A need has been identified to improve the knowledge about extreme slamming loads from breaking waves on vertical columns, such as offshore platforms and wind turbine foundations. Due to strongly nonlinear physical mechanisms and large statistical variability, more and improved experimental data are needed, as well as better qualified design procedures. In this paper, model test data and CFD simulations from a recent study with a fixed vertical column are compared and investigated in more detail. Selected individual extreme slamming events due to energetic breaking waves in 1:40 and 1:125 scaled model tests are presented and considered. Waves correspond approximately to extreme breaking wave occurrences in steep energetic sea states with 10-4 annual probability in the Norwegian sector. Slamming pressures on the column wall are measured in time and space by means of a 7 × 7 pressure sensor array covering 19m2 (full scale). Significant spatial variations are observed. When spatially averaged over the array, the observed highest pressures are typically in the range 1MPa–3MPa (full scale), while smaller measuring areas give higher values. This compares roughly to levels found from recent results in the literature; although exact comparison is difficult due to statistical uncertainty issues. Experiences obtained from parallel CFD and PIV activities are also compared to the experiments, from which free-surface particle velocities up to 25m/s (full scale) are estimated in the worst cases. Finally, a simple empirical formula for a slamming coefficient depending on the actual pressure integration area is suggested based on the results.
The forced heave motion of a dummy ship model with moonpool, including a fixed box-shaped object, was realized experimentally in the Towing tank at MARINTEK. The blockage effect caused by a large object was investigated. Regular and irregular forced heave motions were imposed. In the regular motion tests, four forcing amplitudes, and 11 forcing periods near the piston-mode resonance period were tested. PVC3D (Potential Viscous Code) was used to study the regular heave motion problem numerically. PVC3D is a code developed at MARINTEK, in collaboration with Statoil RDI, which couples a Naviér-Stokes solver with the linear potential flow theory for the free-surface waves. PVC3D has in previous studies proven to be fast, robust and accurate for marine resonance problems. It has not previously been validated for object in moonpool. Here, a validation study is presented. The moonpool response is well predicted by PVC3D both for the case of empty moonpool and moonpool with object. The studied object has a non-negligible blockage effect in resonant condition.
A practical method for prediction of green water and wave impact on FPSO’s in steep irregular waves is described. The relative wave elevation and kinematics are found from combining ship motions, wave diffraction and nonlinear irregular waves. Water heights on deck and related velocities are estimated by simple analytical formulas originally derived from dam-breaking theory but modified in this work to take into account a non-zero water velocity input and the effects from a dynamic and finite wave-determined water reservoir. A bulwark is also included. Deckhouse slamming and bow flare slamming loads are computed by simple formulas from the local velocities and, in the latter case, also the relative angle between the water surface and the flare. Verification against more advanced models and to model test data show promising results. The method is being implemented into a simple research-type software tool.
There is a need for improved analysis and verification tools against wave impact, partly due to updated metocean criteria in various geographical areas, but also due to the complex nature of the problem. Model tests are usually recommended in the final design verification. In this paper, improved engineering methods and procedures in predicting wave impact loads on FPSO's and offshore platforms in severe weather are described. Applications include wave-in-deck on jacket platforms, wave amplification with wave impact on large-volume platforms, green water /bow flare slamming on FPSO, and impact on columns. The resulting theoretical procedures and tools are in particular intended for early stages in the design. State-of-the-art knowledge is combined with systematic analyses of experimental data. New recommended tools and practical procedures for the fast, accurate and robust prediction of wave impact loads due to random waves are established and validated against model tests. Robust and practical methods are emphasized. Basic physical mechanisms and parameters critical to wave impact are pointed out. Experimental data includes existing data sets as well as new experiments. New engineering software tools are developed. Recommendations on model testing procedures are also given. Introduction Hydrodynamic impact loads resulting from random, extreme ocean waves are important factors in the design of ships and offshore structures. This includes extreme global loads such as ringing and whipping, global impact loads on fixed and floating platform type structures, and local impact loads due to green water, bow slamming and wave-in-deck impact forces. An example of the type of practical problems that may occur is the bow flare slamming event that was experienced on the Schiehallion FPSO North of Scotland /1/. The relevance of the wave impact problem is recently also increased due to new metocean criteria being established /2,3/. Because of the strongly nonlinear effects, and limited accuracy or robustness of available theoretical models for such applications, the loads are often estimated by use of model tests in offshore basins. Examples from experiments with structures in extreme waves are shown in Fig. 1.
Sloshing, a violent fluid motion in tanks is of current interest for many branches of the industry, among them gas shipping. Although different methods are commonly combined for analyzing sloshing in LNG carriers, time histories of the pressure in the tanks are most reliably obtained by experiments. Very localized pressures may be important for the structural response of the tank containment system. Moreover, the typical pressure time history duration is similar to the structural natural frequency. Therefore, pressure measurements need to be performed with due account for temporal and spatial distribution. This requires a high sampling resolution both in time and space. Fine spatial resolution becomes especially important when local pressure effects are of interest, such as pressure profile passing a membrane corrugation of Mark III containment or Invar edge of No.96 containment. In this paper experimental approach applied by MARIN-TEK for analyzing sloshing phenomenon is presented. The focus is put on investigating effects of Invar edges. A transverse 2D model of a typical LNG carrier is used. Local pressure effects are investigated based on low filling level tests with different wall surfaces: smooth and with horizontal protrusions representing the surface similar to the No.96 containment system.
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