Abstract
The axial behaviour of an unbonded flexible pipe is highly non-linear and may change throughout the load history and along the pipe. The axial stiffness and expansion coefficients (pressure and temperature) of a flexible pipe are given as two sets: one for the tension state and one for the compression state. Previously the typical interpretation has been that this tension/compression classification referred to the true wall axial force in the pipe. However, this approach is incomplete in several aspects:
• The determining physical factor is whether the tensile armour layers are in contact with the underlying layers.
• An unbonded flexible pipe consists of different layers with different axial force and orientation. It can be argued that the term “true wall axial force” does not make sense for an unbonded flexible pipe.
• To determine the axial force for a given condition, a set of expansion coefficients and axial stiffness must be selected. Hence, it becomes a circular argument.
Based on detailed analysis by the product supplier, a transition plane can be defined. The “Transition Plane Concept” offers an efficient way of implementing a realistic axial behaviour in the in-place design (incl. upheaval buckling). The axial behaviour during the in-place phase can be consistently defined by a transition plane and this can be implemented in the in-place analysis. Reduced risk for upheaval buckling of flexible pipes and removal of excessive conservatism can be achieved.
