Large Wire Rope Mooring Winch Drum Analysis And Design Criteria

Abstract

ABSTRACT Flange splitting is the most common cause in mooring winch failure. Conventionally designed winches have failed on a number of occasions when sizes of, about 3" to 3 1/2" diameter wire ropes in lengths up to 10,000 feet were employed for mooring large construction barges and semi-submersible offshore drilling units. It is believed that this is due to improper approximation of the field loading patterns on the winch, inadequate knowledge of actual forces transmitted onto the flange and drum barrel of the winch and/or defects in the structural joint between the flange and the drum barrel. The available design methods are often empirical, modified or extrapolated from work done using very small wire ropes and drums a decade ago. The application of these techniques to a multilayered winch using large diameter wire rope has proven to be unrealistic. The authors have therefore developed a method to calculate the flange thrust load and the barrel external pressure for winches using large diameter wire ropes. Also, a general guide for design and analysis of such winches and the effect of the lateral modulus of elasticity of wire rope on the reduction in the layer tensions is presented. INTRODUCTION Large wire rope winches are increasingly coming into use for off-shore construction, pipe laying and drilling vessels operating in deep water due to the advantages of mooring with wire or a combination of chain and wire as opposed to mooring with chain only. Winches using about 3" to 3 1/2" diameter wire ropes up to five to ten thousand feet in length and stacked up to 15 or more layers under high tensions have been in use. And even larger winches are being contemplated as the search for hydrocarbons and minerals from and under the oceans expands into deeper waters. An industry-wide survey revealed that several large winches used on lay barges and semi-submersible drilling units have failed in service exposing the owners to millions of dollars in repair or replacement costs on these winches, plus the damaging downtime and delay to the programs on which these units may have been engaged. An in-depth study into the probable causes of these failures revealed that the state of the art in design of large winches, in practice, remained more or less empirical and that in some instances, quality control in manufacture was not being taken as seriously as this equipment warrants. In addition, neither the wire rope nor the winch manufacturers have established wire rope characteristics which are necessary for designing winches. The wire rope characteristics which vary according to rope formation, type of core, rope diameter and material properties of components, the number of layers, the number of wraps on each layer and the operational tension on each layer affect the over-all structural loading on a winch. It is known that as the rope gets larger and stiffer, as the number of layers increase and as the wire winding tension is maintained high, the resulting forces on the barrel and the side flanges likewise increase.