The mooring of vessels and other floating bodies at sea, such as offshore platforms has necessitated the development of specialised moorings technology. The marine renewable energy (MRE) sector is now at a stage in its development whereby floating devices are adding new challenges to the moorings industries. Floating MRE devices are smaller than, for instance offshore platforms, and are usually targeted for deployment in highly energetic environments. The extreme conditions and the highly dynamic response of an MRE device present challenges in terms of peak loading within the mooring system itself and load transfer to the floating body. Compliant mooring systems provide advantages by reducing the peak loads and fibre ropes are an important asset in achieving such compliance. However, the extent to which existing fibre ropes can safely extend axially to provide compliance is insufficient and is strongly associated to the minimum breaking load (MBL) of the rope. A novel fibre rope mooring tether is presented here that provides advantages over existing ropes. The tether employs a hollow fibre rope containing an elastomeric core, this mechanism de-coupling the extension properties from the strength of the line. The load path is carried through the polyester rope which is terminated conventionally by eye splices, thus minimising any new risks to reliability. Very low axial stiffness is achieved and is shown to be selectable within limits. For comparison, the prototype tether’s MBL of 222 kN is assigned to polyester and Nylon reference ropes. The axial stiifness of these ropes are 590 kN and 463 kN respectively when measured by a secant between the origin and 30% MBL; the novel tether displays an axial stiffness of 72 kN by the same method. This enables the novel tether to achieve more than two and a half times the extension of a comparable Nylon rope in its working range. Numerical modelling of a moored installation demonstrates a threefold reduction in peak load magnitude compared to the existing Nylon rope solution. The tether exhibits two distinct stages of extension, the first having very low axial stiffness. It is demonstrated that the extent of this soft phase can be selected by design and that this might add another useful element of control to moorings design work.

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Marine Renewable Energy (MRE) is a promising source of energy for the future. However, it is still under development and many challenges need to be overcome to develop competitive solutions. While the design of the station keeping system of traditional offshore oil and gas structures is driven mainly by their low frequency motions, MRE devices are installed at nearshore locations and move dynamically. Because of these criteria, MRE mooring systems require novel mooring systems and associated standards. MRE mooring standards need to take into account the highly dynamic behaviour of these systems, which can lead to large mooring loads. The nature of these loads needs to be investigated to improve the confidence in mooring design and to improve cost-effectiveness. The aim of this thesis is to develop the understanding of peak mooring loads on highly dynamic mooring systems, in particular, the environmental conditions associated with the loads. In addition, preliminary research into the response of the mooring systems to environmental conditions is presented. Both field tests and tank tests have been conducted. Field tests give insight into the behaviour of a dynamic mooring system in real sea conditions. Measuring the mooring loads and the environmental conditions - wave, and current if available – for several months, a methodology has been developed to detect peak mooring loads and identify the associated environmental conditions in order to compare them with the environmental conditions recorded throughout the field tests. The principal finding is that peak mooring loads occur for sea states with large but not always the highest significant wave height HS. The understanding of the effect of tidal conditions on peak mooring loads requires further work. A tank test of a dynamic mooring system in moderate sea states has been conducted to observe the dynamic behaviour of the mooring system. Tank tests enable detailed observations of the dynamic behaviour of a system in a well controlled environment and allow the calibration of a numerical model. The model can be used to investigate separate physical parameters. The results from this thesis will assist in the development of specific standards for MRE mooring systems. These standards are essential for the evolution of the MRE industry.

The 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.

Abstract The object of this study is to investigate the flow induced vibration of cable structures. Two various methods, Eigenvalue method and Cable dynamics method, are used to evaluate, the natural frequencies of structures. Vortex shedding frequencies are introduced in order to compare with the natural frequencies mentioned above. The results determine whether the phenomenon of resonance or Lock-in occur. This study has four parts. The first part is the dynamic analysis of an underwater cable Depending on the various boundary conditions, the study discusses, the system of towing cable and mooring cable in which the maximum horizontal displacement and the maximum dynamic response amplitude are found in different ocean environment. The second part is a linear Eigenvalue analysis for natural frequencies of cable structures. The third part compare the results based on the two methods , Cable dynamics and Eigenvalue method. The former obtain tension results that are important to determine the natural frequencies of structures by theoretical formula. The results of natural frequencies from the letter are compared to those in the former. The fourth part is mainly to calculate vortex shedding frequencies resulting in the relative motion between structures and fluid. The final results found in the fourth part are necessary to compare with part three, so as to determine whether the resonance or Lock-in occur.

Mooring systems for floating facilities that are used offshore to produce oil and gas, consisting of individual mooring lines and foundations, are currently designed on the basis of individual components and on a case-by-case basis. The most heavily loaded line and anchor are checked under extreme loading conditions (hurricane and loop current) with the system of lines intact and with one line removed. However, the performance of the entire mooring system depends more directly on the performance of the system of lines and foundations rather than on the performance of a single component. In this study, a floating production system design originally developed by the industry consortium, DeepStar, was chosen for study. The mooring system was designed for three different nominal water depths: 1000, 2000 and 3000 m. It is a classic spar with steel mooring lines in 1000 m of water and polyester mooring lines in deeper depths. Based on simulated results of loads on mooring lines and foundations using a numerical model, reliability analyses were conducted using representative probabilistic descriptions of the extreme met-ocean conditions, hurricanes and loop currents, in the Gulf of Mexico. The probability of failure of individual mooring line components during a 20-year design life is calculated first, followed by that of a complete mooring line which consists of top and bottom chains, a steel cable or polyester rope at the middle and a suction caisson foundation, and finally that of the mooring system. It is found that foundations have failure probabilities that are more than an order of magnitude smaller than those for lines under extreme loading. Mooring systems exhibit redundancy in that the failure of the most heavily loaded component during an extreme event does not necessarily lead to failure of the system. The system reliability and redundancy are greater for the taut versus semi-taut systems and is greater for designs governed by loop current versus hurricane events. Although this study concerns about the mooring systems of a classical spar, the methodology of the reliability analysis and the conclusions made in this study may have important implications to the other deepwater mooring systems / text

Floating offshore wind turbines have the potential to become a significant source of affordable renewable energy. However, their strong interactions with both wind- and wave-induced forces raise a number of technical challenges in both modelling and design. This thesis takes aim at some of those challenges. One of the most uncertain modelling areas is the mooring line dynamics, for which quasi-static models that neglect hydrodynamic forces and mooring line inertia are commonly used. The consequences of using these quasi-static mooring line models as opposed to physically-realistic dynamic mooring line models was studied through a suite of comparison tests performed on three floating turbine designs using test cases incorporating both steady and stochastic wind and wave conditions. To perform this comparison, a dynamic finite-element mooring line model was coupled to the floating wind turbine simulator FAST. The results of the comparison study indicate the need for higher-fidelity dynamic mooring models for all but the most stable support structure configurations. %It was also observed that small inaccuracies in the platform motion time series introduced by a quasi-static mooring model can cause much larger inaccuracies in the time series of the rotor blade dynamics. Industry consensus on an optimal floating wind turbine configuration is inhibited by the complex support structure design problem; it is difficult to parameterize the full range of design options and intuitive tools for navigating the design space are lacking. The notion of an alternative, ``hydrodynamics-based'' optimization approach, which would abstract details of the platform geometry and deal instead with hydrodynamic performance coefficients, was proposed as a way to obtain a more extensive and intuitive exploration of the design space. A basis function approach, which represents the design space by linearly combining the hydrodynamic performance coefficients of a diverse set of basis platform geometries, was developed as the most straightforward means to that end. Candidate designs were evaluated in the frequency domain using linearized coefficients for the wind turbine, platform, and mooring system dynamics, with the platform hydrodynamic coefficients calculated according to linear hydrodynamic theory. Results obtained for two mooring systems demonstrate that the approach captures the basic nature of the design space, but further investigation revealed limitations on the physical interpretability of linearly-combined basis platform coefficients.. A different approach was then taken for exploring the design space: a genetic algorithm-based optimization framework. Using a nine-variable support structure parameterization, this framework is able to span a greater extent of the design space than previous approaches in the literature. With a frequency-domain dynamics model that includes linearized viscous drag forces on the structure and linearized mooring forces, it provides a good treatment of the important physical considerations while still being computationally efficient. The genetic algorithm optimization approach provides a unique ability to visualize the design space. Application of the framework to a hypothetical scenario demonstrates the framework's effectiveness and identifies multiple local optima in the design space -- some of conventional configurations and others more unusual. By optimizing to minimize both support structure cost and root-mean-square nacelle acceleration, and plotting the design exploration in terms of these quantities, a Pareto front can be seen. Clear trends are visible in the designs as one moves along the front: designs with three outer cylinders are best below a cost of \$6M, designs with six outer cylinders are best above a cost of \$6M, and heave plate size increases with support structure cost. The complexity and unconventional configuration of the Pareto optimal designs may indicate a need for improvement in the framework's cost model. / Graduate / 0548 / mtjhall@uvic.ca

Previous work on modal analysis of mooring lines has been performed from different theoretical formulations. Most studies have focused on mooring lines of a single homogeneous material, and the effect of added mass and damping produced by the water has not been examined deeply. The variational formulation approach, employed in this research to perform a modal analysis, has been useful to study the behavior of several realistic mooring lines. The cases presented are composed from segments of materials with different mechanical characteristics, more similar to those in current offshore projects. In the newly proposed formulation, damping produced by transverse motion of the mooring line through the surrounding water has been added to the modal analysis. The modal analysis formulation applied in this work has been verified with calculations from commercial software and the results are sufficiently accurate to understand the global behavior of the dynamics of mooring lines with the damping produced by the sea water. Inclusion of linearized drag damping in the modal analysis showed that the modal periods of the mooring systems studied depend on the amplitude of the transverse motion of the mooring line. When more amplitude in the motion is expected more damping is obtained. Two realistic designs of mooring lines were compared: one made up with a main insert of steel rope, called “Steel System”, and one composed by a main insert of polyester, named “Polyester System”. Comparing the natural periods of both systems, the Steel System appears to be safer because its fundamental natural period is more distant from the wave excitation periods produced by storms. The same happens considering the wave excitation periods produced by prevailing seas. In this case the natural periods of the Polyester System are nearer to the wave excitation periods causing fatigue loads. The transverse mode shapes for lateral motions of the mooring lines are observed to be continuous and smooth across material transitions, such as transitions between chain and wire rope and transitions between chain and polyester rope. This behavior is not always observed in the tangential mode shapes for the Polyester System where significant differences in dynamic tension seem to be present in the specific cases studied.

Thesis (Masters Diploma (Civil Engineering)--Cape Technikon, Cape Town, 1987 / Between December 1982 and January 1984 the berthing impacts o~ 50 ore carriers were monitored at the Saldanha Bay ore-jetty. The actual displacement o~ the monitored vessels ranged between 60 and 263 kilotonnes. Only 507. o~ the monitoring runs yielded complete sets o~ data. Approach velocities recorded were high and the design limits were exceeded several times resulting in ~enders being de~lated on ~our occasions. The added mass coe~~icient ~or each impact was calculated. The actual values o~ C" varied between 1 and 7. This agrees with values ~ound in literature. Some o~ the\high values could be attributed to inaccuracies in the measurement techniques. From the values obtained ~or added mass it seems that the value used in the design was very low and that a unitary value ~or CH is not very satisfactory.

The Virginia Tech self-mooring autonomous underwater vehicle (AUV) is capable of mooring itself on the seafloor for extended periods of time. The AUV is intended to travel to a desired mooring location, moor itself on the seafloor, and then release the mooring and return to a desired egress location. The AUV is designed to be an inexpensive sensor platform. The AUV utilizes a false nose that doubles as an anchor. The anchor is neutrally buoyant when attached to the AUV nose. When the vehicle moors it releases the false nose, which floods the anchor making it heavy, sinking both the anchor and AUV to the seafloor. At the end of the mooring time the vehicle releases the anchor line and travels to the recovery location. A prototype vehicle was constructed from a small-scale platform known as the Virginia Tech 475 AUV and used to test the self-mooring concept. The final self-mooring AUV was then constructed to perform the entire long duration mission. The final vehicle was tested successfully for an abbreviated mission profile. This report covers the general design elements of the self-mooring AUV, the detailed design of both the prototype and final AUVs, and the results of successful field trials with both vehicles. / Master of Science