Proposal to modify a subsea pipeline standard
The offshore standard for submarine pipeline systems from Det Norsk Veritas is internationally recognized. This standard was developed based on the limit state design (LSD) criteria and using the load and resistance factors design (LRFD) principle. The combined loading criteria (CLC), applied to pipe members subjected to bending moment, axial force, and internal overpressure, essentially determines the design solutions for a pipeline. This requirement for all cross sections of a pipeline is described in the D505 clause of the DNV standard [1].
The existing CLC does not correctly consider the influence on the combined effect of the axial force for a pressurized pipeline. Equation 5.23 of D505, which describes CLC, includes a design effective axial force (Sd) that determines the global response of a pipeline. See clause 209. The axial force in a pipe wall (“true” force Nd) should be used instead of the effective axial force because equation 5.23 is based on parameters such as the yield stress and the tensile strength. Detailed discussion of “true” and effective axial forces was published previously [2].
The difference between the axial force in a pipe wall and the effective axial force is not only terminology. The quantity is significantly different in value as well as the sign (tensile or compressive). The sign of the axial force, i.e. whether the axial force is tensile or compressive, does not affect the CLC in equation 5.23. This contradicts the Von Mises or Tresca theories of combined (equivalent) stresses used for pressurized pipelines.
The method to determine the “true” steel wall axial force is outside the scope of this column. For a totally restrained and non restrained pipeline, the method is shown in equation 3. The method, which considers the soil resistance and the pipeline deflection for free span analysis is shown in equation 4.
The proposed method is based on classical Von Mises and Tresca combined (longitudinal and hoop) stresses theories, and on the plastic resistance criteria as the limit state. Von Mises and Tresca are used in US Codes B31.4 and B31.8 for offshore pipelines. DNV’s terminology, definitions, and symbols are used for the most part in this column.
The pipeline is modeled as the ring section beam under the bending moment (M) and the axial force in a pipe wall (N). The pipe material has different yield stresses at varying uniaxial tensile and compressive stresses due to the pressure hoop stress (σh). Yield stresses in tensile (σy,t) and in compressive (σy,c) zones in the biaxial stress state (longitudinal and hoop [tangent] stresses) can be related to the yield stress (σy) at the uniaxial tension by equation 1. Equation 1 describes the Tresca and Von Mises tensile and compressive yield surface by the parameter (ψ).
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Volume 68 Issue 9
September 2008
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