Proceedings of the ASME 27th International Conference on Offshore Mechanics and Arctic Engineering OMAE2008 June 15-20, 2008, Estoril, Portugal

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1 Proceedings of the ASME 27th International Conference on Offshore Mechanics and Arctic Engineering OMAE28 June 15-2, 28, Estoril, Portugal CURRENT MODELING EXPERIENCE IN AN OFFSHORE BASIN Bas Buchner and Jaap de Wilde MARIN (Maritime Research Institute Netherlands) Haagsteeg 2 / P.O. Box AA Wageningen, The Netherlands b.buchner@marin.nl, j.dewilde@marin.nl OMAE ABSTRACT A reliable current simulation is vital to achieve the objectives of model testing. In the present paper the development and testing of the current generation in an Offshore Basin is presented. First the objectives of such a development are discussed. Then the actual system as developed is described in detail. Finally the results of current measurements in the basin are presented. It is concluded that with a dedicated current modeling system in a model basin, it is possible to achieve a constant current over the measurement area with low variations in time and space. Figure 1: Wind, wave and current modelling in the Offshore Basin INTRODUCTION With the trends of deep water exploration and production and offshore LNG production and offloading, the present hydrodynamic challenges lie both in deep and shallow water. A combination of model tests, simulations and full scale measurements is needed to tackle these challenges. The present paper focuses on model testing and then on the specific issue of current modeling. It is important to put that issues in the perspective of the overall objectives of model testing, which can be formulated as follows: - Verify the overall behavior of the floater compared to its design requirements: is it fit for purpose? - Determine (statistically reliable) design values (loads and response) - Discover any unexpected effect that was not yet taken into account in the design - Validate and calibrate numerical tools (for instance damping values), so that the test results can be extended reliably to full depth and full scale - Determine aspects that cannot be predicted reliably enough with present day simulation tools, such as green water, slamming and VIV. A reliable current simulation is vital to achieve these objectives. Towing tests for current modelling only, or the application of pre-calculated current loads by a wire is often not sufficient in our view because: - Current has a large contribution of the total load on deep draft structures and mooring/riser systems. Taking into account the complex shape of (for instance) truss SPARs, it is self evident that the current load will have an significant effect on the overall behaviour, see Figure 2. Calculation of this effect will be very difficult due to the interactions between various components in the truss and the heave plates. Figure 2: Current is an important effect for (deep draft) offshore structures such as truss spars [1] 1 Copyright 286 by ASME

2 The current loads on the heave plates will also be depending largely on the actual pitch and roll angle of the structure, an aspect that cannot be modelled with a wire system. Furthermore, the vertical current profile, with its possible strong shear, will affect the current loads significantly. This aspect cannot be taken into account when current is simulated by towing only, which by definition results in a slab type (constant vertical velocity profile) current. - Directly related to that, is the effect of the current on the important issue of Vortex Induced Motions (VIM) of this type of structures. This relates to the VIM of the structure itself, but also to the importance effect of the waves on the VIM, see Ref [1]. Wave action and current action cannot be separated and needs to studied in an integrated way. - A constant current (not varying in time) is necessary because current variations can result in unreal (resonant) motions when the variations occur close to natural periods of the structure in its mooring and riser system - There is a strong interaction between current and waves, which can affect the drift forces, wave impact loads and air gap significantly. occurs in reality, which is typically expressed through the turbulence intensity T. It is defined as the ratio between the standard deviation of the velocity fluctuations (σ) and the required mean velocity (u): T = σ / V (1) The evidence from offshore measurements shows that the turbulence intensity is typically limited to a few percent. Based on this consideration and experience in model basins in the past a challenging target turbulence intensity of 5% was defined by MARIN for a uniform vertical current velocity profile within the measurement area of 2*2 m. If such target cannot be achieved in certain circumstances, tests with the structure in current only should be performed to check the importance of current induced oscillations on the overall behaviour. Taking into account the importance of current modelling, we now should take into account that a model basin is always only part of reality. In reality current is driven by the tide, main global currents or by the wind. In a model basin the current is generated by pumps and should be recirculated outside the basin to prevent re-circulation in the basin itself. It will be clear that the generation of a constant (stable) current in the basin is a real challenge. Based on these challenges, in the development of the new Offshore Basin of MARIN (Maritime Research Institute Netherlands) a lot of attention was paid to the current modeling [2, 3]. The basin has now been in operation since 2 and has been used for the testing of all types of offshore structures, such as TLPs, Semi-submersibles, FPSOs, Spars and other novel structures. The Offshore Basin has been the inspiration for the development of a number of new model basins all over the world [, 5]. It is therefore important to share a number of experiences in the design and use of the current generation system. BASIN DESIGN AND CONSTRUCTION The Offshore Basin measures 6m * 36m and has a movable floor, which is used to adjust the water depth. The maximum water depth measures 1.3m at model scale. The basin also has a deep pit, with a maximum depth of 3m. This is sufficient to test TLP type structures up to depths of 3,m. The cross section and top view of the basin are given in Figure 3. Considering the importance of current modelling for offshore model tests, the current velocity should be as constant as possible in time and space in the basin. As variations in the current velocity close to the natural period of moored vessels can induce unwanted behaviour of the structure, it is important to stay as close as possible to the fluctuation in the current that Figure 3: The cross section and top view of the Offshore Basin Because in deep water the current loading on risers and mooring lines can be large, it is also important to control the vertical velocity profile of the current. The 6 independent current layers available in the Offshore Basin allow a detailed modeling of these current velocity profiles, from hurricane type of current profiles to even reverse type of currents (with the lower layers in the opposite direction of the upper layers). The maximum current speeds in the basin are.5 m/s at the surface and.1 m/s at the bottom. The velocities at model 2 Copyright 286 by ASME

3 scale allow a wide range of full scale current velocities (up to 6-7 knots). As mentioned before, the current flow generation by pumps differs in many ways from the processes that generate natural currents. Pumps transfer a large amount of energy in the very localized volume of the pump house, at the location of the impellers. This process involves high flow velocities, velocity gradients and turbulence production. The natural processes on the contrary, are much gentler. Currents resulting from tidal effects and wind shearing occur at large scales, involve small velocity gradients and have a small turbulence production. inflow section was inclined upward under an angle of 1:, which reduced the size of the re-circulation zone to approximately 3 m. With the wake behind the wave generators it is difficult to create the maximum required flow velocities at the surface of.5 m/s. Because structures in the basin were not allowed, the inflow velocity from upper layers was increased. In this way the velocity loss in the downstream area was compensated. The hydraulic research institute Delft Hydraulics in the Netherlands was contracted for the hydraulic design of the new current flow system. Delft Hydraulics had the required experience on the design of flow loops, test flumes and flow tanks. The design work started in 1996 with a definition study, in which several concepts were evaluated. The final concept was selected as being the most promising for the realization of the requirements. This concept contained several proven elements that have been used successfully in existing facilities at Delft Hydraulics. In Figure 3, a sketch of the flow circulating system is presented, consisting of the basin, outlet section, flow ducts, pump section and inlet section. The in- and outlet sections are the most critical sections in the design of the flow system. The downstream flow coming from the inflow section is largely responsible for the uniformity and the turbulence intensity in the measuring area. The 12.5 m downstream length between the inflow and the measuring area itself is too short for sufficient turbulence decay. Moreover, partly resulting from the low turbulence degree, the mixing process is too small to correct large-scale non-uniformities in the flow field. This required the application of flow improving devices. All these devices, such as guide vanes, perforated walls and turbulence grids had to be situated outside the tank. Structures in the tank itself are not acceptable for their interference with the models and the mooring lines. Certain problem areas of the design were subject to detailed investigation by scale modeling. The main objectives of these model tests were to optimize the design and to define possible pitfalls. In total three models of different specific sections of the overall system were tested at model scales of 1:5, 1:5 and 1:1 respectively. These relatively large model scales were required to avoid Reynolds scale effects and (unknown) scale effects for turbulence. Sufficiently accurate numerical tools (CFD) did not exist at that time for the complex threedimensional flow and the calculation of the turbulence degree. The flow system is divided into 6 individual layers for the adjustment of the different vertical velocity profiles. In Figure a cross-section of the inflow structure is presented. The upper most layer is directly located under the 1.2 m high wave generators, which projects a problem for the flow in the downstream region. The re-circulation zone behind the wave generators will stretch out over a distance of approximately 5 to 1 m. The associated vortex shedding will produce a significant amount of turbulence. To reduce this effect, the Figure. Current inflow with current control systems The uniformity of the flow in the basin is obtained by flow distributing in- and outflow culverts as presented in Figures 3 and. The main function of the components (denoted 1-9 in the figures) is described below: - The diffuser (1) was required for a smooth transition between flow duct and culvert. - The 9 degrees bend (2) was fitted with guide vanes to avoid flow separation. - The tapered culvert (3) was required to ensure constant flow velocity in the culvert. - The flow leaves the culvert through its perforated side wall () and jet breaker (5). The obstruction of the flow by the perforated wall was required to obtain the required uniform flow distribution (+/- 2.5%). The higher its solidity, the more uniform the flow distribution. However, the perforated wall also results in pressure losses. - The mixing chamber (6) was required for quieting the energetic jet flow from the openings in the perforated wall. The relatively large turbulence degree in this section decays sufficiently fast as a result of the small scale of the vorticity, which is related to the size of the jet openings. - The flow is then guided by flow guiding vanes (7) and the inclined inflow section (8). - Fine mesh turbulence grids (9) are used as a final step in the turbulence control. In total 6 inflow and 6 outflow culverts are used, each having specific requirements with respect to the flow rate and 3 Copyright 286 by ASME

4 dimensions. The structural layout presents 1 inflow sections in each layer of 3.6 m wide to cover the total width of 36 m. For the 6 layers this means a total of 6 inflow sections. The solidity of the perforated walls for instance has been optimized for each individual culvert. Also the shape and location of the guide vanes in the 9 degrees bends was carefully optimized. The walls, floors and ceilings are an important factor in the flow guiding and stabilization. The required turbulence level of less than 5% in the measuring section presented the highest challenge for the new flow system. In general the turbulence control becomes increasingly difficult when lower turbulence levels are required. Typical values in high Reynolds number flow are between 2% and 1%. Levels of 2% for instance are found in the wake of blunt objects in the flow. Levels of 1 to 15% are typical for civil engineering, when considering channel or pipe flow. Turbulence levels of less than 1% are only found in special test facilities. In wind tunnels for instance flow contraction and various types of grids and honeycombs are used to obtain such low turbulence levels. As discussed in the previous section, the turbulence control section of the new Offshore Basin in Figure consisted of an energetic jet flow from the perforated wall, a flow breaker behind the perforated wall, a long mixing chamber, an array of guide vanes with 1: angle, a slightly diverging inflow section and a turbulence grid. In Figure 5, the turbulence intensity as measured in the scale model tests is presented as a function of the downstream distance from the inflow. The plot shows a clear decay from 1% turbulence at the inflow and less than 5% at the beginning of measuring section (12.5 m from the inflow openings). This guarantees very constant current flow in the measurement area. Turbulence intensity in % Distance from inflow in metres Measurement area Basin depth [m] Current velocity [m/s] Figure 6. Maximum and minimum current velocity profiles Min velocity Max velocity Figure 6 shows the typical range of current speeds that can be calibrated in the Offshore Basin. It can be seen that at the free surface higher current speeds can be achieved as a result of the lower height of the current flow channels. The minimum current speed is a result of the minimum RPMs the current pump can run to prevent problems with lubrication (of course current pump can also be put off to achieve a zero current at a certain level). In Figure 7 an overview is given of the different components during the construction phase: the construction of the fine mesh turbulence grids, the pumps and the pump room. Figure 5. Turbulence intensity as function of the distance from the current inflow Copyright 286 by ASME

5 Figure 8 Three-axis acoustic current meter is used based on a differential travel time measurement technique However, a current meter is not enough to document the current over the complete depth of the basin. After evaluation of different rigid structures, it was decided to develop a vertical traversing system based on the tension leg principle, see Figure 9: - A heavy weight (5 kg) is placed on the basin floor - Two thin steel wires are pre-tensioned from the top - A light and thin aluminium carriage travels up and down along the pre-tensioned wires This set-up ensures a minimum movement of the current meter during the measurement as a result of current loads and VIV type effects. Two types of measurements are carried out: - Sweep measurements with a small constant vertical velocity of the carriage to determine the vertical profile - Stationary measurements a certain depths to determine the behaviour of the current in time at that position. To calibrate a required current profile the following procedure is applied: Figure 7 From top to bottom: the construction of the fine mesh turbulence grids, the pumps and the pump room CURRENT MEASUREMENT SYSTEM To adjust and document vertical current profiles, after evaluation of a number of systems such as an electro magnetic speed (EMS), nowadays a three-axis acoustic current meter is used based on a differential travel time measurement technique (Nobska MAVS). The current meter takes measurements across acoustic axes to provide a true vector averaged velocity measurement. The accuracy is.3 cm/s, the resolution.3 cm/s and the range 2 cm/s. The actual probe is shown in Figure 8. - The vertical traversing system is positioned at the projected centre of the structure - The RPMs of the six current pumps are set based on the Offshore Basin current database, assuming a linear relation between RPM and current velocity - After 2 minutes a vertical sweep measurement is taken and the measured profile is compared with the required profile. - The ratio between required current speed and measured current speed is applied to the RPM-settings to obtain new RPM-settings. - This procedure is repeated until the measured profile matches the required profile within 5%. - At certain depths a stationary measurement is applied to obtain insight in the current velocity in time 5 Copyright 286 by ASME

6 Current-probe Carriage -1-2 Mean of 3 sweep measurements Stationary measurement 3 hours -3 Water depth [m] Heavy weight -7-8 Figure 9. Measurement system for vertical current velocity profiles. The developed current generation and measurement systems proved to be reliable and efficient. Within a few hours of basin time an accurate current profile can be adjusted Current velocity [m/s] Figure 11. Example of a modelled vertical current profile (full scale values) Figure 1 shows a data based of current profiles that has been calibrated in the Offshore Basin over the years. As the current speed appears to be almost linear with the pump RPMs, it is easy to adjust current profile water depth [m] v cur [m/s] Figure 1. Data base of current profiles adjusted over the years in the Offshore Basin CURRENT MEASUREMENT ANALYSIS We will now consider a modelled vertical current profile in more detail, see Figure 11. A sweep measurement was taken as well as stationary measurements at depths. In Figure 12 the time traces of the stationary measurement points are given over a period of 3 hours prototype: 2 in the upper constant part, 2 in the sheared current part. Figure 12. Time traces of the stationary measurements (full scale values) In the constant upper part the variation is very low (T=5%), in the sheared current the variation is slightly higher (T=7 and 1.9%) but this is a result of the natural processes in sheared current. Measurements directly downstream of the inflow chambers show a highly uniform flow and a low turbulence intensity. The turbulence at this location mainly consists of small scale eddies which rapidly dissipate when traveling downstream. Measured current spectra in the test area of the tank (some 25 m downstream) show very small turbulence levels in the 6 Copyright 286 by ASME

7 higher frequency range (above 1. rad/s model scale). Effectively all turbulence concentrates in the low frequency part of the spectrum (below 1. rad/s model scale, see the typical current velocity spectrum in Figure 13) with overall turbulence intensities between % for slab type currents and around 1% for highly sheared current situations. These turbulence intensities can be considered as very low, compared to MARIN s old offshore tank and other test facilities in the world. Formally the low frequency oscillations should not be considered as true turbulence because it does not relate to localized eddies which dissipate to smaller eddies in a cascading process. This is of course a matter of definition. Current , m 2, m A B C D E Figure 1. Rows for horizontal profile measurements (A, B, C, D and E) and points (1-6) for vertical profile measurements Figure 15 show the measured horizontal profiles in row B, C, D and E over a width of 2m Yposition [m] Current Row B Row C Row D Row E Figure 15. Horizontal velocity profile measurements over rows B, C, D and E Vxcur [cm/s] Figure 13. Current velocity spectrum of a 3 hour measurement (full scale values) The observed turbulence in the basin can have the following backgrounds: The concrete walls of the inflow system with 3.6 m horizontal separation leave some signature on the incoming velocity profile. These small imperfections in the inflow conditions may affect the downstream flow somewhat. This signature of the walls can be found in the velocity profile over the width of the measurement area. This velocity profile over the measurement area was measured with a horizontal sweep measurement over a number of rows (B, C, D and E), as indicated in Figure 1. What can be observed is a very constant current over the measurement area of 2m wide, with a minor reduction in average speed over the width towards the right as expected and a small signature of the inflow walls. For the typical measurement area of a project (a few squared metres) the mean current velocity is within a few procent. The horizontal profile is very similar for the different rows, which indicates that a very stable and controlable current is present in the measurement area. This is confirmed by the measured vertical velocity profile at position 2 and 6 in the basin in Figure Copyright 286 by ASME

8 [ ] [ ] Vxcur [cm/s] The current profiles are not affected significantly by the presence of current. D e p t h m 8 12 point 2 point Figure 16. Comparison of the vertical velocity profile measurements at points 2 and Figure 17 shows the effect of waves on the vertical velocity profile. The vertical current profiles with waves were measured with the same pump RPM settings as the profile without waves. A Jonswap wave spectrum with a significant wave height of.7m and 1.18s peak period was used. The regular wave was.7m in amplitude and 1.s period. D e p t h m Vxcur [cm/s] REFERENCES 1. Truss Spar VIM in Waves And Currents, Finnigan, T., Irani, M. and Van Dijk, R.R.T., OMAE25-675, Halkidiki, June Features of the State-of-the-art Deepwater Offshore Basin, Buchner, B., Wichers, J. E. W. and Wilde, J. J., Offshore Technology Conference (OTC), pp , Houston, May Important Environmental Modelling Aspects for Ultra Deep Water Model Tests, Buchner, B., Cozijn, J.L., and Wichers, J.E.W., Deep Offshore Technology (DOT) conference, Rio de Janeiro, 21.. Hydraulic Performances Of Current Generation System In The New Deepwater Offshore Basin, Lu, H. N., Yang, J. M., and Peng, T., OMAE , San Diego, June The Flow Conditioner of the Current Generation System of Laboceano, Alho, A.T.P., Levi, C.A. and Esperança, P.T.T., Third International Workshop on Applied Offshore Hydrodynamics, Rio de Janeiro, October ITTC Ocean Engineering Committee, Proceedings of the 2 th ITTC conference, Volume I, ITTC, Mathematical Modelling of Wave-Current Interaction in a Hydrodynamic Laboratory Basin, Maragaretha, H., Ph.D. Dissertation, Univ. of Twente, The Netherlands, point 3 without waves point 3 irregular wave point 3 regular wave Figure 17. Comparison of the vertical velocity profile measurements at points 3 with current only, with regular waves and irregular waves Based on these Figures it can be concluded that there is no significant effect of the waves on the current generation for this type of wave heights, which is important for the generation of waves on current and the wave-current interaction effects in the vessel response. However, in steep and large waves it can be important to check this assumption [7]. CONCLUSIONS Based on the present paper, it can be concluded in general that: - A reliable current simulation is vital to achieve the objectives of model testing - With a dedicated current modeling system in a model basin, it is possible to achieve a constant current over the measurement area with low variations in time and space. 8 Copyright 286 by ASME