Browsing by Subject "COUPLE"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item An Efficient Numerical Scheme for Simulating Unidirectional Irregular Waves Based on a Hybrid Wave Model(2012-11-15) Jia, Dongxing 1984-The Unidirectional Hybrid Wave Model (UHWM) predicts irregular wave kinematics and pressure accurately in comparison with its linear counterpart and modification, especially near the free surface. Hence, in using the Morrison equation it has been employed in the computation of wave loads on a moored floating structure, such as Spar or TLP (Tension Leg Platform), which can be approximated by a slender body or a number of slender components. Dr. Jun Zhang, with his former and current graduate students, have developed a numerical code, known as COUPLE, over the past two decades, simulating 6 Degree Of Freedom (DOF) motions of a moored floating structures interacting with waves, current and wind. COUPLE employs UHWM as a module for computing wave loads on a floating structure. However, when the duration of simulating the wave-structure interaction is long, say 3 hours (typically required by the offshore industry for extreme storm cases), the computation time of using UHWM increases significantly in comparisons with the counterpart based upon linear wave theory. This study is to develop a numerical scheme which may significantly reduce the CPU time in the use of UHWM and COUPLE. In simulating irregular (or random) waves following a JONSWAP spectrum of a given cut off frequency, the number of free wave components in general grows linearly with the increase of the simulation duration. The CPU time for using a linear spectral method to simulate irregular waves is roughly proportion to N2, where N is the number of free wave components used in simulating irregular waves, while that for using a nonlinear wave model, such as UHWM, it is roughly proportional to N3. Therefore, to reduce the CPU time, the total simulation duration is divided into a number of segments. However, due to the nature of Fast Fourier Transform (FFT), the connection between the two neighboring surface elevations segments is likely discontinuous. To avoid the discontinuity, an overlapped duration between the two neighboring segments is adopted. For demonstration, a free-wave spectrum is input to COUPLE for simulating the 6 DOF motions of a floating 5-MW wind turbine installed on an OC3 moored Spar and tensions in the mooring lines. It is shown that the CPU time for the above simulation for duration of 2048 seconds is reduced from more than16 hours when the irregular wave elevation and kinematics are calculated without dividing into segments to less than three hours when those are calculated by dividing into five segments.Item Coupled Analysis of the Motion and Mooring Loads of a Spar "CONSTITUTION"(2012-10-19) Li, ChengxiA truss spar, named as 'Constitution' was installed in Gulf of Mexico located at 90.58' 4.8" West Longitude and 27.17'31.9" North Latitude. Since its installation in October 2006, it has weathered multiple hurricanes. After the installation, British Maritime Technology (BMT) installed an Environmental Platform Response Monitoring System (EPRMS). The EPRMS is an integrated system collecting myriad of data that include the significant wave height and peak period of waves, the magnitude and direction of current and wind in the vicinity of the truss spar, its six-degree of freedom (6-D) motions, and tensions in its mooring lines and Top-Tension Risers. With the permission from Anadarko Petroleum Corporation (APC), these data are available to the Ocean Engineering Program at Texas A&M University (TAMU). In this study, the coupled dynamic analysis of the spar interacting with the mooring and riser systems will be performed using a numerical code, named as 'COUPLE'. 'COUPLE' was developed and is continuously expanded and improved by his former and current graduate students and Professor Jun Zhang at TAMU for the computation of the interaction between a floating structure and its mooring line/riser/tendon system in time domain. The main purpose of this study is to exam the accuracy and efficiency of 'COUPLE' in computing offshore structure motions and mooring line tensions and discuss the main issues of the computation. The numerical results will be compared with the corresponding ones obtained using another commercial software, 'Orcaflex', and the corresponding field measurement during Hurricane Ike which occurred on 12th September of 2008 and a winter storm on 9th November of 2009. The satisfactory agreement between the numerical prediction made using 'COUPLE' and field measurement are observed and presented. The results of the comparisons between 'COUPLE' with 'Orcaflex' and field measurements in this study have verified the accuracy and efficiency of 'COUPLE' in computing offshore structure motions and mooring line tensions due to its nonlinear hybrid wave model which could better estimate the second-order difference-frequency wave loading.Item Numerical Model of a Tensioner System and Flex Joint(2013-07-27) Huang, HanTop Tensioned Riser (TTR) and Steel Catenary Riser (SCR) are often used in a floating oil/gas production system deployed in deep water for oil transport. This study focuses on the improvements to the existing numerical code, known as CABLE3D, to allow for static and dynamic simulation of a TTR connected to a floating structure through a tensioner system or buoyancy can, and a SCR connected to a floating structure through a flex joint. A tensioner system usually consists of three to four cylindrical tensioners. Although the stiffness of individual tensioner is assumed to be linear, the resultant stiffness of a tensioner system may be nonlinear. The vertical friction at a riser guide is neglected assuming a roller is installed there. Near the water surface, a TTR is forced to move due to the motion of the upper deck of a floating structure as well as related riser guides. Using the up-dated CABLE3D, the dynamic simulation of TTRs will be made to reveal their motion, tension, and bending moment, which is important for the design. A flex joint is approximated by a rotational spring with linear stiffness, which is used as a connection between a SCR and a floating structure or a connection between a TTR and the sea floor. The improved CABLE3D will be integrated into a numerical code, known as COUPLE, for the simulation of the dynamic interaction among the hull of a floating structure, such as SPAR or TLP, its mooring system and riser system under the impact of wind, current and waves. To demonstrate the application of the improved CABLE3D and its integration with COUPLE, the related simulation is made for ?Constitution? SPAR under the met-ocean conditions of hurricane ?Ike?. The mooring system of the Spar consists of nine mooring lines and the riser system consists of six TTRs and two SCRs.Item Numerical Simulation of Free Standing Hybrid Risers(2014-08-13) Hou, TiancongFree Standing Hybrid Riser (FSHR) is an innovative concept for the riser to transport oil and gas from the seabed to a floating production platform deployed in deep water. A FSHR mainly consists of a buoyancy can, a vertical steel riser, a flexible jumper and an upper riser assembly (URA). This study focuses on the development of the numerical scheme for the dynamic simulation of a FSHR whose flexible jumper is connected to a moored floating production vessel, say floating production, storage and offloading (FPSO). The numerical scheme is mainly based on an existing in-house numerical code, known as COUPLE. In using COUPLE, URA is modeled as a rigid body. The URA connects a vertical steel riser and a flexible jumper, both of which are modeled by beam elements (experienced both tensions and bending moments) in COUPLE. The buoyancy can is connected to the URA through a tether chain, which provides the vertical upright force on the URA and in turn the tension to the vertical riser. The tether chain is modeled by bar elements (experienced only tensions) while the buoyancy can is modeled as a rigid cylinder (beam element). The motions and forces of the two rigid bodies, namely URA and the buoyancy can, are coupled in the simulation. Because the connection between the URA and steel riser is considered as ?rigid?, an accurate numerical scheme modeling a rotational spring connecting the URA and steel riser is developed for calculating the bending moment applied at the upper end of the riser. Given the motion at the upper end of the flexible jumper (connecting to a FPSO), numerical simulations are made for two different FSHRs. The results simulated using COUPLE are in satisfactory agreement with those obtained using OrcaFlex, a commercial code widely used by the offshore industry. Furthermore, the vortex induced motion (VIM) of the buoyancy can is considered in the simulation and its effects on the motion of the FSHR are explored, which is compared with the corresponding model tests. The ability to simulate the dynamics of FSHRs developed in this study may be helpful for the future design of FSHRs.Item Numerical Simulations of a Wave Energy Conversion Device Used for Oceanographic Buoys(2014-07-24) Lee, YongseokMoored buoy systems are often deployed by oceanographers to gather scientific information on local and global changes in the water column, weather patterns and climate change. The data they gather is first transmitted to satellites or passing oceanographic ships prior to transmission land based research facilities. Most buoy designs are powered by battery systems that provide ballast and some can be recharged by solar panels. At-sea maintenance may include regular battery replacement or repairs to the buoy system due to vandalism, each being expensive propositions. In order to reduce the costs and utilize green energy, this thesis research investigates the use of incorporating a pendulum wave energy conversion (WEC) device as a permanent or semi-permanent power source for some oceanographic buoys having an average power consumption that can vary from 0.1W to 6.0W. The main criteria for selecting a WEC device for this application are operational reliability, sustainability during operational and extreme weather conditions, and minimizing the opportunity for vandalism. A general analytical model was developed and simulations of the motions of the buoy were performed using the numerical code COUPLE, which was originally developed to simulate the coupled response behavior of a deepwater floating hull and the associated mooring/riser/tendon systems. Based upon the motion behavior from the numerical simulation, the electrical power output by the selected WEC device is estimated using an iterative scheme to estimate equivalent damping of a hydraulic Power Take-Off (PTO) system. Several illustrative case studies are presented to verify that the electrical power output rate is in the range of the power demands needed by typical oceanographic buoys. It is concluded that the proposed pendulum WEC device is a feasible solution that can be designed to provide an alternative power system to power oceanographic buoys. The research study provides a way to approach the design and utilization of WEC devices to capture wave energy as a natural power source for a wide range of buoy shapes, sizes and configurations for existing and future buoy designs.