ANÁLISIS DE CONFIABILIDAD ESTRUCTURAL DE TUBERÍAS ASCENDENTES DE ACERO EN CATENARIA (SCR)

Abstract

A Steel Catenary Riser (SCR) is an extension of submarine pipeline, which carries oil from wells to floating production platform, with a geometric shape taken by the resultant vertical forces as the weight and buoyancy acting along its length. This SCR is subject to combination extreme tension, bending and pressure throughout its length by the effect of ocean environment and offset of floating system. Under these conditions, the SCR should be evaluated against the collapse and rupture. Pipe materials, geometry and loads introduce uncertainties in the response as the determination of their resistance, due to this fact it is necessary to have tools and methodologies based on structural reliability for limiting the risk to an acceptable consequence of fail. The object of this study is to develop a methodology for structural reliability analysis of steel catenary riser for application in floating production systems in deep water. The methodology is based on obtaining the expected value of the extreme response of the riser for a long term sea state. Through a structural analysis of the 3D hydrodynamic floating system subject to several short-term sea states is obtained history in the time variations of the axial forces and bending moments in each of the riser sections of interest. To consider the uncertainty associated with the geometry, the model analytic and material properties were selected random variables, which are incorporated in a interaction ratio (IR) in order to obtain the dynamic response of the riser for each short-term sea state. The IR is used to evaluate the ultimate limit state of the SCR. This series of IR are used to obtain probability distributions fitted to samples of maxim and extreme responses. Probability functions of the short-term IR are integrated to obtain the distribution of long-term response of the riser. Finally, to evaluate the extreme response of the SCR for a period of time analysis it is considered that short-term storms are a group of independent events over themselves and their numbers of occurrences over time follows a Poisson distribution. This last distribution is conditional on the values of the loads effects and random variables, and it is used to obtain the probability of failure of the SCR using the First Order Reliability Method. The methodology is applied to two SCR in operation on a TLP in the deepwater Gulf of Mexico. The results show that the section of the riser in the touchdown point with the seabed has the highest probability of failure due to a combination of mechanical stresses and external hydrostatic pressure