Figure 5-3. Towline Tension vs. Time.
desirable in the planning or design of a tow, as well as in the actual towing operation. During a tow operation, precise
determination of towline tension requires precision instrumentation. Normally tugs are not equipped with instrumentation
sufficiently accurate to measure T3. Most tugs, however, are equipped with towing machine tension meters sufficiently
accurate for determining steady and quasi-steady tension T3 must be treated as discussed in the following section.
DYNAMIC TENSIONS . Since the dynamic tension is a random function, it must be described in terms of the
statistics of the ship's motion under stated sea conditions. The statistical approach is just beginning to be used in towing
technology. In the past, tow planning and operations dealt with the dynamic tensions by applying large safety factors to
steady tensions when sizing components. These safety factors are based on experience They account for many effects
in addition to dynamic tension, such as towline fatigue, corrosion and wear.
The "safety factor" approach is suitable when the operators have a great deal of experience with the towing system under
consideration. Difficulties with this approach occur when using a new towing system or towing a ship or structure with
which there is no previous experience. Sometimes this happens when operating with synthetic towlines or towing
systems for which the standard design or material has been changed. The more effects that are combined in one factor
of safety, the greater the uncertainty. Nonetheless, recommended safety factors have been established for some known
towing systems. See Table 5-4.
Appendix N presents new work that predicts the statistics of dynamic loading of the towline. As confidence in this
approach is developed, inclusion of dynamic effects in the factors of safety, as listed in Table 5-4, may no longer be
required Removal of the dynamic uncertainty eventually may permit reduction in traditional towing system safety factors.