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Alphatecs Presenting at the Earth and Space Conference

Earth and Space

17th International Conference on Engineering, Science, Construction & Operations in Challenging Environments

Seattle, Washington, USA

April 20 – 23, 2020

Experimental and Numerical Investigations into Fatigue Assessment of Structures in Challenging Environments of Deep Oceans

Sasan Iranpour, Ph.D., P.Eng.

Estimating the fatigue damage of structures in challenging environment such as high winds and ocean waves is an important issue for both design and maintenance of the asset. The interaction of the structure with fluid makes it challenging to properly calculate the dynamic behavior of the structure and provide a real estimate of the stress-time history generated in the structure. Many researches have been completed to date to provide a better estimate of these stresses under such extreme loading conditions. However, the uncertainties in behavior of both the material and the structure under such extreme loading conditions have made it impractical to provide a real estimate of the fatigue life of the asset.

In extreme environments, the structure experiences a Variable Amplitude Loading (VAL) scenario due to its complex interaction with fluid. However, most researches on fatigue life estimation of structures under such loading conditions have been completed using a Constant Amplitude Loading (CAL) scenario, followed by application of simplified fatigue models. In many cases, different numerical methods have been used to calculate a CAL that is equivalent to the VAL experienced by the structure. Although this approach has made it practical to conduct the experimental researches using equivalent CAL scenarios, the limit of its application is often exceeded, resulting in unrealistic fatigue life estimation. These have caused many catastrophic failures, loss of production and costly repairs.

Using extensive field experiments in the Gulf of Mexico, this paper studies the fatigue life assessment of structures under sheared deep ocean current profiles. The real VAL stress-time history from field measurements are used to study the fatigue life of the structure against those calculated based on various numerical fatigue models. The fundamental assumptions and limits of application of CAL fatigue models for cases with VAL is also studied in detail. It is illustrated that the application of simplified fatigue models that are based on CAL scenarios is non-conservative, and has resulted in implementation of high safety factors in the industry. This paper also studies the influence of tensile overloads and compression underloads on the fatigue crack growth of the material. The results from this study can be used during both the design of the structure and establishing the inspection intervals and maintenance criteria, hence reducing the risk of brittle fracture and catastrophic failure in challenging environments.


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