System identification of coupled heave–pitch motion of ships with forward speed in random ocean waves

2010 ◽  
Vol 5 (1) ◽  
pp. 33-49 ◽  
Author(s):  
R. Panneer Selvam ◽  
S. K. Bhattacharyya
Keyword(s):  
System Identification ◽  
Ocean Waves ◽  
Forward Speed ◽  
Pitch Motion ◽  
Random Ocean Waves

System Identification of a Coupled Two DOF Moored Floating Body in Random Ocean Waves

2005 ◽  
Vol 128 (3) ◽  
pp. 191-202 ◽  
Author(s):  
R. Panneer Selvam ◽  
S. K. Bhattacharyya
Keyword(s):  
System Identification ◽  
Mass Matrix ◽  
Ocean Waves ◽  
Added Mass ◽  
Radiation Damping ◽  
Floating Body ◽  
Mooring Line ◽  
Frequency Dependent ◽  
Linear And Nonlinear ◽  
Random Ocean Waves

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the reverse multiple input-single output (R-MISO) method, to coupled surge-pitch response (two-degrees-of-freedom) of a large floating system in random ocean waves with linear and cubic nonlinear mooring line stiffnesses. The system mass matrix has both frequency independent and frequency dependent components whereas its damping matrix has only frequency dependent components. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known that can either be calculated or obtained from experiments. For numerical illustration, a floating half-spheroid is adopted. The motion as well as the loading are simulated assuming Pierson-Moskowitz (PM) spectrum and these results have been analyzed by the R-MISO method yielding frequency dependent coupled added mass and radiation damping coefficients, as well as linear and nonlinear stiffness coefficients of mooring lines satisfactorily.

Nonlinear System Identification in Offshore Structural Reliability

1995 ◽  
Vol 117 (3) ◽  
pp. 171-177 ◽  
Author(s):  
P. D. Spanos ◽  
R. Lu
Keyword(s):  
System Identification ◽  
Nonlinear System ◽  
Structural Reliability ◽  
Transfer Functions ◽  
Nonlinear System Identification ◽  
Simulated Data ◽  
Wave Force ◽  
Ocean Waves ◽  
Offshore Structures ◽  
Analysis And Design

Nonlinear forces acting on offshore structures are examined from a system identification perspective. The nonlinearities are induced by ocean waves and may become significant in many situations. They are not necessarily in the form of Morison’s equation. Various wave force models are examined. The force function is either decomposed into a set of base functions or it is expanded in terms of the wave and structural kinematics. The resulting nonlinear system is decomposed into a number of parallel no-memory nonlinear systems, each followed by a finite-memory linear system. A conditioning procedure is applied to decouple these linear sub-systems; a frequency domain technique involving autospectra and cross-spectra is employed to identify the linear transfer functions. The structural properties and the force transfer parameters are determined with the aid of the coherence functions. The method is verified using simulated data. It provides a versatile and noniterative approach for dealing with nonlinear interaction problems encountered in offshore structural analysis and design.

Modeling and Identification of a Nonlinear SDOF Moored Structure, Part 2—Comparisons and Sensitivity Study

2004 ◽  
Vol 126 (2) ◽  
pp. 183-190 ◽  
Author(s):  
S.C.S. Yim ◽  
S. Narayanan
Keyword(s):  
System Identification ◽  
Small Body ◽  
Ocean Waves ◽  
System Response ◽  
Mooring System ◽  
Nonlinear Structure ◽  
Single Output ◽  
Submerged Sphere ◽  
Multiple Input ◽  
Identification Technique

A system-identification technique based on the Reverse Multiple-Input/Single-Output (R-MI/SO) procedure is applied to identify the parameters of an experimental mooring system exhibiting nonlinear behavior. In Part 1, two nonlinear small-body hydrodynamic Morison type formulations: (A) with a relative-velocity (RV) model, and (B) with an independent-flow-field (IFF) model, are formulated. Their associated nonlinear system-identification algorithms based on the R-MI/SO system-identification technique: (A.1) nonlinear-structure linearly damped, and (A.2) nonlinear-structure coupled hydrodynamically damped for the RV model, and (B.1) nonlinear-structure nonlinearly damped for the IFF model, are developed for an experimental submerged-sphere nonlinear mooring system under ocean waves. The analytic models and the associated algorithms for parametric identification are described. In this companion paper (Part 2), we use the experimentally measured input wave and output system response data and apply the algorithms derived based on the multiple-input/single-output linear analysis of the reverse dynamic systems to identify the system parameters. The two nonlinear models are examined in detail and the most suitable physical representative model is selected for the mooring system considered. A sensitive analysis is conducted to investigate the coupled hydrodynamic forces modeled by the Morison equation, the nonlinear stiffness from mooring lines and the nonlinear response. The appropriateness of each model is discussed in detail.

The Investigation of Non-Linear Effect for a Circular Cylinder Shaped FPSO

2011 ◽  
Author(s):  
Sam-Kwon Hong ◽  
Rae-Hyoung Yuck ◽  
Beom-Seon Jang ◽  
Hi-Seok Kang ◽  
Se-Eun Kim ◽  
...  
Keyword(s):  
Circular Cylinder ◽  
Model Test ◽  
Ocean Waves ◽  
Ocean Engineering ◽  
Natural Period ◽  
Pitch Motion ◽  
Steel Catenary Riser ◽  
Engineering Research ◽  
Motion Performance ◽  
Plane Area

Nowadays, a circular cylinder shaped FPSO appears as a new type FPSO because it does not need the expensive turret system. A circular cylinder shaped FPSO has an excellent motion performance because pitch and roll natural periods are longer compared to traditional FPSO and heave natural period is also longer due to its small water plane area. Samsung heavy industries has developed a circular cylinder shaped FPSO called as S-Line. The feature of S-line has a shape cutting a groove around draft to reduce the water plane area. Through reducing water plane area, the heave natural frequency moves to lower frequency which is far from ocean waves. According to the results of a linear calculation by WAMIT, S-Line showed reduced vertical motion of heave, roll and pitch compared to conventional FPSO. But, S-Line showed unsuspected large pitch response in 1st model test which was carried out with horizontal mooring system by spring in SSMB (Samsung Ship Model Basin). It is conjectured that this large pitch response is caused by 2nd nonlinearity of the wave and/or parametric pitch phenomena. To verify the mooring load and improve the 2nd order pitch motion, model tests were carried out in SSMB and offshore basin of MOERI (Maritime & Ocean Engineering Research Institute) with two kinds of bilge box. This paper describes the nonlinear effect of 2nd order for pitch motion of a circular cylinder shaped FPSO based on model test and numerical simulation. As a result, S-Line developed by Samsung was also verified being able to use SCR (Steel Catenary Riser) due to its improved heave motion performance.

scholarly journals Effect of forward speed on the level-crossing distribution of kinematic variables in multidirectional ocean waves

2021 ◽  
Vol 235 ◽  
pp. 109345
Author(s):  
Romain Hascoët ◽  
Nicolas Raillard ◽  
Nicolas Jacques
Keyword(s):  
Ocean Waves ◽  
Forward Speed ◽  
Level Crossing

scholarly journals PERIODIC THEORY VELOCITY PREDICTION IN RANDOM WAVE

1978 ◽  
Vol 1 (16) ◽  
pp. 18
Author(s):  
John H. Nath ◽  
Koji Kobune
Keyword(s):  
Wave Theory ◽  
Ocean Waves ◽  
Periodic Wave ◽  
Random Wave ◽  
Random Waves ◽  
Wave Kinematics ◽  
Velocity Prediction ◽  
Water Particle Kinematics ◽  
Predictive Theory ◽  
Random Ocean Waves

Large waves in a series of random ocean waves are considered in the design of ocean structures. When random structural vibrations can be ignored, periodic wave theories are used to predict the water particle kinematics for a design wave even though the real wave is irregular. This paper presents the authors' first attempt to quantify the validity of using periodic wave theory for random waves. Measurements of maximum horizontal and vertical velocities were made in laboratory generated periodic and random waves. They compared favorably with predictions from periodic wave theories (even with Airy theory) particularly for the large waves in a series. Since the design wave concept is applied to the largest waves, the conclusion is that periodic wave theory may be adequate, providing an appropriate factor of safety is used to account for the differences between the actual maximum wave kinematics in nature and those in the predictive theory.

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