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.
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.
Scope of Work:
The main tasks of this phase are as follows:
- Develop capability to measure greenwater kinematics for a 3D stationary model in flume
- Simulate experiments in (1) using CFD model to validate agreement
- Develop capability to measure greenwater kinematics for a 3D floating model in flume
- Simulate experiments in (3) using CFD model to validate agreement
Following 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.
Ryu, Y., Chang, K.-A., and Mercier, R. (2007a) “Runup and green water velocities due to breaking wave impinging and overtopping.” Experiments in Fluids, 43, 555-567.
Ryu, Y., Chang, K.-A., and Mercier, R. (2007b) “Application of dam-break flow to green water prediction.” Applied Ocean Research, 29, 128-136.
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, pp. 1912-1915.
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, pp. 3642-3649.
Chen, H.C. and Yu, K. (2007), “CFD simulation of wave-current-body interactions including greenwater and wet deck slamming,” 9th International Symposium on Fluid Control, Measurement and Visualization, September 17-19, Tallahassee, Florida (Keynote Paper).
Ryu, Y. and Chang, K.-A. (2008) “Green water void fraction due to breaking wave impinging and overtopping” Experiments in Fluids, doi 10.1007/s00348-008-0507-3.
Chang, K.-A. & Ryu, Y. (2008) “Void fraction in green water.” The 18th International Offshore and Polar Engineering Conference, Vancouver, Canada, July 6–11, to appear.