Large Wire Rope Mooring Winch Drum Analysis and Design Criteria

Flange splitting (separation of the flange from the barrel) is the most common structural failure in large mooring winches. Conventionally designed winches have failed on a number of occasions when wire ropes 3 to 3.5 in (7.6 to 8.9 cm) in diameter and up to 10,000 ft (3048 m) long were employed for mooring large construction barges and semisubmersible offshore drilling units. It is believed that this is due to improper approximation of the field loading patterns on the winch, inadequate knowledge of 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 a decade ago using very small wire ropes and drums. The application of these techniques to a multilayered winch using large-diameter wire rope has proved to be unrealistic. A method is presented to calculate the flange thrust load and the barrel external pressure for winches using large-diameter mare 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. presented. Introduction Large wire rope winches increasingly are coming into use for offshore construction, pipe laying, and drilling vessels operating in deep water because of the advantages of mooring with wire or a combination of chain and wire as opposed to mooring with chain only. Winches using wire ropes 3 to 3.5 in. (7.6 to 8.9 cm) in diameter, up to 5,000 to 10,000 ft (1524 to 3048 m) long, and stacked up to 15 or more layers under high tensions have been in use. Even larger winches are being contemplated as the search for hydrocarbons and minerals expands into deeper water.An industry-wide survey revealed that several large winches used on lay barges and semisubmersible drilling units have failed in service, exposing the owners to millions of dollars in repair or replacement costs, plus the damaging downtime and delay to the programs on which these units were engaged. An programs on which these units were engaged. An indepth study into the probable causes of these failures revealed that the practical design of large winches remained empirical and that, in some instances, quality control in manufacture was not being taken seriously.Wire ropes, in general, are flattened when lateral pressure is applied. The amount of flattening or pressure is applied. The amount of flattening or compressibility varies according to lateral modulus of elasticity of wire rope, which is defined as the ratio of lateral pressure per unit length of rope to the decrease in rope diameter measured along the lines of pressure. When a wire rope is spooled on a drum, pressure. When a wire rope is spooled on a drum, due to compressibility, the applied line tensions at the middle layers tend to decrease significantly. Thus, the overall structural loading on the winch depends on the lateral modulus of elasticity, number of layers, number of wraps on each layer, and operational tension at each layer. The lateral modulus of elasticity is governed by the rope characteristics such as rope formation, method of weaving, type of core, wire strand and rope diameters, and material properties of core and wire strand. It is known that as the rope gets larger and stiffer, as the number of layers increase, and as the winding tension is maintained at a high level, the resulting forces on the barrel and the side flanges also increase. SPEJ P. 63