Assessing Strength Loss of Abraded and Damaged Fiber Rope
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This paper reports on the results of tests intended to develop criteria for assessing the residual strength of several popular fiber rope types commonly used as mooring lines. The ropes types are: nylon and polyester 8-strand and polyester and HMPE (high modulus polyethylene) 12-strand. Artificial, quantifiable, concentrated and distributed damage was inflicted on rope specimens by cutting yarns to reduce rope area by predetermined proportions. In one set of tests, the same area was reduced in each strand to simulate uniform abrasion. In another set of tests, the area of one or several adjacent yarns was reduced to simulate concentrated damage, and this was done in patterns of yarns with the same twist direction and with opposing twist directions. In another set of tests, the areas of several different yarns were reduced at various separations distances along the rope length. These prepared specimens were loaded to break to determine residual strengths. Loss of strength was then correlated with the percent of lost rope area and to the pattern of damage distribution. Rope retirement criteria were developed for the several types of rope. As a general rule, the ratio of strength loss to area loss is about 1 for polyester and nylon rope, and significantly greater than 1 for HMPE rope. In another set of tests, twist was imposed on undamaged rope specimens. It was discovered that a small amount of twist can significantly reduce the strength of 12-strand HMPE rope. These fiber rope retirement criteria will be published in a new edition of the Oil Companies International Marine Forum (OCIMF) "Mooring Equipment Guidelines", (ref 1) They will be of interest to all users of these ropes for marine mooring lines and for other purposes.