Application of a Hybrid Boussinesq-Panel Model for Motion Predictions of a Moored Sevan-Floater in Finite Water Depth

2017 ◽  
Author(s):  
Jikun You ◽  
Einar Bernt Glomnes
Keyword(s):  
Water Depth ◽  
Low Frequency ◽  
Boussinesq Equations ◽  
Wave Force ◽  
Irregular Waves ◽  
Wave Forces ◽  
Rankine Source ◽  
Panel Model ◽  
Finite Water Depth ◽  
Incident Waves

This paper presents the applications of an efficient hybrid time-domain simulation model for predicting moored Sevan-floater motions in irregular waves and finite water depth. The irregular incident waves are modeled by the extended Boussinesq equations, which can capture wave-wave interactions and the low-frequency long waves accurately in finite and shallow water depth. By imposing the incident wave kinematics on the surface of the floater, a panel model based on Rankine source method is applied for the calculation of wave forces and corresponding floater motions. The contributions from low-frequency components in incident waves as well as their diffraction effects are included in the wave force calculations. Validation of the irregular waves simulated by the present numerical model are performed against experimental data. Then, the simulated moored floater motions are compared with model test results and results based on Newman’s approximation. The general good agreements with experimental results demonstrate the present model can be used as an alternative for this problem while Newman’s approximation shows non-conservative results.

Experimental Study of Wave Force Distribution on a Monopile Structure

2018 ◽  
Author(s):  
Malene H. Vested ◽  
Stefan Carstensen ◽  
Erik Damgaard Christensen
Keyword(s):  
Experimental Studies ◽  
Cost Effective ◽  
Wave Force ◽  
Offshore Wind ◽  
Irregular Waves ◽  
Force Distribution ◽  
Wave Forces ◽  
Total Force ◽  
Wave Flume ◽  
Effective Manner

As the demand for offshore wind energy continues to grow, the strive to understand the wave forces acting on the substructure of the wind turbines continues. In regard to wind turbine design, it is vital to consider not only the total wave force, but also the local wave forces. Local forces are particularly important for the design of secondary structures as e.g. mooring platforms. Typically, however, experimental studies mainly concern total forces or idealized local forces. We present here a rather simple way to measure local forces along a model monopile. The study is conducted in a wave flume of 28 m in length, in which waves are generated by a piston-type wave maker at a water depth of 0.515 m and shoal onto a bed of slope 1:25. A model monopile is installed and subjected to forcing from a series of both regular and irregular waves. In the experimental set-up, the model monopile is fixed at the bottom and the top and consists of seven independent cylindrical sections. The cylindrical sections are connected by force transducers which measure local shear, and so the associated local forces may be determined. The measured local forces are compared to the force distribution given by Morisons equation combined with linear theory and Wheeler stretching, which is a force estimate commonly used in the industry. This study shows that the total force is rather well captured by Morison’s equation. The force distribution estimated from Morison’s equation, however, shows larger discrepancies from the measured forces. This encourages for further measurements. In this study, we show that it is possible to measure force distribution on a model monopile in a simple and cost-effective manner. The aim is here to demonstrate the method and we will later present a larger body of work associated with the outcome of the measurements.

Low Frequency Wave Forces and Wave Induced Motions of a FPSO in Shallow Water

2007 ◽  
Author(s):  
Longfei Xiao ◽  
Jianmin Yang ◽  
Zhiqiang Hu
Keyword(s):  
Shallow Water ◽  
Wave Spectrum ◽  
Low Frequency ◽  
Wave Force ◽  
Wave Forces ◽  
Frequency Wave ◽  
Wave Basin ◽  
The Difference ◽  
The One ◽  
Wave Induced

The low frequency (LF) response of a soft yoke moored 160kDWT FPSO in shallow water is investigated by conducting frequency domain computations and wave basin model tests. An incident wave with Hs = 4.1m and Tp = 8.9s is applied. An obvious LF part appears in the measured wave spectrum at water depth of 16.7m. As a result, the 1st order LF wave force exists and is much larger than the 2nd one. The difference of the spectrums is about one hundred times. The LF wave drift force increases enormously. Consequently, much larger resonant surge response is induced. The LF surge amplitude at h = 16.7m is about 7 times the one at h = 29.0m and 9 times the one in deep water, although the 2nd order response changes a little. Therefore, in very shallow water, LF part of incident waves should be taken into account carefully and LF wave forces and wave induced motions will be very serious.

Computational Fluid Dynamics Simulations of Regular and Irregular Waves Past a Horizontal Semi-Submerged Cylinder

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Shengnan Liu ◽  
Muk Chen Ong ◽  
Charlotte Obhrai ◽  
Sopheak Seng
Keyword(s):  
Free Surface ◽  
Numerical Simulations ◽  
Wave Height ◽  
Stokes Equations ◽  
Wave Force ◽  
Numerical Results ◽  
Irregular Waves ◽  
Wave Forces ◽  
Regular Waves ◽  
Peak Period

Two-dimensional (2D) numerical simulations have been performed to investigate both regular and irregular waves past a fixed horizontally semisubmerged circular cylinder. The 2D simulations are carried out by solving Navier–Stokes equations discretized by finite volume method. Volume of fluid (VOF) method is employed to capture the free surface in the numerical wave tank (NWT). Validation studies have been performed by comparing the numerical results of free surface waves past the cylinder with the published experimental and numerical data. The present numerical results are in good agreement with both the experimental and the other numerical results in terms of hydrodynamic forces and free surface elevation. Subsequently, the effects of the wave height and the wavelength on wave–structure interaction are investigated by conducting numerical simulations on the regular and the irregular waves past a semisubmerged cylinder at different wave heights and the wavelengths. The averaged and maximum vertical wave forces on the cylinder increase with the increasing wave height. The numerical results for the irregular waves are compared with those induced by the regular waves in terms of the maximum and averaged vertical wave forces. When the significant wave height and the spectral peak period of the irregular waves are equal to the wave height and the wave period of the regular waves, the maximum vertical wave force induced by the irregular waves is larger than that induced by the regular waves, meanwhile, the average vertical wave forces have the contrary relationship.

scholarly journals STUDY OF STATISTICAL CHARACTERISTICS OF IRREGULAR WAVE PRESSURE ON A COMPOSITE BREAKWATER

1986 ◽  
Vol 1 (20) ◽  
pp. 131
Author(s):  
Chien-Kee Chang ◽  
Ching-Her Hwang
Keyword(s):  
Pressure Distribution ◽  
Wave Height ◽  
Wave Force ◽  
Shock Pressure ◽  
Irregular Waves ◽  
Statistical Characteristics ◽  
Wave Forces ◽  
Incoming Wave ◽  
Wave Pressure ◽  
Irregular Wave

Wave pressure is the most important external force for the design of breakwater. During recent years, there has been considerable development in the technology of vertical face breakwater; however, there is no reliable method to compute wave forces induced by irregular waves. The purpose of this study is to obtain statistical characteristics of irregular wave pressure distribution from the data of model tests. The results of this study shown that vertical face breakwater under the action of irregular waves, some waves are reflected, so that the next wave breaks a critical distance resulting in a rapidly rising shock pressure on the breakwater. On the average, the wave pressure increase with incoming wave height, but the maximum wave force does not necessarily occur for the largest wave height. It can be occurred for serval larger wave group in an appropiate phase composition. The irregular wave pressure distribution on the breakwater is quite uniform; the ratio of tested and calculated wave pressures decreases with the reduction of relative crest height of breakwater. Coda formula can predict the total horizontal force of the upper part of breakwater quite well except exetreme shock pressure occurred by non-breaking waves. Wave forces calculated by Miche-Rundgren and Nagai wave force formula are about 10% cummulated exceeding percentage of wave force obtained from model test.

scholarly journals THE CLAMP-ON WAVE FORCE METER

2011 ◽  
Vol 1 (7) ◽  
pp. 39
Author(s):  
Lars Skjelbreia
Keyword(s):  
Gulf Of Mexico ◽  
Water Depth ◽  
Wave Force ◽  
Wave Forces ◽  
Oil Well ◽  
Great Effort ◽  
Oil Companies ◽  
Research Corporation ◽  
Cylindrical Piles ◽  
Different Diameters

Because of the tremendous increase in offshore activities, a great effort has been made on obtaining information on wave forces on structural members. Several oil companies have invested large sums of money in the design and construction of full-scale systems for measuring the wave forces. The equipment used for measuring the forces have been single cantilevers or segmented piles designed to make discrete measurements along the pile. For instance, during the last five years, The California Company and California Research Corporation (subsidiaries of Standard Oil Company of California) operated an installation in the Gulf of Mexico with four segmented piles of different diameters. The wave forces were measured by three-foot high force dynamometers located at seven different elevations along the length of each test pile. Each dynamometer was constructed from a section of the cylindrical pile which was attached to a system of flexures on the inside. So far the wave forces have been measured on cylindrical piles varying in diameter from one to four feet and in water depths varying from 30 to 50 feet. As the pile diameter and water depth increase, however, the measurements of wave forces by use of a cantilever or a segmented pile become very difficult and expensive. Therefore, a need exists for investigating other means for measuring the wave forces on a pile. This paper will describe the design and operation of a force meter that may be clamped to an existing pile. In Spring 1960, California Research Corporation installed equipment incorporating eight of the clamp-on meters on an oil well drilling platform in the Gulf of Mexico. The water depth at the location is 100 feet, and two years of operation are planned.

Focused Plunging Breaking Waves Impact on Pile Group in Finite Water Depth

2021 ◽  
pp. 1-17
Author(s):  
Ting Cui ◽  
Arun Kamath ◽  
Weizhi Wang ◽  
Lihao Yuan ◽  
Duanfeng Han ◽  
...  
Keyword(s):  
Water Depth ◽  
Pile Group ◽  
Breaking Waves ◽  
Offshore Structures ◽  
Structural Safety ◽  
Wave Forces ◽  
Pile Groups ◽  
Navier Stokes Equation ◽  
Finite Water Depth ◽  
Numerical Wave

Abstract Accuracy estimation of wave loading on cylinders in a pile group under different impact scenarios is essential for both the structural safety and cost of coastal and offshore structures. Differing from the interaction of waves with a single cylinder, less attention has been paid to pile groups under different arrangements. Numerical simulations of interactions between plunging breaking waves and pile group in finite water depth are performed using the two-phase flow model in REEF3D, an open-source computational fluid dynamics program to investigate the wave loads and flow kinematics characteristics. The Reynolds-averaged Navier-Stokes equation with the two equation k − ω turbulence model is adopted to resolve the numerical wave tank. The model is validated by comparing the numerical wave forces and free surface elevation with measurements from experiments. The computational results show fairly good agreement with experimental data. Four cases are simulated with different relative distances, numbers of cylinders and arrangements. Results show that the wave forces on cylinders in the pile group are effected by the relative distance between cylinders. The staggered arrangement has a significant influence on the wave forces on the first and second cylinder. The interaction inside a pile group mostly happens between the neighboring cylinders.

Mean- and Low-Frequency Wave Forces on Semisubmersibles

1982 ◽  
Vol 22 (04) ◽  
pp. 563-572
Author(s):  
J.A. Pinkster
Keyword(s):  
Low Frequency ◽  
Second Order ◽  
Irregular Waves ◽  
Wave Forces ◽  
Frequency Wave ◽  
First Order ◽  
Wave Drift ◽  
Frequency Components ◽  
Wave Drift Forces ◽  
Drift Forces

Abstract Mean- and low-frequency wave drift forces on moored structures are important with respect to low-frequency motions and peak mooring loads. This paper addresses prediction of these forces on semisubmersible-type structures by use of computations based on three-dimensional (3D) potential theory. The discussion includes a computational method based on direct integration of pressure on the wetted part of the hull of arbitrarily shaped structures. Results of computations of horizontal drift forces on a six-column semisubmersible are compared with model tests in regular and irregular waves. The mean vertical drift forces on a submerged horizontal cylinder obtained from model tests also are compared with results of computations. On the basis of these comparisons, we conclude that wave drift forces on semisubmersible-type structures in conditions of waves without current can be predicted with reasonable accuracy by means of computations based on potential theory. Introduction Stationary vessels floating or submerged in irregular waves are subjected to large first-order wave forces and moments that are linearly proportional to the wave height and that contain the same frequencies as the waves. They also are subjected to small second-order mean- and low- frequency wave forces and moments that are proportional to the square of the wave height. Frequencies of second-order low-frequency components are associated with the frequencies of wave groups occurring in irregular waves.First-order wave forces and moments cause the well-known first-order motions with wave frequencies. First-order wave forces and motions have been investigated for several decades. As a result of these investigations, methods have been developed to predict these forces and moments with reasonable accuracy for many different vessel shapes.For semisubmersibles, which consist of a number of relatively slender elements such as columns, floaters, and bracings, computation methods have been developed to determine the hydrodynamic loads on those elements without accounting for interaction effects between the elements. For the first-order wave loads and motion problem, these computations give accurate results.This paper deals with the mean- and low-frequency second-order wave forces acting on stationary vessels in regular and irregular waves in general and presents a method to predict these forces on the basis of computations.The importance of mean- and low-frequency wave drift forces, from the point of view of motion behavior and mooring loads on vessels moored at point of view of motion behavior and mooring loads on vessels moored at sea, has been recognized only within the last few years. Verhagen and Van Sluijs, Hsu and Blenkarn, and Remery and Hermans showed that the low-frequency components of wave drift forces in irregular waves-even though relatively small in magnitude-can excite large-amplitude low- frequency horizontal motions in moored structures. It was shown for irregular waves that the drift forces contain components with frequencies coinciding with the natural frequencies of the horizontal motions of moored vessels. Combined with minimal damping of low-frequency horizontal motions of moored structures, this leads to large-amplitude resonant behavior of the motions (Fig. 1). Remery and Hermans established that low-frequency components in drift forces are associated with the frequencies of wave groups present in an irregular wave train.The vertical components of the second-order forces sometimes are called suction forces. SPEJ p. 563

CFD Analysis of Waves Over a Submerged Cylinder in Close Proximity of the Free Surface

2014 ◽  
Author(s):  
Harald Ottens ◽  
Alessio Pistidda ◽  
Radboud van Dijk
Keyword(s):  
Inviscid Flow ◽  
Wave Force ◽  
Model Tests ◽  
Irregular Waves ◽  
Ship Motions ◽  
Wave Forces ◽  
Cfd Simulations ◽  
Submerged Body ◽  
Submerged Cylinder ◽  
Water Elevation

Diffraction programs using potential theory are a quick and effective method in calculating wave forces and ship motions. However in cases where a small layer of water is present on top of a submerged body diffraction calculations overpredict motion and wave force RAOs. This shortcoming of diffraction programs is observed after conducting model tests on a captive submerged cylinder and a free floating SSCV. Unrealistic high wave elevations were predicted by diffraction programs on top of the submerged body. In a previous study a damping lid is implemented [1], to decrease the water elevation to realistic values. In this study CFD is used to simulate the captive submerged cylinder in regular waves with different wave heights, wave periods and different submerged drafts. In addition irregular waves are used in the simulation matching the wave spectra used in the model tests. The simulations are transient and require high CPU usage, therefore the influence of numerical settings on wave propagation is investigated. Turbulent, laminar and inviscid flow are applied to evaluate which flow phenomena are important. The forces in heave and surge direction are validated with model test data of the captive cylinder. The numerical water elevation on top of the captive cylinder will be used to gain insight in the fluid flow and can be used as a guideline for the use of damping lids in diffraction programs. This paper will focus on the CFD simulations and the validation with available forces obtained by model tests of the captive submerged cylinder. It will address the use of regular and irregular waves constructing the force RAO for this non-linear phenomenon. Lessons learnt to improve the CFD simulations as well as limitations of constructing RAOs using CFD from an engineering perspective will be addressed as well.

scholarly journals WAVE FORCE ON A VESSEL TIED AT OFFSHORE DOLPHINS

1972 ◽  
Vol 1 (13) ◽  
pp. 92
Author(s):  
Yoshimi Goda ◽  
Tomotsuka Yoshimura
Keyword(s):  
Wave Scattering ◽  
Maximum Amplitude ◽  
Wave Force ◽  
Elliptical Cylinder ◽  
Wave Forces ◽  
Reference Volume ◽  
Wave Approach ◽  
Directional Wave ◽  
The One ◽  
Incident Waves

The solution of wave scattering by a vertical elliptical cylinder is applied to calculate the wave forces exerted upon it. The wave forces in the directions of long and short axes of ellipsis are shown in nondimensional forms as the functions of the angle of wave approach, the diameter-to-wavelength ratio, and the aspect ratio of ellipsis. The results of wave force computed are also shown in terms of the virtual mass coefficients associated with the reference volume of the circular cylinder the diameter of which is approximately equal to the apparent width of the elliptical cylinder observed from the direction of wave approach. Theory is further applied for the wave forces acting upon a vessel moored tight at offshore dolphins and the forces transmitted to the dolphins through the vessel. The vessel is approximated with the fixed elliptical cylinder having the same width-to-length ratio. The computation with directional wave spectra shows that a tanker of 200,000 D.W.T. may exert the force of about 1,400 tons at the one-third maximum amplitude to each breasting dolphin when the tanker is exposed to the incident waves of H,/„=1.0m and T» / =10 sec from the broadside.

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