1. System Identification & Damage Detection of Steel Offshore Structures,
Using Ambient Vibration Tests
Supervisor: Dr. Ali Akbar Aghakouchak
By: Amin Ghafooripour

2. Advisors: Dr. Mohsen Ghafory Ashtiany Dr. Mohammad Taghi Ahmadi
Islamic Azad University
Science & Research Campus
Civil Engineering Department

3. Targets of research: Group I: Group II:
1- System Identification (SI) of Jacket type Platforms in the Persian
Gulf.
2- Development of SI methods for ambient vibration tests.
Group II:
3- Development of vibration based health monitoring for offshore
structures, based on ambient vibration tests with incomplete
measurements.
4- Development of vibration based inspection methods for jacket
type platforms.

4. Proof for importance of targets:
Group I: System Identification
1- System identification of offshore structures will show the behavior
of structure and may verify the theoretical model.
2- The estimated dynamic parameters (frequency, damping values and
mode shapes) can be used for future studies in researches or
engineering design purposes.
3- The accuracy of empirical values for determining natural frequencies
in earthquake practice codes will be studied by these type of tests.
4- There are a few experiences for ambient tests for offshore structures
and the country of Iran is the 5th one in the world which, was done
the vibration tests on offshore structures.
5- The common method does not exist for system identification.

5. Proof for importance of targets:
Group II: Damage detection
1- Vibration based damage detection methods are in progress for using
in structures and a few researches have been done for offshores.
2- Damage detection will reduce the cost of underwater inspections.
3- Damage detection by ambient tests can be used for annual inspection
and as the base for underwater inspections.
4- Today, in the Persian Gulf, all inspections are done by divers. For
deep water diving (exceed 50 m), all divers & underwater inspection
equipments are contracted by foreign companies.
5- No inspection strategies exist for using the vibration tests data.

6. Scope and method of research: i) Objective and scope:
1- Only steel jacket type platforms were studied.
2- Vibrations of platforms are recorded on deck’s levels.(Above water)
3- As built drawings are available.
4- Usual ranger seismometers were used for vibration recording.
5- Dynamic parameters(Damping value, frequencies and mode shapes)
were identified.
6- Only ambient vibrations were used as the source of vibrations.
ii) Method of research:
1- Field tests
2- Analytical modeling of structure.
3- Spectral analysis & signal processing
4- Dynamic residual formulation
5- Statistical formulations

7. Studied items: i) System identification:
1-History and methods of system identification and choosing the low
cost and economic method for the platforms in the condition of
the Persian Gulf.(literature review)
2- Study about spectral analysis methods,(MEM,Welch, MTM,MUSIC)
3- Welch method for estimation of Power Spectral Density Function.
4- Distribution function of data.
5- Efficiency of methods for spectral analysis.
6- Innovations in estimation of half power bandwidth.
7- Cross correlation, phase spectrum and PSD.
8- Effects of stationarity and non stationarity on results
9- Digital filters and Window functions.
10- Bias and random errors.
11- Duration, sampling rate and number of data in records.
12- Wigner-Ville distribution and waterfall display.

8. Studied items: ii) Analytical modeling:
1- ANSYS,ver.5.4, used for structural modeling.
2- Consistent mass matrices, added mass and water structure interaction
were considered.
3- Mass reports and full detail computations of mass were carried out
from contract drawings.
4- SAP 90 modeling was used for comparison of results.
5- Calibration of model to tests’ results.

9. Studied items: iii) Damage detection:
1- Vibration based methods(literature review)
2- Dynamic residual formulation.
3- Inspection strategies for offshore structures.
4- Perturbation method for solving the residual function.
5- Nonstationary effects of damage detection.
6- Relation between damage index and required sampling number for
underwater inspection.

10. Persian Gulf- oil field
Location of platforms:
Persian Gulf- oil field
A view of platforms:

11. Description of tested plaform:
Category
Specification
Type of offshore
Jacket type
Material
Steel
Mud level
-67m
Top of Jacket
+3.05m
Cellar deck lev.
10.36m
Main deck lev.
+19.5m
Deck dimension
~30x30
Dim. Of jacket(bot.)
~33x33
Dim. Of Jacket(T)
~15x15m
No.of Hor. Bracings 6
Mass of Main deck
Ton
Mass of cellar
Ton
Mass of jacket
420 Ton
Mass of boatlanding
9.2 Ton

12. 9 ranger seismometers-SS-1 3 SSR-1 recorders and computers
Data gathering system:
9 ranger seismometers-SS-1
3 SSR-1 recorders and computers

13. Results, Recommendations:
Ambient vibration tests can be used for identification of dynamic
behavior of an offshore structure by proposed modifications on Welch
method. The accuracy of results will be warranty if the statistical
parameters are designed for the specific structure.
There is sufficient ambient excitation in the Persian Gulf environment
for monitoring a range of natural frequencies in winter.
The results show the nonstationary behavior of records. To investigate
the modal parameters of an offshore structure from ambient vibration
tests, the nonstationary analysis of records will be important for further
interpretations.
Partial stationary concept was implemented to use for offshore structures.
The nonstationary analysis of records shows the fluctuation of frequencies
and energy of structure, and for linear structures only the small part of
results may be stationary or stationary with acceptable bias error.
So these types of records, which, the results contain some stationary parts
are called partial stationary.

14. Waterfall display for instantaneous PSD
Typical PSD
Waterfall display for instantaneous PSD
Wigner-Ville instantaneous PSD for
Nonstationary analysis

15. Results-Conitue,....
Analysis of sample offshore structure shows the variation of
frequencies over the time and bias errors should be encountered
for theoretical model verifications.
For estimation of damping values, the record length and
segments of the Periodogram estimation are very important.
Number of data in each segment of the Periodogram should be equal to sub multiple of 2n.
The correlation between model and tests’ results show the accuracy of finite element model with consistent mass matrix, plus added mass, lumped mass should be used for decks with rigid element modeling.
Using the clamps for connecting the oil risers to legs will not
have effects on structural stiffness and they can be modeled for
masses and added hydrodynamic masses with less than 1/100
modulus of elasticity of structure.

16. Results-Conitue,....
Different window functions with interpreted length of segments may
lead the estimation of damping to the different values.
For correct estimation of damping values, Bias errors should not
exceed 2 percent.
For offshore structures, which the ratio of the main lobe energy to the
side lobe energy is maximized, the Bessel-Kaiser window is an
approximation to the prolate window and more efficient than the
other window shape functions.
The adjustable value in Bessel-Kaiser window will be permitted the
function to achieve the best fit for half power bandwidth without
variance and minimum random error with interpreted bias error.
It is observed that the value of bandwidth of window can be vary
between 0.2 to 0.25 of the half power bandwidth in main PSD to
estimate the reliable values of damping.

17. Results-Conitue,....
Using the dynamic residual formulation for offshore structures, shows
the accuracy of results for estimation of damages.
Ambient vibration tests can not used for locating damages in jacket.
The proposed method can be shown the damage index and exigency
of underwater inspection in annual survey of platform.
A new inspection strategy was developed for using the computed
damage index to specify the number of samples for further underwater
inspections. In addition the proposed damage index is the threshold
exigency of using underwater inspections.

18. 2- Proposed thresholds for nonstationary analysis
Innovations:
1- Implement the Welch method for
offshore structures with the new concept
"PARTIAL STATIONARY"
2- Proposed thresholds for nonstationary
analysis
3- Proposed the rule for spectral
analysis based on nonstationary behavior

19. 4- Proposed the Bessel-Kaiser window function instead of other types
for more accuracy in estimation of damping value
5- proposed a method for damage
detection of offshore based on
incomplete measurements,prior to
underwater inspections
6- finally: proposed the new
inspection strategy-(VBIM)

20. Sponsors: * International Institute of Earthquake Engineering
and Seismology(IIEES)
* Ministry of Oil-National Iranian Offshore Oil
Company ,NIOOC
*Islamic Azad University (IAU) Science and Research
Branch
* Islamic Azad University (IAU) Tehran Branch
*Pars Padir Consulting Engineers

21. Total number of references: 215
Selected references:
Total number of references: 215
[94] Chen Chung-Wen, Juang Jer-Nan, Lee G. , [1994], “Frequency Domain State-Space System Identification”, Journal of Vibration and Acoustics, American Society of Mechanical Engineers, Vol. 116, pp. [ ].
[95] Lim Tae W., Kashangaki Thomas A. L. , [1994], “Structural Damage Detection of Space Truss Structures Using Best Achievable Eigenvectors”, AIAA Journal, American Institute of Aeronautics and Astronautics, Vol. 32, No. 5, pp. [ ].
[132] Baruch Menahem, [1997], “Modal Data Are Insufficient for Identification of Both Mass and Stiffness Matrices”, AIAA Journal, American Institute of Aeronautics and Astronautics, Vol. 35, No. 11, pp. [ ].
[145] James G., Zimmerman D., Cao T. , [1998], “Development of a Coupled Approach for Structural Damage Detection with Incomplete Measurements”, AIAA Journal, American Institute of Aeronautics and Astronautics, Vol. 36, No. 12, pp. [ ].
[207] Auger Francois, Flandrin ,P.,Lemoine,O.,Goncalves ,P.,[feb.2002], “Time frequency toolbox for Matlab”, CNRS[National de la Recherché Scientifique] Nantes University ,< , France.
[212] Doebling Scott , Farrar Charles, [march 2002],” Vibration –Based Damage detection “, < >.
[213] Hasselman , Timothy , [Oct. 2001], “Quantification of uncertainty in Structural Dynamic models “,ASCE, Journal of AERO space engineering, vol.14 . No-4 –PP[ ].
[214] Kelly,W. Shawn, [March 1999], “Underwater Inspection Criteria”, Naval Facilities Service Center, Technical report, Port Hueneme, California

22. I. Azad University-TEHRAN BRANCH
THANK YOU FOR YOUR
ATTENTION
AMIN GHAFOORIPOUR
I. Azad University-TEHRAN BRANCH
PARS PADIR CONSULTING
ENGINEERS
PO.BOX /684-TEHRAN,IRAN