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We describe a mechanistic model of Windkessel phenomenon based on the linear dynamics of fluid-structure interactions. The phenomenon has its origin in an old-fashioned fire-fighting equipment where an air chamber serves to transform the intermittent influx from a pump to a more steady stream out of the hose. A similar mechanism exists in the cardiovascular system where blood injected intermittantly from the heart becomes rather smooth after passing through an elastic aorta. In existing haeodynamics literature, this mechanism is explained on the basis of electric circuit analogy with empirical impedances. We present a mechanistic theory based on the principles of fluid/structure interactions. Using a simple one-dimensional model, wave motion in the elastic aorta is coupled to the viscous flow in the rigid peripheral artery. Explicit formulas are derived that exhibit the role of material properties such as the blood density, viscosity, wall elasticity, and radii and lengths of the vessels. The current two-element model in haemodynamics is shown to be the limit of short aorta and low injection frequency and the impedance coefficients are derived theoretically. Numerical results for different aorta lengths and radii are discussed to demonstrate their effects on the time variations of blood pressure, wall shear stress, and discharge. Graphical Abstract A mechanistic analysis of Windkessel Effect is described which confirms theoretically the well-known feature that intermittent influx becomes continuous outflow. The theory depends only on the density and viscosity of the blood, the elasticity and dimensions of the vessel. Empirical impedence parameters are avoided.

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The predicted responses of an offshore structure when the wave kinematics are computed from different estimation methods can change significantly. The sometimes controversial results have recently motivated the development of a new methodology for random wave kinematics prediction known as Hybrid Wave Model (HWM). In this paper, the performance of the new methodology and other methods currently used for kinematics prediction was tested. The (surge) response of two offshore structures designed specially for deep-oil production was estimated using three methods (Hybrid Wave Model, Wheeler "Stretching" and Linear Extrapolation) and compared with the corresponding laboratory measurements. The wave forces were computed from the conventional Morison Equation evaluating the ambient wave kinematics from the wave elevation measurements and the response was computed using a numerical scheme based on a Finite Element time integration technique (Newmark-beta method). The comparisons between measured and predicted responses using kinematics calculated from the Hybrid Wave Model showed excellent agreement, specially for the low frequency components, while those using methods based on linear modifications rendered poor underestimations. The low frequency (peak) responses of these deep-water offshore structures were found to be greatly dominated by very low frequency wave excitations, which are mainly due to the wave-wave interactions. This work will show the necessity of high-order methods to evaluate irregular wave kinematics and induced responses. Introduction While evaluating the response of slender offshore structures, it is a common practice to estimate the induced forces from the Morison Equation with the particle kinematics computed using empirical and semi-empirical methods based on modifications of the Linear (random) Wave Theory. These modifications are intended to avoid force overpredictions induced by the superposition of very low and high frequency wave components. However, in spite of the fact that most of the work in this area has been focused on finding different alternatives to ease the effect of these incorrect estimations, still none of the proposed methodologies is generally accepted. The controversy comes from the fact that each method seems to be adequate only under some specific wave field characteristics. This problem motivated recent research work focused on finding a more reliable approach to estimate the wave kinematics and, as a result, a new method based on high-order wave theory formulations and non-linear decomposition was proposed. The new approach, known as Hybrid Wave Model (HWM), rendered very good agreement with the experiments used for its validation and proved to be more accurate than the empirical and semi-empirical methods commonly used to evaluate the random wave kinematics, but its impact on the estimation of the structural response has not been investigated yet. This paper is devoted to examine the accuracy of the response estimate obtained from the HWM method comparing to model measurements and to the response obtained from other methods of kinematics estimation used by the offshore industry. Two different deep-water production structures will be analyzed using single degree of freedom (SDOF) idealizations and only the surge motion will be considered. P. 339

Mooring systems for floating production systems, including the mooring lines and anchors, are currently designed on the basis of individual components. The most heavily loaded line and anchor are checked under extreme loading conditions with the system of lines intact and with one line removed. However, the performance of the floating production system depends more directly on the performance of the system of lines and anchors rather than on the performance of a single line or anchor. The on-going study conducts the reliability analyses using realistic probabilistic descriptions of the extreme met-ocean conditions (hurricanes and loop currents) for the Gulf of Mexico. Presented in this paper is the methodology of calculating the probabilities of failure of a classical Spar during a 20-year design life for individual components.