Anchors Aweigh - Part 1
New Methods for Installing Synthetic Underwater Pipelines
Many pipelines installed underwater are manufactured from synthetic materials such as high-density polyethylene (HDPE) because of the superior corrosion resistance and, in certain applications, the superior wear resistance of synthetics over iron alloys. Synthetic pipelines are used in many tasks for both industrial and municipal applications. As the depths of the installations and the lengths of the synthetic pipelines are increasing, better methods of installations must be developed.
Pipelines manufactured from synthetic materials do not possess the high tensile and compressive strengths associated with iron alloys. They do possess a relative density that is close to, or less than, that of water. For example, HDPE has a relative density that is less than water, and consequently will float when placed in water. Therefore, synthetics require that large amounts of additional weight in the form of ballast be attached to these pipelines to allow them to sink below the surface and to anchor them firmly on the sea bed. Ballast weight that is attached prior to the installation of the pipeline is the most common type of ballast weighting. Often this ballast weighting consists of precast concrete blocks attached to the pipeline with bolts through openings preformed in the concrete blocks for this purpose.
There are specific problems encountered when installing underwater pipelines, arising from the relationship of the net submerged weight of the pipeline (i.e., the sum total weight of all of the components making up the pipeline) and the material strength of the pipeline itself. This problem is compounded for pipelines produced from synthetic materials as the lack of tensile and compressive strength of synthetic materials can make these pipelines prone to buckling from the forces exerted by the net submerged weight during the sinking process. While this invention aims to enhance the ease of installing synthetic pipelines it also may be applied to pipelines manufactured from metal or other compounds.
The amount of ballast weight attached to pipelines varies with the design of the pipeline and such factors as sub sea terrain, ocean currents, wave action and the type of product or substance the pipeline is designed to carry. A pipeline designed to carry a gas will require a greater amount of ballast weight than a pipeline designed to carry products such as slurry, which has a relative density greater than water. Strong littoral currents or wave induced forces also may dictate that additional ballast weighting be applied to securely anchor the pipeline in specific sections. The amount of ballast weighting installed on the pipeline may not be distributed equally as a function of its length, as the forces of waves or current acting on the pipeline may vary depending on such factors as the variations of the depth and the length of the pipeline. Wave induced forces acting on the pipeline generally will decrease as the depth of water increases. Littoral currents acting on the pipeline generally will increase as the distance from shore becomes greater. Variations of the pipeline elevation due to the sub sea terrain on which it is laid also may dictate that ballast weighting of specific sections must be increased or decreased for a specific section.
The ballast weighting of pipelines forms a significant portion of the total economic value of the pipeline. The amount of ballast weighting influences the method of launching the pipeline from its point of construction as well as the method of sinking the pipeline to the sea bed, both of which can be translated into economic costs.
The amount of weight added as ballast to a pipeline is commonly referred to as the offset weighting and is expressed as a ratio of the amount of ballast weight required to offset the buoyant force of a pipeline assumed to be partially or totally filled with air at atmospheric pressure. For example, an offset weight requirement of 50 percent means that the ballast weight added to the pipeline negates the buoyant force of the pipeline if it were filled to 50 percent capacity with air. An offset weight requirement of 110 percent would mean that the ballast weight added to the pipeline would negate the buoyant force of the pipe if it were possible to fill the pipeline with air to 110 percent of its volume. While filling a pipeline to 110 percent of its volume is not possible, the practice of expressing the weight requirement in this way gives an exact indication of the amount of ballast weight required in relation to the size of the pipeline.
Offset weight requirements in excess of 95 percent generally will require that auxiliary buoyancy in the form of floats, vessels etc., be temporarily attached to the pipeline, to allow it to float on the body of water prior to its placement. In lieu of using auxiliary buoyancy, the pipeline designer may elect to remove some of the ballast weight prior to the pipeline installation and install the deleted ballast weight after installation.
As previously mentioned, it is beneficial to the design of pipelines to vary the amounts of ballast weight applied to specific sections of the pipeline. These sections may be identified as a section of the pipeline located a certain distance from a specific datum or reference point.
As an example of changing ballast weighting, a hypothetical pipeline of 10,000 feet in length terminating at a depth of 500 feet, which is built to discharge a municipal or industrial effluent into an ocean or other body of water, may start at the shore with a 300 percent offset weighting to counter strong wave induced forces. After acquiring a distance of 1,000 feet from the shore and a depth of 75 feet the offset weight may be reduced to 110 percent. At 3,000 feet from the shore the pipeline depth is 200 feet and the offset weight may be further reduced to 100 percent. A change in pipeline elevation, necessitated because of a rise in the sea bed starting at 5,000 feet from the shore, which now exposes the pipeline to littoral or wave induced currents, now may require the offset weight to be increased to 150 percent for a 1,500 foot section of the pipeline. The remainder of the pipeline may now have the offset weight reduced to 100 percent.
The invention allows the designers and installers of pipeline the ability to change the offset ballast weight of any section of the pipeline as dictated by ocean currents or sub sea terrain. This change in ballast weight is accomplished by increasing or decreasing the diameter of the ballast tubes in the specific area in question.
It is common practice for installers of pipelines such as those manufactured from HDPE to construct the pipeline on land, adjacent to the edge of the body of water where it is to be installed, complete with all of its attached ballast weights and, if required, auxiliary buoyancy vessels. Depending on the total length and size of the pipeline as well as the amount of ballast weight attached, it is possible to launch the complete hermetically sealed pipeline into the water to float on its own inherent buoyancy or the combination of its own buoyancy supplemented by the auxiliary buoyancy vessels.
It often is not practical to launch pipelines of great size and length in one piece, and installers of such pipelines may choose to fabricate the pipeline in shorter more easily managed sections. The shorter sections are subsequently launched individually and can be joined together from a barge equipped for this purpose or the sections can be joined together as they enter the water one behind the other. Regardless of the method of launching the pipeline, the same basic principles apply, that is, to position the floating hermetically sealed pipeline over top of the chosen underwater pipeline corridor and then to remove or otherwise delete the buoyancy factor to allow the pipeline to sink to the sea bed.
While laying the pipeline onto the sea bed, the auxiliary buoyancy vessels are flooded or otherwise removed from the pipeline. Auxiliary buoyancy vessels generally are designed to be filled with air and therefore must meet the structural requirements for pressure vessels. Auxiliary buoyancy vessels may not be practical for pipelines laid at great depths because of the significant expense to manufacture them. As previously mentioned, an alternative to attaching auxiliary buoyancy is to delete some of the ballast weight from the pipeline prior to installation. The ballast weight deleted is installed after the placement of the pipeline on the sea bed. This can be accomplished by lowering the ballast weights from the water surface and placing the ballast weights on the pipeline with the use of divers or remotely operated vehicles. However, this method can become economically impractical, depending on the length of the pipeline and the depth of the area requiring post ballasting.
"S" bend sinking is a commonly used method of positioning synthetic pipelines, complete with ballast weighting attached, to the sea bed. "S" bend sinking is accomplished by introducing water at one end of a floating pipeline while simultaneously venting air from the opposite end. As the water is introduced the pipeline loses its inherent buoyancy and the end of the pipeline, where the water is being introduced, sinks to the sea bed. The remainder of the pipeline is floating on the water surface, until the water being introduced propagates further along the pipeline causing the further reduction of buoyancy and allowing the sinking to continue until the entire pipeline rests on the sea bed. During the sinking process, the portion of the pipeline from the last point touching the sea bed to the portion floating on the surface forms the approximate shape of an "S". During the sinking process the pipeline is subjected to bending stresses throughout the "S" which must be controlled by applying axial tension to the pipeline to limit the amount of curvature in the pipeline. Failure to minimize the bending stress in the pipeline by the application of axial tension will result in pipeline buckling.
Pipelines with offset weighting approaching 100 percent and laid in deep water may require enormous amounts of tension to be maintained during their sinking as a means of maintaining the pipeline curvature within the minimum-bending radius as specified by the manufacturer of the pipe. If the proper tension is not maintained throughout the sinking process serious damage to the pipeline will result. Generally, large tugboats or winches are employed to achieve these tension requirements.
Applying large amounts of axial tension, especially in the case of thin wall synthetic pipelines, may not be practical as the tension requirements may exceed the tensile strength of the pipeline composition. In addition, applying axial tension can invoke significant costs and often is not practical from an economical perspective.
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Part 2 will describe the design of the new weighting process.
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