Page 429 - Pipelines and Risers

396 Chapier 21

- Materials
- Installation requirements
- Fabrication requirements
- Inspection and maintenance
- Engineering costs

21.2.2 Currently Used Design Criteria
The riser codes that are used in Norway for metallic risers are taken from regulations issued
by the Norwegian Petroleum Directorate (NPD). Both the Snorre and the Heidrun TLP risers
have been designed on that basis.

For internal overpressure design, the design checks recommended in the guideline or
equivalently the allowable stress method, where the maximum allowable stress is given as a
fraction of the Specified Minimum Yield Strength (SMYS), see Table 21.1 below. Note that
for yield strength above 490 MPa the lesser of SMYS or of the Minimum Specified Tensile
Strength (SMTS) divided by 1.2 is used. The stresses are calculated based on the minimum
wall thickness taking into account corrosion allowance and fabrication tolerances.

Table 21.1 Usage factors, internal overpressure, NPD Pipeline Regulations (1990).
(100 vear) I (10 sumva' vear) I
Load Combination Installation Operation
Phase OOO
Functional Only 0.75 0.6
1.0

The required stress checks include both a pure hoop stress and a von-Mises equivalent stress
that include pipe wall hoop and axial stresses. No equations for calculation of the stress
components are given. This means that there are several possibilities for calculating the hoop
stress (i.e. using internal diameter, mean diameter or outside diameter) and for calculating the
bending stress (i.e. using outside diameter or mean diameter).

For external overpressure design (compressive hoop stress-collapse), the NPD Pipeline
Regulations do not give any design criteria. However, it has been a practice to use the NPD
Load Bearing Regulations, including the guideline. Those are based on limit state design with
partial safety coefficients. Typical factors for ULS condition are 1.3 for axial and bending
stresses and 1.0 for pressure induced stresses with a material factor equal to 1.15.

As for the NPD regulations, the API RP 2RD is a stress based code using the von-Mises yield
criterion for yield strength checks. API RP 2RD provides also design criteria for external
overpressure (collapse) see Table 21.2.