The Effect of High Triaxiality in HPHT Polymers Used in Flexible Pipes

Abstract The increasing trend for higher temperature and higher pressure developments places more demanding design requirements on flexible pipes. More sophisticated polymer modelling becomes important in these conditions. The inner pressure sheath or barrier is the critical pressure containing layer in unbonded flexible pipe. At high temperatures, these materials soften significantly whilst retaining sufficient toughness for standard operation. However, this may result in significant plastic deformation in partially constrained scenarios such as at interfaces between metallic and polymer layers. Traditionally, inner pressure sheath integrity is managed by limiting global strain and ensuring sufficient remaining barrier layer thickness, as specified in API 17J, or through the addition of additional sacrificial layers (tapes or extrusions) where required. Highly localized polymer strains are not normally considered a risk to product integrity. A large local plastic deformation in a relatively thick layer may result in significant residual triaxial tension at the interface between the plastic zone and the elastic zone within the polymer. At low temperatures, the normal polymer material stiffening exacerbates the triaxial stress state, potentially creating the conditions for fracture nucleation. Similar effects may be observed in highly dynamic events, such as rapid decompressions, where the strain rate variation can result in changes in material stiffness analogous to temperature effects. In this paper this phenomenon is discussed further, describing how Enodo and Baker Hughes have performed material testing, developed material models, finite element simulations and defined failure criteria to predict such failures and provide a safe operating envelope for inner pressure sheath design in high temperature and high pressure scenarios.