SEAFLOOR INTERACTION WITH STEEL CATENARY RISERS - Phase III

(Continuation of Prior Phases)

 

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 develop a means for providing quantitative estimates of seafloor stiffness, its variation over the life of the project for various soil and site conditions, and their effects on the fatigue stresses of SCRs.

APPROACH: Ongoing work (Phase 2) on this project involves the development of a seafloor-riser interaction model that features: (1) non-linear load-deflection (P-y) relationships to describe equivalent soil springs supporting the riser pipe, (2) a finite difference seafloor-elastic pipe interaction model to compute bending and shear stresses in the pipe within the touchdown zone, and (3) hydrodynamic studies to establish realistic modes of riser motions at the touchdown point. Key features of the P-y model include the ability to simulate plastic initial penetration, elastic rebound, separation and re-contact between riser and seafloor, and spatial variation in P-y behavior along the length of the touchdown zone. The P-y model can account for the effects of trench depth, trench width, and soil strength variability. Continuance of this research (Phase 3) will involve calibration, evaluation of the model through comparisons to existing data. In addition, the P-y relations will be implemented in the existing numerical scheme for computing the dynamics of a SCR, which in turn will be used to quantify the effects of nonlinear and inelastic support of the seabed on the tension and bending moment of the SCR near its touchdown zone.

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 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. The results can also be used to evaluate the effects of various seafloor conditions – e.g., soil strength, trench geometry – on fatigue stresses.

ANTICIPATED NUMBER OF PHASES: 3

PROJECT PLAN FOR PHASE 3 (2006 -2007):

Scope of Work: The main tasks of this phase are as follows: (1) identify existing sources of data on laboratory measurements of P-y relationships and instrumented riser pipes, (2) compare measurements to model predictions, (3) calibrate the model to measurements, and (4) implement P-y relations in the bottom boundary condition of the existing program, CABLE3D, for computing the dynamics of a SCR. Such calibration can include empirical adjustments to input parameters to match measured data, as well as modifying the form of various analytical components of the model to achieve improved agreement between prediction and measurement. The numerical simulations of a SCR obtained using the extended CABLE3D will be compared with those of the existing CABLE3D and the comparison will reveal the effects of the P-y relation on the fatigue stress and tensions of SCR near its touchdown zone.

Anticipated Results & Deliverable: Anticipated results and deliverables are: (1) a computer code simulating the seafloor-riser interactions that incorporate non-linear soil springs, (2) guidelines on input parameter selection, and (3) a report documenting the research including the theoretical development of the model and illustrative parametric studies.

PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:

PI(s): Charles Aubeny, Giovanna Biscontin & Jun Zhang