The Use of Geotextile Fabric, Anti-Buoyancy Weights for Buried Pipelines

PAPER FOR PRESENTATION at the GAIL Gas Industry O&M Conference at Mumbai on 18th - 19th June-2003

Author: Geoff W. Connors, P.Eng. - President, PipeSak Incorporated

Geotexile fabric weights have been used to control buoyancy of buried pipelines in North America for almost 10 years - and continue to gain market share as they prove to be not only economical but more environmentally friendly than alternative methods. A Canadian company, PipeSak Incorporated, was the first to develop and market fabric weights - the most fundamental advance in pipeline weighting technology in decades.

Background
Buried pipelines have always required a certain level of additional weight to counteract the buoyant force that is often exerted on them from groundwater. Natural gas pipelines have been the most common users, however other pipelines including those transporting oil and water often require additional weighting due to the cyclical nature of their contents - often these pipelines are only partially full which could make them susceptible to floatation.

Early methods of buoyancy control include the use of cast iron and, later, concrete. Weights, pre-formed to match the outer pipe wall, were shipped to the site and bolted directly onto the pipeline prior to lowering into the trench. This was certainly a cumbersome and often risky process - for both the workers AND the pipeline!

Weights that could be set on the pipeline after the lowering-in process soon evolved. Set-on weights greatly aided the installation process as the pipeline could now be welded and lowered into the trench prior to attaching the heavy weights.

Since it is difficult to set on weights in deep-water conditions, methods to concrete-coat pipe were later developed (see Fig. 1). Although the concrete coated pipe was again heavy for installation (as with the old bolt-on weights), it could be dragged into the trench with little risk of damage. Due to the cost, concrete coated pipe was typically used only offshore or for large river crossings where the less expensive concrete set-on weights would be difficult or impossible to use.

Figure 1 - Concrete coated pipe

In the last 10-15 years, concrete set-on weights have fallen out of favor with many pipeline owners due to the propensity of the top-heavy weights to tip off the pipe during and after installation (see Fig. 2), as well as the weight's potential of damaging the pipeline's coating during installation. With no other options available, concrete coated pipe began to slowly replace concrete set-on weights. This has been an expensive option for the pipeline industry. In addition, using concrete coated pipe does not address the environmental concerns of cement, which has been known to leach chemicals into the groundwater. However, in 1993 a Canadian company, PipeSak Incorporated, developed and began marketing a new type of set-on weight that has been quietly taking the conservative pipeline industry by storm-geotextile fabric weights.

Figure 2 - Concrete set-on weights

The Geotextile Polypropylene Saddleweight

Geotextile fabric weights (GFW) are made from very tough, non-biodegradable geotextile polypropylene fabrics that, when filled with local gravel ballast, are set onto a pipeline to achieve buoyancy control. Unlike concrete, fabric weights can be filled quickly, easily installed and, with their low center of gravity, are never at risk of tipping off the pipe. Other advantages include lower overall costs, less risk of cathodic shielding since the type of geotextiles used have a high permeability to water, and increased environmental suitability with local gravel used as ballast.

Geotextile polypropylenes were developed for underground use and as such are considered "non-biodegradable" - with life expectancies well in excess of 100 years. Polypropylene fabrics are considered one of the most inert man-made fabrics and were used in the container industry for many years before civil engineers discovered them. Their reliability and longevity has been proven through decades of use in all forms of buried civil structures, acting as soil stabilizers and separators, while maintaining the flow of surface and ground water.

The geotextile fabric weight is typically loaded from the top and made up of multiple chambers that straddle each side of the pipeline. Multi-chamber construction keeps fabric stresses to a minimum by distributing the load along the pipeline as well as helping the weight to maintain a narrower profile - making installation easier.

How are GFW's used?

Typically, any local gravel can be used as the fill ballast for fabric weights. For remote areas where concrete may be particularly hard to obtain, this is a big advantage. Top loading fabric weights can be filled anywhere that a few loads of gravel can be dropped off. For large numbers of weights, filling can be completed with specially designed filling hoppers with weights ready for use in minutes rather than the days and weeks required for concrete. This saves valuable time in the field, should conditions ever change from design. Leftover weights not filled at the end of the project can be easily warehoused.

Figure 3 - Transporting GFW's

If the filling location is not on site, filled fabric weights can be easily trucked to the site. With their stable footprint, fabric weights travel well (see Fig. 3). Once on site, fabric weights can be strung either directly onto the pipeline or along the ROW for installation later.

Fabric weights can be picked up by sideboom, excavator or crane and placed directly on the pipeline. The bottom portion of the "legs" of the weight are pleated which allow for a gap at the lead edge of the weight. Once centered over the pipeline and quickly lowered, the legs will separate and walk around the pipeline. As the weight is lowered, high tensile polypropylene webbing tensions around the pipes circumference - pulling the legs of the weight tight to the pipe wall.
A few other installation benefits of using fabric weights are;
i) No extra trench depth required. Unlike concrete weights which have most of their weight on top of the pipe, fabric weights have their weight distributed along the sides of the pipeline.
ii) Fabric weights easily conform to the bottom of the ditch decreasing the need to dewater the trench.
iii) Workers are generally not required to be in the trench to release fabric weights, increasing work place safety.
iv) Equipment operators have found that they can install fabric weights almost twice as fast as concrete weights.

Figure 4 - Low profile

Backfilling can commence immediately or be left for days. With their low center of gravity, once fabric weights are installed on the pipeline there is little risk they will tip off. As groundwater slowly percolates back into the trench, water will quickly saturate the weight causing them to snug down even tighter onto the pipeline (see Fig. 4).

Figure 5 - Submerged GFW

Science Behind GFW's

Not only can fabric weights save time and money during installation, but they are considered more environmentally friendly, more pipe friendly, and they weigh more underwater than their concrete equivalents.

Since polypropylene is non-biodegradable and the ballast used is local stone, fabric weights tend to be preferred over concrete weights by environmentalists. There is no risk of chemicals leaching out to either attack the pipe coating or the groundwater.

Fabric weights tend to be much gentler on the pipe. With the heavy grade geotextile fabric used, there is very little risk of ever damaging the pipeline coating - even with fractured stone. Once installed, fabric weights are as permeable as the stone used to fill them - which allows for an easy path for groundwater and cathodic protection. Concrete, although water absorbing has been known to shield cathodic protection. Fabric weights are a soft method of buoyancy control, allowing the pipeline the ability to move slightly while still maintaining buoyancy control - which is particularly important in earthquake zones or areas of other earth instability.

The use of fabric weights can decrease the number of overall weights necessary for any given project. Since the stone ballast used to fill fabric weights generally has a higher specific gravity than concrete, fabric weights tend to weigh more underwater than concrete weights -which means fewer weights to buy AND install.

This is shown by the following example:

The basic equation for saddleweight spacing = Weight of Submerged Saddleweight (kg) / Total Weight Required (kg/m)

As an example, for 762 mm OD pipeline with 9.5 mm wall thickness, you may want a negative buoyancy of 15% in a trench slurry with a specific gravity of 1.15. If we assume both a concrete weight and and fabric weight weigh 3175 kg. and the density of stone* is 2560 kg/m3 (dry bulk density is 1522 kg/m3 ) and concrete is 2242 kg/m3.

*Note: Since GFW's are porous, the void space of stone ballast does not add to the buoyancy of the weight thereby reducing the buoyant volume of the ballast to that of solid block of stone.

Total Weight Required (TWR) = Pipe Buoyancy - Pipe Weight + Negative Buoyancy
Pipe Buoyancy = weight of slurry displaced = 0.456 m3/m X 1150 kg/m3 = 524.4 kg/m
TWR = (524.4 kg/m - 176.6 kg/m) X 1.15 = 400.0 kg/m

Weight of Submerged Saddleweight = Volume of Saddleweight X Submerged Density
For fabric weights = V X (density of stone - density of slurry) = (3175 kg/ 2560 kg/m3) X (2560 kg/m3 - 1150 kg/m3) = 1748.7 kg.
For concrete weights = V X (density of concrete - density of slurry) = (3175 kg / 2242 kg/m3) X (2242 kg/m3 - 1150 kg/m3) = 1546.4 kg.

Finally, with the Center-to-Center Spacing = Weight of Submerged Saddleweight / Total Weight Required
Fabric Weight Spacing = 1748.7 kg / 400 kg/m = 4.37 m
Concrete Weight Spacing = 1546.4 kg / 400 kg/m = 3.87 m
Therefore, for this example, you would use almost 13% FEWER fabric weights than concrete weights.

Additional Benefits

Geotextile fabric weights give the engineer much more flexibility in his design. No longer is the project stuck with the expense of left-over weights or, worse yet, not enough weights. With fabric weights fill only what you need. If any weights are left over, fold them up and warehouse them till the next project - or sell them back to the distributor. Try that with concrete!

Geotextile fabric weights are available for all pipeline diameters with capacities of over six tonnes each and are available world wide.

About the Author

Geoff Connors, P. Eng., has worked in the pipeline industry for over 25 years. He invented the fabric saddle weight while working on the design and construction of some of the largest diameter transmission pipelines in North America. He is now the president of PipeSak Incorporated and continues to revolutionize the pipeline industry with his innovations.

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