A comprehensive series of bend-over-sheave fatigue tests has been carried out on a single design of electrohydraulic umbilical to establish the effect of a number of factors on the measured life The factors which were examined were axial load, hose pressure, sheave diameter and wrap angle.
As part of the Joint Industry Project "Umbilicals - the Future", a comprehensive series of tests has been carried out on a single design of electrohydraulic umbilical The test regime which has been examined is "bend-over-sheave" (BOS) also known as "cyclic-bend-over-sheave" (CBOS) In this style of loading the umbilical is passed around a sheave wheel (or arc of a circular former) with a tensile load applied If the umbilical is passed backwards and forwards around the sheave, the umbilical is subject to an alternating (cyclic) bending, which can lead to fatigue failure of the umbilical components
Bend-over-sheave loading has been generally identified as the most onerous to which an umbilical designed for static service is subject. This loading arises during installation, when, at the lay vessel, the tension occurs by virtue of the weight of the umbilical in suspension to the sea-bed, and the motion arises around the overboarding sheave due to the vessel motion Although in normal installation operations has will not result in a large number of cycles, it is possible that in circumstances where the operation has to be halted and the vessel remains hove-to for a period of time, significant numbers of load cycles can arise
A total of twelve specimens has been tested, enabling a number of variables to be examined
There is a wealth of literature in the field of cables and wire ropes as these are both very widely used. However, papers which address the area of bend-over-sheave testing of cables (rather than ropes) are rare The fatigue behaviour of ropes in the bend-oversheave regime has been widely studied, and some of the results, and mechanisms would seem to have applicability to umbilicals
The most comprehensive paper which addresses the issue of bend-over-sheave testing of cables and ropes is Gibson (Ref 1) This paper describes the concepts involved in the p l m g of bend-over-sheave testing, and defines a suitable terminology Important definitions include the concept of short stroke tests and long stroke tests, the difference between them being whether the stroke of the cable motion is less than or equal to the length of the cable to sheave contact arc (short stroke) or greater than the length of the cable to sheave contact arc (long stroke) The identification that the cable has three possible types of zone, namely zero-bend zone (ZBZ), single-bend zone (SBZ) and double-bend zone (DBZ) are also discussed, with the length, and number of zones depending on the type of test and stroke length