Page 439 - Pipelines and Risers

406 Chapter 21

The RP will include methodology for the design of risers outside the experience range.
Deepwater will be one such area.

21.5.3 Metallic Riser
API issued a new code on design of metallic risers, MI Rp 2RD (API, 1998).

The Norwegian Research Council (NFR) and the Norwegian oil industry are sponsoring a JIP
program to develop a design guideline for deepwater steel risers (Kirkemo et al, 1999).

The Load and Resistance Factor Design (LRFD) is adopted by DEEF'RISER JIP. This format
is based on checking the risers in defined limit states with a set of load and resistance factors
calibrated to give a target reliability of the riser.

There are a number of technical challenges that have to be addressed by the new guidelines
such as:
- Uncertainties in loads, vortex shedding (e.g. vortex-induced vibrations (VIV) and soil
interaction).
- Fatigue life of girth welds.
- Reeling with plastic strain of a dynamic riser influence on fatigue.

21.6 Comparison of IS0 and API Codes with Hauch and Bai (1999)

21.6.1 Riser Capacity under Combined Axial Force, Bending and Pressure
Dynamic, unsupported (catenary) metallic risers are a relatively new development, having
been used by Shell on Tension Leg Platforms (TLPs) since 1994 and more recently by
Petrobras on semi-submersibles since 1998 in water depths of circa 3,000 ft (Silva et al.
1999). Based on this limited practical experience it is very difficult to identifykonfirm in
which areas of strength criteria and methodology the industry is being conservative or un-
conservative.

Dynamic catenary risers will experience a combination of externdinternal pressure, axial
compression/tension and bending moments. As the metallic catenary risers are employed in
deeper water depths with greater diameters, the existing boundaries on acceptable moment
capacities will be challenged - the question is whether the boundaries set by the new codes
are applicable to these ultra deepwater applications (i.e. 10,000 ft - 3,OOOm)

The criterion used to determine when local buckling occurs can be stress based (WSD
approach) or maximum bending capacity (LSD approach), the magnitude of this criterion is a
function of many parameters. The main parameters are as follows: