Page 462 - Pipelines and Risers

Fatigue of Risers 429

the ambitions of the industry - and hence the assumption that the analysis is providing a
similar level of confidence in the deep waters as the shallow waters may not be sound. The
designers should ask themselves when going into deeper waters what the level of confidence
in their analysis is compared to shallower water analyses and review their results accordingly.

Rationalization of Approaches
Upon review of this paper one can question if the predicated pipe responses are realistic and
the stated utilization factors (safety factors) are appropriate. This question is difficult to
answer as historically both the analysis and levels of safety have been built up based on years
of experience. In our case we have no experience of ultra deep waters (10,OOOft) so the
inclination is to be very conservative. However, modem analysis methods using EA such as
a numerical laboratory has increased the confidence in riser designs.

Probably the most rational approach to address this situation is to adopt a ‘Load Resistance
Factor Design’ (LRFD) method. The principal is to look individually at the loads (i.e. weight,
current, vessel motions etc) and the resistance to the loads (i.e. stiffness of the catenary, vessel
support, seabed support etc.) and factor based on our level of uncertainty for each. This level
of uncertainty would include how accurately we know the pipe strength, predict its response
and what loads are being applied. Adopting a LRFD approach for both the local buckling and
fatigue then the following can be observed.

Based on this simplified LRFD approach the local buckling analysis will have relative high
levels of confidence for both the loads and responses, indicating that the utilization factors do
not have to be conservative. However, for the fatigue the levels of confidence for both the
loading and resistance are not high, meriting that the utilization factors are justified as being
high.