Prediction of Burst in Flexible Pipes

Abstract

Usually when a large internal fluid pressure acts on the inner walls of flexible pipes, the carcass layer is not loaded, as the first internal pressure resistance is given by the internal polymeric layer that transmits almost all the loading to the metallic pressure armor layer. The last one must be designed to ensure that the flexible pipe will not fail when loaded by a defined value of internal pressure. This paper presents three different numerical models and an analytical nonlinear model for determining the maximum internal pressure loading withstood by a flexible pipe without burst. The first of the numerical models is a ring approximation for the helically rolled pressure layer, considering its actual cross section profile. The second one is a full model for the same structure, considering the pressure layer laying angle and the cross section as built. The last numerical model is a two-dimensional (2D) simplified version, considering the pressure layer as an equivalent ring. The first two numerical models consider contact nonlinearities and a nonlinear elastic-plastic material model for the pressure layer. The analytical model considers the pressure armor layer as an equivalent ring, taking into account geometrical and material nonlinear behaviors. Assumptions and results for each model are compared and discussed. The failure event and the corresponding stress state are commented.

Finite element modeling of burst failure in unbonded flexible risers

“…(10)(11)(12) with Eq. (8), it is possible to obtained a system of linear equations defined in terms of deformations, ie:…”

“…Neto et al [9,10] proposed a linear analytical formulation, assuming that the pressure armor has a relationship D t > 10, where D and t represents the outer diameter and wall thickness respectively, behave exactly as a thin-walled cylinder. Therefore, the authors suggested that the hoop stress can be represented by:…”

“…This method also considers an added factor so called ''filling fraction'' defined by the effective area of the pressure armor with a ''Z'' profile with the total square area of the profile. Neto et al [9,10] proposed a linear and a non-linear analytical formulation, in the linear model assumed that the pressure armor can be represented as a thin-walled cylinder, considering a thickness equivalent of the ''Z'' section, with reasonably results. The nonlinear model employed by Neto et al [9,10] considered non-linearities of both, the material and pressure armor geometry.…”

“…Neto et al [9,10] proposed a linear and a non-linear analytical formulation, in the linear model assumed that the pressure armor can be represented as a thin-walled cylinder, considering a thickness equivalent of the ''Z'' section, with reasonably results. The nonlinear model employed by Neto et al [9,10] considered non-linearities of both, the material and pressure armor geometry. The non-linearity material considered a bi-linear material represented by the Young's modulus and the tangent modulus model.…”

“…In this sense some works have been developed related to high internal pressure loads (internal overpressure) for different types of risers [5][6][7][8]. Therefore, the pressure armor must be designed to withstand the loads of internal pressure and thus ensure that the pipeline will not failure when high pressure values are presented [9,10]. According to the above, this paper presents a comparison of existing analytical models in the literature with finite element modeling, the finite element models considered the following: (1) one axisymmetric study (with a 2D model) taking into account that the profile of the pressure armor has a pitch near 90°for its characteristic cylindrical shape, (2) a 3D model that considers the interrelation between the winding pitch of the internal pressure armor.…”