1. Onshore pipelines should be buried to protect them from mechanical damage, fires and tempering.
2. The recommended minimum covers are given ANSI/ASME B31.8 Article 841.142.
3. In determining depth cover in agricultural areas, the depth of ploughing and of drain systems shall be considered. A cover of 1 m shall be adequate in most cases.
4. In grazing land, where fencing activities are common, a depth cover of 0.8 m is in general adequate.
5. The location of buried pipelines shall be indentified by markers.
6. In areas where the risk of interference by mechanic excavators is high, a warning table should be installed in the excavation above the pipeline to further lower the risk.
7. Any non-buried pipeline sections shall be justified on an individual basis, and shall be installed clear of the ground to avoid external corrosion.. Pipe supports should be designed according to standard.
1. The section of pipeline within the shore approach should be buried to a depth to ensure that exposure due to erosion will not take place.
2. There is otherwise no requirement to trench or bury offshore pipelines unless necessary in order to achieve pipeline stability, mechanical protection or thermal insulation.
3. It should be noted that protection against dragging anchors from large ships, particularly in soft soils, required significant or even impracticable burial depths.
4. Pipelines of 16” and above, with high impact resistant concrete coating, can generally withstand the impact of fishing gear (e.g. trawl boards) and therefore need not be trenched. However since several installed parameters may vary from case to case, this should be verified on individual basis.
5. When soil properties and the environment conditions are suitable, the pipeline may self-bury within an acceptable in period of time, removing the need for physical burial.
6. The analysis of the self burial process is complex and often based on local records/observations; therefore specialised expertise should be sought.
Buried pipelines operating at high temperatures may be susceptible to upheaval buckling caused by high compressive loads. Upheaval buckling can be prevented either by expansion offsets regularly spaced along the pipeline, or a sufficient burial cover. The use of expansion bellows to accommodate thermal movement is not recommended.
|
Location |
Minimum cover (m) in Normal ground (Note 1) |
Minimum cover (m) in Rock, requiring blasting |
|
Class 1Class 2Class 3 and 4 |
0.81.01.2 |
0.60.81.0 |
|
Public roads and railways crossings |
1.5 |
1.2 |
NOTE 1: The cover refers to the undisturbed ground level.
1. All submerged pipelines, e. offshore pipelines and sections of onshore pipelines in swamps, floodable areas, high water table areas, river crossings, etc., should be stable under the combined action of hydrostatic and hydrodynamic forces. The on-bottom stability can be achieved by increasing the pipe wall thickness, by the application of concrete weight coating, by spaced anchor points, by trenching, or by burial.
2. Special considerations shall be given to pipelines installed in weak soils (e.g. peat), at dyke crossings, etc. where differential settlements may lead to pipeline loss of integrity.
3. The one year return wave and steady state current conditions should be used for the analysis of stability during the installation phase. The one hundred year environmental return conditions should be used for the analysis during the operation phase. The negative buoyancy should be sufficient to prevent unacceptable lateral pipeline displacements.
1. Trench excavation is carried out along staked centreline of pipeline.
2. The width of excavation for trench shall be minimum (pipe diameter plus 200 mm on either side).
3. Depth of trench shall be specified (Normal – 1 Meter, Road : 1.2 M, River 1.5 M, Railway : 1.7 M etc)
4. In case of trenching in rocky areas, blasting is performed with the guidelines and safety rules of chief control of explosives, Nagpur.
5. In case where rock/gravel or murrum/hard soil is encountered in the bottom of trench, padding material (minimum compacted thickness of 150 mm) shall be provided.
6. When up-floating of the pipe line after backfilling is anticipated, antibuoyancy measures are provided in such areas e.g. weighing by applying continuous concrete, installing saddle weights etc.