On the Distribution of Wave Impact Loads on Offshore Structures

2017 ◽  
Author(s):  
Thomas B. Johannessen ◽  
Øystein Lande ◽  
Øistein Hagen
Keyword(s):  
Model Test ◽  
Load Distribution ◽  
Impact Load ◽  
Peak Load ◽  
Offshore Structures ◽  
Impact Loads ◽  
Test Results ◽  
Wave Impact ◽  
The Impact ◽  
Crest Height

For offshore structures in harsh environments, horizontal wave impact loads should be taken into account in design. Shafts on GBS structures, and columns on semisubmersibles and TLPs are exposed to impact loads. Furthermore, if the crest height exceeds the available freeboard, the deck may also be exposed to wave impact loads. Horizontal loads due to waves impacting on the structure are difficult to quantify. The loads are highly intermittent, difficult to reproduce in model tests, have a very short duration and can be very large. It is difficult to calculate these loads accurately and the statistical challenges associated with estimating a value with a prescribed annual probability of occurrence are formidable. Although the accurate calculation of crest elevation in front of the structure is a significant challenge, industry has considerable experience in handling this problem and the analysis results are usually in good agreement with model test results. The present paper presents a statistical model for the distribution of horizontal slamming pressures conditional on the incident crest height upwave of the structure. The impact load distribution is found empirically from a large database of model test results where the wave impact load was measured simultaneously at a large number of panels together with the incident crest elevation. The model test was carried out on a circular surface piercing column using long simulations of longcrested, irregular waves with a variety of seastate parameters. By analyzing the physics of the process and using the measured crest elevation and the seastate parameters, the impact load distribution model is made seastate independent. The impact model separates the wave impact problem in three parts: – Given an incident crest in a specified seastate, calculate the probability of the crest giving a wave impact load above a threshold. – Given a wave impact event above a threshold, calculate the distribution of the resulting peak load. – Given a peak load, calculate the distribution of slamming pressures at one spatial location. The development of the statistical model is described and it is shown that the model is appropriate for fixed and floating structures and for wave impact with both columns and the deck box.

Wave Impact Loads on the Bottom of Flat Decks

2021 ◽  
Author(s):  
Daniel de Oliveira Costa ◽  
Julia Araújo Perim ◽  
Bruno Guedes Camargo ◽  
Joel Sena Sales Junior ◽  
Antonio Carlos Fernandes ◽  
...  
Keyword(s):  
Scale Effects ◽  
Offshore Structures ◽  
Model Tests ◽  
Analytical Models ◽  
Navier Stokes ◽  
Impact Loads ◽  
Wave Impact ◽  
Wave Channel ◽  
Waves And Currents ◽  
The Impact

Abstract Slamming events due to wave impact on the underside of decks might lead to severe and potentially harmful local and/or global loads in offshore structures. The strong nonlinearities during the impact require a robust method for accessing the loads and hinder the use of analytical models. The use of computation fluid dynamics (CFD) is an interesting alternative to estimate the impact loads, but validation through experimental data is still essential. The present work focuses on a flat-bottomed model fixed over the mean free surface level submitted to regular incoming waves. The proposal is to reproduce previous studies through CFD and model tests in a different reduced scale to provide extra validation and to identify possible non-potential scale effects such as air compressibility. Numerical simulations are performed in both experiments’ scales. The numerical analysis is performed with a marine dedicated flow solver, FINE™/Marine from NUMECA, which features an unsteady Reynolds-averaged Navier-Stokes (URANS) solver and a finite volume method to build spatial discretization. The multiphase flow is represented through the Volume of Fluid (VOF) method for incompressible and nonmiscible fluids. The new model tests were performed at the wave channel of the Laboratory of Waves and Currents (LOC/COPPE – UFRJ), at the Federal University of Rio de Janeiro.

Transducer Qualification for Wave Impact Load Measurements Using Wedge Drop Tests

2015 ◽  
Author(s):  
Zhenjia (Jerry) Huang ◽  
Robert Oberlies ◽  
Don Spencer ◽  
Jang Kim
Keyword(s):  
Impact Load ◽  
Research Work ◽  
Offshore Structures ◽  
Model Tests ◽  
Impact Loads ◽  
Dynamic Impact ◽  
Wave Impact ◽  
Impact Forces ◽  
Industry Practice ◽  
Drop Tests

For the design of offshore structures in harsh wave environments, it is essential to accurately determine the wave impact loads on the structure. To date, robust numerical prediction methods / algorithms for determining wave impact forces on offshore structures do not exist. Model testing continues to be the industry practice for determining wave impact forces on offshore structures. Accurate measurements of wave impact loads in model tests have been challenging for several decades. Transducers require the ability to capture the short duration, dynamic nature and high magnitude of impact loads. In order to qualify transducers for these types of measurements, we need to develop a way to physically impose dynamic impact loads on the transducers and to establish benchmark values that can be used to check the effectiveness of their measurements. In this paper, we present our recent research work on transducer qualification for wave impact load measurements, including their development, numerical analysis and wedge drop model tests. Our findings show that wedge drop tests can be used to impose dynamic impact loads for transducer qualification, and that the Wagner solution and / or validated computational fluid dynamics (CFD) simulations that include the effects of viscosity, compressibility and hydroelasticity can provide the appropriate benchmarking values. Numerical simulation results, model test measurements and findings on transducer qualification are presented and discussed in the paper.

An Experimental Investigation of Wave Impact Loads on a Slender Horizontal Cylinder

2014 ◽  
Author(s):  
Joseph F. Haley ◽  
Chris Swan ◽  
Richard Gibson
Keyword(s):  
Water Surface ◽  
Impact Load ◽  
Horizontal Cylinder ◽  
The Body ◽  
Impact Loads ◽  
Wave Impact ◽  
Wide Range ◽  
Wave Event ◽  
Vector Sum ◽  
The Impact

This paper concerns the difficulties arising in the prediction of the impact loads associated with an extreme wave event. A new set of experimental observations are presented. These concern the impact loads arising on a slender horizontal cylinder located at varying elevations above the still water level. The experimental observations incorporate a wide range of wave forms. In each case, data is provided describing (i) the incident water surface profiles, (ii) the incident fluid velocities and (iii) the load components acting on the cylinder. Comparisons between the measured data and the classical impact load solutions confirm a number of important departures. In particular, it is shown that as the wave becomes very steep (approaching the breaking limit) the vector sum of the horizontal and vertical velocity components at the water surface may deviate significantly from the normal to the local water surface. In such cases it becomes unclear exactly what direction the impact force acts. The present data suggests that this is, in part, dependent on the rate of inundation of the body. Furthermore, the present results also show that if the direction of the force is correct modelled, the variations in the predicted loading (or slamming) coefficient are much reduced.

A Study on the Impact Load Acting on an FPSO Bow by Steep Waves

2014 ◽  
Author(s):  
Sam-Kwon Hong ◽  
Jae-Moon Lew ◽  
Dong-Woo Jung ◽  
Hee-Taek Kim ◽  
Dong-Yeon Lee ◽  
...  
Keyword(s):  
Model Test ◽  
Impact Force ◽  
Impact Load ◽  
Impact Loads ◽  
Return Periods ◽  
Seakeeping Performance ◽  
Wave Heights ◽  
Wave Conditions ◽  
Steep Waves ◽  
The Impact

Among offshore floaters used to develop offshore resources, FPSO and FSO have a storage function whereas semi-submersible, Spar and TLP have only production function. The floaters with the storage function such as FPSO and FSO are designed as the typical ship type concept compared to the other floaters with small water plane area. In order to design the floaters for offshore resource development, it is needed to estimate the seakeeping performance under operating condition and survival conditions and then carry out the structural design based on seakeeping performance results. The environment conditions of 1yr, 10yrs, 100yrs or 1,000 yrs return periods are used based on the metocean data of the installation field to evaluate the seakeeping performance under operating and survival conditions. In general, the wave conditions with the maximum wave heights for each return periods are selected on each wave contour lines in the wave scatter diagram. Then the seakeeping performance is evaluated from the seakeeping model test. However, it was observed that the wave with the pitch forcing period, where the wave length is close to the ship length, is more important than the wave with the maximum wave height after several accidents caused by the green water in Northern North Sea and Norwegian Sea. Therefore, it became a common practice to include not only the wave conditions with maximum wave heights for each return period but also the wave conditions with the pitch forcing period to evaluate the seakeeping performance for offshore development floaters. Ship type floaters such as FPSO are more likely to experience higher impact force due to the large frontal area accompanied by large heave and pitch motions in head sea and bow quartering seas. Recently, it was reported that in an accident in North Sea of UK sector, the damage at the bow of the FPSO is caused due to the steep waves. Afterwards, studies on the steep waves have been made in several institutes such as UK HSE. In this study, the effect of the impact load (so called slapping load) by the steep waves acting on the FPSO bow is investigated throughout the model test. For measurement of the pressure and impact force on the frontal area, a bow-shaped panel was fabricated with the pressure and force sensors, and installed on the bow starboard side of the model FPSO. During the model test campaign, the impact load was investigated using the steep waves with Hw/λ greater than 1/16 in addition to the general wave conditions with maximum wave heights. Consequently, it is confirmed in the model test that the impact loads acting on the FPSO bow are significantly increased with the steep waves (Hw/λ > 1/16) compared to the general wave conditions. Therefore, it is necessary to consider whether the steep waves are additionally included in the wave conditions to estimate the seakeeping performance and how to apply the impact loads acting on the FPSO bow from the steep waves in structure design.

Experimental Investigation on the Crashworthiness of Braided Glass/Epoxy Tubes Subjected to Axial Impacting

2014 ◽  
Vol 23 (2) ◽  
pp. 096369351402300
Author(s):  
Ping Zhang ◽  
Liang-Jin Gui ◽  
Zi-Jie Fan ◽  
Jing-Yu Liu
Keyword(s):  
Energy Absorption ◽  
Specific Energy ◽  
Failure Modes ◽  
Impact Load ◽  
Peak Load ◽  
Low Velocity Impact ◽  
Test Results ◽  
Specific Energy Absorption ◽  
The Mean ◽  
The Impact

This paper presented an experimental study on the low-velocity impact response of triaxial braided composite circular tubes, which were fabricated with S-glass/epoxy composite. The impact responses were recorded and analyzed in terms of impact load-displacement curves and specific energy absorption. In addition, four basic failure modes called delaminating, splaying, fragmental fracture and progressive folding were founded. The levels of the mean impact load and specific energy absorption (SEA) are determined by the energy absorption mechanisms, which are related to the dominant failure modes of the tubes. In general, delamination which exhibits the poor energy absorbing performance is the dominant failure mode for all the specimens. Impact test results showed that all three types of tubes had almost the same SEA. Compared to the quasi-static test results, the first peak load and the mean load decrease at about 50% and 10% respectively, SEA generally decreases at an average level 10%.

Load Pulse Determination in Gas Gun Impact Tests

2007 ◽  
Vol 7-8 ◽  
pp. 259-264 ◽  
Author(s):  
Stefan Andreas Ritt ◽  
Alastair F. Johnson
Keyword(s):  
Impact Load ◽  
Axial Strain ◽  
Impact Loads ◽  
Test Results ◽  
Measuring Device ◽  
Gas Gun ◽  
Load Pulse ◽  
Longitudinal Wave Propagation ◽  
Force Time ◽  
The Impact

This paper is concerned with methods to determine the resultant impact load pulse on test structures during gas gun tests. The muzzle of a gas gun carries an instrumented target support designed to measure force-time-pulses of the impact event on structures mounted on the device. The target during impact is supported on two long slender metallic bars with axial strain gauges. By measuring the impact strain pulses in the support bars and assuming longitudinal wave propagation, it is possible to determine the impact load pulse on the target. The paper describes the strain measuring device and methods for determining the impact loads during impact. Gas gun test results with different projectiles are presented and the load pulses compared with a direct load measurement from a target load cell. The paper discusses possibilities and limitations of the device for quantitative force pulse measurements.

Analysis on Fatigue Fracture of the Flame-Quenched 8.8Al-Bronze by Ultrasonic Vibratory Cavitation Erosion

2006 ◽  
Vol 118 ◽  
pp. 463-468
Author(s):  
Sung Mo Hong ◽  
Min Ku Lee ◽  
G.H. Kim ◽  
Chang Kyu Rhee ◽  
K.H. Kim ◽  
...  
Keyword(s):  
Steady State ◽  
Incubation Period ◽  
Load Distribution ◽  
Cavitation Erosion ◽  
Impact Load ◽  
Fatigue Properties ◽  
Impact Loads ◽  
Cavitation Damage ◽  
Number Of Cycles ◽  
The Impact

In this study the fatigue properties due to cavitation damage of flame-quenched 8.8Al-bronze (8.8Al-4.5Ni-4.5Fe-Cu) as well as current nuclear pump materials (8.8Al-bronze, SUS316 and SR50A) have been investigated by using an ultrasonic vibratory cavitation test. For this the impact loads of cavitation bubbles generated by ultrasonic vibratory device quantitatively evaluated and simultaneously the cavitation erosion experiments have been carried out. The fatigue analysis on the cavitation damage of the materials has been made from the determined impact load distribution (e.g. impact load, bubble count) and erosion parameters (e.g. incubation period, MDPR). According to Miner’s law, the exponents b of the F-N relation (Fb N = Constant) at the incubation stage (N: the number of fracture cycle) were 5.62, 4.16, 6.25 and 8.1 for the 8.8Al-bronze, flame-quenched sample, SUS316 and SR50A alloys, respectively. At steady-state, the exponents b of the F-N curve (N: the number of cycles required for a 1μm increment of MDP) were determined as 6.32, 5, 7.14 and 7.76 for the 8.8Al-bronze, flame-quenched sample, SUS316 and SR50A alloys, respectively.

Wave Impact Experiment of a GBS Model in Large Waves

2017 ◽  
Author(s):  
Zhenjia (Jerry) Huang ◽  
Don Spencer ◽  
Robert Oberlies ◽  
Gracie Watts ◽  
Wenting Xiao
Keyword(s):  
Structural Model ◽  
Probability Distributions ◽  
Offshore Structures ◽  
Model Tests ◽  
Wave Crest ◽  
Maximum Pressure ◽  
Impact Loads ◽  
Wave Impact ◽  
Wave Basin ◽  
The Impact

For the design of offshore structures in harsh wave environments, model testing continues to be the recommended industry practice for determining wave impact forces on offshore structures. Accurate measurements of wave impacts in model tests have been a challenge for several decades. Transducers are required to accurately capture the short duration, high magnitude, and dynamic nature of impact loads. The structural model, transducers, and the transducer mountings need to be designed such that mechanical vibrations in the integrated transducer-mounting-structural model system do not contaminate the wave impact measurements. In this work, the dynamic oscillations in the measurements were controlled through the design and fabrication of transducers, their mounting and the GBS model. Wave crest probability distributions were developed that included fully nonlinear effects. These distributions were used as a benchmark to qualify the waves in the wave calibration tests. The highly stochastic nature of impact loads makes it challenging to obtain converged probability distributions of the maximum impact loads (i.e. forces or pressures) from model tests. To increase the confidence in the statistical values of wave impact loads, a large number of realizations were used for a given sea state. Variability of the maximum pressure due to wave basin effects (such as wait-time between tests) was examined with fifteen repeat tests using the same wave maker control signal. These tests provided insights into the random behavior of the impact loads.

Impact Load Measurements in an Erosive Cavitating Flow

2011 ◽  
Vol 133 (12) ◽  
Author(s):  
Jean-Pierre Franc ◽  
Michel Riondet ◽  
Ayat Karimi ◽  
Georges L. Chahine
Keyword(s):  
Scaling Laws ◽  
Pressure Pulse ◽  
Impact Load ◽  
Pulse Height ◽  
Impact Loads ◽  
Similar Data ◽  
Two Dimensional ◽  
Test Results ◽  
Test Technique ◽  
The Impact

Impact load measurements were carried out in a high-speed cavitation loop by means of a conventional pressure sensor flush-mounted in the region of closure of the cavity where maximum damage was observed. The sensor was dynamically calibrated by the ball drop test technique. Pressure pulse amplitudes were measured at different velocities and constant cavitation number and cavity length. It was found that pressure pulse height spectra follow a simple exponential law, which depends upon two parameters interpreted as a reference peak rate and a reference load. By exploring the dependence of both parameters on flow velocity, it was possible to show that the various histograms measured at different velocities can be reduced to a unique non-dimensional one and derive scaling laws, which enable to transpose results from one velocity to another. The measured values of impact loads are compared to similar data in the literature, and the impact load spectra are discussed with respect to pitting test results available from a previous investigation. It is concluded that an uncertainty remains on the measured values of impact loads and that a special effort should be made to compare quantitatively pitting test results and impact load measurements. To evaluate the coherence of both sets of data with each other, it is suggested to introduce two-dimensional histograms of impact loads by considering the size of the impacted area in addition to the measured impact load amplitude. It is conjectured that the combination of impact load measurements and pitting test measurements should allow the determination of such two-dimensional histograms, which are an essential input for analyzing the material response and computing the progression of erosion with exposure time.

A Fully Nonlinear RANS-VOF Numerical Wavetank Applied in the Analysis of Green Water on FPSO in Waves

2014 ◽  
Author(s):  
Anders Östman ◽  
Csaba Pakozdi ◽  
Lucia Sileo ◽  
Carl-Trygve Stansberg ◽  
Daniel Fonseca de Carvalho e Silva
Keyword(s):  
Model Test ◽  
Parametric Design ◽  
Green Water ◽  
Test Results ◽  
Wave Impact ◽  
Measured Time ◽  
Time Histories ◽  
Practical Tool ◽  
The Impact ◽  
Impact Events

This paper presents numerical simulations of Green Water events and wave impact on a FPSO. The simulations are performed at model scale and the results are compared against experimental model test results. The commercial Star-CCM+ CFD software is used in the simulations. The incoming waves are modeled using 5th order Stokes theory, as implemented in the CFD software. Both fixed and free floating FPSO are considered. The moving FPSO are modeled using Chimera overset mesh technology. The vessels is free to move in heave and pitch at 180 (head sea), roll and heave at 270 (beam sea), while roll, pitch and heave is released at 225 (quartering sea). The computed water height on the deck and the relative wave height in vicinity the vessel are compared against model test results at several positions. Also the impact force on load cells blocks located at the deck of the vessel is computed and compared against model test results. The comparison of the time histories of the water elevation and load histories are in reasonable agreement with the measured time series. The number of grid cells range from 7M for the simulations at head sea, where flow is assumed to be symmetric, to 21M for the simulations at quartering sea. Total wall clock simulation time was about 10days for the most computationally demanding cases, which are the quartering sea simulations. This includes simulation of 12 wave periods with the ship fixed, and thereafter 8 wave periods of the free floating vessel. The computations show that CFD tools can be used as a research tool when studying the physics of green water and wave impact events. However, due to time CPU demanding simulations, this type of CFD analysis are not yet a practical tool for parametric design studies and deck structure optimizations. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.

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