through experience, primarily
to account for the dynamic effects in the towline. Nonetheless, failures still occur.
L-2.2 WIRE TOWLINE MOTION . A heavy wire hawser forms a catenary between the tug and tow. In the
steady-state condition, the shape of the catenary is easily estimated. Further, the catenary acts as a spring,
flattening and deepening to compensate for relative motions between the tug and tow. Questions have been
raised recently, however, concerning the cross-flow hydrodynamic resistance on the wire as it rises and falls. It
has been suggested that, for the motion frequencies encountered in most towing situations, the wire towline
does not have time to fully resume its former deep catenary when the tension eases, before the next surge in
tension occurs. The net result over time is that the wire catenary flattens out, thereby providing somewhat less
spring than previously thought. In this scenario, more of the spring remaining in the system can be attributed to
the elastic stretching of the wire itself. See Paragraph 5-4.5.2 for a discussion of the stretch of wire hawsers.
L-2.3 SYNTHETIC TOWLINE BEHAVIOR . Fiber hawsers are much lighter than wire hawsers, and do not form
an appreciable catenary. They rely almost totally on their elastic stretch in the towing scenario. The advent of
strong, highly elastic synthetics, especially nylon, was expected to be a boon to towing, because their elasticity
easily absorbed relative ship motion. Such hawsers could be man-handled and employed with neither a
dedicated towing winch nor an automatic towing machine. However, as the use of nylon became more
prevalent, unexplained failures were reported, often under towing tensions far below the supposed strength of
the towline. Factors of safety were increased to take into account the unsolved problems to the point where the
advantages of nylon over wire hawsers were lost.
A separate problem with nylon was caused by its elasticity. The large amount of energy stored in the stretched
hawser is released explosively when the hawser falls. This often has disastrous effects, especially on personnel
Ul-
L-1


 


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