Snap Loads And Bending Fatigue In Diving Bell Handling Systems

Abstract

Introduction Recent at-sea tests conducted by the Civil Engineering Laboratory (CEL) 1 have found that a suspended object with a low submerged weight and a large virtual mass such as a diving bell is highly susceptible to snap loads in a single lift line system. Tensions as high as seven times the payload weight were measured 'in sea state 2. Such high tensile loading could cause a catastrophic failure in the handling line when the breaking load is exceeded. Since the snap load is basically an impact load, the amplitude is expected to be proportional to the payload mass and the relative velocity of the payloadwith respect to the surface excitation velocity. Therefore, higher snap load tensions are expected for diving bells which have larger volumes and/or operate in higher sea states. Bending fatigue is another problem that is closely associated with the design and selection of a critical lift system such as the diving bell handling system. With such systems, protracted payload suspension over a sheave in the water column is expected. In this mode, the line may fail under repeated snap loads of much lower level than the rated breaking strength of the rope. For fiber and wire ropes, bending fatigue is directly related to external fiberto- sheave abrasion and to internal fiber-to-fiber abrasion. No analytical method is available to predict the fatigue life of a rope/sheave system. Laboratory testing is the only means to generate design data. This paper presents the results of an investigation conducted in 1979 to examine the probability of snap load occurrence, the amplitude of snap loads, and the safety of existing diving bell handling systems. A computer analysis, a sea test, and a laboratory simulation test were made for the U. S. Navy Two-Man Open Diving Bell (ODB). Computer methods were also applied to analyze the handling system of the Deep Diving System/Personnel Transfer Capsule (PTC). The Open Diving Bell (ODB) is essentially a shallow-depth submerged elevator in the water columnto accommodate divers equipped with air/gas supply equipment or hoses. The ODB consists of a hemispherical acrylic dome structure over a diver's transfer stand. It permits compressed air diving to 190 feet and mixed gas diving to 300 feet. For mostoccasions, the diving bell is operated in sheltered areas such as harbors. However, its applications are not limited to calm waters. The ability of the topside deck hands to work safely and effectively will be the controlling factor in stipulating the maximum operational sea state. The ODB manual2 calls for a handling line of non rotating rope capable of handling aload of 4,300 pounds with a safety factor of five. In the past, the diving bell has been handled by a system consisting of the ship capstan, any available standard sheave blocks, and a variety of ropes including 5- to 6-inch-circumference nylon 2-in-l braids and 1/2- to l-inch-diameter wire ropes.