524 Isaacs [chap. 24 



buoy. The stored cable is wound around the large diameter, Dr, of the drum 

 and the bridle is wound around two smaller spools, D Sy on opposite sides. 

 When the tension in the pennant exceeds D s JDr times the buoyancy, B, the 

 float winds down the bridle releasing a length of wire D R jD s times the distance 

 travelled until the stress in the pennant falls below (DsJDr) xB. Models and 

 experiments of this system appear successful, and it is being installed in deep-sea 

 systems. 



5. Mooring Cables 



One of the problems of anchoring or of installing a mooring in deep water is 

 the relationship between the ultimate tensile strength for a steel wire of uniform 

 cross-section and its allowable length in the sea when bearing an additional 

 load equal to its own weight. 



Using this allowable length criterion, wire of an ultimate tensile strength of 

 100,000 lb/in 2 can be safely used to a depth of about 1700 fm, or in about 30% 

 of the oceans. A wire of an ultimate tensile strength of 180,000 lb/in 2 , however, 

 can be employed to 3000 fm, or used in 99% of the sea area. The wire used in 

 recent work had an ultimate tensile strength of about 260,000 lb/in 2 . 



The above relates solely to the quasi-static stresses produced in lowering the 

 mooring wire. Other dynamic stresses become important as soon as the anchor 

 reaches bottom. Also, an allowance must be made for weakening of the wire by 

 handling and by corrosion. 



The area presented by such a wire to the horizontal drag forces can be quite 

 large. For example, 15,000 ft of |-in. diameter wire presents a projected area 

 of about 156 ft 2 of form drag area, or about that of a large barge. Fortunately, 

 water velocities at great depths are low, and hence the large area presented is 

 not a great problem. 



Such horizontal forces must be resisted at the anchor. Thus, as horizontal 

 forces increase, the anchor weight and the lowering stresses must be increased. 

 Also, the excursions of the moorings are functions of the tensile stresses and 

 the horizontal drags. Hence, the strength/drag ratio of the wire is important. 



This can be expressed as : 



'S 



or, R ~st for constant range of Ca and wire construction, where R =strength/ 

 drag ratio, d =coefficient of drag, t =thickness of wire, and s =ultimate 

 strength of wire, lb/in 2 . 



The strength/drag ratio can be considered as a rough measure comparing the 

 minimum obtainable inclinations from the vertical of systems utilizing different 

 size mooring cables of the same construction, and hence this ratio is an inverse 

 comparative measure of horizontal displacement. 



