Weld misalignment study

A parametric study on the effect of weld misalignment on the local buckling response of pipelines, by Aiman Al-Showaiter, MCS Kenny, Houston, TX, USA, and formerly of Department of Civil and Resource Engineering, Dalhousie University, Halifax, NS, Canada; Professor Farid Taher, Department of Civil and Resource Engineering, Dalhousie University, Halifax, NS, Canada; and, Dr Shawn Kenny, Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St John’s, NL, Canada.

Pipelines traverse various terrains and may be subject to ground movements such as subsidence, slope movement, frost heave, thaw settlement, or offshore ice gouging, and it is therefore essential to understand their reliability and integrity. Moreover, the mechanical behaviour of shell structures, such as pipelines, is ‘imperfection sensitive’, and the presence of initial geometric or material imperfections can significantly affect the pipeline load and deformation capacity.

This paper looks at the influence of initial geometric imperfections associated with joint-to-joint offset misalignment that may be present due to the girth welding process when connecting pipeline segments. This investigation was conducted using finite-element methods to assess the effects of internal pressure, axial force, misalignment amplitude, and misalignment orientation on the local buckling response of pipelines. Through this parametric analysis, the moment-curvature response and variation in section geometry with increasing global curvature was examined. Among the authors’ conclusions, they point out that misalignment imperfection has a detrimental effect on both the load capacity and the deformation capacity of the pipeline. Depending on the internal pressure ratio and the misalignment amplitude, a reduction of up to 10 per cent in the moment capacity, and a reduction of up to 50 per cent in the deformation capacity (the curvature at the peak moment) are observed. However, increasing the internal pressure results in reduction in load capacity and increases in the curvature at the loading limit, which gives rise to a more stable pipeline response. Multi-diameter, bi-directional pigging

Multi-diameter, bi-directional pigging for pipeline precommissioning, by Magne Andreas Vik and Alf Åge Kristiansen, StatoilHydro, Haugesund, Norway; Simon Sykes, FTL Seals Technology, Southampton, UK; Steve Hutcheson, Pipeline Pigging Technology, Chesterfield, UK, and, Dr Aidan O’Donoghue, Pipeline Research Ltd, Glasgow, UK.

The paper examines the issues associated with multi-diameter, bi-directional pigging, specifically for pipeline precommissioning. The technique can be used to flood and subsequently dewater a pipeline without the need for temporary subsea traps.

An example of such a technique was used on the 16 km Alve pipeline in Norway. The Alve flowline runs from the Norne platform to the Alve manifold, and includes 10 inch and 12 inch pipe sections. The line was flooded from Norne using pigs with oxygen-scavenged seawater, and then dewatered using nitrogen and produced gas from the well. The pigs needed to have a high sealing efficiency since very low velocities were used to flood the line, and in order to avoid hydrates on dewatering. Multi-diameter wheel pigs were employed with non-buckling disc-type seals. This paper describes the design of the pigs and the seals to achieve the required functionality. The test facility and testing performed to verify the pig performance is also illustrated. Finally, an overview of the offshore pigging operation is provided.

The project showed that the multi-diameter bi-directional approach to precommissioning potentially rules out the need for a subsea pig receiver or launcher and the consequent extra support vessel. The ability to make a single-disc seal work over a wide range of diameters is key to this development, as is the wheel pig to allow the pig to be maintained on the centreline. The Alve pipeline has an approximately 25 per cent increase in diameter, and StatoilHydro is now examining a 12 inch x 16 inch case with a 41 per cent change in diameter, where a similar approach is being adopted with further work on the single seal concept being examined.

External coating assessment

Augmenting ILI tools to assess external coatings, by Dr J Bruce Nestleroth, Energy Systems, Battelle Memorial Institute, Columbus, OH, USA; and, Jason K Van Velsor, FBS, Inc., State College, PA, USA.

The coatings used to protect oil and natural gas pipelines can degrade over time, enabling corrosion and stress-corrosion cracks to initiate and grow. Pipeline companies use in-line inspection (ILI) tools to detect these anomalies, and repair methods to mitigate the result of a failed coating. This project developed inspection sensors that can prevent these anomalies from occurring by monitoring the integrity of the external protective coating of the pipeline. This coating assessment, which could be performed during a typical ILI, could help pipeline owners assess the general health of the coating protecting their pipeline system. A goal of the design was to keep these sensors simple so that an implementation would not add substantial cost or complexity to a typical magnetic-flux leakage (MFL) or caliper survey.

The sensor system was designed to generate the proper wave type and modes to assess coating conditions. Non-contact electromagnetic acoustic transducers (EMATs) were designed to send guided ultrasonic waves around the circumference of the pipe, as a result of which very few sensors were required for implementation. For pipes less than 20inches in diameter, only two sensors are required: one for sending the wave, and a second for receiving. For larger diameter pipes, four EMATs were used for 100 percent circumferential coverage, two EMATs reserved for the excitation of ultrasonic energy and two for receiving the signal. This number of EMATs is an order of magnitude less than the number of bulk-wave ultrasonic transducers that would be required to obtain only partial circumferential coverage.

The coating assessment capability was experimentally demonstrated using a prototype EMAT ILI tool, and all three detection features were shown to perform well in an ILI environment. Improvement to the prototype were made between each test, the most significant of which was the design and construction of a novel set of thick-trace transmitting and receiving printed circuit board (PCB) EMAT coils. These coils were designed very specifically to be capable of handling the high current densities created by the 1,200 V amplifier. Implementation variables, such as moisture and soil loading, were shown to have a minimal influence on the results.

A common approach for assessing coating condition is to assess the amplitude of the received signal. Low amplitude means that the coating is good, because the ultrasonic energy went into the coating and was absorbed, while high signal amplitude means that the coating was not intact. While this works reasonably well, the pitfall in this approach rests with the fact that the amplitude is affected by many inspection variables including surface roughness, pig speed, and debris. By taking a more fundamental approach to the design in this development, new features for assessing coatings were established, one of which was the arrival time of the ultrasonic wave. For the wave type and frequency selected, the wave velocity is different for bare and coated pipe. Therefore, a disbonded or missing coating can be detected by monitoring the arrival time of the ultrasonic wave, a feature that is amplitude independent. Another feature for assessing coatings, the absorption of selective frequencies, was also demonstrated.

Along with the benefit of knowing a coating’s condition, this technology could help justify longer re-inspection intervals for corrosion surveys.