Novel Approach to Pipeline Weighting Reduces Buoyancy, Cost and Materials

About the author: James C. McGill is a consultant with CRC-Evans Pipeline International, Inc. He has been involved in pipeline buoyancy control since 1983, is a registered professional engineer, is a named inventor on 26 patents including a method of concrete coating plastic pipe, and is a member of the Engineering Hall of Fame at the University of Tulsa.

James C. McGill

Clearly, the pipeline construction industry has identified benefits of the filled membrane over traditional concrete weights.

Pipeline weighting was invented in the early 1900s to prevent submerged natural gas lines from floating in rivers or wetlands. The first weights devised were two cast iron halves that bolted together around the pipe. Bolt-on cast iron weights were later replaced by equally effective, less expensive and easier to manufacture cast concrete weights. By the mid-1900s, set-on weights were introduced. These single pieces of cast concrete that set over the pipe were sometimes called doghouse weights because of their shape. Except for the concrete coating of pipe, no significant advance in weighting technology was developed until the end of the 1900s.

During the 1990s, a new means of providing buoyancy control on pipelines was developed. It involved the use of membranes filled with sand or aggregate and draped over or strapped to pipe.

One such device, the saddle bag weight, is manufactured as a strap-on or set-on. The strap-on weight is a replacement for concrete bolt-on weights, and the set-on weight is a replacement for concrete set-on weights. Each design represents a fundamental advance in pipeline weighting technology.

Industry acceptance of filled membrane weights has progressed rapidly. For example, approximately 17,000 saddle bag weights were sold from their introduction in 1999 through November of 2000. By the end of March 2001, the total had grown to more than 50,000. Clearly, the pipeline construction industry has identified benefits of the filled membrane over traditional concrete weights.

Why Filled Membrane Weights?

Geotextile membranes have been used for several years in such applications as dike lining. The materials, when properly applied, will last up to 100 years, according to independent research reports. This is certainly longer than the anticipated life of a natural gas or water line. When used for pipeline weighting, the filled membrane weights have compelling advantages over cast concrete weights:

• Strap-on filled membranes are safer and substantially less expensive to install than bolt-on concrete weights. No bolts need installing or aligning as with a bolt-on concrete weight.
• Filled membranes conform to the bottom of the ditch and less trench de-watering is required.
• No rock shield is needed between the pipe and the bag.
• Multiple lines can be installed in one ditch without the need to place sand bags between the lines.
• Set-on saddle bag weights do not extend above the pipe and thus less ditch depth is required than with set-on type concrete weights, where most of the weight is provided by concrete that extends above the pipe.
• The fabric is not biodegradable, is not attacked by acidic soil, is permeable to water and supports cathodic protection.
• Lifting straps retain their strength and will not rust.
• Movement of the pipeline will not result in damage to corrosion coating.
• In remote areas of the world where concrete is not readily available, substantial savings can be realized by using locally available stone rather than transporting a ready mix plant and cement to the site, or incurring the cost of transporting heavy concrete weights or concrete coated pipe to the site.
• Membranes can be filled and applied to the pipe immediately without any cure time.
• No concern exists with producing and applying the bags at any ambient temperature, while freezing can destroy the structural integrity of concrete.
• Extra bags can be returned to inventory or easily stored and retained for the next project.

Perhaps most significant is that for almost all ballast materials, filled membrane weights require fewer weights to achieve the same buoyancy control as concrete weights. The reasons for this phenomenon can be found in examining the buoyancy properties of the ballast materials that can be used to fill the membrane weights.

Solid Proof

Reference data (Table 1) demonstrate that the density of solids varies widely depending on where the material is mined. CRC-Evans Weighting Systems, Inc., established a laboratory at its Tulsa, Okla., facilities and tested a variety of aggregates. In a typical case, a sample of crushed limestone weighed approximately 105 lbs. /cu. ft.. The difference between the average of 153 lbs. /cu. ft. for solid limestone and the crushed density of 105 lbs. /cu. ft. is due to the amount of void volume in the crushed stone sample. If one uses 153 lbs. /cu. ft. for solid density and 105 lbs /cu ft for dry crushed density, the void volume is calculated as

Void Volume = 100% ¥ [1 – (105/153)] = 31.37%.

However, the issue in buoyancy control is not the weight in air but rather the weight in water or mud. Archimedes’ Principle states that the upward buoyant force acting on an object submerged in water is equal to the weight of the water displaced. A concrete weight with a volume of one cubic foot will displace one cubic foot of water, thus causing an upward buoyant force of 62.4 pounds. However, a filled membrane weight with a volume of one cubic foot will displace less than one cubic foot of water due to the void space between the ballast particles. In the above example, the upward force is 42.8 pounds as only 0.6863 cubic feet of water are displaced. Knowledge of the void volume of various ballast materials is critical to the proper sizing of filled membrane weights.

In order to establish the void volume of any ballast sample, tests were performed on a variety of crushed stone and sand samples to determine the weight in air and weight in water. From those two measurements, the negative buoyancy available in water or mud can be calculated.

Measurement Techniques

The weight of samples of crushed stone or sand was determined in air by weighing an empty container, adding a known volume of dry solids, and then weighing again. The difference between the two measurements determines the dry density of the material. However, to obtain an accurate weight, the samples must be free of moisture. Moisture can introduce substantial error in the measurements, especially with fine solids.

The second measurement, weight in water, is done in the same manner as the weight in air except that the container is submerged in water. However, care must be taken in the filling of the sample container so that the ballast sample is properly compacted. Compaction of crushed solids is a well-known phenomenon. The general rule is that smaller particles experience greater compaction than larger particles. This is easily demonstrated by generating a loose sample of dry sand, filling a bucket with the sample, and then placing the bucket on a vibrating table and watching the level drop. The same experiment will result in little drop if crushed stone is used.

The use of crushed stone alleviates much of the concern about compaction. However, compaction techniques were developed that result in laboratory sample compaction that matches the compaction obtained when membrane weights are filled in the field.

Buoyancy Calculations

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As mentioned previously, laboratory measurements provide accurate weights of crushed stone or sand samples in air and in water. These measurements are used to determine the negative buoyancy provided by a filled membrane weight (Table 2).

These equations can be used to compare the concrete and filled membrane weights in water. If one considers equal weights of concrete and crushed stone, the following is determined using the 105 lbs /cu ft stone example mentioned previously.

Assume 5,000 pounds of concrete and 5,000 pounds of crushed stone. Downward force of concrete in water is calculated as follows.

Concrete Weight = 5,000 – (62.4)*(5,000/140) = 2,771 pounds

Downward force of crushed stone in water is calculated as follows.

Membrane Weight = 5,000 – (62.4)*(0.6863)*(5,000/105) = 2,961 pounds

One can see that equal dry weights of concrete and crushed stone will result in a greater submerged weight for the crushed stone than the concrete and allow wider spacing of the weights. In this example, the crushed stone weighs 6.86 percent more in water than does the concrete. Thus, the number of crushed stone filled membrane weights compared to concrete weights can be reduced by 6.86 percent; this can result in substantial savings when dealing with large numbers of weights.

Conclusions

Some key inherent benefits of the filled membrane weights can be achieved using any type of stone or sand ballast at any location in the world by applying the laboratory methods detailed here to characterize the ballast.