Factors Affecting Wire Rope Life In Amarine Environment

ABSTRACT Laboratory tests of axial tension fatigue and field data on structural applications indicate that there may be an S-N type curve for wire rope. Exposure of steel ropes and strands in marine environments for up to 15 years indicates the benefits to be gained from zinc coatings and plastic jackets. Deployment techniques can seriously damage conventional rope due to rotation. These results are translated into recommendations for providing improved wire rope life in the marine environment. INTRODUCTION Wire rope life in a marine environment is affected by many factors. the most important of which are fatigue. corrosion. deployment methods and combinations of these three. Fatigue tests conducted by U. S. Steel Corporation at facilities in New Haven. Connecticut. and Monroeville. Pennsylvania. plus experience in the Woods Hole buoy program and on certain land installations such as the Golden Gate Bridge have shed new light on the reaction of wire rope to axial tension fatigue. Tests run at International Nickel Company's test site at Kure Beach. North Carolina, Port Hueneme, California, and tests reported in the literature, plus experience at Woods Hole and in the Navy's Seacon II program have provided information on corrosion of wire rope. Knowledge of the properties of wire rope plus experience at Woods Hole has provided guidelines for successful deployment of wire rope in the ocean. DEPLOYMENT The method of deployment depends upon the wire rope construction being deployed. Wire rope life can be severely reduced if care appropriate to the type of wire rope construction is not exercised during deployment. Due to the geometric relationship of the load carrying wires and of the strands to the axis of the applied load, a torque is produced in wire rope when it is loaded. If one end of the rope is free to rotate when a load is applied. the rope will. in most cases. unwind in a direction opposite to the lay of the rope. If the load on the rope is suddenly released. as can happen when the load hits the ocean bottom or when wave action causes the supporting surface ship to pitch, the rope tries to wind back up to its unloaded condition. In many cases this winding occurs too fast for the rope to adjust properly throughout its length and the rope twists up on itself. forming hock1es or tight loops. Upon reapplication of the load, these loops are pulled through and form kinks which are distorted weak spots in the rope. These kinks may reduce rope strength by as much as 90% and lead to premature failure. Figure 1 shows the results of a test conducted by Woods Hole Oceanographic Institution in which a conventional 6 strand rope was attached to a crane and a load lifted off the ground.