1、 The Ageing of PA-11 in Flexible PipesAPI TECHNICAL REPORT 17TR2FIRST EDITION, JUNE 2003The Ageing of PA-11 in Flexible PipesUpstream SegmentAPI TECHNICAL REPORT 17TR2FIRST EDITION, JUNE 2003SPECIAL NOTESAPI publications necessarily address problems of a general nature. With respect to partic-ular c
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15、on, D.C. 20005.iiiCONTENTSPage1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 THE AGEING OF PA-11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1 Background . . . . . .
16、. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 The Effect of Water in Saturated water cut with time; CO2levels;pressure; predicted downtime; crude acidity; hot oil regime;etc. It is also necessary to include estimated cumulative expo-sure to prod
17、uction chemicals, e.g., methanol, acid etc. Tem-perature gradients across the pressure sheath thickness shouldnot be considered.Given the sensitivity of the service life models to tempera-ture, inclusion of the temperature prole of the developmentover eld life in any analysis can improve the reliabi
18、lity andaccuracy of the predictions derived.As a project moves into the operational phase, the extentand accuracy of this data should increase and PA-11 servicelife verication can form part of the overall exible pipeintegrity assurance process.3.3 INITIAL ACCEPTANCE CRITERIAThe initial acceptance cr
19、iterion for PA-11 pressure sheathesis based on the corrected inherent viscosity of the most highlydegraded material on the inside bore of the sheath reaching acertain dened level.It is acknowledged that in practise there may be a gradientof corrected inherent viscosity across the wall thickness of t
20、hepressure sheath. Therefore, depending upon the failure mode,consideration of only the most highly degraded material is aconservative approach.Once the initial acceptance criterion is reached, the pres-sure sheath has aged signicantly, as illustrated in Figure 1. Inorder to continue using the pipe,
21、 an analysis of potential fail-ure modes should be performed. This will aim to evaluate themargin of safety and address risk and criticality, in a remnantlife assessment, considering recovery and re-use if applicable.The initial acceptance criteria is based on a failure criterionderived and validate
22、d from both the examination of piperetrieved from service, detailed in Appendix F, and from labo-ratory mechanical property, fatigue and crack growth testing,detailed in Appendix G.The failure criterion for a PA-11 pressure sheath is a cor-rected inherent viscosity of 1.05 dl/g. Therefore, the initi
23、alacceptance criterion is set at a corrected inherent viscosity of1.2 dl/g. This initial acceptance criterion typically includes atime based safety factor of 1.6 or more over failure, e.g., 1.64at pH = 4, 1.75 at pH = 5, and 2 at pH = 6.It is recognised that this initial acceptance criterion is notb
24、ased on a large statistical base. However, from the eld fail-ure results indicate it is conservative, and it is consistent withlaboratory experience.It should be noted that there is potentially a good case forreducing the initial acceptance criterion to 1.1 dl/g in case ofstatic applications. There
25、are several reasons for this:a.Stress levelsIn static applications, a pressure sheath istypically only subjected to thermal stresses which are lowerthan 3 MPa, e.g., in the case of cool down from 75C to 20C.In a dynamic application, stresses as a result of vessel motion,typically range from 6 to 26
26、MPa, ignoring thermal stresses.b.Number of cyclesA static pipe is typically subjected to1000 to 5000 temperature and pressure cycles over its life-time, whereas a dynamic pipe will see a much larger numberof stress cycles, typically 108.c.Low strain rateIn static applications, strains rates aretypic
27、ally very low. There is a growing body of evidence tosuggest that under such circumstances the ductile / brittletransition occurs at temperatures below those encountered intypical eld conditions.d.Field experienceAs shown in Appendix F, all the fail-ures of static lines have involved PA-11 material
28、withcorrected inherent viscosity lower than 0.95 dl/g.This case for setting a reduced initial acceptance criterionfor static pipe is currently undergoing further renement.3.4 AGEING CURVESThe prediction curves in Figure 2 are derived from the ini-tial acceptance criterion of CIV = 1.2 dl/g.The mathe
29、matical relations for these curves are tabulatedin Table 2.The API RP 17B relations have been taken from closeexamination of Figure 21 in the Third Edition(3). The unsat-urated RP 17B curve is still to be employed in dry service, asdened in 2.2.The Batched methanol treatment relation can be used tog
30、ive a conservative indication of the effect of the batched meth-anol on a PA-11 pressure sheath, by considering cumulativeexposure time and temperature for methanol batch treatments.A worked example of service life prediction using thesecurves is included as Appendix H.6 API TECHNICALREPORT17TR2Figu
31、re 2Time to Reach the Initial Acceptance Criterion of 1.2 dl/gTable 2Service conditionMathematical relation for time to reach initial acceptance criterion of 1.2 dl/gpH 7: ln (Time in years) = 14033/T(K) - 38.383pH 5: ln (Time in years) = 14033/T(K) - 38.974pH 4: ln (Time in years) = 14033/T(K) - 39
32、.259API RP 17B Saturated ln (Time in years) = 8991/T(K) - 23.968API RP 17B Unsaturated ln (Time in years) = 10612/T(K) - 26.300Batched methanol treatment ln (Time in years) = 11147/T(K) - 33.0321.0010.00100.0050 55 60 65 70 75 80 85 90Temperature in deg CAPI RP17B “Unsaturated“API RP17B “Saturated“p
33、H = 7pH = 5pH = 4Time to reach initial acceptance criterion of CIV = 1.2 dl/g / YearsTHEAGEINGOFPA-11 INFLEXIBLEPIPES74 Field Monitoring of PA-11Section 13.2.1 of API RP 17B(3), and other internationalguidelines(9), call for the implementation of a risk basedinspection pressure; pH of the produced w
34、ater;% water cut; and cumulative use of production chemicals(acids, amines, alcohols, and hot oil).Periodic review of all this data to assess the pressure sheathtechnical integrity should form part of the overall exiblepipe Integrity Management System(13). 5 References1. State of the Art Flexible Ri
35、ser Integrity Issues. MCSInternational for UK Offshore Operators Association.UKOOA reference: VES07A, 2002.2.Specification for Unbonded Flexible Pipe,API Speci-cation 17J, Second Edition, November 1999.3.Recommended Practice for Flexible Pipe,API Recom-mended Practise 17B, Third Edition, March 2002.
36、4. ASTM D664-01 Standard Test Method for Acid Num-ber of Petroleum Products by Potentiometric Titration.5.Specification for Subsea Production Control Umbilicals,API Specication 17E, Second Edition, September 1998.6. F. Dawans, J. Jarrin, T. Lefevre Swelling by oil components and water uptake as well
37、 asplasticiser loss; Loss of molecular weight due to a chemical reactionwith water, i.e., hydrolysis.In oileld exible pipes there is an absence of oxygen, andtherefore, hydrolysis is clearly established as the main mech-anism for molecular weight loss. Moreover, as molecularweight goes down, the per
38、formance of PA-11 is eventuallyaffected, in particular elongation at break or impact resistance(see also Appendix 6). There is wide experience within poly-mer science of the concept that material toughness is lost withdecreasing molecular weight (7, 8, 9).A.2 Hydrolysis in Neutral Medium, Water at p
39、H 7The hydrolysis reaction is reversible and can be describedin simplied form as follows:In order to describe the presence of a signicant reversepolycondensation reaction along with the hydrolysis reaction,it is useful to introduce an equilibrium constant which reectsthe ratio of these two rates. It
40、 is dened as follows.where-NHCO- = concentration of amide units,-NH2 = concentration of amine end groups,(-CO2HH = concentration of acid end groups,H2O = concentration of water,kp= rate constant for solid state polycondensation,kh= rate constant for hydrolysis.Four assumptions are made in order to c
41、alculate the overallhydrolysis:Where:1. Concentration of acid and amine end groups areequal, i.e.,-NH2 = -CO2, at the beginning of the reaction.2. Amide concentration is constant, i.e., -NHCO- = constant. This hypothesis assumes that the polymerisation level (conversion of monomer to polymer) is hig
42、h at all times. Only the beginning of the conversion of the hydrolysis reaction is relevant. Example: For an initial number-average molecular weight ( ) of PA-11 around 30000 and a weight average molecu-lar weight ( ) of 60000, there are roughly 150 amide units per chain. To reduce the molecular wei
43、ght by half, it is necessary to hydrolyse one amide unit per chain. Hence, conversion is 1/150parts or 0.67%.3. Concentration of water is constant, i.e., H2O = constant. This hypothesis assumes that the overall reaction is not limited by diffusion of water, and this is justied by the fact that water
44、 diffusion at a given temperature is con-siderably faster than hydrolysis reaction. The hypothesis is conrmed for exible pipe pressure sheath by both analytical and experi-mental approaches. Literature on water absorption and water diffusion in polyamides can be found in References 11, 12, and 13.4.
45、 The inuence of plasticiser is neglected. This hypothe-sis is based on the fact that plasticiser does not interfere inthe hydrolysis reaction - it is chemically inert. Also it doesnot greatly inuence the overall concentrations of differ-ent species considered.CO2H+NH2CNOH+H2OKeq. T()NHCOH2OCO2HNH2-k
46、pkh-=MnMw10 API TECHNICAL REPORT 17TR2The rate equation for the reactions involved is: The two opposing reactions, hydrolysis and recombina-tion, lead to an equilibrium molecular weight being reached.A full derivation of this equation using the above approxi-mations leads to the following equation d
47、escribing the evo-lution of polymer molecular weight change as it decaysfrom its initial value of to an equilibrium value as afunction of time(3, 10):whereJ =2(khkpNCHOH2O) 0.5,and.Here, the notion of concentration must be used with care.Concentration is only well dened in homogeneous media,but soli
48、d PA-11 is not strictly homogeneous as there areamorphous and crystalline regions. The amount of crystallin-ity depends on thermal history but is typically 20% to 30%.As the amorphous regions largely prevail the notion of aquasi-homogeneous material is justied. Furthermore, it hasbeen shown that PA-
49、11 hydrolysis is random along the chainsupporting the assumption that heterogeneity due to the semi-crystalline structure is not important(14).Further basic literature references on polyamide hydrolysiscan be found in References 15, 16, 17, and 18. Literature onpolycondensation or hydrolysis reactions of polyamides inthe solid state can be found in References 19 and 20.Many studies carried out in the context of dening thelifetime of PA-11 in offshore
