scholarly journals Spectral Response of Stationary Jack-Up Platforms Loaded by Sea Waves and Wind using Perturbation Method

2021 ◽  
Vol 28 (4) ◽  
pp. 53-62
Author(s):  
Bogdan Rozmarynowski ◽  
Wojciech Jesien
Keyword(s):  
Dynamic System ◽  
Perturbation Method ◽  
Mechanical Response ◽  
Spectral Response ◽  
Wave Structure ◽  
Stationary Condition ◽  
Offshore Platforms ◽  
Sea Waves ◽  
Non Linear ◽  
Jack Up

Abstract The paper addresses non-linear vibrations of offshore jack-up drilling platforms loaded by sea waves and wind in their stationary condition using the perturbation method. Non-linearity of dynamic equations of motion for fixed offshore platforms yields from two factors. The first is load excitation generating non-linear velocity coupling in a dynamic system. This coupling is inherent in the modified Morison equation, involving the excitation function in the form of the sum of the inertial and velocity forces of sea waves, taking into account relative wave–structure kinematics. Moreover, the wind acting on the exciting side causes similar effects. The second source is the subsoil–structure interaction problem, modelled by a system of springs and dashpots that yields stochastic non-linearity of the dynamic system. The matrix equations of structural motion in FEM terms are set up. The perturbation method is adopted to determine the mechanical response of the system, making it possible to determine response spectra of the first and the second approximations for displacements and internal forces of the platform. The paper is the continuation of research detailed in the paper [1]. It is assumed, that the fluctuation parts of the dynamic loading forces are in line with the direction of sea wave propagation. Sea current and lift forces effects are neglected in this study. A numerical example refers to structural data of the Baltic drilling platform in the stationary configuration, i.e. when three legs support the deck above the seawater level.

Going on Location Study for a Jack-Up Rig

2012 ◽  
Author(s):  
Partha Chakrabarti
Keyword(s):  
Rational Design ◽  
Soil Conditions ◽  
Resistance Force ◽  
Sea Waves ◽  
Leg Length ◽  
Floating Structure ◽  
Non Linear ◽  
Limiting Condition ◽  
Jack Up ◽  
The Impact

A jack-up rig is in a floating condition in contrast with its elevated condition when it is on a field tow and going on or off location. In these conditions it is floating on its hull and is subjected to the sea waves like any other floating structure. This paper investigates the behavior of a jack-up when it is going to a new location and lowers its legs for an eventual touch down at the seabed. When a jack-up unit is in a transit condition either during a field move or during ocean tow, there is always a probability that it could be subjected to a storm. At a specific drilling location it has to lower its legs till touchdown. Although the operation is supposed to be performed in relatively calm weather, due to weather uncertainties it may not be possible to avoid some waves at the location. The mechanics of motions and loads imposed on the legs during this condition and the consequences of this on leg safety prior to touchdown has been previously examined in a paper by the author (OTC 7838, 1995) [1]. When the jack-up rig has to go off the location it has to extricate its legs while floating on the hull. During this process also it could be subjected to weather and consequent strength issues of the leg. This was examined in a previous paper (OMAE2007-29083) [2] by the author. This paper intends to examine the in-between situation when the jack-up legs are at the point of touch down and being contacted by the seabed soil. In this condition if there is some wave, bottom impact would occur at the tip of spud can (TOC) due to the rig motions. The TOC would have both horizontal and vertical motions due to the wave. The magnitude of these motions, in addition to the wave height and period, would depend on the hull characteristics and the length of leg below the hull which is dictated by the water depth. When the TOC impacts the bottom, the resistance force will depend on the soil stiffness characteristics which vary with the soil type. The soil load-deformation behavior being non-linear makes the problem more complex. When the leg impacts the soil it sets up vibrations of the leg which depend on the mass of the leg and the spudcan and the leg length below the hull. This paper analyses the problem for a given jack-up and finally recommends the limiting wave and motion condition for that unit. The limit depends on the leg length, wave period and soil properties. In this study, motions analyses of the floating structure are performed followed by non-linear dynamic analyses for the impact. This analysis is done for a few water depths and soil conditions to get an insight into the complex dynamic behavior. The limiting condition is compared with the traditional DNV criteria (Classification Notes 31.5) [3] usually followed by the industry. The author believes that the methodology and results reported in this paper for a specific jack-up will help to understand the behavior of other jack-ups for a site specific condition as the same design methodology can be applied. This may lead to a more rational design and operational guidelines than available at present.

Automated System Design for Classification of Chronic Lung Viruses using Non-Linear Dynamic System Features and K-Nearest Neighbour

2021 ◽  
Author(s):  
Misha Urooj Khan ◽  
Ayesha Farman ◽  
Asad Ur Rehman ◽  
Nida Israr ◽  
Muhammad Zulqarnain Haider Ali ◽  
...  
Keyword(s):  
Dynamic System ◽  
System Design ◽  
Linear Dynamic System ◽  
Automated System ◽  
Nearest Neighbour ◽  
Linear Dynamic ◽  
Non Linear

The mechanical basis of Drosophila audition

2002 ◽  
Vol 205 (9) ◽  
pp. 1199-1208 ◽  
Author(s):  
Martin C. Göpfert ◽  
Daniel Robert
Keyword(s):  
Mechanical Response ◽  
Dynamic Range ◽  
Near Field ◽  
Sense Organ ◽  
Field Measurements ◽  
Longitudinal Axis ◽  
Acoustic Energy ◽  
Linear Effect ◽  
Response Characteristics ◽  
Non Linear

SUMMARY In Drosophila melanogaster, antennal hearing organs mediate the detection of conspecific songs. Combining laser Doppler vibrometry, acoustic near-field measurements and anatomical analysis, we have investigated the first steps in Drosophila audition, i.e. the conversion of acoustic energy into mechanical vibrations and the subsequent transmission of vibrations to the auditory receptors in the base of the antenna. Examination of the mechanical responses of the antennal structures established that the distal antennal parts (the funiculus and the arista) together constitute a mechanical entity, the sound receiver. Unconventionally, this receiver is asymmetric, resulting in an unusual, rotatory pattern of vibration; in the presence of sound, the arista and the funiculus together rotate about the longitudinal axis of the latter. According to the mechanical response characteristics, the antennal receiver represents a moderately damped simple harmonic oscillator. The receiver's resonance frequency increases continuously with the stimulus intensity, demonstrating the presence of a non-linear stiffness that may be introduced by the auditory sense organ. This surprising,non-linear effect is relevant for close-range acoustic communication in Drosophila; by improving antennal sensitivity at low song intensities and reducing sensitivity when intensity is high, it brings about dynamic range compression in the fly's auditory system.

Rheological Properties of a Natural Estuarine Mud

2003 ◽  
Vol 13 (3) ◽  
pp. 142-149 ◽  
Author(s):  
Thierry Aubry ◽  
Tolotrahasiina Razafinimaro ◽  
Ricardo Silva Jacinto ◽  
Philippe Bassoulet
Keyword(s):  
Rheological Properties ◽  
Material Properties ◽  
Mechanical Response ◽  
Oscillatory Shear ◽  
Cohesive Sediments ◽  
Exponential Functions ◽  
Non Linear

Abstract In this paper, the linear and non-linear rheological properties of estuarine cohesive sediments were investigated. The density of the sediments has been determined by pycnometry. Creep and oscillatory shear measurements have been performed in order to determine i) the transitions in mechanical response to creep and oscillatory shear and ii) the material properties of these natural fluids as a function of their density. For all samples tested, four different rheological transitions have been determined and all material properties have been shown to be satisfactorily fitted by exponential functions of the density.

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