Older pipelines were often designed with additional reserve capacity, and have been capable of serving their demands for many years without upgrading. Most younger pipeline systems were designed for a precise load profile, and even small additional customer connections require system upgrades to accommodate the new flow demands. In some cases, newly designed pipelines have been re-designed at the last minute, or are being looped before they are a few years old.

Modern commercial models usually dictate that the design of a pipeline, or an increase in capacity, is tailored precisely to the known customers, to avoid added capital expense. So it is likely that the advent of pipeline looping, compression and maximum allowable operating pressure (MAOP) upgrade projects will become a more common theme in the industry.

Additional compression capacity

The most common form of capacity increase comes from the addition of compression capacity to a pipeline. This is a relatively easy method for getting small to moderate increases in throughput through a pipeline, and can often be accomplished with minimal interruption to existing operations. Typical time frames to install a new compressor unit are around 18 months. Small flow increases of 5–20 per cent can be achieved through driver power upgrades and optimisation of existing compression units, including re-wheeling. This is done within the boundaries of existing sites and normally without any new connections to the pipeline. New units can also often be added at existing sites, to provide more compression, and provide more reliability.

For greater increments in flow, new compressor units can also be added at new sites along the pipeline. Some pipelines have pre-installed offtake valves to facilitate this. It is important to balance the power of new sites, and their spacing along the pipeline, to ensure even flow profiles and pressure loss along the line. Addition of new units in a balanced way can increase flow between 50 and 150 per cent of the pipeline’s ‘free flow’ capacity. Additional compression also increases the gas inventory capacity of the pipeline; which has benefits to many operators.

The drawback of using additional compression is that the capacity increments are dependent on the mechanical reliability of the units. Although conventional compressor units can achieve better than 97 per cent reliability this can be a commercial (and literal) turn-off for a power station, which is reliant on the added flow for example. The law of diminishing returns also applies to the addition of each new station. After a pipeline is ‘fully compressed’, which is typically seven to ten stations for a long pipeline, it becomes ineffective and costly to add more stations, which would generally require doubling of the compressor stations. Pipelines with this configuration are also very sensitive to flow balancing between stations, and a station trip offline can cause a flow bottlenecks to arise which is difficult to manage.

Looping

The major alternative to additional compression is pipeline looping. This option is essentially to install a parallel pipeline adjacent to the existing line to lower the flow resistance and hence the pressure drop, and to thus increase the throughput and increase the available linepack.

Pipeline looping has been used on systems where higher flow reliability is required than that which can be expected from compression capacity increments. If a new customer is required to fund the pipeline’s expansion, they will normally expect the reliability of the increment to be the same as the existing pipeline system.

Pipeline looping is very flexible from a design point of view, as the length and diameter, and placement of each loop section can be precisely tailored to suit the new load profile. Increases in throughput of over 200 per cent are readily accomplished if the diameter and length of the loops are sufficient. Pipelines can also be looped in stages to suit increasing demands on a defined timetable. The length of time required to loop a whole pipeline or pipeline sections is dependent on the amount of pipe to be installed, so can vary from about 12 to 24 months or longer.

Looping construction can pose a safety threat to the existing pipeline if not properly managed, and can also be a drain on operating resources if increased supervision is required. Construction is occurring adjacent to the live line, and normally hot-taps are required to connect the new sections.

Looping is also normally more expensive than compression addition for flow increases which are less than 50 per cent of free flow. Looping may be cheaper than compression for larger increments depending on the individual pipeline. Looping is the only alternative once the pipeline is fully compressed.

Increasing MAOP

The other option more recently provided by the current revision of AS2885.1 is the possibility to increase the MAOP of a pipeline. Although at present the standard limits the increase up to a design factor of 0.72, some pipelines which have been operating at lower pressures than their original design pressure can utilise this for increased throughput.

Provided the pipeline has been constructed correctly, hydrotested to the correct pressure and maintained well, the MAOP can potentially be increased. This process involves an engineering exercise to demonstrate the new MAOP is safe, given the known integrity of the line. Intelligent pigging will be required as part of this process. Modifications to some parts of the pipeline may need to occur to replace lower pressure capacity components or questionable components. The major capital cost arises from the effect on the connected receipt, delivery and compressor stations, which may need to be upgraded to higher pressure classes, or have pressure limiters installed upstream. A MAOP upgrade can be achieved in 9–18 months, or longer if system modifications are required.

Increments in MAOP deliver reliable increases in throughput, with the increase in flow approximately equal to the percentage change in MAOP. If only minor pipeline modifications are required, and the number of station modifications is low, this can be a cost-effective option for increments up to around 10 per cent of flow.

The major stumbling block for this type of upgrade is that only a few pipelines will fall into the eligible category of being designed for a higher MAOP than they are currently being operated at. Another possible issue comes if the pipeline needs to be shut down to do any modifications which are required or to hydrotest to demonstrate higher pressure capacity.

Pipeline capacity increases are an increasing phenomenon in a maturing high pressure pipeline network in Australia. Engineering the changes required is a complex balancing act of reliability versus cost for each particular pipeline. Constructing the changes is an intricate process, especially while the pipeline is in operation. Numerous projects have been successfully completed, showing the industry’s ability to achieve these balancing acts.