Report Available Comparing Qualities Of Synthetic Ropes
As synthetic fiber ropes are becoming more widely used in various industries and government agencies, there is a need for comprehensive engineering data to compare the various constructions. In a specially prepared technical report for the Offshore Technology Conference, Samson Ocean Systems, Inc. tries to fulfill this need by summarizing the data obtained from a series of systematic tests for strength, elongation and energy absorption. Currently there are three commonly used constructions: twisted (three strand) ; plaited (eight strand), and double braided. Due to its tendency to hockle or kink, twisted rope is seldom used in critical applications where hockling cannot be tolerated. In addition, strength and e l o n g a t i on data for twisted and plaited constructions are similar.
Since there are many synthetic materials available, only the most common material — nylon — was used as the basis for comparing the two constructions. This comparison generally holds true for other materials as well.
The strength, stretch and energy absorption of-double-braided and eight-strand plaited ropes are discussed in detail in the report. The main points may be summarized briefly as follows: 1. Strength (a) Size f o r size double braid is 25 percent to 40 percent stronger. (b) For equal weight, double braid is 15 percent to 30 percent stronger while being slightly smaller in size.
2. Elongation (a) Stretch data for new ropes should not be considered since synthetic fiber ropes stretch permanently and stiffen significantly with only a few load cycles. (b) Plaited eight-strand ropes stretch and stiffen after use much more than double braid. For used ropes, the elongation at the breaking point differs about 9 percent between plaited rope and braided rope. This compares to a difference of 15 percent with new unused rope..(c) Under frequent cycling of load, synthetic fiber ropes stiffen significantly compared to onetime loading. Plaited rope stiffens more than double braid. On an equal size basis, the difference in elongation at the breaking point is about 3 percent. The performance of plaited and double braid is quite close under these conditions, which is representative of actual service conditions, such as mooring and towing operations. 3. Energy Absorption (a) On both a size and weight basis, double braid absorbs more energy than plaited rope when loaded to the breaking strength for all conditions (new, used, cyclic).
(b) On both a size and weight basis, double braid absorbs more energy than plaited at the same percentage of breaking strength for the used condition and the cyclic load condition.
4. Fatigue (a) Under cyclic load conditions, double braid retains its strength much longer than plaited rope.
The complete report, which is an addendum to the Samson Rope Manual, may be obtained from Samson Ocean Systems, Inc., 99 High Street, Boston, MA 02110. For further information,