Kvitebjørn Jacket

Classification: Statoil internal Status: Draft

Kvitebjørn JacketDesign assessment of dynamic amplification

What is measured?

Sverre Haver, StatoilJanuary 2008

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Photo: Halvor Arne Asland

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Wave climate

Platform substructure

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Top side mass

Linear springs modelplatform-soil system

Second order surface process is simulated

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Table 1 Natural period and mode shapes

Mode number Mode shape Natural period (sec)

1 Sway – X 5.09 2 Sway – Y 4.96 3 Torsional 1.92 4 Bending – X 1.30 5 Bending - Y 1.24

First step – eigenvalue analysis

HS = 14.9 m & TP = 16 s

HS = 14.7 m & TP = 15 s

HS = 14.35 m & TP = 14.5 s

HS = 14.0 m & TP = 14 s

HS = 9.2 m & TP = 10 s HS = 4.0 m & TP = 5.0 s

Simulated sea states

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Method used for calculating global characteristic loads

•Design wave method:

There is a well proven metodology available when design wave approach is

used for quasi-static problems (See N-003). Kvitebjørn

is too sensitive to dynamics for thrusting solely on design wave method and a

simple estimate for the dynamic amplification.

•Time domain solution of equation of motion:

Method in principle very adequate for the Kvitebjørn case. However, there is

no detailed recipee for how to do such an analyis.

•Choice of method for predicting design characteristic loads:

Design wave method to determine the 10-2 – probability quasistatic response:

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Time domain analysis for obtaing a equivalent dynamic amplification factor.

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Quasistatic and dynamic simulationfor hs=14.9m and tp=16s.

Gumbel model fitted to sample

A considerable resonant responseis observed.

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Table 1 Quasistatic response compared with design wave response

Base Shear Overturning Moment

Stoke wave with CD = 1.05

With kinematics reduction factor of 0.95 30.9 MN 5380 MNm

HS = 14.9 m & TP = 16 s

Gumbel fitted – 90% fractile 28.8 MN 5160 MNm

HS = 14.7 m & TP = 15 s


HS = 14.9 m & TP = 16 s


HS = 14.7 m & TP = 15 s


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Table 1 Dynamic response


Dynamic response

HS = 14.9 m & TP = 16 s


HS = 14.7 m & TP = 15 s


Dynamic Amplification Factors

HS = 14.9 m & TP = 16 s

Gumbel fitted – 95% fractile 1.42 1.51

HS = 14.7 m & TP = 15 s


The dynamic amplification factor is an equivalent factor. It is the ratio between the 95%3-hour maximum dynamic response and the 95% 3-hour quasistatic response. The extremesdo not necessarily coincide in time.

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Table 1 Dynamic response with current velocity = 0.3 m/s


Dynamic response

HS = 14.9 m & TP = 16 s


HS = 14.7 m & TP = 15 s


Dynamic Amplification Factors

HS = 14.9 m & TP = 16 s


HS = 14.7 m & TP = 15 s


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Table 1 Ratio between linearised and nonlinear response for fatigue

Ratio of standard deviation from nonlinear and linearised analysis Braces in Row 1

HS=3.5m HS=5.5m HS=7.5m HS=9.5m

Level (-) 108 1.02 1.02 1.02 1.03

Level (-) 74 1.02 1.02 1.02 1.04

Level (-) 45 1.02 1.02 1.02 1.03

Level (-) 45 1.02 1.02 1.03 1.05

Level (-) 15 1.02 1.02 1.03 1.04

Level (-) 15 1.03 1.04 1.06 1.10

Level (+) 8 1.05 1.06 1.09 1.20

It is seen for fatigue purposes one may generally utilize a frequencydomain analysis. Exception members close to the free surface.

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Kvitebjørn Jacket

Sea state: HS = 9.2 m TP = 10 s No current

Current vel.

Wave direction: 0o

Remarks: Second order waves

Quasistatic response Dynamic response DAF

Deck displacement (m)

Sample Average 0.12 0.25 2.03

Gumbel - Mode 0.11 0.22 2.00

Gumbel – 90% fractile 0.16 0.32 2.00

Base Shear (MN)


Gumbel – Mode 7.32 13.15 1.79

Gumbel – 90% fractile 10.40 18.76 1.80

Overturning Moment (MNm)

Sample Average 1515 2797 1.87

Gumbel - Mode 1332 2532 1.90

Gumbel – 90% fractile 1901 3563 1.87

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Sea state: HS = 3.8 m TP = 4.9 s No current

Current vel.

Wave direction: 0o

Remarks: Second order waves

Quasistatic response Dynamic response DAF

Deck displacement (m)


Gumbel - Mode 0.012 0.050 4.16

Gumbel – 90% fractile 0.015 0.060 4.00

Base Shear (MN)


Gumbel – Mode 0.77 2.65 3.44

Gumbel – 90% fractile 0.94 3.03 3.22

Overturning Moment (MNm)

Sample Average 157 566 3.67

Gumbel - Mode 147 543 3.69

Gumbel – 90% fractile 180 635 3.53

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• Dimensjon i bunn: 50m x 50m• Dimensjon i topp: 30m x 22,5m• Total lengde: 177,9m• Vekt: 7314 t• Legg nedre del:Ø2900 T = 100/60• Legg øvre del: Ø2000 T = 100/70• Stag nedre del: Ø1200/Ø1300 T=25/30• Stag øvre del: Ø900 T = 65

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Connection Pin• Ø2900 x 95/100• A2: 10,4 m• B2: 8,9 m• A1 og B1: 7.9 m• Weld beads c/c 200 mm

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Innstallasjon• Dokking av øvre del (JUS) ned i nedre del(JBS)• Nivellering og evt. jekking• Aktivisering av grippere• Grouting av hulrom på 165 mm med spesial grout med trykkstyrke på 115 Mpa. Design basert på 80 Mpa.

Kraftfordeling• 60% av trykk kreftene overføres på spiss motstand, og 40% på skjær• Strekk krefter overføres på friksjon (skjær)

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JBS installert: 21.09.02JUS installert: 17.03.03

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Problemer med installasjon av dekket pga dønninger

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Dekket ble installert 16.05.03

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Kvitebjørn platform ble ferdig innstalert 20.05.03

Classification: Statoil internal Status: Draft

Dynamic behaviour of

Kvitebjørn jacket structure

Numerical predictions versus full-scale

measurements

Daniel Karunakaran, Subsea7, Stavanger, Norway

Sverre Haver, Statoil, Stavanger, Norway

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Problem

• Slender steel structure in rather deep water (190m).

• Utilizing values adopted in design, the largest natural period was 5s.

• Hydrodynamic loading is non-linear, i.e. for an ocean wave with period 15s load fluctuations will also be experienced for 7.5s, 5.0s, 3.75s, …

• The wave period of the 10-2- annual probability design wave is around 15s.

• At design ”DAF” = xmaxdyn/xmaxstat

was estimated to be 1.4 – 1.6.

Top derrick

Colour points sensors collectingfull scale data. show positionsof various

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Illustration of 3 exitation

-20

-10

0

10

20

2540 2560 2580 2600 2620 2640 2660

Wav

e el

evat

ion

[m]

Time [s]

Wave elevation time series

2nd order Gaussian

-20

0

20

2540 2560 2580 2600 2620 2640 2660

Bas

e Sh

ear [

MN

]

Time [s]

Total response

Dynamic Quasistatic

-20

0

20

2540 2560 2580 2600 2620 2640 2660

Bas

e Sh

ear [

MN

]

Time [s]

Response around wave peak period

Dynamic around wave peak periodQuasistatic

-20

-10

0

10

20

2540 2560 2580 2600 2620 2640 2660

Bas

e Sh

ear [

MN

]

Time [s]

Resonant response

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Standard deviation – measured versus predicted

Utilizing design assumptions regarding topside weight and soil-structure stiffness

East - West Deck Acceleration, B2 Position

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14

Significant Wave Height (m)

Acc

eler

atio

n (

mm

/s**

2)

Obs. standard deviation

Expected Standard Deviation, 0 deg.


North - South Deck Acceleration, B2 Position

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14

Significant Wave Height (m)

Acc

eler

atio

n (

mm

/s**

2)

Obs. standard deviation



Conclusion: Predictions are well on the conservative side for the extreme sea states.

Most important reason: Less topside weight(?) and stiffer soil-structure interaction.

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Natural periods – design figures versus observations first winter(Tuned model prepared in 2004/2005.)

X-Acceleration - Leg A1 - 1240 - 01012004

1.00E-01

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Frequency [Hz]

Sp

ectr

al d

ensi

ty

Measured Design model

Tuned model

Sway – X 4.03 sec 5.10 sec 4.05 sec Sway – Y 3.99 sec 4.96 sec 3.92 sec Torsion 2.38 sec 2.72 sec 2.39 sec

For tuned model the followingactions were taken:

1) Topside weight reducedfrom 23000tons to 18000tons.

(Comment 2008: Not correct!)

2) Soil-structure stiffness increased by a factor 100.

(Comment 2008: Too large factor.)

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Deck displacement January 1 2004, 12:00 – 13:00 hs=12.3m and tp=13.8s

Quasistatic (mm) Dynamic (mm)

Measured Simulated Measured Simulated

X- A1 16.8 12.5 20.1 19.4

Y- A1 23.3 17.5 25.1 22.6

X- B2 15.4 12.6 18.7 19.9

Y- B2 22.0 17.7 23.5 23.2

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Dynamic leg forces, January 1 2004, 12:00 – 13:00

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Future challenges: Missed events

Observations:

1) Measured waves are not very large, butmeasured responses are rather large.

2) Simulations using measured wave traindo not identify this event.

3) Reasons?

A) Transformed measured wave train at legpositions are not proper.

B) An important load mechanism is not captured by the simulations.

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Conclusions

• Natural periods estimated from measurements are considerably smaller than those

predicted in the design phase, indicating higher soil stiffness and lower deck weight.

• Bearing in mind all uncertainties associated with the measured values, the numerical

simulations using the tuned model compare reasonably well with measurements.

Measured total response is typically on the conservative side.

(Comment 2008: We are presently working on establishing a more correct tuned model)

• Simulated quasi-static leg forces compare better with measurements than the resonant

response. Reason seems to be to large exitation at the natural frequency in the

simulations.

• There are some few large measured response events which are not reproduced by the

present simulation sceme.

Kvitebjørn Jacket

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