1. Modelling Pig Train Dynamics in Natural Gas Pipelines – Aidan O’Donoghue, PPSA Conference Aberdeen, November 2012
The velocity of pigs in natural gas pipelines can be erratic due to the compressible nature of the gas. Low pipeline pressure means that there is less dampening effect. This can lead to high acceleration, high pig velocity and subsequent deceleration of the pig or pig train. Knowledge of the pig train velocity profile is important to allow inspection of gas lines using an Ultrasonic pig train or understanding the start-up characteristics of isolation plug trains following a tie-in or platform bypass for example. Single pigs can be subject to high accelerations that can lead to pig damage or loss of MFL (Magnetic Flux Leakage) data due to high velocity. This can be mitigated to some extent using bypass or controlled bypass using an onboard valve. For pig trains, this is not possible due to the multiple pigs, liquid batches and the need to maintain the spacing between the pigs. This paper examines the motion of pig trains and how this can be determined using modelling. Examples are also examined using case studies.
2. Multi-diameter, bi-directional pigging for pipeline pre-commissioning – Aidan O’Donoghue, PPSA Conference Aberdeen, November 2009
This paper examines the issues associated with multi-diameter, bi-directional pigging specifically for pipeline pre-commissioning. The technique can be used to flood and subsequently dewater a pipeline without the need for temporary subsea traps. An example is the Alve Pipeline in Norway. This 16 km flow-line from the Norne Platform to the Alve Manifold includes 10′ and 12′ pipe sections. The line was flooded from Norne using the pig 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.
3. Pigging as a Flow Assurance Solution – Avoiding Slug Catcher OverflowS – Aidan O’Donoghue, PPSA Conference Aberdeen, November 2005
This paper sets out to provide an initial method of assessing the bypass requirements for pigging of a two-phase gas/liquid pipeline. The use of bypass or high bypass pigging is an established concept that has been discussed many times before. The aim here is to provide an initial indication of where they can work. Given a limited volume at the slug catcher and pump out rate (resulting from economics or other practical considerations), it is possible that a pig will remove too much liquid from the pipeline leading to overload of the slug catcher and subsequently tripping the pipeline. With liquid level control on the slug catcher and slug suppression, the receiving terminal will see a period of no gas. This may be undesirable from a process point of view. Liquid volumes arriving at the slug catcher or separator may be reduced by using an inefficient pig (unpredictable) or by slowing down the pig and aerating the liquid slug using bypass. This paper provides a first pass design method for such pigs, examines the background for their use and provides a case study or example to demonstrate the application.
4. Pipeline Flooding, Dewatering and Venting – Aidan O’Donoghue, PPSA Conference London, December 2004
Flooding, cleaning, gauging, dewatering and venting of offshore oil and gas pipelines during pre-commissioning involves pipeline pigging and expensive deployment vessel time. To aid in the planning of such operations, a number of analyses can be undertaken to determine the duration to perform each of these tasks. The mathematical models can also optimise the equipment required (hoses, pumps, compressors). Problems that could be encountered without a clear knowledge of how the operation will proceed can be avoided. The operation can be monitored by comparing recorded and predicted values, for example inlet pressure. This paper provides an overview of work performed to establish pig velocity, inlet pressure and pigging duration during various pre-commissioning tasks.
5. Pigging as a Flow Assurance Solution – Estimating Pigging Frequency for Dewaxing – Aidan O’Donoghue, Pipeline Pigging Conference, Amsterdam, May 2004
Pigging is used widely for removal of wax build-up on the internal wall of a pipeline. Much information is available on the prediction of wax deposition levels in such lines and pigs play an important role in the removal of this expected wax accumulation. Although these tools are generally much cheaper than chemicals for wax inhibition or suppression, there is very little guidance available for the selection of the correct pig, the sizing of bypass ports and the pigging frequency. Incorrect selection can be dangerous since a significant build-up of wax ahead of the pig can plug the pipeline. This can lead to extended downtime and an expensive pipeline repair. For subsea launching, extending the time between pigging is advantageous since this means that fewer interventions are required to load the subsea launcher. Correct selection of the pigging frequency is therefore very important. This paper aims to provide direction on this subject, using the output from wax prediction models and expected daily wax build-up in the pipeline. The method outline can be used to determine a pigging strategy for pipeline start-ups; pigging at low flow; cleaning normally unpigged lines; progressive cleaning of problem lines and regular pigging of lines with full information on wax deposition or merely an indication of wax percentage in the flow.
6. Why Pigs get Stuck, and how to avoid it – Aidan O’Donoghue, Pipeline Pigging Conference, Amsterdam, October 2002
Operators and contractors have been running pigs successfully for years. Every so often, a problem occurs and one becomes stuck, stalled or damaged in the pipeline. As the need for innovative one-off pigs for specialist applications arises, the risk of this occurring increases. Sufficient planning and analysis should be performed to allow the pigs to run successfully and perform their duty in the line effectively. This includes stringent testing, CAD work and analysis. This paper looks at the main reasons for pigs sticking and stalling in lines and examines what can be done about it. The paper looks at several different categories of failures, identifies the root cause of the problem and looks at how these scenarios might best be avoided. The figures at the back of the paper may be used as guidelines for consideration and avoidance of the problems discussed.
7. Latest Design Techniques for Dual and Multidiameter pigs – Aidan O’Donoghue, Pipeline Pigging Conference, Houston, January 2001
The aim of this work is to show how design and simulation techniques can provide a greater insight into the nature of pig motion and how pigs behave in pipelines. A number of such methods used to investigate and improve the reliability and efficiency of dual and multidiameter pipeline pigs are described. A case study is presented based on actual field examples. The problem of pig selection for a dual diameter application is investigated followed by an analysis of pig efficiency during a dewatering operation. The transient motion of the pig, because of the sudden change in friction when negotiating the reducer, is presented using the Piglab model. Conceptual and detailed design and simulation of pig performance can allow pigs to be built that will perform more satisfactorily in the line, help reduce fear of stalling, and lead to innovative design solutions.
8. Dynamic Simulation of the Norne Heidrun 10″ x 16″ Dewatering Pig – Aidan O’Donoghue, Pipes and Pipeline International, January/February 2001
The 10″ x 16″ Norne Heidrun pipeline presents a considerable challenge in terms of pig development for both pre-commissioning and operations. The RFO (Ready for Operation) Department at Statoil in Stavanger has developed the pre-commissioning concept which employs five 10″ x 16″ Dual Diameter pigs, Figure 1, for pipeline dewatering. The Asgard 42″ x 28″ Dual Diameter pigs are the basis for this concept [1, 2]. These pigs employ a wheel suspension system for centralisation in the large pipeline. Buckle Inducers are used for efficiently folding the 16″ seals into the 10″ line. Correct selection of seal geometry and properties allows the seals to buckle when required and recover sufficiently from compression set. The dewatering pig train consists of four pigs run with a glycol batch between each and a trailing final pig run in dry air or nitrogen. As the final pig exits the 10″ flexible and enters the 16″ line, a potential problem arises. Due to the sudden drop in friction, the pig will accelerate suddenly to a relatively high velocity. Such acceleration can cause the pig to compress the gas in front of it, decelerate and finally reverse. Therefore, the final pig could potentially reverse into the 16″ Y-piece thus damaging either the Y or the pig. This scenario must be avoided.
To investigate this problem, the dewatering operation was modelled using Piglab, a pig motion model from Pipeline Research Limited and the pig train designed to avoid this problem.
9. Multi diameter pigging for Asgard, Commissioning and pigging the 710km 42″ x 28″ Asgard Pipeline – Christian Falck, Aidan O’Donoghue, OPT Conference, Oslo, 2000
Statoil has designed and tested dual diameter pipeline pigs to perform various tasks during operations, precommissioning and commissioning of the 710km 42″x28″ Asgard pipeline. The concepts developed have been put through a major design and testing program to demonstrate that the various functional requirements have been met. The paper describes the development process as well as field experience from the initial pigging runs of the Asgard Pipeline.