OBJECTIVE: Greenwater damage to floating structures results from high pressures and dynamic loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is often associated with use of floating structures in operations or locales for which they were not initially designed. Modification of existing floating structures to prevent greenwater damage is often difficult to achieve, and prevention is generally approached through localized reinforcements or barriers added to the structure and/or modified operating procedures. The objective of this research is to focus on prediction of greenwater through laboratory measurement and numerical simulation, and develop design guidance that will allow designers to avoid or minimize greenwater on new floating structures through design. This research is a continued effort after the successful formulation of greenwater over a 2D platform through laboratory measurement, and a continuation on the development of a 3D computational fluid dynamics (CFD) code on the greenwater simulation. The prior study has shown that the tradition prediction method using the dam breaking model results in significant discrepancy between the model and the laboratory measurements. Since the more realistic 3D prediction model is not yet established, the continuation of the research is critical for the prediction of greenwater and subsequently its mitigation. The project will consider 3D structure geometries such as TLP's, spars, and ship-shaped FPSO's.APPROACH: The interactions between extreme waves and floating structures will be studied both numerically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to mitigate greenwater damage.
Prediction of greenwater over a simplified 2D platform via laboratory experiments applying optic and imaging techniques has been successfully achieved in the prior efforts. Significant progress on the numerical simulation of greenwater has also been achieved with the treatment of the complex surface being overcome in the prior efforts. However, numerical simulation of greenwater, due to its turbulent nature of wave breaking and water splashing, is still a great challenge that needs to be accomplished.
The research is to be carried out by the following procedure: (1) To extend the detailed laboratory measurements from 2D to 3D so the model mimics a more realistic platform, and to obtain a model/formula for 3D greenwater prediction. (2) To continue and complete the computer code for greenwater simulation, and to calibrate and validate the CFD code through comparisons with the 2D and 3D laboratory measurements. (3) To simulate the greenwater effect on the full-scale platform numerically.
DEPLOYMENT OF RESULTS: Design guidance will be synthesized from descriptions of the incident waves, flows and resulting pressures on the structure, and vessel motions. This guidance will provide designers a better understanding of this complex phenomenon, and provide insight and a basis for designing to avoid greenwater damage.
ANTICIPATED NUMBER OF PHASES REMAINING: 2
PROJECT PLAN FOR PHASE 1 (2005 - 2006):
Scope and Plan: The main tasks of this phase is to conduct detailed 3D laboratory measurements, including the water elevation and velocity field, of greenwater, and to complete the development of 2D CFD code for the simulation of greenwater, and to validate the code with the 2D laboratory measurement obtain in the prior research. Scaled down 3D physical models will be constructed and setup in a wave tank in the laboratory. The 2D measurement techniques, called particle image velocimetry (PIV) and bubble image velocimetry (BIV), that were successfully employed in the prior research using lasers and imaging systems will be modified for 3D measurements. The techniques basically track tiny artificial seeding particles and wave breaking bubbles illuminated by a thin laser light sheet. Images of the particles and bubbles were than processed and correlated to obtain the velocity in the entire flow field captured in the images. Typically thousands of velocity vectors were determined in a single realization. Preliminary 3D results will be obtained in the laboratory. In addition, the chimera Reynolds-Averaged Navier-Stokes (RANS) computer code developed in previous investigations of wave runup on offshore structures will be generalized for time-domain simulation of greenwater effects on a two-dimensional platform. Calculations will be performed for two-phase flow including both water and air in the computational domain. A level-set function will be used for accurate tracking of the air-water interfaces. The 2D simulations of free surface greenwater flow with impact and splashing will be completed and validated. The 2D code will be extended to 3D during this phase, but not expected to complete.
Anticipated Results: We expect to be able to predict the 2D greenwater flow both experimentally and numerically. The chimera RANS code will provide instantaneous air-water interface and detailed velocity and pressure distributions around the 2D platform. After the validation of the numerical model, we will test the model on large scale cases and obtain the pressure distribution caused by the effect of greenwater (which cannot be obtained from laboratory measurements). 2D prediction formula, including water elevation, velocity, and pressure field will be established. The results will be disseminated through interim progress reports and the end-of-phase final report.
PROJECT PLAN FOR PHASE 2 (2006 - 2007):
Scope and Plan: The main tasks of this phase is to conduct and finish the detailed 3D laboratory measurement of greenwater, to complete the 3D computer code for the simulation of greenwater, and to validate the code with the 3D laboratory measurements. The measurements will be done by slicing the flow fields to cover the cross tank velocity distribution and therefore obtaining the complete 3D flow field as well as the surface profile. The chimera Reynolds-Averaged Navier-Stokes computer program will be generalized during Phase 2 for time-domain simulation of greenwater effects on more realistic three-dimensional platforms. The CFD code will be used first for the simulation of the laboratory cases to ensure that the code is able to handle the complex 3D turbulent flow of greenwater. The validated 3D code will then be employed for the simulation greenwater effects on a full-scale platform in the ocean.
Anticipated Results: We expect to be able to predict the 3D greenwater flow both experimentally and numerically. After the validation of the numerical model, we will test the model on full-scale cases and obtain the pressure distribution caused by the effect of greenwater. 3D prediction formula, including water elevation, velocity, and pressure field will be established. The results will be disseminated through interim progress reports and the end-of-phase final report.
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI(s): Kuang-An Chang, Hamn-Ching Chen, and Richard S. Mercier
Date: December, 2005
Project Title: Greenwater Prediction and Mitigation: A Combined Experimental and Numerical Approach
Industry Consortia Project TEES Number: 32518-2318A
PI: Kuang-An Chang, Hamm-Ching Chen, Richard Mercier
Estimated Completion Date: 8/31/2007
Project Description: Greenwater damage to floating structures results from high pressures and dynamic loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is often associated with use of floating structures in operations or locales for which they were not initially designed. Modification of existing floating structures to prevent greenwater damage is often difficult to achieve, and prevention is generally approached through localized reinforcements or barriers added to the structure and/or modified operating procedures. The objective of this research is to focus on prediction of greenwater through laboratory measurement and numerical simulation, and develop design guidance that will allow designers to avoid or minimize greenwater on new floating structures through design. This research is a continued effort after the successful formulation of greenwater over a 2D platform through laboratory measurement, and a continuation on the development of a 3D numerical code on the greenwater simulation. The prior study has shown that the tradition prediction method using the dam breaking model results in significant discrepancy between the model and the laboratory measurements. Since the more realistic 3D prediction model is not yet established, the continuation of the research is critical for the prediction of greenwater and subsequently its mitigation. The project will consider 3D structure geometries such as TLP's, spars, and ship-shaped FPSO's.
Progress: The progress stated herein includes only the work after the completion of the greenwater velocity measurements in the laboratory and the final report on the prediction of greenwater velocity field in September 2005. Laboratory experiments were continued for the void fraction measurements using a fiber optic probe developed by one of the PIs (Chang). The purpose is to improve the prediction of the greenwater and to be used later in the validation of the numerical models. Knowing the velocity field along is insufficient in describing the greenwater because, through the laboratory observation, the up-rushing violent greenwater contains a significant amount of bubbles. Since the momentum of greenwater is linearly proportional to the density of the fluid, which is a mixture of water and air, it is necessary to know the distribution of the fluid density, expressed as the void fraction. The void fraction at 3 vertical cross sections above the deck of the model structure and one cross section along the front wall of the model structure was measured. Data at approximately 10 locations were taken at each cross section on the deck. Experiments were repeated 10 to 20 times at each measurement location to result in the ensemble-averaged mean. Preliminary data reduction shows that the void fraction is low and between 0.1 and 0.3 near the surface of the deck, whereas the void fraction gradually increases to between 0.7 and 0.9 at the location close to the greenwater water free surface. Note that the void fraction equals zero means 100% water, and unity means 100% air.
Time-domain simulations of greenwater around offshore structures were performed using a Reynolds-Averaged Navier-Stokes (RANS) numerical method in conjunction with a chimera domain decomposition approach. In order to simulate the complex greenwater effects on the offshore structure and the wave impact on the platform deck, a level-set method was developed to provide accurate resolution of the air-water interface. The level-set method was validated first for several benchmark test cases including the Zalesak’s rotating disk problem and the stretching/shrinking of circular fluid elements. The new interface-capturing method was then employed in conjunction with the chimera RANS method for time-domain simulation of complex free surface problems including dam-breaking, free jet, tank sloshing, and greenwater on offshore platforms. The simulation results clearly demonstrate the flexibility and accuracy of the level-set method for accurate prediction of violent free surface motions including wave breaking, deck slamming, and greenwater on platform deck.
Reports & Publications:
Chang, K.-A., Chen, H.C., Ryu, Y., Yu, K., & Mercier, R. (2005) Mitigating Greenwater Damage through Design. Project final report, OTRC, in press.
Chen, H.C., Yu, K. and Chen, S.Y. (2004) “Simulation of wave runup around offshore structures by a chimera domain decomposition approach,” Civil Engineering in the Oceans VI Conference, Baltimore, Maryland, October 20-22, 2004.
Ryu, Y. & Chang, K.-A. (2005) “Breaking wave impinging and greenwater on a two-dimensional offshore structure.” The 15th International Offshore and Polar Engineering Conference, Seoul, Korea, June 19-24, 2005, pp. 660-665.
Ryu, Y., Chang, K.-A., & Lim, H.J. (2005) “Use of bubble image velocimetry for measurement of plunging wave impinging on structure and associated greenwater.” Measurement Science and Technology, 16, 1945-1953.
Date: June 2005
Project Title: Mitigating Greenwater Damage Through Design
MMS Project: 441 TO Numbers: 85385/35991
PI: Kuang-An Chang, Hamn-Ching Chen, Richard Mercier
COTR: A. Konczvald
Estimated Completion Date: 8/31/2005
Project Description: Greenwater damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is the result of unique combinations of vessel motions and incident wave conditions, e.g. roll, pitch and yaw of a turret-moored FPSO in hurricane seas. The interactions between extreme waves and floating structures will be studied both analytically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to avoid or minimize greenwater through design (e.g., hull shape, protective appurtenances, and strategic reinforcement). In addition to ship-shaped FPSO's, the project will also consider other structure geometries such as spars or TLP's.
Progress: A fixed 2D rectangular structure based on the dimensions of a typical TLP (1:168 scaled down) was tested in the laboratory wave tank using extreme waves breaking and impinging on the structure with greenwater. Velocity fields in the vicinity of the structure, including greenwater, were measured over the entire impinging process using the particle image velocimetry (PIV) technique and a newly developed bubble image velocimetry (BIV) technique that directly tracks the air bubbles and measures velocity in gas-liquid flows. The validation for the accuracy of BIV has been successfully completed with excellent agreement. Detailed PIV/BIV measurements for both plunging and spilling waves impinging on the structure have been compl eted.
To compute greenwater loads, a linear dam-breaking solution is often used to simulate greenwater rushing over a deck. However, our research indicates there are significant discrepancies between the measured greenwater velocities and those simulated by linear dam-breaking solution. The dam-breaking solution and experimental results are compared in Figure 1. The depth averaged velocities along the deck are shown for three different times after the wave inundated the deck The dam-breaking solution over predicts the velocity in the initial stage of greenwater inundation (before t1), matches the velocity at t1,and under predicts the velocity after the initial stage t2 and t3). It also gives an unrealistic velocity distribution along the deck. A study to parameterize greenwater motion based on the measured velocity maps is currently underway, and is expected to result in a more realistic model for greenwater predictions.
In addition to the experimental approach, an interface-preserving level set numerical method was incorporated into the Reynolds-Averaged Navier-Stokes (RANS) method for simulation of greenwater effect. In the level set method, free surface flows are modeled as immiscible air-water two-phase flows and the free surface itself is represented by the zero level set function. In order to maintain a uniform interface thickness between the gas and liquid phases, a reinitialization (or redistancing) algorithm is implemented to ensure that mass conservation is satisfied throughout the entire simulation. Calculations were performed for several two-dimensional greenwater problems including dam-breaking, free jets, and the impingement of dam-breaking flow on a fixed structure. The method is currently being extended for the simulation of nonlinear waves generated by a numerical wave maker.
Reports & Publications:
Chen, H.C., Yu, K. and Chen, S.Y. (2004) “Simulation of wave runup around offshore structures by a chimera domain decomposition approach,” Civil Engineering in the Oceans VI Conference, Baltimore, Maryland, October 20-22, 2004Ryu, Y. & Chang, K.-A. (2005) “Breaking wave impinging and greenwater on a two-dimensional offshore structure.” The 15th International Offshore and Polar Engineering Conference, Seoul, Korea, June 19-24, 2005, to appear.
Ryu, Y., Chang, K.-A., & Lim, H.J. (2005) “Use of bubble image velocimetry for measurement of plunging wave impinging on structure and associated greenwater.” Measurement of Science and Technology, tentatively accepted.
Date: December 2004
Project Title: Mitigating Greenwater Damage Through Design
MMS Project: 441 TO Numbers: 85385/35991
PI: Kuang-An Chang, Hamn-Ching Chen, Richard Mercier
COTR: A. Konczvald
Estimated Completion Date: 8/31/2005
Project Description: Greenwater damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is the result of unique combinations of vessel motions and incident wave conditions, e.g. roll, pitch and yaw of a turret-moored FPSO in hurricane seas. The interactions between extreme waves and floating structures will be studied both analytically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to avoid or minimize greenwater through design (e.g., hull shape, protective appurtenances, and strategic reinforcement). In addition to ship-shaped FPSO's, the project will also consider other structure geometries such as spars or TLP's.
Progress: A fixed 2D rectangular structure based on the dimensions of a typical TLP (1:168 scale) was tested in the laboratory flume using extreme waves breaking and impinging on the structure with greenwater. Velocity fields in the vicinity of the structure, including greenwater, were measured over the entire impinging process using the traditional particle image velocimetry (PIV) technique and a newly developed PIV method that directly tracks the air bubbles. Detailed measurements for plunging wave impinging on the structure with a large air pocket were completed.
These results were compared with the 1-D (depth averaged with only the horizontal velocity) linear solution of dam-breaking theory, which has been widely used to simulate greenwater effect. Preliminary results show that the theory seems to over-predict the velocity in the initial stage of greenwater due to wave impinging and overtopping, and under-predict the velocity after the initial stage. By examining the PIV velocity maps, the discrepancy may be explained by the vertical component of velocity on the deck that is significant at the initial stage but quickly dies down afterward. Future work will be to parameterize the greenwater effect from the PIV velocity maps and modify the existing prediction model. The void fraction of the greenwater will also be measured.
A Reynolds-Averaged Navier-Stokes (RANS) numerical method was developed in conjunction with a chimera domain decomposition approach to simulate greenwater effects and impact loads on floating platforms. Time-domain simulations of wave runup were performed for fixed platforms subjected to regular waves. The predicted wave elevation and velocity field are in good agreement with the corresponding PIV data for 2D rectangular platform.
The method has now been extended successfully to simulate wave runup around three-dimensional offshore structures.
Reports & Publications:
Ryu, Y. & Chang, K.-A., “Extreme waves impinging on an offshore structure.” Civil Engineering in the Oceans VI Conference, Baltimore, Maryland, October 20-22, 2004.
Chen, H.C., Yu, K. and Chen, S.Y., “Simulation of wave runup around offshore structures by a chimera domain decomposition approach,” Civil Engineering in the Oceans VI Conference, Baltimore, Maryland, October 20-22, 2004.
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Date: June 2004
Project Name: Mitigating Greenwater Damage Through Design
Project Number: 441 Task Order: 85385
Principal Investigators: Kuang-An Chang, Hamn-Ching Chen, and Richard S. Mercier
Estimated Completion Date: August 2005
Project Description:
Greenwater damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is the result of unique combinations of vessel motions and incident wave conditions, e.g. roll, pitch and yaw of a turret-moored FPSO in hurricane seas. The interactions between extreme waves and floating structures will be studied both analytically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to avoid or minimize greenwater through design (e.g., hull shape, protective appurtenances, and strategic reinforcement). In addition to ship-shaped FPSO's, the project will also consider other structure geometries such as spars or TLP's.
Progress:
A fixed 2D rectangular structure based on the dimensions of a typical TLP (1:168 scaled down) was tested in the laboratory flume using extreme waves breaking and impinging on the structure with greenwater. Three different types of breaking waves were generated using a wave focusing technique. Velocity fields in the vicinity of the structure, including greenwater, were measured using the particle image velocimetry (PIV) technique over the entire impinging process. The tests were repeated 20 times and the instantaneous and phase-averaged quantities were obtained and analyzed. Detailed measurements have been finished for wave impinging with a large air pocket (plunging type breaking). It is found that the maximum horizontal velocity before reaching the structure is 1.5C, with C being the phase speed of the breaking wave. After the wave hitting the structure and overtopping, the maximum velocity above the structure is reduced rapidly over a short distance (one order of magnitude smaller than the wavelength). The measurements will be used to validate (and modify, if needed) the damn breaking model commonly used by researchers on greenwater studies. Further detailed measurements for the other two types of extreme waves are planned.
A Reynolds-Averaged Navier-Stokes (RANS) numerical method has been developed in conjunction with a chimera domain decomposition approach for simulation of the greenwater effects and impact loads on floating platforms. Time-domain simulations of wave runup were performed for fixed platforms subjected to regular waves. In these simulations, the boundary-fitted platform grid remains fixed, while the free surface and basin grids were updated every time step to follow the movement of nonlinear free surface waves. This enables us to accurately resolve the body boundary layer and wakes without undesirable grid distortions even in the presence of greenwater on the deck. New absorbing beach techniques were developed for both the wavemaker and downstream boundaries to facilitate the long duration simulations without unphysical wave reflections from computational domain boundary. The predicted wave elevation and velocity field are in good agreement with the corresponding PIV data for 2D rectangular platform. The method is currently being extended for simulation of wave runup around three-dimensional offshore structures.
Reports & Publications:
Ryu, Y. & Chang, K.-A., “Extreme waves impinging on an offshore structure.” Civil Engineering in the Oceans VI Conference, Baltimore, Maryland, October 20-22, 2004.
Chen, H.C., Yu, K. and Chen, S.Y., “Simulation of wave runup around offshore structures by a chimera domain decomposition approach,” Civil Engineering in the Oceans VI Conference, Baltimore, Maryland, October 20-22, 2004.
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Date: December 2003
Project Name: Mitigating Greenwater Damage Through Design
TEES Project Number: 32558-58880J MMS Task Order: 85385 MMS Project Number: 441
Principal Investigators: Kuang-An Chang, Hamn-Ching Chen, and Richard S. Mercier
Estimated Completion Date: August 2005
Project Description:
Greenwater damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is the result of unique combinations of vessel motions and incident wave conditions, e.g. roll, pitch & yaw of a turret-moored FPSO in hurricane seas. The interactions between extreme waves and floating structures are being studied both analytically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to avoid or minimize greenwater through design (e.g., hull shape, protective appurtenances, and strategic reinforcement). In addition to ship-shaped FPSO's, the project will also consider other structure geometries such as spars or TLP's.
Progress:
A fixed 2D rectangular structure based on the dimensions of a typical TLP (1:168 scaled down) was tested in the laboratory flume using regular waves with no greenwater and breaking. Velocity fields in the vicinity of the structure were measured using the particle image velocimetry (PIV) technique for 16 phases per each wave period for different wave periods and different wave amplitudes. Both instantaneous and phase-averaged quantities were obtained and analyzed. Further tests were planned with extreme waves breaking and impinging on the structure. Three different types of breaking waves were identified and successfully generated. The velocity field and pressure of the breaking waves on the structure will be measured using PIV and multiple pressure sensors. The velocity and acceleration of the wave front will also be measured using the newly-invented fiber optic reflectometer (FOR) technique.
A Reynolds-Averaged Navier-Stokes (RANS) numerical method has been developed in conjunction with a chimera domain decomposition approach for time-domain simulation of the greenwater effects and impact loads on floating platforms under extreme wave conditions. Time-domain simulations of wave runup were performed for a fixed platform subjected to regular waves. The boundary-fitted platform grid block (including the platform deck) remained fixed during the simulation, while the free surface and basin grids were adjusted at every time step to follow the movement of nonlinear free surface waves. This allowed us to accurately resolve the body boundary layers without undesirable grid distortions even in the presence of greenwater on the deck. For long-duration simulations extending over many wave periods, reflected waves from the structures must be completely absorbed before they reach the wavemaker. Two new absorbing beach techniques were developed to facilitate the wave-structure coupling simulations for long durations with the same incident waves at the wavemaker boundary. The simulation results are currently being compared to the corresponding PIV data to provide a thorough validation of the present method.
Reports & Publications: None
Date: June, 2003
Project Name: Mitigating Greenwater Damage Through Design
Project Number: 32558-58880J Task Order: 85385
Principal Investigators: Kuang-An Chang, Hamn-Ching Chen, and Richard S. Mercier
Estimated Completion Date: 8/31/2005
Project Description: Greenwater damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is the result of unique combinations of vessel motions and incident wave conditions, e.g. roll, pitch & yaw of a turret-moored FPSO in hurricane seas. The interactions between extreme waves and floating structures will be studied both analytically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to avoid or minimize greenwater through design (e.g., hull shape, protective appurtenances, and strategic reinforcement). In addition to ship-shaped FPSO's, the project will also consider other structure geometries such as spars or TLP's.
Progress: A physical model with a simplified 2D rectangular geometry based on the dimensions of a typical TLP was built (1:168 scaled down). The structure is fixed and tested in the laboratory flume using regular waves with no greenwater. Velocity fields in the vicinity of the structure were measured using the particle image velocimetry (PIV) technique for 16 phases per each wave period. Thousands of velocity vectors were obtained in each PIV measurement. Preliminary results show that the effects of turbulence and vorticity are relatively minor during the runup phase except in the region near the lower corner of the structure. On the other hand, more significant effects of turbulence and vorticity were observed during the rundown phase.
A Reynolds-Averaged Navier-Stokes (RANS) numerical method is currently developed in conjunction with a chimera domain decomposition approach for time-domain simulation of the greenwater effects and impact loads on floating platforms under extreme wave conditions. In the first phase of this project, numerical simulations were performed for a fixed platform subjects to regular waves. For simplicity, the boundary-fitted platform grid block (including the platform deck) remains fixed during the entire simulation, while the free surface and basin grids were adjusted to follow the movement of nonlinear free surface waves. This enables us to accurately resolve the body boundary layers without undesirable grid distortions even in the presence of greenwater on the deck. Detailed comparisons of the free surface wave profiles, mean velocity and pressure fields, and turbulence quantities will be made with the corresponding PIV data to provide a thorough validation of the present simulation method.
OTRC PROJECT STATUS REPORT
Date: November 15, 2002
Project Name: Mitigating Greenwater Damage Through Design
Project Number: 32518-1510H CE / 32558-58880J CE Task Order: 85385
Principal Investigators: Kuang-An Chang, Hamn-Ching Chen, and Richard S. Mercier
Estimated Completion Date: 8/31/2005
Project Description: Greenwater damage to floating structures results from high pressures and loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. Greenwater damage is the result of unique combinations of vessel motions and incident wave conditions, e.g. roll, pitch & yaw of a turret-moored FPSO in hurricane seas. The interactions between extreme waves and floating structures will be studied both analytically and experimentally to better understand the complex flows around and over a structure, and the resulting impact loads and pressures that can cause damage. These results will be combined with capabilities to predict vessel motions in order to develop design guidance that will allow designers to avoid or minimize greenwater through design (e.g., hull shape, protective appurtenances, strategic reinforcement). In addition to ship-shaped FPSO's, the project will also consider other structure geometries such as spars or TLP's.
Progress: A Reynolds-Averaged Navier-Stokes (RANS) numerical method was developed recently for time-domain simulation of large amplitude ship motions including capsizing. A general chimera domain decomposition approach was employed to facilitate the simulation of partial ship hull submergence and greenwater on the ship deck. The method successfully predicted the capsizing of a two-dimensional rectangular barge under large amplitude incident waves. In the present project, the chimera RANS method will be further extended for simulation of the greenwater effects and impact loads on floating structures under extreme wave conditions. In order to facilitate the simulation of large waves flowing over stationary or floating offshore structures, a more general free-surface tracking technique is currently being developed to provide accurate description of the wave kinematics. The new free-surface tracking method will be incorporated into the chimera RANS method to provide detailed resolution of the hull pressure and shear stresses acting on the floating structures or ship-shaped FPSO’s.
A physical model (1:168 scaled down) with a simplified 2D geometry based on the dimensions of a typical TLP are under construction for use in the laboratory flume test. A particle image velocimetry (PIV) system will be used to measure the flow velocity distribution in the vicinity of the stationary model. The PIV technique applies optical and imaging technologies to allow non-intrusive full-field velocity mapping. The PIV system has been setup in the laboratory and successfully tested using an existing rectangular barge.
Reports & Publications: None
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