| dc.description.abstract | The stiffness of the soil at the foundation level plays an important role in the global stiffness
of the offshore structure. Beside the static loads, the offshore foundation need also suffer cyclic
loads from both wind and waves. Stiffness and strength degradation in the soil have been
observed under cyclic loading. As the availability of the existing p-y method suggested by the
API regulation has not approved for the application for the large-diameter offshore monopole
foundation. A lot of constitutive numerical models have been developed in order to take the
behavior of the soil under cyclic loading into account.
In this thesis, constitutive models APS-I, APS-II and UCASC are developed to account for the
stiffness and strength degradation of clays under cyclic loading. Instead of following the cyclic
behavior of the clay in the scale of time, the models are based on semi-explicit method in which
the cyclic behavior of clays are described as a function of cyclic number. In this way, the
computational time is more economic and the better precision is reached.
In APS-I model, an accumulated yield surface is used. An accumulated plastic strain
hardening rule and a cyclic transfer rule are formulated to describe the anisotropic
accumulated plastic strain developed in clays under cyclic loading as a function of cyclic stress
ratio, average stress ratio and equivalent cyclic number. Besides, NGI-ADP yield surface is
used in the model to account for the static plastic strain developed. In this way, the average
strength reduction can also be captured as a by-product of the model when the failure strain
is reached. The model is implemented with a backward Euler integration scheme. The validity
of the model is proved by the comparison between the test results and calculation results of
the Drammen clay and Moum clay. Furthermore, in order to improve the precision of the
capture of the average strength reduction, an improved APS-II model is formulated with more
accurate accumulated plastic strain hardening rule and cyclic transfer rule. The improvement
of the APS-II model is also shown from the comparison between the tests results and calculated
results of the Drammen clay and the Moum clay.
In addition, a new UCASC-AV model is proposed. One yield surface is used in the UCASCAV
model to account for both static strain and accumulated plastic strain. An average strength
reduction rule is formulated. Equivalent cyclic number Neq can be updated automatically with
the cyclic stress level and the cycle number increment ΔN. Average stiffness degradation is
predicted as a by-product of the average strength degradation. Thus, the average strain
developed in the clay under cyclic loading can be predicted. In addition, a new UCASC-CY
model is also proposed. A cyclic strength reduction rule is formulated to relate the cyclic
strength reduction to the cyclic number. Nonlinear elasticity is adopted in the model. Thus,
the cyclic strain developed in the clay under cyclic loading can be calculated. The validity of
the UCASC models is proved through the comparison between the test results and calculated
results of the Drammen clay. Finally, a calculation procedure for the combination of the UCASC-AV model and the UCASC-CY model is proposed for the offshore foundation
calculation.
At the end of the thesis, accommodations of the future work are given, including the inclusion
of the excess pore pressure generated during the cyclic loading and the inclusion of the small
strain stiffness. | nb_NO |
