EFFECTS OF COUPLED HYDRODYNAMIC IN THE PERFORMANCE OF A DP BARGE OPERATING CLOSE TO A FPSO

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1 Proceedings of the ASME 3th International Conference on Ocean, Offshore and Arctic Engineering OMAE June 9-4,, Rotterdam, The Netherlands OMAE- EFFECTS OF COUPLED HYDRODYNAMIC IN THE PERFORMANCE OF A DP BARGE OPERATING CLOSE TO A FPSO Daniel P. Vieira Numerical Offshore Tank TPN University of São Paulo São Paulo, SP, Brazil Edgard B. Malta Numerical Offshore Tank TPN University of São Paulo São Paulo, SP, Brazil Fabiano P. Rampazzo Numerical Offshore Tank TPN University of São Paulo São Paulo, SP, Brazil João Luis B. Silva Petrobras Engineering Division Rio de Janeiro, RJ, Brazil Eduardo A. Tannuri Numerical Offshore Tank TPN University of São Paulo São Paulo, SP, Brazil ABSTRACT In ocean systems composed by two or more closing floating bodies, coupled hydrodynamics effects must be considered. Dynamic positioned systems (DP), for example, need an accurate determination of environmental forces to guarantee a safe operation. This work presents a numerical methodology, using the WAMIT code, to evaluate both first order motions and mean drift forces of a system composed by a DP Crane Barge operating close to a turret-moored FPSO. The first order wave forces were evaluated using the code standard method. The second order forces (or mean drift forces) were obtained using the alternative control surface method. The work discussions are centered on the effects of FPSO presence on DP Barge hydrodynamics. Two relative positions between vessels were evaluated as well as three FPSO drafts (full, intermediate and ballasted). The effects of wave incidence angle were also discussed. KEYWORDS Hydrodynamic Interference, Crane Barge, Control Surface, FPSO, Multibody Analysis INTRODUCTION Every day the global offshore industry handles support operations on oil platforms in which one or more vessels are required to accomplish the task. A Dynamic Positioned Barge operating its crane close to a turret-moored FPSO to replace 3 ton equipment is a Brazilian example of these operations (Figure ). Figure Draft of DP Barge operating close to a FPSO. This operation requires the prediction of the forces and motions involved to be safely carried out. Thus, to evaluate these forces, a numerical model in the frequency domain was used. The simulation was conducted using WAMIT, a boundary element method code based on the potential linear theory (Newman, 977). The main properties evaluated by WAMIT Copyright by ASME Downloaded From: on /5/4 Terms of Use:

2 are the wave forces and the hydrodynamic coefficients (additional mass, potential damping and hydrostatic restoring) as well as the Response Amplitude Operator (RAO). It is possible to evaluate these properties for an N floating body system, considering the interference between them. Crane operation requires great precision in the positioning of a DP Barge, so both first and second order forces were calculated. The forces and the hydrodynamic coefficients were used as input to a full operation simulation described in Tannuri et al. () and Rampazzo et al. (). This simulation was conducted in the time domain using TPN, a full coupled nonlinear simulator in which it is possible to simulate the suspension cable mechanics, the DP system, the anchor system, etc. The TPN is also integrated with the WAMIT, since it is able to run a new wave analysis when the relative position between the vessels changes significantly (Queiroz Filho & Tannuri, 9). A comprehensive description of TPN capabilities is presented in Gaspar et al. (9) and briefly in APPENDIX. Thus, the main purpose of the presented numerical model is to provide this input. Additionally, this paper s objective is to provide important information about operation just from the frequency domain model. NUMERICAL MODEL To evaluate the first order forces the code standard method was used. First, the wet surface of each vessel was designed in MULTISURF, computer-aided design (CAD) software in which one can design parameterized surfaces. This software is recommended once it can communicate directly to the WAMIT code to generate the mesh to evaluate the forces (high order method). Figures and 3 present the DP Barge wet surface and the FPSO hull, respectively. cases. Only three loading conditions were used in the analysis: Full, Intermediate and Ballasted. The drafts of each of them were m, 4m and 7m, respectively. Table presents the vessels main dimensions. Table DP Barge and FPSO Main Dimensions. Dimension DP Barge FPSO Full Interm. Ball. Unit Length (L) m Beam (B) m Draft (T) m The next step is to input the mass properties of each vessel. Table shows a summary of these properties for each configuration. Table DP Barge and FPSO Mass Properties. Properties DP Barge FPSO Full Interm. Ball. Unit Mass.7E+4 3.7E+5.99E E+4 t Ixx.4E+6.9E+8 8.E+7 4.7E+7 t.m² Iyy.3E+7.88E+9.33E+9 7.7E+8 t.m² Izz.4E+7.94E+9.37E+9 7.E+8 t.m² In addition, the relative initial positions between vessels were input. In this study two relative positions were evaluated: parallel and transverse. It is necessary to provide the relative positions of each vessel related to the global origin. Here, the global origin was set in the DP Barge waterplane projection of its center of gravity. Figure 4 presents the global origin and the definition of wave incidence angle. Figure DP Barge wet surface. Figure 4 Global Origin and Wave Incidence Angle definitions. Figures 5 and 6 present the relative positions for the two studied cases: Parallel and Transverse case, respectively. Figure 3 FPSO hull. The FPSO hull surfaces were parameterized to enable changes on the draft, supporting the analysis of several different Figure 5 Relative Position for the Parallel Case. Copyright by ASME Downloaded From: on /5/4 Terms of Use:

3 RAO4 (deg/m) Transver se FPSO Parallel formulation to apply this method numerically is described in both Lee (5) and Lee (6). Figure 7 presents the control surfaces designed in MULTISURF used in this analysis. Figure 7 Control Surface used on evaluation of mean drift forces. Figure 6 Relative Position for the Transverse Case. Table 3 summarizes the positions and angles of each configuration local origins from the global origin (,,). The local origins are set in the center of gravity of each vessel, the Ox axis is pointing toward bow, the Oy axis is pointing toward portside and the Oz axis is pointing up. Table 3 Relative positions of each configuration from the global origin. Configuration Position (m) Angle (deg) X Y Z θ DP Barge.3 Full Interm Ball Full Interm Ball Furthermore, to evaluate the main drift forces the alternative control surface method was used. The WAMIT code evaluates the mean drift forces by two different methods: momentum conservation and pressure integration. Initially just the pressure integration method can be used to evaluate the mean drift forces for each body separately. The body panel size has to be coherent with the wave length in order to obtain good results by this method. In low frequency waves this panel size value has to be very small to improve accurate results, but small panel sizes require a large processing time. To use the momentum conservation method and evaluate the mean drift forces separately for each body, it is necessary to define a fluid domain that involves each vessel. This fluid domain is defined by control surfaces. The computational advantage of this method is obtaining more accurate results with less discretization (Ferreira et al., 994). The complete derived According WAMIT Inc. (6), the control surface needs to involve each vessel separately. The method used can evaluate the mean drift forces for the six degrees of freedom of each vessel. For this, it is necessary to involve each vessel in a control surface (including panels on the free surface, too). In the multibody analysis, a control surface can intersect another, but it cannot intersect a vessel surface. FIRST ORDER RESULTS The first order results discussion is centered on the influence of FPSO presence on DP Barge motions. The FPSO has a turret-moored anchor system which implies that the environmental wave conditions incidence will surround the FPSO bow-stern direction. Thus, for each relative position, three incidence angles were studied: 5, 8 and for the Parallel Case; 4, 7 and 3 for the Transverse Case. Figures 8, 9 and show the DP Barge Roll RAOs for the three incidences. The main result presented is the sensitive RAO level reduction for a incidence angle. This can be explained by the FPSO shadow effect, i.e. for wave incidence angle the wave train achieves the DP Barge modified by the FPSO presence. This fact must be considered on the configuration to operate the DP Barge Modelo BGL Roll DP Barge + FPSO Ballasted (T=7m) Figure 8 DP Barge Roll RAO Parallel Case Inc. = 5. 3 Copyright by ASME Downloaded From: on /5/4 Terms of Use:

4 RAO [deg/m] RAO4-Phase (deg) RAO4 (deg/m) RAO4 (deg/m) RAO4 (deg/m) Modelo BGL Roll DP Barge + FPSO Ballasted (T=7m) Figure 9 DP Barge Roll RAO Parallel Case Inc. = Modelo BGL Roll DP Barge + FPSO Ballasted (T=7m) Figure DP Barge Roll RAO Parallel Case Inc. =. Furthermore, the effect of wave incidence angle on DP Barge roll motion is presented in Figure. This figure shows the reduction of roll motions due to the shadow effect on incidences above Wave Incidence [degrees] Period [s] Figure Roll RAO of DP Barge close to FPSO in intermediate draft Parallel Case The other important fact of coupled analysis is presented in Figure 9. The DP Barge Roll uncoupled RAO (blue line) for a 8 wave incidence is null. However, due to the FPSO radiation and diffraction waves, the DP Barge is excited at levels as high as if there was a wave incidence by the vessel side. The presence of a peak about 8 seconds in all cases is clear. This peak is due to the DP Barge roll RAO resonance period in a 9 wave incidence (Figure ). There is also a direct correlation between the FPSO displacement and DP Barge motion performance for 8 o and o incidence angles (Figures 9 and ): the smaller the FPSO draft, the larger are the DP Barge roll motions. This trend may be explained by the proximity between the FPSO heave resonant period to the DP Barge roll resonant period, since for those incidences, the DP Barge roll is mainly excited by the waves irradiated by FPSO motion. For 5 o incidence angle (Figure 8) this correlation is not verified, since in this case the DP Barge does not experience the shadow effect, and the DP Barge is mainly excited by incident waves and FPSO reflected waves Figure DP Barge Roll RAO Inc. = 9. Figure 3 presents the vertical acceleration at the crane suspension point as a function of wave incidence angle. The - accelerations were evaluated for a typical Brazilian sea - condition (JONSWAP 5 spectrum, 5 Tp = 9s, Hs = m), 5 for the 3 Parallel Case and on the intermediate FPSO draft. The drastic acceleration reduction at this critical point is another important shadow effect characteristic that can be achieved numerically ,5,5, Figure 3 Significant Vertical Acceleration (g) at the Crane Suspension Point Parallel Case 4 Copyright by ASME Downloaded From: on /5/4 Terms of Use:

5 F (kn) RAO5 (deg/m) F (kn) RAO5 (deg/m) F(kN) RAO5 (deg/m) The same trend can be observed for DP Barge pitch motion in the transverse case (Figures 4, 5 and 6). Comparing Figure 4 with Figure 6, the coupled pitch motions for a 4 wave incidence are higher than uncoupled motion, but for a 3 wave incidence the coupled motions are lower. Figure 5 presents pitch motion elevation due to FPSO presence DP Barge + FPSO Ballasted (T=7m) Figure 4 DP Barge Pitch RAO Transverse Case Inc. = DP Barge + FPSO Ballasted (T=7m) Figure 5 DP Barge Pitch RAO Transverse Case Inc. = DP Barge + FPSO Ballasted (T=7m) Figure 6 DP Barge Pitch RAO Transverse Case Inc. = 3. MEAN DRIFT RESULTS The mean drift forces evaluation is essential in thruster systems design (Faltinsen, 99). To estimate the required power in Barge DP system for this operation, the accurate evaluation of these forces was essential. Figures 7, 8 and 9 present the surge mean drift forces for the parallel case. In this case, surge is the direction that will require more power from the DP system. 5 5 DP Barge + FPSO Ballasted (T=7m) Figure 7 Mean Drift Surge Parallel Case Inc. = DP Barge + FPSO Ballasted (T=7m) Figure 8 Mean Drift Surge Parallel Case Inc. = DP Barge + FPSO Ballasted (T=7m) Figure 9 Mean Drift Surge Parallel Case Inc. =. 5 Copyright by ASME Downloaded From: on /5/4 Terms of Use:

6 F (kn) F (kn) F (kn) As the first order results, the mean drift forces also reduce due to the shadow effect. Comparing Figure 7 with 9, there is the same trend observed in Figure 4 and Figure 6 comparison. Observing Figure 8, in the coupled case, forces 7% bigger than in the uncoupled case were obtained. This increase in mean drift forces causes a direct impact on the DP system performance. Figures, and present the sway mean drift forces evaluated for the transverse case DP Barge + FPSO Ballasted (T=7m) Comparing these three figures the increase in mean drift forces is less than 3% for the worst case. But if these figures were compared with the parallel case the force levels increased 5 times on average. Figures 3 and 4 present the mean drift force in the DP Barge in the intermediate configuration operating in a Brazilian typical sea (JONSWAP spectrum, Tp = 9s, Hs = m) Figure Mean Drift Sway Transverse Case Inc. = DP Barge + FPSO Ballasted (T=7m) Figure 3 Surge Mean Drift Force (kn) Parallel Case Once more it is possible to observe the shadow effect by acting on the DP Barge. In the first configuration, the asymmetry between portside and starboard forces caused by FPSO presence is notorious. Comparing 5 and there was a reduction about.7 times in the mean drift forces Figure Mean Drift Sway Transverse Case Inc. = DP Barge + FPSO Ballasted (T=7m) Figure Mean Drift Sway Transverse Case Inc. = 3. 9 Figure 4 Sway Mean Drift Force (kn) Transverse Case In this second configuration the mean drift forces have a decrease of 7% comparing 4 and 3. 6 Copyright by ASME Downloaded From: on /5/4 Terms of Use:

7 CONCLUSION AND NEXT TASKS A numerical model, in frequency domain, of a DP Barge operating close to a FPSO was provided. Both first order motions and mean drift forces were evaluated for purposed configurations. The presented results covered the main hydrodynamic interaction effects, for example wave diffraction effects. Results were compared with the uncoupled hydrodynamic of the DP Barge, and important trends about the FPSO presence were achieved. The effect of wave incidence angle was showed and discussed obtaining important information for the operation design. The importance of evaluating coupled hydrodynamic properties in multibody operations is the main conclusion of this work. The next tasks are experimental comparing, other relative positions evaluation and a more comprehensive study of the system operating in real seas to determine a safety operational window. ACKNOWLEDGMENTS The authors would like to thanks Petrobras and the University of São Paulo to support this work as well as the Numerical Offshore Tank (TPN) team for the technical discussions. The first author would like to acknowledge the National Agency of Petroleum (ANP) for the research grant. The fifth author also acknowledges the National Council for Scientific and Technological Development (CNPq) for the research grant (3544/-) and São Paulo Research Foundation - FAPESP (/5348-4). REFERENCES. Faltinsen, O.M. (99). Sea Loads on Ships and Offshore Structures. Cambridge University Press.. Ferreira, M.D., Lee C.-H. (994). Computation of secondorder mean wave forces and moments in multibody interaction. In: Proceedings of Behaviour of Offshore Structures, Cambridge, MA. pp Gaspar, H.M., Fucatu, C.H., Nishimoto, K. (9). Design of Conceptual Offshore Systems based on Numerical Model-Basin Simulations. In: Proceedings of International Maritime Design Conference 9, Trondheim. 4. Lee, C.-H. (5). Evaluation of quadratic forces using control surfaces. In: Report of 5 Annual WAMIT Consortium Meeting, Woods Hole, Massachussetts. 5. Lee, C.-H. (6). On the evaluation of quadratic forces on stationary bodies. In: Journal of Engineering Mathematics, V. 58, p Newman, J.N., (977). Marine Hydrodynamics. MIT Press, Cambridge, Massachussetts. 7. OCIMF (994). Predictions of wind and current loads on VLCCs, Oil Companies International Marine Forum. 8. Rampazzo F.P., Tannuri, E.A., Moratelli Jr, L., Silva, J.L.B., Pacífico, A.P.O., Vieira, D.P. (). Numerical & Experimental tools for offshore DP operations. In: Proceedings of the 3th International Conference on Offshore Mechanics and Arctic Engineering OMAE, Rotterdam, The Netherlands (submitted for publication). 9. Queiroz Filho, A.N., Tannuri, E.A. (9). DP Offloading Operation: a Numerical Evaluation of Wave Shielding Effect. In: 8th Conference on Manoeuvring and Control of Marine Craft (MCMC'9), Guarujá, Brazil.. Tannuri, E.A., Saad, A.C., Morishita, H.M. (9). Offloading Operation with a DP Shuttle Tanker: Comparison between Full Scale Measurements and Numerical Simulation Results. In: 8th Conference on Maneuvering and Control of Marine Craft (MCMC'9), Guarujá, Brazil.. Tannuri, E.A., Silva, J.L.B., Rampazzo, F.P., Malta, E.B., Vieira, D.P., Rossin, B.D. (). Methodology for evaluating DP crane-barge operation in close proximity of a FPSO. In: Proceedings of the 8th IFAC Conference on Control Applications in Marine Systems, CAMS, Rostock-Warnemünde.. WAMIT Inc. (6). WAMIT User Manual, Versions 6.4, 6.4PC. APPENDIX: THE TPN OFFSHORE SYSTEM SIMULATOR The TPN is a time domain numerical procedure designed for the analysis of moored and DP offshore systems. The inputs of the simulator are: Floating body main parameters (dimensions, mass matrix, etc.); Aerodynamic drag coefficients (following standard given by OCIMF (994)); Current coefficients (following standard given by OCIMF (994)) or hydrodynamic derivatives; Hydrodynamic coefficients (potential damping, added mass, first and second order wave force coefficients); Environmental conditions (wave and wind spectra, current); Mooring and risers system characteristics; Thrusters characteristics, saturation and layout; DP modes and parameters. The non-linear time-domain simulation runs in a cluster computational system and outputs time series describing the motions of up to two floating unities (FU) in six degrees of freedom (6dof), tensions on the mooring lines and hawser, propellers thrust and power, etc., and a corresponding statistical summary. 3D visualization outputs are also available. The floating body high frequency motion (HF) due to the wave action can be evaluated in two different ways. In the simpler one the HF motion evaluated by the RAO is added to the low frequency motion (LF) that is calculated by the 3rd order Runge-Kutta integration method. Alternatively, the wave st order forces are applied to the body and all motion components (6dof) are obtained dynamically solving the equations of motion. The current force can be evaluated through 3 different 7 Copyright by ASME Downloaded From: on /5/4 Terms of Use:

8 models: OCIMF Model, Cross flow Model, Maneuvering Model or Short Wing Model (in-house development). It is possible to analyze 3D constant or oscillatory current profile. The simulator allows constant wind and gust wind. The wind spectra implemented in the code are Harris, Wills and API. The wave can be regular and irregular. For irregular waves the spectra formulations available are Pierson-Moskowitz, JONSWAP and Gaussian. The wave first and second-order effects are modeled besides wave-drift damping effects. The wave coefficients are evaluated by WAMIT. Three main classes of algorithms used in commercial DP systems are also implemented in TPN. A low-pass filter, called wave-filter, is employed to separate high-frequency components (excited by waves) from measured signals. Such decomposition must be performed because the DP system must only control low-frequency motion, since high-frequency motion would require enormous power to be attenuated and could cause extra tear and wear in propellers. Furthermore, an optimization algorithm, called thrust allocation, must be used to distribute control forces among thrusters. It guarantees minimum power consumption to generate the required total forces and moment, positioning the vessel. At last, a control algorithm uses the filtered motion measurements to calculate such required forces and moment. Normally, a wind feedforward control is also included, enabling to estimate wind load action on the vessel (based on wind sensor measurements) and to compensate it by means of propellers. Furthermore, the simulator also includes models for propellers, taking into account their characteristics curves, being able to estimate real power consumption and delivered thrust. It also evaluates time delay between command and propeller response, caused by axis inertia. A real-scale DP offloading operation was used for the preliminary validation of the DP system implemented in the simulator (Tannuri et al., 9). In that work, the total and individual thrusts are measured and compared to the values obtained in the simulations. Very good agreement for mean values and spectra was verified, validating the DP algorithms and propeller models included in the simulator. 8 Copyright by ASME Downloaded From: on /5/4 Terms of Use: