Coal seam gas (CSG) gathering systems are a large logistical component of CSG projects and during peak construction many kilometres of pipelines will need to be constructed every day – placing stress on time management, budgets and resource management. Stepping back from the design is like stepping back and looking at spaghetti thrown against the wall. The individual lines that form the gathering system travel in their own directions and have their own individual features. Therefore there is no one formula or rule when planning a CSG gathering system. Each individual project requires a great deal of ingenuity in order to overcome new obstacles faced.
CSG gathering systems are a comparatively new technology and have not been formally addressed by way of an applicable Australian Standard (although the Australian Pipeline Industry Association has recently developed a CSG Code of Practice).
There is, however, another entity within the gas industry that shares the spaghetti analogy: distribution networks. Some of the challenges faced by those in gathering system design have already been dealt with from a network point of view, such as high-density polyethylene (HDPE) welding preferences, material property restrictions for design, trench arrangements to include other services like communications and power cables, and location class for burial depths.
Challenges unique to CSG gathering systems include dealing with significant volumes of CSG water and its associated treatment. The presence of large volumes of water creates problems in the gathering system design, as there is water in the gas and gas in the water, even after the process of separation. In order to combat this low point, drains need to be designed to siphon the water out of the gas, and high-point vents need to be designed to extract the gas from the water. Neither of these tools are an “˜off-the-shelf’ item, and instead require custom fabrication. Working out where to install vents and low-point drains can be a bit of a dark art. For example, a good location to place one of these low-point drains is at the lowest point of a pipeline, which is often in the middle of a creek or stream making access difficult.
Another issue that often arises is whether compression is required at well heads or strategic points to keep the gas and water products moving through the gathering system. In order to achieve this engineers are required to install electrical and communication cables in the gathering system for monitoring purposes. These cables have different installation requirements to plastic gas and water pipe, and the gathering system can bear some resemblance to what you may find under an urban footpath.
HDPE pipe also presents new kinds of obstacles for this type of project. HDPE has wear properties that need to be considered when solids or chemicals form part of the
product within. The life of HDPE is also affected by fluid temperatures above 25°C. Therefore the design life of the gathering system may have to be designed around the expected productive period of a well.
Plastic pipe may present some cost benefits by being cheaper and easier to install, when compared to steel, with some sizes delivered in spools and some installed by ploughing-in the pipe. However the jointing of pipes and fittings in these instances can be difficult. Preferences to either butt welding or using electro-fusion fittings needs to be determined Unlike steel, neither method can be non-destructively tested by x-ray. So the pressure test forms an important part of the construction, and this process needs to be considered in the gathering system design so that pipeline construction does not inhibit access for land owners or locals.
Even if the original concept or front-end engineering and design of the gathering system appears to be prudent and avoids encroachment on residences, industries and farming activities, the design can only be finalised after the pipeline route is walked and inspected manually for flora, fauna and cultural heritage. A majority of national parks, state forests and wildlife corridors are well defined, however cultural heritage is difficult to see from aerial photography, a method most commonly used to design large-scale projects. If an area of cultural heritage, endangered wildlife or vegetation is discovered, then the system design has to be redesigned to avoid this area and reduce the impact. Redoing the design is often a simple enough task, but the modelling of the flowlines consequently need to be rechecked as well as changes to elevation, pipe wall thickness or pressure rating. In addition, placement of features like vents, drains and valves will need to be considered and at times moved to correspond with other changes.
The design of gathering systems is vastly different to that of high-pressure steel transmission pipelines. In fact, designing one steel pipeline can be considered easier than designing a CSG gathering system as there are fewer variables to consider during design. While both may be an iterative process to a degree, retrospective changes to CSG gathering systems present further problems and will consume more design time and resources.
A final problem to consider, is obtaining staff that are familiar with this type of construction, techniques used, material and product properties, and have a good understanding of the process requirements.