CFD for offshore applications
In 2009, MARIN took a major step towards the com- mercial application of Computational Fluid Dynamics (CFD) tools for offshore-related problems. This article outlines achievements so far and the problems currently under investigation. Arjen Koop & Guilherme Vaz,
a.koop@marin.nl
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or current loads on general offshore constructions, such as semi-sub- mersibles, FPSOs, LNG carriers and
shuttle tankers, MARIN can offer services using in-house CFD, as well as model basin tests. Current loads and wind forces have been the two main issues addressed. Current loads have been investigated since the start of the “Current Affairs” JIP in 2007 and although the investigation of wind forces has just started, it profits from the wealth of experience gained since then. Added Value of CFD. The average and time-dependent loads by current and wind are an important design parameter for the mooring of offshore con- structions or for a Dynamic Positioning (DP) system. Usually, these loads are obtained by wind tunnel and/or basin tests, or by empirical methods. CFD can help to calcu- late these loads in a cost-efficient manner. Detailed flow analysis and the resulting forces on the (sub)structures can be obtained, leading to a better understanding of com- plex flow phenomena. Examples include: - Analysis and visualisation of the unsteady vortex shedding from columns of a semi-submersible;
- Determination of shielding effects for columns or vessels in the wake of other structures;
- Scaling effects; - Influence of test facility limitations such as blockage effects.
Achievements in 2009 were possible due to a combination of important assets now
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available at MARIN. These include the in-house CFD code ReFRESCO (old name FreSCo, see [1]), which is optimised, verified and validated exclusively for hydrodynamic applications; High-Performance-Computing clusters with around 1,400 processor-cores; three commercial grid-generation packages, in-house tools and extensive experience. Several researchers continue to be involved in the further development and validation of CFD. Furthermore, MARIN has an extensive experimental database that enhances the validation of our tools. Some examples follow.
Current Loads on a Semi-Submer- sible MARIN carried out time-dependent, model and full-scale calculations for a simplified semi-submersible configuration tested within the “Current Affairs” JIP, (see [2]). The model-scale, time-averaged drag coefficients (Cx) obtained with ReFRESCO are within 4%, when compared to the model-scale tow experiments (see Figure 1a). The full-scale CFD calculations reveal that the time-averaged drag loads are expected to be 15% lower than at model- scale. However, the amplitude of the oscil- lating lift forces (Cy) at full-scale are factor 2 higher than at model-scale (see Figure 1b). These oscillations are a result of the complex vortex shedding around the multiple columns as illustrated in Figure 1c. The full-scale calculations must be validated but agreement between the CFD calculations at model- scale with the experiments gives confidence in the accuracy of the full-scale results.
Current Loads on an LNG Carrier Within the “HAWAI” JIP the current loads were measured on an LNG carrier with bilge keels and rudder in MARIN’s shallow water basin for flow angles between 0 and 180 degrees. Within the Current Affairs JIP and within the MARIN research programme, CFD calculations have been carried out for differ- ent angles. For example, for a bow-quarter- ing current, the CFD results concurred well with the model tests for the steady-state force coefficients, see Figure 2 and [3].
Wind Loads for Tandem-Offloading Within the OO1 and OO2 “Offloading Opera- bility” JIPs, wind tunnel measurements were carried out for tandem and side-by- side offloading to determine the wind force coefficients and the velocity distribution in the wake of the first vessel. This wake has a significant effect on the loads on the second vessel. MARIN has carried out CFD calculations for single ships and for the tandem configuration. This is a difficult problem due to the large geometric com- plexity, grid sizes and the accuracy that has to be achieved regarding the calculation of the wake of the upstream ship. Figure 3a shows the wake field of the FPSO (upstream ship) and indicates that reasonable results have been obtained for the wind loads. Figure 3b shows the tandem configuration and the first qualitative results, see [4]. The work on current loads and wind loads will be extended and in 2010 MARIN will start offering current load calculations to its clients. There are several other offshore- related problems where CFD can make an important contribution. Within MARIN research and the TRUST JIP, DP related issues will be studied. Additionally, CFD analysis of viscous effects on roll damping and offshore-related viscous, free-surface flows offer great pros- pects. We will keep readers posted!
[1] Vaz, G. , Jaouen, F. and Hoekstra, M. “Free-Surface Viscous Flow Computations. Validation of URANS code FreSCo”. OMAE2009, Hawaii, Honolulu, USA. June,2009. [2] Vaz, G., Waals, O., Ottens, H., Fathi, F., Le Souef, T and Kiu, K. “Current Affairs: Model Tests, Semi-Empirical Predictions and CFD Computations for Current Coefficients of Semi-Sub- mersibles”. OMAE2009, Hawaii, Honolulu, USA. June, 2009. [3] Fathi, F., Klaij, C., Koop, A. “Predicting Loads on an LNG Carrier with CFD”, OMAE2010, Shanghai, China. June, 2010. [4] Koop, A., Klaij, C., Vaz, G. “Predicting Wind Loads for FPSO Tandem Offloading Using CFD,” OMAE2010, Shanghai, China. June, 2010.
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