reliability assessment of offshore jacket structures in niger delta

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For the Stand–by parallel, the system is fully redundant. Any one of A or B orcombination A and B in working condition will make the system success. All componentsmust fail for the system to fail.The failure of Stand-by Parallel can be represented in Boolean Logic as:F = A ∩ B(9)The probability of the system failure is given by either:P = P . P(10)RSSAB{( 1 − R )( . − R )}= 1 −1(11)AB2.1.4 Jacket Group Bracing Reliability (Active Parallel)The reliability estimation for group bracing “A” for a jacket structures is illustrated inFigure 4 whose bracing member arrangement is in active parallel mode and can berepresented by Equation (9).R = −[(P + P + P + P −P.P.P.P)](12)AM.A. Salau, D.E. Esezobor, M. F. Omotoso/Petroleum & Coal 53 (4) 291-301, 2011 2941a b c d. a b c dwhere: R A is reliability of bracing group “A” and P a , P b , P c , P d are the failure probabilitiesof each bracing members or member thickness corrosion loss. The reliability of otherbracing groups B, C, D, E, and F will be also estimated according to the formula inEquation (12).L 4L 1L 3L 2(+) 4.0MWLbdacGroup A: Consist of allthe diagonal bracingsbetween Level (+) 4.0mand (-) 7.0mGroup B: Consistof all the horizontalbracings in Level(-) 7.0mEL (-) 7.0Group C: Consist of allthe diagonal & verticalbracings between Level(-) 7.0m and (-) 18.3mEL (-) 18.3Group D: Consistof all the horizontalbracings in Level(-) 18.3mGroup E: Consist of allthe diagonal & verticalbracings between Level(-) 18.3m and (-) 32.0mAEL (-) 32.01BGroup F: Consistof all the horizontalbracings in Level(-) 32.0m2Figure 4 Jacket Structure Diagram showing Bracing Member Groups and Support Legs
M.A. Salau, D.E. Esezobor, M. F. Omotoso/Petroleum & Coal 53 (4) 291-301, 2011 295Bracing Member Groups inParallel Reliability Mode.R AR BR CJacket Platform Legs inSeries Reliability ModeInput R 2 R 3 R 4R 1Out putR DR ER FFigure 5 Jacket Structural Reliability Schematic Diagram2.1.5 Complete Jacket Bracing Reliability (Stand by Parallel)The individual bracing group reliability A, B, C, D, E, and F is represented as:RA, RB, RC ,, RD ,, RE, RF.The reliability for the complete bracing group is parallel inmanner and is represented mathematically in equations (13) and (14).RSG= 1 −{ ( 1−RA)( .1−RB)( 1−RC)( 1−RD)( 1−RF)}(13)R = 1 − P . P . P . P . P(14)where:SGP P . P . P . PABCABECFEF. is the failure probability of individual group bracings2.1.6 Jacket Legs ReliabilityFor a fixed offshore jacket platform, the pile head is assumed to be located at mudline.The legs system reliability is defined as a product of individual leg reliability since everyjacket legs is essential for the successful operation of platform. Accordingly, for a fourlegged jacket platform the system reliability R is shown in equation 15.R SL= R . R(15)1R2. R3.4where:R 1 , R 2 , R 3 ,R 4 , is the correspondent reliability for each jacket platform’s four legs.2.1.7 Jacket System ReliabilityJacket platform structure consists of legs and bracings at different levels, along thestructure length. The jacket system reliability could be obtained by applying networkreduction techniques. The network reduction illustrated in Figure 5 is the most appropriateone for a four legged jacket structures with six bracing groups.R A , R B , R C , R D , R E and R F represent individual bracing groups that are arranged in paralleland R 1 , R 2 , R 3 and R 4 represent the four jacket legs that are arranged in series.Based on Equation (16), “the structural system reliability of a jacket structures due tocorrosion loss is the product of jacket bracings reliability and reliability of jacket platform legs”.R = R . R(16)JSSLSGSL
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reliability assessment of offshore jacket structures in niger delta

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