Simulation of SCR Behaviour at Touchdown Zone - Part II: Testing of a Sectional SCR Model in a Geotechnical Centrifuge
Authors: Clukey Edward Charles , Tognarelli Michael , Li George , Ghosh Rupak , Phillips Ryan Douglas , Zakeri Arash , Elliott Brad , Bhattacharyya Anirban , Sun Qunli 
Published: October 2011

This paper provides an assessment of centrifuge test results and analytical methods used to determine fatigue life of Steel Catenary Risers (SCRs) in the soil touchdown zone. Centrifuge results were obtained on seven tests: five with laboratory clay (Speswhite) and two with Gulf of Mexico (GoM) deepwater clay. Based on previous work described in a companion paper both heave and surge motions, both with wave and slow motion frequencies, were applied in the centrifuge testing. The motions were based on 1-yr winter storm conditions as well as a 10-yr hurricane event (roughly equivalent to a 100-yr. winter storm). The frequencies and amplitudes of these motions were based on computed motions for a semi-submersible with a mean position ~5m (16 ft.) above the seafloor. The centrifuge model then simulated the response of the SCR from this point through the touchdown zone. Important similitude modeling parameters were maintained throughout the testing. These included proper modeling of the strength and stiffness of the soil relative to the pipe stiffness as well as the proper weight of pipe for the 305mm (12-in) and 508 mm (20-in) pipe sections tested. The frequencies for the heave and surge motions, however, for six of the seven tests were four times slower than field prototype conditions. Although inertial considerations were not considered to have a significant impact on the overall test results, the second test on the GoM clay was performed at higher frequencies by either applying smaller amplitude motions or by only providing heave motions.

The centrifuge results were compared to analytical solutions which used elastic soil elements whose stiffness values were based on small amplitude movements resulting in relatively high stiffness coefficients. The centrifuge tests showed that the measured moments during the riser loading produced fatigue lifes greater (factor 1.3 to 1.8) than predicted with the analytical solutions. The equivalent linear springs required to match the centrifuge results were found to be almost an order of magnitude less than the spring coefficients based on small displacements. These results also appear to be consistent with other published data which used nonlinear soil models in their analyses. In addition to direct comparisons with analytical models the impact of storm sequencing (order of large and small storm sequential events) trench formation and comparisons between the laboratory and GoM clay are presented in the paper

Introduction

Determination of the fatigue life in the soil touchdown region is an important part of Steel Catenary Riser (SCR) riser design. Numerous studies (Bridge et al, 2004, Clukey et al, 2005, Clukey et al, 2008) have shown that the soil in this region will experience large deformations sufficient to create trench depths in excess of several pipe diameters. However, current design practice continues to use soil springs that model the soil response at very small displacements (a few millimeters).