Green Water on Ship-type Offshore Structures
In heavy storms, the waves and ship motions can become so large that water flows onto the deck of a ship. This problem is generally known as ‘green water loading’. On ship-type offshore structures green water loading can result in risk for the ship, its crew and its sensitive equipment. Therefore, it should be taken into account in the design of such structures. Based on a historical overview of green water research it was concluded that there is limited insight in the physics of the complex green water problem, which results in a wide range of assumptions in prediction methods. Existing research was also not focussed on moored offshore structures in extreme environmental conditions (‘100 year storms’) and there is very limited insight in the loading process on structures on the deck. Finally, the important problem of green water loading from the side of the vessel has not been studied before. Therefore, the main objective of this study was to develop methods for the evaluation of green water on ship-type offshore structures based on a clear description of the green water physics. To achieve this objective, first the physics of the green water process on the bow were studied using two series of initial model tests. Based on these tests this process was described in the following phases: 1. Motions and relative wave motions 2. Water flow onto the deck 3. Water behaviour and loading on the deck 4. Green water impact on structures It was concluded that in all phases of the green water problem non-linear and highly complex phenomena occur. Consequently, the green water problem cannot be predicted with existing linear prediction methods. New numerical methods still need significant further development, integration and validation before they can be used to predict the green water as a whole within a reasonable timeframe. Therefore, a semiempirical design evaluation method was proposed, to predict the green water problem from the input (extreme relative wave motions) to the output (predicted load levels) based on a clear description of the green water physics. This semi-empirical design evaluation method has been developed using a systematic series of model tests. The building blocks of the method and their relations are presented in detail in the thesis and based on the phases in the green water process. The problem of green water loading from the side of the ship is taken into account as well. The development of the method is completed with a review, together with recommendations for its application in relation with metocean (wind, wave and current) data and structural response analysis. Finally, the numerical prediction of green water loading is discussed. A number of methods have been evaluated based on the specific requirements related to the physics of green water loading. A numerical method for the prediction of green water loading should be able to deal with: • Water entry of a flared bow structure. • Complex flow onto the deck, including the discontinuity at the deck edge. • ‘Hydraulic jump’-type shallow water flow on a moving ship deck. • Meeting water flows on the deck. • Short duration water impact on a structure. • Overturning flow after run-up of the water in front of the structure. The evaluation of a Modified-VOF (Volume Of Fluid) method based on these requirements has shown its ability to simulate the complex green water problem, although a number of numerical details need significant further development and validation. The description of the physics and the model test results presented in this thesis, can provide detailed validation material for this process.