Low Cycle Fatigue in HP/HT Flowlines
Authors: Luk C.H. , Qi Xinhai , Wang Tai-Jun , Brunner Mark 
Published: October 2008

This paper presents two design methods and example analysis results for low cycle fatigue design of HP/HT flowlines. The first method is based on fatigue tests and traditional S-N curves. The second method is based on fracture mechanics in accordance with the Level 2 Engineering Critical Assessment (ECA) procedure as described in the BS7910:1999 guideline [1]. Both methods utilize the same thermal load histograms developed for the flowlineâ??s service life using the ABAQUS finite element program with the expected temperature and pressure history input. The S-N curves are derived from seawater and sour service fatigue tests and are modified to reflect the fatigue behavior of a material in the low cycle and high stress range. Fracture toughness and fatigue crack growth rates are derived from tests conducted in air, seawater, and sour service environments, and are modified to reflect the effects of load frequency, load form, and stress ratio. Also provided in this paper is the calibration result between S-N fatigue constants and crack growth rate constants obtained by the fracture mechanics ECA model.

The design example includes a HP/HT flowline with lateral buckles near the middle. Fatigue damage derived from the S-N approach are below the design value for a 25-year service life. The critical flaws are found to be internal surface and embedded cracks in the circumferential direction in the girth weld along the flowline. A non-destructive testing (NDT) inspection strategy is recommended. Allowable initial flaw sizes are identified for the flowline example. Flaw size sensitivity is also discussed with respect to the crack tip opening displacement (CTOD) and the total stress correction factor (TSCF).

Introduction

The design of high pressure, high temperature (HP/HT) flowlines in deepwater requires the consideration of large pressure and temperature transients resulting from operating cycles such as hydro test, initial startup, and repeated heat up and cool down of the flowline. These operational transients will subject the flowlines to end expansion, lateral buckling and deflections, which in turn may produce repeated high stresses and strains, and hence fatigue damage to the flowlines. Safe lateral buckles are proposed to control the flowline end expansion and fatigue damage by providing localized imperfections where predictable lateral deflections can occur. The basic concept of the flowline safe buckle is that as long as a buckle occurs at the properly designed locations the maximum stresses in the pipe wall will always be within the allowable, and the fatigue life will be predictable.

Brunner et al [2] presented a complete 3-D thermal mechanical finite element model developed with the ABAQUS program for the HP/HT flowline. The maximum longitudinal stresses and stress ranges required in the flowline fatigue damage and ECA calculations are obtained from this FEA model. The FEA model is capable of handling the temperature-dependent material properties, user defined pipesoil interaction with cyclic response, thermal transients, seabed terrains, large deformation due to buckling, and contact surface with the seabed. Figure 1 gives a schematic drawing for the FEA model of the flowline, which contains one buckle initiation section located close to the middle of the flowline.