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Item type:Publication, Access status: without files , Numerical studies of installation and extraction processes of jack-up ships(American Society of Mechanical Engineers (ASME), 2011-06); ; ; New Jack-up ships are built to transport offshore wind turbines from harbor and assemble them with the help of cranes on the ship at the site of installation. Every ship has four stilts, which can be lowered to seabed and jack the ship up. However, penetration and extraction the stilts at the harbor area could jeopardize the existing quay wall construction. In this paper two conceptual designs of foundations, which are used to improve the strength of the seabed in front of the quay wall, are investigated using numerical methods. Through soil exchange in openended cylinder or bucket foundation the strength of the seabed should be improved so that jack-up ships can dock directly at the quay wall and lower the stilts into the foundations. Three dimensional finite element analysis using a coupled Eulerian- Lagrangian method (CEL) are carried out to simulate penetration and extraction process. A hypoplastic constitutive model and the Mohr-Coulomb constitutive model are used to describe the filling. The studies of influences of geometry of foundations as well as the soil parameters on the bearing capacity of stilts and failure mechanism in foundations are carried out as the design basis of the foundations.Publicationtype: Conference Paper25 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Analysis of towing-gear concepts using iceberg towing simulations(American Society of Mechanical Engineers (ASME), 2011-06); ; The paper reports on the development and application of an iceberg towing simulation procedure, whose efficiency allows an on-board use. Different solution techniques for the integration of the governing equations are compared with respect to performance and accuracy. Supplementary, the predictive accuracy of the employed rope models is outlined. Moreover, the merits of two different concepts for the towing gear set-up, i.e. utilizing a floating polypropylene part together with a submerged steel hawser or a single floating line, are discussed. Investigated application examples focus on towing medium sized icebergs with changes in ship speed. Based on a given towing speed profile, the acting forces and accelerations on the iceberg and the rope tension are simulated. Furthermore, the influence of modelling parameters, e.g. added hydrodynamic masses, is reported.Publicationtype: Conference Paper27 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Impact tests in pure and aerated waterImpact loads due to slamming or ditching are an important issue in marine and aircraft engineering. Accordingly, many experimental, theoretical and numerical investigations have been performed. The paper reports the build-up of a test rig, which was designed to study vertical guided motion impact tests, and the results of an experimental study on impact loads into pure and aerated water. Attention is given to the loads experienced by a rigid circular-cylinder section (diameter=0.2 m, length=0.5 m) during guided, constant-speed impact. The investigations involve a variety of entry speeds up to 3m/s and gas-content levels. The obtained data offers a basis for an analysis of the sensitivity of the impact loads on the gas content. Fair compliance with literature reported experimental and theoretical results - using e.g. von Kármán or Wagner theories - has been obtained for pure water impact loads. For impact tests in aerated water, a significant reduction of the measured pressure peaks becomes apparent. Finally, the paper illustrates the feed back of the experimental results into an impact-load model, taking the fluid compressibility into account.Publicationtype: Conference Paper170 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of wave group characteristics on loads in severe seas(American Society of Mechanical Engineers (ASME), 2011-06); ; The precise knowledge of loads and motions in extreme sea states is indispensable to ensure reliability and survival of ships and floating offshore structures. In the last decades, several accidents in severe weather with disastrous consequences have shown the need for further investigations. Besides the sea state behavior and the local structural loads, one key parameter for safe ship design is the vertical bending moment. Previous investigations revealed that different ship design criteria, such as bow geometry and wave board height, affect the global loads significantly. Investigations in regular waves as well as in single high waves of vessels with different bow flares and freeboard heights show that the vertical bending moment increases significantly with increasing bow flare and freeboard height. Furthermore it became apparent that critical loads and motions do not have to come along with the highest wave which results in the main question of this paper: What is the worst case scenario - the highest rogue wave or a wave group with certain frequency characteristics? Which sea states have to be taken into account for the experimental evaluation of limiting criteria? This paper presents investigations in different critical wave sequences, i.e. two real-sea registrations accompanied by results in regular waves to evaluate the influence of the encountering wave characteristics on the vertical bending moment. For the model tests in the seakeeping basin of the Technical University Berlin a segmented RoRo vessel with large bow flare has been built at a scale of 1:70 and equipped with force transducers. The paper proves that critical loads and motions depend most notably on combinations of wave height, wave group sequences, crest steepness, encountering speed and the ships target position: Even small wave heights with unfavorable wave lengths can cause a critical situation.Publicationtype: Conference Paper219 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Formation of extraordinarily high waves in space and time(American Society of Mechanical Engineers (ASME), 2011-06); ; 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.Publicationtype: Conference Paper114 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Stability of temporary submarine slopes(American Society of Mechanical Engineers (ASME), 2011-06); ; Shallow foundation structures for offshore wind turbines offer ecological benefits compared to pile foundations as less noise is emitted at sea floor level during construction process. On the other hand, shallow offshore foundations can rarely be placed on top of the sea floor. Weak soils usually need to be excavated to place the foundation structure on more stable ground and thus, anthropogenic submarine slopes result. Steep but stable slopes meet both economic and ecologic aims as they minimise material movement and sediment disturbance. After Terzaghi [1] the angle b between slope and the horizontal of the ground surface of coarse-grained soil is at most equal to the critical state friction angle jc. However, it can be observed that natural submarine slopes of sandy soils are always much more shallow. Particularly fine-grained, cohesionless or almost cohesionless soils failed in the past, although the slope angle was much smaller than the critical state friction angle jc. Artificial (temporary) slopes do not appear and behave as natural submarine slopes, since the latter are already shaped by perpetual loads of waves, tide and mass movements. Physical simulations of different scales are used to analyse the stability of artificial submarine slopes with sandy soil of the North Sea. The study focuses on gravitational forces and impacts from the excavation processes. The simulations and theoretical considerations result in suggested slope angles for future shallow offshore foundations of wind farms in the North Sea.Publicationtype: Conference Paper52 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Time domain comparison with experiments for ship motions and structural loads on a container ship in abnormal waves(American Society of Mechanical Engineers (ASME), 2011-06); ; ; ; The paper presents experimental results from model tests with a containership advancing in abnormal wave conditions and comparisons with numerical simulations. A nonlinear time domain method based on strip theory is used for the calculation of vertical ship responses induced by abnormal waves. This code combines the linear diffraction and radiation forces with dominant nonlinear forces associated with vertical response arising from Froude-Krylov forces, hydrostatic forces and shipping of green water. The time domain simulations are compared directly with experimental records from tests with a model of a container ship in deterministic waves for a range of Froude numbers. Extreme sea conditions were replicated by the reproduction of realistic abnormal waves like the New Year Wave and abnormal wave from North Alwyn. Head sea condition is considered and the comparisons include the wave elevation, the vertical motions of the ship and the vertical bending moment at midship.Publicationtype: Conference Paper109 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Multiphysics SPH simulations with local particle coarsening(American Society of Mechanical Engineers (ASME), 2011-06); ; The paper reports on Smoothed-Particle-Hydrodynamics (SPH) for multi-physics water/soil interaction computations with dynamic particle coarsening. The procedure is supposed to be applied to harbour and ocean engineering hydrodynamic problems focussing on sediment scouring. This type of simulation usually implies large computational domains, fluid-soil interaction and complex geometries leading to large numbers of particles. To achieve a reasonable time-to-solution even for full-scale simulations, effective strategies to increase the computational performance are needed. We present a dynamic particle refinement/ coarsening strategy based on variable particle masses and spacings. The particle properties are updated in accordance to their current location. Validation studies refer to different water/ soil interaction test cases. Results obtained from the present refinement/coarsening approach show an encouraging predictive and computational performance.Publicationtype: Conference Paper134 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Accidents in situations with large stability values(American Society of Mechanical Engineers (ASME), 2011-06); ; ; ; In the last two years a number of accidents with several dead or heavily injured crew members have been reported to the German Federal Bureau of Maritime Casualty Investigation caused by excessive roll motions with large transversal accelerations. All accidents happened on board of container vessels in situations of large stability and partial draft like in ballast or in laidup situations. Also the accident situations were similar in all cases. During harsh weather conditions the crew tried to keep the ship against the incoming sea at constant slow speed. It was possible to reconstruct the accident events and the occurrence of high transversal acceleration values by numerical seakeeping simulations. To figure out if container vessels have an inherent danger of such problems a numerical study on a group of fifteen container vessels of different sizes was carried out in the mentioned accident situations. In this paper the reported accidents and the numerical investigation on the accidents are described, furthermore the results of the extended numerical study are presented and a phenomenological conclusion is drawn from the results.Publicationtype: Conference Paper98