Experimental and Numerical Hydrodynamic Analysis of a Damaged Ship Section In Waves
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- Institutt for marin teknikk 
In the past few decades, ships have become larger, efficient, durable but the danger of accidental events like grounding, collision etc. continues to be a major issue in ship operations. Following a damage event, a ship can either reach an equilibrium condition after initial flooding or, in the worst case, capsize. In the former case, it is important to analyze damaged ship behavior leading to formulation of protocols for safe return to port. In the latter scenario, it is important to understand the dynamic impact of flooding on the ship just after damage. This can help to implement guidelines in the design stage to mitigate effect of damage. In the present work, the main focus has been on the behavior of a damaged ship after it has achieved a static equilibrium floating condition following a flooding event. From a static point of view, buoyancy and structural strength is lost for a damaged ship. This lost buoyancy/waterplane area leads to increase in natural ship periods due to decrease in metacentric height. However, the floodwater is associated with dynamic effects; effects like vortex shedding, large inflow/outflow through the opening can be dominant in certain scenarios. The floodwater can be associated with sloshing and piston mode resonances which can lead to important phenomena, for example hydraulic-jump like behavior in shallow water conditions. In this work, the effect of these resonance phenomena on a 2D damaged ship model is analyzed experimentally, theoretically and numerically. Dedicated experiments were performed on a midship barge-shaped section in 2D flow conditions. The experiments were conducted in three parts; a) forced heave motions of the model in intact and damaged condition, b) free-roll decay tests in intact and damaged condition and c) freely-floating model in beam-sea waves in intact and damaged condition. Various parameters such as the forcing period/incident-wave period, forcing amplitude/wave steepness, damageopening size, airtightness in the damaged compartment etc. were varied to analyze their effect on floodwater behavior. Effect of transient flooding was also studied for an initially dry freely-floating model. Repeatability analysis was performed for a selected set of experiments. The main focus was on identification of sloshing and piston mode resonances and their effect on damaged ship loads/ motions. These resonances can have serious consequence for smaller Ro-RO ferries, fishing vessels etc. and the local load effects can also be important for larger ships. Images from the experiments are used to identify occurrence of resonance and cross-check with corresponding measurements of ship-section loads/motions and wave elevation inside the damaged compartment. The experiments were complemented with theoretical and numerical analy ses. Theoretical formulations are used to identify sloshing and piston mode resonance frequencies and these values are verified with images from the experiments. Linear potential flow code (WAMIT) is also employed in indirect calculation of piston mode frequencies. An open-source Navier-Stokes solver in laminarflow conditions from the CFD platform OpenFOAM is used for dedicated numerical simulations. It is also used within a simplified strip-theory to calculate motions of the ship model in intact and damaged conditions. Snapshots from the simulations are compared with experimental images. Preliminary study of a domain-decomposition (DD) technique is performed to analyze the potential of such a strategy in maintaining accuracy and achieving efficiency. The proposed DD couples a Navier-Stokes solver from OpenFOAM with an efficient and accurate potential-flow solver based on the Harmonic Polynomial Cell (HPC) method and was applied to the case of a damaged ship section in forced-heave motion. The experiments confirm the occurrence of sloshing and piston mode resonance in a damaged ship. Piston mode had only been described numerically in previous studies. 2D flow conditions help to visualize the sloshing/piston modes and correlate the effect of resonance on loads/motions and internal wave elevation with images. Forced heave motions show that hydrodynamic loads in the damaged condition differ significantly from the intact scenario. Sloshing modes have little effect on the hydrodynamic loads, whereas, piston modes lead to large negative added mass and large damping values. This is associated with a large influx/outflux from the damage opening. An important observation was the effect of airtightness in the compartment, which causes the damaged model to behave similarly to the intact condition. Presence of flow conditions which can lead to local loads was identified and highlighted. Free-roll decay tests also show significant effect of the damage, leading to an increase in both the natural roll period and damping, as documented in previous works. Detailed analysis shows presence of large contribution from the quadratic terms for the roll damping and quantifies its relative importance in the studied cases. In this regard, an important aspect analyzed was the roll damping due to a closed internal tank versus a tank with damage opening. Behavior of a damaged model in waves is also modified significantly as compared to an intact condition. The natural roll and heave periods are modified. Sloshing and piston modes mainly affect sway-roll coupled motions. Airightness in the damaged compartment also affects the heave motions. For a freely-floating dry model with opening lying above the waterline, flooding generally takes place near the natural roll periods. Occurrence of flooding and time to flooding depend on initial loading, incident wave period to natural roll period ratio and wave steepness. Repeatability analysis from the experiments showed acceptable results. Numerical results document acceptable agreement with experimental data. Sloshing and piston mode frequencies calculated from theoretical formulations And WAMIT show a good match with experimental observations. Numerical simulations from OpenFOAM have a reasonable agreement with both forced heave motions and freely-floating experiments. Numerical simulations for forced heave motions confirm large influx/outflux at the opening near piston mode resonance and negligible flow near sloshing modes. Pressure sensors were not used in the experiments, however, simulations confirm the presence of local loading on the deck. The simulations also helped analyze the difference in behavior at shallow and higher filling levels in the damaged compartment. Strip-theory results capture the general trend of freely-floating damaged model motions but over-predict the motions. Results from the domain-decomposition strategy for forced heave motions show good potential for future application in simulation of damage ship behavior.