Fatigue Properties and Analysis of Flexible Riser

Abstract

ABSTRACT Assessment of mechanical durability and prediction of life expectancy constitute important requirements for the safe design of flexible risers. In the present study, fatigue tests of actual pipes were conducted with respect to bending fatigue characteristics, which are most difficult to quantify in the assessment of mechanical durability of actual flexible pipes. Furthermore, a theoretical study of the repeated stress generated in the reinforcements by frictional forces between the reinforcing layers was conducted, and fatigue mechanisms were considered on the basis of this study. Through this theoretical approach the effects of pipe size, structure, internal pressure, and tension upon bending fatigue life were elucidated. On the other hand, fatigue analysis methods combining dynamic behavior analysis and spectral theory, as a means for predicting the degree of fatigue damage inflicted upon riser pipes under irregular natural conditions, will also be briefly described. INTRODUCTION Flexible riser pipes constitute an important structural element in the development of marginal or deep offshore oil fields. Flexible pipes comprise both bonded and unbonded varieties, but only the latter variety will be considered in the present account. Unbonded flexible pipe is a composite formed from thermoplastic resin pipe plus a metal or other reinforcing material, and is characterized by excellent flexibility as well as suitability for continuous fabrication and installation of long sections. An example of the structure of unbonded pipes is shown in Fig. 1. The design of such composite structures in a manner appropriate for the ambient conditions prevailing in the intended applications can provide pipes possessing the required mechanical characteristics [1, 2, 3,4]. Moreover, the intrinsically excellent flexibility of such pipes permits bending deformation following the oscillation of the floating production vessel in extream condition. However, the bending of these pipes is accompanied by relative displacement (slipping) of the constituent elements of the composite, which gives rise to wear as well as repeated frictional stresses in the reinforcing materials, thereby causing degradation of the mechanical characteristics ofthe reinforcing components. The authors have conducted the studies described below with the objective of elucidating these fatigue properties of flexible pipes on the basis of the results of fatigue tests of actual pipes as well as theoretical considerations, and, moreover, the establishment of statistical analytical procedures for the prediction of pipe lifetimes under irregular environmental conditions. DURABILITY EVALUATION TESTS The following durability evaluation tests were performed using as specimens 4" and 8" flexible pipes with working pressure 5000psi for use at submerged depths of 100m. These specimens are standard pipes manufactured by the Furukawa Electric Co., Ltd., incorporating high-density polyethylene tape wound between the tensile reinforcing steel wire and the pressure reinforcement strips as well as between the two layers of tensile reinforcement for the purpose of diminishing friction between the various layers. Furthermore, when required by particularly rigorous environmental conditions, plastic extruded layers may also be incorporated between the abovementioned layers, but were not used in the pipes tested in the present study.