For many offshore oil and gas field developments, subsea pipelines are tied-in to fixed platforms with vertical risers and rigid spools, with structural fastening and stability provided through guides / clamps to the platform. Subsea riser design is driven by functional and environmental loads, and failure states range from short-term over stress due to extreme loading scenarios (ULS) to long-term fatigue loading conditions (FLS).
The wave fatigue assessment of subsea rigid risers during FEED has traditionally been a complicated and timely process. A typical approach has been to use Autopipe, among other tools, for the analysis. However, the use of this software leads to an overly-conservative fatigue life prediction, because it is based on regular wave theory. In addition to being a laborious process, a full wave cycle is accounted for by performing a series of static load cases.
If, when using Autopipe, the fatigue life is predicted to be unacceptable, one needs to adopt more sophisticated FEA modelling techniques and tools such as using ABAQUS, requiring engineering specialists and a significant number of hours. Alternatively, the fatigue life can be met through physical adaptations, such as shortening riser span lengths, which invariably results in addition installation time and cost.
This paper assesses how riser wave fatigue analysis can be performed more accurately and efficiently using Orcaflex, and is demonstrated by running and comparing the results with those from Autopipe.
The paper concludes that Orcaflex provides several benefits over Autopipe; these being:
- Use of a single piece of software, to perform the analysis effectively the first time.
- The ability to model irregular waves.
- Over-conservatism is eliminated.
- Results are more easily interpreted.
- Design duration is reduced.
- Models are readily transferrable to the EPCI Contractor.
The above improve the certainty of FEED, and transfers less risk to the subsequent detailed design stages.