annex d - stw-comflow3

7/21/2019 Annex d - Stw-comflow3

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Technische Wetenschappen

Extreme wave impact on offshore platforms and coastal structures

(Extreme golfkrachten op offshore platforms en kustwerken)

Applicants

prof.dr. A.E.P. VeldmanInstitute for Mathematics and Computing ScienceUniversity of GroningenP.O. Box 4079700 AK Groningenphone: 050-3633988 (office) / 3633939 (secr.)fax: 050-3633800e-mail: [email protected]

prof.dr.ir. R.H.M. HuijsmansDepartment of Ship Hydromechanics and StructuresTechnical University DelftMekelweg 22628 CD Delftphone: 015-2783598 (office) / 2786882 (secr.)fax: 015-2781836e-mail: [email protected]

Project leader prof.dr. A.E.P. Veldman

Key words hydrodynamic wave loading, non-linear surface waves, offshore platforms, coastal protection,CFD, viscous flow simulation, non-reflecting boundary conditions, two-phase flow

1 Summary

1.1 Research

Extreme waves and their impact loading on fixed and floating structures, like production and offloading plat-forms, coastal protection systems and offshore wind farms, have long been subjects that could only be studiedwith experimental methods. The complex, highly non-linear wave kinematics could not be predicted with ex-isting numerical methods (CFD). However, recent research by the partners of this proposal has shown that new

hydrodynamic models based on the Navier–Stokes equations, in combination with a VOF-based method for thedescription of the free-surface dynamics, are able to predict such effects. In two foregoing projects a two-phaseflow model has been developed. Good progress has been made in predicting load forces for flow phenomenalike sloshing, green water loading and wave run-up; also the cushioning effects of entrapped air is included inthe model. On the other hand, experimental validation has revealed aspects in the numerical model that needfurther extension and improvement. This relates to the physical and mathematical aspects of:

– Extreme waves and their propagation (to better model the oncoming waves until impact).– Effect of viscosity in shear layers (to model small-scale flow details of the endangered construction).– Interactive vessel-wave dynamics (to describe the coupled dynamics of wave and vessel motion).

Although first priority is on accurate description of physical phenomena, i.e. the functionality of the simulationmethod, for its daily use also computational efficiency is relevant. Thus another action will be:

– Speed-up through local grid refinement (to limit the number of grid points) and parallelization.

The proposal will focus on these vital and complex modelling issues, and to their computational implementation.Experimental validation with respect to the above-mentioned physical phenomena will also form part of theproject.

1.2 Utilization

The 2004/2005 hurricanes Ivan, Katrina and Rita in the Gulf of Mexico have refocussed attention to extremewaves and their consequences for coastal defense systems and offshore structures [76, 89, 109]. These hurricanescreated huge devastations both on land and at sea, causing many casualties and huge economic damage. Abetter understanding of the consequences of these forces of nature is urgently needed. Even without hurricaneconditions, the impact of extreme waves can be a serious threat to the land behind the dikes and its inhabitants.Also the safety and operability of offshore vessels and the well-being of their crews are jeopardized.



STW proposal: Extreme wave impact 2

Figure 1: Left: Damaged breakwater. Right: After hurricane Ivan, the Ensco offshore platform was found 40 miles from where it was anchored.

As indicated, hydrodynamic wave loading on structures plays an important role in areas such as coastalprotection, harbour design, offshore constructions (production and offloading platforms, offshore wind farms),and mooring systems. In these areas there is a need for knowledge on the prediction and predictability of

hydrodynamic loading, which can be required up to a very detailed level (max./min. pressures, duration of pressure peaks, shear stresses, etc.). In close cooperation with MARIN (Wageningen) and Deltares (formerlyDelft Hydraulics, Delft) two application areas are envisaged in this project. On the one hand, the simulationmethods to be developed will be applied in predicting impact forces on coastal protection structures. Onthe other hand, the simulation methods will be applied to predict the wave forces on offshore platforms andoffloading vessels. Generic examples of the physical phenomena encountered are:

– Wave run-up against fixed and floating platforms and coastal protection systems.– Wave impact loading on breakwaters and dikes.– Sloshing of the cargo inside the hull (e.g. LNG carriers).– Green water loading: sudden and extremely violent water motions on the deck.

The proposal will be a further step forward from the ComFLOW-2 joint-industry project (JIP) that iscurrently being carried out with 20 industrial partners from the offshore industry (oil companies, ship yards,etc.), under coordination of MARIN. Recently Deltares (then Delft Hydraulics) also joined this consortiumto cover coastal protection applications. Currently a follow-up JIP is being defined: ComFLOW-3. It willfocus on the validation of the developed numerical models for advanced engineering applications by improvedfunctionality and speed-up of the algorithms. The forelying proposal will be strongly intertwined with theComFLOW-3 project.

1.3 Summary in Dutch

Onderzoek

Extreme golven en hun belasting op vaste en drijvende objecten, zoals productie- en overslagplatforms, kustbe-schermingswerken en offshore windturbine parken, konden lange tijd uitsluitend met experimentele methodenworden bestudeeerd. De complexe en sterk niet-lineaire golfdynamica kon niet worden voorspeld met bestaandenumerieke simulatietechnieken (CFD). Echter, recent onderzoek door de partners in dit voorstel heeft getoond

dat nieuwe hydrodynamische modellen gebaseerd op de Navier–Stokes vergelijkingen, in combinatie met een opVOF gebaseerde methode voor de evolutie van het wateroppervlak, deze verschijnselen wel kunnen beschrijven.In twee voorgaande projecten is hiervoor een twee-fase model ontwikkeld; goede vooruitgang is geboekt in hetvoorspellen van golfkrachten bij verschijnselen als klotsen, groen-water belasting en oplopende golven. Ookhet ‘luchtkusseneffect’ van ingesloten luchtbellen vormt onderdeel van het model. Aan de andere kant, uitexperimenteel onderzoek is gebleken dat een aantal aspecten van het fysische en numerieke model verdereuitbreiding en verbetering behoeven. Dit betreft fysische en wiskundige aspecten gerelateerd aan:

– Extreme golven en hun voortplanting (om de aankomende golven beter te beschrijven).– Het effect van viscositeit in grenslagen (om kleinschalige stromingsdetails beter te modelleren).– Interactieve vaartuig-golf dynamica (om de gekoppelde beweging van golven en drijvende objecten te

beschrijven).




Hoewel de prioriteit in het voorstel ligt bij een nauwkeurige beschrijving van de genoemde verschijnselen, d.w.z.bij de functionaliteit van het model, is voor dagelijks gebruik ook rekentijd van belang. Een verdere activiteitbehelst daarom:

– Versnellen van de rekenmethode door lokale roosterverfijning (om het aantal benodigde roosterpunten tebeperken) en parallellisatie.

Het voorstel concentreert zich op deze belangrijke complexe modelleeraspecten, en op de implementatie daarvanop moderne multi-processor rekenapparatuur. Daarnaast maakt experimentele validatie op bovengenoemdefysische aspecten deel uit van het voorstel.

Utilisatie

De orkanen Ivan, Katrina en Rita in 2004/2005 hebben de aandacht hernieuwd voor extreme golven en hun gevol-gen voor kustbescherming en offshore constructies [76, 89, 109]. Deze orkanen leidden tot zware verwoestingen,zowel aan land als op zee, met veel slachtoffers en grote economische schade. Een beter begrip van de gevol-gen van dit natuurgeweld is dringend vereist. Zelfs zonder orkaangeweld kunnen extreme golven een serieuzebedreiging vormen voor het land achter de dijken en zijn bewoners. Ook de veiligheid en inzetbaarheid vanoffshore platforms en vaartuigen, evenals de gezondheid van hun bemanning, kan in gevaar worden gebracht.

Zoals geschetst speelt hydrodynamische golfbelasting een belangrijke rol bij kustbescherming, havenontwerp,offshore constructies (productie- en overslagplatforms, offshore windparken) en aanlegsystemen. Op deze ter-reinen bestaat grote behoefte aan meer kennis over het voorspellen van de optredende golfkrachten en golfbe-lastingen. Soms is deze nodig tot op gedetailleerd niveau (maximum en minimum drukken, duur van drukpieken,schuifspanningen, etc.). In samenwerking met het MARIN (Wageningen) en Deltares (vroeger Delft Hydraulics,Delft) bestudeert dit voorstel twee toepassingsgebieden. Aan de ene kant zal de te ontwikkelen simulatieme-thode worden gebruikt bij het voorspellen van golfkrachten en -belastingen op kustbeschermingswerken. Aande andere kant wordt de methode ingezet bij het voorspellen van hydrodynamische krachten en belastingen opoffshore platforms en aanlandingsvaartuigen. Voorbeelden van de te bestuderen verschijnselen zijn:

– Golfoploop tegen vaste en drijvende platforms en kustwerken.– Golfkrachten op strekdammen en dijken.– Klotsen van vloeibare lading (bijv. in LNG tankers).– Groen-water belasting: grote hoeveelheid overslaand water op het dek.

Het voorstel vormt een volgende stap na het ComFLOW-2 ‘joint-industry’ project (JIP) dat de afgelopen jaren is uitgevoerd met 20 industriele partners uit de offshore industrie (oliemaatschappijen, scheepswerven,

etc.), onder coordinatie van het MARIN. Onlangs is ook Deltares (destijds Delft Hydraulics) toegetreden totdit consortium. Momenteel wordt een vervolg JIP gedefinieerd: ComFLOW-3. Het richt zich op de verdereontwikkeling en validatie van simulatiemethoden voor genoemde technologische toepassingen door middel vanverbeterde functionaliteit en versnelling van de onderliggende modellen en algoritmes. Het voorliggende voorstelis sterk verweven met dit ComFLOW-3 project.

2 Research group

The proposed project will be carried out in a close cooperation between the group Ship Hydrodynamics atTU Delft (headed by prof.dr.ir. R.H.M. Huijsmans), and the group Computational Mechanics and NumericalMathematics at the University of Groningen (headed by prof.dr. A.E.P. Veldman). Both groups take part inthe J.M. Burgerscentrum, the Dutch graduate school for fluid dynamics.

Furthermore, the Maritime Research Institute MARIN in Wageningen and the hydrodynamic group of Deltares in Delft will contribute substantially to algorithm development and validation. The major experimentsare carried out at MARIN. The overall project manager will be prof.dr. A.E.P. Veldman. For a further subdi-vision of the tasks carried out by the team members we refer to Section 3.6.2.




The main contributors to the project are:

name affiliation funding % fte task

University of Groningen:

prof.dr. A.E.P. Veldman RUG RUG 20% pro ject manager; supervisor CFDPhD vacancy RUG STW 100% viscous flow modellingPhD vacancy RUG STW 100% numerical efficiencypost-doc (dr.ir. R. Luppes) RUG STW/industry 100% interactive body motion; software mgmt.dr.ir. K.W.A. Lust RUG RUG 5% consultant code implementationdr.ir. R.W.C.P. Verstappen RUG RUG 5% consultant turbulent CFDdr.ir. F.W. Wubs RUG RUG 5% consultant parallel solvers

TU Delft:

prof.dr.ir. R.H.M. Huijsmans TUD TUD 10% sup ervisor hydro dynamicsPhD vacancy TUD STW 100% wave propagation

GTI’s:

dr.ir. T.H.J. Bunnik MARIN MARIN 30% mo del tests; b ody motion; extreme wavesdr.ir. G. Vaz MARIN MARIN 5% turbulence modellingdr.ir. M. Borsboom Deltares Deltares 20% GABC, wave modeling, CFDTBD Deltares Deltares 7% consultant waves/CFD

A highly experienced candidate for the post-doc position is available: dr.ir. R. Luppes. He is currently employed

in the ComFLOW-2 project, where he contributes to algorithm development and to software management(support of developers and users, version control, etc.).

3 Scientific description

3.1 Research objectives

3.1.1 Problem statement and global approach

The 2004/2005 hurricanes Ivan, Katrina and Rita in the Gulf of Mexico have dramatically refocussed attentionto extreme waves and their consequences for coastal defense systems and offshore structures [76, 89, 109]. Thesehurricanes created huge devastations both on land and at sea, causing many casualties and enormous economicdamage. Thus a better understanding of these forces of nature is urgently needed. The Netherlands have a

long tradition in taming the sea, with much expertise on coastal protection collected in Deltares (formerly DelftHydraulics), whereas much experience in offshore engineering is gathered at MARIN.Waves and currents can induce large forces and stresses on and near hydraulic structures. The hydrody-

namic load is responsible for unwanted effects such as vibrations and fatigue, damage to the revetment of bothstructures and their base, and scour around structures [102]. For example, the principal forces that sea dikesand other coastal defence systems have to cope with are the forces due to wave impact, which can be very large.The revetment of sea dikes has to be designed such that it withstands the strong forces that may occur duringa severe storm. Other important phenomena are wave run-up and overtopping, but also the wave-induced flowpatterns near, e.g., intake and outlet structures. Insight in these issues plays an essential roll in studies onsafety, water quality, economic operation and maintanance, and optimization of design. In the Netherlands,substantial investments are involved when it concerns the protection of the land against the sea.

Similar issues apply to the safety of sea-going vessels and offshore constructions (production and offloadingplatforms, wind mill parks) which have to operate under extreme weather conditions [86]. The Discovery

Channel television series “Deadliest Catch” has recorded several instances of the power of nature, e.g. thecapsizing of the Aleutian Ballad due to a 60-foot rogue wave [62]. In these heavy storms, wave and vesselmotions can become so large that solid amounts of (green-colored) seawater flow over the deck of a ship. Thisproblem is known as ‘green water’ loading, as distinguished from ‘white water’ consisting of spray and foam.Green water is considered a serious threat to the safety and operability of naval and merchant vessels. CaptainD. MacIntyre described the situation during ‘The Battle of the Atlantic’ [94]:

”Their hulls whipped and shuddered in the huge Atlantic seas... solid green water swept destructively along their decks... For hour after hour this process repeated itself. Damage mounted, hull plates splitting, boats being smashed, men swept overboard and delicate anti-submarine devices put out of order...”.




At present these phenomena are mainly studied experimentally, and the results are used for the formulationof design rules. There is a growing need for a numerical simulation tool capable of predicting in detail thehydrodynamic load due to waves and currents, and its effect at and near structures (see e.g. [42, 72,95]). Anumerical model has the advantage that a simulation can quickly be adapted to small changes in geometry orconditions, that scaling effects can be avoided, and that detailed insight in the hydrodynamic processes can beobtained. The instantaneous availability of a numerical model is another important advantage.

The tools currently available are hardly capable of predicting such events to an acceptable level of accuracy,

as these tools largely depend on the application of simple models based on e.g. linear potential flow theoryor shallow-water theory. In contrast, the physical phenomena accompanying extreme events are both highlynon-linear and highly dispersive due to the occurring wave steepness, and require new methods as a basis forthe prediction of the water flow and it induced hydrodynamic loads.

The mathematical model for complex water flow dates from the first half of the 19th century already and isknown as the Navier–Stokes equations. However, it is only for about a decade that these field equations can besolved for large-scale complex free-surface flow problems, thanks to novel numerical algorithms and the increasein computer power. This is an important development; in the near future it should provide, besides modeltesting, an additional tool for design problems involving these types of flows. This numerical tool is relativelycheap (in comparison with the operational costs of a model basin), therefore it can be used in an early stage of the design process.

In this research project it is proposed to apply a hydrodynamic flow model based on the Navier–Stokesequations to simulate the steep waves near, at and around fixed and floating structures, e.g. offshore platforms

and coastal breakwaters. The evolution of the free water surface is described by an adapted and (highly)improved version of the Volume-of-Fluid method (VOF) designed originally by Hirt and Nichols [87]. Specifically,use will be made of the ComFLOW code developed at the University of Groningen (RUG) and described infull detail in the, on-line available, PhD theses of Gerrits [6], Fekken [4], Kleefsman [12] and Wemmenhove [33].A global description can be found in the journal papers [13, 23].

3.1.2 History and background

Spacecraft dynamics The first steps towards the development of the free-surface simulation method Com-

FLOW were made in the late seventies, when Veldman and his colleagues at the National Aerospace LaboratoryNLR were studying the influence of liquid propellant (or other liquids) onboard spacecraft. This resulted in theSavof code [25]. It was already in this period that the first contacts were made with Deltares (then WL DelftHydraulics), who extended the NLR-developed simulation method Savof (still two-dimensional) to study wave

impact against coastal protection systems (dikes, etc.): this resulted in the Skylla code [53–55,58]. Later, inthe mid-nineties the development of free-surface simulation methods was continued at RUG (Veldman’s currentemployer) to create a method for fully three-dimensional flow; see e.g. [6–10]. One of the highlights, early 2005,was the flight of the experiment satellite Sloshsat FLEVO. With this NLR-built spacecraft a large series of experiments concerning liquid sloshing under micro-gravity were performed [17, 18, 23].

Under micro-gravity, capillary effects at the free liquid surface are dominantly present. Since these effectsare proportional to the curvature of the liquid surface, high accuracy requirements are put on the descriptionand displacement of the free surface. To meet these requirements, the original VOF treatment for reconstructionand movement of the free surface (with its considerable amount of ‘flotsam and jetsam’; see Fig. 7) has beenextensively redesigned. The use of a local height function was found to be crucial [6].

Maritime applications In the late nineties, MARIN learned of the micro-gravity applications at RUG with

their dynamic free-surface motion. The idea came up to test this approach for violent wave motion at sea. Thecooperation between RUG and MARIN started in 1998 with an MSc project concerning green water loading,where a fixed bow was subjected to a simplified green water event [3,5]. This project was later continuedas a MARIN-funded PhD project, where the physics was extended to include the coupled dynamics betweenthe incoming water and the vessel motion [4, 45]. In 2000 the EU-funded SAFE-FLOW JIP (Joint IndustryProject) on hydrodynamic wave loading was defined. Here, the ComFLOW development was co-supported by aworld-wide consortium of offshore-related companies. The project was aimed at identification of major physicalphenomena relevant for the effects occurring during extreme wave events and to draw conclusions regardingpotentially successful (numerical) methods of analyses. As a model testcase for green water research, the first(one-phase) dambreak results were produced, as shown in Fig. 2. Further results can be found in the PhD thesisof Kleefsman [12], and several publications [13–15, 22, 24, 46–48,50, 56, 59, 60].

As a follow-up, in 2004 the ComFLOW-2 JIP was started; it was partly funded by STW (project GWI.6433).




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Figure 2: Water impact against an obstacle in a dambreak setting: experiment (left) versus computation (right). The middle figure shows a comparison between measured and computed pressure at the front of the box.The two-phase model developed in ComFLOW-2 clearly improves the, spikey, one-phase results.

In this project the functionality of the simulation method was enlarged, as described in Section 3.2.1 below.Since then, several industrial participants have actively been using the ComFLOW program; for a list werefer to Section 4.2. A large number of publications has resulted [21, 23,31–41,43, 44,49, 51,52, 57]. From thescientific point of view, two PhD theses emerge: Wemmenhove has defended his thesis on May 16 at RUG [33];Wellens at TUD is expected to defend his thesis early 2009.

Animations Animations of ComFLOW applications can be found at the website [1].

3.1.3 Numerical framework

ComFLOW belongs to the class of Volume-of-Fluid (VOF) methods. It solves the Navier–Stokes equationson a staggered grid [84], where for each computational cell the ‘VOF’-function indicates which fraction of itis filled with liquid [87]. In principle, this method is strictly mass conserving, which is highly relevant in ourapplications. This is in contrast to the level-set method [103], which is popular in other types of applicationswhere mass conservation is not an important issue.

The computational grid is chosen rectangular; the simplicity of the grid gives an easy geometric frameworkin which the position and slope of the surface can be accurately described. On unstructured grids always somekind of smearing of the surface is necessary to describe the position of the free liquid surface. This creates a

‘spongy’ surface, which will reduce peak pressures during impact. In our application (prediction of wave forces)this is not acceptable.

Rectangular (Cartesian) grids can be generated easily, and allow simpler data structures enabling easierdevelopment. Further, bodies can move freely through the grid. The body description is also of VOF-type,which keeps the shape of the body crisp. Note that an immersed boundary treatment [97] would smear outthe body, which again will suppress pressure peaks. Also, collisions of bodies are allowed, without the gridsbeing squeezed in between; the latter would have been the case when boundary conforming grids were used. Adrawback may be that due to the non-boundary conforming character, the resolution of viscous boundary layersrequires additional attention. However, sufficiently powerful ‘cut-cell’ techniques [2] are available in combinationwith local refinement.

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Figure 3: Sloshing: entrapped air in experiment (left); validation of pressure signal (right).




3.2 Scientific and technological steps forward made in ComFLOW-2

3.2.1 Functionality

The scope of work of the ComFLOW-2 initiative was determined in close cooperation with the participants. Atthe end of the ComFLOW-2 project, in spring 2008, the following has been achieved:

1. The interaction between water and air has been included in the ComFLOW program: two-phase flow.

Two-phase modelling makes it possible to study for example the cushioning effect of entrapped air duringwave impacts related to sloshing in LNG tanks [39, 51]. It required the development of a special typeof discrete density averaging formulas near the free surface (see Section 3.2.3). Figure 3 shows the clearimprovement in force (pressure) prediction that is obtained with two-phase modelling.

2. A new type of generating and absorbing boundary conditions (GABC) has been developed for the inflowand outflow boundaries. These are transparent for the incident waves and absorb the outgoing waves atthe same time. This functionality makes it possible to place the boundaries close to the object, therebyreducing the computational time considerably (see Sections 3.2.3 and 3.3.1).

Figure 4: Interactive motions of Snorre tension leg platform: model test and simulation (StatoilHydro [57]).

3. An interactive coupling between fluid forces, floater motions and mooring systems has been implemented.Fig. 4 shows a simulation of the Statoil Snorre tension leg platform. The algorithmic coupling is fullyimplicit (with subcycling inside each time step) and, hence, stable. However, it cannot be used yet inconjunction with (binary) third-party codes for the body motion, as then only information exchange pertime step is possible (see Section 3.3.4).

3.2.2 Model tests

Figure 5: Validation model tests carried out during the ComFLOW-2 initiative. From left to right: sloshing,wave run-up and CALM buoy motion.

In the ComFLOW-2 project, a well documented and accurate set of validation data has been obtained bymeans of model tests (Fig. 5):

a) Sloshing in an LNG tank (1:10 scale).b) Wave run-up on a semi-submersible.c) Hydrodynamic loads and motion response of a CALM (catenary anchor leg mooring) buoy in extreme

waves.




The first validation studies have already been carried out, in particular for the sloshing experiment shown inFig. 3 [23, 38, 39]. Also some preliminary studies of the wave run-up have been made; see Fig. 6 [31, 32]. Muchexperimental data is still waiting to be analyzed for validation purposes in the ComFLOW-3 pro ject.

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Figure 6: Validation of forces on semi-submersible: water height (middle) and pressure (right)

3.2.3 Algorithmic innovations in ComFLOW-2

Application of, seemingly well-established, numerical techniques to realistic flow problems often ruthlessly re-

veals any flaw in the numerical treatment. In the development of ComFLOW the situation was not muchdifferent. Thus, at several places inside the algorithm, numerical building stones had to be redesigned at a quitefundamental level. We mention some of these innovative ‘highlights’.

Local height function As mentioned above, the VOF method does suffer from ‘flotsam and jetsam’, i.e.loose droplets that separate from the liquid surface as a numerical artefact of the liquid displacement algorithm.This also leads to (sometimes serious) loss of mass, although in theory VOF is fully mass conserving. Tosuppress these unwanted effects a local height function was successfully introduced [6, 12] (Fig. 7). Recently,this approach has also appeared elsewhere in the literature, e.g. [61, 70, 77].

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Figure 7: ‘Flotsam and jetsam’ after a dambreak calculation without (left) and with (right) a local height function. The middle figure shows the mass loss.

Generating and absorbing boundary conditions GABC’s and other non-reflecting boundary conditions

for some generic variable φ are often based on a Sommerfeld-type condition: φt + c(κ)φx = 0, which works finewhen the propagation speed c of the waves (a function of the wavenumber κ) is known [78]. However, in deeperwater dispersion sets in, and any wave becomes a complex mixture of several simple wave components, eachpropagating at its own dispersive wave speed. In ComFLOW-2, a novel idea has been introduced to determine‘local’ wave velocities. The first observation is that a vertical wave profile can be written as φ = C 1eκz+C 2e−κz.The next step is to recognize that κ2 = φzz/φ. The dispersion relation gives the wave velocity c as a function of the wavenumber κ, and the latter observation makes it a function of φzz/φ. Herewith the Sommerfeld conditionbecomes

φt + c (

φzz/φ)φx = 0. (1)

It is solved implicitly in all terms involving φ, which does require some rewriting and approximation of thisexpression. This approach has been developed in close cooperation with Delft Hydraulics (presently part of Deltares) and is believed to be new. For details we refer to the forthcoming PhD thesis of Wellens at TU Delft.




Gravity-consistent discretization Two-phase calculations have to deal with a discrete treatment of thedensity, which varies significantly between the two phases. In grid points near the free surface some form of density averaging has to be used. These averaging formulas may lead to (sometimes large) unphysical velocities,often called spurious or parasitic currents [70,77, 85, 93, 98]. Their origin can be understood by considering anequilibrium configuration, with zero velocity, where a balance exists between body force (F) and pressuregradient, i.e.

1

ρ grad p = F ⇔ grad p = ρF ⇔ 0 = curl(ρF). (2)

The right-most equality also should hold in the discrete approximation, and herewith puts a requirement onthe way in which ρ is averaged. When F denotes gravity, averaging formulas for ρ that satisfy this requirementare called gravity-consistent. In most of the literature, the spurious currents are damped with some form of diffusion, or with other suppression techniques. However, explicit mentioning of the above requirement (2) thatfully prevents these unwanted currents is scarce, although some authors are close, e.g. [77].

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Figure 8: The effect of the gravity-consistent density averaging in a sloshing simulation. Left the ‘usual’ approach; right the newly developed approach.

3.3 Research areas in ComFLOW-3

The presentation in Section 3.2 shows clearly that the ComFLOW code has evolved into a powerful basic

computational tool for pursuing the challenges of violent weather-induced marine and coastal applications.Therefore, the ComFLOW-2 JIP partners are investigating a follow-up for a joint further development. It willstill have a clear research character (step by step discovery of new fields and thorough validation of results), butthere will also be a strong focus on the usability of the developed tool and the speed-up of the computations aswell. The objective of this ComFLOW-3 JIP is:

To further improve, develop and validate the ComFLOW program for complex free-surface flows in the offshore industry and make it useable for advanced engineering applications by improved func-tionality and speed-up of the algorithms.

In a recent progress meeting with the industrial participants (November 2007, Gyeongju, Korea), a numberof areas were identified where major improvements were highly desirable to be pursued in the ComFLOW-3project:

1. GABC’s for all wave-current combinations.2. Less dissipative wave propagation.3. Combined wave-type and viscous effects.4. Interactive body motions.5. Numerical efficiency.

Thus, in the present proposal, it is intended to further enhance and validate the functionality of the simulationmethod with respect to the above-mentioned aspects. We will discuss each of these aspects in more detail below.

3.3.1 Generating and absorbing wave-current boundary conditions

Extreme waves and current Only limited knowledge is available with respect to the modelling of extremesteep waves, sometimes called freak waves. Experiments in a laboratory environment [110] have shown that the




effect of current profiles and or bi-directional current is important. Wave groups whose bandwidth is narroweror spectral slope is steeper can have higher limiting wave steepness. Furthermore, when a wave group meetsstrong opposing currents that block high frequency wave components and steepens the spectral slope, the localgeometric steepness and front-and-rear asymmetry at the breaking onset increase remarkably. The study [112]shows that the steepness of unsteady, incipient breaking waves is altered by the sign and magnitude of currentshear (or vorticity). A current with a positive shear, as would be the case in a wind-driven current, reduces thesteepness of an unsteady incipient breaking wave. A negatively sheared current, such as the jet-like ebb current

at a tide inlet, leads to steeper incipient breaking waves. It is proposed to model this effect of the current onthe formation of freak waves by using the method developed by Van Groesen [63, 82, 83].

GABC The newly developed Generating and Absorbing Boundary Condition (GABC) (see Section 3.2.3)is intended to absorb waves at the inflow and outflow boundaries, and to generate irregular waves at theinflow boundary simultaneously. At the end of the ComFLOW-2 project the method is working well for low(two-dimensional) waves propagating perpendicular to the boundary. The GABC will be extended to (three-dimensional) short-crested outgoing waves and waves coming from different directions. Further, its ability tohandle non-linear waves will be improved. Also, the GABC will be made suitable to include the effects of stationary currents.

3.3.2 Less dissipative wave propagation

Diffraction methods, based on potential flow theory and boundary integral formulations, do a good job inpropagating waves without too much numerical damping. However, they are restricted to (approximately)linear flow regimes. The ability of field methods (like the Navier–Stokes VOF approach in ComFLOW) todescribe arbitrary non-linear wave motion comes with a price: the methods that are used to follow the free-surface evolution introduce spurious dissipation [101]. Several sources of wave dissipation can be distinguished:i) physical viscosity, ii) artificial viscosity due to discretisation of the momentum equations, and iii) the free-surface displacement algorithm. Due to items ii) and iii), the waves damp out faster than they would inreality.

The damping becomes more pronounced for steep waves. This not only influences the wave propagationitself, but also effects such as local wave run-up. Our ambition is to reduce the damping to at most 3% of theheight over one wavelength for all waves up to the break limit. Hereto, the use of higher-order schemes likeWENO [92] or CIP [111] may prove fruitful. We will investigate this in combination with adaptations in thefree-surface displacement algorithm. Currently, the pressure and velocity at the new time level are updatedaccording to the liquid position at the old time level. It is believed that this lag in time may be responsible foran overdose of damping; a more implicit treatment of the free-surface position will be developed.

3.3.3 Combined wave-type and viscous effects

Until now, the ComFLOW projects have primarily focussed on extreme wave effects that were highly momentumdriven and in which effects of viscosity were of relatively small importance. Therefore, boundary layers andturbulent dissipation have had no real attention yet. However, there are several applications in which the effectof viscosity, possibly in combination with wave effects, becomes important. In the ComFLOW-3 project thefollowing applications will be considered:

a. Wave interaction effects between floaters in close proximity (for example side-by-side offloading or offshoredischarge).

b. Sloshing in moonpools (openings in the deck through which the drilling rig passes into the sea).c. Roll damping (in waves) and the effect of bilge keels (to stabilize the ship).d. Seabed and wall friction (e.g. to describe scouring).

With these applications, a first step will be made towards realistic modelling of viscous flows in ComFLOW.The number of grid points required will increase to have sufficient grid resolution of the relevant (turbulent)flow details, making the increase of numerical efficiency (Section 3.3.5) more urgent. In the last decade muchprogress has been made in this respect. A new discretization paradigm has appeared in which discretizationis not tuned to minimizing local truncation error, but instead wants to preserve the most important analyticalproperties of the original differential operators. In particular, convection is a process that distributes energy toother (smaller) length scales – thus creating turbulence – but convection does not destroy or create energy. Inmathematical terms it is a skew-symmetric operator, and this property should also be present in its discreteapproximation. It turns out that such a symmetry-preserving discretization can deal with much coarser and




much more irregular grids than the more traditional methods [26–28, 71, 107]. For flow cases where the gridresolution remains insufficient, the use of partial-slip boundary conditions, as a surrogate shear-layer model,will be investigated.

The same philosophy can be used in designing turbulence models [11, 29]; again numerical and model diffusionshould not interfere with the convective energy cascade. In this way, it appears possible to achieve the quality of Reynolds-averaged (RANS) calculations (or even LES) on grids that are much coarser than before. We expectthat with this approach the influence of viscosity in our the engineering applications envisaged in the project

can be simulated on grids that are quite affordable in terms of computer time and memory.

3.3.4 Interactive body motions

The impact of waves against a ship is obviously highly dependent on the momentary position and motion of the ship’s bow: is the bow low in the water and going up, or does it rise high above the waves. This positionstrongly depends on the foregoing waves, and as such the whole wave group determines whether a special waveevent will be harmful or not. The calculation of interactive (floating) body motion requires the coupling of flowsolver and motion solver. A hierarchical (or weak) coupling between these solvers, i.e. once per time step, willnot always suffice, in particular when there is a large ratio between mass and (hydrodynamic) added mass. Thusthe use of a strong (preferably simultaneous) numerical coupling is required. When multiple floating bodies arepresent, such as in side-by-side moored vessels, large flexibility of the approach is required.

In the ComFLOW-2 JIP a first step has been made with an interactive coupling between the fluid forces

and the floater motion response. Because in this case the floater motion equations are relatively simple, theycould be added to the inner-most loop of the ComFLOW time step (subcycling), achieving effectively a fullysimultaneous (hence stable) coupling. When (often binary) ‘black-box’ software is used, only a weak couplingcan be achieved with information exchange once per time step. Such an approach is not always stable. A wayto overcome this problem is the application of a quasi-simultaneous coupling, where inside the flow solver asimple approximating model of the floater dynamics module is solved simultaneously with the flow. The weakcoupling then only has to cope with the difference between the ‘exact’ dynamics model and its approximation;with a suitable choice for the latter stability can be achieved. This type of approach is being used already for aquarter of century in aerodynamic problems [20, 66, 67], but it has not yet been applied in maritime problems.

As a special deliverable of the project, MARIN will make an interactive coupling between a module tocompute dynamic mooring line loads and ComFLOW.

3.3.5 Improved numerical efficiency

The computation times for a ComFLOW run can be considerable. Especially for three-dimensional simulationsa computation can take several days on a modern PC. Inclusion of viscous effects, with its finer grids, mayfurther increase these computation times. Thus, in order to make ComFLOW useful for advanced engineeringapplications a significant speed-up of the program is necessary. Several ways exist to achieve this. We will focuson the following actions:

1. Decreasing the size of the computational domain through more accurate GABCs.2. Decreasing the number of grid points through local refinement and/or more advanced discretization.3. Application of faster sparse-matrix solvers.4. More efficient implementation including parallelisation.

The ambition of the project is to reduce the computational time by at least a factor 2 by means of each of thenumerical actions 1, 2 and 3, and by a factor 4 (out of 8 cores) by means of parallelisation (action 4).

Ad 1. GABC The development of GABC to decrease the size of the computational domain has been describedin Section 3.3.1.

Ad 2. Local grid refinement and advanced discretization ComFLOW applies a structured rectangular Carte-sian grid. When small objects need to be modelled, this influences the grid spacing in the entire fluid domain.Therefore, it is not possible to model geometries that are small compared to the length scales of the waves with-out seriously increased computational effort. Examples are wave loads on slender structures like deck girdersand subsea structures in the splash zone (Fig. 9), and hydrodynamic loads on bilge keels.

The idea is to borrow from the vast literature on locally refined quadtree/octree grids. A general drawbackof these methods is the loss of structure in the grid, especially when a staggered discretization is used. The




bookkeeping and interpolation required to cope herewith do consume significant computational overhead. Fur-ther, there is as yet little experience with moving a free liquid surface across a refinement boundary. Thus, nearsuch refinement boundaries unwanted wave reflections may appear, destroying the accuracy of the calculations.Here the use of a symmetry-preserving discretization (see Section 3.3.3) will be quite profitable [16]. If theconsequences of the local grid refinement will turn out to be unacceptable (implementation too complex, loss of accuracy too large, insufficient parallel performance) alternative refinement techniques will be considered, likefor example domain decomposition.

Figure 9: Left: Wave loads on slender structures (Ekofisk); right: Subsea structure in the splash zone.

Ad 3. Faster matrix solvers Until recently, ComFLOW used a Successive Over Relaxation (SOR) methodwith automatically optimized relaxation factor to solve the pressure Poisson equation [64]. On vectorcomputersit runs upto 75% of the theoretical top speed, which makes it very tough for more advanced methods (multi-grid, Krylov subspace) to compete. However, the density discontinuity across the free surface in two-phase flow,and the symmetry-destroying properties of the Generating and Absorbing Boundary Condition (GABC, seeSection 3.3.1) cause slow convergence of this method. Therefore, other solvers like Bi-CGSTAB [108] or IDR[104], possibly preconditioned with multi-grid, will be investigated. A peculiarity of the system is created by theGenerating and Absorbing Boundary Condition (GABC, see Section 3.3.1). The occurrence of the second-order

z-derivative in (1) enlarges the usual seven-point stencil for which most iterative solution methods have beendeveloped. Thus, for this unusual stencil a dedicated iterative solver has to be specially designed, while at thesame time keeping in mind its parallelisation properties (Ad 4).

Ad 4. Parallelisation Parallelisation is another option to speed-up the ComFLOW computation. We chooseto port the code to ‘cheap’ shared memory multiprocessors (e.g. 4× quad core, 64 Gb), on which parallelisationis done via ‘smart’ compiler options and OpenMP. The scientific challenge lies in the development of a parallel’almost’-Poisson solver that can handle the time-varying irregular matrix structure due to moving liquid andobjects, due to local grid refinement (Ad 2 ) and due to the unusual stencil induced by the GABC (Ad 3 ).

3.3.6 Overview of algorithmic and modelling challenges

In order to reach the above new functionality and improved efficiency, several basic algorithmic and modelling

building blocks have to be devised. For an overview of these scientific innovations, we list them here once more(between parentheses the workpackage where they are tackled; see Section 3.6.2):

– GABC for extreme waves in three dimensions including the influence of current (WP 1).– A ‘wave-consistent’ free-surface displacement algorithm (WP 1).– Symmetry-preserving turbulence modelling in combination with free-surface flow (WP 2).– Partial-slip boundary conditions in a cut-cell setting (WP 2).– A quasi-simultaneous coupling approach for liquid-body dynamics (WP 3).– Local grid refinement in the presence of a free surface crossing the refinement interface (WP 4).– A parallel Poisson solver that can cope with the GABC-induced irregularity in the matrix entries (WP 4).




3.4 Validation

Model tests Extensive and well documented model test datasets have been obtained in the ComFLOW-2project (see Section 3.2.2). Focus in the ComFLOW-3 project is on further development and extension of functionality, using where possible these existing model test data for validation and benchmarking. A part of the JIP budget will be reserved for an additional series of model tests. The present STW application will alsoinclude a limited set of specific model tests for areas that are being newly developed, in particular devoted to

flow phenomena where viscosity plays a visible role. Amongst others, particle induced velocimetry (PIV) willbe used to measure the velocity fields. Possible options for the experiments are:

a. Wave effects between floaters in close proximity.b. Moonpool sloshing.c. Roll damping and the effect of bilge keels

Also model tests will be carried out that study extreme waves, short-crested waves and waves in stationarycurrent. MARIN is responsible for defining and carrying out these tests. Their precise scope will be determinedduring the project, in close cooperation with the participants.

Benchmarking A very important item in the ComFLOW-3 project is the benchmarking and validation of the ComFLOW program. This will be the primary task of FORCE Technology and will guarantee the qualityof each new ComFLOW version that will be delivered to the participants. We will discuss this issue in moredetail in the utilisation plan in Section 4.3.

3.5 Software development and user support

In a large project the integrity of the software used is of utmost importance. The development team has toadd new functionality to an existing code, which implies that they must understand the structure of the sourcecode and the meaning of many subroutines and variables. Daily assistence should be available, not only on thesoftware side but also on the algorithmic side (choice of flow model including boundary conditions, choice of discretization, etc.).

Further, every year an updated version of the code will be released. Therefore much attention has to be paidto keep the software under control. A clear modular structure is required, such that adaptations and extensionsonly influence a limited part of the code. Strict version management (e.g. CVS) has to be maintained, and usermanuals must be kept up-to-date.

The number of active users of the ComFLOW program is growing. Thereto training sessions (workshops)will be organized for users to become acquainted with the code. Nevertheless, users will inevitably run intoproblems, be it with understanding the user manuals or with bugs in the code. Also they may request newfunctionality, for instance in the postprocessing of the simulation results.

The above activities require a thorough knowledge of the code and its embedded simulation algorithm;they have to be carried out by a ComFLOW ‘connaisseur’. A candidate for performing this task is available:dr.ir. R. Luppes. He has a long-year experience in developing the code for various applications in maritimeengineering, spacecraft technology and cell biology [17–19, 23, 34, 37–39].

3.6 Project organization

3.6.1 Personnel and equipment

The research project will be executed by three PhD students (two at RUG and one at TUD) and one 4-year

post-doc (at RUG). Their funding is requested from STW, with industry co-funding part (0.2 fte/yr) of thepost-doc appointment. The supervisors at RUG and TUD are funded by their respective universities.

Also a signficant financial contribution to the project will be made by MARIN and Deltares. They willprovide a corresponding in-kind contribution in manhours.

Equipment and supporting manpower to carry out the experiments for validation of the simulation modelis available at MARIN.

3.6.2 Workpackages

The above mentioned activities have been grouped into workpackages. In most of these workpackages a combinedeffort is made by several project partners. The individual workpackages are discussed below, including a globaltime planning.




Workpackage 1: Wave generation and propagation

Task Sections 3.3.1 and 3.3.2: Extend the GABC to three dimensions, more extreme waves andthe effects of current. Reduce the numerical damping of wave propagation.

Carried out by PhD student at TU Delft; Deltares; MARINPlanning

Year 1 Deltares will make the PhD student familiar with the concept of GABC’s. Extension of the current 2D implementation to three dimensions (to cover radiated and diffracted wavesgenerated by a structure).

Year 2 Improvement of wave propagation; modified (more implicit) free-surface displacement algo-rithm with local height function; Deltares will act as inspirator. Validation.

Year 3 Extension of GABC’s to short-crested and more extreme waves, and to the influence of current. MARIN will ‘deliver’ the extreme waves.

Year 4 Validation. Preparation of PhD thesis.Funding STW 1 PhD; Deltares 90 ke in-kind; MARIN 50 ke in-kind.

Workpackage 2: Waves and viscous effects

Task Section 3.3.3: Model the effects of viscous shear layers and vortex shedding. More efficientturbulence models. Partial-slip boundary conditions.

Carried out by PhD student at RUG; MARIN; DeltaresPlanning

Year 1 Understanding the numerics of viscous shear layers. Carry out (simple) RaNS calculationsand find out the shortcomings of the present approach; MARIN will assist.

Year 2 Develop and implement efficient (symmetry preserving) turbulence model. Validation.Year 3 Develop and implement higher-order discretizations (with special attention to the cut-cell

boundaries). Assessment of efficiency gain.Year 4 Validation. Preparation of PhD thesis.

Funding STW 1 PhD; MARIN 25 ke in-kind; Deltares 20 ke in-kind.

Workpackage 3: Interactive body motion

Task Section 3.3.4: Development of a flexible, yet stable, fluid-(multiple)body motion algorithm,that can be weakly coupled to ‘black-box’ dynamics software. Validation with CALM buoy

experiments. Example: dynamic mooring lines module.Carried out by 0.4 post-doc at RUG; MARINPlanning

Year 1 More efficient time-integration method.Year 2 Development of unconditionally stable quasi-simultaneous coupling.Year 3 Validation. MARIN prepares module for dynamic mooring lines.Year 4 —

Funding STW 0.4 pd; MARIN 100 ke in-kind.

Workpackage 4: Numerical efficiency

Task Section 3.3.5: Reduction of computational effort by local grid refinement and paralleliza-tion. Special issue is the treatment of the free surface and body movement near refinement

boundaries.Carried out by PhD student at RUG; 0.3 post-doc at RUG; DeltaresPlanning

Year 1 Grid refinement; discretization mass/momentum near interface. Parallelization withOpenMP.

Year 2 Free-surface displacement across interface. Parallel Poisson solver.Year 3 Reduction of reflections at interface. Update of parallelization.Year 4 Validation. Preparation of PhD thesis.

Funding STW 1 PhD + 0.3 pd; Deltares 15 ke in-kind.




Workpackage 5: Model tests

Task Section 3.4: Experiments, e.g. PIV measurements, will be carried out that focus on the newfunctionality of ComFLOW, such as extreme waves, effect of current and viscous effects.

Carried out by MARINPlanning

Year 1 Experiments with (3D) short-crested waves.Year 2 Experiments with viscous effects.Year 3 Experiments with extreme waves and current.Year 4 Validation

Funding STW 100 ke; JIP 150 ke; MARIN 25 ke in-kind.

Workpackage 6: Software development and user support

Task Section 3.5: Daily support of development team (algorithmic CFD issues) and users of ComFLOW. Responsible for integrity of the code, its maintenance and its documentation(user manuals, training sessions, etc.). Yearly release of updated version.

Carried out by 0.3 post-doc at RUGPlanning

Year 1 Introduction of code to new PhD students; CFD algorithmic support.Year 2 User support and maintenance; new release.Year 3 User support and maintenance; new release.Year 4 Final release with workshop.

Funding STW 0.1 post-doc; JIP 0.2 post-doc.

3.6.3 Overview

An overview of the workpackages (including funding) is presented in the table below.

workpackage carried out by funded by

STW JIP in-kind

1: Wave generation and propagation

GABC, waves on current, wave damping AIO-TUD + Deltares 1 PhD 90 keExtreme wave input MARIN 50 ke

2: Waves and viscous flowResolution shear layers; discretization AIO-RUG, Deltares 1 PhD 20 keTurbulence modelling MARIN 25 ke

3: Interactive body motion

Algorithm development; coupling 0.4 pd + MARIN 0.4 pd/yr 25 keApplication: dynamic mooring lines MARIN 75 ke

4: Numerical efficiency

Local refinement AIO-RUG 1 PhDParallelisation; fast solver 0.3 pd + Deltares 0.3 pd/yr 15 ke

5: Model tests and validation

Experiments in model basin MARIN 100 ke 150 keValidation MARIN 25 ke

6: Software development and user support

Algorithmic support of development team; 0.3 pd 0.1 pd/yrcode integrity; new releases 0.2 pd/yr

3.6.4 Progress meetings

In a large project, regular progress meetings are necessary. Based on our experience in the SafeFLOW andComFLOW-2 projects, the following schedule for communication between the project members is planned.

– The post-doc will have the daily supervision of the PhD students.– One day per week, the PhD student at Delft will be visiting Deltares (also in Delft).– The PhD students will visit MARIN when necessary.– Every two weeks there will be a progress meeting between PhD students, post-doc and project leader

(usually in Groningen). If necessary also MARIN and Deltares will attend.– Every two months a progress meeting will be held with all project members from RUG, TUD, MARIN and

Deltares (usually in Wageningen).




– Twice a year, two weeks prior to the FPSO-week meeting (see below), a meeting of the STW User Com-mittee wil be organized (usually in Wageningen); also our Norwegian partner FORCE will be present.

– Twice a year a meeting is organized with all industrial participants in the ComFLOW-3 project; it is heldduring the so-called FPSO week (see the utilisation plan in Section 4.3).

3.6.5 Milestones and deliverables

In Fig. 10 we present the global planning of the project.– It is expected that PhD students can start their research somewhere between spring 2009 and autumn 2009.– The post-doc will start July 1, 2009; until that date, he is partly financed by the ComFLOW-2 JIP. He

will take care of continuity of knowledge towards the new-to-appoint PhD students.

A more detailed description of the project activities can be found in Section 3.6.2.

Figure 10: Global planning of project.

The deliverables of the project will be as follows:

1. Yearly update of ComFLOW executable and user manual; see Section 4.3.2. Model test report and model test data.3. Validation and benchmarking reports.4. Methods to generate extreme waves (memo, software tool).5. Dynamic mooring line module, linked to ComFLOW (executable).6. Scientific publications and presentations in journals and on conferences (OMAE, ISOPE, etc.).7. PhD theses.




3.7 Available infrastructure and embedding in research groups

University of Groningen The work carried out at the University of Groningen will be supervised by prof.dr.A.E.P. Veldman, who heads the research group on Computational Mechanics and Numerical Mathematics in theInstitute of Mathematics and Computing Science. The group consists of one associate professor, two assistantprofessors and one post-doc. Currently the number of PhD students is 7 (most of them externally funded),whereas the number of MSc students averages around 5 students per year.

The research of the group concentrates on the numerical simulation of fluid dynamics and transport phe-nomena (Computational Fluid Dynamics CFD). On the one hand research is focussed on basic advancement of numerical algorithms; on the other hand - through extensive cooperation with external research groups - thesemethods are made available to advance knowledge in other (applied) areas of science and technology. Areas of special attention are: turbulent flow, free-surface flow and sparse-matrix solvers. Applications of our researchare pursued e.g. in maritime engineering, oceanography, aerodynamics, space technology and biomedical engi-neering. Many contacts exist with Dutch industry (including small enterprises), the Dutch large technologicalinstitutes (Deltares, ECN/NRG, MARIN and NLR), and various (non-mathematics) groups at universities.

Technical University Delft The work carried out at TU Delft will be under the guidance of prof.dr.ir. R.H.M.Huijsmans, professor of Ship Hydromechanics and Structures in the department of Marine and Transport Tech-nology. Apart from the section leader prof. dr.ir. R.H.M.Huijsmans, the group consists one part time professorShip Resistance and Propulsion, 2 associate professor, 4 assistant professors and 5 supporting staff member for

the experimental facilities of the group. The experimental facilities consist of two towing tanks and a cavitationtunnel. Currently the group has 13 PhD projects (all externally financed) under their guidance, with an inflowof 12 MSc student per year.

By nature of these PhD projects, the group maintains very close collaborations with the industry and theother knowledge institutes like TNO and MARIN. Also many contacts are with other research groups bothnationally as well as internationally. The research efforts of the group are concentrated on the study of theperformance in seaway of fast ships both from experimental as well as computational point of view. Also animportant aspect of the research focus is the study into very non-linear wave impacts on vessels in extremewaves. The generation of suitable absorbing boundary condition for the VoF solver COMFLOW is part of thisstudy.

MARIN The Maritime Research Institute Netherlands (MARIN) has been dedicated to furthering maritimeunderstanding and knowledge since 1932. It is involved in research programs for governments, the maritime andoffshore industry, and navies. MARIN also provides commercial shipbuilders, owners, propeller manufacturers,naval architects and the offshore industry with state-of-the-art performance predictions, design consultancy,testing services and simulation and training consultancy.

MARIN coordinates several joint-industry projects, of which the SAFE-FLOW and ComFLOW-2 projectshave already been mentioned in the proposal (Section 3.1.2). These projects are a world-wide joint effort(geographically ranging from Northern-America, through Europe upto the Far-East) from almost all mainplayers in the offshore world: oil companies, offshore engineering bureaus, shipyards and classification societies.

Deltares Deltares is the new independent institute for delta technology formed on the first of Januari 2008by Delft Hydraulics, GeoDelft, the Subsurface and Groundwater unit of TNO, and parts of Rijkswaterstaat. Itcombines the knowledge and expertise of Delft Hydraulics on worldwide water issues with GeoDelft expertiseon dikes, roads and underground construction. The new institute also brings together TNO know-how on

the subsurface and groundwater, and the competences of Rijkswaterstaat in the fields of integrated watermanagement, spatial development and administrative processes.Deltares works for and cooperates with Dutch government, provinces and water boards, international govern-

ments, knowledge institutes, and market parties. It provides innovative solutions for water, soil and subsurfaceissues to make living in deltas, coastal areas and river basins safe, clean and sustainable. A part of this taskconsists of developing and making available highly qualified and validated software products that are beingapplied worldwide and by Deltares itself.

3.8 Relation to other research

The study of waves in maritime applications has long been based on potential-flow models; for an overview seee.g. [105]. As a consequence the applications were restricted to mildly non-linear flow phenomena. Only recently,




the step to fully non-linear flow modelling based on the Navier–Stokes equations is being made, following thepioneering work in [87, 100, 101, 111]. For an overview of basic Navier–Stokes methods for free-surface flow werefer to [96]. As always many roads lead to Rome and literature shows a variety of approaches. A motivationfor the choices made in ComFLOW can be found in Section 3.1.3.

‘Competing’ application-oriented research can be found in a number of maritime institutes and departments.In particular, at Principia (France) a simulation tool (called Eole) based on the VOF technique is underdevelopment [99], at INSEAN (Italy) in cooperation with the University of Santa Barbara (USA) programs

based on smoothed particle hydrodynamics are being designed [69, 75, 80, 106], whereas at ECN (France) a non-linear potential flow solver is coupled with a high-Reynolds Navier–Stokes solver [73, 74]. These programs aremore or less successful in delivering impact loads on structures, however their accuracy and validity is not yetestablished. Moreover, for the smooth particle method one also has to consider the high computational effortsrequired.

As for the detailed modelling of free-surface flows in coastal engineering applications, besides previous workby Delft Hydraulics (cf. Section 4.1.2) the work by the department of Civil Engineering of Ghent University isworth mentioning. The techniques used by Li et al. [90, 91] are similar to those used in Skylla and ComFLOW,but there are also differences in the choice of discretization (higher-order) and time-integration (implicit). Theimplicit displacement of the free surface is unclear, however, and validation is limited to surface elevation (novamidation of the important wave-induced forces) of two-dimensional flow (perpendicular to the coastline).Further, unlike ComFLOW, their applied generating-absorbing boundary condition does not include the effectof dispersion.

4 Utilization

4.1 Engineering challenge

4.1.1 Offshore industry

Now that the use of ship-type offshore units for the production and storage of oil has become common, theseunits should be able to survive the most critical environmental conditions occurring as they are unable to fleefor approaching storms. This requires an adequate mooring system, but also attention to the potential problemof green water on the deck. While tankers have an almost empty deck, decks of these floating production andoffloading platforms (FPSOs) carry a lot of sensitive equipment. Consequently green water can cause damageto the vessel’s superstructure and equipment, such as the fluid swivels, piping, turret structure, control valves,

emergency systems, fire detection/protection systems, and cable trays.Similar problems can occur due to wave loading on offshore windmills, often gathered together in large

wind farms. Extreme hydrodynamic loads may cause severe damage to their support structures, and herewith jeopardize their operability.

Figure 11: The Shell Mars tension leg platform before and after hurricane Katrina.

Studies show a steady increase of these green water incidents in offshore operations. The recent hurricanesin the Gulf of Mexico have put broad attention to these forces of nature. Some major incidents:

– An example is wave impact damage to the bow of the Schiehallion FPSO in November 1998 [45, 79] resultingin an evacuation of the personnel and expensive offshore hull repairs and an upgrade of the complete bowstructure. Estimated costs are 87 mill. Euro (78 mill. Euro loss of oil production, 6 mill. Euro damagereparation costs and 3 mill. Euro operational costs without income).




– Hurricanes Ivan, Katrina and Rita have had a devastating influence, not only on land but also at sea. Manyoffshore platforms were thrown from their anchors and severely damaged. E.g. the Ensco offshore platform(Fig. 1) was found 40 miles from where it was originally anchored after Ivan. As another example, thedamage from Katrina on the Shell Mars tension leg platform is shown in Fig. 11.

The problems associated with the behavior of floating structures in extreme wave conditions are experiencedby all operators of such vessels in open seas. As such, the industry has recognized that, being a problem affectingall operators, it should be addressed jointly. This is reflected in the high degree of interest which is being shown

by industry with respect to joint industry projects (JIP) in this field. As a first attempt, in 1997, a JIP on‘FPSO Green Water Loading’ was initiated by MARIN. The problem was studied in detail using an extensiveseries of model tests. The main objective of this study, supported by a wide range of companies in the offshoreindustry, was to develop methods for evaluating green water on ship-type offshore structures based on a cleardescription of the green water physics. It was concluded that in all phases of the green water problem non-linearand highly complex phenomena occur. As a follow-up, the already described SAFE-FLOW JIP (in 2000) andComFLOW-2 JIP (in 2004) were defined to develop methodology to numerically simulate the highly non-lineargreen water events and the consequences of these green water loadings (see Section 3.1.2).

The present proposal is intended to contribute significantly towards the solution of outstanding issues re-sulting from the above-mentioned JIPs. As such, the proposal will be complementary to the third-phase JIPproject ComFLOW-3 (see Section 4.3.1) for which support is being sollicited from the offshore industry. Manyof the ComFLOW-2 participants have already expressed their interest to contribute to the continuation of thisresearch and a project proposal has been issued for their consideration and comments.

4.1.2 Coastal protection

Coastal engineering is of ongoing importance for the development, design, maintenance and adjustment of coastal defense systems. Because of renewed and improved insights, and because of changing conditions (sealevel rise, increasing storm surges due to climate changes), it continues to be necessary to improve and updatethe predictions of the wave climate near the coast and to reduce the uncertainties [81]. Besides by the energyof the wind waves generated at sea, the hydrodynamic load of coastal structures is determined by the complexdynamics of waves when they travel toward the coast. Across the foreshore, processes like shoaling, breakingand refraction may cause significant wave transformations. Studies also continue on the strength of differenttypes of dike revetments as a function of wave impact, mostly experimentally [65, 68]. The data, however, isfar from complete, because of the large number of parameters that is involved and the costs of experimentalresearch.

As a result, there is a large interest in the alternative approach: a suite of computational models capable of reliably and efficiently simulating wave impacts on coastal structures and the damage this leads to. With sucha suite of models, data on wave loads and the resulting structural damage could be generated more cheaply,without scale effects, and for any wave condition and construction geometry. Because of the complex physics,this is far from trivial, but promising results obtained in recent studies show that the approach may becomepractically useful in the near future [58, 88]. Because of the safety and economics aspects involved, a thoroughvalidation of such a model train is of the utmost importance: detailed comparisons of computed results withquality measurement data are required to assess the reliability of the models and to catalog uncertainties andthe effect of model assumptions.

Delft Hydraulics (now part of Deltares) has developed Skylla, a 2DV (two-dimensional in the vertical plane)free-surface Navier-Stokes model built on Savof, a predecessor of ComFLOW (see Section 3.1.2). To make itsuitable for the simulation of breaking waves on coastal structures (Fig.12), the model has been significantly

extended in functionality and extensively validated, see e.g. [58]. However, at present the physical modelingand numerical techniques used in Skylla are not all up to date anymore, while the limitation to 2DV is asignificant limitation in many applications.

To improve its capabilities on detailed near-field free-surface flow modeling, Delft Hydraulics has decided in2006 to join the ComFLOW-2 JIP project and to participate actively in the algorithmic development of Com-

FLOW. This has already led to the current GABC (cf. Section 3.3.1). Deltares intends to use the ComFLOW

simulation program also in other areas where detailed free-surface flow modeling is required, e.g., in offshoreengineering to study the stability of bottom protection and the behavior of scour holes near piles. Besidesthe work that will be invested in the general ComFLOW developments presented in this proposal, Deltares isplanning to invest yearly, beginning in 2008, about 6 man month (100 ke) on the development and validationof ComFLOW for specific Deltares applications. The results of these studies will be made available to theComFLOW-3 partners as far as possible. Likewise, existing data from previous experiments and new data from




Figure 12: Left: Experiment of wave run-up in Deltares wave flume. Right: Skylla calculation of wave run-up against breakwater.

experiments to come will be made available for the validation of ComFLOW in the ComFLOW-3 project if distribution is permitted.

4.2 Potential users

The list of companies that have supported the ComFLOW-2 JIP includes many major oil companies, shipyardsand several offshore-related companies (in alphabetical order):

company country activity ABS USA offshore companyAker Kvaerner Norway offshore companyBluewater Energy Services Netherlands offshore companyBP Amoco UK oil companyBureau Veritas France classification societyChevronTexaco USA oil company

ConocoPhillips USA oil companyDaewoo SME Korea shipyardDeltares Netherlands research instituteDet Norske Veritas DNV Norway classification societyFORCE Technology Norway offshore consultantsHyundai Heavy Industries Korea shipyardGTT France LNG carriersGusto MSC Netherlands shipyardMARIN Netherlands research institutePetrobras Brasil oil companySamsung Korea shipyardSandwell Engineering Canada offshore companySBM Monaco offshore company

Shell Netherlands oil companyStatoilHydro Norway oil company

The motivation for supporting this project lies in the expectation that a rational, effective and efficient methodwill be developed by means of which it will be possible to quantify, in the design stage, extreme motions andloads on a structure floating in extreme waves. This information, in turn, will be used to optimize the design of the floating structure so as to minimize risk of hazardous working conditions for crew members and to safeguardthe integrity of the structure in extreme sea conditions.

Many of these industrial participants are actively using the ComFLOW program, for problems of liquidsloshing, green water loading and wave-in-deck analysis. Several scientific publications of these industrial usershave appeared already. The list below gives an overview:




company application contact person

AkerKvaerner wave-in-deck; run-up T.B. Johannesen([email protected])

Bluewater sloshing; bilge keels J. van der Cammen([email protected])

ChevronTexaco run-up against gravity-based structures(Fig. 13) [49, 52]

T. Finnigan([email protected])

Deltares loads on offshore, coastal and industrialconstructions (Fig. 13)

M. Borsboom ([email protected])

DNV wave-in-deck; sloshing [40, 4 1] J. Birknes ([email protected])

FORCE wave-in-deck [56] B. Iwanowski ([email protected])

Gusto MSC life-boat drop tests J.-H. Westhuis([email protected])

Hyundai sloshing; drop tests Joong Soo Moon ([email protected])

MARIN green water; wave loads; interactive mo-tion [43–51, 59, 60]

T. Bunnik ([email protected])

Sandwell wave flume S. Prasad ([email protected])

Statoil waves against tension leg platforms (Fig. 4)[57]

P. Teigen ([email protected])

The following companies have already expressed their interest in participating in the ComFLOW-3 JIP; itis expected that more companies will follow later this year. Corresponding letters are attached to this proposal(see Appendix):

– Maritime Research Institute (MARIN), Wageningen;– Deltares, Delft;– Bluewater Energy Services, Hoofddorp;– Gusto MSC, Schiedam;– Shell International Exploration and Production, Rijswijk;– Det Norske Veritas (DNV), Hovik (Norway).

Figure 13: Calculations by ComFLOW users. Left: Breakwater calculation by Deltares. Right: Wave run-upagainst vertical column showing ‘roostertail’ as calculated by ChevronTexaco [52].

Suggested members of the user’s committee It is suggested to apppoint the following persons on theusers’s committee:

– dr.ir. T.H.J. Bunnik (MARIN)– dr.ir. M. Borsboom (Deltares)– dr.ir. J.J. van der Cammen (Bluewater Energy Services)– dr.ir. J.-H. Westhuis (Gusto MSC)– dr. S. Masterton (Shell SIEP)– dr. B. Iwanowski (FORCE Technology)




4.3 Utilisation plan

4.3.1 Embedding in ComFLOW-3 JIP

As mentioned above, and illustrated in Fig. 14, the current proposal will be part of the ComFLOW-3 joint indus-try project, which is currently being defined as follow-up of the ComFLOW-2 JIP. The concurrent ComFLOW-3JIP activities are focussed on benchmarking and validating the developed numerical models for wave impactproblems as studied in the proposal, i.e. in the shipping and offshore industry and in coastal regions. Also it will

contribute to the development of user-friendly pre- and postprocessing around the scientific numerical modelsdeveloped in the present project, so that the results can be utilised also by people that do not have exact insightin the numerical models. This ensures that the proposed project itself can focus on the development of thenumerical models, whereas the complementary JIP guarantees the good utilisation of the results.

TUD

industry funded

F

O R C E

MARIN

Deltares

RUG

STW funded

ComFLOW−3 JIP

STW project

Figure 14: Embedding of the STW project (the dash-framed area) within the ComFLOW-3 JIP. The (dark)green-colored part is to be funded by STW; the (light) yellow-colored part will be funded by industry.

Benchmarking A very important item in the ComFLOW-3 project is the benchmarking and validation of the ComFLOW method. This will be the primary task of FORCE Technology and will guarantee that eachnew ComFLOW version delivered to the participants:

– is functioning well, has a clear manual and well-defined input and output;– is able to reproduce previous results;– has been thoroughly tested with respect to new functionality;– has a clear description of the limitation of new functionality, based on validation with measurement data.

A new version of the ComFLOW code will only be released to the project participants when it has passedthese benchmark tests.

Progress meetings During the course of the ComFLOW-3 project, meetings with the project participantswill be held every 6 months to review progress (see also Section 3.6.4). These meetings are scheduled to takeplace during the so-called ‘FPSO week’, in which a number of JIP meetings on a variety of subjects are scheduledto take place at the same venue. The purpose of selecting the same locality for different JIP projects on differentsubjects and with different participants is to facilitate and to encourage contacts between the various groups inorder to stimulate, among others, development of cross-problem thinking. Recent FPSO weeks were organized

in Rio de Janeiro, Brazil (April 2006); Bandol, France (Nov. 2006); Houston, USA (April 2007); Gyeongju,Korea (Nov. 2007) and Trondheim, Norway (April 2008).

4.3.2 Deliverables

At the start of the project the final ComFLOW version of the ComFLOW-2 JIP will be delivered (renamedComFLOW version 3.0) as an executable binary file (for Windows and Linux). It is released on a CD-ROM,accompanied with a comprehensive user manual and worked-out examples of simulations. MARIN, Deltaresand FORCE will receive the source code, such that they can actively participate in the algorithm development.

Thereafter, every year a new version will be delivered with increased functionality and/or efficiency plus acorrespondingly updated user manual. The delivery schedule can be found in Table 1. Prior to the release of a




– At kick-off: ComFLOW 3.0 (= final version of ComFLOW-2 JIP).– After one year: GABC in three dimensions; parallelisation.– After two years: Better wave propagation; interactive body motion.– After three years: Extreme waves and current; viscous effects; local grid refinement.– At end of pro ject: Fully validated version.

Table 1: Delivery schedule of ComFLOW program.

new version it will be extensively benchmarked. This is the responsibility of FORCE Technology, as describedin Section 4.3.1.

User support will be provided throughout the project; MARIN, assisted by the post-doc at RUG, is re-sponsible to handle the requests for support. Also a (password-protected) website is maintained by MARIN,which contains all project reports (manuals, tests, technical notes, etc.) and minutes and presentations fromall progress meetings. At the end of the project a workshop will be organized that focusses on the use of thenewly developed functionality in ComFLOW.

4.4 Past performance

The larger part of the project team has been working together for almost a decade now. Corresponding projectswere:

- “Simulation methods for free surface flow with floating objects”, funded by MARIN, 1999-2003; PhD thesisFekken [4].

- SafeFLOW: “Safe floating offshore structures under impact loading of shipped green water and waves”, fundedby EU, 2001-2004; PhD thesis Kleefsman [12].

- ComFLOW2: “Hydrodynamic wave loading on floating and moored structures in steep waves”, funded bySTW, 2004-2008; PhD theses Wemmenhove [33] and Wellens [30].

5 Contracts and patents

There are no contracts that may influence the progress of the project. Also, there are no patents requested orforeseen in relation to this project.

Similar to the ComFLOW-2 JIP, agreements will be made with respect to the conditions under which thenew software generated in the course of this project may be transferred to the participants in the project.Hereto, a contract will be made between STW and MARIN as the coordinator of the ComFLOW-3 JIP. Inturn, MARIN will make contracts with the individual participants of the JIP. Further, the in-kind contributionsof MARIN and Deltares will be laid down in contracts with STW.

6 Budget

6.1 Personnel

In this proposal we request the funding of three PhD students (two at the University of Groningen and one atTU Delft) and one 4-year post-doc (at RUG). As of July 1st 2008, the cost of one PhD student amounts 174ke, whereas a 4-year post-doc position costs 233 ke. Part of the post-doc appointment (0.2 fte/yr) will befunded by industry; the remainder (0.8 fte/yr) is requested from STW.

6.2 Material

Local travel Regular progress meetings will be held between RUG, TUD, MARIN and Deltares, to maintainintensive contacts between the theoretical development of the simulation method and its experimental validation.Bi-weekly progress meetings with the project team are planned (either in Groningen, Delft or Wageningen).Also, a number of visits to MARIN are planned during the experiment campaigns. We estimate the yearlytravel costs for this in-project cooperation at e 750 per PhD student/post-doc.




PC equipment The simulations to be carried out are processor and memory intensive. The main computa-tions will be carried out at the university computers in Groningen and Delft – with the current university policyno budget is required for the use of these facilities. Also, additional access to the national computer facilities inAmsterdam will be requested. For local desk-top activities, preliminary computations and post-processing, only‘standard’ PC equipment is provided by the universities. Therefore a budget is requested for four well-equipped(with respect to processor speed, central memory, and graphics performance) desktop PC’s, e.g. quad core +16 Gb. Estimated cost e 4000 per PC.

6.3 Foreign travel

Because of the link with the ComFLOW-3 JIP project, twice a year international progress meetings are plannedwith the industrial participants. Typically, one meeting per year will be held in the USA or the Far East, whereasthe other meeting will be held in Europe. Experience in the previous ComFLOW-2 project has learned thatthese two JIP meetings require e 3000-3500 traveling budget per person. Also regular visits to our Norwegianpartner FORCE are foreseen. Finally, the results of the project will be presented at international workshopsand conferences, e.g. the (relatively expensive) OMAE and ISOPE conferences especially dedicated to offshoreengineering. Therefore, a yearly travel budget of e 6000 per PhD student/post-doc is requested. Since theavailable travel budget for faculty members at universities is limited, we request an additional yearly amountof e 4000 per supervisor for attending the above-mentioned progress meetings.

6.4 Use of facilities

The experiments, necessary for validation of the ComFLOW program, will be carried out at MARIN; seeSection 3.4 and WP 5. The costs of these experiments are estimated at 250 ke; these comprise the constructionof the model, energy costs of the basin, and the personnel carrying out the experiments. The industry willinvest more than half of this amount (150 ke, see below); the remainder (100 ke) is requested from STW.

6.5 Contributions third party

The ComFLOW-3 JIP will fund 0.2 fte/yr (≈ 47 ke) of the post-doc activities. Also, it will fund the largerpart (150 ke) of the experiments at MARIN with which ComFLOW will be validated. About 150 ke of thiscontribution (of 197 ke) corresponds with the fee that was contractually agreed in the ComFLOW-2 projectfor using the ComFLOW program after the end of the project (3 years at 2.5 ke/yr per participant, at an

expected number of 20 participants makes 150 ke).

Further MARIN and Deltares will provide in-kind manpower to support the project from the (physical andnumerical) modelling point of view. Section 3.6.2 describes their contribution to the individual workpackages.Their support will guarantee that the model choices to be made throughout the project are relevant for theapplications that are envisaged. In this way MARIN and Deltares will significantly contribute to a succesfullutilization of the results obtained in this project. This support can be capitalized as 200 ke from MARIN and125 ke from Deltares.

6.6 Total project costs

year personnel material foreign use of manhours

PhD post-doc travel facilities MARIN Deltares1 3 PhD 1 fte 19 32 50 60 452 3 PhD 1 fte 3 32 100 60 353 3 PhD 1 fte 3 32 100 60 354 3 PhD 1 fte 3 32 20 10

Total 3 PhD 4 yr pd 28 128 250 200 125 156

325

Table 2: Total project costs (amounts in k e ).




Sections 3.6.2 and 3.6.3 present a detailed specification of the costs of the individual workpackages. Togetherwith the items mentioned in the previous subsections, the project costs are totalized in Table 2. In summarythese amount to

– 3 PhD + 4-year post-doc– 156 ke material + travel;– 250 ke use of facilities, and– 325 ke in-kind manhours from MARIN and Deltares.

A significant part will be contributed by external parties, as specified in Table 3. Again, for a specificationwithin the individual workpackages we refer to Section 3.6.2. In summary, the industry will contribute ≈ 522ke, consisting of

– 4yr 0.2 fte post-doc support (≈ 47 ke);– 150 ke for experiments;– 325 ke in-kind manhours from MARIN and Deltares.

Of the first two items, about 150 ke is funded from the fee that was agreed with the ComFLOW-2 participantsfor the use of ComFLOW after the end of the ComFLOW-2 project (see Section 6.5). For ease of presentation,this fee is considered part of the industrial ComFLOW-3 JIP contribution (as it is provided by the samecompanies).

year post-doc use of in-kind contribution

facilities MARIN Deltares1 0.2 fte 25 60 452 0.2 fte 50 60 353 0.2 fte 75 60 354 0.2 fte 20 10

Total 0.8 yr pd 150 200 125

Table 3: Contribution from third parties (amounts in k e ).

Table 4 gives a summary of Tables 2 and 3. It shows that the total project costs are estimated at ≈ 1.5 Me,of which ≈ 65% (964 ke) is requested from STW, whereas the contribution from third parties will be ≈ 35%(522 ke).

STW third partiesPhD students 3 PhD ⇒ 522 kepost-doc 0.8 fte/yr ⇒ 186 ke 0.2 fte/yr ⇒ 47 kematerial 28 keexternal travel 128 keuse of facilities 100 ke 150 kemanpower third party 325 ketotal 964 ke 522 ke

Table 4: Summary of project funding.

References

Publications by ComFLOW development team

[1] ComFLOW website. URL www.math.rug.nl/∼veldman/comflo/comflo.html.

[2] M. Droge and R. Verstappen. A new symmetry-preserving Cartesian-grid method for computing flow past arbitrarily shapedobjects. Int. J. Numer. Meth. Fluids, 47:979–985, 2005.

[3] G. Fekken. Numerical simulation of green water loading on the foredeck of a ship. Master’s thesis, Department of Mathematics,University of Groningen, 1998.

[4] G. Fekken. Numerical simulation of free-surface flow with moving objects. PhD Thesis, University of Groningen, TheNetherlands, 2004. URL: dissertations.ub.rug.nl/faculties/science/2004/g.fekken.

[5] G. Fekken, A.E.P. Veldman, and B. Buchner. Simulation of green-water loading using the Navier-Stokes equations. In J. Piquet,editor, Proc. 7th Int. Conf. Numer. Ship Hydrodyn., Nantes, France, July 19-22, 1999. Paper 6.3, 12 pages.




[6] J. Gerrits. Dynamics of Liquid-Filled Spacecraft . PhD thesis, University of Groningen, The Netherlands, 2001. URL:dissertations.ub.rug.nl/faculties/science/2001/j.gerrits .

[7] J. Gerrits, G.E. Loots, G. Fekken, and A.E.P. Veldman. Liquid sloshing on earth and in space. In B. Sarler, C.A. Brebbia,and H. Power, editors, Moving Boundaries V . WIT Press, Southampton, 1999.

[8] J. Gerrits and A.E.P. Veldman. Dynamics of liquid-filled spacecraft. J. Eng. Math., 45:21–38, 2003.

[9] J. Gerrits and A.E.P. Veldman. Numerical simulation of coupled liquid-solid dynamics. In E. Onate, G. Bugeda, and B. Suarez,editors, Proc. ECCOMAS 2000 , Barcelona, Spain, 2000. Paper 575.

[10] J. Gerrits and A.E.P. Veldman. Transient dynamics of containers partially filled with liquid. In B. Sarler and C. A. Brebbia,editors, Moving Boundaries VI , pages 63–72, 2001.

[11] J. Helder and R.W.C.P. Verstappen. On restraining convective subgrid-scale production in Burgers’ equation. Int. J. Numer.Meth. Fluids, 56:1289–1295, 2008.

[12] K.M.T. Kleefsman. Water impact loading on offshore structures - a numerical study . PhD thesis, University of Groningen,The Netherlands, 2005. URL: dissertations.ub.rug.nl/faculties/science/2005/k.m.t.kleefsman.

[13] K.M.T. Kleefsman, G. Fekken, A.E.P. Veldman, B. Iwanowski, and B. Buchner. A Volume-of-Fluid based simulation methodsfor wave impact problems. J. Comput. Phys., 206:363–393, 2005.

[14] K.M.T. Kleefsman, G.E. Loots, A.E.P. Veldman, B. Buchner, T. Bunnik, and E. Falkenberg. The numerical solution of greenwater loading including vessel motions and the incoming wave field. In Proc. 24th Int. Conf. Offshore Mech. Arctic Eng.,Halkidiki, Greece, 2005. Paper OMAE2005-67448.

[15] K. M. Theresa Kleefsman, Geert Fekken, Arthur E.P. Veldman, and Bogdan Iwanowski. An improved Volume-of-Fluid methodfor wave impact type problems. In J.S. Chung, K. Izumiyama, M. Sayed, and S.W. Hong, editors, Proc. 14th Int. Offshore and Polar Eng. Conf. ISOPE2004, Vol. I , pages 334–341, 2004.

[16] A.J.A. Kort, R.W.C.P. Verstappen, F.W. Wubs, and A.E.P. Veldman. Symmetry-preserving discretizations for local gridrefinement. In M. Oberlack et al., editor, Progress in Turbulence 2 , volume 109 of Springer Proceedings in Physics, 2007.ISBN 3-540-32602-2.

[17] R. Luppes, J.A. Helder, and A.E.P. Veldman. Liquid sloshing in microgravity. In Proc. 56th Int. Astron. Congress, Fukuoka,2005. Paper IAC-05-A2.2.07.

[18] R. Luppes, J.A. Helder, and A.E.P. Veldman. The numerical simulation of liquid sloshing in microgravity. In P. Wesseling,E. Onate, and J. Periaux, editors, Proc. Europ. Conf. Comput. Fluid Dyn.: ECCOMAS CFD 06 , Egmond aan Zee, 2006.ISBN 909020970-0 (CD-ROM) paper 490.

[19] C. Nouri, R. Luppes, A.E.P. Veldman, J.A. Tuszynsky, and R. Gordon. Rayleigh instability of the inverted one-cell amphibianembryo. Physical Biology , 5(1):015006 (2008).

[20] A.E.P. Veldman. New, quasi-simultaneous method to calculate interacting boundary layers. AIAA J., 19:79–85, 1981.

[21] A.E.P. Veldman. The simulation of violent free-surface dynamics at sea and in space. In P. Wesseling, E. Onate, and J. Periaux,editors, Proc. Europ. Conf. Comput. Fluid Dyn.: ECCOMAS CFD 06 , Egmond aan Zee, 2006. ISBN 909020970-0 (CD-ROM)paper 492.

[22] A.E.P. Veldman, G. Fekken, and K.M.T. Kleefsman. Numerical simulation of hydrodynamic wave impact. In B. Oskam, editor,Flow induced unsteady loads and the impact on military applications, pages RTO–AVT 123 (CD–Proceedings), Budapest, 2005.

[23] A.E.P. Veldman, J. Gerrits, R. Luppes, J.A. Helder, and J.P.B Vreeburg. The numerical simulation of liquid sloshing on boardspacecraft. J. Comput. Phys., 224:82–99, 2007.

[24] A.E.P. Veldman, K.M.T. Kleefsman, and G. Fekken. Numerical computation of wave impact. In P. Bergan et al., editor,Computational Methods in Marine Engineering , pages 323–332, CIMNE, Barcelona, 2005.

[25] A.E.P. Veldman and M.E.S. Vogels. Axisymmetric liquid sloshing under low gravity conditions. Acta Astronautica , 11:641–649,1984.

[26] R.W.C.P. Verstappen and A.E.P. Veldman. Direct numerical simulation of turbulence at lesser costs. J. Eng. Math., 32:143–159, 1997.

[27] R.W.C.P. Verstappen and A.E.P. Veldman. Spectro-consistent discretization: a challenge to RANS and LES. J. Eng. Math.,34:163–179, 1998.

[28] R.W.C.P. Verstappen and A.E.P. Veldman. Symmetry-preserving discretization of turbulent flow. J. Comput. Phys., 187:343–

368, 2003.

[29] R.W.C.P. Verstappen. On smooth, symmetry-preserving approximations of turbulent convection in a plane channel. Physicsof Fluids. Accepted for publication.

[30] P.R. Wellens. In preparation . PhD Thesis, Technical University Delft, The Netherlands, 2009.

[31] P.R. Wellens, J.A. Pinkster, A.E.P. Veldman, and R.H.M. Huijsmans. 3D diffraction based boundary conditions. In R. Beck,editor, Proc. NSH-2007 , Ann Arbor, MI, USA, August 2007.

[32] P.R. Wellens, J.A. Pinkster, A.E.P. Veldman, and R.H.M. Huijsmans. Numerical wave run up calculation on GBS columns.In J. Chung, editor, Proc. ISOPE-2007 , Lisbon, July 1-6, 2007.

[33] R. Wemmenhove. Numerical simulation of two-phase flow in offshore environments. PhD Thesis, University of Groningen,The Netherlands, 2008. URL: dissertations.ub.rug.nl/faculties/science/2008/r.wemmenhove.

[34] R. Wemmenhove, E. Loots, R. Luppes, and A.E.P. Veldman. Modelling two-phase flow with offshore applications. In Proc.24th Int. Conf. Offshore Mech. Arctic Eng., Halkidiki, Greece, 2005. Paper OMAE2005-67460.




[35] R. Wemmenhove, E. Loots, and A.E.P. Veldman. Hydro dynamic wave loading on offshore structures simulated by a two-phaseflow model. In Proc. 25th Int. Conf. Offshore Mech. Arctic Eng., Hamburg, Germany, 2006. Paper OMAE2006-92253.

[36] R. Wemmenhove, E. Loots, and A.E.P. Veldman. Numerical simulation of hydrodynamic wave loading by a two-phase model.In P. Wesseling, E. Onate, and J. Periaux, editors, Proc. Europ. Conf. Comput. Fluid Dyn.: ECCOMAS CFD 06 , Egmondaan Zee, 2006. ISBN 909020970-0 (CD-ROM) paper 517.

[37] R. Wemmenhove, G.E. Loots, R. Luppes, and A.E.P. Veldman. Simulation of green water loading by a three-dimensionaltwo-phase numerical model. In J. Grue, editor, 20th Int. Workshop on Water Waves and Floating Bodies, University CentreSvalbard, Spitsbergen, 2005. 4 pages.

[38] R. Wemmenhove, R. Luppes, A.E.P. Veldman, and T. Bunnik. Numerical simulation and model experiments of sloshing inLNG tanks. In Proc. Int. Conf. Comp. Meth. Marine Eng. MARINE 2007 , Barcelona, 2007.

[39] R. Wemmenhove, R. Luppes, A.E.P. Veldman, and T. Bunnik. Numerical simulation of sloshing in LNG tanks with acompressible two-phase model. In Proc. 26th Int. Conf. Offshore Mech. Arctic Eng., San Diego, 2007. Paper OMAE2007-29294.

Publications by ComFLOW users

[40] B. Brodtkorb. Prediction of wave-in-deck forces on fixed jacket-type structures based on CFD calculations. In Proc. 27th Int.Conf. Offshore Mechanics and Arctic Eng., OMAE2008 , Estoril, Portugal, June 15-20, 2008. Paper OMAE2008-57346.

[41] B. Brodtkorb. Prediction of increased jacket substructure loads due to wave-in-deck diffraction based on CFD calculations.In Proc. 27th Int. Conf. Offshore Mechanics and Arctic Eng., OMAE2008 , Estoril, Portugal, June 15-20, 2008. PaperOMAE2008-57361.

[42] B. Buchner. Green water on ship-type offshore structures. PhD Thesis, University of Delft, Delft, The Netherlands, November

2002.[43] B. Buchner and T. Bunnik. Extreme wave effects on deepwater floating structures. Houston, 2007. OTC paper 18493.

[44] B. Buchner, T. Bunnik, and A.E.P. Veldman. The use of a Volume of Fluid (VOF) method coupled to a time domain motionsimulation to calculate the motions of a subsea structure lifted through the splash zone. In Proc. 25th Int. Conf. Offshore Mech. Arctic Eng., Hamburg, Germany, 2006. Paper OMAE2006-92447.

[45] B. Buchner, G. Fekken, T.H.J. Bunnik, and A.E.P. Veldman. A numerical study on wave run up on an FPSO bow. In Proc.20th ASME Conf. Offshore Mech. Arctic Eng., Rio de Janeiro, Brasil, 2001.

[46] T. Bunnik and B. Buchner. Experimental investigation of subsea structures during installation and the related wave loads,added mass and damping. In J.S. Chung, K. Izumiyama, M. Sayed, and S.W. Hong, editors, Proc. 14th Int. Offshore and Polar Eng. Conf. ISOPE2004, Vol. I , pages 291–296, 2004.

[47] T. Bunnik and B. Buchner. Numerical prediction of wave loads on subsea structures in the splash zone. In J.S. Chung,K. Izumiyama, M. Sayed, and S.W. Hong, editors, Proc. 14th Int. Offshore and Polar Eng. Conf. ISOPE2004, Vol. I , pages284–290, 2004.

[48] T. Bunnik and B. Buchner. Simulation of the dynamic motions of complex sub-sea structures in the splash zone during

deepwater installations. In Proc. Deep Offshore Technology 2005 , Vitoria (Brazil), November 8-10, 2005.

[49] T. Bunnik, B. Buchner, A.E.P. Veldman, M.-Y. Lee, T. Finnigan, and A. Moises. Numerical simulation of complex greenwaterand wave loads on offshore structures. In Offshore Technology Conference OTC2006 , Houston, 1-4 May, 2006. Paper OTC17853.

[50] T. Bunnik and R. Huijsmans. Validation of wave propagation in numerical wave tanks. In Proc. 24th Int. Conf. Offshore Mech. Arctic Eng., Halkidiki, Greece, 2005. Paper OMAE2005-67221.

[51] T. Bunnik and R.H.M. Huijsmans. Large scale LNG sloshing model tests. In Proc. 17th Int. Offshore and Polar Eng. Conf.ISOPE2007 , Lisbon, 2007.

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Appendix : Letters of conformationThe appendix contains conformation letters from:

– Maritiem Research Institute MARIN, Wageningen– Deltares, Delft– Bluewater Engineering Services, Hoofddorp– Det Norske Veritas, Hovik (Norway)– Gusto MSC, Schiedam– Shell SIEP, Rijswijk



E

GustoMSC

Prof. A,E.P. Veldman

University

of

Groningen

lnstitute

of Mathematics

and Computing

Science

PO

Box 407

9700

AK

Groningen

Gusto

B.V.

Karel

Doormanweg

66

-

31 15 JD Schiedam

The Netherlands

Telephone:

+31

(0)102320000-Telefax:

+31

(0)102320

1O1

Subject:

STW

proposal

-

Extreme

wave impact

on

offshore

platforms

and coastal

structures

Dear Prof. Veldman,

We have read

your

proposal

for

the

project Extreme

wave impact

on offshore

platforms

and coastal

structures with

great

interest.

We consider

this

research

important

and valuable

for several reasons.

Firstly, as

engineers of floating

offshore structures

we are interested

in

the

development

of accurate and validated

methods

to be able to

determine extreme wave

loads in

order

to

design saver and

more

robust structures.

Secondly,

we support the

scientific efforts

that are needed

to make these

complex

calculations models

suitable for

use

in

day-to-day work.

This will further

maximize

the

utilization

of the

preceding

research

efforts.

Thirdly, the collaboration

between Dutch

universities,

research institutes

and offshore

companies

that

will

result

from

this research will further

strengthen

our national

competitiveness

in

this industry.

We are therefore willing

to support

and contribute

to this research

and recommend

it

to

be selected

for

STW support.

Dr.

ir.

Jaap-Harm

Westhuis

Manager Naval Architecture Department

Gusto B.V.

Our

ref. :

NAD-wsh

Project.

:

STW-Comflow3

Your ref. :

Schiedam,

30 May 2008

PO. Box

11

-

3100

AA

Schiedam

-The

Netherlands

NL-Trade Register 24173568

wtr-csr-431

. 2

Rev-

24

May 2005

Yours

faithfully,



@

Prof.dr.A.E.P. Veldman

Shell

International Exploration and

Production B.Y.

Kesslerpark 1

2288

GS Rijswijk ZH

The Netherlands

Tel 31 70447 3532

Fax 31 70447 5019

Email [email protected]

Internet http://www.shell.com/eandp-en

Dr.ir.Bas Buchner

Institute of Mathematics and Computing Science

University of Groningen

P.O. Box 407 9700 AK Groningen

June 2008

Subject: ComFLOW3 proposal

Thank you for sending the comFLOW proposal which we have now read with great interest. We

believe the proposed work will enhance the program and increase the value of the code significantly.

We would like to continue to be involved in the development and to contribute financially to the joint

industry project but this will of course be subject to us securing funding.

Yours Faithfully

~~~

Dr. Kevin Ewans

Metocean Engineer EPT -PNR

Shell International Exploration and Production RV.

Gevesftgd le Den Haag: Carel van Bylandrlaan 30 2596 HR Den Haag

Handelsregister Den Haag 27259906



E

GustoMSC

Prof. A,E.P. Veldman

University

of Groningen

lnstitute

of Mathematics

and Computing

Science

PO

Box 407

9700

AK Groningen

Gusto

B.V.

Karel

Doormanweg

66

-

31 15 JD Schiedam

The Netherlands

Telephone:

+31

(0)102320000-Telefax:

+31

(0)102320

1O1

Subject:

STW

proposal

-

Extreme

wave impact

on

offshore

platforms

and coastal

structures

Dear Prof. Veldman,

We have read

your proposal

for

the

project Extreme

wave

impact

on offshore

platforms

and

coastal

structures with

great

interest.

We consider

this

research

important

and valuable

for several

reasons.

Firstly, as

engineers of floating

offshore structures

we are

interested

in

the

development

of accurate and validated

methods

to be able to

determine extreme wave

loads in

order

to

design saver and

more

robust structures.

Secondly,

we

support

the

scientific efforts

that are needed

to make these

complex

calculations models

suitable for

use

in

day-to-day work.

This will further

maximize

the

utilization of the

preceding

research

efforts.

Thirdly, the collaboration

between Dutch

universities,

research institutes

and offshore

companies

that

will

result

from

this research will further

strengthen

our national

competitiveness

in

this industry.

We

are therefore willing

to support

and contribute

to this research

and recommend

it to

be selected

for

STW support.

Dr.

ir.

Jaap-Harm

Westhuis

Manager Naval Architecture Department

Gusto

B.V.

Our

ref. :

NAD-wsh

Project.

:

STW-Comflow3

Your ref.

:

Schiedam,

30 May 2008

PO. Box

11

-

3100

AA

Schiedam

-The

Netherlands

NL-Trade Register 24173568

wtr-csr-431

. 2

Rev-

24

May 2005

Yours

faithfully,



@

Prof.dr.A.E.P. Veldman

Shell

International Exploration and

Production B.Y.

Kesslerpark 1

2288

GS Rijswijk ZH

The Netherlands

Tel 31 70447 3532

Fax 31 70447 5019

Email [email protected]

Internet http://www.shell.com/eandp-en

Dr.ir.Bas Buchner

Institute of Mathematics and Computing Science

University of Groningen

P.O. Box 407 9700 AK Groningen

June 2008

Subject: ComFLOW3 proposal

Thank you for sending the comFLOW proposal which we have now read with great interest. We

believe the proposed work will enhance the program and increase the value of the code significantly.

We would like to continue to be involved in the development and to contribute financially to the joint

industry project but this will of course be subject to us securing funding.

Yours Faithfully

~~~

Dr. Kevin Ewans

Metocean Engineer EPT -PNR

Shell International Exploration and Production RV.

annex d - stw-comflow3

Documents