Maintenance and servicing are terms often associated with relatively complex mechanical systems. Most people, for example, understand the importance of having a car serviced at regular intervals to ensure any wearing parts can be replaced before they actually fail and possibly

result in additional damage and expense. Pipeline systems are mechanically relatively simple and the need for regular

maintenance may seem less important; however, carrying out regular pipeline maintenance activities can form an extremely effective part of a pipeline integrity management system, both for ageing assets and for more recently installed pipelines. Typically, these maintenance activities may be intended to manage internal or external corrosion issues, to improve flow conditions and reduce potential process

Case studies in pre‑inspeCtion

Lucy Lingard and Gavin West, project Managers at pipeLine engineering & supply Co. Ltd, uK, present effective pipeline pigging.

cleaning

july 2012 | Reprinted from World pipelines

upsets within the wider production system, or to facilitate periodic inspections of all or part of the pipeline system.

Pigging services for pipeline maintenanceWhile some external inspection and maintenance activities are carried out on pipelines, the primary integrity concern for a pipeline system generally relates to the transported fluid

and therefore requires internal intervention into the pipeline. Pipeline pigging therefore forms a key part of many pipeline maintenance schedules, both during routine operations and as part of more complex interventions such as inspections or local repairs.

For pigging to be an effective form of pipeline maintenance it is also important that the pigging tools are suitably designed to address the expected issues within each individual pipeline. There are many instances where production issues have been caused or aggravated by an incorrect or inappropriate pigging strategy. For example, insufficient bypass capacity when pigging lines with water dropout may result in a large slug of water being received with the potential to swamp the slug catcher or separator causing a production trip. In waxy pipelines, insufficient bypass can, in the worst case, result in a wax candle building up in front of the pig and plugging the line.

In other cases, poorly designed pigs, or an inappropriate pigging schedule, may mean that no real benefit is obtained from regular pigging. While inefficient pigging is not necessarily detrimental to the overall condition of the pipeline, it can provide false confidence in the line condition and may lead to unexpected operational upsets or delays. For example, when planning an inspection campaign, a ‘regularly pigged’ line may suddenly be found to require intensive cleaning to allow an inspection tool to obtain full line data, resulting in delays to the inspection schedule and an increase in the overall cost of the inspection campaign.

Maintenance of the pigging tools themselves should also be considered within the pigging strategy, as poorly maintained pigs will also loose efficiency over time. Sealing and scraper discs that contact the pipe walls will wear, and enhanced cleaning components such as brushes will degrade (Figures 1 and 2). Regular inspection of the pigging tools can also increase the identification of distinctive wear or damage patterns, which may be indicative of restrictions or obstructions within the pipeline. In the worst case, poorly maintained pigging tools may eventually fail in service, leaving debris in the line.

Changes to the pipeline process conditions or flowrates may also reduce the efficiency of previously suitable cleaning pigs and in the worst case, may result in process upsets or a stuck pig scenario. It is therefore sensible to review cleaning pig designs and pigging frequencies regularly to ensure that routine pipeline maintenance programmes remain effective.

Pigging for inspection and interventionPeriodic pipeline inspection and intervention activities also frequently require pigging support, either in the form of an intensive pipeline cleaning programme, or through the design of specific tools for pipeline de‑oiling or isolation. While some pre‑inspection pipeline cleaning programmes can be carried out using routine cleaning pigs, on an enhanced pigging schedule, it is often necessary to provide a series of specific pigs for pre‑inspection cleaning; either to reduce the logistical difficulties associated with retrieving the pigs and returning them to the launch point in the required timescale, or to remove larger quantities of wax and debris from the line with each pig and reduce the overall duration of the pigging programme (Figure 3).

Figure 1. Pigging tool showing worn discs.

Figure 2. Pigging tool refurbished with new discs.

Figure 3. High levels of wax retrieved during progressive pipeline cleaning.

Reprinted from World pipelines | july 2012

During a recent North Sea pipeline cleaning campaign carried out by the pipeline services division of Pipeline Engineering & Supply Co. Ltd (PE), it was found that the specifically designed pre‑inspection cleaning pigs were capable of removing more than double the wax volume per pig than was being achieved with the routine cleaning pigs normally used to remove wax from the target pipeline (Figure 4). Relying on the

routine cleaning pigs alone to condition the line for inspection would therefore have been likely to result in either a prolonged cleaning campaign or a failed inspection, either of which would have been extremely costly to the pipeline operator.

Effective pigging requires a good understanding of the line conditions and variables such as temperature, flowrates, pipeline diameter. Understanding the variables enables companies to design and deploy pipeline pigs that will provide the best fit for the required pigging operation. It is also important for service companies to work with operators to develop pigging programmes specific to individual pipeline requirements.

Determining how clean a pipeline is following pigging Carrying out an inspection run is a critical process in assessing the condition of a pipeline and it is important therefore to obtain meaningful and reliable information/data from the running of an inspection tool. Prior to running an inspection tool, it is to be anticipated that, following the running of a series of cleaning pigs, the line will be sufficiently clean for inspection, but how can one be sure that this will be the case?

PE recognised that a specialist pigging tool capable of measuring residual debris deposits in a pipeline would benefit pipeline inspection and they were actively encouraged by some of the world’s leading oil and gas operators to develop such a tool. Subsequently, the company started a research programme into different technologies to assess the cleanliness of pipelines that could be built into a pigging tool, and in particular the use of Hall Effect‑based sensor technology, configured to detect proximity to the pipewall. Based on this research, PE developed the PECAT™ pipeline cleanliness assessment tool, which is now available as a service when assessing the internal condition of pipelines either before the use of an inspection tool, or to assess the pipeline cleaning results from a progressive pigging programme.

Pipeline pigging servicesPE has extensive experience in the design, manufacture and operation of pigging tools and provides pipeline pigging services across a range of pigging activities, pipeline inspection campaigns and scheduled and non‑scheduled pipeline interventions. The case studies below illustrate ways in which pigging can form a key part of the regular maintenance and servicing of pipelines.

Case study 1 – Marathon Oil UK pre‑inspection cleaningMarathon Oil operates the Brae Field in the North Sea, including an 18 in. pipeline system exporting gas condensate from East Brae and Brae Bravo to Brae Alpha, which is regularly pigged to control wax build up. PE has supported the routine pigging of this pipeline system for some time, carrying out regular inspection and refurbishment of the cleaning pigs to facilitate an effective maintenance pigging programme, incorporating alternate pig launches from the exporting platforms. Data from the pigging programme has been periodically fed back into the design of the tools, and Marathon Oil review the pigging schedule on a regular basis.

Figure 4. Chart showing wax volume removal and measurement of residual wax in the line.

Figure 5. Debris build‑up recorded by PECAT interface.

Figure 6. Pig retrieved from pipeline.

july 2012 | Reprinted from World pipelines

When planning an inline inspection of the pipeline sections from Brae Bravo to Brae Alpha, Marathon Oil realised that additional pig runs would be required to give the inspection tool the best opportunity of recording full inspection data and developed a revised pigging schedule based around these requirements. The intensity of the revised pigging schedule required the pigs to be transferred directly between the platforms with little opportunity for inspection or refurbishment of the tools. It was also recognised that successful completion of the programme would involve the co‑ordination of a number of functions and facilities on board both platforms, including operations personnel, deck crew, lab technicians, material controllers and third party contractors.

PE was able to increase its support to Marathon Oil for this campaign by mobilising offshore pigging co‑ordinators. The company was tasked with reporting on the condition of each pig received and the returned wax volumes, inspecting and refurbishing the pigging tools as required, co‑ordinating the transfer of the pigging tools between platforms and reporting on progress of the operation. It was also able to service platform pigging equipment to improve the efficiency of future operations (Figures 6 and 7).

Data from the pre‑inspection cleaning campaign was fed into both the Marathon Oil pigging programme to assess the

line cleanliness prior to launching the inline inspection tool and PE review to confirm that the most effective cleaning pig design is employed, thus ensuring the 18 in. condensate line continues to be pigged safely and effectively.

Case study 2 – Centrica Mallard cleaning and inspectionThe Mallard pipeline system forms part of the Greater Kittiwake Area operated by Centrica Energy Upstream (CEU) in the Central North Sea. It produces oil from a single subsea wellhead back to the Kittiwake platform via an 8 in. pipeline. There are also two further subsea seawater injection wells fed via a parallel 8 in. pipeline. The two pipelines are connected by means of a pigging loop that effectively bypasses the Mallard template.

As part of the integrity assessment of the system, there was a requirement to carry out an intelligent internal inspection of both pipelines. PE was contracted by CEU to carry out this work.

Prior to this inspection requirement, the pipelines had only been pigged during pre‑commissioning and as such, a substantial progressive cleaning programme needed to be developed. The programme detailed the types and running sequence of the cleaning pigs to gradually build up the aggressiveness of the pigs, so that the anticipated levels of wax/scale could be removed successfully without causing any potential blockages to the system (Figure 8).

On completion of the cleaning programme, the inline inspection tool was run and the resulting data confirmed that the cleaning pigs had functioned to their specification, removing sufficient debris within the system to allow a detailed and successful inspection run.

PE provided a full pigging team and a project manager acting as the client representative offshore assisting CEU and the Kittiwake platform duty holder with the oversight of the whole programme. The company also supplied and operated pig monitoring equipment so that constant monitoring of the pigs’ passage through the system could be maintained. The pigging team were responsible for loading and removing each of the pigs, evaluating their performance and deciding on the next pig to be run.

The data recovered by an inline inspection tool was used as part of a formal assessment of the integrity condition of the two pipelines.

ConclusionAs highlighted by these case studies, providing appropriate cleaning pigs for a pipeline is essential if effective pigging is to be achieved. Also, if the success of a pigging campaign can be measured, not just in terms of quantity of debris removed from a line, but also in terms of residual debris left in the line, through the use of tools such as PECAT™, the whole process is improved and both contractor and operator can be more confident that meaningful data is likely to be captured if intelligent tools are to be deployed.

Figure 7. Pig refurbished onsite including replacement of worn discs.

Figure 8. Studded cup pig for aggressive cleaning attached to a towing pig.

Reprinted from World pipelines | july 2012