Slamming Force Research Articles

Application of Bragg grating sensors in the characterization of scaled marine vehicle models

Experiments using two different fiber-optic Bragg grating strain sensor systems to measure dynamic loads, slamming forces, and bending moments on a scaled catamaran model are reported. The system designed to measure slamming forces is based on dynamic locking of the laser diode wavelength to the peak wavelength of a Bragg grating mounted in a force cell. The sensor system has a dynamic range of 27 dB (1-550 N) and a bandwidth in excess of 1 kHz. For the measurement of bending moments, a time-domain multiplexing system, which used a unique pulsed rf modulation technique, of two Bragg grating sensors mounted on a steel beam was designed. The dynamic range of this system was also 27 dB [17 Nm (Newton meter) with a 0.03-Nm resolution]. The bandwidth was limited to approximately 50 Hz.

Bifurcation behaviour and wave slamming force on a compliant horizontal cylinder

The effect of a horizontal wave slamming force on the dynamic behaviour of a horizontal compliant cylinder is studied from the point of view of bifurcation analysis. It is found that, even though the time history responses of the system may change significantly, the patterns of bifurcations are not very sensitive to the slamming force.Therefore, the parameter ranges for the bifurcations can be reasonably determined without a complete knowledge of the exact values of the parameters. An understanding of the bifurcation behaviour of the system may have important practical relevance, e.g. in forecasting the possible parameter ranges in which the nonlinear system can be approximated by an equivalent linear system.

Sea loads on large buoyant cargo during ocean transport

This paper presents a semi-empirical procedure for the evaluation of sea loads on large buoyant cargos during ocean transport by heavy lift ships. Theories and underlying assumptions used in the procedure are discussed. The procedure predicts kinematics of the cargo and waves, the occurrence of coupling between waves and cargo, and the coupling forces in time domain. Air gaps between the cargo and wave surface are traced on the basis of linear theories of ship motion and waves to detect the occurrence of couplings. The coupling process is then treated as a local problem in the sense that it does not affect overall ship motion. The coupling forces are assumed to be a linear sum of buoyancy, Froude-Krylov force, added mass, and drag. The first two components are computed in a nearly exact method, whereas the latter components are approximated by empirical equations. The slamming force, which occurs only briefly at the moment of water entry, is computed separately. The required force coefficients are determined empirically. The underlying assumptions are verified with hydraulic model measurements, the results of which confirm the present approach for engineering applications.

Offshore structural system reliability under changing load pattern

This paper considers structural system reliability analysis when the spatial force pattern changes as the load increases, specifically when the nodal forces on a steel offshore jacket structure change as the base shear increases under increasing wave height. This paper demonstrates the so-called “fragility approach” to systems analysis, wherein the reliability of the system is obtained for each of a set of increasing wave heights. These probabilities are conditional on the wave height. These results are multiplied by the (discrete) probability levels of these wave heights, and the products are summed to obtain the (total) system reliability. The method is applied to analyze a fixed offshore jacket in 140 feet of water and designed for a 100-year (63 foot) wave. The change of the relative importance of different member-failure sequences with wave loads corresponding to different wave heights is studied. In this case study, the inclusion of wave slamming forces on the deck at higher wave heights led to an increase of the failure probability by an order of magnitude. The use of an improved fixed wave-load pattern treatment is explored. A comparison of the member-level and system-level failure probabilities is presented in terms of factors measuring different system effects.

On the initial-stage slamming of a rigid sphere in a vertical water entry

Analytic expressions are obtained for the small-time slamming coefficient and wetting factor of a rigid spherical shape in a vertical water entry. The theoretical model neglects gravity, compressibility and viscous effects with respect to inertia, which is usually the dominant effect during the early stages of water impact. Replacing the free-surface by a flat equipotential surface enables us to solve the incompressible inviscid flow problem about a double spherical bowl for the Stokes stream function expressed in terms of toroidal coordinates 12 . A semi-Wagner approach is then used to compute the wetting factor and the Lagrange equations are employed in order to determine the slamming force from the kinetic energy of the fluid. Good argeement between theoretical model and available experimental measurements has been obtained, both for the early-stage impact force and the free-surface rise at the vicinity of the three-phase contact line (wetting correction factor).

Cushioning of Slamming Impact by Elastomeric Layers

The effectiveness of layers of elastomeric materials bonded to the undersurface of rigid plates in cushioning their impact on calm water was investigated under two-dimensional conditions. It was shown that an elastomeric layer of optimal modulus of elasticity could significantly reduce the slamming forces on flat bottoms falling vertically on a water surface. An elastomeric layer tended to increase the slamming force on wedge-shaped bottoms, but the magnitudes of maximum impact forces on wedge-shaped bottoms either with or without an elastomeric layer were considerably smaller than those values for the flat bottom under identical drop conditions and diminished sharply with increasing deadrise angle.

Analysis and prediction of flat bottom slamming impact of advanced marine vehicles in waves

This article treats the problem of the analytical-computational determination of slamming forces, and resultant structural loads, for flat-bottom centerbody surfaces of advanced marine vehicles (such as hullborne SES, SWATH and similar catamaran vessels) in waves. The analysis uses the concept of fluid momentum theory. A three-dimensional model eliminates the infinite force behavior attaching to conventional two-dimensional strip theory methods applied to impact problems for flat surfaces. Theoretical results are in good agreement with model test data. The range of values possible in peak slamming load measurements in regular wave is discussed, as are the contributing causes for such effects.

Computations of overturning waves

The numerical method of Longuet-Higgins & Cokelet (1976), for waves on deep water, is extended to account for a horizontal bottom contour, and used to investigate breaking waves in water of finite depth. It is demonstrated that a variety of overturning motions may be generated, ranging from the projection of a small-scale jet at the wave crest (of the type that might initiate a spilling breaker) to large-scale plunging breakers involving a significant portion of the wave. Although there seems to be a continuous transition between these wave types, a remarkable similarity is noticed in the overturning regions of many of the waves.Three high-resolution computations are also discussed. The results are presented in the form of interrelated space-, velocity- and acceleration-plane plots which enable the time evolution of individual fluid particles to be followed. These computations should be found useful for the testing of analytical theories, and may also be applied, for example, to studies of slamming forces on shipping and coastal structures.

The Ubiquitous Barge: Some Trends in Barge Design and Construction in British Columbia

This paper briefly reviews barge construction in British Columbia, Canada in the 1970's. Changes in the material handling systems for barges serving the forest products industry are outlined. The design of the fore end structure of unmanned barges to withstand slamming forces in heavy seaways is discussed. Progress in the design of the bow form and skeg configuration of larger barges and specific resistance characteristics of a wide variety of barges are examined. The practical effect of the application of some aspects of the 1966 Load Line Convention to unmanned barges and the possible effect of recently promulgated Canadian Interim Stability Standards for this type of vessel are discussed. Finally, trends in the use of radio-controlled remote operation of anchors and ballasting pumping systems, emergency/pickup gear, and paint coating systems favored by the Canadian West Coast tug and barge industry are outlined.

ESTIMATION OF FLUID LOADING ON OFFSHORE STRUCTURES.

This paper presents a working guide to methods and associated data for estimating loading on offshore structures due to waves and currents. Its primary concern is with existing practice in which the formula known as J. R. Morison's equation is used extensively, but some attention is given to diffraction theory methods adopted for large monolithic types of structure and a section on the influence of marine roughness on loading is included. Regarding Morison's equation, a comprehensive review of published data on the relevant coefficients is presented. The considerable uncertainties which still exist in some areas are stressed but advice is offered on the best values to be used in the light of current knowledge. Other important questions discussed include the selection of appropriate wave theories and associated particle kinematics, the effects on fluid loading of proximity and inclination of structural members and the special problems of estimating impact of slamming forces.

Recent Contributions Under the Bureau of Ships Fundamental Hydromechanics Research Program

The third main nautical objective, Seakeeping, refers to that aspect of ship performance in which the seaway enters in a dominant manner and affects fundamentally the character of the problem. In the section on Control [3],1 a pattern was introduced according to which seakeeping was related to the uncontrolled as well as the controlled oscillations which take place with reference to a ship's inertial system. But, more broadly speaking, seakeeping includes all of the following subjects:Description of the seaway.Determination of the forces imposed by the seaway on the vessel—the excitation (hydrodynamic loadings, wave bending moment, slamming forces, and so on).Determination of the response of the ship in her six degrees of freedom (ship motions).Prediction of the perils to which a ship may be exposed (capsizing, foundering, safety at sea).Prediction of the loss in speed she will sustain in heavy weather.Evaluation of the amount of stabilization necessary to prevent unacceptable or undesirable displacements and accelerations. The elastic response and strength of a vessel's structure, though not included under seakeeping, depend, nevertheless, in an essential manner thereon.