Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities

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1 Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities Lee Sing-Kwan and Seah Ah Kuan American Bureau of Shipping Presented at the 2 nd International MTEC 2007 Conference, held in Singapore, September 2007, and reprinted with the kind permission of the Maritime Port Technology and Development Conference (MTEC) Abstract Model tests have been a principal tool in estimating complex environmental loading for marine and offshore structure design, however, cost, time and scale effect uncertainty are still the Achilles heels. More recently, with advances in computational technology, first-principle calculations based on Computational Fluid Dynamics (CFD) methodology have been looming as an effective alternative to model tests. In this paper, existing potential flow method for offshore and marine hydrodynamic will be reviewed with a view to identify some of their weaknesses and limitations. On-going developments proposed by ABS, to introduce new CFD based solutions will be discussed. 1. Introduction As the exploitation of hydrocarbon resources moves to deeper waters and harsher environments, the design of Mobile Offshore Drilling Units (MODUs) and Floating Production Units (FPSs) are constantly challenged by the need to more comprehensively account for the hydrodynamic loading imposed on such structures in these environments. In high seas, breaking waves, wave impact spray, wave run-up on offshore structure, green water on deck and sloshing in LNG tankers (Figure 1a, b, c, d) are only a few examples of flow with violent free surface motion, behind which complex physics are involved. Many of these flow phenomena occur commonly offshore and have profound impacts on marine structure design. (1a) Spray due to wave impact (1b) Run-up and wave breaking on semi column (1c) Greenwater on desk In addition to waves, current below the sea surface is also a complex issue to offshore structure safety. As is well known, current can cause complex VIV (Vortex Induced Vibration) problem to risers. The complexity of the problem can be illustrated by its complex flow behind. Figure 2 shows streamwise vorticity patterns generated by uniform flow passing a single and multi risers. Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities 177

2 (1d) Sloshing in LNG tank CFD simulation As seen (Figure 2a), a steady uniform current (2D flow) can cause extreme complex unsteady 3D flow behind a cylindrical riser. When multi-risers are used, the interaction among the risers becomes more complicated. It is generally agreed that RANS (Reynolds Averaged Navier-Stokes) simulation with standard turbulent model such as k-ε model is not sufficient to handle this complex flow. Rather, LES (Large Eddy Simulation) has been considered essential. Compared to turbulent model RANS simulation, LES is much more computationally intensive. (2a) Unsteady 3D flow behind a cylinder (2b) Multi-risers interaction Figure 2. Streamwise vorticity generated in riser VIV 2. Current potential flow based simulation 2.1 Potential flow based simulation In marine and offshore industry, potential flow based softwares are widely used as they are less computational intensive and based on more tangible mathematics. Basically, most of potential flow based tools are BEM (Boundary Element Method) type software, for example, WAMIT, AWQA, MOSES, MLTSIM, LAMP to name a few. In these softwares, wave nonlinearity, if included, is usually handled by perturbation wave theory such as second or higher order Stokes waves. Usually, in most of BEM type software simulations, body motion excited by waves is solved by linear method, which means the motion problem is linearized at the equilibrium position and solved only for small amplitude motion excluding large motion situation. Although, in principle, time domain large amplitude motion simulations can be preformed by some BEM softwares, for instance, LAMP, this kind of simulation is unavoidably very computationally demanding but still incapable of handling wave breaking situation, which is critical for wave impact problem. 2.2 Limitations of potential flow model & BEM It is generally agreed that most off-the-shelf engineering tools today are not capable of accurate or robust modeling of highly non-linear problems. In offshore hydrodynamics for floating structures, the nonlinear effect may be said to be originated from three sources, namely i) wave nonlinearity, ii) large motion, and iii) viscous effects. Strong (or highly) nonlinear waves commonly occur along the free surface, which can at times become violent and even generating breaking waves (see Figure 1d for sloshing in LNG and 1b wave breaking on semi 178 Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities

3 column), especially when waves are interacting with structures or when water depth becomes shallow. Offthe-shelf engineering software, generally speaking, cannot handle such phenomena as they are designed based on only weakly non-linear wave theories (up to certain order of wave nonlinearity). In order to handle the highly non-linear waves, fully non-linear waves capability is essential. Also, due to the restriction of BEM itself, tracking the moving water surface is often a computation bottleneck in highly nonlinear waves simulations. Furthermore, due to the numerical instability it is often very challenging for BEM to simulate over-turning waves robustly. For wave breaking, it is generally agreed that it is totally beyond BEM capability. In addition to highly non-linear waves, large amplitude motion is yet another critical phenomenon that introduces nonlinearity to wave structure interaction analysis. Usually, large motion for floating structure is unavoidable under high sea condition. In BEM potential flow based simulation it is still a very difficult task to combine both highly non-linear waves and large motions together to model the large floating structure behavior in high waves. Although there are some successful applications (for ships) in large amplitude motion simulation under linear and weakly non-linear waves by using research-type program such as LAMP, there are still very few attempts to simulate problems with highly non-linear waves with large motion. Besides, there is also physics-related limitation in potential flow method. As is well-known, potential flow model lacks of the capability of accounting for viscous effect involved in VIM and VIV problems. In order to include the viscous flow, the governing equations will become the Navier-Stokes equations. This causes not only the boundary conditions (at non-linear free surface and large moving body surface) but also the governing equations themselves to be non-linear. Basically, the reliable treatments of the aforementioned three nonlinearities are not fully available in most of potential flow based BEM-type softwares. In hostile sea environment commonly involving high waves and strong current, nonlinear effects will become extremely dominant and hydrodynamic load and structural response based on linear methods will differ significantly from the real fluid/structure behavior based on nonlinear solutions. 3. Emerging CFD based analysis With advances of computational technology, numerical simulation of physical phenomena has been gaining momentum. Computational Fluid Dynamics (CFD) based simulations have been looming as, at least, a contending engineering tool. 3.1 Validity of CFD based model - Navier-Stokes equations As CFD is based on a realistic physical model Navier-Stokes (NS) equations, the physics related to fluid viscosity can be represented. It is generally accepted that the NS equations are valid even for the smallest turbulence scale. In principle, NS equations can be used directly to solve any real flow problems instead of using turbulent models along with them. However, because of the extremely large range of spatial and time scales in turbulent flow, solving them directly is still far beyond today s computer capability. Instead of NS equations alone, in CFD simulation RANS (Reynolds Averaged Navier-Stokes) equations is used along with the turbulent models such as k-ε, k-ω, or Reynolds stress models for turbulent flow simulation. Since turbulence model is introduced into the governing equations, it brings in uncertainty to CFD simulations. In addition, in CFD community, grid quality and numerical method are two other main uncertainties affecting the simulations accuracy. While CFD can boast many successful applications, for new applications it still needs experimental data for validation if such new applications are to gain any following. 3.2 Essential capabilities required in marine/offshore problem Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities 179

4 To apply CFD to marine and offshore problems, some essential capabilities are required. In this paper, we will highlight three of them - the violent free surface, the large motion, and the viscous effect. These three are also the main difficulties in BEM potential flow simulations Surface capturing method for violent free surface waves Historically, there are many schemes developed for violent free surface flow in CFD simulation such as the interface tracking and capturing schemes. To illustrate the CFD capability on violent free surface problem, an ABS benchmark test case for LNG sloshing problem 1 based on an interface capturing method is presented here. left sensor (Ch.11) right sensor (Ch.11) left sensor (Ch.15) right sensor (Ch.15) (3a) Numerical pressure sensor locations (3b) Impact pressure at Ch. 15 (3c) Violent free surface pattern 1 st peak impact instant (3d) Violent free surface pattern 2 nd peak impact instant Figure 3. CFD simulation for violent free surface problem - LNG tank sloshing In this benchmark test, the filling level of the tank is 30% height with water. A comparison between simulation and measurement for impact pressure at the channel 15 (see Figure 3a) is plotted in Figure 3b. As indicated in the figure, CFD simulation captures the impact pressure in very good agreement with experimental data. Even more, details and insight of the flow physics can be discovered through the CFD results. To illustrate this aspect, physical explanations for the distinct doublepeak pattern exhibited in the impact pressure time history (Figure 3b) are briefly described here based on further CFD results post-processing. According to the two violent free surface patterns at the two peak pressure instants (Figure 3c and 3d), it is found that after the first primary impact (Figure 3c), the water continues to rush up the tank side wall (Figure 3d) resulting in the increase of the water head on the submerged pressure sensor (channel 15). This is the main cause of the 2 nd pressure peak Overset grid technique for large motion To handle the body motion problem, the overset grid technique is commonly regarded as a effective solution, especially when structural grid is adopted in the CFD simulations. In our pervious CFD simulations for propeller off-design flows (Lee 2, 2006), overset grid method was applied. The simulation results show quite promising results in comparison to available experimental data. Figure 4a shows the overset grid used for these propeller flow simulations. In Figure 4b, velocity flow field is plotted for a crash-astern operation case. As seen in the figure, the famous vortex ring pattern is captured in this CFD simulation. 180 Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities

5 (4a) Overset grid configuration (4b) Flow pattern under crash-astern Figure 4. Overset grid technique for propeller flow analysis For problems involving coupled fluid and structure motions such as VIV and wave-current-body interactions, the overset grid CFD simulations has to combine with a six-degree-of-freedom motion program to facilitate the time-domain simulation of fluid-structure interactions. This method has been used with considerable success for a wide range of problems including internal cooling of turbine blades, film cooling, vortexinduced vibration of marine risers, bridge pier scour, contraction scour, abutment scour, channel migration, submarine hydrodynamics, ship berthing operations, large amplitude ship roll motions and capsizing, multiple-ship interactions in navigation channels, and propeller-ship interactions. To show the capability of the method, Figure 5 is a demonstration example for a floating cylinder inside a sloshing tank. As seen, even though very violent free surface and a floating structure are strongly interacted with each other, the method still can handle it very well. Figure 5. Overset grid method for large motion violent free surface waves problem 3.23 Viscous-inviscid coupled approach for complex flows Although, in principle, CFD simulation can simulate quite a wide range of flow problems, for some applications due to the intensive computational time requested, purely RANS simulation may become impractical and even prohibitive. To reduce the computations to an acceptably practical level, certain appropriate simplifications are essential. For offshore hydrodynamic problems, as quite often the flow domain is an open sea with deep waters, viscous effect has no dominant effect on the wave dynamics in the open sea area other than at the boundary layer region nearby the structure surface. In other words, if potential flow model is used in the region far away enough from the offshore structure, the potential flow simulation will still be good enough. To take advantage of this, a viscous-inviscid coupled method can be a tool to reduce intensive computation encountered in RANS simulations. Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities 181

6 In fact, the viscous-inviscid coupled method has been successfully applied in ship waves and waves induced seabed boundary layer 3 problems. Basically, in this hybrid method, the potential flow (Laplace equation) for nonlinear waves is combined with the RANS simulation for turbulent incompressible flows. 4. Concluding remarks This paper identifies the limitations of the existing computational engineering tools. These limitations are founded mainly in the simplified flow physics by assuming invicid flow on the one hand and in the use of boundary element computational method on the other. While CFD a technology based on Navier-Stokes equations and turbulence modeling seems to be the natural choice to move ahead as a computational tool of choice, many challenges remain, not least of which is the validation of the computational results itself, which should be the subject of more rigorous studies. This paper discussed how some of these computational challenges in CFD technique could be overcome in the context of offshore and marine structures. In fact many promising but disparate computational techniques can be brought together to engineer just the tool for the trade. Three such examples are given: surface-capturing method for violent free-surface waves, overset-grid technique for large motion, and viscous-inviscid coupled approach for complex flows. The authors believe that judicious coupling of such techniques could yield potent and efficient tools that are not available in current commercial CFD packages, particularly in solving offshore and marine engineering problems in ever challenging operational environments. An effort is now underway by ABS and research institutions in Singapore to bring this to fruition. 5. References [1] Chen, H.C. and Yu, K, (2007) Chimera FANS simulation of sloshing impact pressure inside a LNG tank: Benchmark tests & code validation, ABS Technical Report. [2] Lee, S.K. (2006) CFD simulation for propeller four-quadrant flow, SNAME Propeller 2006, Virginia beach, Virginia, USA, June, 2006 [3] Lee, S. K. and Cheung, K. F. (1999) Laminar and turbulent bottom boundary layer induced by nonlinear water waves, Vol. 125, No. 6, pp , Journal of Hydraulic Engineering 182 Simulation Technology for Offshore and Marine Hydrodynamics Status Review and Emerging Capabilities