MATERIAL. III-1 Mechanical Finite Element Analysis and Engineering Critical Assessment Study

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1 BRINGING THE STRUCTURAL INTEGRITY OF ALLOY 36 (36% NICKEL) PIPES A STEP AHEAD FOR LNG TRANSPORTATION WITH CRYOGENIC PIPE IN PIPE READY FOR EPCI PROJECT PURPOSES Laurent Pomié Jeffrey O Donnell Technip Paris France, Technip Houston USA Guillaume Graindor Julien Lavaux, Serimax Mitry-Mory France Jan Van der Ent Mireille Boot Dennis Zaal Applus RTD Rotterdam, The Netherlands ABSTRACT TECHNIP has successfully developed and qualified a technology of LNG transfer lines over moderate distances [1], based on Pipe in Pipe concept. The use of Alloy 36% Nickel material as the core of the technology allows designing straight lines, thereby suppressing expansion loops required by conventional stainless steel material. As a result of 36%Ni steels low coefficient of thermal expansion, the Technip LNG transfer systems can be buried or laid subsea like conventional pipelines and facilities separated by longer distances can be reliably connected, under more secure conditions. To convince operators of the operational reliability of the concept, Technip pushed further, a qualification program on structural integrity of the alloy 36 pipes, more than recommended by usual design and construction codes. Technip, with the support of Aperam - Alloys Imphy has developed statistical and numerical approaches and models to predict the mechanical and fracture resistance of alloy 36 base material and weldments. All these models were qualified by extensive base metal and weldments testing programs performed in partnership with Serimax (Vallourec Group) including fracture and fatigue properties testing. By optimizing the chemistry of the filler metal and appropriate welding strategies, the overmatching at cryogenic LNG temperature has been reached for the very first time. As a direct benefit, the mode of failure at cryogenic temperature of weldments has been established as a 'leak before burst' fully ductile tearing mode and has been verified on medium scale tests. Technip conducted an exhaustive 'engineering critical assessment' program aiming at verifying the integrity sensitivity to different parameters: fracture toughness, misalignment (hi-lo), residual stresses and precommissioning conditions. Moreover, Technip, in collaboration with a non-destructive testing company Applus RTD, has qualified a more sensitive technology, based on digital radiography, for the inspection of welding flaws in alloy 36% nickel pipes 1

2 I. BACKGROUND As evident causes of services failures, the integrity of a structure depends on a combination of factors, as illustrated in the below triangle. Failure will only occur if critical conditions corresponding to all three corners come together. Assessment based on fitness for purposes addresses all such factors. In contrast, traditional design only uses two of the corners, usually applied stresses and material yield strength to represent strength. The fitness for purpose program that Technip developed with its different partners aims at improving performances and reliability of the different factors that may impact the integrity of alloy 36% Nickel pipes. Technip has developed finite element analysis models to quantify primary, secondary and residual stresses, to to derive stress intensity factors at welding flaws tips [2]. Aperam - Imphy alloys has developed a specific welding consumable INVAR M93TS to guarantee mechanical overmatching at cryogenic LNG service temperature. Serimax carried out an extensive welding qualification program to make construction procedures as robust as possible while guarantying soundness welds. Applus RTD, has qualified their digital radiography technology RAYSCAN to improve performances, sensitivity and detection threshold on alloy 36 weldments. MATERIAL STRESS FLAW II. OBJECTIVES OF THE PAPER This paper presents the content of the fitness for purpose program and give the demonstration of how this program has improved the structural integrity of alloy 36% Nickel pipeline for LNG transfer, now being recognized by the new revision of ASTM A 333 [3]. The integrity improvement has been verified at different scales: at the low scale : the mechanical overmatching of alloy 36 weldments at cryogenic temperature has been achieved on tensile test specimen. In addition fracture toughness and fatigue crack growth rate have been characterized with outstanding properties at an intermediate scale: Curve wide plate tests have shown a failure mode of leak before burst at cryogenic temperature (LNG temperature). III. DEVELOPMENT AND METHODOLOGIES III-1 Mechanical Finite Element Analysis and Engineering Critical Assessment Study Coefficient of thermal expansion mismatch between base material and filler metal can induce thermal secondary stresses at welding joints. These thermal stresses coupled with stresses due to misalignment can 2

3 create hot spot stresses, specifically during cool down between the ambient and cryogenic temperature (LNG temperature). These hot spot stresses levels have been derived from Finite Element Analysis (Fig 02) for the long seam and the girth weld with a meshing appropriate to each area of interest (Base Metal, Heat Affected Zone, Weld Metal) See Fig 01 a-b Figure 01 a:) Etched image of the seam weld Figure 01b): FEA model of the seam weld Figure 02: FEA on residual stress with CTE & misalignment sensitivity Detailed finite element analyses (Fig 03) were done to determine the effect that those residual stresses have on stress intensity values at the crack tip for different levels of flaw depths (1.5 mm, 3.0 mm and 4.5 mm) and different misalignments (0, 1.5 and 3 mm). In addition, a comparison was done with the solutions generated by BS 7910 [4] (Fig04) to determine their applicability 3

4 Figure 03: FEA for stress intensity derivation Flaw depth and misalignment sensitivity K (MPa-m 1/2 ) FEA BS 7910 K (MPa-m 1/2 ) FEA BS 7910 K (MPa-m 1/2 ) FEA BS Nominal Stress (MPa) Nominal Stress (MPa) Nominal Stress (MPa) no misalignment 1.5mm misalignment 3.0mm misalignment Figure 04: Stress intensity factors (K I ) from finite element analyses and BS7910 for 1.5mm flaw with zero, 1.5mm and 3.0mm misalignment 4

5 Fracture and Fatigue engineering critical assessment studies have been conducted based on: Tested material properties Loading and stresses conditions for linepipes welding joints (girth and long seam) Fatigue crack growth rate as characterized fracture and fatigue testing program Flaw sizes: 50 mm length by half thickness depth Sensitivity analyses have been then conducted on following parameters: Fracture toughness Weld misalignment Residual stress level at different temperatures: ambient, LNG operating temperature C, and liquid nitrogen temperature -196 C Wall Thickness Years 150 Years Critical Initial Flaw Height (a; mm) Wall Thickness Critical Initial Flaw Height (a; mm) Years 600 Years Flaw Acceptance Criterion 3 Life = 0 Years 3 Life = 150 Years 2 Life = 300 Years 2 Life = 600 Years 1 Flaw Acceptance Criterion Critical Initial Flaw Length (2c; mm) Critical Initial Flaw Length (2c; mm) (a) (b) Figure 5: ECA on service life for girth weld (a) and long seam (b) III-2 Mechanical overmatching of weldments at cryogenic temperature Targeting overmatched welds allows to obtain a safer fracture control. In case of applied remote strain, if the yield strength of weld metal is over the one of base metal, the weld metal will be shielded, deformations in the area adjacent to welding flaws defects are minimized. Hence, overmatching welds permit larger allowable defects compared to under-matching welds. To achieve the mechanical overmatching at cryogenic temperature on alloy 36% nickel weldments, a fine chemistry trade off (Figure 6) of the filler metal shall be found. This trade off is defined as the Nickel over Titanium ratio and shall satisfy the different criteria: the weldability the overmatching at room temperature the overmatching at cryogenic LNG temperature the stability against phase transformation at cryogenic temperature under plastic deformation 5

6 Figure 6: Nickel and Titanium content influence on integrity of welding joints Aperam, Imphy alloys, based on his expertise and perfect knowledge of Fe-Ni alloys has optimized the chemistry of the existing filler metal to develop a new product: INVAR M93 TS guaranteeing mechanical overmatching properties and excellent toughness at cryogenic temperatures (LNG temperature). Overmatching properties have been characterized by superimposition of the stress strain curves of base metal and welding joints. Elasto-plastic properties have been characterized at different temperatures and a verification of the rupture out of the weld has been verified on transverse tensile tests specimen. In addition, fracture and fatigue resistance properties have been characterized on CTOD, J-R curves and Fracture Crack Growth Rate testing. III-3 Welding strategies and qualification Joint Development Agreement between TECHNIP and SERIMAX has been established for girth weld automatic welding for Main Line assembly and manual welding for repair & Tie-ins of alloys 36% Nickel linepipes. In order to set up the appropriate manual and automatic welding procedures the following parameters have been determined: Chamfering parameters: radius land and root face, bevel angle Sensitivity to misalignment and quality of fit up Quality of back purge and parameters (backing purge cycle.. ) Shielding gas: mixture (binary/ternary) and component content Welding direction: uphill vs downhill Mechanized (automatic) vs manual welding Endurance and repeatability tests 6

7 SERIMAX welding engineering has consisted in: Developing specific beveling & clamping equipment (Picture 1 a c) Optimizing weld preparation Reducing as much as possible heat input to limit grain growth in Heat Affected Zone Checking quality, repeatability and productivity (Picture 1b) All procedures have been qualified with respect to overmatching properties. a b c Picture 1: Welding bevelling and welding equipment at SERIMAX III-4 Inspection and non-destructive testing technologies Technip and Applus RTD have agreed to conduct a joint development aiming at pre-qualifying Real time Digital Radiography (Digital RT) technologies and associated equipment RAYSCAN adapted for ALLOY 36% Nickel girth weld testing. The main benefits of RAYSCAN system (Picture 02) are: Equipment designed for fast production External rotation of X-ray source and sensor Direct image processing with dedicated software 2 to 30 pipe range extendable to 36 Sensitivity better than 1% Picture 02: Applus RTD s digital radiography equipment 7

8 Digital radiography trials have been performed on alloy 36% nickel pipes and performances have been compared with conventional X-Ray radiography for several configurations: IV. RESULTS Girth weld partially welded for the inspection after root and hot passes welding Girth weld fully welded for final inspection. The following conclusions can be drawn from the finite element analyses of 36%Ni steel welds: 1. The peak residual stresses that develop during cooling from ambient to LNG service temperature do not exceed 50% of the yield strength; the size of the hot-spot is small relative to the weld and significant flaws. 2. The effect that the residual stresses that develop during cool-down from ambient to LNG service temperature have on crack driving force and stress intensity value is sufficiently small to be captured by the existing conservatism of the closed form solutions of BS Thermal cycles between ambient and cryogenic temperatures have been applied to simulate plastic accommodation due to CTE mismatch between weld metal and base metal. The superficial residual stress levels near the weld toe increased after thermal cycling but remains at a quite low value (30 % of SMYS) Engineering Critical Assessment studies allow to determine safety margin on the different factors governing the integrity of alloy 36 weldments: material toughness required properties allowable residual stress maximum acceptable misalignment at long seam and girth weld welding flaw acceptance criteria for long seam and girth weld Fatigue crack growth rate testing showed the average and mean + 2 standard deviation crack growth rates of 36% Ni steel were approximately 50% that of what is recommended by BS7910 for metals other than steel when data is not available. Fracture toughness testing was done to characterize toughness and stable tearing. The 36% Ni steel welds were significantly tougher than anticipated and the clip gauge reached its capacity before any appreciable tearing was characterized and peak load was reached. As a result, minimum values of CTOD and J-integral (crack driving force) are based on the maximum clip-gauge reading, these measurements are lower than the actual toughness. Recorded CTOD values ranged from 0.64 mm to 1.30 mm. All samples displayed 100% ductile failure and substantial blunting. 8

9 Factors for welding key success have been evaluated and can be summarized as follows: Bevelling quality Back purging quality Clamping quality to compensate pipe out of roundness and to limit the hi-lo at girth weld The digital radiography (DR) is superior to conventional radiography (RT) in image quality, sensitivity, and flaw detection threshold. The digital radiographs resolved wire W15 or compared to wire W12 on the conventional radiographs V. CONCLUSIONS Wide plate testing was done to characterize failure modes based on a more structurally relevant geometry and a surface breaking notch. In every case the notch tore through thickness before failure and 100% ductile failure which indicates that leak before burst mode of failure has been achieved ( Figure 7a). Most of wide plate samples indicated a substantial amount of blunting occurred prior to tearing, and tearing propagated preferably through the based metal over the weld, (Figure 7b), which indicates that the weld metal overmatches the based metal. a) b) Fig 7a-b: Demonstration of leak before burst failure and overmatching during curved wide plate testing As recommended by the DNV OS F101 [5] to confirm the overmatching of welding joints, the rupture out of weld of transverse tensile testing have been verified at different temperatures (Figure 8 a-b-c) for long seam and girth weld. 9

10 a) b) c) Figure 8: Transverse tensile testing specimen of girth welds a) ambient temperature b) -163 C c) -196 C REFERENCES [1 ][Technip] Cox, P., Risi, R., How the Use of Cryogenic Piping Can Reduce the Impact of LNG Transfer Terminals on Environment and Local Communities, While Increasing Site Safety and Security, 2009 Offshore Technology Conference, Houston, TX USA, 2009 [2] [Anderson] Anderson, T. L., Fracture Mechanics Fundamentals and Applications, Third Edition, CRC Press, FL, USA, 2005 [3] [ASTM] ASTM A : Standard Specification for Seamless and Welded Steel Pipe for Low- Temperature Service [4] [BS7910] BS7910: 2005, Incorporating Amendment 1, Guide to methods for assessing the acceptability of flaws in metallic structures, British Standards Institution, London, 2005 [5] [DNV] DNV OS F101: 2010 Submarine Pipeline System, Det Norske Veritas. 10