Günther F. Clauss

Hydrodynamic shape optimization of large offshore structures

Abstract The paper presents a hydrodynamic shape optimization procedure applicable at an early design stage to develop offshore structures with improved seakeeping qualities. Nonlinear programming algorithms are used to find minima of the selected objective function which qualifies the design. Evaluation and comparison of different designs are based on significant double amplitudes of forces and motions which are calculated for a given design sea spectrum. The hydrodynamic analysis is combined with long term wave statistics to minimize expected downtime. Due to a powerful automated shape generation and discretization technique, the procedure allows us to optimize different types of offshore structures. Results for various types of offshore structures are presented and verified by model tests.

Bending Moments of an FPSO in Rogue Waves

The increasing numbers of reported rogue waves with extreme crest and wave heights and unusual group pattern with the consequence of severe damages raise the question if such exceptional events have to be considered routinely for the design of ships and offshore structures. For the investigation of the effects of rogue wave impacts time domain simulation methods are required in addition to traditional frequency domain methods which may not be sufficient to consider these extreme events. In this paper the vertical bending moments at the midship section of an FPSO are investigated using state of the art numerical simulation tools in combination with experiments. For the seakeeping tests the extremely high New Year Wave (registered in the North Sea) is generated in the wave tank, and motions and structural forces are analyzed at model scale. For validation the results are evaluated deterministically and compared to numerical simulations. The time domain calculation allows to artificially change local wave characteristics. The steepness of the selected rogue wave is varied and the influence on wave induced loads is studied. A comparison with standard procedures of seakeeping analysis and classification rules closes the paper.Copyright

Task-related wave groups for seakeeping tests or simulation of design storm waves

Abstract The chaotic wave field of a natural seaway can be decomposed into an infinite number of independent harmonic waves, and its spectrum follows from the associated wave amplitudes and frequencies. If superimposed with random phase we register the well-known irregular sea, which is characterized by its significant wave height and zero-up-crossing period. As rare events very high waves are observed accidentally. Since RAOs of wave/structure interactions are independent of the random phase shift between superimposing component waves this parameter can be selected arbitrarily to compose an optimum and short-duration transient wave train which allows the precise determination of all response amplitude operators within the relevant spectral range. Applications of the wave group technique are presented for: (i) standard seakeeping tests of stationary or moving (self-propelled) marine structures; (ii) simulation of design storm waves for the investigation of coastal and offshore structures. The paper illustrates the generation of task-related wave packets, the determination of the associated acceleration, velocity and pressure fields, as well as the related energy flux. Based on the dispersion relation the propagation behavior is exactly predictable. Consequently, the kinematics and dynamics of the wave field can be determined at any position and time. If the converging wave group approaches its concentration point the associated particle motions are analyzed by a nonlinear procedure using coupled Lagrangian expansion equations. The efficiency and the limitations of the transient wave technique are demonstrated by presenting typical test examples. These include the determination of the RAOs of stationary offshore structures and towed or self-propelled ships as well as the investigation of coastal structures in 100-year waves. As the entire process is deterministic, the action/reaction chains can be evaluated in detail. The paper demonstrates that the wave group technique is a reliable and efficient tool for all standard investigations related to wave/structure interactions, and opens a new area for the analysis of transient processes in the sea, e.g. dynamic stability of floating vessels or design wave impacts on coastal or offshore structures.

Numerical Wave Tank: Simulation of Extreme Waves for the Investigation of Structural Responses

For the deterministic analysis of extreme structure behavior, the hydrodynamics of the exciting wave field, i. e. pressure and velocity fields, must be known. Whereas responses of structures, e. g. motions, can easily be obtained by model tests, the detailed characteristics of the exciting waves are often difficult to determine by measurements. Therefore, numerical wave tanks (NWT) promise to be a handy tool for providing detailed insight into wave hydrodynamics. In this paper different approaches for numerical wave tanks are introduced and used for the simulation of rogue wave sequences. The numerical wave tanks presented are characterized by the following key features: a) Potential theory with Finite Element discretization (Pot/FE); b) Reynolds-Averaged Navier-Stokes Equations (RANSE) using the Volume of Fluid (VOF) method for describing the free surface. For the NWT using the VOF method three different commercial RANSE codes (CFX, FLUENT, COMET) are applied to calculate wave propagation, whereas simulations based on potential theory are carried out with a wave simulation code developed at T echnical U niversity B erlin (WAVETUB). It is shown that the potential theory method allows a fast and accurate simulation of the propagation of nonbreaking waves. In contrast, the RANSE/VOF method allows the calculation of breaking waves but is much more time-consuming, and effects of numerical diffusion can not be neglected. To benefit from the advantages of both solvers, i. e. the calculation speed (Pot/FE-solver WAVETUB) and the capability of simulating breaking waves (RANSE/VOF-solver), the coupling of both simulation methods is introduced. Two different methods of coupling are presented: a) at a given position in the wave tank; b) at a given time step. WAVETUB is used to simulate the propagation of the wave train from the start towards the coupling position (case A) or until wave breaking is encountered (case B). Subsequently, the velocity field and the contour of the free surface is handed over as boundary (case A) or initial values (case B) to the RANSE/VOF-solver and the simulation process is continued. To validate these approaches, different types of model seas for investigating wave/structure interactions are generated in a physical wave tank and compared to the numerical simulations.Copyright

FORMATION OF EXTRAORDINARILY HIGH WAVES IN SPACE AND TIME

The existence of freak waves is indisputable due to observations, registrations, and severe accidents. The occurrence of extreme waves, their characteristics and their impact on offshore structures is one of the main topics of the ocean engineering research community. Real sea measurements play a major role for the complete understanding of this phenomenon. In the majority of cases only single point registrations of real sea measurements are available which hinders to draw conclusions on the formation process and spatial development in front of and behind the respective registration points. One famous freak wave is the “New Year Wave”, recorded in the North Sea at the Draupner jacket platform on January 1st, 1995. This wave has been reproduced in a large wave tank and measured at different locations, in a range from 2163 m (full scale) ahead of to 1470 m behind the target position — 520 registrations altogether. Former investigations of the test results reveal freak waves occurring at three different positions in the wave tank and these extreme waves are developing mainly from a wave group. The possible physical mechanisms of the sudden occurrence of exceptionally high waves have already been identified — superposition of (nonlinear) component waves and/or modulation instability (wave-current interaction can be excluded in the wave tank). This paper presents experimental and numerical investigations on the formation process of extraordinarily high waves. The objective is to gain a deeper understanding on the formation process of freak waves in intermediate water depth such as at the location of the Draupner jacket platform where the “New Year Wave” occurred. The paper deals with the propagation of large amplitude breathers. It is shown that the mechanism of modulation instability also leads to extraordinarily high waves in limited water depth. Thereby different carrier wave length and steepnesses are systematically investigated to obtain conclusions on the influence of the water depth on the modulation instability and are accompanied by numerical simulations using a nonlinear potential solver.Copyright

Tailor Made Freak Waves Within Irregular Seas

The paper presents a nonlinear technique for generating high and highest freak waves by using deterministic wave packets which are also embedded into irregular seas. First, the technique is developed for small water elevations with a linear description of the wave. Afterwards the method is extended to a nonlinear description based on the introduction of a “wave information” which includes the (linear) information of waves as Fourier Transforms. Combined with a modified wave celerity (information speed) the propagation of high waves can be developed by using Stokes higher order solutions or semi empirical expansion equations. As a result, the nonlinear wave contours, registrations and associated wave data are derived from the respective wave information. As the mentioned methods are not only used as a theoretical approach for describing steep wave trains a wide range of applications for generating high deterministic waves in model basins is presented. Thus extreme waves with heights of more than 3 m have been generated. This technique has also been adapted for the demands of computer controlled seakeeping tests. As both model and wave probe do not necessarily move at the same velocity, a time domain procedure is introduced for transforming the registration of the wave elevation from the wave probe posi-tion to the model position which leads, especially in astern and quartering seas, to extraordinary wave contours experienced by a moving vessel.Copyright

MOTION BEHAVIOUR OF A NEW OFFSHORE LNG TRANSFER SYSTEM AT HARSH OPERATIONAL CONDITIONS

Today, the demand of natural gas from offshore fields is on a high level and still increasing. Floating turret moored terminals receive gas directly from the field via risers and liquefaction is achieved by on-board processing plants. The LNG (liquefied natural gas) is transferred to periodically operating shuttle carriers for onshore supply. This paper presents an innovative offshore LNG transfer system, based on newly developed flexible cryogenic pipes of 16” inner diameter, which allow fast loading/offloading procedures in tandem configuration (see Fig. 1), even in harsh environmental conditions. The motion characteristics of the proposed concept are investigated in detail by the potential theory programmes WAMIT and ANSYS AQWA, respectively, with the focus on the dynamic behaviour of the multi-body system in waves. Each vessel is generating its own radiation and diffraction wave field affecting the motions of the adjacent vessels and vice versa. Results from calculations in the frequency and time domain are compared and show good agreement. Tolerable relative motions between terminal and carrier are limited by maximum torsion and bending of the flexible transfer pipe.

Experimental Optimization of Extreme Wave Sequences for the Deterministic Analysis of Wave/Structure Interaction

For the deterministic analysis of wave/structure interaction in the sense of cause-reaction chains, and for analyzing structure responses due to special wave sequences (e.g., three sisters phenomenon or other rogue wave groups) methods for the precise generation of tailored wave sequences are required. Applying conventional wave generation methods, the creation of wave trains satisfying given local wave parameters, and the generation of wave groups with predefined characteristics is often difficult or impossible, if sufficient accuracy is required. In this paper we present an optimization approach for the experimental generation of wave sequences with defined characteristics. The method is applied to generate scenarios with a single high wave superimposed to irregular seas. The optimization process is carried out in a small wave tank. The resulting control signal is then transferred to a large wave tank considering the electrical, hydraulic and hydrodynamical response amplitude operators (RAOs) of the respective wave generator in order to investigate wave/structure interaction at a large scale.

Influence of the Bow Shape on Loads in High and Steep Waves

To ensure survival of floating structures in rough seas, a precise knowledge of global and local loads is an inevitable integral part for safe design. One of the key parameters is the vertical bending moment. Not only vertical forces but — as previous investigations revealed — also longitudinal forces significantly contribute to the vertical wave bending moment. Three segmented ships, equipped with force transducers, are investigated systematically in high and steep regular waves and in harsh wave environments at several cruising speeds to identify the structural loads. The model tests are carried out in the seakeeping basin of the Technical University Berlin at a scale of 1:70. To cover possible influences of the bow geometry, three different types of vessels are chosen, a bulk carrier with a full bow, a Ro/Ro vessel and a container vessel with a V-shaped frame design. For identifying the influence of the wave height and steepness on the vertical bending moment, model tests in regular waves with different crest/trough asymmetries are performed with the Ro/Ro vessel and the bulk carrier. The program can be subdivided into three parts, each characterized by the same wave lengths with varying wave steepness. The first test series includes regular waves with small amplitudes, thus linear wave theory can be applied. In the second part the same (regular) wave lengths have been generated with increased wave heights, i.e. increasing crest/trough asymmetries and wave profiles within Stokes II domain. During the last part of the experimental program the wave heights are further increased to reach wave profiles within Stokes III domain. For the evaluation of the test results in regular waves — in particular in high steep waves — the results are compared to the respective Response Amplitude Operator determined by the transient wave package technique. Here the focus lies on the asymmetry of the hogging and sagging loads with respect to the wave steepness and the bow geometry of the investigated ship models. Furthermore, the influence of the freeboard height on the vertical bending moment is analysed. For this purpose a container vessel is investigated with two different freeboard configurations in a harsh wave environment.Copyright

Critical Situations of Vessel Operations in Short Crested Seas—Forecast and Decision Support System

The encounter of extreme waves, extreme wave groups, or unfavorable wave sequences poses dangerous threats for ships and floating/stationary marine structures. The impact of extreme waves causes enormous forces, whereas an unfavorable wave sequence—not necessarily extreme waves—can arouse critical motions or even resonance, often leading to loss of cargo, ship, or crew. Thus, besides a well thought-out maritime design, a system detecting critical incoming wave sequences in advance can help avoiding those dangerous situations, increasing the safety of sea transport or offshore operations. During the last two years a new system for decision support onboard a ship or floating/fixed marine structure named CASH—Computer Aided Ship Handling—has been introduced. The preceding papers showed the step wise development of the main components of the program code—3d-wave forecast and 3d-ship motion forecast . These procedures provide a deterministic approach to predict the short crested seas state within radar range of the ship, as well as resulting ship motions in six degrees of freedom. Both methods have been enhanced with special focus on the speed of calculation to ensure a just-in-time forecast. A newly developed component is the adaptive 3d-pressure distribution . This method calculates the pressure distribution along the wetted surface of the ship hull using a newly developed stretching approach. With the end of the joint project Loads on Ships in Seaway (LaSSe), (funded by the German Government) the paper presents the CASH system, giving the possibility to detect critical situations in advance. Thus not only decision support onboard a cruising ship can be provided, but also time windows for offshore operations are identified well in advance.