An Experimental Investigation of Hydrodynamic ...

Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering OMAE2014 June 8-13, 2014, San Francisco, California, USA

OMAE2014-23530

AN EXPERIMENTAL INVESTIGATION OF HYDRODYNAMIC IMPACTS OF MARINE GROWTH ON MID-WATER ARCH SYSTEM Michael Farmakis Australian Maritime College University of Tasmania Launceston, Tasmania, Australia Shuhong Chai Australian Maritime College University of Tasmania Launceston, Tasmania, Australia

Yuting Jin Australian Maritime College University of Tasmania Launceston, Tasmania, Australia Henri Morand Technip Perth, Western Australia, Australia

Cecile Izarn Technip Perth, Western Australia, Australia

ABSTRACT

INTRODUCTION

The presence of marine growth modifies hydrodynamic effects to subsea structures and could lead to incorrect modelling if not properly accounted for. Widely-used design practice codes do not contain any specific guidelines or recommendations to account for the effects of marine fouling on complex subsea structures and due to the desired longevity of oil and gas constructs, considerable amounts of marine biofouling can accumulate. In the experimental investigation described in the paper, the impacts of different marine growth severities, current velocities and current directions on the hydrodynamic drag were carried out in the Flume Tank at the University of Tasmania. A 1:15 scale mid-water arch (MWA) was employed during this investigation. Several marine biofouling severities were tested as well as the structure without marine growth, representing scenarios based on realistic MWA operating conditions. Physical modelling was validated with numerical simulations using computational fluid dynamics. Experimental results gathered show a rise in drag forces when the artificial marine growth is attached. The highest force magnitudes were observed when the marine growth severity was at its maximum roughness. This has been complemented by numerical results, with input parameters coming from 3D scans of the artificial marine growth.

The worlds increased dependence on fossil fuels has forced subsea engineering to new frontiers in search of hydrocarbons. This has pushed the offshore industry to develop innovative means of ensuring a long life span of their installation. MWA systems have been developed to meet this need. A MWA is a submerged buoyance canister that supports flexible riser systems within the water column. The key design purpose of such structure is to reduce topside loading and maintain a constant bend radius in the risers. This is achieved by laying the risers over the arch, forming a lazy or steep S riser configuration as shown in Figure 1.

Figure 1: Typical configuration incorporating a MWA: Lazy-S configuration

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Copyright © 2014 by ASME

However being immersed in water the structure is susceptible to sever biological fouling as shown in Figure 2. The accumulation of marine growth can bring adverse effects on the hydrodynamic performance of the MWA structure by exerting unexpected surface roughness and distorting its mass distribution, natural frequency, dynamic response and geometry. Therefore, the MWA system is normally designed against the expect marine growth level, if in service these are found to be exceed, cleaning of the MWA can be performed during the field service life.

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Assessing marine growth impact on the drag force Validating results against findings from numerical and previous studies Testing was following the body fixed co-ordinate system as outlined in Figure 3.

Normal

Axial

Figure 3 - Co-ordinate system used during model testing

Figure 2. Example of marine growth accumulation on a MWA Studies into modelling and experimentation of marine biofouling have been performed in the past. Baarholm et al [2] applied different exposures of artificial marine growth onto sections of helical strake risers. Their findings showed significant increases in drag coefficients as a result of the riser’s surface covered by 60-100% in artificial fouling. Wolfram and Theophanatos[3] performed tests to investigate the effect of varying types of marine growth on cylinders. Their research showed that not only did the drag coefficient increases due to marine growth, but that the coefficients will react differently depending on the type of fouling growing. For example, soft marine kelps will cause coefficients to oscillate depending on the Reynolds number while hard mussels and barnacles will keep the coefficients relatively constant. Investigations into the hydrodynamic properties of an un-fouled MWA have been performed numerically by Russel and Vignaud[4]. It was concluded that higher drag coefficients were observed at angles normal and axial to the structure. Hill et al[5] additionally conducted model tests and numerical studies on the hydrodynamic response of a MWA. These tests were undertaken on a clean hull where fouling was not present. Model testing was performed at the Circulating Water Channel (CWC) in Australian Maritime College (AMC) which allowed for the model to be tested in simulated current conditions. The scope of work covered within this paper is aimed at achieving the following objectives:  Developing feasible model testing techniques to simulate marine growth on a complex structure  Determining the drag force acting on the MWA structure

NOMENCLATURE AMC CWC MWA DOF Rt Ra

Australian Maritime College Circulating Water Channel Mid Water Arch Degree of Freedom Mean Roughness Height Arithmetic Average Roughness

EXPERIMENTAL SETUP MWA Model

A 1:15 scale physical model of a Technip MWA was employed in the experimentations. Dimensions of the full and model scale are provided in Table 1.

Table 1 - MWA dimensions Full Scale Model Scale Length 17400 1160 Width 10770 718 Height 7305 487 Mass 121000 35.85

Parameter

Unit mm mm mm kg

The model MWA illustrated in Figure 4 was constructed from aluminium sheeting and PVC piping. Arch plates were made from sheet aluminium that was bent to form the curved outer sides. This was then MIG welded to aluminium pipes to strengthen and maintain curvature. The PVC tubing acts as the structures buoyancy canister with threaded end caps allowing for the addition or removal of ballast as needed.

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which houses the data acquisition and tank control equipment used throughout experimentation. Test Configurations

Physical experimentation of the model was conducted from a fixed position. Fixed tests were conducted using an AMTI MC3A-250 6 degree of freedom (DOF) load cell attached to top centre of the model, as shown in Figure 6. Using this setup the model was able to be rotated about this point to obtain a full spread of load data. Orientations of key interest were measured from the body fixed coordinates at 90° and 0°, labeled normal and axial in Figure 3.

Figure 4 - Scale model under bare hull condition The model required 17.30kg of solid ballast to be installed to bring it to the desired weight outlined in Table 1. Implementation of the Biflar suspension test indicated that the ballast needed to be placed as far from the transverse centreline as possible. Experimental Modelling of marine growth

Artificial turf was employed to simulate soft marine growth such as seagrasses and algae as these species are predominately found on shallow water MWA systems. Figure 5 illustrates one of the tested artificial fouling conditions. The turf was selected as the length and shape allows for accurate modelling of the organisms expected at the design depth. Information gathered indicated a typical end of service life marine growth thickness on the structure was 100mm which scale to 7mm in the model testing. A secondary thickness (3mm) of artificial turf was chosen to simulate an intermediate (50mm) thickness of marine growth to obtain a more complete set of data on marine fouling effect.

Figure 6 - Fixed testing arrangement Drag coefficients could then be derived from forces measured at the load cell to determine the marine growth impact on the MWA structure. Data acquisition

Data provided by the load cells were extracted by a NI LabVIEW program and converted into .dat files. The files came equipped with calibration factors that were determined for each load cell axis prior to testing. Averages of all the runs were taken to better preserve accuracy and even out any anomalies that may have occurred during tests. Projected area at differing angles was measured using AUTOCAD and was used to calculate drag force coefficients of the model. Repeatability tests were also conducted to estimate the expected error in the experiment setup. Test Matrix

Figure 5: Artificial marine growth on MWA Test Facility

Experimental tests were carried out following the test matrix specified in Table 2. Due to physical limitations set by the model and test basin, Froude scaling laws were employed to scale model the test conditions. The orientations mentioned in the table are taken using the body fixed co-ordinate system outlined in Figure 3.

Testing was conducted at the AMC CWC. The facility can achieve flow speeds of up to 1.7ms-1 through four independently controlled turbines. Fixed configurations were performed by connecting the model to the testing carriage 3


Table 2 - Experimental test matrix Flow Speed (ms-1) Orientation () Full Scale Model Scale 0.50 0.13 0 22.5 45 67.5 1.00 0.26 0 22.5 45 67.5 1.50 0.39 0 22.5 45 67.5 2.00 0.52 0 22.5 45 67.5

90 90 90 90

ROUGHNESS CHARACTERIZATION OF MARINE GROWTH 3D scanning method was employed to determine the surface property of the tested artificial marine growth. A 300  300 mm sample from each artificial marine growth was scanned by using a laser arm. Three-dimensional surface coordinates were generated every 0.1mm. Topography and two-dimensional roughness profiles of the sample pieces are presented in Figure 7. From which, the arithmetic average roughness values are evaluated as shown in Table 3. The full scale roughness is achieved by applying a Froude scaling factor of   15 . It was found that the heavy artificial marine growth obtained a lower arithmetic average roughness value even though its mean roughness height is almost two times that of the intermediate marine growth.

roughness to these area. The roughness inputs were evaluated from the 3D scanning results indicated in Table 3. The velocity field of the flow in the vicinity of the MWA under bare hull conditions is shown in Figure 8. The flow captured by the CFD simulations appears to be fully turbulent and indicates significant amount of wake existing behind the structure. The turbulent boundary layer detaches from the wall when the fluid impacts the MWA and recirculation has developed at the surrounding area of the bluff body

(A) (B) Figure 8. Contour plot of flow velocity field (A. Normal direction; B. Axial direction) RESULTS Drag Force

The drag forces acting on the bare hull MWA model for varying flow speeds are presented in Figure 9. The largest drag force is observed when the flow is at 67.5 degrees. Numerical simulations conducted concurrently showed the same trend. This occurrence can be justified by the complex geometry of the MWA structure. When the fluid flow impacts the structure at 0 degree, only a single gutter plate is directly subject to the incident flow, where the downstream are shielded. As the structure orientates, the surface area that is directly subject to the flow increases and reaches the maximum value at 67.5 degrees as well as the drag force.

(A) (B) Figure 7. 3D scanning images of artificial marine growth (A. Heavy marine growth; B. Intermediate marine growth) Table 3. Technical roughness measurements of the artificial marine growth Marine Growth Thickness Ra Thickness (mm) (mm) None 0 0 Medium 3.33 1.28 High 6.67 1.15 NUMERICAL SIMULATION A steady flow analysis of the drag coefficient of a model scale MWA was performed by using computational fluid dynamics software ANSYS CFX. Modelling of marine growth was achieved by: 1) Intensifying the thickness of marine growth accumulated surfaces; 2) Imposing equivalent

Figure 9: Experimental drag force of bare hull MWA

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The experimental test results indicate that the tested intermediate marine growth condition produced the largest force which led to higher drag coefficients. A percentage of difference between the bare hull and artificially fouled coefficients is shown in Table 4 below. The table shows the largest difference occurring when the model was orientated at an angle of 67.5°, this also correlates where the largest forces were measured from.

Table 4 - Percentage difference of drag coefficient when compared to bare hull Marine Growth Roughness (mm) Orientation 1.28 1.15 0 9.0% 1.2% 22.5 1.3% 1.3% 45 -0.1% -1.7% 67.5 16.6% 4.8% 90 4.0% 0.8% Drag forces at two typical operational flow speeds are presented graphically in Figure 10 which further illustrates how intermediate drag is dominant with respect to other marine growth conditions. The graph demonstrates the average drag force gathered at the each orientation.

Figure 11: Drag Forces vs Froude Number in normal flow Below in Table 5 is a comparative summary of values found experimentally, numerically and within industry for drag force acting normal and axial to the MWA. Results gathered from experimentation are similar to those found by Technip. It was noticed that the numerical simulation overly predicted the drag force on the structure when compared to experimental values.

Table 5 - Drag force of MWA in 0.26m/s normal flow Comparison Marine Growth Drag (N) Roughness (mm) Normal Axial 0 11.9 5.9 Experimental 1.28 12.6 6.7 1.15 12.1 6.0 0 15.1 7.4 Numerical 1.28 15.9 7.4 1.15 15.8 7.2 CFD Simulation 0 12.4 6.7 by Technip 1.33 12.6 6.9 Figure 10. Drag forces vs model orientation Results Validation

A focused analysis of the drag force with flow passing normal to the model representing most realistic MWA operating condition is illustrated in Figure 11. The experimental results gather in this study were compared with numerical findings and previous work performed by Hill[5].

Repeatability

A repeatability study was conducted during testing. These tests were used to check that the experimental error was within an acceptable limit. Table 6 shows the percentage error against the force from a flow fixed x-axis. It can be seen that the repeatability for this experiment was excellent with the maximum variation being 0.5%.

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Table 6 – Repeatability results from flow fixed Co-ordinates Run Fx from % from Fx from % from 67.5° mean 12.5° mean 1 60.5 0.1% 18.4 0.5% 2 60.6 0.1% 18.3 0.2% 3 60.4 0.2% 18.3 0.1% 4 60.5 0.1% 18.2 0.3% 5 60.8 0.4% 18.4 0.4% 6 60.5 0.2% 18.2 0.3% Mean 18.3 60.6 DISCUSSION Results show that marine growth affects the hydrodynamic characteristics of a MWA. It has been found that this correlates with the surface roughness characteristics of the fouling present on the arch. Table 4 features the first indication that the intermediate marine growth severity will impede hydrodynamic performance more so than heavy and bare hull conditions. Although the difference between heavy and intermediate surface roughness is quite small, it plays a large role in the results since it is assumed that pressure drag is constant through all marine growth phases. Furthermore the results from this experiment show that surface roughness is less dependent on the marine growth thickness and is more influenced by the roughness number of the marine fouling. Table 4 and Figure 10 illustrate how the roughness has affected the arch at different orientations. Coefficients measured at 67.5° yielded the largest percentage difference when compared to bare hull figures as this orientation produced the largest drag forces. Figure 10 shows that the largest overall drag force at 67.5°. When comparing values of drag force gathered in this paper against results from Hill et al[5] in Figure 11, similar trends arise to support the findings. Both studies show the drag force increase with a change in the Reynolds number. Moreover, the obtained drag coefficients from these two studies do follow similar trends observed by Wolfram and Theophanatos[3], where drag coefficients oscillated until Reynolds number was large enough that fluctuations ceased. The comparisons shown in Table 5 indicate that numerical results are slightly higher from ones found experimentally. This is deemed to be due to the different methods when assessing the roughness of the surface. CFX utilises the equivalent sand grain roughness which is an approximation of the surface texture. The program is also unable to model soft marine growth properly and instead renders hard bio-fouling such as mussels and barnacles.

CONCLUSIONS An experimental investigation of the marine growth impact on the hydrodynamic loads acting on a MWA structure was performed. The physical tests followed scaling laws to achieve similitude between the model and its full size counterpart with the model being suspended from a 6DOF load cell in order to quantify drag coefficients. The following conclusions can be drawn from this study:  The conducted experimental work provided a conclusive approach to investigate marine growth impact on subsea structures.  Hydrodynamic loads acting on the MWA for varying flow directions has been investigated. The largest drag force was observed at a flow attack angle of 67.5°.  The tested intermediate marine fouling condition, which obtained the greatest surface roughness, is found to have the largest drag force.  Surface roughness rather than marine growth thickness was proven to be an assignable factor when investigating marine growth impact on hydrodynamic loads acting on subsea structures.  The numerical simulation predicted the drag force of the MWA with acceptable accuracy which supported the findings obtained from the experimental work. FUTURE WORK The experimental work performed to investigate marine growth impact on the hydrodynamic loads acting on the MWA can be implemented further to achieve better outcomes. Suggestions that could contribute to a more comprehensive investigation are provided:  A wider range of marine fouling severities are recommended to be studied in the future in order to obtain more comprehensive data for identifying marine growth impact.  Physical modelling of hard marine growth can be conducted to provide a comparative study on different marine fouling.  Scale-Adaptive Simulation method can be employed together with Shear Stress Transport turbulence model in the numerical simulation to predict the drag force of the MWA more precisely.  Model tests incorporating flexible risers can be carried out to investigate marine fouling impact on the hydrodynamic response of an integral S-shaped riser system. ACKNOWLEDGEMENTS The authors would like to thank Mr Rowan Frost and Mr Alan Faulkner for their assistance at the AMC Circulation Water Channel Facility. The authors would also like to acknowledge Technip for providing the opportunity to conduct research and their support throughout the research collaboration with the Australian Maritime College. 6


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Russell, C. and B. Vignaud. Hydrodynamic Loading on Mid Water Arch Structures. in ASME Conference Proceedings. 2011. Baarholm, R. and K. Skaugset, Modelling and Characterization of Artificial Marine Growth. OMAE, 2008. 27(June 15-20, 2008). Wolfram, J. and A. Theophanatos. The effects of marine fouling on the fluid loading of cylinders: some experimental results. in Offshore technology conference. 1985. Russell, C. and B. Vignaud. Hydrodynamic Loading on Mid Water Arch Structures. 2011: ASME. Hill, J., et al. Experimental Investigation of Hydrodynamic Loads on Subsea Structures. in 18th Australasian Fluid Mechanics Conference. 2012.

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