Influence of a Boatlanding and J-tubes on Wave Loads and Wall

Thickness of the Monopile Support Structure Design

M.L.A. Segeren

Offshore Wind, Faculty of Civil Engineering and Geosciences, Delft University of Technology

Stevinweg 1, 2628 CN Delft, the Netherlands

Tel.: +31 15 27 88934, Fax: +31 15 27 85124, Email: m.l.a.segeren@tudelft.nl

November 28, 2011

Abstract

Keywords: Support structure, Monopile, Design limitation, Steel reduction, Secondary steel, Appur-

tenances

Support structures for offshore wind turbines play a significant part in the cost of offshore wind en-

ergy. With current access systems a conventional boat landing is not necessary. Secondary steel, such

as a boat landing and J-tube, influences wave loads. A way of taking the secondary steel items into

account in the wave load calculation is through modifying the hydrodynamic coefficients. Wave loads are

calculated using four design standards and the design of the monopiles of the Prinses Amalia Wind Farm.

The wave loads results with and without secondary steel items can be used for a buckling check at the

mudline for the specific case. This gives insight in the influence of the secondary steel items on wave load

and wall thickness. Determining the drag coefficient using different standards will result in differences

of the drag coefficient between 18% and 22%. A boat landing and j-tube will result in a wall thickness

increase of 8-12% at the mud line location of the monopile pile design of the case study. This result can

be used for considerations in the design of future support structures for offshore wind turbines.

1 Introduction

1.1 Background

Support structures for offshore wind turbines play a significant part in the cost of offshore wind energy.Support structures have to deal with hydrodynamic and aerodynamic loads but also provide access to theturbine. On the support structure secondary steel items are attached, also called appurtenances. Secondarysteel items, such as boat landings and J-tubes, cause additional loads and introduce stress concentrations.Reducing loads will potentially reduce the costs of the support structure and therefore it is interesting toknow the influence of the secondary steel items on wave loads. With the current access systems, such as theAmpelmann, conventional boat landings are not necessary anymore to provide access. This research is partof the FLOW research program which strives to reduce the cost of offshore wind energy.

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1.2 Problem description

Wave load calculations are not new and there are multiple design standards that provide guidelines tocalculate wave forces. These standards also give a guideline to incorporate the marine growth in the waveload calculation. Although the standards mention that the appurtenances should be taken into account inthe Ultimate Limit State calculation they do not specify in what way. The goal of this paper is thereforeto gain insight in the influence of secondary steel items on the wave loads and wall thickness of monopilesupport structures using different design standards.

1.3 Approach

To investigate the influence of secondary steel, various design standards are used to calculate the wave forces.First, the design standards are used to determine hydrodynamic coefficients in case of a single cylinderwithout appurtenances. The wave forces are calculated on the monopile support structure designs of thePrinses Amalia offshore Wind Farm(PAWF), formerly known as Q7. The results will show the differencesin wave loads following the different standards. The influence of the marine growth roughness guidelines ofeach design standards on the hydrodynamic coefficients, is illustrated by comparing the wave load calculationresults.

The different standards do not present a guideline to incorporate appurtenances in the hydrodynamic coeffi-cients, therefore a way of incorporating the appurtenances in the hydrodynamic coefficient is presented here.The modified hydrodynamic coefficient approach is used to calculate the wave forces and moments for thecase study and different design standards. The force and moment results are used for the global bucklingcheck at the mudline. Unity ratios of the global buckling check are given for the different combination ofdesign standard guidelines and appurtenances options. The effect of the appurtenances on the wall thicknessis shown if the wall thicknesses of the different cases with and without appurtenances are adjusted in orderto obtain equal buckling unity ratios. The difference in wall thickness at the mud line location between thetwo cases gives an indication of influence of appurtenances on the weight of the structure. This is the caseif the global buckling check governs the design. In Figure 1 the approach is indicated by means of a flowdiagram.

Figure 1: Approach for determining the influence of secondary steel on wave loads and wall thickness ofmonopiles

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1.4 Hydrodynamic coefficient determination with different standards

In [1] five methods to determine the hydrodynamic coefficient are given. One time series analyzed withthe different methods all gave different results and they concluded that it is impossible to determine theexact values for these coefficients. Various design standards specify or suggest appropriate values for thehydrodynamic coefficients Cm and Cd as functions of the Reynolds(Re) number, relative roughness (ǫ) of thepile and the Keulegan Carpenter (KC) Number. In this paper the hydrodynamic coefficients are determinedaccodring to the following design standards:

• Det Norske Veritas - DNV-OS-J101 -Design of Offshore Wind Turbine Structures (DNV)

• Germanischer Lloyds - Guideline for the Certification of Offshore Wind Turbines (GL)

• American Petroleum Institute - Recommended practice 2A-WSD(RP 2A-WSD) (API)

• International Organization for Standardization - Petroleum and Natural Gas Industries,Fixed SteelOffshore Structures (ISO 19902)

1.5 Modified coefficient method to incorporate secondary steel influences in

wave load calculations

The design standards do not supply a method to modify the hydrodynamic coefficients to incorporate theinfluence of appurtenances. The drag and inertia force are proportional respectively to the diameter andsquare of the diameter. If we consider that the drag force of the main pile plus the drag force by theappurtenance are equal to a equivalent drag force with a modified drag coefficient and using the main pilewe can obtain equation 1. For the inertia force and inertia coefficient we can do the same and we can obtainequation 2.

CDmod = CD

DMP +DSS

DMP

(1)

CMmod = CM

D2

MP +D2

SS

D2

MP

(2)

In which:

DMP = Diameter of the monopile including marine growth [m]DSS = Diameter of the secondary steel item [m]

The advantage of this method is that you can apply it to the coefficients for the height for which the modifiedcoefficient is applicable and that the appurtenance does not have to be modeled. The Morrison equation uses2D kinematics and does not incorporate the disturbance of the flow in the kinematics. Using equation 1 and2 we neglect the influence of the items on the flow and thus on each other. This method is used in this paperonly to get insight in the influences of appurtenances.

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2 Wave load case study on the PAWF

2.1 Introduction

In this section the effect of using a guideline on loads and eventually primary steel, based on ULS, is shown bycalculating the wave forces on the monopile design of the Prinses Amalia Wind Farm (PAWF). The designsdetails are obtained from the original design documents [2]. The modified coefficient method of section 1.5is used for calculation of the influence of secondary steel using the different design standards. This is a wayof illustrating the effect of secondary steel on wave loads.

2.2 PAWF design

2.2.1 Introduction to the PAWF

The PAWF is a wind farm for the coast of IJmuiden in the Netherlands. It consist of 60 2.0 MW Vestas V80offshore wind turbines. The monopile support structure is used and its stands in water depths ranging from19m to 24m. In Figure 2(a) a drawing of pile type 2 of the monopile design of the PAWF is shown.

2.2.2 Dimensions

The monopiles of the PAWF have an outer diameter of 4m. The transitions piece has an outer diameterof 4.226m and extends from -4m LAT to +14.75m LAT where the diameter is 3.38m. In Figure 2(b) thediameter over height of the PAWF support structure is given.

(a) Type 2 (b) Diameter over the height

Figure 2: Pile type 2 monopile design of the PAWF

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2.2.3 Calculation parameters

Four designs have been made of the monopile for the PAWF in [2] for the different combination of waterdepth and soil conditions. The ’worst’ case, type 2 in combination with poorest soil profile (WP42) andlargest water depth is taken as a case study. For more information about the different design reference ismade to to [2]. In Table 1 the design parameters are given.

Table 1: Design parameters

Parameter Value Unit Parameter Value Unit Runs from

Water depth 24.5 [m] Marine growth 200 [mm] +1.5m LAT to mudlineHmax 13.73 [m] Dboatlandingtube 0.325 [m] +6m to -2m LATTHmax 9.62 [s] Dj−tube 0.324 [m] +15m to -24m LAT

Ucurrent 0.6-1.24 [m/s] Max. Wind force 410 [kN] At hub height=+57 m LAT

3 Results

3.1 Wave load results using different design standards without influence of ap-

purtenances

In this section the wave load results are given for a maximum wave passing the PAWF monopile. The waveload calculation is done in Matlab using linear wave kinematics and the Morison equation. In Table 2 thetotal horizontal wave force and the total overturning moment at the mudline are given for the different designstandards without influence of secondary steel.

Table 2: Maximum total wave loads at mudline using different design standards

Design standard Roughness Fmax Mmax

marine growth [kN] [kNm]

API Low 1990 3.51E+04API High 2580 4.45E+04DNV Low 2080 3.71E+04DNV High 2370 4.19E+04GL 2340 3.92E+04

From the table it can be seen that there is difference in the results. This means that it matters which designstandard is used. The range of roughness of the marine growth given by the different standards standards inwave force differences up to 30%.

3.2 Wave loads on the PAWF design using different design standards with mod-

ified coefficients

The results of the previous section did not include the influence of secondary steel. The next section willgive the results of the hydrodynamic loads and coefficients using the modified coefficient approach given insection 1.5. The influence of secondary steel is taken into account using the modified coefficient method forthe following cases with KC determined with umax at wave crest level:

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1. One boatlanding and two j-tubes

2. One boatlanding

3. One j-tube

It is assumed that the umax should be determined for each time step in the calculation of the wave forces withthe Morison equation and linear wave theory. In Table 3 the results of the above cases are given. The resultsshow that the hydrodynamic loads can increase up to 17% over the case without influence of secondary steelitems.

Table 3: Increase of hydrodynamic loads due to different secondary steel options using the modified coeffi-cient method

Case 1 1 Boatlanding 2 Jtubes 1 Boatlanding 1 J-tube

Roughness Fmax Mmax Fmax Mmax Fmax Mmax

DNV Low 112% 115% 103% 105% 104% 105%High 114% 117% 104% 106% 105% 105%

API Low 111% 114% 103% 106% 104% 105%High 114% 117% 104% 106% 105% 106%

GL 109% 114% 109% 102% 109% 103%

3.3 Weight differences at mudline

3.3.1 Increase in wall thickness due to secondary steel influences

In the previous section the influence of secondary steel on the hydrodynamic coefficients and loads weregiven. In this section the effect on primary steel is given. The unity ratios for global buckling are calculatedaccording to GL at the mudline location. The wall thickness is then changed until it meets the required unityratio. This is done in order to compare the results. In Table 4 the increase of wall thickness at mud line dueto the increase of the hydrodynamic coefficients based on the modified coefficient method using the globalbuckling check according to GL.

Table 4: Wall thickness using different standards and secondary steel options

Unity ratio 0.6 Roughness No BL+No J BL+2J BL JDesign standard marine growth t [mm] t t t

DNV low 80 +9% + 5% + 5%DNV high 92 +12% + 4% + 4%API low 83 +8% + 2% + 2%API high 89 +10% + 3% + 3%GL 85 + 8% + 4% + 2%BL=Boat landing J= J-tube

The table shows that with the modified coefficient method the wall thickness increases up to 12% for amonopile with a boatlanding and 2 J-tubes. Using either DNV or API in combination with the high roughnessassumption on marine growth will require larger wall thicknesses. It also indicates that the assumption onthe roughness of marine growth will influence the results. Up to +10% difference in wall thickness is foundusing the range of roughness suggested by the API standard. The results are meant to be an indication ofthe influence of secondary steel on wall thickness. The method of modifying the hydrodynamic coefficients

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neglects the influence of the cylinders on eachother. This influence of the cylinders on eachother is notinvestigated in this paper.

4 Discussion of results

In this paper it is shown that determining the hydrodynamic coefficients using one standard or another caninfluence the outcome of the wave load calculations. The range of roughness of the marine growth given bythe different standards results in wave force differences up to 30%. This difference in the hydrodynamic forcesis a result of the difference between the hydrodynamic coefficients. In Figure 3(a) and 3(b) the hydrodynamiccoefficient for the umax determined at crest height is given. It can be seen that there is difference betweenthe coefficients using different standards.

0 0.5 1 1.5 2−30

−20

−10

0

10

20

30

40

50

60

value cd

Hei

ght

Hydrodynamic drag coefficient

Germanischer LloydsDNV low roughnessDNV high roughnessAPI low roughnessAPI high roughness0.1*D

pile

(a) CD

0 0.5 1 1.5 2−30

−20

−10

0

10

20

30

40

50

60

value Cm

Hei

ght

Hydrodynamic inertia coefficient

Germanischer LloydsDNV low roughnessDNV high roughnessAPI low roughnessAPI high roughness0.1*D

pile

(b) CM

Figure 3: Hydrodynamic coefficients development over height using different standards and umax at crestlevel

From Figure 3(b) it can be seen that the values for the hydrodynamic inertia coefficient of DNV and APIhave similar trends and values. Determining the drag coefficient using the DNV and API standards willresult in differences of the drag coefficient between 18% and 22%.

Using the modified coefficient method to take the influence of secondary steel on wave loads into account willincrease the wave loads between 9%-17%. This will result in larger wall thicknesses based on the bucklingchecks of GL. A boat landing and j-tube will result in a wall thickness increase of 8-12% at the mud linelocation of the PAWF monopile using the modified coefficient method.

The influence on the cylinders on each other is neglected and it is recommended to perform more researchon this. The results shown in this paper are therefore intended to give an indication what the influence ofsecondary steel is on the wave loads and wall thicknesses using different design standards for the determinationof the hydrodynamic coefficients. The monopile was checked at the mudline for buckling. The influence ofsecondary steel along the entire pile and the influence on the penetration depth has not yet been checked.However based on the results presented in this paper it can be worth while to reconsider the conventionalboat landing and external J-tubes.

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References

[1] J.M.J. Journee, W.W. Massie, Offshore Hydromechanics, 2001

[2] JHessels & van Rooij Engineering B.V., Design of tower and foundation pile for a V80-2.0MW wind

turbine at location Q7-WP., 2005

[3] Germanischer Lloyds, Rules and Guidelines, Guideline for the Certification of Offshore Wind Turbines,edition 2005.

[4] Offshore Standard Det Norske Veritas DNV-OS-J101, Design of Offshore Wind Turbine Structures,September 2011

[5] API Recommended Practice 2A-WSD (RP 2A-WSD), Recommended Practice for Planning, Designing

and Constructing Fixed Offshore PlatformsWorking Stress Design, December 2001

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