Nonlinear Random Wave Loading on Fixed Offshore Platforms

1986 ◽  
Author(s):  
N. Spidsoe ◽  
H.P. Brathaug ◽  
O. Skjåstad
Keyword(s):  
Random Wave ◽  
Offshore Platforms ◽  
Wave Loading ◽  
Nonlinear Random Wave

Study on a magnetorheological elastomer-base device for offshore platform vibration control

2018 ◽  
Vol 30 (2) ◽  
pp. 243-255 ◽  
Author(s):  
Dingxin Leng ◽  
Haiyan Xiao ◽  
Lei Sun ◽  
Guijie Liu ◽  
Xiaojie Wang ◽  
...  
Keyword(s):  
Wave Spectrum ◽  
Offshore Platform ◽  
Random Wave ◽  
Offshore Platforms ◽  
Wave Loading ◽  
Magnetorheological Elastomer ◽  
Control Robustness ◽  
Resonance Control ◽  
Wave Induced ◽  
Strong Control

Wave loading is one of the leading factors contributing to fatigue damage of offshore platforms. Vibrations in marine platforms due to nonlinear hydrodynamic forces can reduce platform productivity, endanger safety, and affect serviceability. This article presents numerical evaluation of a magnetorheological elastomer device for wave-induced vibration reduction of offshore platform. Random wave loadings are estimated by wave spectrum analysis and Morison’s equations. By altering field-induced stiffness of magnetorheological elastomers and non-resonance control strategy, the wave-induced vibration of offshore platform is effectively reduced, and the magnetorheological elastomer device presents strong control robustness under various wave loadings. This work indicates that magnetorheological elastomer-base device may open a new insight for vibration mitigation of ocean structures.

Extreme Response Prediction for Fixed Offshore Structures by Monte Carlo Time Simulation Technique

2016 ◽  
Author(s):  
M. K. Abu Husain ◽  
N. I. Mohd Zaki ◽  
M. B. Johari ◽  
G. Najafian
Keyword(s):  
Monte Carlo ◽  
Statistical Properties ◽  
Simulation Technique ◽  
Random Wave ◽  
Wave Loading ◽  
Offshore Structure ◽  
Sampling Variability ◽  
Time Simulation ◽  
Wide Range ◽  
Extreme Response

For an offshore structure, wind, wave, current, tide, ice and gravitational forces are all important sources of loading which exhibit a high degree of statistical uncertainty. The capability to predict the probability distribution of the response extreme values during the service life of the structure is essential for safe and economical design of these structures. Many different techniques have been introduced for evaluation of statistical properties of response. In each case, sea-states are characterised by an appropriate water surface elevation spectrum, covering a wide range of frequencies. In reality, the most versatile and reliable technique for predicting the statistical properties of the response of an offshore structure to random wave loading is the time domain simulation technique. To this end, conventional time simulation (CTS) procedure or commonly called Monte Carlo time simulation method is the best known technique for predicting the short-term and long-term statistical properties of the response of an offshore structure to random wave loading due to its capability of accounting for various nonlinearities. However, this technique requires very long simulations in order to reduce the sampling variability to acceptable levels. In this paper, the effect of sampling variability of a Monte Carlo technique is investigated.

scholarly journals Comparison of Extreme Surface Elevation for Linear and Nonlinear Random Wave Theory for Offshore Structures

2018 ◽  
Vol 203 ◽  
pp. 01021
Author(s):  
Nurul 'Azizah Mukhlas ◽  
Noor Irza Mohd Zaki ◽  
Mohd Khairi Abu Husain ◽  
Gholamhossein Najafian
Keyword(s):  
Probabilistic Approach ◽  
Wave Theory ◽  
Offshore Structures ◽  
Random Wave ◽  
Linear Wave ◽  
Random Waves ◽  
Sea State ◽  
Higher Order Terms ◽  
Structural Responses ◽  
Nonlinear Random Wave

For offshore structural design, the load due to wind-generated random waves is usually the most important source of loading. While these structures can be designed by exposing them to extreme regular waves (100-year design wave), it is much more satisfactory to use a probabilistic approach to account for the inherent randomness of the wave loading. This method allows the statistical properties of the loads and structural responses to be determined, which is essential for the risk-based assessment of these structures. It has been recognized that the simplest wave generation is by using linear random wave theory. However, there is some limitation on its application as some of the nonlinearities cannot be explained when higher order terms are excluded and lead to underestimating of 100-year wave height. In this paper, the contribution of nonlinearities based on the second order wave theory was considered and being tested at a variety of sea state condition from low, moderate to high. Hence, it was proven that the contribution of nonlinearities gives significant impact the prediction of 100-year wave's design as it provides a higher prediction compared to linear wave theory.

Optimal Vibration Control Strategy for Offshore Platforms Accounting for AMD Constraints

2004 ◽  
Author(s):  
Chunyan Ji ◽  
Qingmin Meng
Keyword(s):  
Control Algorithm ◽  
Feedback Gain ◽  
Offshore Platforms ◽  
Control Force ◽  
Wave Loading ◽  
Offshore Structure ◽  
H2 Control ◽  
Input Control ◽  
The Mathematical Model ◽  
H2 Optimization

In order to control the excessive vibration of offshore platforms under wave excitations, an H2 control algorithm was presented in this paper. In the present study, noise terms for generating filtered wave loading and accounting for model uncertainty are separated. In addition, in H2 optimization problem, AMD’s capacities are considered by setting the limits of AMD stroke and maximum input control force. And the formulations of such algorithm are described. In order to investigate the feasibility and effectiveness of the proposed method, a numerical example applied to an offshore platform is presented in this paper. The numerical results demonstrate that the proposed algorithm is effective in reducing the vibration of offshore structure when there are some uncertainties in building the mathematical model of the structure. In addition, AMD designed by the proposed method can keep its operation by choosing appropriate feedback gain among several gain candidates based on the AMD limits.

Efficient Derivation of the Probability Distribution of the Extreme Values of Offshore Structural Response: Comparison of Three Different Methods

2013 ◽  
Author(s):  
M. K. Abu Husain ◽  
N. I. Mohd Zaki ◽  
G. Najafian
Keyword(s):  
Probability Distribution ◽  
Extreme Values ◽  
Structural Response ◽  
Offshore Structures ◽  
Simulation Technique ◽  
Random Wave ◽  
Wave Loading ◽  
Sampling Variability ◽  
Time Simulation ◽  
Extreme Responses

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. To this end, the conventional (Monte Carlo) time simulation technique (CTS) is frequently used for predicting the probability distribution of the extreme values of response. However, this technique suffers from excessive sampling variability and hence a large number of simulated extreme responses (hundreds of simulated response records) are required to reduce the sampling variability to acceptable levels. In this paper, three different versions of a more efficient time simulation technique (ETS) are compared by exposing a test structure to sea states of different intensity. The three different versions of the ETS technique take advantage of the good correlation between extreme responses and their corresponding surface elevation extreme values, or quasi-static and dynamic linear extreme responses.

The Influence of Alternative Wave Loading and Support Modeling Assumptions on Jack-Up Rig Response Extremes

1996 ◽  
Vol 118 (2) ◽  
pp. 109-114 ◽  
Author(s):  
L. Manuel ◽  
C. A. Cornell
Keyword(s):  
Nonlinear Models ◽  
Random Wave ◽  
Force Model ◽  
Wave Loading ◽  
Force Modeling ◽  
Hydrodynamic Damping ◽  
Extreme Response ◽  
The Absolute ◽  
Non Gaussian ◽  
Jack Up

A study is conducted of the response of a jack-up rig to random wave loading. Steady current and wind load effects are also included. The effects of varying the relative motion assumption (in the Morison equation) and of varying the bottom fixity assumptions are investigated. One “fixity” model employs nonlinear soil springs. Time domain simulations are performed using linearized as well as fully nonlinear models for the jack-up rig. Comparisons of response statistics are made for two seastates. Hydrodynamic damping causes the rms response to be lower in the relative Morison case. The absence of this source of damping in the absolute Morison force model gives rise to larger resonance/dynamic effects—this tends to “Gaussianize” the response. Hence, the relative Morison model leads to stronger non-Gaussian behavior than the absolute Morison model. This is reflected in moments as well as extremes. The different support conditions studied are seen to significantly influence extreme response estimates. In general, stiffer models predict smaller rms response estimates, but also exhibit stronger non-Gaussian behavior. The choice of the Morison force modeling assumption (i.e., the relative versus the absolute motion formulation) is seen to have at least a secondary role in influencing response moments and extremes.

Mechanics of nonlinear random wave groups interacting with a vertical wall

2008 ◽  
Vol 20 (3) ◽  
pp. 036604 ◽  
Author(s):  
Alessandra Romolo ◽  
Felice Arena
Keyword(s):  
Vertical Wall ◽  
Random Wave ◽  
Wave Groups ◽  
Nonlinear Random Wave

Nonlinear Dynamic Response of a Compliant Tower Under the Effect of Steady and Unsteady Sea States

2017 ◽  
Author(s):  
Konstantinos Chatziioannou ◽  
Vanessa Katsardi ◽  
Apostolos Koukouselis ◽  
Euripidis Mistakidis
Keyword(s):  
Structural Analysis ◽  
Dynamic Response ◽  
Deep Water ◽  
Nonlinear Dynamic ◽  
Wave Theory ◽  
Random Wave ◽  
Wave Loading ◽  
Nonlinear Dynamic Response ◽  
Steady Wave ◽  
Compliant Tower

The purpose of this work is to highlight the importance of considering the actual nonlinear dynamic response for the analysis and design of fixed deep water platforms. The paper highlights the necessity of applying dynamic analysis through the comparison with the results obtained by the authors by applying static nonlinear analysis on the structure under examination. The example treated in the context of the present paper is a compliant tower, set-up in deep water. Nonlinearities are considered both for the calculation of the wave loadings and the structural analysis. The wave loading is based on linear random wave theory and comparisons are provided with the steady wave theories, Airy and Stokes 5th. The former solution is based on the most probable shape of a large linear wave on a given sea-state; the auto-correlation function of the underlying spectrum. On the other hand, in the field of structural analysis, two cases are considered for comparison, static analysis and time history dynamic analysis. For both types of analysis, two sub-cases are considered, a case in which geometric nonlinearity and nonlinearities related to the modelling of the soil are considered and a case in which the corresponding linear theories are employed (reference cases). The structural calculations were performed using the well-known structural analysis software SAP2000, which was enhanced by a special programming interface that was developed to calculate the wave loading and to directly apply the generated loads on the structural members. The results show that the consideration of the particle velocities associated with the linear random wave theory in the wave loading lead to significant differences with respect to the steady wave theories in terms of the displacements and stresses of the structure. Moreover, irrespectively of the adopted wave theory, the nonlinear analyses lead to significant discrepancies with respect to the linear ones. This is mainly associated with the nonlinear properties of the soil. Another source of discrepancies between the results of static and dynamic analyses stems from the change of the effective natural frequency of the structure when nonlinearities are considered.

Probabilistic Modelling of Extreme Offshore Structural Response due to Random Wave Loading

2013 ◽  
Author(s):  
Y. Wang ◽  
H. Mallahzadeh ◽  
M. K. Abu Husain ◽  
N. I. Mohd Zaki ◽  
G. Najafian
Keyword(s):  
Extreme Values ◽  
Structural Response ◽  
Offshore Structures ◽  
Random Wave ◽  
Wave Loading ◽  
Empirical Distributions ◽  
Generalized Pareto ◽  
Economical Design ◽  
Ocean Environment ◽  
Extreme Responses

Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. This paper investigates the suitability of the Gumbel, the Generalized Extreme Value (GEV), and the Generalized Pareto (GP) distributions for modelling of extreme responses by comparing them with empirical distributions derived from extensive Monte Carlo time simulations. It will be shown that none of these distributions can model the extreme values adequately but that a mixed distribution consisting of both GEV and GP distributions seems to be capable of modelling the extreme responses with very good accuracy.

Simplified Method to Assess Dynamic Response of Jacket Type Offshore Platforms Subjected to Wave Loading

2004 ◽  
Author(s):  
B. Asgarian ◽  
A. Mohebbinejad ◽  
R. H. Soltani
Keyword(s):  
Dynamic Response ◽  
Dynamic Characteristics ◽  
Offshore Structures ◽  
Center Of Gravity ◽  
Simplified Model ◽  
Mode Shapes ◽  
Stokes Wave ◽  
Wave Crest ◽  
Offshore Platforms ◽  
Wave Loading

Dynamic response of offshore platforms subjected to wave and current is of fundamental importance in analysis. The first step in dynamic analysis is computing dynamic characteristics of the structure. Because of pile-soil-structure and fluid-structure interactive effects in the dynamic behavior, the model is very complex. In this paper a simplified model for dynamic response of jacket-type offshore structures subjected to wave loading is used. Since wave loads on offshore platforms vary with time, they produce dynamic effects on structures. In the model used in this paper, all of the structural elements are modeled as vertical equivalent cylinders that are in the direction of the wave crest. In the simplified model, the degrees of freedom are considered at the seabed, jacket horizontal elevations and topside center of gravity. The stiffness properties of the model are computed considering the stiffnesses of the vertical bracings, legs and piles. The structural mass is considered as lumped nodal masses at horizontal elevations and topside center of gravity. The hydrodynamic added mass in addition to the structural masses was modeled at jacket horizontal elevations. In the simplified model, for computing wave loading, the projected areas of all members in the direction of the wave crest are considered. For the wave loading calculation, Morison equation is considered. The fluid velocities are calculated for the submerged portions of the structures using a computer program developed for this purpose. In this program both Airy and Stokes wave theories can be used. This model can be used to assess dynamic properties and responses of jacket type offshore structures. The model is used to assess the response of three jacket-type offshore platforms in Persian Gulf subjected to loadings due to several waves. The results in terms of dynamic characteristics and responses were compared with the more accurate analysis results using SACS software. The results are in a good agreement with the SACS analysis outputs, i.e. structural periods, mode shapes and dynamic response.

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