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Volume 9, Number 1, March 2019

Latching control was applied to a Wave Energy Converter (WEC) buoy with direct linear electric Power Take-Off (PTO) systems oscillating in heave direction in waves. The equation of the motion of the WEC buoy in the time-domain is characterized by the wave exciting, hydrostatic, radiation forces and by several damping forces (PTO, brake, and viscous). By applying numerical schemes, such as the semi-analytical and Newmark B methods, the time series of the heave motion and velocity, and the corresponding extracted power may be obtained. The numerical prediction with the latching control is in accordance with the experimental results from the systematic 1:10-model test in a wave tank at Seoul National University. It was found that the extraction of wave energy may be improved by applying latching control to the WEC, which particularly affects waves longer than the resonant period.

Key Words
heave motion; model test; power extraction; latching control; latching duration

Jeongrok Kim and Il-Hyoung Cho: Department of Ocean System Engineering, Jeju National University, Jeju, Republic of Korea
Moo-Hyun Kim: Department of Ocean Engineering, Texas A&M University, College Station, Texas, USA

In this paper, the Finite-Analytic Navier-Stokes (FANS) code is coupled with an in-house finite-element code to study the dynamic interaction between a floating buoy and its mooring system. Hydrodynamic loads on the buoy are predicted with the FANS module, in which Large Eddy Simulation (LES) is used as the turbulence model. The mooring lines are modeled based on a slender body theory. Their dynamic responses are simulated with a nonlinear finite element module, MOORING3D. The two modules are coupled by transferring the forces and displacements of the buoy and its mooring system at their connections through an interface module. A free-decay model test was used to calibrate the coupled method. In addition, to investigate the capability of the present coupled method, numerical simulations of two degree-of-freedom vortex-induced motion of a CALM buoy in uniform currents were performed. With the study it can be verified that accurate predictions of the motion responses and tension responses of the CALM buoy system can be made with the coupling CFD-FEM method.

Key Words
Catenary Anchor Leg Mooring (CALM) buoy system; computational fluid dynamics (CFD); unsteady Navier-Stokes equations; coupling mooring analysis; Large Eddy Simulation (LES)

Haoyuan Gu: Department of Civil Engineering, Texas A&M University, College Station, Texas, USA
Hamn-Ching Chen: Zachry Department of Civil Engineering and Department of Ocean Engineering, College Station, Texas, USA
Linyue Zhao: Saipem America Inc., Houston, Texas, USA

A local-analytic-based Navier-Stokes solver has been employed in conjunction with a compound ocean structure motion analysis program for time-domain simulation of passing ship effects induced by multiple post-Panamax class ships in the exact condition of a real waterway. The exact seabed bathymetry was reproduced to the utmost precision attainable using the NOAA geophysical database for Virginia Beach, NOAA nautical charts for Hampton Roads and Norfolk harbor, and echo sounding data for the navigation channel and waterfront facilities. A parametric study consists of 112 simulation cases with various combinations of ship lanes, ship speeds, ship heading (inbound or outbound), channel depths, drift angles, and passing ship coupling (in head-on or overtaking encounters) were carried out for two waterfront facilities at NAVSTA Norfolk and Craney Island Fuel Terminal. The present paper provides detailed parametric study results at both locations to investigate the site-specific passing ship effects on the motion responses of ships moored at nearby piers.

Key Words
passing ship effects; site-specific; computational fluid dynamics; moving overset grids; coupled motion analysis

Hamn-Ching Chen and Chia-Rong Chen: Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas 77843, USA
Erick T. Huang: Engineering and Expeditionary Warfare Center, Naval Facilities Engineering Command,
Port Hueneme California 93043, USA

The material analysis of an offshore structure is generally conducted in the contract design phase for the price quotation of a new offshore project. This analysis is conducted manually by an engineer, which is time-consuming and can lead to inaccurate results, because the data size from previous projects is too large, and there are so many materials to consider. In this study, the piping materials in an offshore structure are analyzed for contract design using a big data framework. The big data technologies used include HDFS (Hadoop Distributed File System) for data saving, Hive and HBase for the database to handle the saved data, Spark and Kylin for data processing, and Zeppelin for user interface and visualization. The analyzed results show that the proposed big data framework can reduce the efforts put toward contract design in the estimation of the piping material cost.

Key Words
piping material; big data analysis; offshore structure; contract design

Min-Jae Oh: Research Institute of Marine Systems Engineering, Seoul National University, Seoul, Republic of Korea
Myung-Il Roh: Department of Naval Architecture and Ocean Engineering, and Research Institute of Marine Systems Engineering, Seoul National University, Seoul, Republic of Korea
Sung-Woo Park: Interdisciplinary Program in Offshore Plant Engineering, Seoul National University, Seoul, Republic of Korea
Do-Hyun Chun: Department of Naval Architecture and Ocean Engineering, Seoul National University, Republic of Korea
Sehyun Myung: School of Automotive & Mechanical Design Engineering, Youngsan University,
Yangsan, Republic of Korea

Accurate calculation of the mean drift forces and moments is necessary when studying the higher order excitations on the floater in waves. When taking the time average of the second order forces and moments, the second order potential and motion diminish with only the first order terms remained. However, in the results of the first order forces or motions, the irregular frequency effects are often observed in higher frequencies, which will affect the accuracy of the calculation of the second order forces and moments. Therefore, we need to pay close attention to the irregular frequency effects in the mean drift forces. This paper will discuss about the irregular frequency effects in the calculations of the mean drift forces and validate our in-house program MDL Multi DYN using some examples which are known to have irregular frequency effects. Finally, we prove that it is necessary to remove the effects and demonstrate that the effectiveness of the formula and methods adopted in the development of our program.

Key Words
irregular frequency effect; drift force; hydrodynamics; body-wave interaction

Yujie Liu and Jeffrey M. Falzarano: Marine Dynamics Laboratory, Department of Ocean Engineering, Texas A&M University, College
Station, Texas 77843, USA

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