Hydraulic and earth engineering – Subterranean or submarine pipe or cable laying – retrieving,... – Submerging – raising – or manipulating line of pipe or cable...
Reexamination Certificate
1999-07-21
2001-08-14
Pezzuto, Robert E. (Department: 3671)
Hydraulic and earth engineering
Subterranean or submarine pipe or cable laying, retrieving,...
Submerging, raising, or manipulating line of pipe or cable...
Reexamination Certificate
active
06273642
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to a self-propelled underwater trenching apparatus. More particularly, the present invention is concerned with a self-propelled, underwater buoyant apparatus for burying pipelines or cables laid on the floor of a body of water.
Underwater trenching apparatus are well known in the art. These apparatus are typically used to bury either pipelines or cables under the floor of a body of water, such as across lakes, seas and oceans. Before the invention of underwater trenching apparatus, pipelines and cables were simply submersed under water and due to the high density of steel pipelines and cables, they sink to the bed of the sea.
The pipeline may be laid in a straight line and therefore presumed to remain in a uni-direction so that the trenching apparatus may easily follow and bury the pipeline but underwater current or other disturbances that may for instance be caused by storms vary the direction of the pipeline, often forming them into tight curves or even loops. Such abrupt changes in direction of the pipeline will not permit the apparatus to be steered quickly enough to the new direction and the water jets will tend to continue along the straight path to cut a straight rather than curved trench. The result is that the pipeline is not centerlined with the trench.
Even if the bottom is relatively level and the pipeline in a desired direction, obstacles often are found on the bottom that obstruct the forward movement of the apparatus as it is guided along the pipe. In most instances, the trenching apparatus known in the art requires a diver to be at hand if not riding upon the apparatus to remove such obstacles and prevent damage to the pipe or the apparatus. The requirement of the presence of the diver is not only added expense, but an inefficient method of detecting obstacles since the diver's vision is seriously limited by reason of the bottom cuttings being cast up upon the apparatus as well as the lack of light at the usual depth. A substantially self-sufficient trenching apparatus would avoid the frequency of diver inspection.
While guidance along the pipeline is logical and in most cases an effective method of directing the trenching apparatus, the pipeline is frequently not smooth, either due to cable slings permitted to remain on the line or because of various cathodic protection devices secured to the pipeline to prevent corrosion. These irregularities have in the past prevented guide rollers from passing over such irregularities. If the means of propulsion is a drive roller such drive roller often wears away the corrosion-resisting coating on the pipeline as it attempts to pass over the irregularity. In any event, the stoppage requires the immediate attention of the diver who usually must disassemble the apparatus from the pipe so that it can pass over the irregularity. Substantial loss in time and efficiency inherently occurs.
Numerous other problems arise in an effort to provide a self-propelled underwater trenching apparatus for burying pipeline and the like that requires the minimum amount of attention by the diver and maximum control in the supervising barge or ship. In order to provide a properly functioning trenching and burying apparatus having these characteristics, the attitude of the apparatus is of great importance to assure that it proceeds in the desired course. Means for effecting buoyancy and attempts to control the attitude of the apparatus have been utilized with limited success in the past.
Also of great importance is the requirement that no damage be caused to the pipeline or to its protective coating. When the apparatus is of great weight as is usually true of prior art apparatus, the pipeline and coating are frequently found to be scraped or marked. Lighter weight apparatus would be desirable provided they are capable of withstanding the very high pressures that may for instance exceed 1,000 psi.
DESCRIPTION OF PRIOR ART
There are many underwater trenching apparatus that have been commercially useful to both dig the trench and bury the pipelines. Among those apparatus in the prior art are the trenchers patented by Robert M. Norman, entitled Self-Propelled Underwater Trenching Apparatus and Method (U.S. Pat. No. 4,274,760), Buoyancy and Attitude Correction Method and Apparatus (U.S. Pat. No. 3,926,003), Oscillating Jet Head Underwater Trenching Apparatus (U.S. Pat. No. 4,389,139), Underwater Trenching Apparatus Guidance System (U.S. Pat. No. 3,877,237), and Buoyant Self-Propelled Underwater Trenching Apparatus (U.S. Pat. No. 4,087,981).
The Norman underwater trenching apparatus are all designed to be self-propelled along a pipeline by powered rollers maintained in rolling contact with the pipeline. Essentially, the pipeline serves as a rail-track allowing the underwater trenching apparatus to be guided to dig the trench properly beneath the pipeline. A drawback of such an arrangement is that the powered rollers abrade or otherwise damage the protective coating of the pipeline, thus, curtailing its life span.
As is well known to one skilled in the art, the weight of the apparatus exerts a force on the pipeline and there is a normal reaction force by the pipeline. To move the apparatus along the pipeline, the forces exerted longitudinally along the pipeline by the friction generated between the rollers and pipeline must be greater than the product of normal force and the coefficient of friction of the pipe surface.
On land, air friction is typically ignored because it does not play a significant role at low speeds. But under water, fluid fiction plays a significant role even at a low speed. Furthermore, when the pipeline forms a sharp turn or a loop by water currents, the normal force must be increased again to generate sufficient friction allowing the apparatus to be propelled forward. Inevitably, the enormous amount of friction and normal force exerted between the rollers and surface of the pipeline either abrades the pipeline coating, or deforms the shape of the pipeline, neither of which result is acceptable.
An early apparatus was patented by Urban A. Henry, Jr. entitled Underwater Pipeline-Burying Apparatus (U.S. Pat. No. 3,576,111). This apparatus is operable only when a diver is present serving as an operator. This invention is notably different from the Norman patents because the apparatus moves along the pipeline solely through thrusting nozzles and cutting nozzles instead of powered rollers. To move forward, thrusting nozzles generate a greater thrusting force than the reaction force of the cutting nozzles. While this invention does not erode the protective coating of the pipeline, it has numerous drawbacks. First, the apparatus can only be operated when an operator is present, which escalates the overall operating cost. Second, such a high pressure cutting environment is extremely dangerous to a human operator. Third, because the apparatus is operable only when a human operator is present, many safety precautionary features must be installed, again escalating the overall operating cost.
Johnson et al. discloses an invention entitled Embedding Cablelike Members (U.S. Pat. No. 4,812,079). Essentially, this invention plows a trench on the ocean floor for burying cables. The plow is a soil embedment depressor assembly. The plowing force comes from a winch being pulled against a fixture, such as a riverbank. Such advice would not be useful over large bodies of water to bury pipelines.
Saint E. Saxon discloses an invention entitled an Underwater Trenching System (U.S. Pat. No. 5,456,551). Since this invention also uses powered rollers on a pipeline, it has the same problem of abrading the pipeline protective coating as those of Norman patents.
Charles F. Martin discloses an invention entitled Underwater Trenching Apparatus (U.S. Pat. No. 4,516,880). This invention also contributes to abrading of pipeline protective coating by powered rollers. This invention teaches using two lateral fan thrusters in a weight control tank to provide lateral control.
Gerald G. Reuhl discloses an
Lalos & Keegan
Pezzuto Robert E.
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