Paulo Gioielli

Development of the SENT Test for Strain-Based Design of Welded Pipelines

Strain-based design (SBD) pipelines are being considered to develop hydrocarbon resources in severe environments. As part of a research program to develop a SBD methodology, work was conducted to develop a suitable fracture mechanics test that can be used as part of a strain capacity prediction technique. The single edge notched tensile (SENT) specimen geometry has been chosen due to the similarity in crack-tip constraint conditions with that of defects in pipeline girth welds. This paper describes a single-specimen compliance method suitable for measuring ductile fracture resistance in terms of crack tip opening displacement resistance (CTOD-R) curves. The development work included investigation of the following items: specimen geometry, crack geometry and orientation (including crack depth effects), direct measurement of CTOD. The results demonstrate that toughness measurements obtained using a B = W configuration (B = specimen thickness, W = specimen width) with side grooves are similar to those using a B = 2W configuration without side grooves; however, specimens with side grooves and B = W geometry facilitates even crack growth. Studies of crack depth have shown that ductile fracture resistance decreases with increasing ratio of the initial crack depth to specimen width, a0 /W. Studies of notch location and orientation (outer diameter (OD) and inner diameter (ID) surface notches and through-thickness notches) have shown an effect of this variable on the CTOD-R curves. This has been partly attributed to crack progression (tearing direction) with respect to weld geometry and this effect is consistent with damage modeling predictions. However the experimentally observed difference of CTOD-R curves between ID and OD notches is believed to be primarily due to the material variability through the pipe thickness.Copyright

VIV Response of a Subsea Jumper in Uniform Current

Subsea jumpers are susceptible to in-line and/or cross-flow vortex induced vibration (VIV) fatigue damage due to sea bottom currents. However, there is no proven industry standard design analysis methodology currently available specifically for assessing subsea jumper VIV response.In 2012, ExxonMobil conducted a jumper VIV model test to assess the validity of potential jumper VIV prediction approaches. A towing test rig was used to expose a small scale jumper model to flow conditions simulating uniform bottom currents. The jumper model was instrumented to acquire acceleration, bending strain and end connection load data. Several accelerometers and strain gauges were installed to enable reconstruction of static and dynamic deformations and bending deflections along the jumper model. Towing tests at different orientations and tow speeds were performed on both a bare pipe model and a straked pipe model. The data were analyzed to examine the frequencies and amplitudes of the jumper vibration. The data from these experiments provide a benchmark for validating jumper VIV prediction approaches.In this paper, the model test program is presented including model testing philosophy, jumper design and fabrication, and high level model test results.Copyright

SCR Application for Turret Moored FPSOs in West Africa

Turret moored Floating Production Storage and Offloading (FPSO) systems with flexible risers have been successfully used in many deepwater developments in West Africa. However, Steel Catenary Risers (SCR) have not been used on a turret moored FPSO because of concerns with the riser fatigue and compression loading due to the large FPSO motions at the turret location. ExxonMobil has initiated a development program to establish the feasibility of SCRs on turret moored FPSOs. The goal of this work effort is enable the use of SCRs on West African turret-moored FPSOs and thereby expand the riser concept options to bring potential economic advantage for future prospects. Additionally, the SCRs may be needed to comply with local content requirements. The development program consists of identifying the limitations of a conventional SCR design, establishing feasible SCR concepts that meet the strength and fatigue requirements, and pre-qualification of a high fatigue performance welding technology. The program focuses on alternate SCR concepts including SCRs with locally weighted and buoyant sections. This paper presents the SCR concepts and the associated analysis results which demonstrate the elimination of compressive loads in the touch down area and the achievement of adequate weld fatigue performance throughout the riser. This paper further presents the results of the inconel weld pre-qualification program including fatigue tests on large and small diameter pipe to demonstrate enhanced fatigue performance.Copyright

Evaluation of Welding Flaws Under Ratcheting Fatigue

Hydrocarbon-carrying lines can be subjected to cyclic loads superimposed on monotonically increasing mean strains well into the plastic domain, resulting in tearing and tearing fatigue of initial welding flaws. This combined demand is referred to here as ratcheting fatigue. Examples of these loads are frost-heave in pipelines and thermal cycling of flowlines. This paper presents the experimental verification of a fracture mechanics model of monotonic and cyclic crack extension under ratcheting fatigue loads and its calibration to small-scale tests. The model is an extension of one currently used to predict tearing and tear-fatigue due to reeling. Crack driving forces (J-solutions) under load- and displacement-control conditions were derived and used with the model to predict test results. A total of 24 single-edge notched bend (SEN-B) specimens, taken from a welded riser, were tested for crack extension under combined monotonic and cyclic loads. Comparisons of predicted to measured fatigue crack-growth rates, and alternatively cyclic J-R curves, provide quantitative and qualitative validation of the model. However, calibration to large–scale tests are needed before the model can be used for design. ExxonMobil has already completed the first set of large-scale pipe tests under ratcheting fatigue loading, including internal pressure.Copyright

Full Size Fatigue Crack-Growth Testing for Girth Welds

Fatigue crack-growth modeling has a significant impact in establishing defect acceptance criteria for the inspection of fracture-critical, girth-welded components, such as risers and tendons. ExxonMobil has developed an experimental technique to generate crack-growth data, in actual welded tubulars, that account for the particular material properties, geometry, and residual stresses. The technique is fully compatible with conventional fracture mechanics models. It uses a series of pre-designed notches made around the welds on a production quality, full-scale specimen that is tested efficiently in a resonant fatigue setup. The crack development from notches is monitored during testing and evaluated post-mortem. Given its simplicity and high loading frequency, the technique provides growth data germane to the component at hand at a lower cost and faster than standard, small-scale tests.Copyright