Date of Award
Naval Architecture and Marine Engineering
Naval Architecture and Marine Engineering
Uttam K Chakravarty
Dropped objects are among the top ten causes of fatalities and serious injuries in the oil and gas industry. Objects may be dropped during lifting or any other offshore operation. Concerns of health, safety, and the environment (HSE) as well as possible damages to structures require the prediction of where and how a dropped object moves underwater. This study of dropped objects is subdivided into three parts. In the first part, the experimental and simulated results published by Aanesland (1987) have been successfully reproduced and validated based on a two-dimensional (2D) theory for a dropped drilling pipe model. A new three-dimensional (3D) theory is proposed to consider the effect of axial rotation on dropped cylindrical objects. The 3D method is based on a modified slender body theory for maneuvering. A numerical tool called Dropped Objects Simulator (DROBS) has been developed based on this 3D theory. Firstly, simulated results of a dropped drilling pipe model using a 2D theory by Aanesland (1987) are compared with results from 3D theory when rolling frequency is zero. Good agreement is found. Further, factors that affect the trajectory, such as drop angle, normal drag coefficient, binormal drag coefficient, and rolling frequency are systematically investigated. It is found that drop angle, normal drag coefficient, and rolling frequency are the three most critical factors determining the trajectories. In the second part, a more general three-dimensional (3D) theory is proposed to physically simulate the dynamic motion of a dropped cylindrical object underwater with different longitudinal center of gravity (LCG). DROBS has been further developed based on this 3D theory. It is initially applied to a dropped cylinder with LCG = 0 (cylinder #1) falling from the surface of calm water. The calculated trajectories match very well with both the experimental and numerical results published in Aanesland (1987). Then DROBS is further utilized to simulate two dropped cylinders with positive LCG (cylinder #2) and negative LCG (cylinder #3) in Chu et al. (2005), respectively. The simulated results from DROBS show a better agreement with the measured data than the numerical results given in Chu et al. (2005). This comparison again validates and indicates the effectiveness of the DROBS program. Finally, it’s applied to investigate
the effects of varying LCG on the trajectory and landing points. Therefore, the newly developed DROBS program could be used to simulate the distribution of landing points of dropped cylindrical objects, as is very valuable in the risk-free zone prediction in offshore engineering. The third part investigates the dynamic motion of a dropped cylindrical object under current. A numerical procedure is developed and integrated into Dropped Objects Simulator (DROBS). DROBS is utilized to simulate the trajectories of a cylinder when dropped into currents from different directions (incoming angle at 0o; 90o; 180o; and 270o) and with different amplitudes (0m/s to 1.0m/s). It is found that trajectories and landing points of dropped cylinders are greatly influenced by currents. Cylinders falling into water are modeled as a stochastic process. Therefore, the related parameters, including the orientation angle, translational velocity and rotational velocity of the cylindrical object after fully entering the water, is assumed to follow normal distributions. DROBS is further used to derive the landing point distribution of a cylinder. The results are compared to Awotahegn (2015) based on Monte Carlo simulations. Then the Monte Carlo simulations are used for predicting the landing point distribution of dropped cylinders with drop angles from 0o to 90o under the influence of currents. The plots of overall landing point distribution and impact energy distribution on the sea bed provide a simple way to indicate the risk-free zones for offshore operation.
Xiang, Gong, "Motion Dynamics of Dropped Cylindrical Objects" (2017). University of New Orleans Theses and Dissertations. 2340.