Page 420 - Pipelines and Risers

Design of Deepwater Risers 387

20.3.2 Design Codes
The most applicable design guidance, for metallic catenary risers, is fragmentized between a
number of Codes and Recommended Practices. Rationalization of these is currently the
subject of other forums, in America (MI), and Europe (ISO).

Riser maximum equivalent stresses during extreme storm conditions are limited to 80% yield
stress. 100% yield stress is acceptable during abnormal conditions such as a mooring line or
tether failure. This approach has been adopted on other (vertically tensioned) riser systems
and is in line with API RP 2RD and the ASME Boiler Code. However, the question arises as
to whether higher allowable equivalent stresses can be considered for metallic catenary
applications.

Higher stress allowables are particular interest at the Touch Down Point (TDP) where stresses
are largely displacement controlled. Langner et al. (1997) propose a stress of 1.0 and 1.5 times
yield stress for extreme and abnormal conditions. Whilst this offers some scope to the
designer to address extreme storm response, caution must be exercised. Designing with higher
utilization may lead to an unacceptable fatigue life and the validity of assuming that TDP
response is displacement controlled is not always correct. This is particularly true where low-
tension levels are observed. Additionally, the effects of plastic deformation on weld fatigue
performance must be investigated before higher utilization levels can be adopted with
confidence.
20.3.3 Analysis Parameters

Hydrodynamic Loads
There are uncertainties related to vortex-induced vibration (VIV). If the stresses are above the
endurance limit of the material then fatigue may take place. In addition, VIV may result in
drag amplification that may result in increased stresses. Finally the hydrodynamic interaction
between risers may result in riser crashing loads which must be considered. Which of these
effects that can be acceptable for a design and what measures should be taken to control such
effects are not yet fully understood. It should, however, be mentioned that VIV suppression
has been used on most MCR’s and that MCR systems with narrow riser spacing have so far
not been installed in deepwater.

Material Properties
The metallic material to be used in deepwater MCR’s offshore is likely to be steel of API
grade X65 or above. Alternatively, other high strength steel such as 13% Cr or Super Duplex
may be applied. Titanium alloys are also very attractive to deepwater applications.

The long-term properties for the base material are relatively well known. The main
uncertainty lies in the effect of welding combined with plastic strain (reeling and laying).
Testing is presently ongoing. Until validated S-N curves (Stress range versus Number of
cycles to failure curves) are available, MCR design has to be based on conservative
assumptions which may limit the use and complicate installation.