Cyclic Stiffness Degradation in Nonlinear Jackup Dynamics
- Edward T.R. Dean (Soil Models Limited) | Rich Metters (Geotechnical Engineering and Marine Surveys Limited)
- Document ID
- Society of Petroleum Engineers
- SPE Projects, Facilities & Construction
- Publication Date
- June 2010
- Document Type
- Journal Paper
- 89 - 96
- 2010. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 4.5 Offshore Facilities and Subsea Systems, 5.1.5 Geologic Modeling, 4.3.4 Scale, 1.6 Drilling Operations
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The load-deflection response of the elevated hull of an installed jackup, subject to environmental loading, and the stresses at the leg/hull connections, depends on the stiffness of the spudcan foundations and thus on the moduli of the soils supporting them (Osborne et al. 1991; Dean et al. 1992; Temperton et al. 1997; Martin and Houlsby 1999; Nelson et al. 2000; Cassidy et al. 2002; Dier et al. 2004). Typically, soil moduli decrease with increasing cyclic-strain amplitude (Atkinson 2000). Soil stiffness can also change as a result of cyclic-strain history and because of buildup of excess pore-water pressures, both because of a sequence of storms, and during a design storm (Hsu 1998).
SNAME (2002) uses the concept of a yield envelope to determine combinations of vertical and horizontal loads and moments on a spudcan that cause significant elastoplastic responses. The envelope is used as if it were a failure envelope in the Step 2 bearing capacity check, which includes an option to allow for rotational foundation stiffness. If the jackup fails Step 2, it can still be accepted if it passes a Step 3 displacement check, which can include allowance for hardening. Sophisticated models of this type include Schotman (1989), Dean et al. (1997a; 1997b), van Langen et al. (1997), Cassidy et al. (2004), Bienen et al. (2006), and others.
This paper explores the guidelines using a simplified jackup response model. Results are found to be potentially conservative if stiffness degradation is ignored. Stiffness degradation is shown to produce significant nonlinearity in dynamic responses. An implication appears to be that observations in mild seastates do not extrapolate linearly to more severe seastates.
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