Page 440 - Pipelines and Risers

Design Codes and Criteria for Risers 407

Pipe characteristics:
0 Diameter over wall thickness ratio @/t);
Material work hardening characteristics;
Material imperfections;
0 Welding (Longitudinal and circumferential welds);
Dents;
Initial out-of-roundness;
0 Reduction in wall thickness due to corrosioderosion;
Cracks (in pipe andor welding);
0 Local stress concentrations due to coating;

Loads applied:
External and internal pressure;
Axial tensionlcompression;
Temperature;
Bending moment.

21.6.2 Design Approaches
To meet the new challenges being placed on the industry two new riser codeslguidelines have
recently been issued, these are:
IS0 13628-7, (1999) “Petroleum and natural gas industries - Design and operation of
subsea production systems”, Part 7: “Completiodworkover riser systems”, International
Standardisation Organisation ;
0 API RP 2RD, (1998) “Recommended Practice for Design of Risers for Floating
Production Systems and TLP’s”, First Edition.

These two codes adopt different approaches, the first being a “Limit State Design” (LSD)
approach and the second an “Allowable Stress Design” (ASD) approach. Both these
approaches are valid, however the because of the different approaches adopted direct
comparison is difficult. In addition to these codes Bai and Hauch have been investigating over
the last 3 years the local strength characteristics of pipe under combined loads based on
detailed Finite Element Analysis (EA) with comparison with physical testing. The three
approaches are subject to a comparative review. See Langford et al. (2000).
21.6.3 Application of codes
The riser design codes vary in the way they interpret the allowable loads on the riser (Le.
ASD vs. LSD) and hence differences are expected between the two approaches. To answer
the question of whether the approaches generate consistent levels of safety over the full range
of axial loads, bending moments and pressures, all methods have been normalized based on
allowable bending moments (Jensen, 1999).