Introduction
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.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
10 February 2009
- Meeting paper published:
5 May 2009
- Manuscript approved:
19 July 2009
- Published online:
10 June 2010
- Version of record:
10 June 2010
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