Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
Preliminary Determination Of Hot Sour Brine Environmental Effect On Fatigue Crack Growth Rate In Grade 29 Titanium Pipe Base And Weld Metal
Schutz, Ronald W. (RTI Energy Systems, Inc. ) | Thodla, Ramgopal (DNV Columbus, Inc. ) | Baxter, Carl F. (RTI Energy Systems, Inc.) | Caldwell, Christopher S. (RTI Energy Systems, Inc.)
ABSTRACT Grade 29 Titanium is an established, prime candidate material for critical dynamic offshore riser components; particularly for deepwater and/or corrosive, sour High Pressure/High Temperature (HPHT) field developments, stemming from its unique, synergistic combination of properties. These include an elevated strength-to-density, low elastic modulus, and exceptional resistance to corrosion and corrosion fatigue in seawater and sour environments even at high temperatures. Recently published laboratory S-N testing of Grade 29 titanium Gas Tungsten Arc-welded riser pipe specimens in air vs. sour brine at 150°C (302°F) revealed no significant knockdown in fatigue life under sour conditions, in contrast with steels. The next pertinent question relating to riser fatigue/fracture mechanics design must address the influence of sour environments on Grade 29 titanium's fatigue crack growth (FCG) resistance. The laboratory study described in this paper utilizes “frequency scans” to compare Stage 2 FCG rates of Grade 29 Ti pipe base and weld metal at various loading frequencies in air versus sour brine at 150°C (302°F), as measured at a fixed stress intensity factor range ?K value and high, closure-corrected stress ratio. Test results indicate no discernable effect of the hot sour environment and loading frequency on FCG rate of Grade 29 Ti pipe base and weld metal INTRODUCTION The growing development of deeper, hotter, and increasingly sour oil/gas well reservoirs worldwide is creating serious design challenges for cyclically loaded offshore steel components. Traditional quench and tempered low-alloy steel (e.g., 448 MPa (65 ksi) minimum yield strength) pipelines used in dynamic risers and subsea flowlines can be highly susceptible to corrosion fatigue while exposed to H2S in sour crudes or sour brine well fluids. The primary mechanism involves corrosion of the steel by acidic H2S (with or without CO2) gas, and enhanced by-product nascent hydrogen absorption induced by the sulfide species (i.e., cathodic hydrogen embrittlement). Specifically, steel pipeline base and weld metal can suffer a 10-100 factor reduction (i.e., knockdown factor) in S-N (cyclic stress-cycle to failure) fatigue life1,2 and similar increase in fatigue crack growth (FCG) rate,3-11 depending highly on steel hardness/strength, loading frequency, H2S partial pressure, and media pH. This often makes safe fatigue design life very difficult to achieve with steel in these sour field developments. One viable solution to this steel sour service limitation involves the incorporation of titanium alloy components into areas of dynamic offshore riser and flowlines systems that experience significant fatigue loading. For example, Figure 1 reveals primary candidate areas of high bending/fatigue stresses in offshore steel catenary risers (SCRs) and top-tensioned risers (TTRs) systems where titanium's combination of high strength, low elastic modulus, and exceptional corrosion resistance to seawater and bore well-fluids makes it a logical choice.12,13,14 In fact, more than eighty titanium alloy taper stress joints (TSJs) used for termination of offshore SCRs and TTRs deployed worldwide demonstrate practical and successful utilization for bend- and fatigue-critical oil/gas production components.
INTRODUCTION ABSTRACT Based on its exceptional corrosion and fatigue resistance, high strength-to-density ratio, and low elastic modulus, Grade 29 titanium represents an attractive candidate material for dynamic offshore steel catenary riser components such as taper stress joints (TSJs), and touch-down zones (TDZ) sections, and deepwater intervention and production risers for HPHT and XHPHT service. Traditional steel alloy riser components are known to exhibit substantial S-N fatigue life knockdown (i.e., corrosion fatigue) when exposed to sour-rich well fluids, making safe fatigue design life difficult or even unachievable. Similar to its documented corrosion fatigue resistance in hot seawater, Grade 29 titanium welded pipe joint specimens tested in this lab study exhibited neither statistically significant corrosion, hydrogen absorption, nor reduction in S-N fatigue life when exposed to a sour NaCl brine at 150°C (302°F). A test frequency effect was observed, which probably stems from sustained-load strain occurring during the time when peak stresses exceed the alloy's proportional limit in the loading cycle. As such, normalization of air and sour test frequencies at the lower value would have further diminished any difference between the two population means in this study, suggesting little or no sour knockdown for Grade 29 Ti pipe welds in this lower cycle regime study. Additional S-N testing in the higher cycle regime at normalized, low frequencies is recommended to establish a design curve. The growing development of deeper, hotter, and increasingly sour oil/gas well reservoirs worldwide is creating serious design challenges for cyclically loaded offshore steel components. Traditional quench and tempered low-alloy steel (e.g., X65) pipelines used in dynamic risers and subsea flowlines can be highly susceptible to corrosion fatigue while exposed to H2S in sour crudes or sour brine well fluids. The primary mechanism involves corrosion of the steel by acidic H2S (with or without CO2) gas, and enhanced by-product nascent hydrogen absorption induced by the sulfide species (i.e., cathodic hydrogen embrittlement). Specifically, steel pipeline base and weld metal can suffer a 10-100 factor reduction (i.e., knockdown factor) in S-N fatigue life and/or similar increase in fatigue crack growth rate, depending highly on H2S partial pressure and pH. This often makes safe fatigue design life very difficult to achieve in these sour field developments. One viable alternative or solution to this steel sour service limitation involves incorporation of specific titanium alloy components into dynamic offshore riser and/or flowlines system areas where these limitations occur. For example, Figure 1 reveals primary candidate areas of high bending/fatigue stresses in offshore steel catenary risers (SCRs) and top-tensioned risers (TTRs) systems where titanium's combination of high strength, low elastic modulus, and exceptional corrosion resistance to seawater and bore well-fluids makes it a logical choice. In fact, more than eighty titanium alloy taper stress joints (TSJs) used for termination of offshore SCRs and TTRs developed worldwide demonstrate practical and successful utilization for critical dynamic oil/gas production components. A recent JIP investigated the titanium alternative to steel in the touchdown zone (TDZ) section of a deepwater production riser (SCR) in the Gulf of Mexico.
- Research Report > New Finding (1.00)
- Research Report > Experimental Study (0.86)
- Geology > Geological Subdiscipline > Geomechanics (0.54)
- Geology > Mineral (0.49)
Abstract Titanium alloys have been used for stress joints at the top of SCRs for approximately eight years. The impact on the riser design, hang-off structure and installation of a stress joint is discussed. More severe loading and internal fluid conditions have also given rise to interest in using titanium in the TDZ, and several studies have shown great advantages, mainly in improved fatigue life. Practical methods of incorporating the alloy into an SCR are presented. Many design codes do not fully address titanium alloys, and some characteristics of the material require consideration when establishing design load capacities and fatigue behavior. These are discussed and recommendations for safe design are given. Finally, the operational requirements to ensure integrity throughout the design life are presented. Introduction Titanium alloys have a unique combination of properties, such as high strength, low elastic modulus and density, excellent fatigue resistance together with high chemical resistance, which make them attractive for use in offshore riser systems. Titanium alloys are now routinely used for tapered stress joints at the upper termination of SCRs, and at the subsea wellhead of some TTRs, as listed in Table 1. In these applications, they are normally located in the highest load and fatigue zone of the riser, and those used on production SCRs are continually exposed to produced well fluids. Recently, extension of production into deeper water and harsher environmental conditions has generated interest in extending the application of titanium alloys to the TDZ where substantial improvements in fatigue life can be achieved over steel. This increased interest has also been due to the discovery of substantial HPHT reservoirs, often associated with more corrosive and sour well flows. This paper is aimed at the riser designer. It presents a brief guide to the selection of titanium alloys and their properties, and solutions to issues arising from integrating them into an SCR. Guidance is given on the design of tapered stress joints (TSJs), and support structures at the platform. Aspects of manufacturing of which the designer should be aware, and typical installation methods are discussed. Similar information is provided for titanium alloy riser sections at the TDZ. Finally, guidance is offered concerning the compatibility of relevant titanium alloy components with typical injected and workover well chemicals, and completion fluids. Design Codes A recommended practice(1) for the design of titanium risers was published by DNV in 2002. The increasing use and acceptance of titanium as a riser material is reflected in its inclusion in the new API/ISO riser design code (ISO 13628- 12) currently being drafted. Other codes used for riser design could be used for titanium riser design, but the DNV recommended practice has been ratified by testing, and the results will be incorporated into the new API/ISO code. Alloy Selection & Properties Alloy Types There are primarily two grades of titanium commonly used for catenary risers:ASTM Grade 23 Titanium: (UNS R56407) Nominally Ti-6%Al-4%V ELI (extra-low interstitial, 0.13% max. O) ASTM Grade 29 Titanium: (UNS R56404) Nominally Ti-6%Al-4%V-0.1%Ru ELI (extra-low interstitial, 0.13% max. O)
- Materials > Metals & Mining > Titanium (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.95)