Günther F. Clauss

Rogue Waves: From Nonlinear Schrödinger Breather Solutions to Sea-Keeping Test

Under suitable assumptions, the nonlinear dynamics of surface gravity waves can be modeled by the one-dimensional nonlinear Schrödinger equation. Besides traveling wave solutions like solitons, this model admits also breather solutions that are now considered as prototypes of rogue waves in ocean. We propose a novel technique to study the interaction between waves and ships/structures during extreme ocean conditions using such breather solutions. In particular, we discuss a state of the art sea-keeping test in a 90-meter long wave tank by creating a Peregrine breather solution hitting a scaled chemical tanker and we discuss its potential devastating effects on the ship.

Gaussian wave packets — a new approach to seakeeping testsof ocean structures

The transient wave train techniques has been widely used in testing ocean structures. The methodpresented here is based on a special Gauss-modulated amplitude spectrum. These wave groups of limited length can be superimposed, the actual surface elevation being a function of packet characteristics and initial time lag. The application of this technique is demonstrated in three cases presenting a semisubmersible,an articulated tower and a floating oil skimmer. It is shown, that a particular problem needs a tailored wave packet containing sufficient energy all over the relevant frequency range. These packets are designed by superimposing individual Gaussian wave groups. The new seakeeping test techniques has the following advantages u - The wave train is well defined at any location of the tank. - The wave elevation and spectrum can be standardised or easily adapted to any specific problem. - The duration of the test is very short; reflections (beach) do not interfere the results. At theculmination point the length of the wave train is small. This facilitates short duration seakeeping and manoeuvring tests. - The results show high resolution and are in good agreement with regular wave test data. Summarising, the Gaussian wave packet method is a versatile technique yielding precise and highly resoluted results in a short time.

Robust Pareto-optimum routing of ships utilising deterministic and ensemble weather forecasts

Sophisticated routing of ships is increasingly recognised as an important contribution to safe, reliable and economical ship operation. The more reliable the weather forecasts and the performance simulation of ships in a seaway become, the better they serve to identify the best possible route in terms of criteria like estimated time of arrival (ETA), fuel consumption (FUEL), safety (of ship, crew, passengers and cargo) and comfort. This establishes a multi-objective, non-linear and constrained optimisation problem in which a suitable compromise has to be found between opposing targets. For its solution, a new optimisation approach to select the most advantageous route on the basis of hydrodynamic simulations and sophisticated weather forecasts is posed. Transfer functions are employed to assess the operating behaviour of a ship in waves. Probabilistic ensemble forecasts are applied to account for the stochastic behaviour of weather. A multi-objective genetic algorithm turns out to be a suitable optimisation method to identify Pareto-optimum routes for a sustainable support of a conscious decision-making process.

Simulations and experiments of short intense envelope solitons of surface water waves

The problem of existence of stable nonlinear groups of gravity waves in deep water is considered by means of laboratory and numerical simulations with the focus on strongly nonlinear waves. Wave groups with steepness up to Acrωm2/g ≈ 0.30 are reproduced in laboratory experiments (Acr is the wave crest amplitude, ωm is the mean angular frequency, and g is the gravity acceleration). We show that the groups remain stable and exhibit neither noticeable radiation nor structural transformation for more than 60 wavelengths or about 15-30 group lengths. These solitary wave patterns differ from the conventional envelope solitons, as only a few individual waves are contained in the group. Very good agreement is obtained between the laboratory results and numerical simulations of the potential Euler equations. The envelope soliton solution of the nonlinear Schrodinger equation is shown to be a reasonable first approximation for specifying the wave-maker driving signal. The short intense envelope solitons possess ver...

Laboratory and numerical study of intense envelope solitons of water waves: Generation, reflection from a wall, and collisions

The investigation of dynamics of intense solitary wave groups of collinear surface waves is performed by means of numerical simulations of the Euler equations and laboratory experiments. The processes of solitary wave generation, reflection from a wall, and collisions are considered. Steep solitary wave groups with characteristic steepness up to kAcr ≈ 0.3 (where k is the dominant wavenumber and Acr is the crest amplitude) are concerned. They approximately restore the structure after the interactions. In the course of the interaction with the wall and collisions, the maximum amplitude of the wave crests is shown to enhance up to 2.5 times. A standing-wave-like structure occurs in the vicinity of the wall, with certain locations of nodes and antinodes regardless the particular phase of the reflecting wave group. A strong asymmetry of the maximal wave groups due to an anomalous setup is shown in situations of collisions of solitons with different frequencies of the carrier. In some situations of head-on col...

Application of Higher Order Spectral Method for Deterministic Wave Forecast

This paper addresses the Higher Order Spectral (HOS) method as very fast and accurate non-linear method for deterministic wave forecast. The focus of the paper lies on wave propagation, with the objective to draw conclusions on the applicability of the HOS method for deterministic wave forecast. Systematic numerical and experimental investigations are conducted. The investigations comprise exact solutions of the nonlinear Schr ¨ odinger equation (NLS) as special non-linear wave groups, which are used as initial conditions for the subsequent simulations. Different parameters such as relative water depth, wave steepness and propagation distance are varied to evaluate the applicability of the HOS method. The results obtained are validated by experiments as well as fully non-linear simulations. It is shown that the HOS method is capable for non-linear wave forecast due to high accuracy and fast calculation time at once.

Application of Breather Solutions for the Investigation of Wave/Structure Interaction in High Steep Waves

During the design process of floating structures, different specifications have to be aligned such as the range of application, the warranty of economical efficiency as well as the reliability and are an inevitable integral part of the evaluation process during the design stage. The validation of the performance by means of model tests in terms of sea state behavior and the associated local and global structural loads are an important milestone within this process. Therefore it is necessary to determine an adequate test procedure which covers all essential areas of interest. Thereby one field of interest are limiting criteria of the design such as maximum local and global loads as well as maximum accelerations due to the impact of extraordinarily high waves, at which the floating structure has to survive. Different alternatives are available to conduct model tests in high, steep waves — transient wave packages, regular waves, irregular waves with random phases or more sophisticated deterministic tailored irregular wave sequences such as reproductions from numerical simulations and real-world measurements.This paper introduces a new approach for the systematic investigation of wave/structure interaction in high, steep waves. Exact solutions of the nonlinear Schrodinger equation — the so called breather solutions — are implemented for the generation of extraordinarily high waves. Three types of breather solutions are investigated in the seakeeping basin and to cover the full range of interest, each solution has been used to generate freak waves at certain frequencies.To evaluate the applicability of breather solutions for model tests two types of ships — a LNG Carrier and a Chemical Tanker — are investigated in the seakeeping basin. The ships are segmented and connected with strain gauges to detect the vertical wave bending moment. Furthermore, green water probes are installed on deck to evaluate the local impact on the bow of the freak waves. The obtained results are compared to investigations in regular waves with certain frequency and steepness as well as in real-world freak wave reproductions.Copyright

Response Based Identification of Critical Wave Scenarios

In the last years the identification and investigation of cri tical wave sequences regarding offshore structure responses became one of the main topics in the ocean engineering community. Thereby the area of interest covers the entire field of applic ation spectra at sea - from efficient and economic offshore operati ons in moderate sea states to reliability as well as survival in e xtreme wave conditions. For most cases, the focus lies on limiting criteria for the design, such as maximum global loads, maximum relative motions between two or more vessels or maximum accelerations, at which the floating structure has to operate o r to survive. These criteria are typically combined with a limit ing characteristic sea state ( Hs, Tp) or a rogue wave. For the investigation of offshore structures as well as the identification o f critical wave sequences, different approaches are available - most of them are based on linear transfer functions as it is an efficie nt procedure for the fast holistic evaluation. But, for some ca ses the linear method approach implies uncertainties due to nonlinear response behavior, in particular in extreme wave conditions. This paper presents an approach to these challenges, a response based optimization tool for critical wave sequence detection. This tool, which has been successfully introduced for the evaluation of the applicability of a multi-body system based

Gap Effects at Side-by-Side LNG-Transfer Operations

The current demand of liquefied natural gas (LNG) from remote marine locations pushes the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred between shuttle carrier (LNGC) and terminal. Even if the tandem configuration is the primary choice for LNG transfer at rough offshore locations, side-by-side configurations would be the preferred option because of existing midship coupling manifolds on the present carrier fleet (no need for manifold modifications) as well as standard mooring systems and transfer-process-chains similar to oil-transfer. Therefore, the operation conditions at rough seas have to be improved to allow side-by-side LNG-transfer and to reduce offloading downtime.Within the SOTLL-project, side-by-side LNG transfer up to HS = 3 m is reached as a transfer limit using a new flexible pipe design, the advantages of sheltered areas at the leeside of the terminal barge and an optimized ship transfer position due to a flexible longitudinal offloading position. In addition to the evaluation of the hydrodynamic characteristics of this multibody system, one key aspect is the analysis of the exciting forces and motions due to wave amplification between the ships. In the gap between the hulls, the incoming wave field is amplified and changes dramatically. Depending on gap width, longitudinal offset, wave heading and length, large wave amplifications, standing waves and other resonance phenomena are observed which may result in high relative motions and increased forces of the entire mooring system. In this paper, the gap effects are investigated in detail with numerical approaches in frequency domain, validated by model tests at TU Berlin. A typical offloading scenario with barge and carrier is investigated for different gap sizes to identify suitable transfer configurations and ensure safe LNG offshore transfer up to HS = 3 m.Copyright

SLOSHING: FROM THEORY TO OFFSHORE OPERATIONS

The current demand in liquefied natural gas (LNG) from remote marine locations drives the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred to shuttle carriers (LNGC). During the loading procedure, which takes about 18-24 hours for a standard sized LNGC, free fluid surfaces and varying filling levels occur inside the internal cargo tanks. This condition is critical since the seakeeping behavior of the LNGC — especially the roll motion — is strongly influenced and varying. In order to estimate and forecast the LNGC motions, numerical methods based on potential theory are the most efficient and appropriate method. The selected approach is validated by model tests at 30% water filling height inside four prismatic tanks. In-depth analyses, including force and moment measurements between tanks and hull, revealed a discrepancy between the analytical natural modes of a prismatic tank and the resonance frequencies for four prismatic tanks mounted to a LNGC hull. This effect is caused by the ratio of rigid to added mass of the system as well as the fact that the tanks are mounted to a standard hull shape featuring a longitudinal bow-stern asymmetry. In order to investigate this phenomenon