ON THE LOADING AND SCOUR OF THE JACKET TYPE OFFSHORE WIND TURBINE FOUNDATION

A 1:36 scale model tests were carried out in the Medium Wave Flume (MWF) and Near-shore Wave Basin (NSWB) at the Tainan Hydraulics Laboratory (THL) with the jacket type offshore wind turbine foundation located in the test area. The loading of typhoon wave with current on the jacket type offshore wind turbine foundation was investigated in the MWF with fixed bed experiment. Meanwhile, the scour around the jacket type offshore wind turbine foundation exposed to wave and current was conducted in the NSWB with the moveable bed experiment. Two locations (water depth 12m and 16m) of the foundations are separately simulated in this study. Based on the analysis from the former NSWB experimental results, the suitable scour protection of a four-layer work around the foundation is also proposed to the impact of scour. Finally, a four-layer scour protection is tested and found to be effective in preventing scour around jacket type foundation of offshore wind turbines at water depth 12m and 16m.


INTRODUCTION
The growing concern in the 1990s over CO 2 -forced global warming has given new life to the prospects for greater use of wind turbines because of their credentials as non-polluting generators powered by winds created by solar energy, a renewable resource.Therefore over the past two decades, on-shore wind energy technology has seen a ten-fold reduction in cost and is now competitive with fossil and nuclear fuels for electric power generation in many areas of the world.As a direct consequence of the Kyoto Protocol there is a new impetus in developing the capacity of offshore wind farms to provide a significant percentage of the target renewable energy quota.Offshore wind energy began in shallow waters of the North Sea where the abundance of sites and higher wind resources are more favorable by comparison with Europe's land-based alternatives.All today's offshore wind farms producing energy (start 2007) are located in shallow waters; they are either founded on monopoles or on gravity based caissons.However transitional substructure of the offshore wind turbines will be replaced by fixed bottom systems that use a wider base with multiple anchor points like those frequently used in oil and gas industry.In order to investigate the loading on the jacket type offshore wind turbine foundation, the effect of typhoon wave together with current was conducted in the Medium Wave Flume (MWF,200m×2m×2m) by 1:36 scale in this study.Meanwhile, the stability of the seabed around the wind turbine foundations is another one of the major challenges in offshore wind farm design.The design of the offshore turbines has to include an assessment of the possible long-term morphological development occurring in the design period, and an assessment of the local erosion caused by the presence of the foundations themselves.
Within the TAIWAN a range of seabed locations from Chang-Hwa to Yun-Lin, mid western coast of Taiwan, have been licensed for development of offshore wind farms.One of the key factors in the sitting and construction of offshore wind farms is the influence of the seabed geology and sedimentary environment on the foundation design and stability over the lifetime of the structure.Therefore, the objective of this study is to improve understanding of scour behavior and to aid the design of scour protection systems for offshore jacket type foundations located on sandy seabed.A physical model test study has been performed on the occurrence and prevention of erosion holes (scour) around jacket type foundations of offshore wind turbine on sandy soils.The scour around the jacket type offshore wind turbine foundation exposed to wave and current is conducted in the Near-Shore Wave Basin (NSWB, 150m×60m×1.5m) of Tainan Hydraulics Laboratory (THL) with the 1:36 scale mobile bed experiment.Two locations (water depth 12m and 16m) of the foundation are separately simulated in this study.The maximum scour depth and the potential impact scour area around the jacket type offshore wind turbine foundation is analyzed from the result of the mobile bed experiment.In order to prevent the erosion, the suitable scour protection with rock dump layer is also proposed to reduce the impact of scour.

HYDRODYDYNAMIC AND MORPHOLOGIC BACKGROUND
Offshore wind turbines are presently perceived as one of the most environmentally friendly sources of electrical power, being a non-polluting renewable resource that causes minimal human, ecological and environmental impacts.Offshore wind farms are now being proposed for, or built in, increasingly hostile hydrodynamic environments.Many candidate sites for offshore wind turbine parks are located on seabed of

LOADING ON THE JACKET TYPE FOUNDATION
Two test items are investigated in the fixed bed experiment, including the total horizontal hydrodynamic force on wind turbine and wave pressure loading distribution along the leg of foundation.Sketch of the fixed-bed experimental setup and process of model construction are shown in Fig. 2 and Fig. 3.The scale ratio is 1:36 and the Froude law is obeyed in the fixed bed experiment.Experimental analysis of the force measurements is focused on the evaluation of the hydrodynamic force and on the prediction of the maximum breaking wave load on the jacket type offshore wind turbine foundation under shallow water condition.The measurement system of the fixed bed experiment includes wave gages, ADV current meters, pressure gages and the loading table (to see Fig. 2).From the fixed bed experimental result, it shows that the vertical wave pressure P 1/3 and P 1/10 , measured on the pile base under typhoon waves within depth of 12m and 16m have a highest values on the water surface and with a decreasing trend on going both up and down sides.But the maximum wave pressure (P max ) seems to happen on above the water surface rather than near the water surface (to see Fig. 4).It is due to the wave breaking on the cylinder that caused the impact pressure going that high.From the result of loading measurement, the maximum forces (F max ) under typhoon waves have a higher result while the tide gets higher, which means within the depth of 12m and 16m the results, will be 1360.1 KN and 1747.2KN.However, if with including current issues the results will increase to 1538.7 KN and 1975.3

SCOUR AROUND THE JACKET TYPE FOUNDATION WITHOUT PROTECTION
Two series of physical model tests are performed in this study.The aim of the first test series is to investigate the greatest magnitude of local scour and potential scour area in the sand bed around the jack type foundation with no scour protection.The other series of tests is aimed at the level of protection for proposed scour mitigation under design wave and current conditions and to assess the extent of scouring around the foundations.Layout of 27m× 19m wave-current basin for scour testing is shown in Fig. 5.The wave flume passes from right to left and the current-generation flume circulates from downside to upside.The test area with jack type foundation is at the junction of these two flumes.The physical modeling is carried out in this wave-current basin at the Tainan Hydraulics Laboratory (THL) with a scale of 1:36.The sediment used in this experiment is light density (specific gravity,  =2.02) coal of a median grain size of 50 d =0.18mm and the model sand bed is 0.35m deep, 7.0m long and 3.0m wide.The equipments and measurement system used in this experiment include irregular wave maker, current-generation system, electrical capacitance wave gauges, ADV current meters and ultrasonic bottom profiler moved by the carriage table (7m×3m).Before and after the wave-current action for several runs in each test, the bottoms of model sand bed are scanned line by line along the carriage table.Then the topography evolution sea bottom and the scour around jacket type foundation can be analyzed from the measurement data of ultrasonic bottom profiler.Scour is herein considered to be the lowering of the bed in the vicinity of a marine structure due to local accelerations and decelerations of the near-bed velocities and the associated turbulence (vortices) leading to an increase of the local capacity for sediment transportation.Generally, the near-bed flow around the vertical pile consists of the horseshoe vortex generated at the upstream side of the pile and vortices generated at the lee-side of the pile.Currently with 181 out of 295 foundations for offshore wind turbines, the monopole is the preferred foundation option (Schachner, Josef, 2004).Of these foundations, 169 are driven in sandy soils, which can be more or less susceptible to scour.As a rule of thump, confirmed by experience with other structures, the scour hole can reach a depth of 1.5 times the pile diameter (D).Based on experimental data, Sumer et al. (1992) have found for circular pile with its maximum scour depth ,max The length of the scour hole was 4D upstream and 6D downstream of the pipe for combined current and wave.
Compared to monopole foundation, jacket type foundation has more complex structure and still no further discuss on its maximum scour depth and the length of the scour hole.Therefore, a physical model test study has been performed on the occurrence and prevention of erosion holes (scour) around jacket type foundation of offshore wind turbine on sandy soils.From Table 3 and Table 4, the experimental results show that the maximum scour depth and the potential impact erosion area around the jacket type foundation located at water depth 12m under different hydrodynamic conditions.The diameters (D) of four legs of the foundation are all the same, D equals to 2.08m for each leg.
For the jacket type foundation located at water depth 12m, case 12TLLCM is the most serious scour event exposed to the lowest water level, action combined current and 1:100 year return period typhoon wave.The maximum scour depths around four legs of the jacket type foundation are 1 / F D =1.11, 2 / F D =1.31, 3 / F D =1.13 and 4 / F D=1.26 respectively.Fig. 6 and Fig. 7 show the evolution of maximum scouring depth and the potential impact erosion area around four legs of the jacket type foundation under the case 12TLLCM.In this experiment, the angle between the incident wave and current is 90°.The results show that more serious scour will be induced at legs 2 F and 4 F on the up-current side of the foundation.From observations it is estimated the length of the scour holes around 4 legs of the foundation is approximated up to 4~6 times the leg diameter.However when foundation location changed to 16m water depth, the maximum scour depth and the length of the scour hole are less than those obtained from the tests at 12m water depth (to see Table 5 and Table 6).It is because not only erosion but also re-deposition will occur within the experimental process under the location of 16m water depth.

CONCLUSION
The loading of typhoon wave with current on the jacket type offshore wind turbine foundation is investigated by the fixed bed experiment in this paper.The fixed bed experimental analysis is focused on the evaluation of the hydrodynamic force and on the prediction of the maximum breaking wave load on the jacket type offshore wind turbine foundation under shallow water condition.The results of fixed bed experiment indicate the maximum horizontal forces on the wind turbine foundation have a good agreement with those from the designed values and also validate the structure design of the engineering consulting company.
This paper also presents the results of a study into the scour around the jacket type offshore wind turbine foundation being considered for use in the mid-western coastal wind farm development of the TAIWAN.The scour and scour mitigation investigation is carried out in a wave-current basin to provide a deep understanding of flow-structure interaction.The ratio of maximum scour depth to leg diameter (D) of jacket type foundation and the potential impact scour area can be obtained from the results of this physical model tests.Meanwhile, a four-layer scour protection is tested and found to be effective in preventing scour around jacket type foundation of offshore wind turbine.Although it is common practice to apply scour protection at sites with a potential for local scour, the analysis of this study indicate that the function of protection is likely to provide a technically acceptable solution.Furthermore, the study of design solutions with and without scour protection will be investigated by the comparison with respect to technical feasibility, risks and costs in the near future.

Figure 1 .
Figure 1.Two locations (water depth 12m and 16m) of the jacket type wind turbine foundation.

Figure 5 .
Figure 5. Sketch of the mobile bed experimental setup.
studied the scour process near a vertical pipe in current and wave conditions.The bed material was natural sand with 50 d =0.2mm.The water depth in the experiment was 0.3m.The depth-averaged velocity upstream of the pipe was 0.4m/s (mobile bed, a current alone,  =1.0 for current and non-breaking waves,  =1.9 for current and breaking waves.

Figure 6 .
Figure 6.The evolution of maximum scour depth around four legs of the jacket type foundation under the case 12TLLCM.

Figure 8 .
Figure 8. Sketch of the proposed scour protection.

Figure 10 .
Figure 10.Comparison of maximum scour depth with the case 12TLLCM (without scour protection) and the case 12TLLCPM (with scour protection).

Figure 11 .
Figure 11.The potential impact erosion area around scour protection of the jacket type foundation under the case 12TLLCPM.

Table 2. Test conditions of the mobile bed experiment.
LL : Lower Low Water Level (L.L.W.L.) M(1st) : Mean Water Level (M.W.L.) L : Low Water of Ordinary Spring Tide (L.W.O.S.T.) C : current; M(last) : mobile bed KN.The results of fixed bed experiment indicate the maximum horizontal forces on the wind turbine foundation have a good agreement with those from the designed values and also validate the wind turbine foundation structure design of the engineering consulting company.