a study on lateral displacement of monopile of offshore wind turbine ...

International Conference on“Structural Engineering &Construction Management” SECM-2016

Organized byDepartment of Civil Engineering MarAthanasius College of Engineering, Kothamangalam, Ernakulam, Kerala

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Int. J. Engg. Res. & Sci. & Tech. 2016 Neenu Maria Jose and Alice Mathai, 2016

A STUDY ON LATERAL DISPLACEMENT OFMONOPILE OF OFFSHORE WIND TURBINE IN A

LAYERED SOIL

A numerical study using Finite Element Analysis is performed to evaluate displacement behaviorof monopile and surrounding soil of Offshore Wind Turbine in medium dense sand. In this studymonopile, turbine tower and soil modeled as 3 D solid model in ANSYS. Soil modeled as layerswith different material properties in each layer. An explicit dynamic analysis, considering soil asan explicit material and all loads on turbine as static loads. The study shows that pile displacementdepends on soil properties on each layer and on soil depth along pile embedded length.

Keywords: Monopile, Wind turbine, Finite element analysis

INTRODUCTIONThe wind resources are even more abundant andof better quality at sea as compared to onshore.Offshore Wind Turbines (OWT) are providing anincreasing proportion of wind energy generationcapacity because these sites are characterisedby stronger and more stable wind conditions thancomparable onshore sites.

The design and construction of foundations foroffshore turbines are challenging because of theharsh environmental conditions. There arebasically 6 types of support structures. They aremonopile structure, tripod structure, latticestructure, gravity structure, tripile foundation, andfloating structure.1 Assistant Professor, KMP College of Engineering, Cherukunnam, Kerala, India.2 Associate Professor, M. A College of Engineering, Kothamangalam, Kerala, India.

Int. J. Engg. Res. & Sci. & Tech. 2016

ISSN 2319-5991 www.ijerst.comSpecial Issue, Vol. 3, No. 1, April 2016

International Conference on “Structural Engineering &Construction Management” SECM-2016

Organized by: Department of Civil Engineering Mar AthanasiusCollege of Engineering, Kothamangalam, Ernakulam, Kerala

© 2016 IJERST. All Rights Reserved

Research Paper

The choice of monopiles results from theirsimplicity of installation. Monopile foundation isopted when water depth from 10 m to 30 m.Hollow monopiles are used with 0.05 to 0.06 mthickness and soil is plugged inside the pile.

For monopile the overturning moment isresisted by horizontal soil reaction along theembedded length shown in Figure 1. The pile mustbe long enough to mobilise enough soil over itslength to transfer all loads. Offshore wind turbinessupported on monopile foundations aredynamically sensitive because it cannot toleratemore than 0.5 degrees tilt.

The study deals with pile displacementbehavior under layered soil of different material

Neenu Maria Jose1* and Alice Mathai2

*Corresponding Author: Neenu Maria Jose [email protected]

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properties for a laterally loaded wind turbinetower.

SITE AND SOIL FEATURESAbout 80% of offshore wind turbine supported onmonopiles. A location at Rameswaram,Tamilnadu has been selected based onenvironmental data obtained. Wind and wave dataobtained from National Institute of OceanTechnology (NIOT) Chennai (Ramana Murthy andAtmanand, 2013). The water depth of the selectedlocation is 10 m approximately, hence monopilefoundation is selected. Soil properties depend onsoil type and depth of soil from mud-line shownin Table 1.

WIND TURBINECHARACTERISTICSMaterial of turbine tower and monopile is steeland material properties depend on material type.Dimension of turbine tower is 80 m height andcross section diameter of 4.5 m and dimensionof monopile is 4.2 m cross section diameter and30 m height (Ramana Murthy and Atmanand,2013).

LATERAL LOADS ONMONOPILEWave LoadFor slender structures, such as jacket structurecomponents and monopile structures, Morison’sequation can be applied to calculate the waveloads (Robert Dean and Robert Dalrymple, 1991;and Swagata Bisoi and Sumanta Haldar, 2014).

Wave force = Drag force + inertia force

F = CD . ½ . ρ . D . |U| . U + ρ . CI D2 /4 . ax ...(1)

CD = Drag coefficient

= mass density of sea water

D = projected area normal to cylinder axis/unitlength

CI = inertia coefficient for smooth circularcylinder

U = component of velocity vector of water dueto wave normal to axis of the member in m/s

|U| = absolute value of U in m/s

Wind LoadWind load on turbine blades (Swagata Bisoi andSumanta Haldar, 2014)

Fb = 0.5a RT²V²CT(S) ...(2)

Fb = the wind load acting on the hub in N

Figure 1: Transfer of Horizontal Loads andMoments in Multi- and Monopile Structure

Soil Description Depth (m)Internal

Friction ( 0 )

Grey fine sand 0 – 3 32-33

Grey silty fine sand 3- 7.5 33-39

Grey silty clay withcalcareous sand stone

7.5-10.5 33-34

Crushed pieces of rockand calcareous sand

stone10.5-12 34-44

Silty fine sand 12- 13.5 31-44

Fine sand with whitecolor small stone

13.5-16.5 31-41

Fine silty sand 16.5-30 34-42

Table 1: Soil Characteristics

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V = the wind speed at the hub height in m/s

RT = the rotor radius in m

a = the air density which equals to1.23 kg/m3

at 15.1 °C at 1 atm

CT = force coefficient depend on shape ofstructure

CT (s) = is the thrust coefficient which is afunction of the tip speed ratio (s)

Wind load on turbine tower (Swagata Bisoi andSumanta Haldar, 2014);

Ftower = ½..².Cs.A ...(3)

F = wind force (kN)

= Air density = 1.23 kg/m3 wind speed inm/s

Cs = shape coefficient or force coefficientdepend on shape of structure

A = Projected area in m2

SOIL MONOPILE INTERACTIONMonopile under lateral loading the response ofthe soil is described in terms of P – y curve whichrelates the soil resistance to the pile deflection atvarious depths below the ground surfaces(James Wilson, 2003). These curves arenonlinear and depend on several parameters,including depth, and soil properties. For differentsoil layers and soil depth the P-y curve vary basedon the equation below.

Ultimate bearing capacity (RP2AWSD, 2000);

Pus = [C1 x H + C2 x D] x x H ...(4)

Pud = C3 x D x x H ...(5)

where,

Pu = ultimate resistance (force/unit length)(kN/m)

= effective soil weight (KN/m3)

H = depth (m)

= angle of internal friction of sand

D = average pile diameter from surface todepth (m)

Soil resistance (RP2AWSD, 2000);

P = A x Pu x tan h[{(k x H)/(A x Pu)}xy] ...(6)

A = factor to account for cyclic or static loadingcondition.

Pu = ultimate bearing capacity at depth H (kN/m)

k = initial modulus of subgrade reaction (kN/m3)

y = lateral deflection (m)

H = soil depth (m)

Figure 2: P-y Curves for Soil Layers

LATERAL SOILINTERACTION AND PILEDEFORMATIONDue to the lateral loads and overturning momentmonopile has a tendancy to tilt at a pivot pointwhile considering pile has a rigid behaviour. Themaximum permissible tilt of monopile in sandy

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soil is 0.5 degree (Swagata Bisoi and SumantaHaldar, 2014). For 30 m embedded lengthmonopile pivot point lies within the pile at a depth22 m from mud-line shown in Figure 3 (KhalidAbdel-Rahman and Martin Achmus, 2006).

FINITE ELEMENTMODELLING IN ANSYSA 3 dimension FE model of monopile, tower andsurrounding soil with 20 m width around the pileand 30 m depth below the pile modeled in Ansys.Both monopile and turbine tower material propertyis structural steel. Soil modeled as an explicitmaterial with strength material propertiesPoison’s ratio, density, cohesion and modulus ofelasticity depends on P-y curve. Modulus ofelasticity is increase with depth. Soil divided as 9layers along pile embedded length and a hardstrata layer is provided below pile tip shown inFigure 4.

The contact friction is applied at the pile soilinterface by contact – target pairs. Since thebottom layer is a hard strata so it restraint the

movement in any direction hence a fixed boundarycondition is provide at the bottom of soil solidcube. At the lateral side of soil solid cube soil freeto moves in the corresponding direction.

RESULTS AND DISCUSSIONFrom the lateral direction, z directionaldeformation it is inferred that, when loads are inthe positive z direction, pile displaced in positive

Figure 3: Maximum Displacement at 0.5degree Tilts at Pile Top and Bottom

Figure 4: Model of Monopile, Tower and Soilin Ansys

Figure 5: Directional Deformation – zDirection

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CONCLUSIONMonopile is modelled as flexible member in Ansysand soil is modelled as layers with differentmaterial properties.

• Directional deformation in lateral load direction,due to lateral load pile is tilted about a pivotpoint within the pile close to pile toe.

• Tilt angle obtained with in permissible limit,hence pile in safe condition.

ACKNOWLEDGMENTThe work was supported by Department of CivilEngineering, Mar Athanasius College ofEngineering, Kothamangalam. The source ofsupport from second author is greatlyappreciable. Athors really thankful to the reviewersfor their critical comments which have been helpfulto complete the work.

REFERENCES1. American Petroleum Institute (2000),

“Recommended Practice for Planning,Designing and Constructing Fixed OffshorePlatforms – Working Stress Design”, APIRecommended Practice 2A-WSD(RP2AWSD), 21st Edition, December.

2. James F Wilson (2003), “Dynamics ofOffshore Structures”, John Wiley & SonsInc., New Jersey.

3. Khalid Abdel-Rahman and Martin Achmus(2006), “Behaviour of Monopile and SuctionBucket Foundation Systems for OffshoreWind Energy Plants”.

4. Ramana Murthy M V and Atmanand M A(2013), “Feasibility Studies on Offshore WindDevelopment in India”, National Institute ofOcean Technology, Ministry of EarthSciences, Chennai.

z direction and soil at right hand side of pile iscompressed and soil at left hand side loose andmoves in loading direction at the initial time stepshown in Figure 5. Hence soil has a negativedisplacement at right hand side of pile and apositive displacement at left hand side of pilealong embedded length of pile up to a point frommud-line. That point is the pivot point is locatedwithin the pile to a depth from mud-line. After thepivot point pile slip backward and moves oppositeto the loading direction.

Pile is modeled as flexible member, itsdeformation is depends on the soil property. Dueto the increase in the modulus of elasticity withrespect to depth both bending and tilting ishappened to pile. From the results positivedeformation at mud-line and negative deformationat pile toe, hence it is inferred that the pivot pointis located within pile embedded length close topile toe shown in Figure 6. The tilt angle obtainedis 0.0965 less than permissible tilt angle 0.5degree.

Figure 6: Z Directional Deformation Patternof Rigid and Flexible Pile and Pivot Point

Location

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5. Robert G Dean and Robert A Dalrymple(1991), “Wave Water Mechanics forEngineers and Scientist”, Advanced Seriesof Ocean Engineering, Vol. 2, WorldScientific Publication.

6. Swagata Bisoi and Sumanta Haldar (2014),“Dynamic Analysis of Offshore Wind Turbinein Clay Considering Soil–Monopile–TowerInteraction”, Soil Dynamics and EarthquakeEngineering, Vol. 63, pp. 19-35.

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