Offshore Technology Research Center

 

OTRC Project Summary

Project Title:

Mitigating Greenwater Damage through Design

Prinicipal Investigators:

Kuang-An Chang, Hamn-Ching Chen, and Richard Mercier

Sponsor:

Minerals Management Service and Industry Consortium

Completion Date:

November, 2005

Final Report ID#

A156(Click to view final report abstract)

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.

PROJECT PLAN FOR PHASE 1:

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:

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.

Related 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, 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.

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