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CONTENTS
Volume 6, Number 4, December 2016
 

Abstract
Wave load prediction at zero forward speed using finite depth Green function is a well-established method regularly used in the offshore and marine industry. The forward speed approximation in deep water condition, although with limitations, is also found to be quite useful for engineering applications. However, analysis of vessels with forward speed in finite water depth still requires efficient computing methods. In this paper, a method for analysis of wave induced forces and corresponding motion on freely floating three-dimensional bodies with low to moderate forward speed is presented. A finite depth Green function is developed and incorporated in a 3D frequency domain potential flow based tool to allow consideration of finite (or shallow) water depth conditions. First order forces and moments and mean second order forces and moments in six degree of freedom are obtained. The effect of hull flare angle in predicting added resistance is incorporated. This implementation provides the unique capability of predicting added resistance in finite water depth with flare angle effect using a Green function approach. The results are validated using a half immersed sphere and S-175 ship. Finally, the effect of finite depth on a tanker with forward speed is presented.

Key Words
green function; shallow water; forward speed; potential theory; added resistance; flare angle

Address
Amitava Guha and Jeffrey Falzarano: Marine Dynamics Laboratory, Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA 77840-3136

Abstract
Using 3D computational fluid dynamics techniques in recent years have shed significant light on the Vortex Induced Vibrations (VIV) encountered by deep-water marine risers. The fatigue damage accumulated due to these vibrations has posed a great concern to the offshore industry. This paper aims to present an algorithm to predict the crossflow and inline fatigue damage for very long (L/D > 103) marine risers using a Finite-Analytical Navier-Stokes (FANS) technique coupled with a tensioned beam motion solver and rainflow counting fatigue module. Large Eddy Simulation (LES) method has been used to simulate the turbulence in the flow. An overset grid system is employed to mesh the riser geometry and the wake field around the riser. Risers from NDP (2003) and Miami (2006) experiments are used for simulation with uniform, linearly sheared and non-uniform (non-linearly sheared) current profiles. The simulation results including inline and crossflow motion, modal decomposition, spectral densities and fatigue damage rate are compared to the experimental data and useful conclusions are drawn.

Key Words
computational fluid dynamics; vortex-induced vibration (VIV); crossflow; inline; fatigue; riser

Address
Chetna Kamble: Department of Ocean Engineering, Texas A&M University, USA
Hamn-Ching Chen: Department of Civil Engineering, Texas A&M University, USA

Abstract
This study aims to develop the numerical method to estimate level ice impact load and investigate the dynamic interaction between an arctic Spar with sloped surface and drifting level ice. When the level ice approaches the downward sloped structure, the interaction can be decomposed into three sequential phases: the breaking phase, when ice contacts the structure and is bent by bending moment; the rotating phase, when the broken ice is submerged and rotated underneath the structure; and the sliding phase, when the submerged broken ice becomes parallel to the sloping surface causing buoyancy-induced fictional forces. In each phase, the analytical formulas are constructed to account for the relevant physics and the results are compared to other existing methods or standards. The time-dependent ice load is coupled with hull-riser-mooring coupled dynamic analysis program. Then, the fully coupled program is applied to a moored arctic Spar with sloped surface with drifting level ice. The occurrence of dynamic resonance between ice load and spar motion causing large mooring tension is demonstrated.

Key Words
ice interaction; arctic spar; level ice; sloped surface; coupled dynamics; bending failure; breaking phase; rotating phase; sliding phase; ice-induced resonance; time domain analysis; platform motions; mooring forces

Address
H.K. Jang, H.Y. Kang and M.H. Kim: Department of Ocean Engineering, Texas A&M University, College Station, TX 77843, USA

Abstract
Large numbers of submarine pipelines are laid as the world now is attaching great importance to offshore oil exploitation. Free spanning of submarine pipelines may be caused by seabed unevenness, change of topology, artificial supports, etc. By combining Iwan\'s wake oscillator model with the differential equation which describes the vibration behavior of free-span submarine pipelines, the pipe-fluid coupling equation is developed and solved in order to study the effect of both internal and external fluid on the vibration behavior of free-span submarine pipelines. Through generalized integral transform technique (GITT), the governing equation describing the transverse displacement is transformed into a system of second-order ordinary differential equations (ODEs) in temporal variable, eliminating the spatial variable. The MATHEMATICA built-in function NDSolve is then used to numerically solve the transformed ODE system. The good convergence of the eigenfunction expansions proved that this method is applicable for predicting the dynamic response of free-span pipelines subjected to both internal flow and external current.

Key Words
free-span submarine pipeline; vortex-induced vibration; internal flow; integral transform; pinned-pinned

Address
Tongtong Li: Institute for Ocean Engineering, China University of Petroleum, Beijing, 102249, China;
College of Mechanical and Transportation Engineering, China University of Petroleum, Beijing, 102249, China
Menglan Duan and Chen An: Institute for Ocean Engineering, China University of Petroleum, Beijing, 102249, China
Wei Liang: College of Mechanical and Transportation Engineering, China University of Petroleum, Beijing, 102249, China


Abstract
Dropped objects are among the top ten causes of fatalities and serious injuries in the oil and gas industry (DORIS, 2016). Objects may accidentally fall down from platforms or vessels during lifting or any other offshore operation. Proper planning of lifting operations requires the knowledge of the risk-free zone on the sea bed to protect underwater structures and equipment. To this end a three-dimensional (3D) theory of dynamic motion of dropped cylindrical object is expanded to also consider ocean currents. The expanded theory is integrated into the authors\' Dropped Objects Simulator (DROBS). DROBS is utilized to simulate the trajectories of dropped cylinders falling through uniform currents originating from different directions (incoming angle at 0o, 90o, 180o, and 270o). It is found that trajectories and landing points of dropped cylinders are greatly influenced by the direction of current. The initial conditions after the cylinders have fallen into the water are treated as random variables. It is assumed that the corresponding parameters orientation angle, translational velocity, and rotational velocity follow normal distributions. The paper presents results of DROBS simulations for the case of a dropped cylinder with initial drop angle at 60 through air-water columns without current. Then the Monte Carlo simulations are used for predicting the landing point distributions of dropped cylinders with varying drop angles under current. The resulting landing point distribution plots may be used to identify risk free zones for offshore lifting operations.

Key Words
dropped cylindrical object; landing point distribution; Monte Carlo simulation; risk free zone; current

Address
Gong Xiang, Lothar Birk and Xiaochuan Yu: School of Naval Architecture and Marine Engineering, University of New Orleans, New Orleans, LA, USA
Linxiong Li: Department of Mathematics, University of New Orleans, New Orleans, LA, USA
Yong Luo: School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai, China


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