STUDIES ON THE NONLINEAR INTERACTIONS ASSOCIATED WITH MOORED SEMI SUBMERSIBLE OFFSHORE PLATFORMS

The design of moored semi submersible systems constitutes a challenging engineering
problem in which, the platform offset, stability, payload and system-optimized cost
requirements are to be met simultaneously. This problem is complicated by the
incomplete understanding of the nonlinearities associated with the multiple
interactions such as wave to wave, wave to platform, platform to mooring, fluid to
mooring and mooring to seabed. In this study, an attempt has been made to probe into
these nonlinearities through numerical, experimental, and parametric studies.
In the numerical study, moored semi submersibles were analyzed in the time
domain. The dynamic equilibrium conditions were satisfied through a set of coupled
nonlinear differential equations for the six DOF motions. For representing the
platform to mooring nonlinear interactions, the 6x6 mooring stiffness matrix was
derived based on the mooring stiffness and on the fairlead coordinates relative to the
structure CG. For the evaluation of the slow frequency horizontal motions of the
platform, the second order wave forces resulting from the second order temporal
acceleration and the structural first order motions were formulated. For the
assessment of the fluid to mooring and mooring to seabed nonlinear interactions, a
deterministic approach for the dynamic analysis of a multi-component mooring line
was formulated. The floater motion responses were considered as the mooring line
upper boundary conditions. Lumped parameter approach was adopted for the
mooring line modeling. Mooring to seabed nonlinear interactions were modeled
assuming that the mooring line rested on an elastic dissipative foundation. A
numerical dynamic analysis method in the time domain was developed and results for
various mooring lines partially lying on different soils were validated by conducting a
comparative study against published results. The contribution of the soil
characteristics of the seabed to the dynamic behavior of mooring line was investigated
for different types of soil.
Two phases of experimental studies were conducted to provide benchmark data
for validating the numerical methods. In the first phase, the seakeeping performance
of a semi submersible with eight circular columns was studied. The model was built
to scale of 1:100 using Froud’s law of similitude. The tests were conducted for head,
beam and quartering seas. In the second phase, a semi submersible with six circular
columns was modeled using the same scale as for the first semi submersible. Linear
mass-spring system was arranged to facilitate measurements of the horizontal drift
forces. The system natural periods, still water damping, nonlinear viscous damping,
drag coefficient and inertia coefficient information were evaluated from the free
decay tests. Seakeeping tests were conducted for head and beam model orientations.
The measured drift forces were compared to available formulae in the literature to
assess the available semi-empirical methods for evaluation these forces. In both
experimental phases, twin-hulled conventional semi submersibles were considered.
By comparing the results of the numerical and experimental models, the validity of
the numerical method was established.
Based on the validated numerical algorithm, a number of parametric studies were
conducted for investigating the contributions of various design parameters on the
dynamics of moored semi submersibles. The effects of pretension, mooring line
configuration, clump weight, cable unit weight, elongation, breaking strength and
pretension angle on the behavior of multi-component mooring line, were investigated
by using an implicit iterative solution of the catenary equations. On the other hand,
using linearized frequency domain analysis, the contributions of platform payload,
platform dimensions, number of columns, number of mooring lines, the wave
environment mathematical model, the wave characteristics and the operating (intact or
damage) conditions to the responses of moored semi submersibles were investigated.
The experimental and published results verified the efficiency of the developed
numerical model for prediction of the wave frequency and low frequency motions and
mooring dynamic tension responses of the semi submersible. Moreover, experimental
results indicated that in addition to the modeling of the mooring system stiffness,
typical or hybrid modeling of the mooring system and attachments are necessary for
the critical assessment of the mooring system damaged conditions.