Overturning Stability of Offshore Wind Power Substructure with Bucket Foundation

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1 Website: (ISSN , ISO 9001:2008 Certified Journal, Volume 5, Issue 10, October 2015) Overturning Stability of Offshore Wind Power Substructure with Bucket Foundation Young-Jun You 1, Youn-Ju Jeong 2, Min-Su Park 3, Du-Ho Lee 4 1,2,3,4 Korea Institute of Civil Engineering and Building Technology, Structural Engineering Research Institute, Goyang, Gyeonggi, Republic of Korea Abstract There are many types of foundation for offshore wind power system and these days a suction bucket foundation has been emerged as a fast and cost effective foundation. This study is for analysing the overturning stability of a substructure with a suction bucket foundation for offshore wind power. A concrete substructure which was designed for supporting a 5 MW wind turbine and a tower was considered. Based on the analysed results of 5 MW wind turbine by GH-Bladed, overturning was checked with various environmental forces. The designed substructure had a stability for overturning by environmental forces. In disadvantageous condition for increasing safety, additional resistant moment is to be secured by filling sand in the empty space in the suction bucket foundation after completing of installation. Keywords Wind power, offshore, foundation, suction, overturning. I. INTRODUCTION Installation of wind power system foundation take much part in construction cost distribution and the part becomes larger for offshore wind power as shown in Figure 1 and it reaches up to 40% in some cases [1]. Until now, most offshore wind turbines are founded on monopoles. These days suction buckets has been tried for offshore wind turbine foundation. Suction buckets are considered as a really great innovation for offshore wind power foundation since it can be installed faster and at lower costs than conventional foundations. The bucket foundation which meteorological masts was deployed firstly for in United Kingdom in 2013 [3] and DONG Energy successfully installed the world s first offshore wind turbine jacket with suction buckets as a proof-of-concept full-scale prototype in 2014 at 25 meters water depth in the Borkum Riffgrund 1 offshore wind farm, 37 km off the German island Borkum [4]. There are some types of suction buckets for offshore wind power. Generally one bucket is adapted like left figure in Figure 2 and applied type with three buckets like right figure in Figure 2 is also adapted. Figure 1. Capital cost breakdown for typical onshore and offshore wind system [2] Figure 2. How the suction bucket works [5] Figure 3 shows a working way of the suction bucket. When the bucket is installed in seabed, inner space of the bucket is closed. As water in the space is sucked by a pump, negative pressure increases. Collaboration of the negative pressure in inner space, hydrostatic pressure on the bucket, and self-weight of the bucket push the structure into seabed. 48

2 [ Unit: m ] Figure 3. How the suction bucket works [6] In this paper, the overturning stability of a substructure with a suction bucket foundation for offshore wind power was studied. A concrete substructure which was designed for supporting a 5 MW wind turbine and a tower was considered. Based on the analyzed results of 5 MW wind turbine by GH-Bladed, overturning was checked with various environmental forces. II. SUBSTRUCTURE WITH BUCKET FOUNDATION A. Geometry of substructure The target turbine was the 5 MW turbine of NREL [7]. Aerodynamic analysis was performed for the turbine by a commercial analyser GH-Bladed. Based on the results, a substructure with a suction bucket type was. It was planned to be constructed with concrete. Substructure consists of a cylinder at upper part, a cone shape at lower part, and a bucket of cylinder type. The height of cylinder and cone is 32.7 m and that of the bucket is 14.0 m. The outer diameter of the bucket is 17.0 m and wall thickness is 0.5 m. A layout of the substructure is shown in Figure 4. B. Loads Based on the analysis results by GH-Bladed, horizontal load acting on the top of the substructure was given with kn (this paper does not deal with the process of analysis because it is apart from this subject). An installation area was considered as west-south of Republic of Korea. The height and period of extreme wave was m and seconds, respectively Height from seabed (m) H.A.T (6.55 m) M.S.L (3.28 m) L.A.T (0.00 m) Figure 4. Layout of substructure with bucket foundation H.A.T M.S.L L.A.T 0.8 B L.A.T M.S.L H.A.T A ,000 1,500 Wave force (kn) Figure 5. Waver force acting on substructure Wave force was calculated by an in-house code developed based on eigenvalue expansion method [8]. Wave forces for L.A.T. (Lowest Astronomical Tide), M.S.L. (Mean Sea Level), H.A.T. (Highest Astronomical Tide) were plotted in Figure

3 It showed that wave force was the biggest at lower part which had the largest projected area. The smaller the projected area was, the smaller wave force became. All structures was loaded from external forces and these external forces act with combination. Engineers should design a structure considering possible combinations of those external forces. API [9] recommends offshore jacket structures to be designed considering load combinations listed in Table I. TABLE I LOAD COMBINATIONS [9] Conditions D1 D2 L1 L2 Wo We factored gravity loads operating wind wave & current load extreme conditions when the actions effects due to permanent and variable actions are additive extreme conditions when the actions effects due to permanent and variable actions oppose D1 = self-weight of the structure D2 = the load imposed on the platform by weight of equipment and other objects L1 = live load including the weight of consumable supplies and fluids in pipes and tanks L2 = the short duration force exerted on the structure from operations Wo = the owner defined operating wind wave and current load We = the force applied to the structure due to the combined action of the extreme wave (typically 50-yr return period) and associated current and wind III. OVERTURNING STABILITY A. Resistant moment Forces by self-weight, buoyancy of the substructure and hydrostatic pressure above the substructure were tabulated in Table II. In Table II, calculated values included load factor listed in Table I. If the substructure is overturned by external forces, the rotational axis on side view would be A or B point in Figure 4. Therefore, the moment arm would be a half of the diameter of the bucket and calculated moments for each part were tabulated in Table II. In Table II. (-) means a force which disturbs substructure stability. 50 TABLE II FACTORED ACTING FORCE AND RESISTANT MOMENT OF SUBSTRUCTURE Item Force (kn) Moment Super-structure self-weight 10,972 93,262 Substructure self-weight 24, ,929 Substructure HAT -18, ,277 buoyancy MSL -18, LAT -18, ,429 Bucket self-weight 11, ,834 Bucket buoyancy -3,560-30,257 Hydrostatic pressure on cone base Sum (when ignoring hydrostatic pressure) HAT 31, ,099 MSL 25, ,901 LAT 19, ,253 HAT 568,590 (299,491) MSL 516,321 (300,420) LAT 464,592 (301,339) TABLE III FACTORED MOMENT ACTING ON SUBSTRUCTURE Item A point B point Wind -159, ,478 Wave HAT -82, ,982 MSL -74,730-97,149 LAT -69,084-89,810 Boat collision HAT -17,380-27,016 MSL -15,129-24,766 LAT -12,871-22,508 Sum HAT -258, ,477 MSL -248, ,393 LAT -241, ,796 B. Acting moment Environmental forces considered in this study were wind and wave forces. Acting moments by them listed in Table III for A and B points. All moments were calculated considering load factor in Table I. In Table I, (-) means an overturning moment. In order to consider the lowest safety condition, a situation that a boat collides with the substructure. The displacement and draft of a considered boat were 20 gross tone and 1.5 m, respectively and impact force was kn [10].

4 Moment C. Stability check International Journal of Emerging Technology and Advanced Engineering Table IV shows the summary of acting and resistant moments calculated from earlier sections. In order to consider an extremely unsafe condition, a result that hydrostatic pressure above a part of cone type was not considered was also shown. (+) values in the right column in Table IV means that the substructure is stable against external forces and (-) values means that the substructure would be overturned. As shown in Table IV, the substructure is stable for the rotation axis of A point whether hydrostatic pressure is considered or not. However, for the rotation axis of B point, if hydrostatic pressure is not included, the substructure would be overturned. In this case, additional resistant moment can be secured with additional self-weight by filling sand in the empty space of the substructure. Filling sand in the empty space up to 3 m in height would make the substructure stable. TABLE IV SUMMARY OF ACTING AND RESISTANT MOMENT Point Hydro- Moment static pressure Resistant Acting Margin A HAT Considered 568, , ,782 MSL 516, , ,328 LAT 464, , ,502 HAT Not 299, ,808 40,683 MSL considered 300, ,993 51,427 LAT 301, ,090 60,249 B HAT Considered 568, , ,113 MSL 516, , ,928 LAT 464, , ,795 HAT Not 299, ,477-64,986 MSL considered 300, ,393-51,973 LAT 301, ,796-41,457 D. Stability by ground settlement of installing error Since substructures for offshore wind power is installed underwater, it is very difficult and dangerous for worker to approach to it and work there. Moreover, seabed is not even so levelling work should be precede. Since the height of towers of offshore wind power reach to 100 m or more, even a small inclination leads to a big displacement of RNA (Rotor Nacelle Assembly). Operation of wind power at this state would cause structural problems as well as mechanical problems. Even though seabed is even, the substructure could be inclined in process of suction. It could be unsure to secure the straightness of the substructure after perfect construction due to ground settlement in wind power operation. The inclination of the structure results in the change of the centroid where self-weights act on and thereby the decrease of resistant moments. In this section, overturning stability of the substructure according to inclined angles. An overturning moment acting on the substructure was takes as the maximum value in Table IV (364,477 kn-m). When the hydrostatic pressure above the substructure is not considered, the substructure does not resist the overturning moment by its self-weight. Thereby, two cases was considered here. One case is that the cone part of the substructure is filled with sand up to a half of the cone height and another is fully filled case. 700, , , , , , ,000 0 Acting moment Stable Overturned Inclined angle (degree) Figure 6. Overturning stability according to inclined angle Figure 6 shows the calculated results according to inclined angles. When the cone part of the substructure is filled with sand up to a half of the cone height, the substructure could resist the overturning moment even though it inclined approximately up to five degrees. However, this result does not mean that the offshore wind power system could be operated in the inclined state because engineers would not allow such inclination and the state could cause many problems as stated early but mean that the substructure is stable for overturning for the inclined angle. 51

5 IV. CONCLUSIONS In this study, stability check of an offshore wind power substructure with a suction bucket was performed. The substructure was designed to support a 5 MW wind turbine and a tower and planned to be fabricated with concrete. Based on an aerodynamic analysis and the environmental conditions in a specific sea area, the overturning stability of the substructure was checked. As results of this study, the substructure had a sufficient overturning resistance. Even though the worst condition was considered to secure sufficient safety, the shortage of overturning resistance could be secured by filling sand into the empty space of the substructure. A state that the substructure was inclined by construction error or ground settlement was also checked and overturning did not occur if the substructure was inclined up to five degrees when the empty part was filled with sand up to a half of cone height. Consequently, the designed substructure with a suction bucket is stable for overturning moment at a specific sea are. Acknowledgements This study was supported by the Korea Institute of Marine Science and Technology Promotion (KIMST), Project No: , and by the Ministry of Trade, Industry, and Energy, Project No: (Development of hybrid substructure system for offshore wind power). REFERENCES [1] Houlsby G. T. and Byrne B. W., Suction Caisson Foundations for Offshore Wind Turbines and Anemometer Masts, Wind Engineering, Vol. 24, No. 4. [2] IRENA (International Renewable Energy Agency) 2012 RENEWABLE ENERGY TECHNOLOGIES: COST ANALYSIS SERIES. IRENA working paper, Vol. 1, Issue 5/5, pp. 43 [3] [4] [5] [6] [7] NREL (National Renewable Energy Laboratory) Definition of a 5-MW Reference Wind Turbine for Offshore System Development [8] Park, M. and Koo, W., Mathematical Modeling of Partial-Porous Circular Cylinders with Water Waves, Mathematical Problems in Engineering, [9] API (American Petroleum Institute) Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms- Loads and Resistance Factor Design. [10] MOS (Ministry of Oceans and Fisheries) Design standard for Harbor and Fishery port (Appendix) pp.376 (in Korean) 52