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An investigation on the motion responses of a Floating Production, Storage and Offloading (FPSO) vessel moored in irregular waves has been carried out based on both frequency- and time-domain approaches. In the frequency-domain approach a three-dimensional panel method was employed in order to calculate the first-order hydrodynamic forces and moments such as added masses, potential damping and wave excitation forces and moments and of the resulting the first-order motions and mean second-order forces and moments on the vessel in six degrees of freedom behaviour. A spectral analysis was carried out in order to estimate both the significant and the extreme values of the first-order motions. Additionally Pinkster’s approximation was used to find the mean-square values of slow drift motions, in order to calculate wave-induced extreme excursions and the resulting tensions on the mooring lines of the vessel. Two different methods were used in the time-domain approach for undertaking a mooring analysis. One method used a fast practical time-domain technique that calculates the first-order motion responses in random waves based on the frequency-domain response amplitudes and simulated seas, and also solves the uncoupled second-order motion responses of the FPSO induced by second-order forces, based on Newman’s approximation in irregular seas. The other method is by solving six coupled equations of motion based retardation functions transformed from potential damping for the FPSO and induced by the first-order and second-order wave excitations in random seas. The results of the wave-induced extreme excursions and the mooring line tensions obtained by means of the frequency- and time-domain methods are compared and discussed. As the selected FPSO is operating in deep water, the effect of the mooring line inertia may be significant. The equations of motion of line dynamics were formulated and numerically solved to investigate the importance of line dynamics for deep water mooring. Comparisons between the results of the line tensions both with and without the effects of line dynamics are made and discussed.

Offshore structures are exposed to random wave loading in the ocean environment and hence the (long-term) probability distribution of their extreme responses to wave loading is of great value in the design of these structures. Due to nonlinearity of the drag component of Morison wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, I simple techniques for derivation of the probability distributions of the extreme responses are not available. Conventional time simulation (CTS) method is a convenient technique to achieve this objective as it is capable of accounting for various nonlinearities. The main shortcoming of the CTS method is that it is computationally very demanding as reliable estimates of an extreme event with a very low probability of exceedence require extensive simulations to reduce the sampling variability to acceptable values. The purpose of this project is to replace the complex offshore structural system with a much simpler system to make simulations much less costly; consequently, reliable estimates of extreme events can be made for design purposes. Finite-memory nonlinear systems (FMNS) are extensively used in establishing a simple relationship between the output and input of complicated nonlinear systems. This thesis is devoted to the development of an equivalent finite-memory nonlinear system for efficient prediction of the response of an offshore structure to (random) Morison wave loading. For validation, responses from the equivalent FMNS model have been compared with corresponding responses from the CTS procedure in the time, frequency and probability domains. In particular, the 100-year responses from the FMNS and CTS methods have been compared. Overall, 216 different cases have been investigated consisting of six responses (drag-induced, inertia-induced and total base shear together with drag-induced, inertia-induced and total overturning moment), four different structures (quasi-static, JCP2, JCP5 and JCP8), three different sea states (H, = I5m, 10m and 5rn), and finally, three different current situations (zero, positive and negative currents = ±0.9m/sec). This was necessary to ensure that the conclusions of this study are comprehensive and have wide application. JCP2, JCPS and JCP8 refer to three test structures with first mode natural frequencies of0.40Hz, 0.1 9Hz and O.l2Hz, respectively. The dynamic effect on the responses of the JCP2 structure is relatively small. On the other hand, the dynamic effects for JCPS and JCP8 responses are moderate and large, respectively. It should, however, be considered that the sea surface is not stationary and that it can best be represented by a large number of sea states, each having its own specific probability of occurrence. It is, therefore, the 100- year responses derived from the long-term (accounting for the effect of all the sea ; states at the site of the structure) distribution of the extreme responses which is required for probabilistic analysis of offshore structures. It was observed that the 100- year responses from the FMNS method (and a more accurate variation of it) are accurate within a few percent of its value from the less efficient CTS method. Linear random wave theory (LRWT) is a generally acceptable method for determining water particle kinematics below mean water level (MWL) as it is found to predict sensible kinematics. However, water particle kinematics at points above MWL, calculated from LRWT, suffer from unrealistically large high-frequency components. A number of empirical techniques have been suggested to provide a more realistic representation of near surface wave kinematics. Each of these methods is intended to calculate sensible kinematics above the MWL, yet they have been found to differ from one another in the results yielded. Although it is well known that different methods of simulating water particle kinematics lead to different values of extreme responses, no systematic study has been conducted to investigate their effect on the magnitude of the l00-year responses, which are required for design. Using conventional time simulation method, it has been shown that the Wheeler and the vertical stretching methods, both popular in the industry, lead to significantly different estimates of the l00-year responses. The ratio between l00-year responses from the Wheeler and the vertical stretching methods has been found to be as low as 0.66 in some cases. It is, therefore, desirable to come up with a method that resolves this problem. To this end, two new techniques, i.e. the effective node elevation and the effective water depth methods, have been introduced in this study. Water particle kinematics in the near surface zone from the effective node elevation and the effective water depth methods, lie between those from the Wheeler and the vertical stretching methods. This is promising as there is some evidence that the water particle kinematics under crests from the Wheeler method are underestimated and that those from the vertical stretching method are somewhat exaggerated. Furthermore, it has been shown that the effective node and the effective water depth procedures lead to lOO-year responses which lie between those predicted from the Wheeler and the vertical stretching methods, and hence may be more suitable for design. However, further research-is required to determine which method is more appropriate. The foregoing ratios between l00-year responses were also calculated by the FMNS method to demonstrate that both CTS and MFMNS methods lead to similar results and conclusions. The FMNS is, however, much more efficient than the CTS method, and therefore, can pave the way for comprehensive parametric studies such as investigating the effect of leg spacing and natural frequency on the magnitude of the l00-year responses. This will pave the way for the optimal design of offshore structures.

Offshore structures are subject to a wide variety of environmental loads (such as wind, wave and current) all of which exhibit a high degree of statistical uncertainty. The dominant load, however, is normally due to wind-generated random waves. Probabilistic procedures can account for these uncertainties by establishing the statistical properties of loads and responses and hence are necessary for risk-based assessment of these structures. The major obstacle in establishing the probabilistic properties of the response is the nonlinearity of the wave load mechanism and/or the structural system which leads to non-Gaussian distribution for response. The problem is further compounded by current and by intermittent loading on members in the splash zone, which have a significant effect on the statistical properties of response. The most versatile and the most reliable technique for predicting the statistical properties of the response of an offshore structure to random wave loading is the time simulation technique. However, this technique requires very long simulations in order to reduce the sampling variability to acceptable levels. If this technique can be speeded up, then it would be an ideal technique because it does not suffer from any of the inadequacies of the alternative methods and is applicable to all different types of structures. This research is concerned with reducing the computational demand of the conventional time simulation (CTS) method. To this end, a more efficient version of the time simulation technique (ETS) has been introduced which divides the simulated response extreme values into a number of groups based on the magnitude of the extreme values of their associated surface elevation or linear response records. The probability distribution of extreme responses for each group is then calculated individually based on a relatively small number of simulations, and then the total probability theorem is used to derive the probability distribution of response extreme values. In this study, the probability distribution of response extreme values together with the 100-year responses from the ETS models have been compared with corresponding distributions and lOO-year responses from the CTS procedure to examine the accuracy and the efficiency of the efficient time simulation technique. Overall, four different structures, three different sea states and three different current situations have been investigated. This was necessary to ensure that the conclusions of this study are valid for a broad range of conditions. The ETS technique was found to be many times more efficient than the CTS method.

The demand for floating offshore platforms has been increased dramatically in the last decade as the sectors for oil/gas exploration and production are moving into remote areas and deeper waters. Semi-submersible platforms are floating offshore systems which are designed to have very low vertical motion responses in seaways. Over the last several years their popularity has rebounded due to the demand from industry. The MultiPurpose Semi-Submersible concept could be considered as one of the preferred floating offshore units in the near future because of considerably acceptable its motion performances and the competitive design for productivity. This thesis describes effective design evaluation methods and tools for the Multi-Purpose Semi-Submersible platform suitable for use at a preliminary design stage in hydro-structural aspects together with the validation of the proposed design by means of the assessment of the safety against design requirements and failure due to excessive maximum load effects.

The prediction of Vortex Induced Motion (VIM) of compliant offshore structures such as Spars is one of the most challenging areas in the offshore industry. It has been observed that though, numerous experimental and numerical studies have been conducted in an effort to improve the understanding of this phenomenon, the outcomes still often appear incoherent. Models and data, where available, are often proprietary and understandably being kept confidential (because of the cost involved). Presented here is a new Semi-Empirical Method, a wake oscillator model derived assuming span-wise constant flow velocity, in which attempts were made to replicate or capture the flow phenomena observed in VIV experiments. The model involves coupling, through a displacement parameter, two non-linear equations namely, the Duffing equation and the Van der Pol equation. Steady state harmonic solutions were sought for the resulting system of non-autonomous nonlinear differential equations through Multiple Scale (Many- Variable/Derivative Expansion Procedure approach) Perturbation theory, an asymptotic method developed by Nayfeh (1968). Three scenarios usually encountered in VIV were examined in this work, (1) for a Cylinder mounted rigidly, (2) for a Cylinder mounted elastically [i.e. free], and (3) for a Cylinder mounted elastically and forced externally. Of most interest is the unforced rigid cylinder mounted elastically, as it forms the basis of VIV/VIM problem formulation for floating structures such as Classical and Truss Spars. Among other analysis carried out, the new model was used to investigate the effect of nonlinearity in the stiffness coefficient of the system of an elastically mounted rigid cylinder and to study its contribution to the hysteresis (jump) phenomenon. Over all the model demonstrated adequate qualitative and quantitative results for phenomena A Semi-Empirical Model for Vortex Induced Motion of a Cylinder normally observed in a VIV experiment, such as a clear resonance peak, a lock-in range, and the structure phase lag(φ) passing through lock-in with an overall phase jump of π. The uniqueness of the present model could be stated as follows: Firstly it is not dependent on lift and drag coefficients data as usually obtained from forced VIV experiments, demerits of which have been clearly stated in this work. Secondly, inputted Strouhal parameter has not been fixed to the subcritical value of 0.2, as have been done in all the other VIV models. But rather in the present model it has been obtained from a well established Strouhal-Reynolds relationship estimated from the data of Morkovin (1964) and Lienhard (1966) which have been expanded to an extremely high range of Reynolds number regime of Re = 4.5 x 10 7 based on vortex shedding data obtained from the Space Shuttle Solid Rocket Motor during re-entry, therefore making the model to be directly applicable in real engineering scenario. Thirdly, all the empirical parameters used have been clearly defined with expressions linking such parameters to the physical mass and damping parameter that governs the oscillatory response. The present model has been successfully utilized in the prediction of other systems resulting in very good qualitative and quantitative correlations.

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