This invention relates to the laying of underwater pipelines and is particularly concerned with the installation of offshore pipelines from floating vessels.
A number of methods have been developed for the laying of underwater pipelines in recent years. These include (1) the bottom-pull method wherein the line is fabricated on shore, launched into the water, and then pulled into position along the bottom by means of winches mounted on a barge or the opposite shore; (2) the flotation method in which long strings of pipe fabricated ashore are towed to the site on floats or pontoons, connected by lifting adjacent ends out of the water with the cranes on a tie-in barge, and then lowered into place by systematically removing the floats or pontoons; (3) the reeled pipe method in which the pipe is unwound from a large reel mounted on a barge and lowered into place on the bottom; and (4) the lay barge method wherein individual sections of pipe are welded together on a barge and then lowered into position over a stinger at the stern as the barge advances. Each of these methods has limitations which restrict its application. It has been found that the bottom-pull method is generally practical over only short distances; that the flotation method is difficult to control and may result in buckling or the loss of long pipe sections at sea; and that the reeled pipe method is normally restricted to pipe of relatively small diameter. Most operations requiring the laying of relatively large diameter pipe in the open sea have therefore been carried out by using the lay barge method.
The lay barge method normally requires the use of a barge provided with multiple work stations arranged to permit handling of the pipe in an assembly line manner. Each section of pipe is welded into place, the weld is inspected, and the field joint is coated before it moves down the stinger at the stern of the barge. The stinger at least partially supports the line between the barge and the ocean floor to prevent the generation of excessive bending stresses. Wheel or track-type tensioning devices on the barge hold the pipe in tension and thus further aid in controlling stresses. Experience has shown that this method is generally slow because of the time consuming pipe handling and welding operations carried out on board the barge, that it is expensive because of the long periods over which marine equipment must be made available, and that it is difficult to use in deep water because of limitations on the amount of tension that can be applied to the pipe as it is lowered into place. Costly delays are often necessary because of changes in the weather before a job can be completed. Efforts to overcome these and related disadvantages have in the past been only partially successful.
The present invention provides an improved system for laying underwater pipelines which avoids many of the problems encountered heretofore and generally permits much better control of the lowering operation and faster pipelaying than would otherwise be possible. The method involves the use of a long preassembled floating pipeline section which extends in front of a lay barge and is lowered to the sea floor from the stern of the barge as it advances. The suspended portion of the pipeline between the sea floor and the end of the floating section is maintained in tension by a second vessel positioned in front of the barge. When the lay barge reaches the end of the floating section, tension is transferred to the lay barge by clamping the pipe in a fixed position and the second vessel then moves off. Another section of pipe is connected in place by welding the ends together on board the barge, tension is applied to the new section by the second vessel, tension is released by the lay barge, and laying of the line is resumed. This eliminates the necessity for storing and handling individual pipe sections on board the lay barge; reduces the number of welds or other connections that must be made at sea; simplifies inspection, testing and coating of the pipe; permits the application of tension to the pipeline without relying on conventional tensioning devices; makes possible the laying of pipelines in deeper water than would otherwise be feasible; facilitates the laying of pipelines over trenches that are difficult to traverse by conventional methods; simplifies mooring problems during pipelaying operations; and has other advantages.
In carrying out the invention, long preassembled pipeline sections are first made up on shore or in shallow water and are capped or plugged at their ends. Buoys may be attached to help support the sections if necessary. These long floating sections are then towed to the pipeline site by one or more tugs or other vessels. A lay barge to be used in installing the line will normally be moored at the site near an offshore platform or other installation from which the line is to extend. The first floating pipeline section is aligned with this barge so that it extends outwardly in front of the barge. One end of this first section is pulled onto a pipe ramp or similar device at the bow of the lay barge while a tug or pulling barge holds the other end of the floating section in tension and maintains its alignment with the barge. If necessary, tugs or barges can also be stationed at intermediate points to help maintain alignment. Traction on the line is continued by means of winches on the lay barge until the end of the line extends over the barge to a stinger at its stern. After the end of the pipeline has been lowered to the ocean floor, it is secured to the platform or bottom in the conventional manner.
Once the initial pipeline section is in position on the lay barge and arrangements have been made for securing the end of the section at the sea floor, laying of the line may be commenced. The floating end of the line is held in tension by the tug or other vessel positioned in front of the lay barge as the barge moves forward along the floating pipeline section, lowering the line into place behind it. When the lay barge reaches the end of the first section, it takes over the tension from the tug or pulling barge. This is done by clamping the line in place on the lay barge pipe ramp so that it cannot move with respect to the barge. A second floating pipeline section which has been towed from shore or shallow water and aligned with the lay barge is then pulled up the pipe ramp at the bow of the barge and the ends of the two sections are welded or mechanically connected together. After the connection has been made and the joint has been inspected and coated, tension is transferred to the tug or crane barge and the lay barge again moves forward, lowering the pipeline into place over its stern. This procedure is continued until the entire line has been laid.
As pointed out earlier, the method of the invention substantially reduces the cost of installing offshore pipelines. Marine equipment required for the pipelaying operation is often needed for much shorter periods of time than in conventional operations and hence very significant savings are possible. In some cases it also permits pipelaying operations to be carried out in deep water where conventional methods are ordinarily not feasible. These and other advantages of the method make it attractive for use under a wide variety of conditions. It is particularly useful in areas such as the Artic Ocean and the North Sea where the working seasons are short and periods of good weather may last for only a few days at a time.
FIG. 1 in the drawing illustrates an initial stage in the installation of a light weight offshore pipeling in accordance with the invention;
FIG. 2 is a top view of a lay barge suitable for use in carrying out the method shown in FIG. 1;
FIG. 3 is a longitudinal view of the barge of FIG. 2;
FIG. 4 illustrates an early stage in the laying of a negatively buoyant pipeline by the method of the invention; and
FIG. 5 represents a later stage in the operation shown in FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The apparatus depicted in FIG. 1 of the drawing includes a long preassembled pipeline section 10 which has been fabricated on shore or in shallow water, capped or plugged at each end to prevent the entry of water, and towed out to the pipeline location by one or more tugs not shown in the drawing. The pipe used in fabricating section 10, which may be of any desired diameter, is welded together or mechanically connected by means of threaded joints, inspected for the detection of leaks, and then given an external coating of an asphaltic material or other agent for the prevention of corrosion. An internal protective coating and an outer sheath of concrete or similar weighting agents sufficient to give the desired specific gravity may also be provided if needed. The pipe should be sufficiently light to float at the surface 11 of the water when it is filled with air and hence heavily weighted pipe will not normally be used. The length of each pipeline section will depend in part on the size of the pipe used, the sea conditions, and the distance over which it is to be towed but will normally range from about 300 feet to about 3,000 feet or more. Generally speaking, the lengths should be the maximum compatible with wave height, current velocities, and equipment capabilities. One of the advantages of the invention is that these long preassembled sections can be made up in an assembly line operation at a protected onshore base and that several long sections can be prepared simultaneously if desired. This use of an onshore base permits fabrication much more rapidly and at considerably less expense than would be incurred if the pipeline sections were made up at sea.
A lay barge 12 is positioned at the pipeline construction site. This barge, shown in greater detail in FIGS. 2 and 3 of the drawing, includes an elongated barge hull 13 containing a hold in which supplies, fuel and equipment may be stored. A superstructure 14 including control equipment and the like is positioned on the deck of the barge as shown. The barge employed may be moved by a spread mooring system technique using winches 15 or may be provided with a propulsion system which does not appear in the drawing. Such a system will normally include a propeller at the bow and stern of the vessel for providing forward thrust and two or more laterally directed propellers near the bow and stern for maintaining the desired heading or changing the lateral position of the barge as necessary. These "side thrusters" may be operated selectively to permit precise control of the vessel's position under various wind, wave and current conditions.
On one side of the barge is a double pipe ramp 16 which slopes downwardly from points near the middle of the barge to points near the water level at the bow and stern. The ramp is provided with rollers 17 or other means for guiding and supporting the pipeline section 10 as it moves over the ramp. A working area 18 is located amidships between the bow and stern portions of the pipe ramp to permit the welding or mechanical connection of adjacent pipeline sections to one another and the inspection and coating of the joints. The floor of the working area may be recessed with respect to the adjacent deck as shown to permit ready access to the full pipe circumference. In lieu of this, the pipe ramp may be made higher so that the working area is flush with the deck. Hydraulically or mechanically actuated pipe supports 19 and 20 are located within the working area. These supports, which may be of conventional design, are fitted with rollers on which the pipe rests and with clamps 21 and 22 for holding it in position. Each of the supports can be adjusted to permit precise alignment and abutment of the ends of two adjacent pipe sections. Davits 23, 24, 25, 26 and 27 are located at intervals along the deck above the pipe ramp to facilitate handling and positioning of the pipe as required. Pipe handling equipment of this type is described in the literature in connection with lay barges of conventional design and will therefore be familiar to those skilled in the art. The barge may be used in conventional pipelaying operations by installing a temporary extension of the deck over the forward end of the pipe ramp so that pipe sections can be moved from the deck onto the ramp.
The lay barge 12 may also be equipped with a pipe tensioning device 28 of conventional design if desired. Such a device is not normally employed in carrying out the improved method but may be provided for use in conventional operations. If provided, this device will normally include tracks or wheels which engage the pipe and generate friction to restrict its motion relative to the barge. The tensioning device will normally be located near the stern of the barge between work station 18 and stinger 29. The stinger is an elongated buoyant member provided with rollers or similar members 30 which is hinged to the barge and supports the pipe as it is lowered to the sea floor. Any of a wide variety of stingers of conventional design may be employed. Only a portion of the stinger is shown in FIGS. 2 and 3. A crane 31 and other conventional equipment not shown in the drawing may also be provided to facilitate the movement of pipeline sections onto and off of the barge. It will be understood that the method of the invention is not restricted to use of the particular barge shown and that barges fitted with outrigger devices or other means in lieu of a double pipe ramp can be employed if desired.
After the first preassembled long pipeline section has been towed to the pipeline construction site, it is positioned by the tugs used in the towing operation so that it extends in the direction the pipeline is to be laid with one end near the bow of the lay barge. One or more lines is then extended from the lay barge to the near end of the pipeline section and the section is pulled onto the pipe ramp at the bow of the barge. Here the end of the pipeline section is connected to a line from a winch on the deck and the section is pulled up the ramp and onto the barge. The cap or plug is then removed from the end of the pipeline section.
Concurrently, a pulling barge 34, a tug, or a similar vessel is positioned near the other end of the floating pipeline section. A line 35 from the pulling barge is attached to the end of the pipeline section and the end is lifted clear of the water. The cap or plug in this end of the pipeline section is then also removed. The purpose in raising the end of the section and removing the cap or plug is to permit the escape of air from the pipe and thus permit the entry of water. In lieu of lifting the pipe, the end may be connected to a buoy-supported hose or the like. Tension is applied to the floating line by means of a winch 36 on the barge to maintain alignment with the lay barge. At this point, assuming that the pipeline is to extend from an offshore platform 37 or similar installation near the lay barge, a cable may be run from a winch on the deck of the barge to an underwater sheave on the platform and brought back to the end of the pipeline section to assist in positioning the end of the section when it reaches the sea floor. The pipe is then lowered over stinger 29 and pulled into position adjacent the base of the platform by means of the winches on the lay barge.
As the pipeline section enters the water at the stern of the barge, the water rises within it so that the level is substantially the same as the outside water level. Since the suspended end of the pipe is thus filled with water, it is considerably heavier than most pipelines installed from conventional lay barges. For this reason, the stinger shown in this particular embodiment of the invention is designed to support the pipeline between the lay barge and a point near the sea floor. If desired, the stinger may be provided with buoyant members to further aid in supporting the line and in shallow water may be designed to drag along the bottom. The design considerations for stingers are well known to those skilled in the art and need not be set forth in detail to permit an understanding of the invention. The tension maintained on the line by pulling barge or similar vessel 34 after the end has been secured to the platform or sea floor reduces the bending stresses in the line between the end of the stinger and the sea floor and hence often permits the use of a somewhat shorter stinger than might otherwise be required.
After the open end of the pipeline has been positioned adjacent the base of the platform or similar structure 37, it may be anchored to the bottom or tied to the platform if desired. Once the end is in place, the lay barge begins to move forward in the water toward pulling barge 34 while the pipeline section moves across the lay barge ramp, down the stinger 29 and onto the bottom. The lay barge may be propelled by means of its mooring system or dynamic positioning equipment or by one or more tugs. Tension is maintained at the floating end of the line by means of a winch on the pulling barge. This forward movement of the lay barge and laying of the line is continued until the lay barge reaches a point near the pulling barge. The suspended end of pipeline section 10 is then lowered into the water by the crane on barge 34. The lay barge moves forward until the end is in position on the barge and is ready for the addition of another pipeline section. Tension is maintained by the pulling barge until the end has been secured by pipe clamp 21. This maintains the necessary tension and thus permits release of the pipeline by the pulling barge. The pulling barge then moves forward to a point about one pipeline section length in advance of the lay barge.
After the first pipeline section has been positioned as described above, a second pipeline section which has been towed to the construction site from the shore base is brought up by tugs to a position in front of the lay barge. This section, like the previous one, is capped or plugged at both ends so that it will float. A line from the lay barge is connected to the near end of the second section and the end is pulled up the pipe ramp onto the barge into a position where it can be clamped in place adjacent working area 18. At the same time, a line from the pulling barge or other vessel is affixed to the other end of the second section and the end is raised from the water. The caps or plugs are then removed from both ends of the second pipeline section.
The ends of the two pipeline sections on the lay barge are aligned by adjusting pipe supports 19 and 20 and clamping them in place. The two ends can then be welded or mechanically connected together. In the interest of speed, it is generally preferred that the sections be joined by means of automatic welding equipment, but other means may be used if desired. Once the connection has been made, the pulling barge applies tension and clamps 21 and 22 on the lay barge are released. The lay barge once more begins to move forward toward the pulling barge. As the barge moves, the pipe is lowered to the ocean floor at the stern of the barge. Because of the long pipeline sections used and the relatively small number of joints that have to be made up at sea, laying of the line normally progresses very rapidly. The marine equipment is generally required for a much shorter period of time than would be the case if individual sections of pipe were welded together aboard the lay barge and then installed. The use of a pulling barge or similar vessel to apply tension to the line as it is laid permits the application of much greater force than can generally be obtained with the conventional constant tension restraining devices that are used on barges where the pipe is carried aboard the barge and made up one joint at a time. The use of a pulling barge for the application of tension permits operations in much deeper water than would otherwise be possible.
The pipeline installed as described above can be used for the transmission of crude oil or other liquids but will normally not be satisfactory as a gas line unless it is anchored to the sea bottom. Because of the low weight, the line will tend to rise if filled with gas. Anchors can be employed if desired but it is generally preferred to employ an alternate method where the installation of a heavier line to be used for gas transmission purposes is necessary or where the line must be laid in deep water.
FIGS. 4 and 5 of the drawing illustrate an alternate embodiment of the invention suitable for the laying of heavier pipe in relatively deep water. The area depicted in FIGS. 4 and 5 is one in which the pipeline route crosses an ocean floor trench which may be 5 miles or more in width and from 1,500 to 2,000 feet or more in depth. Such trenches, common in many areas, present severe problems in conventional pipelaying operations because of difficulty in mooring the lay barge in deep water and because of limitations on the amount of tension that can be applied to the line with conventional track or wheel type tension devices. At present, such devices can seldom be used to apply more than about 150,000 lbs and in many cases this is insufficient to permit laying of the line in water more than a few hundred feet deep.
The method shown in FIGS. 4 and 5 is similar to that described earlier in that it is carried out by first constructing long preassembled pipeline sections 40 at a shore base where assembly line equipment is available. Each pipeline section will again be from about 300 feet to about 3,000 feet or more in length. The pipe may be of any desired diameter. Each pipeline section is capped or plugged at the ends to prevent the entry of water. The pipe employed is sufficiently heavy that it will tend to sink even though filled with air and hence buoys 41 are attached at intervals along each section. Any of a variety of different types of buoys may be used. The spacing of the buoys will depend upon the size of the individual buoys and the size and weight of the pipe employed. Each pipeline section should be as long as possible, considering wave heights, current velocities, and equipment capabilities. Two or more tugs or other vessels will normally be used to tow each pipeline section to the construction site.
A lay barge 42 similar to that shown in FIGS. 2 and 3 of the drawing may be used in carrying out the method depicted in FIGS. 4 and 5. This barge will normally be fitted with a stinger 43 but the use of such a stinger is not always essential. External floats or other means can be used to support the line without a stinger if desired.
After the first pipeline section has been towed to the construction site, it is positioned with one end near the front of lay barge 42 and the other end extending along the pipeline route in front of the barge. A line 44 extends from the forward end of the section to a winch 45 on pulling barge 46 to assist in maintaining the floating section in the proper position. The pulling barge is moored by means of lines 47 and 48. Once the floating pipeline section is in position, one end is pulled up the forward end of the pipe ramp on the lay barge. Lines extending from winches on the deck of the barge may be used for this purpose. The other end is held with sufficient tension to maintain alignment of the pipeline section in front of the lay barge. The buoys attached to the pipeline section are removed as the section comes aboard the lay barge and are stored for return to the shore base. The lay barge then begins to move ahead while pushing the capped or plugged end of the pipeline down the stinger at the stern of the barge. The air contained in the pipeline section will contribute buoyancy and thus aid in supporting the section between the barge and the ocean floor. Once the end of the pipeline reaches the sea bottom and is secured to the bottom or a platform, sufficient tension must be applied by the pulling barge at the other end to keep the pipe supported and prevent excessive bending. The tension needed under particular operating conditions can be calculated. If the end of the pipeline is to be located near a platform or similar installation, it can be positioned adjacent the platform by running a cable from a winch on the lay barge to a pulley attached to the base of the platform and back to a padeye on the end of the pipeline. This will permit pulling of the end of the pipeline section to the platform if necessary. Other means for securing the end of the first pipeline section include the use of anchors and pilings. The tension required to prevent excessive bending of the line can be supplied by means of a pulling barge or tug as shown.
After laying of the pipeline has been started as described above, the lay barge continues to move forward along the floating pipeline section, removing buoys, until it reaches a point near the end of the floating section. The pulling barge maintains tension in the pipeline until it has been secured by clamp 21 on the lay barge. The pulling barge then releases the line and moves forward to receive the next pipeline section. The cap or plug on the end of the line is then removed. Since the line is capped at the bottom, it remains filled with air. By this time, a second pipeline section has been towed into position in front of the barge. A line is attached to the end of the second section and it is pulled up the pipe ramp at the bow of the lay barge by means of winches on the barges. The cap or plug is removed from the end of the second section and the two sections are aligned. Tension is maintained in the second pipeline section by means of a line extended from the pulling barge to the forward end of the section. The two pipeline sections are then welded or mechanically connected together. Once this has been done, the lay barge begins to move forward again while the pulling barge maintains tension in the pipeline. The buoys attached to the floating line in front of the barge are removed as the line is lifted from the water onto the barge. These buoys are returned to the shore base for use in supporting additional pipeline sections as they are moved from the base to the construction site.
The method of FIGS. 4 and 5 permits the crossing of deep trenches such as that shown without the difficulties encountered in conventional pipelaying operations. In FIG. 4, the pulling barge is moored on the far side of the trench and the floating pipeline extends across the trench to the lay barge. Behind the lay barge, the pipeline extends down the stinger and onto the ocean floor. Since the pulling barge maintains the line in tension, the lay barge can be moved forward by towing it along the floating pipeline. This eliminates the necessity for using mooring lines to move the lay barge and avoids difficulties normally associated with the placement of anchors in very deep water. As much as 400,000 lbs of tension can be applied to the pipeline as it extends down the stinger to the sea floor at the bottom of the trench and hence excessive bending of the pipe can be avoided. Once the line has been laid across the trench and the lay barge has reached the other side as shown in FIG. 5, the amount of tension applied to the line can be reduced and operations can be continued as described earlier.