On the Influence of the UOE Process on Collapse and Collapse Propagation Pressure of Steel Deepwater Pipelines Under External Pressure
Authors: Toscano R. , Raffo J.L. , Mantovano L. , Fritz M. , Silva R.C. 
Published: October 2008

Large diameter pipes manufactured by UOE SAW process utilizing heavy plates are generally used for onshore and offshore applications. The heavy plate is pressed along its edges, formed into a U-shape and then pressed into an Oshape between two semicircular dies. The pipe is welded by SAW process and then expanded to obtain a circular shape. A 2-D finite element model can be used to numerically model the UOE forming process. The model can simulate the effects of process parameters of each forming step on the final geometry and mechanical properties of the pipe. The FEA method is an adequate and reliable tool for the above mentioned studies. By using Finite Element Models, our purpose in this paper is to model the UOE process and analyze its influence on the collapse behavior of the pipes, study how the process and material model affects to collapse behaviour.

Introduction

The UOE process is characterized by a forming stage, SAW welding and expansion. During the forming stage, the plates are bent into a circular shape by an Edge press, and then deformed with the "U" press, and afterwards with the "O" press. Then the formed plate is welded to produce the pipe. Finally this welded pipe is expanded with a mechanical expander. This manufacturing process introduces plastic deformations and residual stresses in the initial unstrained plate material. A bi-dimensional finite element model is developed to describe the UOE process, following CONFAB specifications (process and tooling), in order to analyze the influence of each stage of the process on the material plastic deformations and residual stresses and the structural behavior of the pipes. The manufacture process of a 12.75" OD 0.5" WT X60 and a 18.0" OD 1.0" WT X60 UOE welded pipes are modeled. A kinematic hardening model is considered in order to incorporate a description of the Bauschinger effect on the final pipe collapse pressures. A sensitivity analysis aimed at the investigation of the effect, on the UOE pipe properties, of the steel strain hardening, is performed using the developed finite element model.

Finite element model

For the numerical simulation of the UOE process, a finite element model using the Q1-P0 plane strain element, in the ADINA general-purpose code [1-2] was developed. The numerical model was developed using a material and geometrical nonlinear formulation, taking into account large displacements/rotations but small strains [2]. Regarding the material, we use an elasto-plastic bi-linear material model and von Mises associated plasticity with kinematic hardening. The main characteristics of the material model are Young's modulus: 206010 MPa; Poisson coefficient: 0.3; yield stress: 522.7 MPa, assumed isotropic in the unstrained material; several hardening modulus values are considered for the strain hardening: 0.2 %, 0.3 %, 0.4 %, 0.5 % and 1.0 % of the Young's modulus for the 12.75" OD pipe and 0.2 %, 0.5 % and 1.0 % of the Young's modulus for the 18.0" OD pipe. During the collapse tests performed at C-FER Technologies (Edmonton, Alberta, Canada) the tensile / compressive hoop yield stress was determined for fibers located close to the OD and ID respectively. Hence, we use as the yield stress of the unstrained material: