Sascha Kosleck

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

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.

Forecast of Critical Wave Groups From Surface Elevation Snapshots

Reports on damages of ships, cargo and structures during heavy seas have been increasing within the last years. The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of crew, ship and cargo. Dangerous situations can be predicted by a forecast of encountering wave trains and the identification of critical wave groups. The paper presents a method to calculate the wave train a ship will encounter from surface elevation snapshots of the surrounding sea, taken by the ship radar. The time-dependent surface elevation snapshot far ahead of the ship is transferred into frequency domain by the use of Fast Fourier Transformation (FFT). The resulting complex Fourier spectrum given over the inverse wave length 1/L is converted into an amplitude spectrum and a phase spectrum. By shifting the phase spectrum to the position of the cruising ship the encountering waves can in turn be calculated in advance — depending on speed. The permanent processing of incoming snapshots delivers a continuous prediction of the water surface elevation at the position of the cruising ship. Based on these data the expected ship motion behaviour can be calculated continuously in time domain. In addition the response spectra, resulting from the wave spectrum and the relevant RAOs, are also evaluated. As wave data far ahead of the ship are used, it allows a forward glance, and dangerous situations, particularly resonance and parametric resonance are detectable before the ship is encountering this wave train. Consequently, the procedure can be used by the master as an assistance support system.Copyright

Forecast of Critical Situations in Short-Crested Seas

The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of ship, cargo and crew. One approach to avoid dangerous situations is to adjust heading and cruise speed. To identify critical situations well in advance the forecast of the incoming wave train is essential. Concerning the method to predict the wave train a ship will encounter within the near future — some minutes ahead — the so far unidirectional WAVE FORECAST method, pre-calculating an encountering wave train from surface elevation snapshots of the surrounding sea — taken by radar — has been improved. This paper presents a method to predict the entire sea state within the surrounding area of the vessel considering multidirectional waves. Thus the evolution of critical waves coming from various directions can be predicted. In addition the SHIP MOTION FORECAST method — pre-calculating the vessel response — has also been enhanced. Taking into account the encounter angle of the incoming wave components, depending on time and course angle of the vessel, the ship-fixed compass rose is divided into a number of sectors. The corresponding encountering wave train for every sector is derived by superimposing all wave components coming from certain directions. With a set of directional R esponse A mplitude O perators (RAOs) for the six degrees of freedom the sector-wise vessel responses can be calculated as well. The response spectra are derived in frequency domain and transferred into time domain by the use of I nverse F ast F ourier T ransformation (IFFT). Thus the overall vessel response is obtained by superimposing the time domain responses for every sector and degree of freedom, delivering a comprehensive data base for the analysis of critical situations in advance.Copyright

Computational Fluid Dynamics for the Simulation of Oil Recovery Systems at High Seas

The paper presents multi-phase CFD-Calculations for simulating oil skimming processes in heavy seas. During the last years tanker catastrophes showed the shortcomings of existing oil recovery systems, especially while operating in heavy seas. For developing new and more efficient devices complex and expensive model tests must be conducted under special conditions to prevent environmental pollution. To minimize these costs CFD-tools for multi-phase flow simulations have been developed, and are applied to analyse and optimize oil recovery devices. The analysis of local flow phenomena dependent on the motion of an oil recovery system in a given sea state are the basis for the development of an optimized oil recovery device. For this purpose, existing nonlinear numerical methods used for stationary and unsteady viscous computation (based on Volume of Fluid (VOF) methods and Reynolds Averaged Navier Stokes Equations (RANSE)) are enhanced and combined to simulate two-phase (air, water) and three-phase-flow (air, water, oil). New methods for simulating motions in three (2D) and six degrees (3D) of freedom as well as for the generation of waves — regular and irregular sea states — are developed. To increase the speed of calculation the RANSE/VOF-method is coupled with a Potential theory method using Finite Element discretization (Pot/FE). Combining the advantage of the Pot/FE-solver, i.e. calculation speed, with the possibilities of the RANSE/VOF-solver to simulate multi-phase flow and free body motion offers the opportunity to simulate a complete test in reasonable time. To validate the procedure, the numerical simulations are compared to WAMIT-calculations and model tests carried out in a physical wave tank.Copyright

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

The encounter of extreme waves, extreme wave groups or of unfavourable wave sequences is a dangerous thread for ships and floating/fixed marine structures. The impact of extreme waves causes enormous forces whereas the encounter of an unfavourable wave sequence — not necessarily extreme waves — can arouse critical motions or even resonance, often leading to loss of cargo, ship and 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 (see [1] and [2]) a new system for decision support on board a ship or floating/fixed marine structure named CASH — C omputer A ided S hip H andling — 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 6 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 [3]. With the end of the joint project LaSSe — Loads on Ships in Seaway (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 on board a cruising ship can be provided, but also time windows for offshore operations are identified well in advance.Copyright

NUMERICAL ANALYSES OF SCALING EFFECTS AND THE FLOW FIELD INSIDE AN INNOVATIVE SEA STATE INDEPENDENT OIL SKIMMING SYSTEM — SOS

Due to improved regulations and high safety standards, the quantity of oil spilled in tanker accidents is notably decreasing over the last years. Despite this progress, the factor of human failure and hence the eventuality of an oil spill with catastrophic ecological and economical consequences can never be excluded. Current oil recovery systems are able to operate in wave heights up to 1.5 m. In severe weather they have to wait until conditions are improving. To prevent emulsification and weathering processes, it is necessary to skim the oil film off the sea surface shortly after the accident. This can only be achieved by an oil recovery system with high transit velocities on the one side, and the capability of operating in rough seas on the other side. A S ea state-independent O il S kimming System (SOS) that satisfies these requirements has been developed and gradually optimized in various numerical and experimental analyses. The skimming process was already successfully validated in compliance with Froude’s law, but scaling effects were observed. In order to understand the complex flow phenomena around and through the SOS, numerical analyses based on a RANSE/VOF (R eynold-A veraged N avier-S tokes E quations / V olume O f F luid) approach are conducted. Here, in addition to Froude’s and Morton’s law, viscous effects and the physical characteristics of oil and water are also scaled correctly according to Reynolds’ law. The results of these investigations present essential information for the transfer of the entire oil skimming process to full scale. This paper presents a further step of development towards a marketable system (see Fig. 1). The latest innovations — which are already applied for a patent — comprise a hermetically closed moon pool with pressure regulation and a hatch system with separate inlet and outlet flaps. In addition to the improved hull strength, this new design enables better individual control of the oil skimming process — hence the efficiency in dependency of environmental conditions — by pressure control of the water fluid level inside the moon pool.Copyright

Three Phase Flow Simulations of an Oil Recovery Ship in Various Sea States

The analysis of local flow phenomena, in particular the analysis of the oil flow and the oil-water separation process in a three phase flow simulation (air, water, oil), including the free water surface, is a basic need for the development of an efficient oil recovery system such as the S eaway Independent O ilskimming S ystem (SOS). As the oil separation process is highly dependent on the ships motions, its seakeeping behaviour needs to be simulated accurately. The paper presents two-phase flow simulations (air, water) of the seakeeping behaviour in three and six degrees of freedom (two- and three-dimensional — 2D/3D). The vessel motions simulated in various sea states are validated by model tests conducted in a physical wave tank. The grid resolution as well as the flow parameters of the simulation have been varied to find a fast and reliable solution. The need for three dimensional simulation runs is questioned, as two dimensional simulations give nearly the same results and are far less time intensive. Oil is introduced as the third phase. The associated analysis illustrates the oil-water separation process and yields the systems efficiency in dependency of the sea state conditions. Based on the results of three-phase simulations, the operational range of the Seaway Independent Oilskimmer is determined and recommendations for the system optimization can be made.Copyright

Oil Skimming Efficiency of the SOS: Scaling From GeoSim Model Tests to Full Scale Prototype Operations

The severe ecological and economical aftermath of the 2010 ‘Deepwater Horizon’ catastrophe in the Gulf of Mexico clearly shows the insufficiency of current oil recovery systems which cannot operate in wave heights above 1.5m. To prevent emulsification and weathering processes, it is necessary to skim the oil film off the sea surface shortly after the accident. The autonomous SOS (Sea State-independent Oil Skimming System) developed within the framework of the research project SOS3 features high transit velocities, the capability of operating in rough seas and a massive intake of oil polluted water — and is therefore a unique technology. The oil water separation process of the SOS is purely based on hydrodynamic principles involving vortex evolution and a special flow pattern inside the internal moon pool. These requirements for efficient oil skimming operations depend on various hydrodynamic effects that would imply model testing in compliance with Froude’s and Reynolds’ law simultaneously — a physically impossible condition. Therefore GeoSim model tests with the SOS at model scales of 1:16, 1:25 and 1:36 are conducted with discrete particles of the correct density substituting the oil phase. The tendencies in flow pattern evolution and oil skimming efficiency are compared and extrapolated to full scale. Results from open water tests with the prototype of the SOS in the mouth of river Elbe serve for validation of the extrapolated results.Copyright

A HYBRID ANALYSIS METHOD FOR INVESTIGATING OIL CLEANUP OPERATIONS AT SEA

The severe ecological and economical aftermath of the 2010 ‘Deepwater Horizon’ catastrophe in the Gulf of Mexico clearly shows the insufficiency of current oil recovery systems which cannot operate in wave heights above 1.5 m. To prevent emulsification and weathering processes, it is necessary to skim the oil film off the sea surface shortly after the accident. The autonomous SOS (Sea State-independent Oil Skimming System) developed within the framework of the research project SOS3 features high transit velocities, the capability of operating in rough seas and a massive intake of oil polluted water — and is therefore a unique technology. Numerical analyses of realistic oil skimming operations require three-dimensional transient three-phase flow simulations in order to take into account wave-induced ship motions. Due to the computational effort, a simplified approach is chosen for systematic investigations. Model tests at a scale of 1:25 are conducted in two different irregular sea states. In order to represent oil cleanup operations as realistically as possible, towing tests at constant velocities are replaced by a series of experiments with a free-running model. A self powered tug pushes the oil skimming barge and experiences interactions with waves, influencing the velocity and therefore the skimming performance of the SOS. Data from optical motion measurements of the barge is then used as input for two-dimensional CFD simulations. The numerical analyses are focusing on the oil-water-separation process of the realistically moving coupled system in different irregular sea states.Copyright