Page 64 - Failure Analysis Case Studies II

49

0 multi-strand constructions can combine sufficient bending flexibility and crushing resistance to
operate on multi-layer winches, and possess good torque balance;
0 these ropes have smaller outer wires than other ropes of comparable strength, so tend to have
inferior corrosion resistance;
0 because of the high contact stresses at the crossed contacts between the strands in different layers,
fatigue damage in these ropes tends to develop internally rendering them prone to high levels of
strength loss without externally visible signs;
0 these ropes are significantiy more expensive than conventional six strand ropes of similar strength.

Multi-strand ropes have been used for mooring a floating offshore production platform, but with
limited success.

4. The torsional response of wire rope

Under conditions of rotational restraint, conventional six strand ropes develop a torque which
is approximately proportional to the tensile load, however this torsional response is modified by
twisting or untwisting the rope. This characteristic is illustrated in Fig. 2 in which axial rope torque
(M) is plotted as a function of axial tension (F) for different levels of twist (d$/ dz), where a positive
twist implies a reduction in lay length. As would be expected, a reduction in lay length increases
the torque generated by applied tension. The offsets on the torque axis show the torque at zero
tension: this is a function of the torsional stiffness. A typical characteristic for a torque balanced
rope is also shown for comparison: note that torque balance is never perfect due to the various
geometrical changes associated with tensile deformation (both radial and axial).
Simplifying these torque characteristics for six strand rope to a set of parallel lines makes it
possible to describe the relationships by an equation of the form:

I
Fig. 2. A schematic representation of axial torque as a function of rope tension showing the characteristics typical of
six strand rope (as manufactured, twisted up, and untwisted), and a nominally torque balanced rope for comparison
(the dashed line).