A 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.

The conditions of a moored container ship are examined by a physical model in a wave basin and by a numerical simulation. Each condition, wave period, significant wave height and wave direction, was isolated and tested for a 50:1 scale model of a 710 ft ship and model dock. The dock construction, solid sheet wall or pile supported, was varied to add another aspect of a moored vessel. Mooring lines were modeled using 14 springs in typical mooring line arrangement to simulate the elastic properties. Loads experienced on mooring lines and fenders during different wave conditions were recorded using in line force transducers. Each wave property increased the loads on the mooring lines and fenders as it intensified, except in few conditions. The loads throughout the ship also decreased for the tests run with a pile constructed dock. The bow line received the greatest load and the greatest range of loads of all the lines. The greatest average load was 175 kips experienced by the bow line during a 20 second period, 6 feet wave coming perpendicular to the ship. The results of the solid wall dock setup were compared to the results from the numerical simulation data, aNySIM. Numerical results showed similar trends as the experimental but at a lower magnitude, with a maximum percent difference of 36%.

The nonlinear response of an ocean system subjected to random excitations can exhibit very complex dynamic behaviors including jump phenomena and coexistence of attractors. In this study, the stochastic system response behavior of a simple (Duffing) oscillator under narrow-band random excitations is first examined in the subharmonic resonance region. A semi-analytical procedure based on the nonlinear response characteristics of the corresponding deterministic system is developed to derive the response transition probabilities within individual attraction domains and among finite attraction domains under the assumptions of stationarity and Markov process. Overall response amplitude probability distributions are obtained by applying the Bayes formula to the two different types of response transition probability distributions. To validate the prediction capability of the semi-analytical method, numerical simulation of the responses of the Duffing system are generated and statistical characteristics of the response behavior are compared with prediction results. It is shown that the semi-analytical procedure provides more accurate predictions than other approximate methods available in the literature. A parametric study on the effects of variations in excitation intensity and degree of narrow-bandedness is conducted. Results confirmed that the nonlinear response characteristics including jump phenomenon and co-existence of attraction domains are preserved under narrow-band random excitations. The semi-analytical prediction method developed above is then applied to analyze the stochastic response behavior of a nonlinear mooring system subjected to random ocean waves. For modeling of the structural system, a nonlinear�� structure, nonlinearly-damped (NSND) model is employed and a reverse multiple�� input/single-output technique is applied to identify the system coefficients. To verify the accuracy and capability of the semi-analytical method in predicting the complex behaviors of the nonlinear mooring system, analytical predictions are compared with experimental results and numerical simulations. System response amplitude probability distributions predicted by the semi-analytical procedure are shown to be in good agreement with experimental and simulation results. / Graduation date: 2001

Verifying the design of floating structures adequately requires both numerical simulations and model testing, a combination of which is referred to as the hybrid method of design verification. The challenge in direct scaling of moorings for model tests is the depth and spatial limitations in wave basins. It is therefore important to design and build equivalent mooring systems to ensure that the static properties (global restoring forces and global stiffness) of the prototype floater are matched by those of the model in the wave basin prior to testing. A fit-for-purpose numerical tool called STAMOORSYS is developed in this research for the design of statically equivalent deepwater mooring systems. The elastic catenary equations are derived and applied with efficient algorithm to obtain local and global static equilibrium solutions. A unique design page in STAMOORSYS is used to manually optimize the system properties in search of a match in global restoring forces and global stiffness. Up to eight mooring lines can be used in analyses and all lines have the same properties. STAMOORSYS is validated for single-line mooring analysis using LINANL and Orcaflex, and for global mooring analysis using MOORANL and Orcaflex. A statically equivalent deepwater mooring system for a representative structure that could be tested in the Offshore Technology Research Center at Texas A&M University is then designed using STAMOORSYS and the results are discussed.

Due to the complexity of body-shape, the investigation of hydrodynamic forces on mooring lines, especially those comprised of chain segments, has not been conducted to a sufficient degree to properly characterize the hydrodynamic damping effect of mooring lines on the global motions of a moored offshore platform. In the present study, an experimental investigation of the hydrodynamic characteristics of various mooring elements is implemented through free and forced oscillation tests. Since no direct measurement capability for distributed hydrodynamic forces acting on mooring line segments such as chain and wire rope is available yet, an indirect measurement technique is introduced. The technique is based on the fact that hydrodynamic forces acting on a body oscillating in still water and on a stationary body in an oscillatory flow are equivalent except for the additional inertia force, the so-called Froude-Krylov force, present in the latter condition. The time-dependent displacement of a slender body moving in calm water is acquired through optical tracking with a high speed camera. The distributed hydrodynamic measurements are then used to obtain the force by solving the equation of motion with the boundary condition provided from tension measurements. Morison’s equation is employed along with Fourier analysis to separate the inertia and drag components out of the total fluid force. Given the experimentally-derived information on hydrodynamic behavior, the resistance provided by a mooring line to a floating structure is briefly studied in terms of damping and restoring force in a coupled dynamic system.

With 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.

A 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.

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

For oblique and normal water waves at various frequencies, the use of moored cylinders as breakwaters is considered numerically using linear three-dimensional analysis. The breakwater can be used by itself for protection of small structures or as a series of cylinders to protect a harbor, shoreline, or moored vessel from the destructive energy associated with incident water waves. The breakwater is completely submerged below the free surface and is attached to the ocean floor with six symmetrically configured mooring lines. The cylinder is filled with air and the mooring lines remain taut during the structure's motion. Six degrees of freedom describe the motion of the breakwater and additional degrees of freedom are introduced as the cables are modeled with the use of lumped masses connected with springs. The fluid is assumed to be inviscid and incompressible, so that the velocity field can be written as the gradient of the velocity potential. A boundary integral method is used to solve the integral equations that define the external fluid flow. Free vibrations of the cylinder in both air and water are considered and "dry " and "wet" natural frequencies are computed. Motions caused by water waves are studied to establish the effect of certain parameters on the effectiveness of the breakwater. The transmission coefficient is shown to be somewhat misleading when compared to plots that show the spatial variation of the wave amplitude. / Master of Science