Conversion of an Oil Tanker into FPSO. Strength Analysis using ABS Rules

Transcription

1 National Technical University of Athens School of Naval Architecture & Marine Engineering Division of Marine Structures Conversion of an Oil Tanker into FPSO. Strength Analysis using ABS Rules Diploma Thesis Dimitris Georgiadis Supervisor: Prof. Emmanuel Samuelides 26 January 2017 The Greek Section of the Society of Naval Architects and Marine Engineers (SNAME) 1

2 1. Introduction - Object of this thesis 2

3 Introduction First contact with FPSOs FPSO F Floating P Production S Storage O Offloading FPSO is a type of floating tank system designed to receive all the crude oil from wells, process it and store it until the oil can be offloaded to shuttle tankers or be transported through pipelines to shore. Main advantages: reliable solution for deepwater marginal fields where the construction of pipeline is not cost effective or practical, redeployment capability, cost-effective solution over fixed platforms August FPSOs on operation worldwide 70% conversions / 30% purpose built FPSOs 3

4 Introduction Worldwide Distribution of FPSO Vessels Source: 4

5 Introduction General Layout Offloading system Living quarters & helicopter deck Power generation Process modules Flare tower Mooring system & risers Source: 5

6 Introduction Mooring System The most commonly used mooring systems for FPSO are: Spread Mooring system Turret Mooring System (weather-vaning) o External turret Internal turret o 6

7 Introduction FPSO vs trading Tanker FPSO hull structure arrangement general similar generally similar to trading tankers Similar number and size of cargo holds Longitudinal stiffened plating Same connection details 7

8 Object of this Thesis Conversion of a tanker to FPSO Investigate how the operating site affects the requirements and consequently the decision to proceed with the conversion or not. For this purpose, two sites have been chosen for installation of unit and two shipyards as a departure location for transit. Methodology: Collection of environmental data Selection of mooring system Evaluation of corrosion and fatigue during tanker service life Strength assessment according to ABS Rules 2016, Floating Production Installations 13H T 28 s p 8

9 2. Conversion Procedure 9

10 Conversion Procedure Flow Chart 10

11 Conversion Procedure Selection of tanker for FPSO conversion Main dimensions of VLCC tanker: L= 320m B= 60m D= 30.5m T = 22.5m C = b 11

12 Conversion Procedure Main Dimensions & Mooring System selection Spread-mooring (Nigeria) External turret (North Sea) 4x3 line catenary system 8 line catenary system The selection of mooring system was based on past experience showing that turrets are used frequently in North Sea and spread mooring in Nigeria. 12

13 Conversion Procedure Lightship weight distribution for FPSO The lightship weight is classified into the following basic groups: Hull steel Accommodations & Helideck Hull outfitting Hull machinery Electric Miscellaneous Mooring & risers Topsides (wet & dry) 13

14 Conversion Procedure Compartmentation of FPSO tanks Software AVEVA has been used for the compartmentation of the vessel VESSEL CAPACITIES (m 3 ) N.SEA Stabilized product 303,071.8 Water Ballast 96,293.7 Slop 14,874.1 Diesel oil 2,547.4 Fresh Water 1,244.3 Methanol 4,022.7 Sludge TOTAL 422,173.3 VESSEL CAPACITIES (m 3 ) NIGERIA Stabilized product 324,662.7 Water Ballast 111,897.1 Slop 14,874.1 Diesel oil 2,547.4 Fresh Water 1,244.3 Methanol 4,022.7 Sludge TOTAL 459,

15 Conversion Procedure Loading Conditions for FPSO Draft and Trim variation check Minimization of free surfaces with appropriate loading/offloading pattern Check of Stability in intact condition (IMO Resolution A.167) 15

16 3. Strength Analysis 16

17 Strength Analysis Procedure FPSO conversion Structural assessment Reassessment of vessel scantlings Hull girder strength Local strength of plates & stiffeners Calculation of Renewal scantlings Buckling check Determination of replacements/repairs needed Evaluation of remaining fatigue life Thickness measurements 17

18 Strength Analysis Procedure 18

19 Strength Analysis SEAS Sea Environment Assessment System (SEAS) FPSO Wave Conditions ( ) H, T, S ω s p ESF, β-type On-site Transit Ls β = Lu Ls = the most probable extreme value based on environmental conditions on-site & transit Lu = the most probable extreme value based on N. Atlantic environmental conditions (unrestricted service environment-basis of tanker design) 19

20 A) Strength Analysis Global SM calculation The most unfavorable loading conditions η Full Ballast (max hogging) η Full Load (max sagging) ( ) M = k1β CLB 1 C wv sag VBM b M wv hog =+ k β C L BC VBM 1 b 10 SM 1 = M + M σ sw all wv Mtotal = Msw + Mwv B) β VBM SM 2 < SM S.V.R. 0.7< β VBM <1 (0.5+ β VBM /2) SM S.V.R. 175MPa k 2 SM SV.. R. = kc1c 2L B ( Cb + 0.7) > 1 SM S.V.R. ( ) SM = max SM, SM req 1 2 N. Sea SM req. = max {(a),(b)} = max {94.3,78.0} = 94.3 m 3 Nigeria SM req. = max {(a),(b)} = max {72.6, 66.6 } = 72.6 m 3 SM Tan ker = m 3

21 Strength Analysis Local Scantling requirements Plates Tertiary stresses Bryan formula ( ) 1/2 t= 0.73s kp f 1 1 SM Stiffeners > SM Rule, net actual, net e External Pressures ( β ) p = ρg h+ kh s EPS / EPP u de Pressures Internal Pressures p= kρ g( η + kh) + p 0 i s u d o Sloshing Pressures p = k ρgh 0 is s e Static Dynamic Static Dynamic Ship motions and Accelerations a = C β k a g Pitch v v VAC v o a= Cβ ka g l l LAC l o a= Cβ ka g t t TAC t o ( ) 1/4 ϕ = β k 10 / C / L PMO 1 b Roll θ = CRβRM O ( 35 kc θ di /1000)( Tr ) 21

22 Strength Analysis Criteria for the assessment of hull structure Application: keel plate - Nigeria As-Built 22 mm Reassessed mm mm (Ant. Corrosion) mm (Ant. Corrosion) mm mm mm mm mm Actual Yard req. Rule req. Substantial Renewal 22

23 Strength Analysis Buckling check Buckling Plates Stiffeners Compression Shear Column buckling Torsional buckling Web and Flange buckling 1. Actual Stresses (N. Sea, Nigeria) 2. Ideal Elastic Buckling of plates & stiffeners 3. Critical Buckling Stresses 4. Buckling Check 23

24 Strength Analysis Fatigue Fatigue life of FPSO conversion Tanker phase: accumulated fatigue damage during tanker operation for actual trading routes Transit phase: transit environment FPSO phase: High cycle fatigue: operating condition, on-site environment Low cycle fatigue: loading and off-loading Tanker phase FPSO phase Transit phase 24

25 Strength Analysis Fatigue Application of the Palmgren-Miner cumulative damage rule for conversion FPSO can be expressed as: S L where, PriorConv L R, PostConv + = L P,Pr iorconv P,PostConv S PriorConv L P, PriorConv L R, PostConv L P, PostConv or LR, PostConv = LP, PostConv ( 1 SPriorConv / LP, PriorConv ) (1) (2) = service years prior to conversion = predicted (design) fatigue life prior to conversion = remaining fatigue life for on-site operation, post conversion = predicted (design) fatigue life for on-site operation, post conversion Design life L R, PostConv ( S α ) ( S α ) ( S α ) 20 = 1 DM L L L Route i Route i HisSite i HisSite i Transit Transit P,Tan ker P, Site P, Transit (3) High cycle + Low cycle

26 Strength Analysis Fatigue Fatigue Life, DM Structural Detail Stress Range S-N Curves A. Primary Stresses B. Secondary Stresses I. Vertical BM loads M M S z z ws wh ( ) = ( n ) RG, v i Iv II. Horizontal BM loads S ( ) RG, h i 2 M = h y I M m C L DC h 2 3 H =± hβhbm Κ 3 1 b 10 σ = C C M SM M 2 t y / = 2 ( Cd psl ) /12 The reference stresses used in the S-N curves are the nominal stresses. C. Total Stress Range ( ) S = kc S + S R p f RG RL 26

27 Strength Analysis Fatigue Calculation of Cumulative Damage DM & Fatigue Life The resultant cumulative fatigue damage that is predicted to occur on-site, is to be taken as: (4) DM = 0.15DM DM DM DM (5) DM = f DM + f DM + f DM + f DM i i,1 2 i,1 2 i,3 4 i,3 4 i,5 6 i,5 6 i,7 8 i,7 8 i=1-4 (6) DM ( 0.01S Ri ) n ( ln N ) m NL i, j k µ / γ i K2 R m = Γ 1+ γ B case C case DM 1 = DM 2 = DM 3 = DM 4 = (3) L R, PostConv 20 = [ 1 0.5] = 25 years DM DM=0.41 The detail withstands as concerns Fatigue 27

28 4. Conclusions 28

29 Main conclusions Most critical loading conditions in strength study are the two extreme conditions, Ballast at minimum draft & Load at full draft. Transit condition needs to be considered for offshore Nigeria. significant effect on resulting SWBM Mooring system static loads increase the total weight of vessel Regarding fatigue special attention must be paid in harsh environment because of the: preexisting fatigue damage of vessel as a trading tanker increased magnitude of stress range induced by waves compared to N. Atlantic 29

30 Main conclusions Life cost analysis and comparison of the two alternatives; conversion vs new build 30

31 31

32 Notes 32

33 Anticipated corrosion wastage of VLCC hull structure at the time of conversion Considering: 10 year service life as a trading tanker 5 year duration of coating life The next 5 years the corrosion rate is given from measurements and time-dependent corrosion wastage models 33

34 34

35 Buckling check Actual stresses (Working stresses) Compression Shear σ β M + M Μ VBM w sw Fm = y ή σα = c5 y Ι I t n F I τ = q = τ t q = m+ qc Ι Maximum stresses at bottom and deck Maximum shear stresses near to neutral axis 35

36 Buckling check Ideal Elastic Buckling of plates & stiffeners Plates Compression Shear tb σ E = 0.9 me s 8.4 m = 0 Ψ 1 Ψ+ 1.1 tb τ E = 0.9 ke t s k t s = l Stiffeners Column buckling ΕΙ σ Ε = α c Al Torsional buckling π ΕΙ K I σ Ε = m E 10cI l m I 2 w p Web and flange buckling t w σ E = 3.8CE h w 2 t p b t f f 15 36

37 Buckling check Critical buckling stresses τc Compression = τ Ε τ F = τ F 1 4 τ Ε τ Ε τ F 2 τ Ε > τ F 2 σc Shear = σ Ε σ F = σ F 1 4 σ Ε σ F σ E 2 σ F σ E > 2 Buckling check σ τ c c βσ τ α α The buckling criteria are satisfied for both locations 37