Experimental Measurement of Touchdown Zone Stiffness for SCR in Gulf of Mexico Clay

The location near the touchdown zone (TDZ) of a Steel Catenary Riser (SCR) onto the seabed is a primary "hot spot" for fatigue-assessment, with seabed stiffness having a major influence on the predicted fatigue life. This paper presents the results of laboratory model tests evaluating the effects of cyclic loading, loading rate, load amplitude, riser embedment depth and consolidation on the vertical stiffness of the soil supporting the riser in the TDZ. Monotonic and cyclic model tests using single gravity, displacement-controlled laboratory model tests of soil stiffness were conducted. All tests used high plasticity Gulf of Mexico clay with undrained shear strength corresponding to typical normally consolidated conditions. A test program was developed to evaluate: (1) rate effects for monotonic and cyclic loading, (2) vertical stiffness for displacement amplitudes ranging from 0.02-0.05 D (where D = riser outer diameter), (3) soil stiffness degradation with increasing in the number of cycles, (4) stiffness recovery during rest periods, and (5) stiffness recovery under prolonged cyclic loading. The results show that the cyclic resistance during a shallow embedment, 0.5 D, and large amplitude-cycles, 0.05 D, drops significantly during the first few cycles in the displacement-controlled cyclic loading. The resistance will continue to drop and approaches zero after about one hundred cycles for both the penetration and the extraction resistance. By contrast, for small amplitude-cycles, 2% D, soil resistance degrades during the first few cycles, but reaches an apparent steady state at larger cycles. After the pause periods, ranging from 1 to 13 hours, and after each series of 100 cycle parcels, the vertical displacements occurring during consolidation were recorded. Consolidation following rest periods leads to a short-term increase in the soil secant stiffness, which quickly declines to pre-rest period levels after the resumption of cyclic loading. However, under prolonged cyclic loading the soil stiffness gradually trends upward. The test program adds to the database evaluating the influence of the major variables (displacement magnitude, embedment depth, etc.) affecting soil stiffness for SCR fatigue-assessment. Additionally, it presents new results on stiffness recovery during rest periods, stiffness reduction after resumed cyclic loading, and gradual gains in stiffness under prolonged cyclic loading. The findings on stiffness recovery are particularly important, as this has been a major source of uncertainty in prior investigations.