Mons Hauge

A notched cross weld tensile testing method for determining true stress–strain curves for weldments

Abstract Cross weld tensile testing is widely used in the industry to qualify welds. In these conventional testing fracture load is measured and the location of fracture (weld metal, base metal or heat affected zone) is evaluated. Because the load-elongation curve depends on the location of fracture and the initial gauge length, it cannot be utilized in the failure assessment of weldments. Failure assessment of weldments requires input of true stress–strain behaviour for each material zone. In this paper, a notched cross weld tensile testing method is proposed for determining the true stress–strain curve for each material zone of a weldment. In the proposed method, cylindrical cross weld tensile specimens, with a notch located either in the weld metal, base metal or possibly heat affected zone are applied. Due to the notch, plastic deformation is forced to develop in the notched region. A load versus diameter reduction curve is recorded. It has been shown that the true strain at maximum load is independent of the notch geometry. Furthermore, the materials true stress–strain curve can be determined from the recorded load versus diameter reduction curve of a notched cross weld tensile specimen by dividing a geometry-factor G, which is approximated by a quadratic function of the specimen diameter to notch radius ratio and a linear function of the true strain at the maximum load. It is found that G is independent of the material zone length when the homogenous material length is larger or equal to the minimum diameter.

Constraint effects on crack tip stress fields for cracks located at the fusion line of weldments

Abstract The case of a deeply notched (a/W=0.3) surface crack positioned at the fusion line of a weldment is considered. The tensile properties of the base material and the heat affected zone (HAZ) are kept constant, while the weld metal properties are changed. First the weld metal yield strength overmatches both the base material and the HAZ, and in the second case there is yield strength evenmatch between the HAZ and weld metal and overmatch with respect to the base material. The effect of these strength mismatch conditions has been examined for two fusion line geometries: straight and slant fusion line. The effect of the crack tip constraint has been characterized with the J–Q–M approach where both geometrical and material mismatch effects can be considered with respect to the critical conditions for cleavage fracture initiation.

J-Q-M Approach for Failure Assessment of Fusion Line Cracks: Two-Material and Three-Material Models

The theoretical background for the J-Q-M approach for quantifying the constraint in weldments for fusion line cracks is presented. In this model, Q quantifies the geometry effects and M the material mismatch effects. Initially the approach was developed for a two-material modified boundary level (MBL) model, but later was extended to include three materials: weld metal, heat-affected zone and base material, and more realistic specimen geometries. The analysis with MBL models showed that the effect of mismatch was rather independent of the T-stress for both bi- and tri-material models, indicating that Q and M could be treated independently. However, analysis of fracture mechanics tension specimens made of three materials revealed that the mismatch effect in some cases could depend on the geometry effects. New calculations have demonstrated that the dependence/independence is related to load level, ratio of mismatch, and the local geometry.

Fatigue of 28-inch Titanium Riser: SN Data and Defect Assessment

Qualification testing was carried out on full scale titanium pipes for a gas export riser from the Asgard B platform. Prototype pipe sections with girth welds produced by the TIG process were delivered from three different suppliers. The wall thickness was 30 mm. The pipes were sectioned into fatigue specimens and tested in constant amplitude loading in air. The loading was transverse to the weld, simulating axial and bending loading of the pipe. Secondary stresses were measured on each specimen. SN data were obtained for the welds and compared with proposed design curves. The fracture surfaces were investigated with scanning electron microscopy, and defects were categorised and sized. Fracture mechanics analysis was carried out and the results were compared with experimental data. Problems relating to fracture mechanics modelling for fatigue life prediction and defect assessment of titanium welds were discussed.© 2002 ASME

Cost Effective Fabrication of Large Diameter High Strength Titanium Catenary Riser

A large diameter high strength titanium free-hanging catenary riser was evaluated by the Demo 2000 Ti-Rise project, from initiative of the Kristin Field development license. In order to reduce the uncertainties related to the schedule, cost, and special technical issues identified in the work related to a similar riser for future installation on the Asgard B semi-submersible platform, a fabrication qualification of a full scale riser in titanium was run. Several full-scale production girth welds were made in an in-situ fabrication environment. The welding was performed on extruded titanium grade 23 (ASTM) pipes with an ID of 25.5″) and wall thickness of 30 mm. The main challenge was to develop a highly productive TIG orbital welding procedure, which produced welds with as low pore content as possible. It is well known that sub-surface pores often are initiation sits for fatigue cracks in high strength titanium welds. This paper describes how a greatly improved productivity was obtained in combination with a high weld quality. NDT procedures were developed whit the main on the reliability to detect and locate possible sub-surface weld defects, volumetric defects such as pores and tungsten particles and planar defects such as lack of fusion. The results from the actual Non Destructive Testing (NDT), the mechanical testing, and the fatigue testing of the subjected welds are presented. The response of the catenary is optimised by varied distribution of weight coating along the riser’s length. A satisfactory weight coating with sufficient strength, bond strength, and wear properties was developed and qualified. The riser is planned to be fabricated from extruded titanium pipes, welded together onshore to one continuous piece. The field coating is added and the riser is loaded into the sea and towed offshore and installed.Copyright

Material Challenges in Arctic Areas

There is a lack of rules and standards that provide guidelines for material selection and qualification of materials for offshore and onshore structures in arctic areas. Many current standards for low temperature applications such as cryogenic piping and process systems do not reflect the need for low-cost bulk materials for large volume applications such as pipelines and production facilities. The growing focus on oil and gas exploration in arctic areas has raised the need for new standards and industry practice that supports cost effective and safe installation and operation of production and transport facilities in the cold climate. There are materials today that are applicable for low temperature conditions. The grades are often highly alloyed (typically 3–9% Ni) with good toughness properties, but these alloys are expensive compared to conventional steel material grades. Such materials may not be applicable in pipelines, structures and process plants. This challenge can be met in two ways. First, structural steels that are capable of being welded and operated in the cold climate should be developed and qualified. Second, materials for forged and casted components that can be welded to the structural steels should be developed and qualified to fit into the integrated structure or pipeline system. Some actions have been taken to develop new standards e.g. within ISO19906, and actions are being taken in Russia to harmonize their specifications with the international standards, but this is a comprehensive job and the work must be executed in parallel with the development of new steels and welding technology.Copyright

Fracture Control — Offshore Pipelines: Current Status of Fracture Assessment for Pipelines, Limitations and the Need for Development

Recent development plans envisage the exploitation of very deep offshore reservoirs as well as transport of hydrocarbons at temperature and pressure conditions far more severe than in past projects. Technical feasibility of such projects requires higher material utilisation, and the design guidelines need to be improved to allow for the new design conditions. Fracture assessment methods have been used in the evaluation of pipeline integrity for several years. In particular, the verification of acceptable defect sizes for installation and operational loads are now widely used and assessment methods are referenced in pipeline standards and guidelines. However, design guidelines are still missing the calibrated safety factors and stringent design format required to let the fracture failure mode be consistent with the other failure modes in the pipeline design such as bursting, local buckling and fatigue. The Fracture Control Offshore Pipelines Project is a Joint Industry Research and Development Project, whose objective is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. Due to the envisaged high loading condition and the high costs of recent offshore pipeline projects it is important, with an accurate defect assessment analysis, to avoid delays caused by unnecessary repairs or failures because of flaws that should have been detected and repaired. The final objective is the development of specific design criteria in the form of a design guideline to be used in the verification and design of offshore pipelines against the fracture/plastic collapse failure of a defected girth weld. The design criteria are based on the application of reliability methods to calibrate the partial safety factors in compliance with the safety philosophy established by DNV OS-F101 and will include the rational application of new NDT techniques. The JI Project is carried over 5 years and has started in 2002. The JI project is sponsored by the industry (BP, ENI Norge, Hydro and Statoil) and by the Norwegian Research Council. This paper describes the current status of existing fracture assessment procedures for pipelines with particular attention to their limitations and the needs for development and a brief overview of the results obtained in the project so far as well as the challenges to be solved in the project.Copyright