Challenging pipeline conditions for inline inspection (ILI) can be caused by several different parameters, including pipeline length, diameter changes, installations and operating conditions. Very often, the real challenge results from a combination of these influencing parameters — each single individual parameter does not necessarily constitute a challenge.
When preparing an existing tool for an ILI run, the properties must be aligned with the basic pipeline and operational parameters. Due to the flexibility and high performance of modern ILI tools, many pipelines can be inspected utilizing existing tool configurations. However, despite the continuous development, there will always be applications which exceed the abilities of available solutions due to combined or conflicting parameters, which require further modifications than a standard tool configuration.
PARAMETERS AND CONSEQUENCES
A 36”, 700-kilometer-long, natural gas subsea pipeline with high wall thickness, high pressure, high flow rates, tight bends, and a subsea wye piece, required an inline inspection for metal loss.
The individual parameters in this case, provide the following consequences:
- The long length of the pipeline means that the batteries in the tool must have a higher capacity, and the sealing cups must withstand higher wear.
- The high wall thickness of 0.935 inches (23.8 mm) requires a stronger magnetization and has a limiting effect on the maximum tool velocity.
- The high flow rate, reaching up to 1.3 Bcf/d, causes a velocity in the tool which effects the MFL magnetization ability.
- The 3D bend radius limits the length of the segments and requires special setup of the sealing elements.
- The wye piece requires the tool to have high stability over a certain distance and sealing elements
Although each of these elements require special consideration, an existing tool configuration would be able to overcome the consequences of the first four parameters, by including the following elements:
- Higher magnetization
- Speed control unit
- Higher battery capacity
- Bend capability and passage valves
However, the subsea wye-piece introduces conflicting parameters to the previously-mentioned elements, thus exceeding the abilities of a standard ILI tool configuration.
The wye-piece in this particular pipeline is located around 30 kilometers from the receiver, and poses special challenges relating to safe tool passage.
A typical wye-piece is a symmetrical junction of two supply lines, usually with inclined branched of 15 degrees each. Two properties of a wye-piece can inhibit the passage of a cleaning or ILI tool: the lack of guidance and the lack of sealing.
Long tools with an adapted setup of sealing elements are capable of successfully negotiating a wye-piece. However, this tool setup is not capable of negotiating tight bends, such as 3D, and therefore is unsuitable for this pipeline.
A segmented tool with rear sealing capabilities would be another viable design option. However, as the tool passes through the wye-piece, the front unit may not receive the necessary guidance or achieve the appropriate sealing, which can introduce the risks of buckling, jackknifing or blockage. Therefore, special consideration must be given to the design of the tool joint and sealing elements.
The solution consisted of three units; a pull unit with a speed control valve; a MFL measuring unit; and a trailer carrying all electronics and batteries. These units were connected by universal joints.
The three-unit design allows for the navigation of tight bends, and, at the same time, safe and stable passage through the wye piece. The front cup on the first unit re-gains sealing after traversing the wye-piece, before the rear cup on the sensor unit has unsealed from the line. The specially-designed universal joint between the first two units contained a large internal diameter for bypass flow through the units to the speed control valve. Furthermore, it is designed to stabilize the tool passage through the wye-piece.
The speed control valve controlled the tool velocity throughout the pipeline, its algorithm was adapted to continuously monitor the tool speed, keeping it in a consistent velocity range. Every time the tool velocity left the target range, the speed control valve would open or close, to bring the tool back to the target velocity.
Combined pipeline parameters have an effect on the design of pipeline inspection tools. Single, usually uncritical, features can become critical in combination. In this case, challenges were caused by the combination of a high flow rate, high wall thickness, tight bends (3D) and the presence of a wye-piece.
A tailored three-unit MFL solution was developed, consisting of elements to address each of the challenging parameters. The inline inspection was performed to expectations, the tool successfully navigated the wye-piece, and velocity remained within the MFL tolerance range of ± 0.4 m/s.