Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Reexamination Certificate
2000-07-18
2002-03-19
Bagnell, David (Department: 3673)
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
C405S158000, C138S030000, C138S031000
Reexamination Certificate
active
06357966
ABSTRACT:
BACKGROUND OF THE INVENTION
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 a wide variety of 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.
Unlike pipelines manufactured from iron alloys, pipelines manufactured from synthetic materials do not possess the high tensile and compressive strengths associated with iron alloys. Unlike pipelines manufactured from iron alloys, synthetic pipelines possess a relative density which is close to, or less than, that of water. HDPE for example, has a relative density which is less than water, and consequently will float when placed in water. Because the relative density of synthetics is close to that of water they 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 upon 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 pre-cast 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, that is 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 may also 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 may also 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 upon the pipeline may vary depending upon such factors as the variations of the depth and the length of the pipeline. Wave induced forces, acting on the pipeline, will generally decrease as the depth of water increases. Littoral currents, acting on the pipeline, will generally increase as the distance from shore becomes greater. Variations of the pipeline elevation due to the sub sea terrain, on which it is laid, may also dictate that ballast weighting of specific sections must also 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. As an example, an offset weight requirement of 50% means that the ballast weight added to the pipeline negates the buoyant force of the pipeline if it were filled to 50% capacity with air. As another example, an offset weight requirement of 100% 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% of it's volume. Filling a pipeline to 110% of its volume is not possible although 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% will generally require that auxiliary buoyancy, in the form of floats, vessels etc., be temporarily attached to the pipeline, to allow it to float upon the body of water prior to its placement on the bed of the body of water. 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 from a specific datum or reference point. This variation of ballast weight, on these sections of the pipeline, can increase or decrease. 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% 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%. At 3,000 feet from the shore the pipeline depth is 200 feet and the offset weight may be further reduced to 100%. 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, may now require the offset weight to be increased to 150% for a 1,500 foot section of the pipeline. The remainder of the pipeline, after the 1500 foot section over the rise in sea bed elevation, may now have the offset weight reduced to 100% for the remaining 3,500 feet.
The invention allows the designers and installers of pipelines 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, for pipelines with offset weight designed at or exceeding 100%, auxiliary buoyancy vessels. Depending upon 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 upon its own inherent buoyancy or the combination of its own buoyancy supplemented by the auxiliary buoyancy vessels.
It is often not practical to launch pipelines of great size and length in one piece, because of the sheer bulk of the total aggregate weight of the pipeline and its components, 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 on
Bagnell David
Mitchell Katherine
LandOfFree
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