PROCEDURES FOR IDENTIFICATION OF OFFSHORE …PROCEDURES FOR IDENTIFICATION OF OFFSHORE PLATFORM...

PROCEDURES FOR IDENTIFICATION OF OFFSHORE

PLATFORM STRUCTURAL DAMAGES

Paula F. Viero and Ney RoitmanCOPPE/UFRJ - Civil Engineering DepartmentE-mail: [email protected]

Abstract — The present work reports on the evaluation of the performance ofsome damage identification methods applied on a small scale hydroelasticmodel of a fixed offshore platform designed and constructed according to theSimilitude Theory. Experimental tests were carried on the model in order toverify the behavior of the structure due to damage and to deck mass changesand in order to evaluate the feasibility of the application of the used methods.

NOMENCLATURE

(<i>x)j • j* mode shape of the undamaged structure (x)(<|>p)j : j mode shape of the damaged structure (p)J X,L '• J component of the undamaged structure mode shape ( %) corresponding

to the mode shape LJ P,L • J* component of the damaged structure mode shape (<|>p) corresponding to

the mode shape Lfsj : natural frequency of the model obtained using the Similitude Theory (Hz)fp : natural frequency of the prototype (Hz)N : number of included coordinatesL max • number of experimentally measured mode shapes

INTRODUCTION

The increasing depth of water in offshore oil exploitation made it hardto perform the visual inspection of structural damages. These problems led tothe development of simpler monitoring techniques for damage identification by

Transactions on the Built Environment vol 29, © 1997 WIT Press, www.witpress.com, ISSN 1743-3509

266 Offshore Engineering

inspecting at changes in the offshore structure modal characteristics. This ideais being developed since the early 1970s .

Recently, the number of technical references on damagedetection/location and fault diagnostics using experimental and theoreticalmodal analysis has been increased. One believes that a periodic evaluation ofexisting methods and procedures is desirable so that useful advantages andlimitations of these methods can be emphasized. In such a manner, this paperevaluates the performances of some damage detection/location methods, usingeigenvectors, on a small scale hydroelastic model of a fixed platform designedand constructed using the Similitude Theory. The following methods wereused: Modal Assurance Criterion (MAC/, Coordinate Modal AssuranceCriterion (COMAC)*, Modal Scale Factor (MSF/ and Change in ModalVector Perpendicular to Predominant Modal Direction .

The modal characteristics of the perfect model were taken as a referencefor comparison with the results obtained from the same model when some jointdamage was imposed. In both cases the modal characteristics were obtainedthrough white noise type random excitation. The results were analyzed byusing the 'Least Squares Complex Exponential' (Prony Method/ and the maindynamic parameters relating to the first three flexural and to the first torsionalvibration modes were determined.

A preliminary analysis of these results concluded that the MAC andMSF methods showed sensitivity to damage. Although the COMAC methodshowed sensitivity to damage, it was unable to clearly identify the damagelocation. Changes in Modal Vector Perpendicular to Predominant ModalDirection seems to be of great importance to identify damage in offshoreplatforms. Tests carried on the model considering only deck mass changesshowed differences between the behavior of the structure due to damage and todeck mass changes. Results obtained from the tests carried on the model inorder to evaluate the feasibility of the application of the used methods showedthat the tests could be applied on the prototype.

MODEL DESCRIPTION

The model constructed for observation of the physical behavior in airwas a 1:85 scaled model of a fixed platform with four legs in 300 m depthwater designed by Petrobras-Brazilian State Oil Company.

A general view of the model constructed with ABS, polystyrene andpolyurethane tubes is shown in Figure 1, and the main geometric and physical


Offshore Engineering 267

characteristics of the prototype and of the model are given in Table 1. Figure 2shows the frontal view and one typical section of the model.

Figure 1 - General view of the model

Table 1 - Prototype and Model Main Characteristics

Characteristics

Height of towerBase dimensionsTop dimensionsJacket weightDeck weightDuct weightRisers

88258

3

Prototype

313.50m.59 x 90.59 m.00 x 25.00m5 645.38 kN90 000 kN7 751.35 kN

10

Group of 36

Mod

3.6904 mx.29 mx0.5050.3640.011

ducts

el

m1.060.29kNkNkN

mm

The physical similitude conditions necessary for modeling an offshorestructure have been presented previously in referenced These conditions lead tothe scale factors presented in Table 2.



0.294

Damages:

• H5 - Face A• D6, D7, D9 - FaceB

FaceB0.483

Face ADimension in meters

5th Seccion

Figure 2 - Frontal view, typical cross section of the model-damage location

Table 2 - Model Scale Factors

Kg,

Modeled physical parameters:

Length, LOutside diameter, DWall thickness, dAxial stiffness, EADamping factor, £Bending stiffness, EJImmersed or dry weight

| Scale factor*

KLKL

KL\(Kc)-'Kp f • KL

1Kp f . KLKp f . KL

Related fundamental parameters: ||

Cross-section area, AMoment of inertia, IModulus of elasticity, EFrequency, fTime, TAcceleration (ac)

KL'KL'

Kp f . KL(KL)-"(KL)'"

1

are respectively geometric, elastic modulus and fluid densityscales.



The design, construction and assessment of the first natural frequenciesof the model were carried out in parallel with an adjustment through anintegrated theoretical numerical experimental analysis. Details of thisdevelopment can be found in reference .

The natural frequencies of the model obtained using the Similitude

Theory (f§j = V85 xfp), numerically and experimentally (last column) are

presented in Table 3. One can see the good correlation obtained. More detailsabout the model design and construction can be found in reference®.

Table 3 - Natural Frequencies (Hz) of the Model

Vibration Modes

1" Flexural- X1" Flexural -Yl^Torsional2™ Flexural - Y2™* Flexural - X3™ Flexural - Y3™ Flexural - X

SimilitudeTheory

2.402.436.569.169.04**

NumericalModel

2.382.436.999.408.8817.3416.82

ConstructedModel

2.552.606.968.508.2214.6714.57

* These values weren't identified until the 40 analyzed mode

DAMAGE DETECTION TECHNIQUES

In this item, some of the proposed methods to detect/locate damage inoffshore structures are described.

Modal Assurance Criterion (MAC)The Modal Assurance Criterion (MAC/ correlation technique takes,

two sets of eigenvectors from either test and finite element analysis or test andtest. In this work, the MAC was used to compare the eigenvector from theperfect structure to the damaged structure, in order to identify the damage.

The MAC is defined as:

MAC(p,x) =

N

N

I(+p)Lj=i J

(1)



This procedure results in a matrix - order m%number of undamaged modes and nip is the number of damaged modes.

x nip - where is the

The leading diagonal of the MAC matrix will indicate the similitude (ornot) between two sets of eigenvectors. If the correlated mode shapes aresimilar, the leading diagonal of the MAC matrix will be approximately unity. Ifthey are different, the leading diagonal will be less than unity and major orequal to zero.

Although it is indicated that there is a disparity between the two sets ofmode shapes, it does not show explicitly where the damage is located in thestructure. Obviously, it is of greater importance to know the position, or atleast, the region of the damage(s). In order to locate the damage, the CoordinateModal Assurance Criterion (COMAC) has been developed.

Coordinate Modal Assurance Criterion (COMAC)The Coordinate Modal Assurance Criterion (COMAC/ calculates a

correlation factor for the undamaged and damaged experimental coordinates inall mode shapes for a specific DOF (Degree of Freedom) j:

COMAC(j) =

Z(j*x,:LL=I

faf /2. j+p

LL=1

(2)

This complete procedure results in a list of COMAC values ofmagnitude between zero and the unity that can be analyzed by the samecriterion as MAC.

Modal Scale Factor (MSF)The Modal Scale Factor (MSF)* represents the 'slope' of the best

straight line through the points for a pair of modes shapes, in this case, theundamaged and the damaged mode shapes. This quantity is defined as:

N

(3)

It should be noted that this parameter gives no indication as to thequality of the fit of the points to the straight line, simply its slope.



Change in Modal Vector Perpendicular to Predominant Modal DirectionThis procedure is based on the fact that the damage in offshore

structures can be detected by analyzing the changes in the modal vectorperpendicular component to the predominant modal direction measured only onthe deck of the platform to the first flexural and first torsional modes'*'*. Themodal vector component in the Y direction of the first flexural mode in the Xdirection, the modal vector component in the X direction of the first flexuralmode in the Y direction and the modal vector perpendicular component of thefirst torsional mode present substantial changes when damages occur. Thesechanges are probably caused by the decrease of the platform stiffness, inducingplatform eccentricity mainly on the deck.

TEST PROGRAM

Preliminary tests were carried out on the model vibrating in air.Frequency Response Functions (FRFs) were then obtained for the three firstglobal flexural modes in the X direction (see Fig. 2) and for the first torsionalmode of the small scale model under two distinct conditions: with and withoutstructural integrity. For the latter condition joint damages were intentionallyimposed to the model.

The damages were chosen according to a fatigue analysis that indicatedthe members with major probabilities of damage occurrence after ten years ofplatform operation.

Figure 2 indicates these damaged tubular members with one of their endjoints entirely cut out. The damage is classified according to the locations ofthe damaged members along the height of the tower and where the damagedmembers are located (face or horizontal section) as follows:- damage DJ : the damaged member is located on one face of the platform

(diagonal member)- damage HJ : the damaged member is located on a cross section (horizontal

member).where J = 5, 6, 7, 9 indicates the level along the tower.

The tests were performed separately for each one of the above damagescases. After each test the damaged members, in the level considered forinvestigation, were repaired by gluing firmly and the integrity of the wholemodel was thoroughly verified before the next test on the model with newdamages imposed.

The FRFs were obtained from forced random excitation of the whitenoise type. The dynamic force was measured by a piezoelectric load-cell and



the structure response was measured by micro-accelerometers of the same type.Figure 3 indicates schematically the sensors location. Two differentinstrumentation were used according to the applied method:- instrumentation along the height of the tower on one of their legs (MAC,COMAC and MSF);

- instrumentation on the deck (Change in Modal Vector).

--Shaker

Load-cell" AccelerometerD1X

Instrumentation on the deck Instrumentation along the height of thetower

Figure 3 - Sensors location

Figure 4 shows a sketch of the equipment used for data acquisition andprocessing of the dynamic signals.

In order to verify changes in the behavior of the structure due to deckmass changes some experimental tests were carried on the model. Twodifferent typical situations of deck mass changes were considered:- situation 1: 16.7 % of the total mass of the deck was evenly removed;- situation 2: 14% of the total mass of the deck was removed according to the

sketch showed in figure 5.

TEST RESULTS AND ANALYSIS

The exciting force and acceleration response signals were processed byusing a two channels spectrum analyzer HP 35660A model, with an average of



50 samples and a frequency resolution of ± 0.08 Hz. The coherence functionbetween force and response signals was always verified.

T I

Spectrum Analyzer,jH| Power Amplifier

Output - White Noise HP-IB Board

Accelerorneter

Figure 4 - Data acquisition and processing system of the dynamic signals

Removed mass

Remained mass

Deck

Figure 5 - Change of the deck mass in situation 2

The FRFs processed by the spectrum analyzer were automaticallytransferred to a microcomputer using a software developed for this purpose.Afterwards, these FRFs were analyzed using the 'Least Squares ComplexExponential' (Prony Method/ implemented by a software package developedfor this purpose^, obtaining the modal characteristics of the structure.



Damage IdentificationTable 3 shows the elements on the leading diagonal of the MAC matrix

for the small scale model when damages are imposed. The values presented inthe second column are obtained for the model on similar conditions (withoutdamages). The major changes of the MAC values are enhanced.

Table 3 - MAC Diagonal Values

Mode

1" Flexural2™ Flexural3™ Flexurall^Torsional

Withoutdamage

1.001.001.000.99

H5

0.990.990.991.00

D6

0.991.000.961.00

D7

0.990.990.860.99

D9

1.000.970.991.00

One can observe from Table 3 that the MAC values for the first flexuraland for the first torsional modes are almost insensitive to the imposed damage.For the second and third flexural modes the MAC values are, in most of thecases, significant when it is compared to the MAC values on the similarconditions. The MAC values are able to indicate the presence of damage inmost of the cases when the second and third flexural modes are analyzedtogether. Existence of damage is not shown, only for case H5.

COMAC values are shown in table 4. The underlined values indicatethe level of the imposed damage, as can be seen in Figure 2. The nodespresented in the following tables are shown in Figure 3. It can be seen (fromthis table) that the COMAC values presented changes for cases D6, D7 and D9.Although COMAC values showed sensitivity to damage, they can onlyindicated the region of the damage. They were unable to identify the exactdamage position.

Table 4 - COMAC Values

Node

12345678910

Withoutdamage

0.990.980.970.991.001.001.001.000.990.99

H5

0.991.000.990.990.980.980.990.990.980.99

D6

0.980.990.970.980.980.990.990.990.960.99

D7

1.001.000.890.990.880.860.870.940.950.93

D9

0.990.990.990.960.970.990.950.940.950.99



The results presented on tables 3 and 4 could be better if the number ofmeasured points were greater. This fact would increase the accuracy of the

mode shapes.

Table 5 shows that the MSF values can indicate existence of damage inall cases. One can observe from this table that only the first flexural mode isalmost insensitive to the imposed damage.

Table 5-MSF Values

Mode

jFFJexuraT2™ Flexural3™ Flexurall" Torsional

Withoutdamage

1.011.000991.04

H5

0.980.941.02132

D6

1.040.960.871.23

D7

1.050.981.500.93

D9

0.960.780.991.34

The mass-normalized modal shape measured on the deck of theplatform for the first flexural and for the first torsional modes are presented ontables 6 and 7, respectively. These tables also present the percent changesobserved in the modal vector.

Table 6 - Modal Vector Values on the deck (Kg""*) -1" Flexural Mode

Mode

l*Flex.XDir.

1" Flex.YDir.

Accel.

D1XD2XD3YD4YD3YD4Y

Withoutdamage

15.0314.48*

3.043.573.38

H5

14.4814.776.077.48**

%

3.72.0-

146.1-*

D6

14.8913.425.594.21*

3.96

%

1.07.3-

38.5-

20.7

D7

14.8014.134.105.165.144.35

%

1.52.4-

69.744.032.6

D9

14.1814.324.035.873.874.61

%

5.71.1-

93.18.440.6

* These modal coordinates were not identified

Table 7 - Modal Vector Values on the deck (Kg"*'*) -1" Torsional Mode

Accel

D1XD2XD3YD4Y

Withoutdamage

1.961.941.801.74

H5

2.684.084.053.91

%

36.7110.3125.0138.4

D6

3.493434.073.85

%

78.1102.6126.1134.8

D7

3.253.523.813.63

%

65.881.4111.7121.3

D9

1.661.855.037.41

%

15.34.6179.4351.8

Table 6 shows that the modal components measured in the Y direction(perpendicular to predominant modal direction - X) for the first flexural modein X direction present substantial changes in their values when damages are



imposed. These parameters measured in predominant modal direction - X,present small changes.

It can be seen from table 7 that the modal vectors of the first torsionalmode measured in both directions present substantial changes when damageswere imposed, mainly in the Y direction.

These changes can be seen in figure 6 that shows a comparison betweenthe FRFs obtained from the undamaged model and when some damages wereimposed.

Deck Mass ChangesThe MAC and MSF values obtained when two different typical

situations of deck mass changes were considered are presented on table 8. Itcan be seen that these values showed sensitivity not only to damage but also todeck mass changes.

Table 8 - MAC and MSF Values - Deck mass change

Mode

l" Flexural2™ Flexural3"* Flexural1" Torsional

MACWithoutdamage

1.001.001.000.99

Situation 1

1.000.970.970.95

Situation 2

1.000.960.96

L_ 0.99

MSFWithoutdamage

1.011.000991.04

Situation 1

1.040.800.992.08

Situation 2

1.000.790.932.42

The COMAC values and the percent changes (%) observed in the modalvector measured on the deck of the platform for the first flexural and for thefirst torsional modes obtained from the tests with deck mass changes arepresented on tables 9 and 10, respectively.

Table 9 - COMAC Values - Deck mass change

Node

12345678910

Withoutdamage

0.990.980.970.991.001.001.001.000.990.99

Situation 1

0.990.960.950.930.940.950.910.850.810.85

Situation 2

0.950.900.890.880.900.890.890.830.850.87



One can observe from table 9 and table 10 that the COMAC values inalmost all DOF (degrees of freedom) and the modal components measured inthe X direction (predominant modal direction) for the first flexural mode in Xdirection showed sensitivity to deck mass changes. Thus, the situation of deckmass changes can be identified by analyzing the COMAC values and the modalvectors measured on the deck of the platform for the first flexural mode in theX direction.

Table 10 - Percent Change (%) in the Modal Vector measured on the deckDeck mass change

Mode

1* FlexuralX Direction

1" FlexuralY Direction

l^Torsional

Accel

D1XD2XD3YD4YD3YD4YD1XD2XD3YD4Y

Situation 1

+13.4+15.7*

-37.2+8.7+2.6-3.3+31.1-14.8-11.4

Situation 2

+9.0+10.9*

-38.6-30.5-32.2+19.4+3.7+16.3+19.7

* These modal coordinates were not identified

FINAL REMARKS

The results shown in this paper lead to conclude that the MAC andMSF methods were sensitive not only to the imposed damages but also to deckmass changes.

The behavior of the structure due to damage and to deck mass changescan be distinguished by analyzing the COMAC values and the modal vectorsmeasured on the deck of the platform. In the damage situation, the maximumchanges of COMAC values occurred only in the vicinity region of the damage,while in the deck mass changes, the COMAC values showed sensitivity inalmost all DOF. For the first flexural mode in the X direction, the modalvectors measured in the X direction did not present substantial changes in theirvalues when some damage were imposed, while the same components in thesame direction (X) showed sensitivity to deck mass changes.

The COMAC values showed sensitivity to damage in most of the cases,but they were unable to clearly indicate the damage location. Here, themaximum changes of COMAC values occurs in the vicinity region of thedamage, so it does not occur exactly at the defect position.



The Change in Modal Vector Perpendicular to Predominant ModalDirection seems to be of great importance to identify damage in offshoreplatforms, since with only a deck instrumentation it would be possible to detectdamages. The modal vectors measured in the Y direction for the first flexuralmode in X direction presented substantial changes in their values when somedamage were imposed. The modal vectors of the first torsional mode measuredin both directions also present substantial changes when damages wereimposed.

Tests carried on the model showed that this procedure could be appliedon a prototype by using a hydraulic actuator with a stroke of 30 cm and areaction mass of = 10 ton, located on the deck of the platform. It could bepossible to excite the prototype up to the third flexural mode and measured thestructure response with great accuracy.

REFERENCES

[1] Vandiver; J.K., "Detection of Structural Failure on Fixed Platforms byMeasurement of Dynamic Response", Offshore Technology Conference,paper 2267, Houston, 1975.

[2] Coppolino, R.N. and Rubin, S., "Detectability of Structural Failures inOffshore Platforms by Ambient Vibration Monitoring", OffshoreTechnology Conference, paper 3865, Houston 1980.

[3] Shahrivar, F. and Bouwkamp, J.G., "Damage Detection in OffshorePlatform Using Vibrations Information", International Offshore Mechanicsand Arctic Engineering, 1984.

[4] Idichandy, V.G., Ganapathy, C. and Rao, P.S., "Structural Integrity ofFixed Offshore Platforms", IABSE Colloquium, Bergamo, 1987.

[5] Ewins, D.J., "Modal Testing: Theory and Practice", Research Studies PressLtd., London, 1984.

[6] Lieven, N.AJ. and Ewins, D.J., "Spatial Correlation of Mode Shapes, theCoordinate Modal Assurance Criterion (COMAC)", Proceedings, 6*IMAC, Vol. I, pp. 690-695, Kissimme, Florida, USA, 1988.

[7] Mergeay, M., "Least Squares Complex Exponential Method and GlobalSystem Parameter Estimation Used by Modal Analysis", Proc. of the 8*Intl. Modal Analysis Sem., Leuven, 1983.

[8] Viero, P.F., "Procedures for Identification of Offshore Platform StructuralDamages", D.Sc. Dissertation (in Portuguese), COPPE/UFRJ, Rio deJaneiro, Brazil, 1996.

[9] Cameiro, F.L.L.B., "Some Aspects of The Dimensional Analysis Appliedto the Theory and Experimentation of Offshore Platforms", OffshoreEngineering, Vol. 2, p. 542-558, 1980.

[10]Rosa, L.F.L., "Modal Parameter Estimation using an OptimizationTechnique", D.Sc. Dissertation (in Portuguese), COPPE/UFRJ, Rio deJaneiro, Brazil, 1996.



0.4

0.3

0.2

0.1

0.08

0.06 .

0.04 _

0.02 _

0.25

0.2

§0.15(N

i 01u.

0.05

0

l^flexuralmode

2.5 5 7.5Frequency (Hz)

10 12.5

—Undamaged Damage D7 ... Damage D9

5 7.5Frequency (Hz)

_ Undamaged Damage D6 _. Damage H5

[D4Yl|

12.5

1" torsional mode

r'flexuralmode

i

5 7.5Frequency (Hz)

_ Undamaged Damage D7 .. Damage D9

12.5

Figure 6 - FRF measured on the deck


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