Transmission pipeline systems exist ‘mid-stream’ in the oil and gas value chain, which begins ‘upstream’ with exploration and production (E&P), or the receipt of products, and terminates ‘downstream’ with a distribution network that moves the product to end users. The adjacent images show the elements that comprise typical natural gas (NG) and hazardous liquid (HL) systems.
Elements of Transmission Pipeline Systems
It is apparent there that transmission systems involve end-points and distributed storage in various forms, as well as processing and treatment plants prior to the product moving into the transmission system. Delivery into the downstream aspects follows transmission. Key elements in the transmission system include prime-movers, meters and pressure regulator (M&R) stations, and main-line valves (MLVs) which can be remotely operated. Other related facilities and technologies include a supervisory control and data acquisition (SCADA) system.
E&P and receipt lie to the left margin of these images, with those aspects followed by processing and treatment, which prepares the product for entry to the transmission pipeline. The end-users lie to the right side of these images. NG and HL systems differ in regard to the transported product, with clear differences evident between NG and HL systems. NG systems all tend to be similar because they ship the same product – marketable methane. The same cannot be said for HL systems, as some transport crude (brown) products while others move refined (white) products. Some HL systems move highly volatile fluids, with some among those being high-vapor-pressure liquids that must be moved under managed temperatures and pressures.
The system elements noted above can be discriminated in regard to their location and function. These elements are either 1) located at discrete sites (e.g., facilities, compressor/pump stations, storage), or 2) are part of the often remote lineal aspects (e.g., pipelines, main-line valves). In regard to function, and its implications for integrity, the elements either do not involve moving parts (i.e., they are static, such as the line pipe), or they involve moving parts (like prime movers and other equipment). What follows is specific to the network of transmission line pipe, and how that line pipe was built into and managed as a pipeline.
In its current usage the term component means any element of the pipeline system – like stations, storage, M&R, MLVs, etc, which sometimes are termed facilities. Fittings and transitions serve to connect the components, and should be designed if possible to direct any potential failure process back into the line pipe. While the transmission pipeline is the focus of this M&C chapter, it is clear that all such elements must be equally and adequately safe with respect to the public and the environment. As stations and other facilities are typically manned, the applicable Codes and Regulations impose a more stringent design factor for those aspects – which is generally 0.50.
Transmission Pipeline Loadings
For present purposes the term loading is used generically to identify all external and internal drivers that stress or deform a pipeline. In the simplest context a buried onshore pipeline is landlocked and shortly after construction exists buried and contained by its surrounding soil. Lines offshore may be buried, or exposed depending on the circumstances, and can experience currents that tend to move or excite the pipeline, including the effects of scour. Depending on the product being shipped, the loadings due to that product involve the pressure required to move the product, and the temperature associated with its transmission. These must be adequately addressed during design. Pipelines in river crossings can experience similar loadings to those offshore, with scour potentially being more severe during periods of flooding. Pipelines also can suffer external loadings due to ground movement laterally, as well as by subsidence or compaction. Other case-specific external loadings also occur.
Depending on the transported product and its prior processing, loadings can develop due to surge and slug-flow for the HL systems, whereas the compressibility or mechanical stored energy of some products can open to unanticipated loadings – depending on the circumstances in an upset state. While slug-catchers and other design innovations help to manage such upsets, the point to be made is that all such plausible events need to be identified, considered, and addressed in the design phase.
The key point made in this QR segment is that the pipeline and the fittings and transitions to the pipeline are open to loadings other than pressure. Care therefore must be taken during design to address these.