Dynamic Analysis Of Offshore Structures Research Articles

Fluid-structure interaction analysis of offshore structures based on separation of transferred responses

A new partitioned fluid-structure interaction (FSI) method is proposed for dynamic analysis of offshore structures. The main idea of the developed approach is to provide the separation of different components caused by different contributions (such as transferred conditions and updated loadings). One theoretical development is that separated Laplace-domain responses can be obtained from the previous responses. The other is that the structural responses in time domain can be easily calculated by inverse Laplace transform. Once solutions to the structural responses at the time of interest are obtained, the fluid loadings can be updated to consider the impacts of the responses on the geometry of the fluid domain. More importantly, the proposed method is able to improve the estimation accuracy of the structure responses, and hence, may provide better updating results of fluid loadings when the FSI mechanism is considered. To demonstrate the correctness and accuracy of the approach, three numerical examples are used in this paper. The first example is a single degree-of-freedom (DOF) system subject to an irregular dynamic loading. The computation results show that the system responses at previous time steps can be successfully separated into typical components that correspond to different contributions such as transferred conditions and updated loadings. The system responses at current time, obtained by combining previous responses, are in a good agreement with analytical results and those obtained by the Newmark-β method. Then, the second example of a four-DOF lumped system is explored to indicate the superiority of the proposed method over the Newmark-β method in terms of accuracy. Finally, in the third example of an elastic upright cantilever beam in water, results show that the developed method has the ability to calculate dynamic responses with high precision when solving FSI problems, which implies a potential application in ocean engineering.

Stochastic dynamic analysis of marine risers considering fluid-structure interaction and system uncertainties

Stochastic dynamic analysis of offshore structures considering both fluid-structure interaction and system uncertainties is a challenging research task. This paper proposes a novel method to evaluate the statistical characteristics of offshore structural responses with consideration of uncertainties in the fluid and structure. The fluid behavior is simulated by a finite number of particles with particle finite element method (PFEM), and the dynamic behavior of the offshore structure is modelled with finite element method (FEM). A PFEM-FEM scheme is used to model the fluid-structure interaction. To evaluate the statistical characteristics, spectral representations method is used for uncertainty propagation. The output of fluid particles position/pressure, structural vibration, etc. are represented by using polynomial chaos (PC) expansion, and their coefficients are obtained from the least squares method. Statistical characteristics of the responses, such as mean value and variance, can be evaluated with the obtained PC coefficients. Three numerical examples are studied in this paper. The first example is a simple structural model, which is used to demonstrate the convergence and accuracy of the uncertainty analysis method. The second example is a benchmark dam break problem. Statistical characteristics of the fluid particles position due to uncertainties in the mass density are evaluated. Numerical results are verified with experimental data and observations. In the third example, PFEM-FEM scheme is used to conduct fluid–structure interaction analysis. The marine riser structure is modelled with beam elements. In the fluid domain, PFEM is used. Uncertainties in both the fluid domain and structural domain are considered. Results demonstrate that the proposed approach can be used to evaluate the statistical characteristics of responses in the fluid-structure interaction analysis accurately and efficiently.

Dynamic analysis of offshore structures with non-zero initial conditions in the frequency domain

The state of non-zero conditions is typically treated as fact when considering the dynamic analysis of offshore structures. This article extends a newly proposed method [1] to manage the non-zero initial conditions of offshore structures in the frequency domain, including new studies on original environmental loads reconstruction, response comparisons with the commercial software ANSYS, and a demonstration using an experimental cantilever beam. The original environmental loads, such as waves, currents, and winds, that act on a structure are decomposed into multiple complex exponential components are represented by a series of poles and corresponding residues. Counter to the traditional frequency-domain method, the non-zero initial conditions of offshore structures could be solved in the frequency domain. Compared with reference [1], an improvement reported in this article is that practical issues, including the choice of model order and central-processing-unit (CPU) time consumption, are further studied when applying this new method to offshore structures. To investigate the feasibility of the representation of initial environmental loads by their poles and corresponding residues, a measured random wave force collected from a column experiment at the Lab of Ocean University of China is used, decomposed, reconstructed and then compared with the original wave force; then, a numerical offshore platform is used to study the performance of the proposed method in detail. The numerical results of this study indicate that (1) a short duration of environmental loads are required to obtain their constitutive poles and residues, which implies good computational efficiency; and (2) the proposed method has a similar computational efficiency to traditional methods due to the use of the inverse Fourier transform technique. To better understand the performance, of time consumption and accuracy of the proposed method, the commercial software ANSYS is used to determine responses from the time history analysis. Comparisons of the results between the proposed method and the ANSYS results demonstrate that the proposed method produces high accuracies that are similar to the time-domain method and good computational efficiencies, which are similar to the traditional frequency-domain method. Finally, a steel cantilever beam is used to demonstrate the proposed method.

Coupled dynamic analysis for wave interaction with a truss spar and its mooring line /riser system in time domain

A full time-domain analysis program is developed for the coupled dynamic analysis of offshore structures. For the hydrodynamic loads, a time domain second order method is developed. In this approach, Taylor series expansions are applied to the body surface and free-surface boundary conditions, and the Stokes perturbation procedure is then used to establish the corresponding boundary value problems with time-independent boundaries. A higher-order boundary element method (HOBEM) is developed to calculate the velocity potential of the resulting flow field at each time step. The free-surface boundary condition is satisfied to the second order by fourth order Adams–Bashforth–Moultn method. An artificial damping layer is adopted on the free surface to avoid the wave reflection. The mooring-line/tendon/riser dynamics are based on the rod theory and the finite element method (FEM), with the governing equations described in a global coordinate system. In the coupled dynamic analysis, the motion equation for the hull and dynamic equations for mooring-lines/tendons/risers are solved simultaneously using the Newmark method. The coupled analysis program is applied for a truss Spar motion response simulation. Numerical results including motions and tensions at the top of mooring-lines/risers are presented, and some significant conclusions are derived.

Mass perturbation influence method for dynamic analysis of offshore structures

The current work presents an analysis algorithm for the modal analysis for the dynamic behaviors of offshore structures with concepts of mass perturbation influence term. The mass perturbation concept by using the term, presented in this paper offers an efficient solution procedure for dynamical response problems of offshore structures. The basis of the proposed method is the mass perturbation influence concepts associated with natural frequencies and mode shapes and mass properties of the given structure. The mathematical formulation of the mass perturbation influence method is described. New solution procedures for dynamics analysis are developed, followed by illustrative example problems, which deal with the effectiveness of the new solution procedures for the dynamic analysis of offshore structures. The solution procedures presented herein is compact and computationally simple.

Catchment Regions of Multiple Dynamic Responses in Nonlinear Problems of Offshore Mechanics

Abstract Consideration of nonlinear effects in the dynamic analysis of offshore structures leads to complex phenomena even for relatively simple deterministic mathematical models. Nonlinearity in the stiffness or restoring force of a structure can be conveniently incorporated into a differential equation of motion, which can then be solved either analytically, using perturbation techniques, or numerically for various parameter ranges and forcing conditions. A particular feature of nonlinear dynamics is the appearance of multiple, competing steady-state oscillations which depend crucially on initial conditions. These coexisting stable solutions can be thought of as attractors for transient motion. A central question is therefore: How does the final long-term behavior of a dynamical system depend on the starting conditions? This paper concentrates on the computation of the domains of attraction or catchment regions using numerical techniques based on Poincare mapping ideas. The three specific examples illustrated are the roll motion of a ship, the oscillations of an articulated column and the surge response of a moored semi-submersible. This work may be considered an extension of previous work by the same authors.

An approximate method for dynamic analysis of offshore structures to wave action

A simplified approximate method for calculation of the dynamic response of offshore structures to wave action is developed based on the assumption that waves are narrow-banded. Using linear, Gaussian waves and the Morison equation for wave forces, taking into consideration the relative motion between wave-induced fluid particles and the structure, the stochastic nonlinear differential equations of motion of the structure, idealized as a lumped mass system, are solved by the statistical equivalent linearization method and response statistics are given in closed form. Numerical values of response statistics of four offshore structures ranging from 475 to 1075 feet in height are obtained for a wide range of sea states. It is shown that the maximum error incurred by assuming narrow-band waves is no more than 23%, suggesting that the narrow-band wave model can be used to advantage for preliminary design.

Nonlinear Dynamic Analysis of Offshore Towers in Frequency Domain

A frequency domain iterative method is proposed to solve the nonlinear dynamic equation of motion encountered in the dynamic analysis of offshore structures under regular and random waves. The nonlinearities due to the relative velocity-squared drag term and variable submergence are considered and treated using an approximate Newton-Raphson method. In order to facilitate the computation, the formulation makes use of the normal mode theory. The resulting iterative scheme, which introduces a modal hydrodynamic damping on both sides of a decoupled equation of motion in modal coordinates, is shown to be highly efficient for the analysis of fixed base offshore structures. The numerical study shows that only 3 – 5 iterations are required to obtain the converged solutions for hydrodynamic loadings on the structure produced by random and regular waves. The computational time is marginally greater than that required for the linearized methods which in certain cases may introduce considerable error in the results.

Book Reviews

Book reviewed in this article:Dynamic analysis of offshore structures, edited by C.L. KirkProbabilistic offshore mechanics, edited by P.D. SpanosFatigue and crack growth in offshore structures, edited by W D. Dover, G. Glinka and A.G. Reynolds

The application of the Lanczos Mode Superposition Method in dynamic analysis of offshore structures

In this work the application of the Lanczos Mode Superposition Method in dynamic analysis of offshore structures is studied. It is shown, in two typical examples, that this approach reduces remarkably the computational effort spent on dynamic response computations without loss of accuracy. Furthermore, by an adequate choice of the Lanczos algorithm starting vector, it is also verified that the localized effects, which depend on the higher modes, are well represented.

Development of a stochastic wave force function on ocean-structures

The wave forces exerted on structures are of vital importance in the design of marine structures. Circular piles are frequently used to provide most or all of the support for such structures. The forces exerted by waves of similar properties vary greatly. These variations are the result of fluctuations in fluid particle velocities and accelerations as well as by eddies and turbulence around piles caused by rapidly reversing flow. Consequently, a deterministic approach to wave force prediction appears to be impossible. As an alternative, a probabilistic approach was developed in this paper. The method applied here utilizes a multiple linear regression analysis to develop a relationship between wave parameters which are relatively easy to observe and the parameters used in the Morison force equation. They include the velocity and acceleration of water particles and the drag and mass coefficients and are considered to be random variables. The assumption of their lognormal distribution was reasonably well verified. Using Monte Carlo simulation these regression relations were then utilized to generate the frequency function of the wave forces. The wave force function can best be described as stationary periodic process. It can then be used as an input function for a probabilistic dynamic analysis of offshore structures.