Page 102 - Pipelines and Risers

Limit-state based Strength Design 75

4.7 Ratcheting
Ratcheting is described in general terms as signifying incremental plastic deformation under
cyclic loads in pipelines subject to high pressure and high temperatures (WEIT).
The effect of ratcheting on out of roundness, local buckling and fracture is to be considered.
Two types of ratcheting are to be evaluated and the acceptance criteria are as below:
1. Ratcheting in hoop strain (the pipe expands radially) as a result of strain reversal for pipes
operated at high internal pressure and high temperature. The accumulative hoop strain
limit is 0.5%.
2. Ratcheting in curvature or ovalisation due to cyclic bending and external pressure. The
accumulative ovalisation is not to exceed a critical value corresponding to local buckling
under monotonic bending, or serviceability. The accumulative ovalisation is to be
accounted for in the check of local buckling and out-of-roundness.
A simplified code check of ratcheting is that the equivalent plastic strain is not to exceed
0.1%, based on elastic-perfectly-plastic material and assuming that the reference for zero
strain is the as-built state after hydro-testing.
In case the simplified code check is violated, a finite element analysis may be applied to
determine if ratcheting is a critical failure mode and quantify the amount of deformation
induced by ratcheting.

4.8 Dynamic Strength Criteria

Stress criteria (Le. allowable moments, allowable stresses elc.), or strain criteria should be
specified for the dynamic stresses or strain expected during vortex induced vibrations (VIV).
At the maximum amplitude of vibrations, the strength criteria defined in this Chapter should
be satisfied.

4.9 Accumulated Plastic Strain
If the yield limit is exceeded, the pipe steel will accumulate plastic strain. Accumulated
plastic strain may reduce the ductility and toughness of the pipe material. Special strain aging
and toughness testing must then be carried out.

Accumulated plastic strain is defined as the sum of plastic strain increments irrespective of
sign and direction. The plastic strain increments are to be calculated from the point where the
material stress-strain curve deviates from a linear relationship, and the accumulated plastic
strain are to be calculated from the time of fabrication to the end of lifetime. Limiting
accumulated plastic strain is to ensure that the material properties of the pipe will not become
sub-standard. This is especially relevant for the fracture toughness.

Accumulated plastic strain may also increase the hardness of the material and thus increase its
susceptibility to stress corrosion cracking in the presence of H2S. Stress corrosion cracking is
also related to the stress level in the material. If the material yield limit is exceeded, the stress
level will necessarily be very high. Plastic deformation of the pipe will also impose high
residual stress in the material that may promote stress corrosion cracking.