Browsing by Author "Zhang, Jun"
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Item Adapting MIMO networks to manage interference(2009-12) Zhang, Jun; Andrews, Jeffrey G.Multiple-Input Multiple-Output (MIMO) communication uses multiple transmit and receive antennas to improve the throughput in wireless channels. In cellular networks, self-interference greatly degrades MIMO's potential gain, especially in multiuser MIMO systems where multiple users in each cell share the spatial channel in order to maximize the total throughput. In a multiuser MIMO downlink, the two main causes of this self-interference are residual inter-user interference due to imperfect spatial separation between the users and other-cell interference due to cochannel transmissions in other cells. This dissertation develops adaptive transmission strategies to deal with both residual inter-user interference and other-cell interference in cellular MIMO networks. For the residual inter-user interference caused by imperfect channel state information at the transmitter, we explicitly characterize the impact of channel quantization and feedback delay. Achievable ergodic rates for both single-user and multiuser MIMO systems with different channel state information are derived. Adaptive switching between single-user and multiuser MIMO modes is proposed to improve the throughput, based on the accuracy of the available channel information. It is then extended to a multi-mode transmission strategy which adaptively adjusts the number of active users to control residual interference and provide additional array gain. To adaptively minimize the other-cell interference, two practical base station coordination strategies are proposed. The first is a cluster based coordination algorithm with different coordination strategies for cluster interior and cluster edge users. It performs full intra-cluster coordination for enhancing the sum throughput and limited inter-cluster coordination for reducing the interference for cluster edge users. A multi-cell linear precoder is designed to perform the coordination. The second is an adaptive intercell interference cancellation strategy, where multiple base stations jointly select transmission techniques based on user locations to maximize the sum throughput. Spatial interference cancellation is applied to suppress other-cell interference. Closed-form expressions are derived for the achievable throughput, and the proposed adaptive strategy is shown to provide significant average and edge throughput gain. The feedback design to assist the interference cancellation is also discussed.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 Analysis of WACSIS data using a directional hybrid wave model(Texas A&M University, 2007-04-25) Zhang, ShaosongThis study focuses on the analysis of measured directional seas using a nonlinear model, named Directional Hybrid Wave Model (DHWM). The model has the capability of decomposing the directional wave field into its free wave components with different frequency, amplitude, direction and initial phase based on three or more time series of measured wave properties. With the information of free waves, the DHWM can predict wave properties accurately up to the second order in wave steepness. In this study, the DHWM is applied to the analyses of the data of Wave Crest Sensor Inter-comparison Study (WACSIS). The consistency between the measurements collected by different sensors in the WACSIS project was examined to ensure the data quality. The wave characteristics at the locations of selected sensors were predicted in time domain and were compared with those recorded at the same location. The degree of agreement between the predictions and the related measurements is an indicator of the consistency among different sensors. To analyze the directional seas in the presence of strong current, the original DHWM was extended to consider the Doppler effects of steady and uniform currents on the directional wave field. The advantage of extended DHWM originates from the use of the intrinsic frequency instead of the apparent frequency to determine the corresponding wavenumber and transfer functions relating wave pressure and velocities to elevation. Furthermore, a new approach is proposed to render the accurate and consistent estimates of the energy spreading parameter and mean wave direction of directional seas based on a cosine-2s model. In this approach, a Maximum Likelihood Method (MLM) is employed. Because it is more tolerant of errors in the estimated cross spectrum than a Directional Fourier Transfer (DFT) used in the conventional approach, the proposed approach is able to estimate the directional spreading parameters more accurately and consistently, which is confirmed by applying the proposed and conventional approach, respectively, to the time series generated by numerical simulation and recorded during the WACSIS project.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 Dynamic simulation of polyester mooring lines(Texas A&M University, 2004-09-30) Kim, Min SukA numerical scheme, known as CABLE3D, originally developed for the simulation of dynamics of steel chain-wire mooring lines is extended to allow for the large elongation in a mooring line, the dependence of the modulus on tension, and energy dissipation of a polyester rope under mean and cyclic loads. The modified CABLE3D is then integrated into a numerical package, known as COUPLE6D, for computing the interaction between a floating structure and its hybrid polyester mooring system. The Deepstar Spar is chosen in this study to represent the floating structure. By considering large elongation in polyester ropes in numerical simulation, the static offset curve of a polyester mooring system is softer than that calculated under the assumption of small elongation. That is, about 10% reduction in restoring force at the mean offset position of the Spar under the impact of 100-year hurricane storm. The effects of the mean loads on the modulus of polyester ropes are much greater than those of the dynamic loads. Hence, the former is more important in the simulation of the response of floating structures. The energy dissipation in polyester ropes under cyclic loading does not play significant roles in the responses of the Spar and tensions in a polyester mooring system. The above findings, although observed based on the numerical simulation of a particular floating structure, namely Spar, may have implications to other floating offshore structures moored by a polyester mooring system. Two systems are simulated in two different met-ocean conditions: hurricane and loop current condition. To study the properties of polyester, numerical simulation were conducted in three ways. Those are related to the modulus based on mean load, mean and dynamic loads, and structural damping respectively. Through the simulation, statistics of motions of the hull and tension in the mooring lines are compared with those of a conventional steel mooring system.Item Essays in Corporate Finance(2014-06-02) Zhang, JunThe first essay, ?Local Institutional Investors and the Maturity Structure of Corporate Debt?, examines the relation between the geographic proximity of a firm?s institutional shareholders and its debt maturity choices. Local institutional investors may pressure firms to employ short-term debt as a means of disciplining managers to reduce equity agency costs, and to reduce debt agency costs associated with debtholder-stockholder conflicts caused by monitoring of local institutional investors. Thus, we hypothesize that firms with local institutional investors choose shorter debt maturity structures. Using dynamic GMM estimators to account for endogeneity and dynamic relations between debt maturity structure and institutional proximity, we find that firms with local institutional investors have shorter maturity debt. Similar results obtain for the maturity of new debt issues. To help establish causality, we use Sarbanes-Oxley Act as a quasi-natural experiment, conduct a nearest neighbor matching analysis that holds location constant, and employ a sample of firms? headquarter relocations as quasi-exogenous shocks to the locality of institutional investors. The results demonstrate the importance of local institutional investors in affecting firms? debt maturity policy choices. In the second essay, ?The Effect of Algorithmic Trading on Firm Value?, we study the overall impact of algorithmic trading on firm value. Extant literature has found mixed evidence on the impact of algorithmic trading on market quality, and it is still under intense public debate and controversy that whether algorithmic trading is beneficial or not. Using an algorithmic trading proxy based on electronic message traffic, we find a positive relation between algorithmic trading and firm value. The relation is stronger for firms with lower stock liquidity, higher idiosyncratic volatility, higher analyst coverage, and greater information asymmetry, which suggest that the value increases occur through market quality channels. The results are robust to various model specifications, reverse causality test using NYSE automated quote dissemination as an exogenous shock, and endogeneity concerns. The results imply net benefits of algorithmic trading to firms.Item Longshore sediment transport rate calculated incorporating wave orbital velocity fluctuations(Texas A&M University, 2006-10-30) Smith, Ernest RayLaboratory experiments were performed to study and improve longshore sediment transport rate predictions. Measured total longshore transport in the laboratory was approximately three times greater for plunging breakers than spilling breakers. Three distinct zones of longshore transport were observed across the surf zone: the incipient breaker zone, inner surf zone, and swash zone. Transport at incipient breaking was influenced by breaker type; inner surf zone transport was dominated by wave height, independent of wave period; and swash zone transport was dependent on wave period. Selected predictive formulas to compute total load and distributed load transport were compared to laboratory and field data. Equations by Kamphuis (1991) and Madsen et al. (2003) gave consistent total sediment transport estimates for both laboratory and field data. Additionally, the CERC formula predicted measurements well if calibrated and applied to similar breaker types. Each of the distributed load models had shortcomings. The energetics model of Bodge and Dean (1987) was sensitive to fluctuations in energy dissipation and often predicted transport peaks that were not present in the data. The Watanabe (1992) equation, based on time-averaged bottom stress, predicted no transport at most laboratory locations. The Van Rijn (1993) model was comprehensive and required hydrodynamic, bedform, and sediment data. The model estimated the laboratory cross-shore distribution well, but greatly overestimated field transport. Seven models were developed in this study based on the principle that transported sediment is mobilized by the total shear stress acting on the bottom and transported by the current at that location. Shear stress, including the turbulent component, was calculated from the wave orbital velocity. Models 1 through 3 gave good estimates of the transport distribution, but underpredicted the transport peak near the plunging wave breakpoint. A suspension term was included in Models 4 through 7, which improved estimates near breaking for plunging breakers. Models 4, 5 and 7 also compared well to the field measurements. It was concluded that breaker type is an important variable in determining the amount of transport that occurs at a location. Lastly, inclusion of the turbulent component of the orbital velocity is vital in predictive sediment transport equations.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 prediction of mobile offshore drilling unit drift during hurricanes(2009-05-15) Tahchiev, Galin ValentinovHurricanes Ivan, Katrina, and Rita tracked through a high-density corridor of the oil and gas infrastructures in the Gulf of Mexico. Extreme winds and large waves exceeding the 100-year design criteria of the MODUs during these hurricanes, caused the failure of mooring lines to a number of Mobile Offshore Drilling Units (MODUs) in the Gulf of Mexico. In addition to the damage MODUs undertook during these severe hurricanes, drifting MODUs might impose a great danger to other critical elements of the oil and gas industry. Drifting MODUs may potentially collide with fixed or floating platforms and transportation hubs or rupture pipelines by dragging anchors over the seabed. Therefore, it is desirable to understand the physics of the drift of a MODU under the impact of severe wind, wave, and current and have the capabilities to predict the trajectory of a MODU that is drifting. In this thesis, a numerical program, named ?DRIFT,? is developed for predicting the trajectory of drifting MODUs given met-ocean conditions (wind, current, and wave) and the characteristics of the MODU. To verify ?DRIFT,? the predicted drift of two typical MODUs is compared with the corresponding measured trajectory recorded by Global Positioning System (GPS). To explore the feasibility and accuracy of predicting the trajectory of a drifting MODU based on real-time or hindcast met-ocean conditions and limited knowledge of the condition of the drift, this study employed a simplified equation describing only the horizontal (surge, sway, and yaw) motions of a MODU under the impact of steady wind, current, and wave forces. The simplified hydrodynamic model neglects the first- and second-order oscillatory wave forces, unsteady wind forces, wave drift damping, and the effects of body oscillation on the steady wind and current forces. It was assumed that the net effects of the oscillatory forces on the steady motion are insignificant. Two types of MODU drift predictions are compared with the corresponding measured trajectories: 1) MODU drift prediction with 30-minute corrections of the trajectory (every 30 minutes the simulation of the drift starts from the measured trajectory), and 2) continuous MODU drift prediction without correction.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.Item Numerical Study On A SPAR Type Floating Offshore Wind Turbine Using COUPLE-FAST Code(2015-03-03) Peng, ChengFloating offshore wind turbine (FOWT) attracts more and more attention for harnessing wind power over the surface of relatively deep ocean water, where steady and strong wind occurs. Although it has been shown that the knowledge gained from the development of floating platforms for oil and gas production is helpful for the development of FOWTs, it alone is insufficient for understanding dynamic interactions between the supporting platform and the wind turbine. Therefore, it is desirable to conduct numerical simulations of a FOWT under the impact of different combinations of winds, waves and currents. In this study, a numerical code named as COUPLE-FAST has been developed to investigate the motions of a selected FOWT and the tensions in its mooring lines. The selected FOWT mainly consists of a 5MW NREL wind turbine and OC3-Hywind Spar support platform. COUPLE-FAST is made based two existing codes COUPLE and FAST. The former is an in-house code developed and being continuously expanded for the simulation of an offshore floating platform positioned by a mooring-line/tendon system. FAST is an open-source code capable of predicting both the extreme and fatigue loads of two- and three- bladed horizontal-axis wind turbines [1]. In COUPLE-FAST, COUPLE module is used to calculate the external loads on the support floating platform, mooring line forces and its motions, and FAST module to calculate the aerodynamic loads and flexible responses of the wind turbine. The displacements, velocities and accelerations predicted in COUPLE are transferred to FAST. The forces at the tower bases calculated by FAST are transferred to COUPLE. Total 25 cases with different combination of winds, waves and currents are simulated for calculating the motions of the FOWT and tensions in its mooring lines. Among many interesting observations made based on these simulations, it is confirmed that when the mean wind speed is above the rated wind speed the blade pitch control system may induce resonant interaction (also known as ?negative damping?) between the surge of the FOWT and dynamic wind loads induced by the adjustment of blade pitch angle. However, the resonant effects on surge of the FOWT in the case of turbulent winds are not as significant as in the case of steady winds of the corresponding wind speed.Item Numerical Study on Transverse Friction of a Slender Rod Contacting the Seabed(2012-10-19) Lu, HangWith the increasing developments of exploiting oil and natural gas in deep water and harnessing renewable (wave and wind) energy in the sea, mooring lines and risers are widely deployed to position the related floating structures. Subject to environmental loads, a mooring line or riser connected to floating structure, moves up and down, back and forth, and sometimes from the left to the right. In computation of the dynamics of a mooring line or riser, it is often modeled as a flexible slender rod. While the bending moment of a chain or a rope is neglected, that of a riser is considered and specified by characteristics of the riser. Existing numerical codes for simulating the dynamics of a slender rod, such as CABLE3D, allow for the vertical support force and longitudinal (along the direction of the rod) friction from soils of the seabed while the transverse (in the direction transverse to the slender rod) friction between the rod and the seabed soils is not considered. In this study, we extend the current version of CABLE3D to allow for the transverse friction applied on the portion of a slender rod contacting the seabed soil, which is time-varying when it is moving. The friction between a slender rod and the seabed soil is computed based upon a Coulomb model originally developed for the simulation of the friction in all dry contact mechanical systems. In applying the Coulomb model, the transverse friction depends on the transverse displacement and/or velocity of a slender rod contacting the seabed. In addition, vertical bottom support of the seabed soil is calculated based on the shear stress of the seabed soil. The simulations of the dynamics of a few typical mooring lines are made given their motions at their fairleads and the results are compared with the corresponding results obtained using Orcaflex, a commercial code, and the existing version of CABLE3D.Item Simulated Fatigue Damage Index on Mooring Lines of a Gulf of Mexico Truss Spar Determined from Recorded Field Data(2012-07-16) Kiecke, Adam FullerThe Constitution Truss Spar, operated by Anadarko Petroleum Corporation (APC), is located in Green Canyon Block 679 and 680 in a water depth of 1,500 m. It was installed in October of 2006 and has since weathered multiple hurricanes and other storms. The platform is equipped with an Environmental Platform Response Monitoring System (EPRMS) which records real-time motions, environmental parameters and loads. These measurements were used to hind-cast the platform mooring tensions and estimate fatigue damage index accrued over the short life (install to start of study, July 2010) of the platform. The study found that extreme events such as Hurricane Ike (~100 yr storm) accounted for considerably higher fatigue damage index than the total caused by other small storms likely to occur in the 20 year service life of the vessel. It is therefore a recommendation of this study that a design criterion for fatigue damage accrued during extreme events such as 100 yr hurricanes be considered in the design of station keeping systems in a similar manner to the guidelines found in API RP 2T (2010) for design of tension leg platforms.Item Static Stability of Tension Leg Platforms(2010-07-14) Xu, NingThe static stability of a Tension Leg Platform (TLP) with an intact tendon system is principally provided by its tendons and hence quite different from those of a conventional ship or even a floating structure positioned by its mooring system. Because small deformations in tendons are capable of providing sufficient righting moment to a TLP, the contribution from the inclination of its hull is relatively insignificant, especially when its tendon system is intact. When the tendon system of a TLP is completely damaged, the static stability of a TLP behaves and is calculated in a similar manner as those of a conventional ship. In the case of a TLP with a partially damaged tendon system, the stability of a TLP may be provided by the deformation of its tendons and to a certain extent the inclination of its hull. Several hurricanes in recent years have raised concerns about the feasibility and the robustness of the TLP concept in the deep water Gulf of Mexico. To the best of our knowledge, existing publications on the research of static stability of TLPs are limited. This study investigates the static stability of different types of TLPs representing those deployed in the Gulf of Mexico, under three different scenarios. That is, a TLP with 1) an intact tendon system, 2) a partially damaged tendon system, and 3) a completely damaged tendon system. The four different types of TLP chosen for this study are 1) a conventional four-leg TLP, 2) three-leg mini TLP, 3) extended four-leg TLP and 4) mini four-leg TLP. To avoid buckling and yielding occurring in a tendon, we define that the maximum righting moment provided by an intact or partially damaged tendon system is reached when the tension in one or more tendons on the down tension leg becomes zero or when the tension in one or more tendons on the up tension leg starts to yield. This definition leads us to identify the most dangerous (or vulnerable) directions of met-ocean conditions to a TLP with an intact or partially damaged tendon system. Hence, our finding may also be used in the study on the pitch/roll dynamic stability of a TLP. The righting moments of each TLP in the three different scenarios are respectively computed and compared with related wind-induce static upsetting moment at certain velocities. By comparing their ratios, the static stability of a TLP and the redundancy of its tendon system may be revealed, which has important implication to the design of a TLP.Item Statistics of Met-Ocean Conditions Between West and Central Gulf of Mexico Based on Field Measurements(2012-07-16) Su, LinStatistics of met-ocean conditions including wind, current and wave at the location between west and central Gulf of Mexico (GOM) are derived based on about three year of field measurements. Two-parameter Weibull distribution has been employed to fit wind speed at 10m over sea level and current speed in various depth. The joint probability contour was derived based on First-Order Reliability Method. In addition, the joint distribution of wind speed and direction was visualized by wind-rose diagram. The results provided in this study may provide essential information to the probability distribution of met-ocean condition in the particular location and can be used as a reference in the future designs.Item Studies on Dynamics of Suction Piles during Their Lowering Operations(2010-10-12) Huang, LiqingSuction piles are used for anchoring the mooring lines at the seafloor. One of the challenges of their installing is the occurrence of the heave resonance of the pile-cable system and possibly the heave induced pitch resonance during the lowering process. When the heave and/or pitch frequency of the vessel which operates the lowering of the pile matches the heave natural frequency of the pile-cable system, the heave resonance may occur, resulting in large heave oscillations of the pile and thus significantly increasing loads on the lowering cable and lowering devices. Furthermore, the large heave may resonantly induce the pitch of a pile. To predict and possibly mitigate the heave/pitch resonance of the pile-cable system during the lowering process, it is crucial to under the mechanism of heave induced pitch resonance and estimate the added-mass and damping coefficients of the pile-cable system accurately. The model tests of the forced heave excitation of pile models were first conducted to investigate the added-mass coefficient for a pile model with different opening area ratios at its top cap at the Haynes Coastal Engineering Laboratory of Texas AandM University. In the model tests, it was observed that the resonant heave may occur if the heave excitation frequency matches the related heave natural frequency and the pitch resonance may be induced by the heave resonance. The results of the following theoretical analysis and numerical simulation of the heave excitation of the pile-cable system are found to be consistent with the related measurements, which is helpful to further understand the physics of lowering a pile-cable system. The results of this study may be used to determine the magnitudes of total heave added-mass and damping coefficient of a pile and the heave natural frequency of the pile-cable system based upon its main characteristics. The heave induced resonant pitch is found to occur when 1) the pitch natural frequency is roughly equal to one half of the heave natural frequency and 2) the heave excitation frequency is approximately equal to the heave natural frequency. If only one of the two conditions is satisfied, no significant pitch resonance will occur. These results may have important implications to the operation of lowering offshore equipment to the seafloor in deep water.Item Study of the Fatigue Life of Steel Catenary Risers in Interaction with the Seabed(2011-02-22) Nakhaee, AliThe fatigue life of a Steel Catenary Riser (SCR) near its touch-down zone is substantially affected by its interaction with the seabed. Hence, accurate estimate of its fatigue life requires the understanding and realistic modeling of the interaction between them. The interaction between SCR and the seabed depends on many factors, such as soil properties, riser characteristics, and the development of trenching at the seafloor. Existing approaches for modeling the seabed in interaction with a SCR approximate the seabed soil by a linear or nonlinear spring and a dashpot which respectively represent the stiffness and damping of the soil. However, they do not take into account certain phenomena resulting from plastic deformation or degradation of the seabed soil, such as trenching. In this study, a more realistic approach is developed for simulating the interaction between a SCR and the seabed soil. In addition to the use of a realistic P-y curve (where P stands for the supporting or resistance force of the seafloor and y for the vertical penetration of the riser into the soil) to simulate the soil deformation during its interaction with a riser, it considers the development of a trench caused by continuous poundings of a riser on the seabed and then its feedback effect on the variation of the bending moment along the riser. In this study, it has been found that trenching underneath a SCR may decrease the maximum variation of bending moment near its touch-down zone. Since the variation of the moment dictates the fatigue damage to the SCR, the results based on this approach indicate that the trenching development at the seabed may increase the fatigue life of a SCR and therefore, it may have important application to the design of a SCR.