R.H.M. Huijsmans

Advances in the Hydrodynamics of Side-by-Side Moored Vessels

In this paper a comparison between model basin experiments and results of diffraction computations on side-by-side moored LNG carriers is presented. The computations are based on a new lid method in diffraction codes to suppress non-realistic high wave elevations between the two floating objects. This lid method was originally formulated by Chen (2005). In this method a damping value is added to the free surface by means of a damping parameter. Since no theoretical solution can be found to establish the required value of the damping parameter, model basin experiments have been performed to determine this value. However from the results of the model basin experiments it is shown that it is difficult to obtain one unique value of the lid damping (for the 4m or small gap). The way of tuning the damping value of the lid is crucial. Tuning the damping based on first order results, like motions or wave height RAO’s will lead to a much larger variation in the estimate of the second order sway wave drift force transfer function. INTRODUCTION LNG carriers are used to transport the cargo from the floating LNG-FP(S)O to the shore or from terminal to terminal. ownloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 02/03/2016 T Offshore floating LNG systems become a more and more viable economic solution. The need for an accurate assessment of the hydrodynamic performance of LNG offloading systems is an important issue. LNG production offshore give rise to the need for a remote offshore berthing location, without the benefit of coastal protection against wind, waves and current. Calculation of the wave elevations at the waterline of the two floating bodies is necessary to calculate the wave drift force quadratic transfer functions (QTF’s) using a pressure integration scheme. In the model test campaign the wave heights are measured at the centerline between the two vessels, effectively at the wall of the model test basin. Unrealistic wave heights at the centerline will obviously also indicate their effects at the waterline between the two vessels. Conventional near field calculations result in unrealistic free surface elevations in the gap. Far field estimates of the QTF’s do not apply on the individual bodies of a multi body diffraction calculation. The middle field equation technique, to avoid spurious effects in pressure integration techniques is described by Chen (2006). The complex hydrodynamics of two floating bodies in close proximity has been a research topic for over two decades. Two main issues are to be considered. 1.The resonance behavior of the waves in the gap between the two vessels tends to be overestimated using standard diffraction programs. E.g due to non-linear effects in the physics of the waves like breaking tend to reduce the extreme wave elevations. 2. Viscous effects, neglected in diffraction theory, may be more dominantly present at the bilges of the keel due to the resonant flow between the two floating bodies. In our approach we disregards also any viscous effects at the tank wall and the hull of the vessel. For the viscous flow effects the current set-up is not applicable, then a full representation of the two side-by-side 1 Copyright

High order geometric methods with exact conservation properties

Conservation laws in, for example, electromagnetism, solid and fluid mechanics, allow an exact discrete representation in terms of line, surface and volume integrals. In this paper, we develop high order interpolants, from any basis that constitutes a partition of unity, which satisfy these integral relations exactly. The resulting gradient, curl and divergence conforming spaces have the property that the conservation laws become completely independent of the basis functions. Hence, they are exactly satisfied at the coarsest level of discretization and on arbitrarily curved meshes. As an illustration we apply our approach to B-splines and compute a 2D Stokes flow inside a lid driven cavity, which displays, amongst others, a point-wise divergence-free velocity field.

Real Time Wave Forecasting for Real Time Ship Motion Predictions

This paper presents results of a validation study into a linear short term wave and ship motion prediction model for long crested waves. Model experiments have been carried out during which wave elevations were measured at various distances down stream of the wave maker simultaneously. Comparison between predicted and measured wave elevation are presented for 6 different wave conditions. The theoretical relation between spectral content of an irregular long crested wave system and optimal prediction distance for a desired prediction time is explained and validated. It appears that predictions can be extended further into the future than expected based on this theoretical relation.© 2008 ASME

Extreme Wave Impact on Offshore Platforms and Coastal Constructions

Hydrodynamic wave loading on structures plays an important role in areas such as coastal protection, harbor design and offshore constructions (FPSO’s, mooring), and there is a need for its prediction up to a detailed level (max./min. pressures, duration of pressure peaks, shear stresses, etc.). In close cooperation with industry, long-year joint-industry projects are carried out to develop a numerical simulation method: the CFD method ComFLOW. The two major application areas are the prediction of extreme wave forces on offshore platforms and offloading vessels, and the prediction of impact forces on coastal protection structures. The paper will present a short overview of the method, some recent results and future plans.Copyright

Simple Wake Models for Tidal Turbines in Farm Arrangement

From wind turbines it is known that the wake, induced by a turbine, has a negative impact on the energy production of downstream devices. Basically, the wake is a zone with reduced velocity behind a turbine. Further downstream, the velocity recovers gradually by turbulent mixing with the ambient flow. In order to optimize the design of a tidal farm, the aim of this paper is to find simple relations that can be used to predict the energy output of a given farm configuration. The energy output of a turbine depends on its inflow velocity. Therefore, the strategy is to find a model that is able to predict the velocity field in the tidal farm. Such ‘wake models’ exist already for wind turbines and thruster-thruster interaction. In this research, the applicability of these wake models to tidal turbines is investigated by comparing their results to reference data of tidal turbines. Only limited measurement data for tidal turbines are available; therefore a CFD model of a tidal turbine is used to generate the reference data. The velocity in the wake is simulated for different conditions with the CFD model. The CFD model is validated with the available data in the literature. The velocity in the wake for a single turbine is predicted accurately for different initial conditions. Modeling of the turbulence showed some discrepancies in the far wake, consequently the wake of turbines in farm configurations is predicted less accurate. Three wake models, selected from the literature, are compared to the CFD simulations of the wake behind a single turbine. The wind turbine wake model of Jensen performed best; the velocity in the wake is calculated accurate for different situations. Mutual interaction of wakes will occur inside tidal farms. Several methods from wind turbines theory are used to estimate the velocity in interaction situations. Three basic situations of wake interaction are distinguished: tandem operation, wake interference and overlapping inflow. The interaction methods are tested with CFD reference data for each situation separately. Most methods compared reasonably well; the most suitable interaction methods are selected. A small tidal farm case study is performed to test the combination of wake model and interaction methods. The flow in the cluster of 5 turbines is predicted satisfactorily by the wake model for different inflow velocities. All results indicate that the principle of applying wind turbine wake models to tidal turbine has good potential. However the number of test cases conducted in the thesis is limited and the incorrect turbulence modeling of the CFD model caused some uncertainties for multiple turbine situation.© 2010 ASME

Turbulence Modeling, Local Grid Refinement and Absorbing Boundary Conditions for Free-Surface Flow Simulations in Offshore Applications

To study extreme hydrodynamic wave impact in offshore and coastal engineering, the VOF-based CFD simulation tool ComFLOW is being developed. Recently, much attention has been paid to turbulence modeling, local grid refinement, wave propagation and absorbing boundary conditions. The turbulence model has to cope with coarse grids as used in industrial applications. Thereto a blend of a QR-model and a regularization model has been designed, in combination with a dedicated wall model. Local grid refinement is based on a semi-structured approach. Near refinement interfaces special discretization stencils have been designed. The computational domain is restricted to the close environment of the objects studied. To suppress unphysical reflections, special generating and absorbing boundary conditions have been designed. The combined performance of the new ingredients will be demonstrated with several applications. For validation, experiments have been carried out at MARIN.

A Novel Method for Generating Continuously Surfable Waves: Comparison of Predictions With Experimental Results

In this paper, a novel idea to produce continuous breaking waves is discussed, whereby a pressure source is rotated within an annular wave pool. The concept is that the inner ring of the annulus has a sloping bathymetry to induce wave breaking from the wake of the pressure source. In order to refine the technique, work is being conducted to better understand the mechanics of surfable waves generated by moving pressure sources in restricted water. This paper reports on the first stage of an experimental investigation of a novel method for generating continuously surfable waves utilising a moving pressure source. The aim was to measure and assess the waves generated by two parabolic pressure sources and a wavedozer [I] for their suitability for future development of continuous breaking surfable waves. The tests were conducted at the Australian Maritime College (AMC), University of Tasmania (UTas) 100 metre long towing tank. The experimental results as variations in wave height ( H ) divided by water depth ( h ) as functions of depth Froude number ( Fr h ) and h , together with predictions from both methods, are presented in this paper. Finally, measures of the wave making energy efficiency of each pressure source, and the surfable quality of the waves generated by it, were developed and are presented.

Towards a General-Purpose Open Boundary Condition for Wave Simulations

For the design of FPSO’s in harsh environments an accurate assessment of the ability of the platform to survive in extreme sea conditions is of prime importance. Next to scaled model tests on the FPSO in waves also CFD capabilities are at the disposal of the designer. However even with the fastest computers available it is still a challenge to use CFD in the design stage because of the large computational resources they require. In that respect to use a small computational domain will improve the turn around time of the computations, however at the expense of various numerical artifacts, like reflection on artificial boundaries in the computational domain. In order to mitigate the reflection properties new absorbing boundary conditions have been developed. The work in this paper is constructed on the previous study about the generating and absorbing boundary condition (GABC) in the ComFLOW project. We present a method to apply the GABC on all the boundaries in a three dimensional domain. The implementation of the GABC in ComFLOW is explained in detail.Copyright

‘Wet Handshake’: Workability Study of an Offshore Thruster Exchange Operation

Thruster exchange operations are performed when an azimuth thruster on a DP operated vessel needs to be replaced for repair or maintenance purposes. At present these operations are performed either during a dry-dock call or in sheltered waters with the assistance of a shearleg or heavy lift vessel moored alongside the DP operated vessel. In order to reduce downtime of the DP operated vessel a clear trend is observed of operations being carried out more offshore and in deeper waters by means of a wet handshake between a heavy lift vessel (HLV) and a DP operated vessel. To get insight into the workability of such offshore thruster exchange operations it is important to study the dynamic interactions between the bodies involved. This paper describes the development of a methodology which accurately determines the workability of an offshore thruster exchange operation. The methodology is developed by BigLift Shipping, which has performed several thruster exchange operations in sheltered waters in the past few years. Furthermore the results of a workability analysis of a thruster exchange operation offshore Ghana are presented. In order to determine workability, first the hydrodynamic behavior including interaction effects of the coupled configuration of the DP operated vessel, the heavy lift vessel and the azimuthing thruster is calculated for a range of seastates, which results in the response spectra of the vessels. Then for each seastate the response is compared with the defined operational criteria to determine whether this seastate is workable, critical or non-workable, which results in a workability diagram. The workability can be quantified by combining this workability diagram with a wave-scatter diagram of the location of interest. In general this is a thoroughly studied topic. In order to achieve a higher level of accuracy of the workability prediction the focus of the methodology described in this paper has been pointed on two aspects that can be of significant influence. • First the persistency of a certain seastate is investigated. In the approach described above any variation of the environmental conditions over the duration of the operation is not considered. The effect of changing weather in time can be of significant influence on the overall workability. In order to take into account persistency information a numerical dataset of 3 hours statistics over a period of 10 years is used. Combining this information with the duration of all consecutive steps of the operation, results in a more realistic workability prediction. Similar approaches in seakeeping of ships are shown by Dallinga et al. (2004) [1], Naito et al. (2006) [2]. • Secondly the influence of a frequency domain approach compared to a time domain approach is analyzed. The nonlinear effects that occur in the coupled configuration of multiple bodies can not be taken into account in the frequency domain approach. Therefore the motion analysis is performed in the time domain. As an example case a thruster exchange operation offshore Ghana has been studied. The results of the study show that the workability prediction based on scatter diagram metocean data is influenced substantially when persistency information of metocean data is taken into account. The effect of the duration and criteria of independent operational steps on the workability are clearly visible in the persistency approach. This enables a to-the-point approach in improving the workability. Although time domain analysis is taking into account nonlinear effects, the difference between time and frequency domain analysis can be neglected for the presently studied configuration.Copyright

Ship Motion Predictions: A Comparison Between a CFD Based Method, a Panel Method and Measurements

In the past 50 years the research into the behavior of ships in seaway has shown a great deal of progress. From analytical solutions of 2-d hydrodynamics as derived by Ursell in 1949 to complex 3-d CFD numerical solutions that can be used nowadays. The first consistent approach for ship motions in time domain was derived by prof. T.F.Ogilvie [] as presented in his seminal paper in Bergen in 1964. Throughout the years however ship hull forms developed and the need for validation of computational procedures for the calculation of the ship motions has never gone away. In this paper we present a selection of results of a benchmark study performed by some 12 companies and universities using their state of the art computational tools. In this benchmark study the results of model tests of a modern container vessel are used. The results presented in this paper show that the panel method as described in this paper, based on the disturbed steady flow, leads to acceptable transfer functions for ship motions. The CFD approach used in this paper also produces acceptable motion transfer functions. However the results from the CFD computation for the internal load transfer functions do show a larger scatter when comparing with the results from model test.Copyright