Currently accepted codes advocate limits on allowable design stresses based on elastic stress analysis, together with strain limits which are not well defined This leads to inconsistencies with pipeline steel specifications which include plasticity effects A constitutive model for pipeline steels IS Introduced which allows the calculation of stresses and strains for significant design cases to be accomplished in a rational manner The procedures suggested should allow the question of the acceptability of current stress limits to be explored further
The design of submarine pipelines is based on a variety of codes and regulations, which are, in many cases, open to conflicting interpretation This is particularly true in relation to the fundamental question of the choice of allowable hoop stress. Operating pressure and temperature, and also pipe diameter are effectively determined by product delivery considerations The outstanding question for the pipeline designer having chosen a material specification is the choice of wall thickness For a given diameter, steel grade and pressure, this effectively translates to the selection of hoop stress The combination of hoop stress with the axial stress arising from pressure, temperature, and bending requires the formulation of an appropriate theory of failure under multi-axial stresses and the definition of corresponding safety margins The choice of margin is complicated by the intervention of plasticity effects These are often ignored by design codes (1,2,3), which rely on elastic analysis to determine axial stress even when plasticity is known to be present The effect of plasticity is sometimes (2) allowed for by setting an arbitrary limit on strain without defining the means of calculation The purpose of this paper is to introduce a consistent procedure for the introduction of both elastic and plastic multi-axial stress-strain effects into the design process.
