Page 98 - Pipelines and Risers

Limit-state based Strength Design 71

PD6493 FAD (Failure Assessment Diagram) gives critical stress for the given defect and
material. It is necessary to convert the critical stress to critical strain and for this the Ramberg-
Osgood relationship is used as defined below:
O 3 0
E = - (1 + -(-)n-l} (4.13)
E O0.7

where:
o0., = 430 MPa for X65 at 20°C
n = 26 for X65 at 20°C

The allowable strain criterion used in this report is conservative due to:

The stress-strain curves used in converting stress to strain are based on the lowest yield
stress and lowest ultimate stress.
PD6493 has been derived for load-controlled situations, but is here applied to both load
and displacement-controlled situations.
The flow stress is in PD6493 defined as the average of yield and tensile stress.

Corrosion in girth welds can significantly reduce the critical tensile strain of the girth welds if
a flaw is assumed to be in the surface of the corroded weld. Fracture mechanics assessment of
existing pipelines has shown that the critical strain can be between 0.1% for heavily corroded
pipes and 0.5% for pipes with shallow corrosion defects. However, we shall not assume the
combination of corrosion and cracks in the girth welds, although some pitting could occur in
the BAZ (Heat Affect Zone). If corrosion takes place, it will occur over a certain number of
years after entering into service, when the maximum strain load became lower due to reduced
operating pressure and temperature, and “shakedown” of peak stresdstrain levels in a number
of shut downs.

In the technical report “Update of laying criteria for pipelines”, Denys and Lefevre, it is stated
that the failure of welds under displacement controlled situations is highly dependent on the
weld matching (in particular), and on the ratio of yield to tensile strength. They gave an
allowable strain of 0.61x1.2A.5 = 0.50 % (a safety factor of 1.5 is applied to the critical
strain) for defect length dl.2 and depth 3 mm, assuming that the weld is matched and the ratio
of yield to tensile strength is 0.87. They also reported that the results are very sensitive to
weld matching (over-matching will increase the allowable strain considerably, and under-
matching will reduce it).

The Dutch code NEN 3650 states that, normally, a tensile strain of 0.5% will not pose any
problems for material and welding in accordance with their specifications. If it can be
demonstrated that the ductility of the material is greater, higher strains can be tolerated
accordingly.