LOADS DUE TO EXTREME WAVE CRESTS
OBJECTIVE: In approximately one year from September 2004 to September 2005, three Category 5 hurricanes (Ivan, Katrina, Rita) hit the Gulf of Mexico. Well over 80% of the 4,000 oil and gas production platforms in the Gulf were directly impacted by the hurricanes. The hurricanes destroyed or caused extensive damage to 190 platforms. In most cases the platform damage was caused by greenwater wave loading on the deck. 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. In situations where one must consider the possibility of wave overtopping and greenwater rushing onto the deck, the mechanics of wave loading become very complex and are poorly understood. One of the major sources of uncertainty is the velocity field of the greenwater flow itself.The objective of this research is to develop a robust procedure to estimate local and global greenwater loads on structures due to extreme wave crests. Through the combined efforts of laboratory measurements and numerical simulation, the result will allow designers to avoid or minimize the impact of greenwater on new floating structures through design, and help the industry and regulators to develop associated design guidance. 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 traditional prediction method often used in design, i.e., 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 structure/vessel motions in order to develop design guidance that will allow designers to mitigate greenwater damage.Prediction of greenwater over a simplified 2D platform through laboratory experiments applying newly developed optic and imaging techniques has been successfully achieved in the prior efforts by the PIs. 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, 3D velocity measurement and numerical simulation of greenwater, due to its turbulent nature with wave breaking and water splashing, are still a great challenge that needs to be accomplished.
The research is to be carried out by the following plan: (1) Develop a measurement technique capable of measuring 3D velocity distribution. (2) Extend the detailed laboratory measurements from 2D to 3D so the model mimics a more realistic platform. (3) Extend the model study from a fixed platform to a more realistic floating platform. (4) Complete the computer code for greenwater simulation, and to calibrate and validate the CFD code through comparisons with the 2D and 3D laboratory measurements. (5) Simulate the greenwater effect on the full-scale platforms numerically.
DEPLOYMENT OF RESULTS: Prediction procedure of greenwater loads on a structure will be synthesized from descriptions of the incident waves, flows and resulting velocity and pressure on the structure, and vessel motions. This procedure will provide designers a better understanding of this complex phenomenon, and provide insight and a basis for designers to avoid or minimize greenwater damages.
PROJECT ORGANIZATION & TIMING: The project will be accomplished in 2 phases. Each phase will last for one year. The first PI (Dr. Chang) with one graduate student will be responsible for the development of measurement techniques and laboratory work, while the second PI (Dr. Chen) with another graduate student will work on the numerical code development and simulations. The third PI (Dr. Mercier) will bridge the proposed research with the need of MMS and the industry and provide inputs.
ANTICIPATED NUMBER OF PHASES: 2
PROJECT PLAN FOR PHASE 1 (2006 -2007):Scope of Work: The main tasks of this phase are as follows:
1. Develop capability to measure greenwater kinematics for a 3D stationary model in flume
2. Simulate experiments in (1) using CFD model to validate agreement
3. Develop capability to measure greenwater kinematics for a 3D floating model in flume
4. Simulate experiments in (3) using CFD model to validate agreementFollowing the previous successful development of the 2D image-based measurement techniques (particle image velocimetry or PIV, and bubble image velocimetry or BIV), the PIs will extend the techniques to 3D. The techniques basically track tiny seeding particles, wave breaking bubbles, and air-water interfaces. Images of the particles, bubbles, and interfaces are than processed and correlated to obtain the velocity in the entire flow field captured in the images. Typically thousands of velocity vectors are determined in a single realization. Subsequently, the PIs will construct a scaled down fixed 3D model in the laboratory and conduct detailed measurements on the model, including the water elevation and velocity field of greenwater. The PIs will also complete the development of the CFD code for the simulation of greenwater, and to validate the code with the 2D and 3D laboratory measurements. The chimera Reynolds-Averaged Navier-Stokes (RANS) computer code developed in previous investigations for wave runup and wave impingement will be generalized for time-domain simulation of greenwater effects on 2D and 3D platforms. A level-set function will be incorporated into the chimera RANS code to facilitate accurate tracking of the air-water interfaces. Calculations will be performed for two-phase flow including both water and air in the computational domain. Validation will be done by comparing with the laboratory measurements.
Anticipated Results & Deliverable: We expect to complete the development on both the laboratory measurement technique and numerical simulation for 3D greenwater measurement and simulation. We expect to be able to predict the 3D greenwater flow on a fixed platform both experimentally and numerically. We also expect to establish the capability and obtain preliminary results for a 3D floating platform both experimentally and numerically. The results will be disseminated through interim progress reports and the end-of-phase final report.
Proposed Period of Performance for Phase 1: September 1, 2006 - November 30, 2007.
PROJECT PLAN FOR PHASE 2 (2007 - 2008):Scope of Work: Activities in this phase are predicated on having successfully established the capability to measure and compute greenwater effects on a floating body in the flume. With that milestone reached, the main tasks for Phase 2 are as follows:
5. Perform experiments in the OTRC wave basin to measure interactions of extreme wave crests with moored FPSO's.
a. Measurements (video, wave probes, BIV)
b. Study variability and repeatability
6. Simulate wave basin experiments using CFD model
7. Develop robust procedure for estimating global and local wave impact loadsThe main focus of this phase is to conduct and finish the detailed 3D laboratory measurement of greenwater in the OTRC wave basin, and to validate the 3D computer simulation using the laboratory measurements. The imaging techniques developed in Phase 1 will be implemented in the OTRC basin to allow the measurements of structure movement and the greenwater velocities, in addition to the wave conditions. A range of wave parameters will be tested and the repeatability of the tests will be examined and ensured. 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 of greenwater effects on a full-scale platform in the ocean.
Anticipated Results & Deliverables: We expect to be able to predict the 3D greenwater flow both experimentally and numerically on a fixed or floating structure. The numerical simulation on full-scale cases will result in the pressure distribution caused by the effect of greenwater, in addition to other kinematics such as velocity distribution and water elevation. A robust procedure for estimating global and local impact loads due to extreme wave crests will be established through the measurement and simulation. The results will be disseminated through interim progress reports and the end-of-phase final report.
Proposed Period of Performance for Phase 2: September 1, 2007 - November 30, 2008 (work on Phase 2 will begin while Phase 1 project report is under preparation).
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI(s): Kuang-An Chang, Hamn-Ching Chen, and Richard Mercier
DATE: June, 2007Project Title: Loads on Structures due to Extreme Wave Crests
MMS Project: 571 TO Number: 39807
Project PI: Kuang-An Chang, H. C. Chen and Richard Mercier
COTR: M. Else
Estimated Completion Date: 11/30/2007
Project Description:
In approximately one year from September 2004 to September 2005, three Category 5 hurricanes (Ivan, Katrina, Rita) hit the Gulf of Mexico. Well over 80% of the 4,000 oil and gas production platforms in the Gulf were directly impacted by the hurricanes. The hurricanes destroyed or caused extensive damage to 190 platforms. In most cases the platform damage was caused by greenwater wave loading on the deck. 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. In situations where one must consider the possibility of wave overtopping and greenwater rushing onto the deck, the mechanics of wave loading become very complex and are poorly understood. One of the major sources of uncertainty is the velocity field of the greenwater flow itself.The objective of this research is to develop a robust procedure to estimate local and global greenwater loads on structures due to extreme wave crests. Through the combined efforts of laboratory measurements and numerical simulation, the result will allow designers to avoid or minimize the impact of greenwater on new floating structures through design, and help the industry and regulators to develop associated design guidance. 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 traditional prediction method often used in design, i.e., 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 TLPs, spars, and ship-shaped FPSOs.
Progress:
In the experimental approach, the in-house developed bubble image velocimetry (BIV) method that is to be used in the present study to measure the velocity field in the highly turbulent, multi-phased greenwater flow has been further refined. The method is based on correlating the acquired images in the laboratory for greenwater velocity determination. The in-house developed image processing software used in BIV has been modified and improved for faster velocity processing. Part of the original code has been converted to Fortran from MATLAB. The BIV post-processing software, also developed in house, has been improved to deal with the highly chaotic flow for better detecting stray velocity vectors in BIV results and replacing the vectors using the Kriging method. The improved software is ready to be employed. In addition, the void fraction measurement using fiber a optic reflectometer (FOR) on a 2D structure has been analyzed to obtain the void fraction distribution, overtopping flow rate, overtopping volume, and momentum flux of green water on the deck for force prediction.In the numerical approach, we have developed a Finite-Analytic Navier-Stokes (FANS) method in conjunction with an interface-preserving level-set method for the simulation of greenwater on two- and three-dimensional offshore structures. In numerical wave tank simulations, open boundaries enclosing the fluid domain are artificial and essentially arbitrary. In order to prevent unphysical wave reflections from the computational domain boundaries, a damping function was implemented on the downstream and sidewall boundaries to reduce the wave amplitude in the absorbing beach zone. For long duration simulations, it is also necessary to prevent the reflected and diffracted waves from reaching the wavemaker boundary. In this study, concurrent computations were performed for the incident wave field (without structure) and the total wave field (with structure) simultaneously. A second damping function was then implemented to suppress the differences between the total and the incident wave fields in front of the wavemaker. This enables us to absorb the true wave reflection and diffraction from the offshore structures.
During this reporting period, considerable effort has also been devoted to the generation of random waves and breaking waves. A piston-type wavemaker was used successfully for the generation of random wave fields around a vertical circular cylinder in a numerical wave tank. Breaking waves were also successfully generated in the numerical wave tank by the superposition of a long wave and a group of shorter waves. Time-domain simulations are currently being performed for the impingement of the breaking waves on a two-dimensional rectangular platform. The simulation results will be compared with the experimental data to provide a detailed validation of the level-set FANS method. The method will then be employed for the simulation of breaking waves and greenwater effects on a three-dimensional platform.
Reports and Publications:
1. Yu, K. and Chen, H.C. (2007) “Chimera RANS simulation of slamming forces and wave overtopping around offshore structures.” 17th International Offshore and Polar Engineering Conference, July 1-6, Lisbon, Portugal, to appear.
2. Chang, K.-A., Mercier, R., and Ryu, Y. (2007) “Validation of applicability of dam-break flow to green water prediction.” 17th International Offshore and Polar Engineering Conference, July 1-6, Lisbon, Portugal, to appear.
3. Ryu, Y., Chang, K.-A., and Mercier, R. (2007) “Runup and green water velocities due to breaking wave impinging and overtopping.” Experiments in Fluids, to appear.
4. Ryu, Y., Chang, K.-A., and Mercier, R. (2007) “Application of dam-break flow to green water prediction.” Submitted to Applied Ocean Research.
Date: December, 2006Project Title: Loads on Structures due to Extreme Wave Crests
MMS Project: 571 TO Number: 39807
Project PI: Kuang-an Chang, H. C. Chen and Richard Mercier
COTR: M. Else
Estimated Completion Date: 8/31/2008
Project Description:
In approximately one year from September 2004 to September 2005, three Category 5 hurricanes (Ivan, Katrina, Rita) hit the Gulf of Mexico. Well over 80% of the 4,000 oil and gas production platforms in the Gulf were directly impacted by the hurricanes. The hurricanes destroyed or caused extensive damage to 190 platforms. In most cases the platform damage was caused by greenwater wave loading on the deck. 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. In situations where one must consider the possibility of wave overtopping and greenwater rushing onto the deck, the mechanics of wave loading become very complex and are poorly understood. One of the major sources of uncertainty is the velocity field of the greenwater flow itself.
The objective of this research is to develop a robust procedure to estimate local and global greenwater loads on structures due to extreme wave crests. Through the combined efforts of laboratory measurements and numerical simulations, the results will allow designers to avoid or minimize the impact of greenwater on new floating structures through design, and help the industry and regulators to develop associated design guidance. This research is a continued effort from the successful study of greenwater over a 2D platform through laboratory measurement, and a continuation of the development of a 3D computational fluid dynamics (CFD) code for greenwater simulation. The prior study has shown that the traditional prediction method often used in design, i.e., the dam-break 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 TLPs, spars, and ship-shaped FPSOs.
Progress:
In the experimental approach, the recently developed Bubble Image Velocimetry (BIV) method will be used to measure the velocity field in the highly turbulent, multi-phased overtopping flow. The method is based on correlating the acquired images in the laboratory for greenwater velocity determination. The in-house developed image processing software used in BIV has been modified and improved for faster velocity processing. This is due to the need of processing a very large number of images expected to be acquired in the proposed 3D laboratory measurements. Past experience found it will take months to process the images if the original code is been used. Part of the original code (the minimum quadratic difference (MQD) method for BIV image processing and velocity determination) that was written in MATLAB has been converted to Fortran. The processing time has been significantly reduced from months to days. In addition, the design of the model structure and wave conditions for the 3D flume tests has been progressing and is nearly finished.
In the numerical approach, we have developed a Finite-Analytic Navier-Stokes (FANS) method in conjunction with an interface-preserving level-set method for the simulation of greenwater on two- and three-dimensional offshore structures. Preliminary simulations have been performed for a rectangular offshore platform in a three-dimensional wave tank. The physical dimensions of the 3D rectangular platform and the wave tank are the same as those used for the experimental investigation. The numerical results successfully predicted the overtopping of incident waves around the three-dimensional platform. The incident wave impinges on the front face of the platform with greenwater spreading over the platform deck. Along the platform center plane of symmetry, the overtopping wave pattern is similar to that obtained earlier for a two-dimensional platform with infinite width. Around the edges of the three-dimensional platform, however, the incident wave was able to move around the platform edges and reduce the total amount of overtopping flow. The simulation results will be compared with the available experimental data after the completion of the experimental investigation.
Reports and Publications:
Ryu, Y., Chang, K.-A. & R. Mercier (2006) “Breaking wave impinging and greenwater on a two-dimensional offshore structure.” 16th International Offshore and Polar Engineering Conference, May 28-June 2, 2006, San Francisco, California.Chen, H.C. and Yu, K., “Numerical Simulation of Wave Runup and Greenwater on Offshore Structures by a Level-Set RANS Method,” 16th International Offshore and Polar Engineering Conference, May 28-June 2, 2006, San Francisco, California.
Ryu, Y., Chang, K.-A. & Mercier, R. (2006) “Runup and green water velocities due to breaking wave impinging and overtopping” Submitted to Experiments in Fluids.
Ryu, Y., Chang, K.-A. & Mercier, R. (2006) “Application of dam-break flow to green water prediction.” Submitted to Applied Ocean Research.
Progress Reports: June 2007 December 2006