Design Considerations for the Use of Ultrasonic Phased Array Probes in Weld Inspection

Ultrasonic testing of metal welds has been in use for many years. Scanning methods using both contact and immersion methods are often used at the time of manufacture and also during periodic in-service inspection programs. But because of a variety of component configurations and potential flaw geometries it is often necessary to perform several inspections, each with a different probe configuration to assure adequate defect delegability. It is possible that a properly designed phased array probe can perform several different inspections without changing hardware thereby reducing inspection times. This presentation reviews the design and operation of ultrasonic phased array transducers and the necessary features to achieve the desired performance. Situations in which these probes have already been implemented effectively are also discussed.

Rational Modeling of Ultimate Pipe Strength Under Bending and External Pressure

The capacity of pipelines to resist collapse or local buckling under a combination of external pressure and bending moment is a major aspect of offshore pipeline design. The importance of this loading combination increases as oil and gas projects in ultra deep-water, beyond 2,000-m water depths, are becoming reality. The industry is now accepting, and codes are explicitly incorporating, limit state design concepts such as the distinction between load controlled and displacement controlled conditions. Thus, deep-water pipeline installation and limit state design procedures are increasing the need to understand fundamental principles of offshore pipeline performance. Design codes, such as API 1111 (1999) or DNV (1996, 2000), present equations that quantify pipeline capacities under combined loading in offshore pipelines. However, these equations are based on empirical data fitting, with or without reliability considerations. Palmer (1994) pointed out that “it is surprising to discover that theoretical prediction [of tubular members under combined loading] has lagged behind empirical prediction, and that many of the formula have no real theoretical backup beyond dimensional analysis.” This paper addresses the ultimate strength of pipelines under combined bending and external pressure, especially for diameter-to-thickness ratios, D/t, less than 40, which are typically used for deep water applications. The model is original and has a rational basis. It includes considerations of ovalization, anisotropy (such as those caused by the UOE pipe fabrication process), load controlled, and displaced controlled conditions. First, plastic analysis is reviewed, then pipe local buckling under pure bending is analyzed and used to develop the strength model. Load controlled and displacement controlled conditions are a natural consequence of the formulation, as well as cross section ovalization. Secondly, external pressure effects are addressed. Model predictions compare very favorably to experimental collapse test results.

Leak Detection Systems for Short Pipelines

This paper describes procedures developed by PETROBRAS Research & Development Center to assess a software-based leak detection system (LDS) for short pipelines. These so-called “Low Complexity Pipelines” are short pipeline segments with single-phase liquid flow. Detection solutions offered by service companies are frequently designed for large pipeline networks, with batches and multiple injections and deliveries. Such solutions are sometimes impractical for short pipelines, due to high cost, long tuning procedures, complex instrumentation and substantial computing requirements. The approach outlined here is a corporate approach that optimizes a LDS for shorter lines. The two most popular implemented techniques are the Compensated Volume Balance (CVB), and the Real Time Transient Model (RTTM). The first approach is less accurate, reliable and robust when compared to the second. However, it can be cheaper, simpler, faster to install and very effective, being marginally behind the second one, and very cost-efective. This paper describes a procedure to determine whether one can use a CVB in a short pipeline.

Pipeline Compressor Redesign With the Consideration of Both Noise and Performance

A complete effort to redesign the aerodynamic characteristics of a single-stage pipeline compressor is presented. The components addressed are the impeller, diffuser region, and the volute. The innovation of this effort stems from the simultaneous inclusion of both the noise and aerodynamic performance as primary design parameters. The final detailed flange-to-flange analysis of the new components clearly shows that the operating range is extended and the tonal noise driven by the impeller is reduced. This is accomplished without sacrificing the existing high efficiency of the baseline machine. The body of the design effort uses both Computational Fluid Dynamics (CFD) and vibro-acoustics technology. The predictions are anchored by using the flange-to-flange analysis of the original design and its experimental performance data. By calculating delta corrections and assuming that these deltas are approximately the same for the new design, the expected performance is extrapolated.

Freezing Technique Applied to an Offshore Pipeline at Campos Basin in Brazil

This paper describes an offshore pipeline freezing technique that was used to isolate a gas pipeline to effect repairs and to and perform associated tests. The freezing technique was used after evaluation and land testing and was successful in returning the pipeline to production much sooner that the other compared conventional methods. This technique reduced the amount of water used in the gas pipeline and enabled the repairs to be pressure tested prior to returning the pipeline to service.

A Pipeline Dent Assessment Model Considering Localised Effects

The Canadian Pipeline Design Standard (CSA Z662) [1] requires the repair of smooth dents with depths exceeding 6% of the pipeline’s outside diameter. This limit on dent depth is reduced in the presence of additional localised effects such as pipe wall gouges, corrosion or planar flaws. Furthermore, it has been observed that pipe wall metal loss, planar flaws and weld seam interaction with dents can significantly reduce the service life of a dented pipe segment. A previously developed pipeline dent assessment model, based on the actual dent profile and in-service pressure history applied to non-linear pipe finite element model with a fracture mechanics crack growth algorithm, has been used to explore the consequences of these localised effects. The effects of corrosion (uniform or local pitting), weld seams (including their weld toe stress concentration effects and residual stress fields), planar flaws (cracks) and gouges on the service life of a dent are reviewed in this investigation. The performance of the model is demonstrated based on its agreement with field observations. The dent assessment model application and validation processes has indicated that the model presented here can be reliably used to predict the service life of dented pipelines in the presence of various localised effects.

The Business of Pipeline Simulation

Pipeline simulation is the creation and operation of a “virtual pipeline” that closely replicates the physical pipeline and its operation. Pipeline simulation uses mathematics, specifically the laws of fluid mechanics and conservation of mass and energy to represent the hydraulic behavior of the fluid. A simulation can be as simple as calculating pressure drop in a single pipeline segment, or as complex as taking live measurement data from an operating pipeline network and using these data to drive a virtual pipeline side-by-side with its real-world counterpart. A virtual pipeline is also an extremely effective tool for training pipeline operators. Critics of Information Services (IS) departments coined the phrase “islands of automation” to characterize the uncoordinated evolution of their Information Technology (IT) infrastructures. Pipeline simulation, as a compute-intensive application, has been critically dependent on the available IT platforms, and has been marooned on these islands for years. This paper charts how the application of simulation technology has changed over time as the available IT platforms have evolved. It offers suggestions as to how, in the coming years, pipeline simulation applications will be freed from these technology shackles as IS departments embrace new architectures in response to corporate business requirements.

Hydrogen Permeation Behavior of X-70 Pipeline Steel in a Near-Neutral pH Soil Environment

This paper reports the hydrogen permeation behavior of an X-70 pipeline steel in a synthetic near neutral pH field solution under both galvanostatic and potentiostatic conditions. The hydrogen flux through the steel exposed to the solution is influenced by the chemical reactions at the steel surface on the charging side. Results from tests done under potentiostatic and galvanostatic conditions were compared to predictions based on different boundary conditions assumed in solving Fick’s diffusion equations. Constant concentration boundary conditions gave the best fit. It was also found that the diffusible hydrogen generated in a near neutral pH soil environment is likely to be 3 to 10 times lower than the minimum hydrogen concentration required to initiate hydrogen blistering in pipeline steels.

Effect of Surface Scratch Roughness and Orientation on the Development of SCC of Line Pipe Steel in Near Neutral pH Environment

This research has focused on the effect of surface roughness and surface scratch orientation on the development of neutral pH SCC on pipeline steels. The susceptibility to neutral pH SCC was assessed in this study using slow strain rate testing on X-65 line pipe steel. The surfaces of the test samples were ground using either #240 or #600 sandpaper to introduce different scratch roughness. Scratches were produced with an orientation parallel, perpendicular and inclined at 45° to the loading direction, respectively. The test samples were exposed to a synthetic neutral pH ground water at both open circuit potential (OCP) and −800 mV (SCE). It has been found that the reduction in ductility due to a near neutral pH environment was more than 20% larger for the specimens with perpendicular scratches than those with parallel scratches, either at OCP or −800 mV (SCE). Roughness appeared to have little effect on the ductility of the specimen with parallel scratches. However, it has some effect on the sample with perpendicular scratches. For these samples, a finer scratch caused more reduction in ductility than the rougher scratches, particularly under cathodically protected conditions. The reduction in ductility for the specimen with scratches 45° inclined to stress axis was in between those with parallel and perpendicular scratches. The difference in ductility arising from scratch roughness and orientation was consistent with the observation of the surface conditions after test. For samples with perpendicular and angle scratches, cracks were seen to coincide with scratch lines. For those with parallel scratches, only short cracks developed in a direction approximately 45° to the stress axis. Mechanisms concerning this scratch-facilitated crack formation are discussed.

Mechanical Factors Affecting Stress Corrosion Crack Growth Rates in Buried Pipelines

Over the past several years, investigations have been carried out into the rate of crack growth in pipeline steels in simulated, near-neutral pH, groundwater environment (NS4 solution). Pre-cracked specimens were subject to constant amplitude loading under various frequencies, maximum loads and R-ratios (minimum/maximum load). Test times varied from about 20 to 400 days. Transgranular crack features, similar to those found in service, have been observed. The extent of crack growth was monitored using either electrical potential drop or detailed metallographic examinations at two laboratories. The resulting crack growth rates from both labs are consistent with a superposition model based on a summation of fatigue (Paris Law) and static (SCC) crack growth rates. Differences between the results at the two laboratories are discussed.