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OTC17236 API RP 2A Tubular Joint Design

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  1. 1. OTC 17236The New API RP 2A, 22nd Edition Tubular Joint Design PracticeD.I. Karsan, Paragon Engineering Services Inc., P.W. Marshall, MHP Systems Engineering, D.A. Pecknold, U. of Illinois atUrbana – Campaign, W.C. Mohr, EWI, J. Bucknell, MSL Services CorporationCopyright 2005, Offshore Technology Conference RP 2A in 1969. In the 3rd edition of API RP2A, issued in 1972, some simple recommendations were introduced basedThis paper was prepared for presentation at the 2005 Offshore Technology Conference held inHouston, TX, U.S.A., 2–5 May 2005. on punching shear principles (Marshall, 1974). In the 4thThis paper was selected for presentation by an OTC Program Committee following review of Edition, factors were introduced to allow for the presence ofinformation contained in a proposal submitted by the author(s). Contents of the paper, as load in the chord and the brace-to-chord diameter ratio (Beta =presented, have not been reviewed by the Offshore Technology Conference and are subject tocorrection by the author(s). The material, as presented, does not necessarily reflect any β). In the 9th edition, issued in 1977, differentiation wasposition of the Offshore Technology Conference, its officers, or members. Papers presented atOTC are subject to publication review by Sponsor Society Committees of the Offshore introduced in the allowable stress formulations for the jointTechnology Conference. Electronic reproduction, distribution, or storage of any part of thispaper for commercial purposes without the written consent of the Offshore Technology and loading configuration i.e. T/Y, X, and K. In the 14thConference is prohibited. Permission to reproduce in print is restricted to a proposal of not Edition, the punching shear stress formulations weremore than 300 words; illustrations may not be copied. The proposal must contain conspicuousacknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. considerably modified and included a more realisticBox 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. expression to account for the effect of chord loads as well asAbstract providing an interaction equation for the combined effect of brace axial and bending stresses. Also introduced in the 14thSince the early 1990s, API sponsored a series of research Edition was the alternative nominal load approach, whichprojects to develop advanced formulation for design of non- gives equivalent results to the punching shear method (Yura,overlapping tubular T, Double T (DT-X), and K offshore 1980). The static strength guidance then essentially remainedplatform joints (API RP2A Upgrade Plan, 1990). The unchanged for all editions up to the 21st, although furtherUniversity of Illinois at Urbana-Champaign, sponsored by recommendations were added on load transfer through theAPI, developed nonlinear finite element models and verified chord in the RPRA Edition 20 issued in 1993. No tubular jointthem against available test results. These models were then revisions were made since then, until now.used to simulate a wide variety of geometries and staticloading conditions, establishing a broader database than Much further knowledge, including both experimental dataavailable test results. Advanced closed form parametric and numerical studies, has been gained on the behavior offormulations were developed and verified against these joints since the API RP 2A 14th Edition. Over the period 1994analytical results to reduce scatter and generate more reliable to 1996 a joint industry project (JIP) organized by MSLformulation than provided in the API RP 2A WSD 14th Engineering undertook an update to the tubular joint databasethrough 21st Editions. One significant feature of the new static and guidance (MSL 1996, Dier 1995, Lalani 1993). This workdesign formulations is better representation of the chord load and more recent studies, notably by API/EWI and theand geometry effects and the boundary conditions that are not University of Illinois, have formed the basis of the tubularpossible with physical testing (Pecknold 2000, 01, 02, 05). joint strength provisions of the draft ISO/CD 19902, 2001 Code. Being in LRFD format, the ISO Committee took, as aThe Task Group also evaluated and upgraded the tubular joint starting point for drafting, the relevant provisions from APIfatigue design procedures. The welded joint X and X’ fatigue RP2A LRFD (similar to API RP2A WSD 20th Edition). Fordesign curves in the RP 2A Editions 11 thru 21 have been the purposes of the update for the API RP2A 22nd Edition, thereplaced by a basic SN curve with a slope m=3 that changes to draft ISO/CD 19902 provisions, in turn, have been used as a5 at ten million cycles. Fatigue life correction factors for starting basis. However, the ISO provisions were greatlyseawater, thickness, and use of weld profile control, grinding, modified during the drafting process to take account of theand peening have also been introduced. In addition, the greater knowledge. Additional studies, not available at thecomprehensive Efthymiou 1988 equations replaced the Alpha time of the preparation of the draft ISO guidance, have beenKellogg Stress Concentration Equations. The Simplified incorporated into the API RP2A Edition 22. The majorFatigue Design Procedure has been maintained, with the updates in converting from the 20th to 22nd editions areallowable peak hot spot stresses rechecked for the new SN outlined in the following paragraphs and detailed in Pecknoldcurve. SN curves and fatigue design requirements for cast 2005 and the commentary to the new API RP2A Edition 22nodes and grouted joints have also been introduced. but, in summary, involve a relaxation of the 2/3 limit on tensile strength, additional guidance on detailing practice,Introduction removal of the punching shear approach, new Qu and QfThe API tubular joint static design technology has been formulations, new provisions for grouted joints, and a changeunder continuous development since the first edition of the API in the form of the brace load interaction equation.
  2. 2. 2 OTC 17236Fatigue has long been recognized as an important introduced. In addition, the comprehensive Efthymiou 1988consideration for designing offshore structures. The first equations have replaced the Alpha Kellogg Stressedition of RP 2A gave some general statements regarding Concentration equations, which account for chord stressfatigue and brittle fracture. More specific criteria were adopted effects. The Simplified Fatigue Design Procedure has beenin 1971 and appeared in the 3rd edition. These criteria maintained, with the allowable peak hot spot stressesincluded a static stress limitation of 20 ksi (138 Mpa) on rechecked for the new SN curve. SN curves and fatigue designcyclic nominal stress, coupled with recommendations that requirements for cast nodes and grouted joints have also beensimple joints are designed to meet the punching shear criteria provided.and that complex joints are detailed with smooth flowing lines. More information on new Tubular Joint design proceduresThis simple approach sufficed to relegate fatigue to the status recommended in the API RP2A Edition 22 are summarized inof secondary considerations, for typical Gulf of Mexico the following paragraphs and details are provided in the twostructures. However, it was recognized that using higher companion OTC 05 papers by Marshall OTC 17295 anddesign stresses [corresponding to steels with over 50 ksi (345 Pecknold OTC 17310.MPa)] yield or more severe loading experience, e.g., dynamicamplification or North Atlantic type wave climate) would Geometric Requirementsrequire specific treatment of the fatigue problem (API RP2AUpgrade Plan 1990 and AWSD1.1). The 11th edition API RP 2A Edition 22 did not introduce any major change inexpanded the allowable cyclic stress guidelines to assure the dimensional design requirements for static or fatigue design. However, several Draft ISO/CD 19902 figures haveample fatigue lives as part of the normal design process for thelarge class of structures, which do not warrant detailed fatigue been adopted to assure harmony and ease of transition fromanalyses. API to ISO practice. RP 2A Section 4 has as also been fully rewritten, and made compatible with the ISO proposed text.The years 1974-89 saw a resurgence of research interest intubular joints and fatigue, particularly in the North Sea area New API Figures 4.2-2 and 4.2-3 adopted from ISO (Figures 1 and 2) shows the in-plane joint detailing requirements. These(Back 1981, TWI 1978, Wordsworth 1981). These large-scaleefforts have significantly increased the amount of available are summarized below.data, and have prompted reexaminations of fatigue criteria. In a- The joint can length should extend past the outside edge ofparticular, the endurance limits in the original AWS criteria the bracing a minimum distance of one quarter the chordwere questioned in light of seawater environments, random diameter or 12 inches (excluding the chord taper)loading, and fracture mechanics crack growth conditions. A whichever is greater. Note that, in all earlier editions, thenumber of designers and agencies have been using modified can length included the taper. Even larger extensions maycriteria, which defer or eliminate the endurance limit. These be required to comply with API Eq. 4.3-4. Thiswere reflected in the 11th edition when API included its own conservative revision is introduced in order to stay inS-N curves for tubular joints. In addition, large-scale test harmony with the Draft ISO practice.results emphasized the importance of weld profile and b- The stubs at brace ends should extent past the brace crownthickness. A lower set of S-N curves was included to bracket heel a minimum of one brace diameter or 24 inchesthe range of fatigue performance, which can result from (excluding the stub taper); whichever is the greater.typical variations in fabrication practice. An improved c- The gap between the crown toes of two braces forming asimplified fatigue analysis approach replacing the allowable “k-braced” joint configuration should not be less than 2cyclic stress guidelines was adopted in the 17th edition, along inch for non-overlapping braces.with changes to the provisions for detailed fatigue analysis d- The joint offset (eccentricity = e) may be as much as one-reflecting greater consensus regarding preferred methods of fourth the chord diameter (D/4).analysis, description of sea states, structural frame analysis, S- e- If there should be a circumferential (girth) weld on a jointN curves and stress concentration factors. can, this should be located at a location where it will not beNew Gulf of Mexico guideline wave heights were adopted in crossed by a brace to chord weld. If this weld crossingthe 20th edition. Therefore, the simplified fatigue analysis cannot be avoided, the girth weld should be locatedprovisions were recalibrated in 1992. In addition to adjusting between the saddle and crown heel of the lightest (or thethe Allowable Peak Hot Spot Stress values for the simplified least loaded) brace location (such as a small horizontalfatigue analysis provisions, RP2A Edition 20 included changes brace).to the detail fatigue analysis provisions to the effect that only f- Longitudinal seam weld on a chord should be offset athe spectral analysis techniques should be used for minimum of 6 inch from the point of intersection of anydetermining stress response. Thickness as well as profile brace, measured along the brace surface (Figure 2). Whereeffects were explicitly considered. this is not possible, see (e). g- A tangential intersection of brace footprint and can seamIn the RP2A Edition 22, the Offshore Tubular Joint Task should always be avoided, as this sets up fatigue cracks togroup (OTJTG) replaced the welded joint X and X’ fatigue grow with a substantial part of their length residing in adesign curves used in Editions 11 thru 21 by a basic SN curve local brittle zone of the seam weld.with a slope m=3 that changes to 5 at ten million cycles.Fatigue life correction factors for seawater, thickness, and use The method proposed by API Edition 20 for classifying jointsof weld profile control, grinding, and peening have also been into K, T/Y, and Cross Double T / X joints based on their
  3. 3. OTC 17236 3geometry and axial load transfer mode within a plane formed Strength Factor Qu varies with the joint and load type, asby the brace and the chord tubulars has been maintained. given in Table 1 (4.3-1 in API RP2A Edition 22).Details on overlapping and other joint types and their Chord Load Factor Qf , which accounts for the presence ofgeometric requirements are provided in the RP2A Edition 22 nominal loads in the chord, has been fully reformulated toCommentary. produce a much better representation of the chord stressThe API Edition 21 limitation of the chord steel yield strength effects for a wide variety of joint types and loadings. ChordFyc to 2/3 tensile strength, if less, is modified to 0.8 times the stress effects are difficult to simulate in physical tests. Thetensile strength of the chord for materials with a yield stress of finite element models used for the Edition 22 formulation72 ksi (500MPa) or less. enabled better representation of the chord stress effects, resulting in the following formulation:Static Strength Design Qf = ⎡ ⎛ FSPc ⎞ ⎜ ⎟ ⎛ FSM ipb ⎞ ⎜ ⎟ ⎤ API Eq. (4.3-2)The newly developed API static strength design formulation ⎢1 + C1 ⎜ ⎟ − C2 ⎜ M ⎟ 2 − C3 A ⎥ ⎢ ⎣ ⎝ Py ⎠ ⎝ p ⎠ ⎥ ⎦required a full rewrite of the API RP 2A Section 4.3 text andtables. The punching shear design formulation has been fully The new A parameter is defined as follows:eliminated. The joint load capacity and interaction equations 0.5 A = ⎡ ⎛ FSPc ⎞ ⎛ FSM c ⎞ ⎤ 2 2based design philosophy and the basic formulation has been API Eq (4.3-3) ⎢ ⎜ ⎟ +⎜ ⎟ ⎥maintained but the formulas for calculation of its Qu, Qβ, Qg, ⎢ ⎜ Py ⎟ ⎜ M ⎟ ⎥and Qf parameters have been fully revised as outlined below. ⎣ ⎝ ⎠ ⎝ p ⎠ ⎦Validity Range. As before, the new formulation is based on (Where 1/3 increase applicable, FS=1.20 in API Equationsan interpretation of data, both experimental and FEA. 4.3-2 and 4.3-3).Therefore, a validity range has been imposed, although its Pc and Mc are the nominal axial load and bending resultant 2 2 2implication in general is minimal since the range covers the (i.e. M c = M ipb + M opb ) in the chord,wide spectrum of geometries currently used in practice. Joint Py is the yield axial capacity of the chord,designs outside the below parametric ranges are permitted, but Mp is the plastic moment capacity of the chord, andrequire special investigation of design and welding issues. C1, C2 and C3 are coefficients depending on joint and load type 0.2 ≤ β = d/D ≤ 1.0 as given in the following Table (RP2A Table 4.3-2). 10 ≤ γ = D/2t ≤ 50 VALUES FOR C1, C2 and C3 30° ≤ θ ≤ 90° Joint Type and Loading C1 C2 C3 Fy ≤ 72 ksi (500 MPa) K joints under brace axial loading 0.2 0.2 0.3 g/D > -0.6 (for K joints) T/Y joints under brace axial loading 0.3 0 0.8Basic Capacity Equations. The general form of the brace X joints under brace axial loading * β ≤ 0.9 0.2 0 0.5load and moment based capacity equations have been β = 1.0 -0.2 0 0.2maintained. However, the 0.8d multiplier in Edition 21’s All joints under brace moment loading 0.2 0 formulation has been eliminated and absorbed into *Linearly interpolated values between β=0.9 and β=1.0 for X jointsthe Qu coefficient. The 1.7 factor has been replaced with FS under brace axial loading.enabling transportability between the WSD design approachcommonly used by API and the LRFD approach proposed in The average of the chord loads and bending moments on eitherthe ISO 19902 Code, as follows: side of the brace intersection should be used in the API Equations 4.3-2 and 4.3-3. Chord axial load is positive in F yc T 2 tension, chord in-plane bending moment is positive when it Pa = Qu Q f API Eq. (4.3-1a) FS sin θ produces compression on the joint footprint. The chord thickness at the joint should be used in the above calculations. F yc T 2 d Ma = Qu Q f API Eq. (4.3-1b) FS sin θ For further details of static tubular joint capacity equations development, see Pecknold OTC 17310 (2005)(plus 1/3 increase in both cases where applicable) Joints with Thickened Cans. While the 0.25D joint canWhere: extension (as detailed beyond the brace footprint) still suffices Pa = Allowable capacity for brace axial load. for K-joints, the following formulation is introduced for Ma = Allowable capacity for brace bending moment. calculating the capacity of the simple, axially loaded Y and X Fyc = The yield stress of the chord member at the joint joints where a thickened joint is specified: (or 0.8 of the tensile strength, if less), ksi (MPa) Pa = [r + (1 - r) (Tn / Tc)2] (Pa)c API Eq. (4.3-4) FS = Safety factor = 1.60 where:For axially loaded braces with a classification that is a mixture (Pa)c= Pa from API Eq. 4.3-1a based on chord canof K, Y and X joints, take a weighted average of Pa based on the geometric and material properties, including Qfportion of each in the total load. Note that the FS value is calculated with respect to chord canreduced from 1.7 to 1.6.
  4. 4. 4 OTC 17236Tn = nominal chord member thickness known static loads. However, for further study, a modestTc = chord can thickness reduction of the WSD safety factor to 1.6 was chosen.r = Lc / (2.5 D) for joints with β ≤ 0.9 Whereas API’s existing WSD safety factor of 1.7 = (4β - 3) Lc / (1.5 D) for joints with β >0.9. corresponded to an LRFD resistance factor of 0.95, a WSDL = effective total length. Figure 4.3-2 gives examples for safety factor of 1.62 (rounded off to 1.6) would correspond to calculation of Lc an LRFD resistance factor of 1.0.Possible mitigations for Y-joints are discussed in Pecknold Figure 3- API RP2A Edition 20, with SF=1.7.(2005). Dead load only (static) betas for compressive axial load testsStrength Check: The arcsine joint interaction ratio, IR, are safely in the range of 5 to 6, and most of the experimentalrecommended by the 21st and earlier editions for axial loads betas (shaded in black) meet the target criteria. The test resultsand/or bending moments in the brace has been replaced by a are what the Edition 20 criteria were originally based upon.parabolic relation which is found to result in better correlation The finite element results cover a wider range of chord loadingwith the finite element results and adopted by the draft cases (Qf effect) than was previously considered, and containISO19902 Code: some bad news. There is tremendous scatter, and most of the 2IR = P + ⎛ M ⎞ + M ⎜ ⎟ ≤ 1.0 API Eq (4.3-5) finite element betas fail to meet the targets. Pa ⎜ M a ⎟ipb M a ⎝ ⎠ opb Storm betas tell a similar story for the old criteria.Reliability of the New Simple Joint Formulation and Compressive axial load tests are all acceptable, but some ofComparison to API RP2A Edition 21 the experimental results, and almost all of the finite elementThe API RP 2A, Edition 21 tubular joint design formulation cases, are not.statistics are compared against the new API RP2A Edition 22 Figure 4-OTJRC Static Strength Criteria, with SF=1.6.in table 3 for K, X, and Y joint configurations. The tableprovides Mean Bias, Coefficient of Variation (COV) and For the new criteria, the dead load only (static) betas for testnumber of data points for the physical test database used in and finite element results are all acceptable, and their range ofAPI Edition 21 against the Finite Element Analysis (FEA) scatter is much reduced. Three cases out of 20 are lessdatabase used in the API Edition 22. No FEA axial tension conservative than existing API; these are the experimentaldata is reported because joint tension failures cannot yet be axial compression cases.reliably predicted by numerical methods due to the The wave load only (storm) betas are all acceptable, and fall inunavailability of an appropriate and accepted failure criterion a tight cluster, except for the notionally more conservativefor ductile tearing. In general, when only the physical test tension test results. This is because the large storm loaddatabase are taken into consideration, both formulations result uncertainty overwhelms the small COVs on joint strength,in not too different mean biases and COVs. However, when making mean bias and safety factor (both elements of reservethe FE data is taken into consideration, striking differences are strength) more important.observed. Conclusion. The WSD safety factor of 1.6 has been adoptedFor the balanced K joints, when FEA Database is considered for use with the new OTJRC static strength criteria. Staticthe new Edition 22 formulation biases are generally higher betas greatly exceed target values from precedent, benefitingwhile the COVs reduced significantly. The most striking COV from reduced scatter, but they do not govern. When the one-reductions are for the balanced axial and in plane bending load third increase is used for storm loadings, the safety factorcases. A similar trend is observed for the cross X and Y joints becomes 1.2. Storm betas are clustered on the safe side of theespecially for the X joint axial compression case where the API-WSD precedent.COV is decreased almost tenfold, from 1.33 to 0.12. Fatigue Strength DesignThe safety index (also beta=β) is the ratio of total safetymargin to total uncertainty. The 1988 safety calibration of Stress Concentration Factors (SCF). Efthymiou’s (1988)API RP2A found that the existing RP2A had betas of 3.4 for SCF equations are included as part of the RP2A Edition 22.90% static load, and 2.1 (lifetime) for 80% storm loading These are based on maximum principal stress, rather than(100-year design storm). The higher safety level was deemed strain normal to the weld toe, and are some 15% to 20% moreappropriate for periods when the platforms are manned and conservative than the alpha (ovalizing) Kellogg formulasloads are under human control. Moses (1988) proposed a recommended in RP2A editions 11 through 21. Efthymiou istarget beta of 2.44 across the board for RP2A-LRFD. more accurate for planar T, Y, K, X, connections and the ovalizing term is retained in Efthymiou’s more generalIn Figures 3 and 4, the safety index for the new API Edition 22 multiplanar cases.and the old Edition 20 formulation is compared against eachother and the AWS-ASCE and API-WSD target safety indices, The alpha (length) factor in the Efthymiou equations for T-now calculated for the 100% Dead Load and 100% Wave and Y-connections captures beam bending as it occurs in a testLoad cases. frame or an isolated FEM analysis, but is difficult to reconcile with bending moments realized in design frame analysis,Because of the lower scatter (COV), huge reductions in the particularly where shears result from distributed wave orsafety factor would have still given acceptable betas for gravity loads along the chord member. One should use either
  5. 5. OTC 17236 5the chord bending from Efthymiou, or that from the frame Seawater Effects. The basic design S-N curves given in Tableanalysis, but not both. The effect of average chord axial load 4 are applicable for joints in air and submerged coated joints.should always be added. More details on application of the New RP2A Edition 22 recommends that the basic allowableEfthymiou’s SCF equations to offshore tubular joint design are cyclic stresses should be corrected empirically for seawaterprovided in Marshall OTC 17295 (2005) and commentary to effects (Hart, 1981). For welded joints in seawater withthe RP2A Edition 22. adequate cathodic protection, the m=3 branch of the S-N curve should be reduced by a factor of 2.0 on life, with m=5 branchSCFs for grouted tubular joints are also provided in Edition remaining unchanged and the position of the slope change22. These are generally same as those recommended by the adjusted accordingly. For free corrosion, the reduction factorDraft ISO/CD 19902 Code. The design of cast nodes is based is 3.0 on the m=3 branch, with no slope change. Theseon local stress rather than structural hotspot stress, and a recommendations are similar to that recommended in the Draftdifferent S-N curve applies. ISO/CD 19902 Code. Cathodic protection does notNew Basic Tubular Joint Design S-N curve completely restore in-air fatigue performance in the low cycleIn the 22nd Edition, the Offshore Tubular Joint Task Group range, where significant crack growth is involved.(OTJTG) replaced the welded joint X and X’ fatigue design Thickness effect. The basic as-welded S-N curve is for acurves of Editions 11 thru 21 by a basic SN curve with a slope reference thickness of tref = 5/8 inch (16 mm), which obscuresm=3 that changes to 5 at ten million cycles as formulated the fact that the criteria become more onerous for typical jointbelow. can thickness used offshore. For material thickness above theLog10(N) = Log10 (k1) – m Log10 (S) API Eq. 5.4.1-1 reference thickness, the following thickness effect is applied for as-welded joints:where N is the predicted number of cycles to failure under S = So if tref < 5/8 inchstress range S, k1 is a constant, and m is the inverse slope of S = So (tref /t)0.25 if tref > 5/8 inchthe basic S-N curves. These values are given below: where S = allowable stress range, Basic Tubular Joint Design S-N Curves So = the allowable stress range from the S-N curve, and t = member thickness for which fatigue life is predicted.Curve Log10 (k1) m If the weld has profile control as defined in API RP2A Figure 7Welded Tubular Joints 9.95 ksi (12.48 Mpa) 3 for N<10 11.1.3d, the exponent in the above equation may be taken as 7(WJ) 11.92 ksi (16.13 Mpa) 5 for N>10 0.20. If the weld toe has been ground and peened or a cast 7 joint is used, the exponent in the above equation may be takenCast Joints 11.80 ksi (15.17 Mpa) 3 for N<10(CJ) 13.00 ksi (16.13 Mpa) 5 for N>10 7 as 0.15. For cast joints, reference thickness tref is 1.5in (38mm).The basic tubular joint design S-N curve recommended in the Weld improvement Techniques. For welded joints,22nd Edition was the subject of extensive OTJTG discussions improvement factors on fatigue performance can be obtainedand considerations. While all task group members were in by a number of methods, including controlled burr grinding ofgeneral agreement with the shape of the low cycle S-N curve the weld toe, hammer peening, or as-welded profile control to(k1 = 9.95 ksi, m =3), there was some disagreement on the produce a smooth concave profile which blends smoothly withtransition point of this curve to the high cycle (k1 = 11.92 ksi, the parent metal. Below table shows improvement factors thatm =5) S-N curve. The draft ISO/CD 19902 and the earlier can be applied, provided adequate quality control proceduresMSL study recommended a transition point at N =108 stress are followed. The grinding improvement factor is notcycles. However, there was no observed tubular joint fatigue applicable for joints in seawater without cathodic protection.failure for N > 107 cycles to support this high cycle transitionpoint. Claim of fatigue failure of a non-joint configuration Factors for Weld Improvement factorsweld (a riser butt weld connection) below the first fatiguecurve, in N > 107 cycles was made, but no such test result was Weld Improvement Improvement Improvement Technique Factor on S Factor on Nmade available to OTJTG. Based on these observations,OTJTG resolved to specify the transition point at N = 107 Profiled to merge smoothly τ - 0.1 Variescycles, with the provision that the use of the Edition 22 fatigue Weld toe burr grind 1.25 2design curves are limited to simple tubular joint designs only. Hammer peening 1.56 4More information on the justification of the basic S-N curvesare provided in Marshall OTC17295 (2005), Commentary to For welds with profile control, where the weld toe has beenthe RP2A Edition 22, Bomel (1995), and Dimitrakis (1995). profiled (by grinding if required) to merge smoothly with theFatigue life correction factors for seawater, thickness, and use parent metal, and magnetic particle inspection demonstratesof weld profile control, grinding, and peening have been the weld toe is free of surface and near-surface defects, theintroduced as summarized below. For more details see improvement on fatigue performance can be considered asMarshall OTC17295 Paper (2005) and commentary to the shown in the Table, where τ is the ratio of branch/chordRP2A Edition 22. thickness. This improvement is in addition to the use of hotspot stress at the actual weld toe location, and the reduced
  6. 6. 6 OTC 17236size effect exponent. Either the factor on S or on N should be tubular design provisions. George Rodenbush the Chairmanused, but not both. and Andy Radford, The API Senior Associate, are specifically acknowledged for their support. Shell International E&P,Welding assemblages with fully ground radius profiles and Paragon Engineering Services Inc., MSL, and ExxonMobilstress relief may be considered the equivalent of castings with provided considerable manpower and logistical support duringweld repairs. the preparation of the API revisions and this manuscript.Cumulative Damage. As in previous editions, API RP 2A ReferencesEdition 22 specifies the Miner’s Rule for the calculation ofcumulative fatigue damage. This subject is further examined American Petroleum Institute, API (2000), Recommended Practice for Planning, Designing, and Constructing Fixed Offshorein Marshall (2005). Platforms-Working Stress Design, API RP2A-WSD, Edition 22,Simplified Fatigue Design Procedure. The Simplified December 2000.Fatigue Design Procedure has been maintained, with the American Petroleum Institute, API (1990), Proposed API RP2Aallowable peak hot spot stresses rechecked for the new SN Upgrade Plan, 1990 - 1999, for Joint Strength and Fatiguecurve. Its re-calibration as a function of the shape of the long- Provisions, API Committee Chaired by N Zettlemoyer, 1990.term stress distribution is described by Marshall (2005), American Welding Society. Structural Welding Code, AWS D1.1, ANSI Document.Safety Factors. The API RP2A Editions 11 through 21 in Back, J de (1981), Strength of Tubular joints, Special and Plenarygeneral required that the design fatigue life of each joint Sessions, PS7, Proc of the 2nd Intl Conf Steel in Marine Structures,should be at least twice intended service life of the structure Paris.(i.e., Safety Factor =2.). It also recommended that, for critical BOMEL (1995), Design and Reassessment of Tubular Joints forelements whose sole failure could be catastrophic, larger Offshore Structures, Chapter 5: Fatigue life assessment, S-Nsafety factors should be considered, In concert with the draft approach, BOMEL report C6060R09.07 Rev A, February 1995ISO 1992 Code and recent safety studies, RP2A Edition 22 Dexter, RJ and Fisher JW (1997), Fatigue and Fracture, Chapter 24 inrecognized the effect of failure consequence (i.e. criticality) Chen, WF, Handbook of Structural Engineering, CRC Pressand the in-service inspectability of a tubular joint design in Dier, A. F (1995). and Lalani, M. Strength and Stiffness of Tubularmore detail. Critical elements are those whose sole failure Joints for Assessment/Design Purposes, Paper OTC 7799, Offshorecould be catastrophic. For failure-critical and non-inspectable Technology Conference, Houston, May 1995.connections, increased safety factors are recommended as per Dimitrakis, SD, Lawrence, FV, and Mohr, WC (1995), S-N curvesTable below. Reduced safety factors can be used for Category for tubular joints, final report to OTJRC, American Petroleum Inst.L-2 and L-3 (Low consequence and hurricane evacuated or Efthymiou, M (1988), Development of SCF formulae and generalizedunmanned) conventional steel jacket structures on the basis of influence functions for use in fatigue analysis, Recentin-service performance data (for redundant framing inspected Developments in Tubular Joint Technology, OTJ88, Octoberby divers or ROV, SF of 1.0 and half the numbers in the table 1988, London, plus updates.for the other cases). Hartt, WH (1981) Fatigue of welded structural steel in seawater, OTC Fatigue Life Safety Factors 3962, Proc Offshore Tech Conf, May 1981 Failure Critical Inspectable Not Inspectable Hartt, WH and Lin, N (1985), Variable deflection fatigue properties No 2 5 of welded steel as applicable to offshore structures, Florida Yes 5 10 Atlantic Univ. final report to API. Hartt, WH (1989), Weld Profile and Plate Thickness Effects inFor more details see Marshall OTC17295 (2005), commentary Fatigue as Applicable to Offshore Structures, API 87-24 progressto the RP2A Edition 22, Bomel (1995), Dexter (1997), report, Florida Atlantic University, May 1989Dimitrakis (1995), Hart (1981, 85, 89), Marshall (1989, 92), ISO/CD 19902, Draft E June 2001, International StandardsTrembath (1995), and Vosikovsky (1991). Organization, Petroleum and Natural Gas Industries – Offshore Structures – Part 2: Fixed Steel Structures.Conclusions ISO/DIS 14347 (2002), Fatigue Design Procedure for Welded HollowThe upgraded tubular Joint design procedures are expected to Section Joints – Recommendations, International Standardsresult in more reliable designs under static and fatigue Organization, Geneva (as proposed by IIW-XV-E).loadings. The reduced scatter in the new static design Lalani, M (1993), Nichols, N.W. and Sharp, J.V. The Static Strengthformulation justifies a reduction of the static load safety factor and Behavior of Joints in Jack-Up Rigs, Conference on Jack-up rigs,from 1.7 to 1.6. Further reductions may be considered in the City University, London, August 1993.future, with more reliability calibration. Marshall, PW (1989), Recent developments in fatigue design rules in the USA, Fatigue Aspects in Structural Design, Delft Univ. PressThe new API RP 2A tubular joint design procedures improvetubular joint design procedures and provide better alignment Marshall, PW (1992), API Provisions for SCF, S-N, and Size-Profile Effects, OTC 7155, Proc Offshore Tech Conf, May 1992.with the ISO/CD 19902 draft design procedures. Marshall P. W (2005), Bucknell, J, Mohr W.C, “Background to NewAcknowledgements RP2A Fatigue Provisions”, OTC05 Paper No: 17295, Houston TX, 2005Authors acknowledge API and the members of the API SC2on Offshore Structures who encouraged and made most funds Marshall P. W (1974), Toprac, A. A., Basis for Tubular Joint Design) Welding Journal, Research Supplement, May 1974available for the research and development of the new API
  7. 7. OTC 17236 7Moses, F (1988), and Larabee, R. D., Calibration of the Draft API Moment Loads, Report to the American Petroleum Institute, EWI RP2A-LRFD for Fixed Platforms, Proc OTC 5699, May 1988. Project No. 42705-CAP, Edison Welding Institute, 2003.MSL Engineering Limited (1996). Assessment Criteria, Reliability Pecknold D. A (2005), Marshall, P.W, Bucknell, J, “New API RP2A and Reserve Strength of Tubular Joints, Doc. Ref. C14200R018, Tubular Joint Strength Design Provisions”, OTC05 Paper No: Ascot, England, March 1996. 17295, Houston TX, 2005Pecknold, D.A (2000), Ha, C.C. and Mohr, W.C. Ultimate Strength Trembath, V (1995), Review of thickness effect in profiled welded of DT Tubular Joints with Chord Preloads, Proceedings of the 19th joints, MaTR 0238, Material Tech Support Unit (UK), June 1995. International Conference on Offshore Mechanics and Arctic Yura, J.A. (1980), Zettlemoyer, N. and Edwards, I.F. Ultimate Engineering, New Orleans, 2000. Capacity Equations for Tubular Joints, OTC 3690, HoustonPecknold, D.A (2001), Park, J.B. and Koeppenhoefer, K.C. Ultimate Vosikovsky, 0 and Bell, R (1991), Attachment Thickness and Weld Strength of Gap K Tubular Joints with Chord Preloads, Profile Effects on the Fatigue Life of Welded Joints; Proc. 1991 Proceedings of the 20th International Conference on Offshore OMAE, Stavangar Mechanics and Arctic Engineering, Rio de Janeiro, 2001.Pecknold, D.A (2003), Chang, T-Y, and Mohr, W.C. Static Strength of T Tubular Joints with Chord Preloads under Brace Axial and TABLE 1- API RP 2A EDITION 22 TABLE 4.3-1-VALUES FOR Qu Brace Load Joint Classification Axial Axial In-plane Out-of-Plane Bending Tension Compression Bending K (16+1.2γ)β1.2 Qg but ≤ 40 β1.2 Qg 2.8 + (20+0.8γ)β1.6 T/Y 30β (5+0.7γ)β1.2 2.5+(4.5+0.2γ)β2.6 but ≤ 2.8+36 β1.6 X 23β for β ≤ 0.9 [2.8 + (12+0.1γ)β]Qβ 20.7 + (β - 0.9) (17γ - 220) for β > 0.9 The following notes apply to Table 1: (a) Qβ is a geometric factor defined by: Qβ = 0.3 for β >0.6 β (1 − 0.833β) Qβ = 1.0 for β ≤ 0.6 (b) Qg is the gap factor defined by: Qg = 1 + 0.2 [1 – 2.8 g/D]3 for g/D ≥ 0.05 but ≥ 1.0 Qg = 0.13 + 0.65 φ γ0.5 for g/D ≤ -0.05 where φ = t Fyb/(T Fy) Linear interpolation between the limiting values of the above two Qg expressions may be used for -0.05 < g/D < 0.05. Fyb = yield stress of brace or brace stub if present (or 0.8 times the tensile strength if less), ksi (MPa) (c) The Qu term for tension loading is based on limiting the capacity to first crack. The Qu associated with full ultimate capacity of tension loaded Y and X joints is given in the Commentary. (d) The X joint, axial tension, Qu term for β > 0.9 applies to coaxial braces (i.e. e/D ≤ 0.2 where e is the eccentricity of the two braces). If the braces are not coaxial (e/D > 0.2) then 23β should be used over the full range of β. (e) Where the working points of members at a gap connection are separated by more than D/4 along the chord centerline, or where a connection has simultaneously loaded branch members in more than one plane, the connection may be classified as a general or multi-planar connection, and designed as described in the Commentary.
  8. 8. 8 OTC 17236 TABLE 2 API RP 2A TUBULAR STATIC STRENGTH STATISTICS. NEW EDITION 22 vs. PREVIOUS EDITION 21 FOR TEST AND FINITE ELEMENT ANALSIS DATABASES Brace K Joints Loading Statistical Test Database FE Database Parameter Edition 22 Edition 21 Edition 22 Edition 21 Mean Bias 1.34 1.38 1.14 1.18 Balanced Axial COV 0.17 0.18 0.11 0.42 Number 161 440 Mean Bias 1.47 1.29 1.32 0.94 In-Plane Bending COV 0.15 0.09 0.17 0.50 Number 6 242 Mean Bias 1.54 1.15 1.2 0.84 Out-of-Plane Bending COV 0.19 0.14 0.11 0.14 Number 7 306 X Joints Mean Bias 1.17 1.16 1.31 1.47 Axial Compressio COV 0.09 0.11 0.12 1.33 n Number 65 339 Mean Bias 2.40 2.65 Axial COV 0.28 0.54 Tension Number 34 Mean Bias 1.55 1.27 1.35 0.97 In-Plane Bending COV 0.19 0.21 0.11 0.35 N Number 17 40 Mean Bias 1.39 1.13 1.52 0.75 Out-of-Plane Bending COV 0.06 0.20 0.23 0.23 N Number 6 80 Y Joints Mean Bias 1.21 1.45 1.18 1.24 Balanced Axial COV 0.11 0.20 0.14 0.32 Number 64 46 Mean Bias 2.56 3.45 Axial COV 0.29 0.29 Tension Number 16 Mean Bias 1.41 1.00 1.34 0.90 In-Plane Bending COV 0.16 0.32 0.10 0.34 Number 29 18 Mean Bias 1.45 1.07 1.31 0.89 Out-of-Plane Bending COV 0.26 0.26 0.08 0.17 Number 27 18
  9. 9. OTC 17236 9 SEE SEC. 3.4 1 1 4 4 D/4 or 12in. (300mm) MIN. SEAM WELD d2 or 24in. 1 (600mm) MIN. 4 2 d GAP 2in. d2 or 24in. (50mm) MIN. (600mm) MIN. CAN GIRTH WELD. d2 /4 or 6in. (150mm) MIN. 1 D 4 1 SEE SEC. 3.4 d1 or 24in. d (600mm) MIN. 4 1 Figure 1. API RP 2A Edition 22, Figure 4.2-2- In-Plane Joint Detailing D X. A M 4 D/ 6in. (150mm) MIN. GAP 2in. (50mm) MIN. d2/4 or 6in. (150mm) MIN. d2 or 24in. (600mm) MIN. d 1 4 1 d2 Figure 2. API RP 2A Edition 22 Figure 4.2-3- Out- of-Plane Joint Detailing
  10. 10. 10 OTC 17236 Fig 3 Old API RP 2A Edition 21 Formulation (New API Fig Fig 4 New API RP 2A Edition 22 Formulation (New API FigC4.3.2.2)