Pipelines
The pipeline system that conveys the individual-well production or that of a group of wells from a central facility to a central system or terminal location is a gathering pipeline. Generally, the gathering pipeline system is a series of pipelines that flow from the well production facilities in a producing field to a gathering “trunk” pipeline.
Transmission pipelines
“Cross-country” transmission pipelines will collect the product from many “supply” sources and “deliver” to one or more end users. There are three general categories of transmission pipelines:
- Natural gas- Carry only natural gas
- Product
- Carry a number of processed or refined petroleum products such as:
- Processed natural gas liquids- butane and propane
- Gasoline
- Diesel
- Refined fuel oils
Convey unrefined crude oil from producing areas to large storage areas or directly to refineries
Transmission pipelines will generally require much larger pipe than gathering systems. Transmission systems normally are designed for long distances and will require pressure-boosting equipment along the route.
Pipeline construction process
Conventional, onshore pipeline construction process is described next.
ROW clearing/preparation
Before initiation of construction activities, any sedimentation, erosion control, construction fencing, and other preparation is completed. All vegetation is cleared and grubbed, topsoil is removed (if required), and the working ROW is graded.
Pipe stringing
Once the ROW has been prepared, the pipe is loaded on flatbed trucks. Before unloading, pipeline skids (typically 4-in. × 6-in. × 4-ft hardwood timbers) are dropped along the ROW to be placed under the pipe. The trucks are driven down the ROW, and the pipe is unloaded, joint by joint/end to end, by sidebooms or cranes.
Ditching
The ditch is excavated along the pipeline centerline using ditching machines, excavators, backhoes, and other excavation equipment. Pipelines are normally buried with a minimum of 36 in. of cover (DOT regulatory requirement). In consolidated rock, the minimum cover varies between 18 and 24 in. The cover for Class 1 locations is 18 in.; the cover for Classes 2 to 4 (railroads, highways, and public roads) is 24 in.
Welding
The pipe strung along the ROW is welded in a progressive manner. Sidebooms will work along the ROW lifting the pipe while a crew aligns the pipe in preparation for the “stringer bead” weld. Generally, a welder or welders (depending upon the size of the pipe) will work with the alignment crew, align the pipe, and apply the initial weld “bead.” A group of welders will follow immediately behind the stringer welder(s) and apply the “hot pass” bead or seal weld. Additional welders will follow to apply the final passes of weld material.
Field joint and anticorrosion coating and inspection
When the welding is completed, field joint crews clean the weld areas and the short, adjacent bare steel on either side of the weld, and apply the field joint coating. Any nondestructive testing of the welds, such as X -ray, will be completed before application of the field joint coating. Following the completion of the field joint coating, the pipe is inspected with “holiday” detection equipment (low-voltage DC equipment that shows where the pipe coating and field joints have failures or breaches), and anomalies and breaches in the coating are repaired.
Pipe lowering
Upon completion of the field joint application and coating inspection, the pipe is lowered and placed into the ditch by sidebooms or other lowering equipment.
Backfill, cleanup, and restoration
Following completion of the pipe lowering, the ditch is backfilled, and the ROW is cleaned and dressed. The ROW is finely dressed, grass and vegetation replanted, and any special remediation measures or cleanup requirements are completed.
Highway, road, railroad, and river Crossings
Highway, road crossings are seldom installed using conventional, open trench methods. Typically, these crossings are installed using a wet bore or dry bore method. The boring is done by rigs that are similar to very small drilling rigs, laid horizontally, and placed in pre-excavated boring level “pits.” The boring rig drills underneath the crossing area, and the pipe or casing is installed. The wet method uses a boring rig and circulates water or drilling fluid through a drill stem to open a small pilot hole, then pulls a pipe or casing-sized cutting head back to the rig, cutting a hole large enough to place the pipe or casing. The dry bore method is similar, but the casing or carrier pipe is fitted with a cutting head and is used to drill the hole and is left in place when the drill is completed. The hole is drilled dry and does not use any water or fluid to assist the drilling operation. Railroad crossings are never open cut and are always bored. Typically, railroads require that the borings be made with the dry bore method. Both wet and dry bore methods are limited on the distance that they are effective and practical.
River crossings are now typically installed using the HDD method. Open-cut trenching of rivers may be allowed by the U.S. Corps of Engineers, but HDD installations have become more economical. The HDD method uses a computer-controlled rig that controls a directional wet-bore pilot drill that can be accurately steered from the rig. The directional drill can bore a pilot hole up to a mile or more and ream a hole back to the rig large enough to install the carrier pipe. The “drill” string or pull section of pipe is welded together on the drill exit side, pretested, then pulled back to the rig side following the backreamer.
The HDD method may be used to install long highway and road crossings, such as interstate highways and freeways. The wet- and dry-bore methods are limited to several hundred feet in length, which requires multiple borings to cross the distances typically required to cross interstate highways and freeways.
Tie ins
A crew, or crews, is typically deployed that makes all pipeline tie-ins along the construction corridor. The tie-in crew makes the final welds at junctures where the progressive welding cannot make the final welds. Tie-ins are made at locations such as highway, road, railroad, and river and creek crossings and at drag sections, etc. The tie-in crew typically has excavation and pipe handling equipment and dedicated welders.