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.

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.

Estimating Wave Induced Kinematics Underneath Measured Time Histories of Surface Elevation

2020 ◽  
Author(s):  
Thomas B. Johannessen
Keyword(s):  
Boundary Condition ◽  
Wave Field ◽  
Nonlinear Wave ◽  
Velocity Potential ◽  
Laboratory Data ◽  
Surface Elevation ◽  
Surface Boundary ◽  
Wave Impact ◽  
Measured Time ◽  
Time Histories

Abstract The present paper is concerned with the accurate prediction of nonlinear wave kinematics underneath measured time histories of surface elevation. It is desired to develop a method which is useful in analysis of offshore measurements close to wind turbine foundations. The method should therefore be robust in relatively shallow water and should be able to account for the presence of the foundation and the shortcrestedness of offshore seastates. The present method employs measurements of surface elevation time histories at one or a small number of locations and solves the associated velocity potential by minimizing the error in the free surface boundary conditions. The velocity potential satisfies exactly Laplace’s equation, the bed boundary condition and (optionally) the boundary condition on the wall of a uniform surface piercing column. This is achieved by associating one wavenumber with every wave frequency thereby sacrificing the possibility of following the nonlinear wave evolution but ensuring a good description of the wave properties locally. For shortcrested waves, the direction of wave component propagation is drawn from a known or assumed directional spectrum. No attempt is made to calculate the directional distribution of the wave field from the surface elevation measurements since this is usually not realistically possible with the available data. The method is set up for analysis with or without a uniform current, for shortcrested or longcrested waves and with or without a surface piercing column in the wave field. It has been compared with laboratory data for steep longcrested and shortcrested waves. The method is shown to be in good agreement with measurements. Since the method is based on a Fourier series of surface elevation, however, it cannot model overtopping breaking waves and associated wave impact loading. For problems where wave breaking is important, the method may serve as a screening analysis used to select wave events for detailed analysis using Computational Fluid Dynamics (CFD).

Compressive Behavior of a Polyurea Elastomer

2014 ◽  
Vol 900 ◽  
pp. 7-10
Author(s):  
Fabrizia Ghezzo ◽  
Xi Geng Miao ◽  
Chun Lin Ji ◽  
Ruo Peng Liu
Keyword(s):  
Material Behavior ◽  
Loading Conditions ◽  
Test Results ◽  
Metallic Structures ◽  
Compression Load ◽  
Existing Structures ◽  
The Past ◽  
Severe Shock ◽  
The Impact ◽  
Impact Events

The application of elastomeric coatings for improving the ability of already existing structures to dissipate the energy released by impact events has been investigated by many researchers in the past decade and is today an area of considerable interest. In recent years, polyurea has been successfully applied as a coating material for enhancing the impact protection of buildings and it has also demonstrated a considerable improvement of the survivability of metallic and non-metallic structures subjected to severe shock and impact loading conditions. Given its remarkable properties in terms of impact energy mitigation, life endurance and corrosion resistance, this material is currently of interest for its application in many fields of engineering. This paper presents and discusses the results of the mechanical characterization conducted on a polyurea elastomer fabricated following two different procedures and subjected to varying strain rates of compression load. The tests were conducted to verify the sensitivity of the material behavior to the varying loading conditions and to verify how the fabrication of the material in the laboratory can influence the test results.

Finite-Element Modeling and Model Verification of Steel W-Beam Guardrails Subject to Pendulum Impact Loading

1998 ◽  
Vol 1647 (1) ◽  
pp. 147-157
Author(s):  
T. Russell Gentry ◽  
Lawrence C. Bank
Keyword(s):  
Finite Element ◽  
Impact Loading ◽  
Model Verification ◽  
Federal Highway ◽  
Time Histories ◽  
Tension Capacity ◽  
The Impact ◽  
Impact Events ◽  
Pendulum Impact ◽  
Good Agreement

The experimental and simulated response of steel W-beam guards to pendulum impact loading for impact velocities of 20 km/h, 30 km/h, and 35 km/h are presented. The guardrails were supported by four posts and cable-anchored at each end to ensure that the full tension capacity of the rail could be developed. Experiments carried out with a 912-kg impact pendulum are compared with LS-DYNA finite-element simulations of the impact events. Pendulum tests were completed at the Turner Fairbank Highway Research Center of the Federal Highway Administration. Acceleration, velocity, and displacement time histories are compared for the pendulum impact test and the LS-DYNA simulations. Comparison of the experimental and simulation acceleration records is made using the Numerical Analysis of Roadside Design time-domain statistics. The comparative statistics show that the simulations are in good agreement with the experiments. Guardrail tension data and cable tension data are presented from the LS-DYNA simulations. Results show that the guardrail was close to its tension yield point when impacted an initial velocity of 35 km/h.

Green Water on FPSO Predicted by a Practical Engineering Method and Validated Against Model Test Data for Irregular Waves

2014 ◽  
Author(s):  
Rafael Vergara Schiller ◽  
Csaba Pâkozdi ◽  
Carl Trygve Stansberg ◽  
Douglas Gustavo Takashi Yuba ◽  
Daniel Fonseca de Carvalho e Silva
Keyword(s):  
Model Test ◽  
Model Tests ◽  
Irregular Waves ◽  
Green Water ◽  
Wave Impact ◽  
Irregular Wave ◽  
Beam Seas ◽  
Practical Engineering ◽  
The University ◽  
Wave Induced

This paper presents a series of numerical analyses performed with the potential theory-based Green Water engineer tool KINEMA3. KINEMA3 was designed to predict wave-induced impact loads on FPSOs in steep irregular waves, and for use in design load analysis. The purpose of the study presented herein is to validate KINEMA3 green water (deck overtopping) predictions in nonlinear irregular waves with results from model tests performed at the TPN (Tanque de Provas Numérico) laboratory at the University of São Paulo, Brazil. Comparisons are made for a selection of irregular wave cases, for two choices of anchoring conditions (free floating vessel and fixed vessel) and for three wave headings (180°, 225° and 270°: head, quartering and beam seas, respectively). KINEMA3 statistical green water predictions present a general good agreement with observations from the TPN model tests for all wave cases, headings and mooring conditions. Overall, observed trends for occurrence of green water and standard deviation/maximum of relative wave height are successfully reproduced by KINEMA3. In agreement with model test results, it is predicted that green water occurs more frequently for a free floating vessel and for beam seas. Additional comparisons between KINEMA3 predictions using different FPSO panel models (low-order and high-order models) present negligible differences with respect to green water estimates. The results presented herein demonstrate the robustness of the tool towards the prediction of green water for variable wave headings and sea states, and highlight the capability of KINEMA3 to be employed as an engineering-like tool for fast and multiple estimates of green water in early design studies. This work is a part of the research project “Green Water and Wave Impact on FPSO” carried out for and in cooperation with PETROBRAS.

New Combined CFD and Model Testing Technique for Identification of Wave Impact Loads on a Semisubmersible

2017 ◽  
Author(s):  
Csaba Pakozdi ◽  
Anders Östman ◽  
Bjørn C. Abrahamsen ◽  
Ole D. Økland ◽  
Tone M. Vestbøstad ◽  
...  
Keyword(s):  
Time Series ◽  
Test Data ◽  
Model Test ◽  
Impact Force ◽  
Hydrodynamic Conditions ◽  
Wave Impact ◽  
Platform Motion ◽  
Matching Procedure ◽  
Wave Matching ◽  
The Impact

A procedure is presented describing how to estimate realistic loads using combined numerical and model test data. Measured platform motions are imposed on the structure during the CFD analysis. The combination of the wave matching procedure with the imposed measured platform motion gives a very good numerical reproduction of the observed extreme event. The numerical reproduction of model test events provide all necessary information on the hydrodynamic loads for further structure analysis. This represents an improvement in industry design applications. Imposing the measured motion from regular wave model test into CFD simulation is validated by comparison of relative wave height time series. This comparison shows a very good agreement between the measured and the simulated time series. Existing model test data from irregular model test and CFD generated numerical wave are compared. A wave matching procedure has been developed, which shows very promising results with respect to reproducing critical hydrodynamic conditions observed during the model tests. This paper presents a case study how CFD can be used to enhance model test data in an efficient way to provide the critical hydrodynamic conditions for structure analysis. Comparison of the measured free surface elevation of the calibrated waves with the time series of the numerical waves, as well as the measured and simulated relative wave probes time series and the slamming load time series show that the applied numerical wave events show similar physical conditions as those observed in the model test. The effect of the platform motion on the impact force is identified by comparison of the impact force time series of the simulation with and without platform motion against model test time series. The results demonstrate that the approach provides a clear improvement compared to numerical or model testing alone. The observed steep wave events are numerically reproduced in a simplified manner, instead of trying to reproduce measured events directly. This approach significantly reduces the computational time, as well as computational costs, to an industrially acceptable level. Traditional load estimation is not able to provide such reliable detailed local load history for structural design purpose at areas exposed to wave impacts. This new procedure, where CFD simulates realistic breaking waves with coupling to measured vessel motion, offers new possibilities for the design of structures subject to risk of wave impact loading.

Useful Indicators for Screening of Sea States for Wave Impacts on Fixed and Floating Platforms

2018 ◽  
Author(s):  
Tim Bunnik ◽  
Carl Trygve Stansberg ◽  
Csaba Pakozdi ◽  
Sebastien Fouques ◽  
Luke Somers
Keyword(s):  
Model Test ◽  
Screening Methods ◽  
Statistical Uncertainty ◽  
Probability Level ◽  
Critical Events ◽  
Wave Impact ◽  
Design Load ◽  
Design Loads ◽  
Design Value ◽  
The Impact

Design approaches for wave impact on marine structures in storm sea-states are being reconsidered due to events related to the safety of North Sea offshore structures, both fixed and floating (Valhall QP extended lifetime; COSL Innovator accident). There has been a strong research and tool development within the field during the last decade, both within model testing and numerical analysis, including CFD. However, there is still a lack of efficient methods and tools to properly analyze these phenomena and their probably of occurrence. One major aspect in this is to reduce the statistical uncertainties that are naturally arising in estimates of design loads related to extreme waves. In order to estimate the design loads it is common practice not to investigate all possible sea states (i.e. long-term analysis) but to investigate a few sea states and assume that the design value occurs at a prescribed probability level in the sea states with the same probability level (i.e. contour line approach). The estimate of the design value at that probability level is then based on results from a limited number of random realizations of these sea states. For linear or weakly nonlinear response types it is possible to estimate design loads accurately with a quite limited number of realizations. For strongly nonlinear/badly behaved problems however this is not possible due to the large variations in the tail of the distribution of the impact load, and many more realizations are required. This means that much more of these extreme, rare impact-related events should be collected to reduce the statistical uncertainty in the design load. This approach is restricted by time and costs and eventually one may have to accept an estimated design load with a large statistical uncertainty and account for the uncertainty with a higher safety margin. In this paper an improved methodology for estimating design loads related to extreme wave impacts will be presented. The methodology is based on screening many 3-hour realizations of the design sea states with simplified, fast but sufficiently accurate methods and to focus only on the potentially critical events with a model containing a more complete description of the physics. This can be either a model test or a non-linear impact simulation (i.e. CFD analysis). By doing this many more rare/critical events can be assessed, reducing the statistical uncertainty in the estimate of the design load. The main challenge is to find suitable screening methods, which may be different for different structures (fixed, floating, etc.). Several screening methods/wave impact indicators will be discussed, and their capability will be illustrated by analyzing existing model test data for fixed and floating structures, showing the correlation between indicator and actual impact events.

Shallow Water STL Mooring and Riser System

2009 ◽  
Author(s):  
Qinzheng Yang ◽  
Muthu Chezhian ◽  
Geir Olav Hovde
Keyword(s):  
Shallow Water ◽  
Wave Height ◽  
Model Test ◽  
Water Depth ◽  
Significant Wave Height ◽  
Mooring Line ◽  
Test Results ◽  
Sea State ◽  
Significant Wave ◽  
The Impact

A shallow water disconnectable STL turret mooring and riser system has been developed for water depth between 30 and 50 m. This technology is based on APL’s disconnectable STL (Submerged Turret Loading) and STP (Submerged Turret Production) technologies which had been widely applied for water depth between 70 m to 2600 m for FPSOs and LNG offshore terminals. The advantage of disconnectable system is that the mooring and riser system can be designed to a preferred sea state. When the sea state is higher than design sea state (like hurricane), the vessel can be disconnected and sail away. The shallow water STL system consists of STL buoy, mooring lines, riser system and landing pad. The interface with vessel is the same as traditional STL system. The mooring and riser system are connected to the vessel through STL buoy and can be pulled into vessel by using ship winch. Unlike traditional STP and STL buoys, the shallow STL buoy has a net weight and will stay on the landing pad when disconnected from vessel. The landing pad is designed to support the impact load from STL buoy and supply enough friction for the STL buoy to stay in position during 100-year storm. The mooring system design has taken the advantage of directionality of weather when close to the shore by using different mooring line length in different directions. Further an innovative Hold-Back-Wave riser configuration has been developed for shallow water system. The riser configuration has a larger flexibility compared to traditional wave configuration and has proved to be feasible for significant wave height at least 7 m when connected to the vessel and 10+ m when disconnected from the vessel. Model test for the disconnectable shallow water turret mooring and riser system had been performed in MARINTEK, Trondheim with a LNG re-gasification vessel model at 30 m water depth. For connected system, significant wave height Hs = 6 m and 8 m has been tested. The mooring and riser system perform well, as predicted. For disconnected system (when the buoy sitting on the landing pad), significant wave height Hs = 10 m has been tested. The STL buoy is sitting on the landing pad without significant movement and the riser system performs well. SIMO program has been used to calibrate the model test results with numerical simulations. By adjusting surge, sway, yaw damping and 2nd order wave drift force, the calibrated SIMO model agrees well with model test results and can be used for similar development.

scholarly journals Wave Front Steepness and Influence on Horizontal Deck Impact Loads

2020 ◽  
Vol 8 (5) ◽  
pp. 314
Author(s):  
Carl Trygve Stansberg
Keyword(s):  
Model Test ◽  
Incident Wave ◽  
Offshore Platforms ◽  
Rise Velocity ◽  
Wave Impact ◽  
Near Surface ◽  
Design Storm ◽  
Impact Phenomena ◽  
The Impact ◽  
Random Scatter

In design storm sea states, wave-in-deck forces need to be analysed for fixed and floating offshore platforms. Due to the complex physics of wave impact phenomena, numerical analyses should be complemented by model test data. With a large statistical variability, such experiments usually involve running many 3-h storm realisations. Efforts are being done to establish efficient procedures and still obtain improved statistical accuracy, by means of an initial simplified screening based on parameters derived from the incident wave record only. Here, we investigate the vertical rise velocity of the incident wave elevation at a fixed point in space, which indirectly measures both the local slope and the near-surface orbital velocity. A derived simple deck slamming model is also suggested, to support the check of the physical basis of the approach. Correlation against data from a GBS wave-in-deck model test is used for checking this model. The results show that, although there is a significant random scatter in the measured impact forces, especially in the local slamming forces but also in the global forces, there is a correlation to the rise velocity. Comparisons to the simple load model also show promising results when seen on background of the complex physics and random scatter of the impact problem.

Wave-in-Deck Impact: Comparing CFD, Simple Methods, and Model Tests

2010 ◽  
Author(s):  
Timothy E. Kendon ◽  
Csaba Pakozdi ◽  
Rolf J. Baarholm ◽  
Petter A. Berthelsen ◽  
Carl-Trygve Stansberg ◽  
...  
Keyword(s):  
Model Test ◽  
Cfd Analysis ◽  
Impact Event ◽  
Regular Waves ◽  
Wave Impact ◽  
Test Setup ◽  
Vertical Loading ◽  
Design Basis ◽  
Simple Potential ◽  
Impact Events

The slamming of waves on the lower deck of large volume offshore platforms has received increased attention over recent years. For many existing platforms, the problem of insufficient air-gap clearance has become more acute of late due to more extreme weather conditions than was used in their original design basis and/or due to issues such as subsidence of gravity based structures. To investigate this problem, MARINTEK’s Wave Impact Loads JIP has, in one of its sub-tasks, focused towards an idealised model test setup of a rectangular block in regular waves. The block is fixed at a distance h above the calm water line. Both 2D and 3D model test experiments of the block in regular waves have been carried out in Phase 1 of the JIP (2008). This paper considers results from the 2D model test setup, and compares the measured vertical loading on the deck against two simple potential theory based methods (Baarholm, OMAE 2009-79560) and against results from a CFD code (STAR-CCM+). The results demonstrate that a second impact event closely following a first impact event can have a much flatter free-surface profile (and stronger water entry force) as a result of its interaction with the (deck) diffracted wave from the first impact event. The importance of resolving this diffracted wave in the CFD analysis is demonstrated. The paper concludes that for isolated impact events the simple potential flow based models, which do not consider the influence of one impact event on another, are adequate to predict the vertical loading on the deck. However, from a design basis criteria, if there is the strong likelihood of steep wave groupings resulting in closely following wave-in-deck impact events, then the presented simple methods may be non-conservative, and a CFD (Computational Fluid Dynamics) analysis or model test may be advisable to predict the vertical wave-in-deck loading. However the horizontal loading was significantly under-predicted in the CFD analysis compared to the measurements, so more work still needs to be done in this respect.

Sign in / Sign up

Export Citation Format

Share Document