Industrial codes state that ratcheting of high pressure and high temperature flowlines (HP/HT) during start-up and shut-in/shut-down cycles places a stringent limit and allowable equivalent plastic strain is 0.1%. Unfortunately, this allowable limit neglects the effect of strainhardening and large deflection. The aim of this study is to investigate the ratcheting behavior of HP/HT (High Pressure/High Temperature) flowlines using a finite element approach. The analysis procedure is at further development of a procedure applied by the author to pipe local buckling/collapse under combined pressure, tension and bending load. The extension of the analysis procedure consists of applying cyclic pressure and bending loading in order to study ratcheting. The finite element approach has been validated using experimental and analytical results. The allowable limit for ratcheting failure mode of an unburied HP/HT flowline was obtained by simulating its ratcheting behavior.
As flowlines operate at higher temperatures and internal pressure, conventional design criteria that limit equivalent stress to the elastic region of the material stress.strain curve result in overly conservative design. Ratcheting is a general term signifying incremental plastic deformation under cyclic loads. Pipeline design rules state that it is necessary to consider the effects of ratcheting and its effect on local buckling and plastic strain of longitudinally welded pipes. On ratcheting of high temperature flowlines, the work from Klever et al (1994) is known in the pipeline industry. It states that cyclic ratcheting during start-up and shut-in/shut-down cycles for a buried pipeline places the most stringent limits on allowable loading and 0.1% equivalent plastic strain is the allowable limit. Unfortunately, this allowable limit was based on analytical plasticity that neglected the effect of strain-hardening and large deflection. In Bai and Damsleth (1997), failure modes and limit-state design criteria are discussed in detail for design of HP/HT flowlines.
