Minimum Bending Radius (MBR) Tests of Flexible Pipes: An Experimental Approach Via Optical Motion Capture and Image Processing

Abstract A new optical measurement approach for minimum bending radius (MBR) tests of flexible pipes is presented. The method is based on an accurate tracking of the elastica through image processing. The new approach shows to be not only appropriate to the standard measurement requirements but also enables a comprehensive understanding of the structural mechanisms embedded in the problem. Samples were made from two distinct flexible pipes, with inner diameters of 2.5 and 4 inches. A progressive set of tests was performed using five types of samples, obtained by dissecting each flexible pipe up to a certain layer. Sample 1 is made up with the internal carcass only. Sample 2 is constructed up to the liner (the internal plastic sheath covering the carcass). Sample 3 is mounted up to the pressure armor. Sample 4 is manufactured up to the external tensile armor. Sample 5 is made from the entire pipe, up to the external polymeric sheath. The experimental methodology is given in detail. The samples are bent through the pulling of a cable linked to the extremities. The traction is measured by a load cell. A special stereoscopic system is used to optically obtain the geometry of the elastic line of the bent sample. From the measured elastica, the curvature is calculated. Results of bending moment at any section may be also calculated from the elastic line. In particular, bending moment vs. curvature, measured at the vertex of the arch, are calculated for each sample. Introduction Flexible Pipes are increasingly used for the transport of oil & gas, due to their flexibility, reliability and high resistance to the several types of loads inherent to this type of application. These loads include high pressures, both internal and external, tensions and curvatures and the flexible pipe resistance to these different loads is obtained by a construction with many component layers, each one responsible for the resistance to a given load (See Figure 2 for the different layers, represented by the different samples used in the test). Since they are slender structures, a significant amount of bending is induced during their operation and must be taken into account in its design. This paper describes an experimental methodology to relate bending moment and imposed curvature, which is specially suited for the validation of a design methodology, since its results allow the developer of such methodology to have insights into details of the mechanical behavior of the tested structure, on a layer-by-layer basis. The test was applied to obtain the maximum curvatures that different layers of two flexible pipes can reach without damage.