OMAE 2007: SeaFlex published a paper on fatigue in tensile wires of flexible pipes using irregular waves
SeaFlex, represented by lead engineer Christoffer Nilsen-Aas, recently presented the FlexStress methodology at the 26th international conference on Offshore Mechanics and Artic Engineering (OMAE) held in San Diego, California. The publication was the result of a comprehensive R&D project with SeaFlex and Exxon Mobil Development Company as main contributors. The authors consist of MSc. Rune Haakonsen, Dr. Robert Bjærum and MSc. Christoffer Nilsen-Aas from SeaFlex, and Dr. Krassimir Doynonv and Dr. Wan Kan from Exxon Mobil Development Company. The industry responded very well to the presented methodology and the general feedback in the wake of the presentation was that service life assessments of flexible pipes using irregular wave philosophy is the natural step forward for all flexible pipe suppliers.
The FlexStress methodology is a bridge between validated programs for local and global analytical tools for flexible pipe analysis such as BFLEX and RIFLEX. By relying on advanced mathematical formulas and powerful interpolation algorithms it is possible to describe the stress development in the tensile wires of non-bonded flexible pipes as function of global riser response. The concept of the methodology is to use the local analysis tool in a limited number of runs in order to describe all possible load combinations established in the global analysis tool. Sufficiently long time series of riser response can then be efficiently transformed into time series of tensile wire stress, maintaining the consistent statistics of the real sea environment.
The paper describes in short the methodology and the underlying assumptions. Great focus is given a case study whereby the traditional regular wave methodology is compared with the more advanced irregular wave methodology. The conclusions from the case study show that the uncertainties related to selecting a limited set of regular waves from the full scatter diagram is high and the quality of the regular waves depends on the experience and competence of the analyst. This uncertainty can strike both ways, but in most cases waves are selected on a high conservative level and used as basis for the fatigue calculations. Hence, due to the logarithmic nature of the service life estimate, much can be gained by applying the riser system with a set of realistic irregular waves. This may not only be important for maintaining the integrity and safety of flexible riser systems, but also improved lifetime estimates may be the key factor for opening for flexible riser technology in deeper and harsher environments.
The paper is available on the OMAE proceedings database.