Action Of Sea Waves Research Articles

Fuzzy control of a swells canal system

This work forms a part of the research undertaken on the maritime developments behaviour submitted to the action of sea waves. To succeed in this objective, we have conceived and achieved a system of artificial wave generation in a swells channel (channel of 37.5 m of length, 1 m of width and 1 m of depth). Indeed, the simulation of swells on scale model has many advantages. We can mention the construction of dykes for the protection of ports, the didactic interest of conception and generation of new models of waves and the construction of boat shells. The installed canal has to achieve tests of developments stability in the domain of the maritime hydraulics. Every work or scale model is constituted of several layers of ripraps of shapes and well-determined relative density. Thus, the survey of stability consists in reproducing in similitude (scale 1/20) in the swells canal to observe their effects on the holding of riprap (accumulation of boulder) or the artificial blocks of the scale model. Otherwise, the modelling of the process of swell generation is described by complicated physical laws and requires important means of computation. To solve this problem, we have conceived a system based on fuzzy logic permitting to assure the generation of coherent command data with the statistical parameters characterizing the oceanic waves, and to guarantee its conformity to the model of Pierson Moskowich spectrum.

Regulation of a Manned Sea-Surface Vehicle via Stochastic Optimal Control.

A control system is proposed for the regulation problem of the roll-motion of a manned sea-surface vehicle. Taking into account the roll-ship equations coming from the Conolly theory, a novel stochastic model has been proposed for the uncertainties driving the total mechanical torque acting on the vehicle, deriving from the wind and/or the sea-wave action. The proposed model results in a bilinear stochastic system to which a feedback controller is applied, giving linear-optimal performance with respect to a classical quadratic index. Open loop simulations carried out on real data validate the choice of the stochastic model of the uncertainties, producing a ship-roll time evolution which resembles the real data. Closed loop simulations on a virtual ship show the effectiveness of the proposed control scheme.

Dynamic analysis of structure/foundation systems

A review of current procedures being used in engineering practice to analyze the response of structure/foundation systems subjected separately to different types of dynamic excitation, such as earthquake, sea-wave action, wind, or moving wheel loads, is presented. Separate formulations are given for analyzing systems in the time and frequency domains. Both deterministic and stochastic forms of excitation are treated. A distinction is made between demand and capacity analyses.

Power conversion mechanisms for wave energy

It is easy to make a device that will respond vigorously to the action of sea waves. Indeed, it is quite hard to make one that will not. However, the conversion of the slow, random, reversing energy flows with very high extreme values into phase-locked synchronous electricity with power quality acceptable to a utility network is very much harder. This paper describes a range of different control strategies of varying degrees of sophistication and then describes possible conversion equipment for high-pressure oil and water and low-pressure air. Like many renewable energy sources, waves would benefit from some form of energy storage, particularly if they are to be used in weak island networks. Flywheels, gas accumulators, submerged oil/vacuum accumulators, thermal stores and reversible fuel cells are discussed, with emphasis on the coupling hardware. This leads on to a description of a new type of hydraulic machine with digital control which has been specially designed for high efficiency and flexible control of multiple erratic sources. An explanation of the rather low performance of air turbines in the random air flows caused by sea waves suggests the need for a variable-pitch version of the Wells turbine and the design of a reciprocating-flow wind-tunnel with recirculating energy recovery in which it could be tested.

Power conversion mechanisms for wave energy

It is easy to make a device that will respond vigorously to the action of sea waves. Indeed, it is quite hard to make one that will not. However, the conversion of the slow, random, reversing energy flows with very high extreme values into phase-locked synchronous electricity with power quality acceptable to a utility network is very much harder. This paper describes a range of different control strategies of varying degrees of sophistication and then describes possible conversion equipment for high-pressure oil and water and low-pressure air. Like many renewable energy sources, waves would benefit from some form of energy storage, particularly if they are to be used in weak island networks. Flywheels, gas accumulators, submerged oil/vacuum accumulators, thermal stores and reversible fuel cells are discussed, with emphasis on the coupling hardware. This leads on to a description of a new type of hydraulic machine with digital control which has been specially designed for high efficiency and flexible control of multiple erratic sources. An explanation of the rather low performance of air turbines in the random air flows caused by sea waves suggests the need for a variable-pitch version of the Wells turbine and the design of a reciprocating-flow wind-tunnel with recirculating energy recovery in which it could be tested.

The Viscoelastic Effect of the Cable on the Dynamics of an Underwater Towed System Suspended from a Buoy

A method and algorithm are proposed to analyze the dynamics of spatial viscoelastic flexible structures in a liquid medium with allowance for slackening in some structural elements. As an example, the dynamics of the structure upon change in the towing speed is studied with account taken of the action of sea waves. The relations between the buoy and wave periods are established. The kinematic and force characteristics of the structure are determined for various wave parameters and changes in the towing speed and direction

Complete two-dimensional analysis of sea-wave-induced fully non-linear sloshing fluid in a rigid floating tank

A time-independent finite-difference method and a fifth-order Runge–Kutta–Felhberg scheme were used to analyze the dynamic responses of sea-wave-induced fully non-linear sloshing fluid in a floating tank. The interaction effect between the fully non-linear sloshing fluid and the floating tank associated with coupled surge, heave and pitch motions of the tank are analyzed for the first time in the present pilot study. For the analysis of fluid motion in the tank, the coordinate system is moving (translating and rotating) with tank motion. The time-dependent water surface of the sloshing fluid is transformed to a horizontal plane and the flow field is mapped on to a rectangular region. The Euler equations as well as the fully non-linear kinematic free surface condition were used in the analysis of the sloshing fluid. The strip theory for linearized harmonic sea-wave loading was adopted to evaluate the regular encounter wave force. In addition, the dynamic coefficients used in the dynamic equations of tank motion were also derived based on strip theory and a harmonic motion of the tank. The characteristics of free and forced tank motions with and without the sloshing effect are studied. By the damping effect, the response of free oscillation will damp out and that of forced oscillation will approach a steady state. Without sea-wave action, the contribution of the sloshing load would enlarge the angular response of tank motion as well as the rise of free surface and the sloshing effect will delay the damping effect on angular displacement. On the contrary, under sea-wave action, the sloshing effect will decrease the dynamic response of tank motion and rise of free surface. The interaction, sloshing and coupling effects are found to be significant and should be considered in the analysis and design of floating tanks.

Calculation of the dynamic behavior of branched cable systems

A technique for the study of the dynamic behavior of three-dimensional elastic branched cable structures is proposed. Parametric local spline functions of spatial variables is used to reduce the boundary-value problem to a Cauchy problem. A graph of a three-dimensional cable structure is used to construct the solution algorithm. A study of the dynamic behavior of a hydrophysical anchored buoy system affected by the action of sea waves is performed, taking into account relaxation effects in individual elements.

Dynamic response and reliability analysis of large offshore structures

Offshore structures, when provided with large deck areas, may be utilized for new purposes other than the conventional oil and gas exploration works. They may be used as airports, power stations, waste treatment plants, etc. Since these structures have very heavy dead loads, reaction forces on the base foundation become severe and necessitate very firm foundation system. If the reaction forces can be reduced by some method, the concept of large offshore structures may become popular. This paper investigates the dynamic response and reliability analysis of such a structure under the action of sea waves, currents and earthquakes. The severe reaction forces on the foundation system have been reduced by adopting large-diameter members of buoyancy-type near the sea bottom. The equation of motion of the structure is obtained by the substructure method. The response analysis is carried out using frequency-domain random-vibration approach. Sensitivity studies are performed to study the effects of parametric uncertainties on the response values. The long-term behavior of the offshore structure is examined with first-passage probabilities of level crossing of extreme responses and reliability indices.

DETERMINATION OF LONG TERM DISTRIBUTION OFSTRESS RANGES FOR FATIGUE RELIABILITYANALYSIS OF A TLP DESIGNED FOR CAMPOS BASIN

The tension leg platform (TLP) is a type of structural system for exploitation of oil and gas fields below sea floor and must be designed to avoid fatigue damage due to cyclic action of sea waves. Fatigue is a phenomenon which presents high randomness, making a probabilistic study of major concern. In this work, a methodology to obtain the probability distribution function of time to fatigue failure is applied to a point in the hull and to one tendon of a TLP designed for Campos Basin, offshore Brazil. The short term responses at these points are obtained through a stochastic analysis in frequency domain and Miner's rule is used to assess the fatigue damage. Long term statistics are introduced through a scatter diagram of Campos Basin. Different alternatives of fitting known distributions to the calculated long term distribution of stress ranges are presented. Wirsching's model, assuming only lognormal variates, is compared to more general formats, where the method FORM is employed to assess the probability of failure. Assuming minimum acceptable reliability indices over the service life of the platform, maximum intervals of time to first inspection at referred points are determined.

Ship-based Weather Routing Using Dynamical Meteorology

Many vessels trading on North Atlantic routes are equipped with weather facsimile receiving devices of various types. Ship-masters engaged on regular voyages in this area are thus able to conduct their own passage planning using the information readily available from these intruments. The capital cost in relation to other ship operation costs is negligible; in US dollars (1983) it is likely to be of the order:There would be no problem in planning a vessel's route across an ocean if it were possible to forecast with total accuracy the actual sea state existing for the duration of a voyage. Such an ideal circumstance does not yet exist and a method must thus be used which incorporates flexibility to modify a voyage plan as actual weather varies from the prognosis. This means that a subjective approach to storm avoidance may, on occasions, be preferable to the stringent demands of a least-time track. The obsession of routing agencies with the latter method may be one reason why weather routing has not achieved the success it perhaps deserves.A ship may be damaged or delayed by the action of sea waves on her hull or structure. Hull form and ship dimensions are usually selected on the basis of calm-water performance, rather than for sustained sea-speed.

The Age and Origin of the Plateaus around Torquay

Having lately paid special attention to the hills and valleys of the Torquay district, I have been struck with the fact that the hills fall into two categories: first, those which rise above the level of a certain inclined plain or plateau; secondly, those which seem to have been carved out of this plateau. In the first category are the Warberry and Lincombe Hills, and the high ground above Kilmorie which bears no special name, but may, for the sake of convenience, be called ‘Kilmorie Hill.’ In the second category are nearly all the other hills within the Torquay watersheds; few of these hills rise much above 300 feet, and many of them are flat-topped, such as Daison Hill and Lummaton Hill. Besides these flat-topped eminences, there are also definite plateaus which cannot be called ‘hills,’ but are obviously portions of one or more ancient plains. As instances of these, I need only mention the Babbacombe Downs, and the smaller tract which is known by the rustic appellation of Daddy-Hole Plain. The plateaus around Torquay, and the problems which they offer to the student of physical geography, did not escape the notice of William Pengelly, who referred to them ill 1864 as ‘terraces of denudation.’ He seems to have regarded them as submarine platforms produced by the planing action of sea-waves at a time ‘not very remote geologically—when the entire district was below the sea-level.’ In 1866 he developed the idea still further in a paper on ‘Lithodomous Perforations... in