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Kvitebjørn Jacket

Kvitebjørn Jacket. Design assessment of dynamic amplification What is measured?. Sverre Haver, Statoil January 2008. Photo: Halvor Arne Asland. Wave climate. Platform substructure. Top side mass. Second order surface process is simulated. Linear springs model platform-soil system.

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Kvitebjørn Jacket

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  1. Kvitebjørn Jacket Design assessment of dynamic amplificationWhat is measured? Sverre Haver, StatoilJanuary 2008

  2. Photo: Halvor Arne Asland

  3. Wave climate Platform substructure

  4. Top side mass Second order surface process is simulated Linear springs modelplatform-soil system

  5. First step – eigenvalue analysis Simulated sea states

  6. 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). Kvitebjørnis 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:+Time domain analysis for obtaing a equivalent dynamic amplification factor.

  7. Quasistatic and dynamic simulationfor hs=14.9m and tp=16s. A considerable resonant responseis observed. Gumbel model fitted to sample

  8. 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.

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

  10. 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

  11. 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

  12. 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)

  13. JBS installert: 21.09.02 JUS installert: 17.03.03

  14. Problemer med installasjon av dekket pga dønninger

  15. Dekket ble installert 16.05.03

  16. Kvitebjørn platform ble ferdig innstalert 20.05.03

  17. Dynamic behaviour of Kvitebjørn jacket structure Numerical predictions versus full-scale measurements Daniel Karunakaran, Subsea7, Stavanger, Norway Sverre Haver, Statoil, Stavanger, Norway

  18. 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. Colour points sensors collectingfull scale data. show positionsof various

  19. Illustration of 3w exitation

  20. Standard deviation – measured versus predictedUtilizing design assumptions regarding topside weight and soil-structure stiffness Conclusion: Predictions are well on the conservative side for the extreme sea states.Most important reason: Less topside weight(?) and stiffer soil-structure interaction.

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

  22. 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

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

  24. Future challenges: Missed events • Observations: • Measured waves are not very large, butmeasured responses are rather large. • Simulations using measured wave traindo not identify this event. • Reasons?A) Transformed measured wave train at legpositions are not proper.B) An important load mechanism is not captured by the simulations.

  25. 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.

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