Summary
Objective
The research will improve the current state of understanding regarding the basic mechanisms affecting the seafloor stiffness within the Steel Catenary Riser (SCR) touchdown zone, and (2) develop a means for providing quantitative estimates of seafloor stiffness and damping and their variation over the life of the project for various soil and site conditions.
Approach
The geotechnical studies in this project will use finite element analyses to develop load deformation relationships to describe soil-pipe interactions for various conditions of soil properties, trench conditions, and characteristics of riser motions. The hydrodynamic studies will investigate riser-structure-fluid interactions for various sea states, floating system characteristics, and seafloor conditions. The parallel studies will be coordinated throughout the project to ensure proper accounting of interaction effects.
Benefits to MMS & Industry
Results will allow a more accurate estimation of SCR fatigue life in the touchdown area.
Deployment of Results
The studies described above will be documented in a final report. Guidelines and recommendations will be provided for appropriate seafloor boundary conditions (stiffness, damping) for use in fatigue stress analyses of steel catenary risers for a practical range of seafloor conditions likely to be encountered in the Gulf of Mexico.
Project Organization & Timing
Year 1 of this project has provided an overall study framework and plan, validation of FEM tools for use in this project, and an initial identification and assessment of the important parameters for modeling SCR’s and developing boundary conditions in the touchdown area for use in estimating the fatigue life of SCR’s. Year 2 will include model development, FEM calibration, and parametric studies for various seafloor and loading conditions. Year 3 will evaluate model predictions in light of laboratory measurements and field observations, recalibration/refinement of the model as necessary, and formulation of final guidelines.
Project Plan
Scope and Plan: Develop a numerical model, perform parametric studies, and identify conditions most influential on the magnitude of bending stresses in the riser pipe. Parameters to be evaluated included soil strength and stiffness, trench geometry, the effects of trench collapse and infilling, strain rate effects, and non-linear effects associated with a range of amplitudes and velocities of riser motions. The range of riser motions near the touchdown zone will be approximately estimated using a numerical code accounting for the interaction among a floating structure, its riser/mooring system and the seafloor.
The parametric model developed in Year 1 will be compared to available laboratory experiments and field data, and calibrated as needed to improve agreement with the data. Additional experiments may need to be undertaken to improve and/or validate the models developed in Year 2. The model may be also need to be refined or extended to 3D to more accurately predict SCR behavior in the touchdown area. Recommended boundary conditions (stiffness and damping) for use with riser analysis programs such as CABLE3D or other commercially available riser analysis programs in use by the industry will be recommended.
Related Publications
Aubeny C.P. and Biscontin G. (2007) “Seafloor-Riser Interaction Model,” Proceedings of the International Workshop on Constitutive Modelling – Development, Implementation, Evaluation, and Application, Hong Kong, China, January 12-13, 2007.
Biscontin, G., Jiao, Y., and Aubeny C.P. (2007) “Degradation of P-y Model for Seafloor-Steel Catenary Riser Interaction,” Proc. 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, California, 10-15 June, 2007.
You, J.-H., Aubeny C.P., and Biscontin, G. (2007) “Seafloor Interaction with Steel Catenary Risers,” Proc. 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, California, 10-15 June, 2007.
Nakhaee, A. and Zhang, J. (2007) “Dynamic Interactions between A SCR and the Seabed,” Proc. 26th International Conference on Offshore Mechanics and Arctic Engineering, San Diego, California, 10-15 June, 2007, OMAE2007-29638.
Aubeny, C., Gaudin, C., Randolph, M. (2008) “Cyclic tests of model pipe in kaolin,” Society of Petroleum Engineers, paper SPE 123131, December.
Aubeny C.P. and Biscontin G. (2008) “Interaction model for steel compliant riser on soft seabed,” Society of Petroleum Engineers, paper SPE 120077, September.
Langford, T.E. and Aubeny, C. (2008) “Model tests for steel catenary riser in marine clay,” Offshore Technology Conference, Houston, Texas, paper OTC 19495, May.
Nakhaee, A. and Zhang, J. (2008) “Effects of Interactions with The Seafloor on The Fatigue Life of A SCR ,” Proceedings of the 18th ISOPE Conference, Vancouver, Canada, July 6-11.
Langford, T.E. and Aubeny, C. “Large scale soil-riser model testing on high plasticity clay,” Proceedings of the 18th ISOPE Conference, Vancouver, Canada, July 6-11.
Aubeny, C.P. and Biscontin, G. (2009) “Seafloor-riser interaction model,” ASCE International Journal of Geomechanics, Vol. 9, No. 3., pp. 133-141.
