Janou Hennig

A Phase-Amplitude Iteration Scheme for the Optimization of Deterministic Wave Sequences

For the deterministic investigation of extreme events like capsizing, broaching or wave impacts, methods for the generation of deterministic wave sequences are required. These wave sequences can be derived from full scale measurements, numerical simulations or other sources. Most methods for the generation of deterministic wave sequences rely as a backbone on linear wave theory for the backwards transformation of the wave train from the target position in the wave basin to the position of the wave maker. This implies that nonlinear wave effects are not covered to full extend or they are completely neglected. This paper presents a method to improve the quality of the generated wave train via an experimental optimization. Based on a first wave sequence generated with linear wave theory and measured in the wave basin, the phases and amplitudes of the wave maker control signal are modified in frequency domain. The iteration scheme corrects both, shifts in time and in location, resulting in an improved deterministic wave train at the target location. The paper includes results of this method from three different basins with different types of wave generators, water depth and model scales. In addition, this method is applied to a numerical wave tank where the waves can be optimized before the actual basin testing.Copyright

NEW INSIGHTS IN EXTREME CREST HEIGHT DISTRIBUTIONS (A SUMMARY OF THE 'CREST' JIP)

The objective of the CresT JIP was ‘to develop models for realistic extreme waves and a design methodology for the loading and response of floating platforms’. Within this objective the central question was: ‘What is the highest (most critical) wave crest that will be encountered by my platform in its lifetime?’ Based on the presented results for long and short-crested numerical, field and basin results in the paper, it can be concluded that the statistics of long-crested waves are different than those of short-crested waves. But also short-crested waves show a trend to reach crest heights above second order. This is in line with visual observations of the physics involved: crests are sharper than predicted by second order, waves are asymmetric (fronts are steeper) and waves are breaking. Although the development of extreme waves within short-crested sea states still needs further investigation (including the counteracting effect of breaking), at the end of the CresT project the following procedure for taking into account extreme waves in platform design is recommended: 1. For the wave height distribution, use the Forristall distribution (Forristall, 1978). 2. For the crest height distribution, use 2nd order distribution as basis. 3. Both the basin and field measurements show crest heights higher than predicted by second order theory for steeper sea states. It is therefore recommended to apply a correction to the second order distribution based on the basin results. 4. Account for the sampling variability at the tail of the distribution (and resulting remaining possibility of higher crests than given by the corrected second order distribution) in the reliability analysis. 5. Consider the fact that the maximum crest height under a complete platform deck can be considerably higher than the maximum crest at a single point.Copyright

Vertical Wave Impact Loading on a Fixed Platform Deck

The TLP model tests in CresT [1] showed that there is a significant difference in the maximum load events due to long-crested and short-crested waves of same peak period and significant wave height. This decrease in load amplitudes for increasing spreading was not dominated by the reduction in crest heights, but related to a change in wave excitation. In ShorT-CresT wave-in-deck model tests were carried out with the focus on the physics of impact loading. The primary objective of the platform tests was to link crest height and wave impact with local and global loading on the deck. The model test results showed that the global vertical loads in short-crested waves can be similar to long-crested events, if the wetted deck area is comparable. In other words, the platform deck loading corresponds to the relative short-crestedness of the sea state: if the crest length is at least as large as the characteristic deck dimension, the loads are significantly larger than for lower crest lengths (step change). In this paper the results of the wave-in-deck model tests are presented and discussed. The analysis of the model tests is focused on a comparison between short-crested and long-crested impacts and a comparison of the measurements to a simplified loading model.Copyright

Qualitative and Quantitative Validation of a Numerical Code for the Realistic Simulation of Various Ship Motion Scenarios

There is an ongoing discussion on safety guidelines to be considering more recent developments in ship design. Numerical simulations of ship motions are considered as powerful tool for the safety evaluation of a given design. However, the consequent use of numerical codes calls for their thorough validation which has to be performed both qualitatively and quantitatively. This paper focuses on a code used and further developed by the Flensburg Shipyard. For its validation, the capsizing scenario in steep wave sequences is realized in the wave tank first. The dedicated computer controlled experimental technique ensures the exact phase correlation of wave excitation and resultant ship motions. Thus, the registered wave and the track of the ship model in the model test serve as input to the numerical simulation which results in the specific motion time traces. These are now directly compared to the motion registrations from the model tests. First results of the validation by direct comparison of time series have been presented in earlier publications, still with the restriction that only a few cases have been investigated. In this paper, the promising method is applied to another scenario in a long-crested sea state including steep wave combinations. Different aspects are discussed which results in the conclusion that the method is feasible for free running ships in stern and stern quartering seas.Copyright

Worst Sea–Best Sea Wave Group Spectra From Random Sea States

The paper presents a proposition called the worst sea-best sea (WS-BS) method that intends to identify wave group spectra that yield time series that lead, respectively, to the highest wave group excitation (worst sea) and the smallest wave group excitation (best sea) to the second order slowly varying response for a floating structure model submitted to random waves in a wave basin. The methodology is based on the estimation theory. Based on the fact that even with the same wave spectrum it is possible to devise different wave group spectrum, the paper proposes a method to identify the WS-BS time series. Both simulations and concrete model testing are shown indicating the feasibility of the approach.© 2008 ASME

Effect of Short-Crestedness on Extreme Wave Impact: A Summary of Findings From the Joint Industry Project “ShorTCresT”

Long-crested waves are typically used in the design of offshore structures. However, the corresponding statistics, kinematics and loading are significantly different in short-crested waves and up to date, there is no state-of-the-art methodology to apply short-crested models instead. The objective of the “ShortCresT” Joint Industry Project was to take into account short-crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver (empirical) design recommendations and methods. This paper gives an overview of the findings of ShorTCresT regarding wave crest and height distributions, a comparison of basin and field data, the role of wave breaking, the most realistic directional model, hindcast models as well as the related platform loading.Copyright

New Methods and Insights in Advanced and Realistic Basin Wave Modelling

For model tests, the correct generation of the most realistic representation of the natural wave field is of greatest importance as the environmental conditions denote the starting point for all following analyses of any behaviour of a marine structure. Thus, it has to be defined first what “reality” is, followed by a thorough analysis of the inherent limitations of basin wave fields. Furthermore, all realistic aspects of the wave field have to be modelled at sufficient accuracy involving the wave maker control, flap geometries and appropriate analysis techniques. This paper gives an overview of the most recent developments in advanced basin wave modelling including a wide range of aspects as realistic wave spreading, deterministic wave generation, focusing waves, directional wave analysis, spurious waves and shallow water wave generation.Copyright

EXPERIMENTAL VARIATION OF FOCUSING WAVE GROUPS FOR THE INVESTIGATION OF THEIR PREDICTABILITY

In deterministic model testing, focusing wave groups are used for the simulation of dedicated wave environments. They are characterized by the transient appearance of one relatively steep wave crest. The phasing of the wave components which leads to an exact focusing in one point in time and space is strongly dependent on the correct modeling of the wave phase velocity while the position of the focusing point depends on the wave group celerity. For wave generation purposes, the calculation of a wave maker control signal based on a target wave train at a desired position in the tank (inverse or backward modeling) is of crucial importance. Numerical wave tanks and empirical approaches are often calibrated based on wave characteristics measured in a particular tank. This paper presents model test results for the variation of frequency range, steepness and focal point of focusing wave groups at intermediate water depth. The measured characteristics are compared to predicted parameters.Copyright

ShorTCresT: Directional Wave Measurements at MARIN

The objective of the ShortCresT JIP Joint Industry Project was to take into account short-crestedness in the design of offshore structures against extreme waves based on a good description of their spectral characteristics, statistics, kinematics, breaking and loading and to deliver a concrete (empirical) design methodology. The second order wave crest distribution showed to be a good basis for the estimation of a design wave crest. However, depending on sea state steepness and directional spreading, crests may exceed the second order distribution in some severe seas by around 10 %. On the other hand, the very highest crests may be limited by breaking and even fall below the second order model. This paper addresses experimental results from the MARIN where directional wave measurements were carried out in two dimensions. Conclusions with respect to evolution of wave spectra in the basin, directional analysis and calibration, sampling variability, local variability of measured crest heights and measured crest height distributions due to different types of spreading are evaluated. Furthermore, the influence of the type of instrumentation and the effect of the spectral high frequency tail on the crest distributions are assessed.Copyright

Systematically Varied Rogue Wave Sequences for the Experimental Investigation of Extreme Structure Behavior

For an improved design of ships and offshore structures with regard to their behavior under severe weather conditions, wave height and steepness as well as the shape of the wave profile have to be considered. In this paper, the extreme New Year Wave as documented in numerous publications is varied with respect to wave height and period. These varied wave sequences are realized and measured in a model tank and applied to the investigation of motions and bending moments of an FPSO ship. The results are compared to the responses in the original wave train. An investigation of the riskiness of extreme wave sequences in comparison with existing rules concludes the paper.Copyright