Dr Hubert Lindner reports on the joint development of a series of 14-in/18-in ILI tools and the application survey subsequently conducted in a high-pressure, heavy-wall offshore pipeline
In-line corrosion inspections incorporating various technologies such as magnetic flux leakage (MFL), ultrasonic testing (UT) and, increasingly, ultra-high-resolution geometry inspection based on eddy current methods (EC) have become standard in pipeline integrity assessment worldwide.
In the past, about 40 per cent of pipelines, often multi-diameter offshore lines, were classified as 'unpiggable', since they could not be inspected with off-the-shelf inspection tools.
Due to significant improvements in inspection technology, the range of unpiggable pipelines is on the decrease.
Here we look at joint development of a series of 14-in/18-in ILI tools by Rosen and the application survey subsequently conducted in a high-pressure, heavy-wall offshore pipeline.
Project requirements
In addition to its multi-diameter design, the pipeline posed several other challenges, its essential properties being as follows:
- Length: 163.9km
- 14-in segment: 11.9 km (wall thickness: 20.6mm-22.2mm).
- 18-in segment: 151.9km (wall thickness: 22.2mm-28.6mm).
- Bends: 14-in and 18-in, 5 D.
- Maximum water depth: 1900m.
- Maximum pressure: 290bar.
- Known minimum ID: 300mm.
- Surface: internally coated.
- Medium: gas.
Furthermore, the pipeline had several subsea appurtenances (notably check valves, connectors, ball valves, tees and reducers) as well as two jumpers in the subsea connection segment and an adjacent wye-piece with a transition from 14-in to 18-in.
Project planning
The operator's goal was to ensure the integrity and safety of all pipeline operations while maintaining the flow of gas throughout the inspection process.
Due to the particular challenges of the pipelines and the potentially serious consequences in case the pigging operation failed, the project plan featured an extensive test programme as well as the following components:
- Tool development: development and manufacture of three tools (gauging, extended geometry, MFL).
- Testing: Tool component, pull and pump tests.
- Preparation of on-site activities: The precise launching and receiving procedure was defined and a contingency plan for unwanted tool behaviour drawn up.
- Data Evaluation: Specialists at Rosen USA and the Rosen Technology & Research Center (RTRC) in Germany made meticulous preparations for the data evaluation immediately following the inspection run.
- Project communication: Throughout the project, a policy of very close communication between the operator and the involved branches of Rosen was implemented.
Tool design
In terms of tool design, the main challenges were posed firstly by the wide range of different diameters from about 300mm to 415 mm, secondly the high pressure (290bar), and thirdly the wye-piece passage.
The first of these challenges was overcome by developing a petal disc solution for the gauging tool.
In addition, a particular segmented multi-diameter driving unit had to be developed in response to the high loads of the intelligent tools (Figs. 1 and 2).
The driving unit of the MFL tool, for example, has to pull two heavy-wall (25mm) multi-diameter MFL units plus an electronic trailer (Fig. 2).
To meet the second challenge, all components of the tools were designed to withstand pressures of up to 300bar.
The wye-piece challenge was overcome by the sealing lengths of the driving units.
Testing
The testing phase was divided into three parts: pressure test of standard components and vessels (300bar); tool component tests (pulling force, flip-over test); testing of the finished tools.
The main focus was on testing the finished tools. All tools were pulled through the 14-in, 16-in and 18-in test lines.
The tests were conducted using the full size 14-in/18-in test loop at the RTRC which features all the crucial properties of the original pipeline (ie jumpers, valves, tees and a wye-piece).
During the various test runs, pipeline flow and pressure were measured to evaluate tool performance and optimise tool behaviour.
In combination, the differential pressure and the tool inclination provide reliable information on tool performance within the pipeline, thereby enabling tool behaviour optimisation where required.
Gauging and inspection runs
At the end of June 2008, the actual pigging operations started with a successful initial run of the gauging tool. Since neither the tool nor the gauge plate showed any damage and wear on the polyurethane (PUR) was low, the geometry tool was launched.
It went through the pipeline without any problems, and the data collected was complete and of very high quality.
On-site analysis showed one extreme measurement caused by a check valve between the jumpers. A detailed comparison of this measurement and the tool design was conducted before approval for the MFL tool passage was given.
The MFL tool was launched in a tube with two segments poking out which had to be pushed into the reducer because of the length of the launcher (Fig. 3). Again, the run was completed without any problems, and the data collected was complete and of high quality.
Summary
Faced with the challenging task of inspecting a pipeline system featuring high pressure, high wall thicknesses, and subsea installations, Rosen developed and produced two in-line inspection tools (extended geometry and MFL).
Due to detailed and comprehensive preparations at a 130m test loop specially built at the Rosen facility in Lingen, the pipeline system was successfully inspected despite its problematic properties: not only were all tools launched and received without any incidents or time delays, but they supplied complete data sets of very high quality.
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Dr Hubert Lindner is an ILI Tool Specialist with ROSEN Technology & Research Center, Lingen, Germany,
www.roseninspection.net
