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Ocean Engineering, Volume 88, 2014, Pages 63–80.
Lucile M. Quéau, , Mehrdad Kimiaei, Mark F. Randolph.
Centre for Offshore Foundation Systems, The University of Western Australia, Crawley WA 6009, Australia.
Abstract
Steel catenary risers (SCRs) are dynamically sensitive structures and their fatigue design in the touchdown zone is challenging. The dynamic response of SCRs is traditionally assessed by performing a series of long time history analyses but a simplifying approach has recently been proposed. The simple method is based on the use of dynamic amplification factors that quantify the dynamic response for a given perturbation at the hang-off point relative to the static response. The determination of the static response of SCRs is therefore a prerequisite to this approach. In this paper, an existing analytical model is extended to accommodate the displacement at the hang-off point of the SCR and predict the static stress range. The results of this analytical model are validated against numerical simulations. Then, using this simple and efficient analytical model, various sensitivity analyses are performed to explore the impact of key dimensionless groups on the static stress range in the touchdown zone.
Significant statement
Steel catenary risers (SCRs) have been used extensively in the past decades for offshore oil and gas developments in deep water. They are a cost effective solution but are very sensitive to the hydrodynamic loading and the vessel motions, which generate fatigue damage concentrated at the vessel hang-off point (HOP – where the riser is connected to the floating facility) and in the touchdown zone (TDZ – the area of dynamic riser soil interaction). An accurate estimation of the fatigue life of SCRs is fundamental as failure would have high economical and environmental impacts. The structural response of SCRs is usually assessed by carrying out dynamic time history analyses but they are time consuming and they need high computational effort.
In an attempt to simplify the early stages of fatigue design (i.e. conceptual or preliminary design stages), the authors have proposed an approach based on dynamic amplification factors (DAFs). DAFs are defined as the ratio of the maximum dynamic stress range to the maximum static stress range occurring in the TDZ under application of given wave packs. They are an efficient alternative to explicit numerical analysis as they allow determination of the maximum dynamic response amplitudes directly from the static response.
The main aim of this paper is to establish an accurate analytical method to assess the static response of oscillating SCRs, as this is a fundamental input for the DAF approach. The developed model accounts for a linear soil stiffness and the boundary layer effect systematically. It also evaluates the maximum static stress range by assessing the static stress distribution along the riser length, combining changes in both tension and bending moment, when the HOP is relocated under a cycle of static loading. Validation of the analytical model was performed by comparison against numerical simulations using OrcaFlex software.
This work was also extended to assess the static stress range in the touchdown zone for lazy-wave catenary risers (Quéau, L.M., Kimiaei, M., Randolph, M.F. (2013). Lazy wave catenary risers: scaling factors and analytical approximation of the static stress range in the touchdown zone. In: Proceedings of the ASME 32nd International Conference on Ocean, Offshore and Arctic Engineering, OMAE2013-10273, Nantes, France.)
This research is supported by the Lloyd’s Register Foundation which helps to protect life and property by supporting engineering-related education, public engagement and the application of research.
