R.G. Tickell

Morison wave force coefficients for application to random seas

Abstract Results are presented from the analysis of the data collected from the Christchurch Bay Tower Compliant Cylinder when tested in random waves in the De Voorst flume in The Netherlands during 1987. Attention is focused on the use of rigid and flexible member (relative motion) forms of Morisons equation for the estimation of drag and inertia forces under random wave excitation. Notwithstanding some inconsistencies, many attributable to the ill-conditioning of much of the data for Cd determination, the coefficients show reasonable agreement to values obtained from earlier studies. The work suggests that Cd and Cm coefficients from large scale measurements may not be so sensitive to their method of estimation as has previously been thought and that they are applicable to the relative velocity form of the loading mechanism. For non-deterministic analyses, Morisons equation is found to reproduce well the probability distribution of loading associated with given sea state conditions. Furthermore, no simple expansion/extension to the equation has been forthcoming and no mechanistic allowance for history effects or vortex shedding has been found.

Measurements of particle velocities in laboratory scale random waves

The paper describes measurements of water particle velocities in laboratory-scale irregular, non-periodic surface waves. The measurements were taken over a range of elevations using Laser Doppler anemometry and included observations of particle kinematics at two points separated in plan. The observed statistical and spectral properties were compared with those predicted by both traditional and intermittent linear random wave theory. For elevations which are always submerged, the measured properties were in good agreement with both theoretical approaches. This was not the case for points near mean water level, where the observed properties were approximated more closely by the intermittent approach. However, some departure between observations and the intermittent approach was evident for elevations above mean water level.

The UK Christchurch Bay Compliant Cylinder Project : analysis and interpretation of Morison wave force and response data

Abstract The main purpose of the Project was to investigate wave loading on and dynamic response of a compliant (flexible) cylinder in real sea conditions, at a sufficiently large scale (Re>100,000 with Kc

Interaction of short-crested random waves and large-scale currents

Methods are outlined for determining the transformations to directional wave spectra induced by large-scale currents. The problems considered are those where waves move from quiescent water on to a current, or from one current region to another. Situations involving wave generation on currents are not discussed. The principle of wave action conservation is used to relate the wave energy densities in the two regions, and an equilibrium range constraint is applied to the high frequency tail of the transformed spectrum in instances where wave action is not conserved and energy is dissipated by wave breaking. Examples are presented which highlight how current-induced wave refraction and energy dissipation may have important consequences for the transformed spectrum.

STORM STATISTICS IN THE NORTH SEA

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.

An efficient technique for derivation of the kurtosis of offshore structural response due to random morison wave loading

Nonlinear wave loading leads to non-Gaussian offshore structural response so that higher-order statistical moments, such as kurtosis, are often necessary for its probabilistic description. The existing models for determination of these moments are computationally very demanding. Consequently, the distributed wave loading on the structure is idealised by a relatively small number of nodal loads, requiring care and experience in the representation of the continuous loading on (complex) structures with many structural elements. These shortcomings are successfully overcome by an approximate approach, as described herein, offering a dramatic reduction in computational effort so that the distributed loading can be idealised more realistically by a large number of nodal loads. The effectiveness of the proposed procedures, which have arisen from a UK EPSRC-sponsored project, are demonstrated by applying them to a test structure under different environmental conditions. With these improved tools, designers can now consider incorporation of more robust and precise probabilistic analysis into their evaluation procedures for structural behaviour, without facing onerous computational effort.© 2004 ASME