Analysis of a Flexible-Riser Top Connection With Bend Restrictor

Abstract

ABSTRACT The connection of a flexible riser to the floating production system is a critical area with respect to extreme deformation and fatigue. One of the proposed means of preventing failure of the flexible pipe at this point is to apply a tapered polyurethane collar around the pipe, termed a 'bend restrictor'. For the design and analysis ot this bend restrictor two models are developed: one based on slender beam theory and a full finite-element model. The slender beam model provides an efficient design tool, although finite-element analysis remains necessary to check the? final design. 1. INTRODUCTION The riser system is a vital part of a floating production/storage unit, particularly in a harsh environment such as the North Sea. Compliant flexible risers are undoubtedly a suitable lliethod for transporting hydrocarbons between the sea bed and the floating facility. They have numer0US advantages over vertical steel risers, the most important being permanent connection, lighter weight and no need fur heave-compensating facilities. The attractiveness of such systems is reflected by the large number of publications on the design and analysis of flexible riser systems over the last five years, e.g [l]-[61. The mechanics of flexible risers is complicated, not 0nly because of the compliant nature of the system and the consequent complex loading, large displacements and geometrical nonlinearities, but also because of the composite structure of the flexible pipes used in these applications. These pipes derive their strength from reinforcing wires, wound helically around a polymer or elastomer oil/gas conduit. As a result, they are strong and stiff inresponse to tension, torsion and internal pressure, while being compliant in bending and having a large curvature capacity. The mechanics of flexible pipes is the subject of many papers, e.g. [7]-[1]. One of the critical areas is the top connectionof the flexible riser to the floating production platform. A rapid transition from a rigid connection to a very compliant pipe structure is undesirable, because it may lead to kinking at that location. This can also occur at the sea-bed connection in a steep wave configuration as shown in Fig. 1. In principle there are two solutions available:a hingedconnection,a clamped connection in combinationwith a means of preventing kinking. The first solution requires a reliable mechanical arrangement with sufficiently low restraint; the second involves a belled restrictor of some kind. While adequate mechanical arrangements are not readily available, there is some choice in bend restrictors. A steel trumpet (bell-shaped riser entry) can be attached to the riser connection point, but is not favoured from an operational point of view, particularly if a large number of risers have to installed. The preferred alternative is a solid polymer cone, fitted around the top section of thepipe. The cone would be moulded around this section or prefabricated and applied later. This paper deals with the analysis of the top section of a flexible riser with a polyurethane bend restrictor of a conical shape.