Offshore structures are subjected to wind loads, wind generated wave excitations, and current forces. In this paper we focus on the wind generated wave excitations as the main source for the external forces on the structure. The main objective of the paper is to provide a tool for using deck acceleration measurements to predict the value of the force and moment acting on the offshore structure foundation. A change in these values can be used as an indicator of the health of the foundation. Two methods of
analysis are used to determine the relationship between the force and moment acting on the foundation and deck acceleration. The first approach uses neural networks while the other uses a Fokker-Planck formulation. The Fokker-Plank approach was used to relate the variance of the excitation to the variance of the deck acceleration. The total virtual mass of the equivalent SDOF of the structure was also determined at different deck masses.
Ahmed A. Elshafey : Faculty of Engineering, Minoufiya University and a PDF, Memorial University of Newfoundland, Canada
M.R. Haddara : Faculty of Engineering, Memorial University of Newfoundland, Canada
H. Marzouk : Faculty of Engineering, Architecture and Science, Ryerson University, Toronto, Canada
Impact pressure due to sloshing is of great concern for the ship owners, designers and builders of the LNG carriers regarding the safety of LNG containment system and hull structure. Sloshing of LNG in partially filled tank has been an active area of research with numerous experimental and numerical investigations over the past decade. In order to accurately predict the sloshing impact load, a new numerical method was developed for accurate resolution of violent sloshing flow inside a threedimensional LNG tank including wave breaking, jet formation, gas entrapping and liquid-gas interaction. The sloshing flow inside a membrane-type LNG tank is simulated numerically using the Finite-Analytic Navier-Stokes (FANS) method. The governing equations for two-phase air and water flows are formulated
in curvilinear coordinate system and discretized using the finite-analytic method on a non-staggered grid. Simulations were performed for LNG tank in transverse and longitudinal motions including horizontal, vertical, and rotational motions. The predicted impact pressures were compared with the corresponding experimental data. The validation results clearly illustrate the capability of the present two-phase FANS method for accurate prediction of impact pressure in sloshing LNG tank including violent free surface motion, three-dimensional instability and air trapping effects.
LNG tank sloshing; compressible two-phase flow; computational fluid dynamics; Navier-Stokes equations; level-set function; impact pressure.
Hamn-Ching Chen : Ocean Engineering Program, Zachry Department of Civil Engineering, Texas A & M University, USA
The objective of this work is to investigate the possibility of using the longitudinal strain on the surface of a pipe to determine the inception of dangerous free spanning. The long term objective is to develop an online monitoring technique to detect the development of dangerous free spanning in subsea pipelines. This work involves experimental study as well as finite element modeling. In the experiments, the strains at four points on a cross section of a pipeline inside the free span zone are measured. Pipes with different boundary conditions and different diameter to length ratios were tested. The pipe is treated as a simple beam with fixed-fixed or simply supported boundary conditions. The variation of the strains
as a function of the diameter to length ratio gives a pointer to the inception of dangerous free spanning.
The finite element results agree qualitatively with the experiments. The quantitative discrepancy is a result
of the difficulty to replicate the exact boundary conditions that is used by the finite element program.
free spans; pipelines; subsea; monitoring - strain.
Ahmed A. Elshafey, M.R. Haddara :Faculty of Engineering, Memorial University of Newfoundland, St. John
Large deck openings of ultra large container ships reduce their torsional stiffness considerably
and hydroelastic analysis for reliable structural design becomes an imperative. In the early design stage
the beam model coupled with 3D hydrodynamic model is a rational choice. The modal superposition
method is ordinary used for solving this complex problem. The advanced thin-walled girder theory, with
shear influence on both bending and torsion, is applied for calculation of dry natural modes. It is shown
that relatively short engine room structure of large container ships behaves as the open hold structure with
increased torsional stiffness due to deck effect. Warping discontinuity at the joint of the closed and open
segments is compensated by induced distortion. The effective torsional stiffness parameters based on an
energy balance approach are determined. Estimation of distortion of transverse bulkheads, as a result of
torsion and warping, is given. The procedure is illustrated in the case of a ship-like pontoon and checked
by 3D FEM analysis. The obtained results encourage incorporation of the modified beam model of the
short engine room structure in general beam model of ship hull for the need of hydroelastic analysis,
where only the first few natural modes are of interest.
Increasing numbers of floating offshore wind turbines are planned and designed these days due to their high potential in massive generation of clean energy from water depth deeper than 50 m. In the present study, a numerical prediction tool has been developed for the fully-coupled dynamic analysis of FOWTs in time domain including aero-blade-tower dynamics and control, mooring dynamics, and
platform motions. In particular, the focus of the present study is paid to the dynamic coupling between the rotor and floater and the coupled case is compared against the uncoupled case so that their dynamic coupling effects can be identified. For this purpose, a mono-column mini TLP with 1.5MW turbine for 80m water depth is selected as an example. The time histories and spectra of the FOWT motions and accelerations as well as tether top-tensions are presented for the given collinear wind-wave condition.
When compared with the uncoupled analysis, both standard deviations and maximum values of the floater responses/tower-accelerations and tether tensions are appreciably increased as a result of the rotorfloater
dynamic coupling, which may influence the overall design including fatigue-life estimation especially when larger blades are to be used.
renewable wind energy; FOWT (floating offshore wind turbine); rotor-floater-tether coupled dynamics; coupled vs. uncoupled analysis; mono-column mini TLP; tower elasticity; blade control/aerodynamics; floater responses; tower accelerations; tether tension; fatigue life.
Y.H. Bae :Texas A&M University, College Station, TX, USA
M.H. Kim :Civil Engineering, Texas A&M University, College Station, TX, USA