Submerged pipeline stability is a crucial design element that ensures a pipeline remains in place when subjected to buoyant forces from water or saturated soil. The primary goal is to counteract this upward lift force by adding sufficient weight, keeping the pipeline stable in a cost-effective manner.
๐ ๐ฒ๐๐ต๐ผ๐ฑ๐ ๐ณ๐ผ๐ฟ ๐๐๐ผ๐๐ฎ๐ป๐ฐ๐ ๐๐ผ๐ป๐๐ฟ๐ผ๐น
There are three main strategies to control buoyancy:
Use of Density Anchors
๐น ๐๐ฒ๐ป๐๐ถ๐๐ ๐๐ป๐ฐ๐ต๐ผ๐ฟ๐ (๐๐ผ๐ป๐ฐ๐ฟ๐ฒ๐๐ฒ ๐ช๐ฒ๐ถ๐ด๐ต๐๐) This is the most common method and involves adding concrete weights to the pipeline.
- Swamp Weights: Inverted โUโ-shaped weights placed over the pipe in wet areas like swamps and bogs. They are economical and easy to install.
- River Weights: Two-piece weights that are bolted directly onto the pipe. They are used for river crossings where the pipe is assembled with weights before being moved into place.
Example of Rigid Weights
3. Continuous Concrete Coating: A reinforced concrete jacket that completely surrounds the pipe. It offers excellent weight and mechanical protection, making it ideal for major river crossings or rocky environments.
Use of Backfills
๐น ๐๐ฎ๐ฐ๐ธ๐ณ๐ถ๐น๐น This method uses the mass of the soil placed over the pipe to hold it down. Itโs a viable option if the trench can be kept dry during construction, but the area may become saturated later.
Use of Mechanical Anchors
๐น ๐ ๐ฒ๐ฐ๐ต๐ฎ๐ป๐ถ๐ฐ๐ฎ๐น ๐๐ป๐ฐ๐ต๐ผ๐ฟ๐ These devices, often screw-like anchors, are driven into the soil and attached to the pipeline with straps. They use the shear strength of the soil to provide a holding force and are useful in swamplands where heavy concrete is impractical.
๐๐น๐๐ฒ๐ฟ๐ป๐ฎ๐๐ถ๐๐ฒ ๐ช๐ฒ๐ถ๐ด๐ต๐๐ถ๐ป๐ด ๐ ๐ฒ๐๐ต๐ผ๐ฑ๐
Other options for adding weight include:
- ๐น Increased Pipe Wall Thickness: This is generally only economical for small-diameter pipes.
- ๐น Geotextile Bags (PipeSaksยฎ): These are fabric bags filled with heavy material like gravel and draped over the pipe like a saddle, offering a flexible weighting solution.
Final selection of the type and extent of buoyancy control measures should be made on a site-specific basis, taking the following into consideration: type of terrain, type of soil, ditch conditions (dry or wet), construction season, cost (economics), availability of materials, access to site, ease of handling during transport and construction, and limitations of equipment.
References:
- Handbook of Pipeline Engineering
- PIPELINE DESIGN & CONSTRUCTION: A Practical Approach