You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION  
       [0001]     Hydrocarbons from a deepwater field (in a sea of at least 100 meters depth) can be produced by the use of a FPU (floating production unit) that carries equipment for removing sand, stones, water, etc. from hydrocarbons that are produced from subsea wells, storing the hydrocarbons, and offloading the stored hydrocarbons to a tanker at intervals. As a field is developed, risers are laid on the sea floor to connect the FPU to additional wells. A large field may be developed over a period of years, as the drilling and completion of wells progresses. Construction vessels that have been used to lay down steel pipe in a pipe string that extends from a well to the FPU, are large and costly, with working rates that may exceed $200,000 per day. A lower cost way to install a pipe string between the FPU and each of a plurality of seafloor well heads over a long period of time, would be of value.  
       SUMMARY OF THE INVENTION  
       [0002]     In accordance with one embodiment of the invention, a method and apparatus are provided for installing risers in the form of pipe strings that extend between a FPU (floating production unit) and sea floor well heads, in an economical manner. The method includes connecting pipe sections at the FPU and lowering them with their axes initially extending primarily vertically, into the sea to create a pipe string composed of the pipe sections. While pipe sections are added at a near end of the pipe string that lies at the FPU, the far end of the pipe string which is opposite the near end, is pulled by a tug boat toward the well head to which the pipe string is to be connected. The far end of the pipe string is attached to a tug line that extends to the tug boat. The tug line initially is long and extends in a double catenary curve, and is pulled in by a tug winch to pull the far end of the pipe string until the pipe string extends in a double catenary curve. The tug boat moves forward to continue to pull the far end of the pipe string, with pipe string bending controlled by maintaining the near end of the pipe string at an angle to the vertical which is within a limited range such as between 6° and 12°.  
         [0003]     When a portion of the pipe string rests on the sea floor, the tug boat continues to pull the far end of the pipe string while the pipe string slides along the sea floor. The near end angle of the pipe string is maintained within the limited range by moving the tug boat in steps every time a number of new pipe sections is added to the near end of the pipe string. When the far end of the pipe string lies close to the intended well head, the FPU lowers the near end of the pipe string so it lies deep under water. The tug pulls the far end of the pipe string and lowers it to connect it to the well head. The FPU then lifts the near end of the pipe string up to the FPU. This results in the near end angle increasing as from the range of 3°-12° to 20°, which results in greater pipe string tension in the fully installed pipe string.  
         [0004]     The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is an isometric view of a production system of the invention, showing a FPU (floating production unit) and the near end of a pipe string, and showing a tug boat.  
         [0006]      FIG. 2  is a plan view of the production system of  FIG. 1 .  
         [0007]      FIG. 3  is a side elevation view of the production system of  FIG. 1 , showing the tug boat and pipe string at four different position during the installation of the pipe string.  
         [0008]      FIG. 4  is a side elevation view of the production system of  FIG. 3 , shown at four later positions during the installation of a pipe string.  
         [0009]      FIG. 5  is a side elevation view of the FPU of  FIG. 3  during final stages in the installation of the pipe string. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]      FIG. 1  illustrates a hydrocarbon production system  10  which includes a FPU (floating production unit)  12  that has a vessel hull  14  that is anchored to the sea floor as by mooring though multiple mooring lines  16 . A J-lay tower  22  is mounted on the hull, at one side  24 , with the J-lay tower extending beyond the hull side. The J-lay tower includes a winch  25  that is designed to hold a long pipe section  21 , with the axis  23  of each pipe section being primarily vertical as its lower end is connected to the upper end of the last pipe section. The following description of the installation of a pipe string assumes the use of steel pipes of 10 inch diameter that come in lengths of 24 or 37 meters. A string of pipe sections may have a total length of 1000 meters or more, so a large number of pipe sections will be connected in series to create a pipe string that is commonly referred to as a riser after it is installed.  
         [0011]      FIG. 1  shows a tug boat  30  which applicant uses to pull out the pipe string to install it. The tug boat, or tug, is a dynamically positioned boat with a GPS (global positioning system) unit, so it can maintained itself at a desired position and can be moved in any direction on the sea surface. The tug boat has a motor-powered winch  32  that allows it to pay out and pull in a tug line  34 , such as a steel cable. The PDU has hydrocarbon processing facilities  40  as to remove sand and water from hydrocarbons, with ports  42  for receiving hydrocarbons from installed pipe strings, or risers.  FIG. 2  shows that the FPU may be connected to a large number of pipe strings, or risers  44  that each extends to a different well head  46  that connects to a well in a sub sea reservoir  50 .  
         [0012]      FIG. 3  shows four different stages in the installation of a riser. In a first stage, the rear of the tug is at position A, and a first pipe section  52  has been lowered from the J-lay tower  22  while a connection is being made to the next pipe section  54 . The upper end of the first pipe section is considered to be the near end  56  of the pipe string (now consisting of a single pipe). The tug line  34  extending rearwardly R from the tug, extends in a double catenary curve  60  (portion  62  extends at a forward-downward incline and portion  64  extends at a forward-upward incline) to the lower end of the first pipe section  52 . The rear end of the tug line is attached by a connector  65  to the front or far end of the pipe string. As additional pipe sections are attached in tandem to the first one, the tug operates its winch  32  to pull in the tug line so the pipe string extends along progressively greater percentages of the combination  66  of pipe string and deployed length of pull line, until the pipe string portion at  68 B extends in a double catenary curve as at tug position B. Each portion of the pipe string can be referred to as a pipe string because it includes a string of pipes, or as a pipe string portion because it is a portion of the final, longer pipe string, or riser.  
         [0013]     During the lengthening of the pipe string by attaching additional pipe sections, applicant controls the orientation of the pipe string portion extending down from the FPU by closely monitoring the deployment angle, or near end angle  70  of the pipe string. Applicant maintains the angle  70  within a predetermined range between about 3° and 12° (2° to 15°) from a vertical line  72 . The near end angle  70  can be measured as the angle of the pipe string under the FPO or the average angle of the top two pipe sections that lie below the J-lay tower, or the angle at a depth of 50 meters below the sea surface and close to the FPU.  
         [0014]     When the tug pull line  34  has been sufficiently shortened at tug position B and pipe sections are added, the tug is moved forward in steps and maintained in a stationary position between those steps. By now, at tug position B, the connection  65  that lies at the rear of the tug pull line, lies along a forward-upward incline. When sufficient pipe sections have been added that the near end angle  70  has decreased to the lower end of the range such as 3°, the tug is moved forward F until the near end angle has increased as to 10° to 12°. The tug is then held in position while additional pipe sections are added and the angle decreases to 6°, when the tug is advanced again.  FIG. 3  shows the tug at positions B and C with the tug line  34  at a constant length, and with the pipe string portion at  68 B and  68 C being of progressively greater lengths as additional pipe sections have been added. If the final riser is long enough compared with the depth of the sea, then the tug will reach the position D at which the pipe string portion  68 D has a part  74  that rests on the sea floor  76 . The tug continues to move forward in steps while pipe sections are added, with the middle pipe string part  74  sliding along the sea floor. The sea floor has been surveyed to make sure that the pipe string part on the sea floor will not be damaged.  
         [0015]     In one installation method that applicant has designed for a 10 inch steel pipe, the tug initially lies at position A ( FIG. 3 ) which is 300 meters forward of the FPU  12 , with a 750 meter length of tug line  34  extending to the FPU. The pipe string lengthens as pipe sections are added, and the tug pulls in its pull line, and then moves forward in steps with pipe sections added between steps. A tug pull line length of a plurality of hundreds of meters will be required in most systems.  
         [0016]      FIG. 4  shows the tug as it moves progressively more forward through positions E, F and G until it reaches a position H. Then, the winch on the tug is operated to pull in its tug pull line  34  so the connector  65  lies at only a small depth. A pipeline end termination  80  is attached to the far end of the pipe string, and the pipe string is lowered to the sea floor to connect to a connection  82  of a well head platform  84  of a well head  86  at the sea floor that has been previously installed. A remotely operated vehicle is usually used to accompany the pipeline end termination down to the well head and to help make the connection.  
         [0017]     Before the far end of the pipe string, which is held by the tug is pulled to the position at G or H, activity takes place at the near end  56  ( FIG. 5 ) of the pipe string, at the FPU (floating production unit)  12 .  FIG. 5  shows that applicant attaches a line  90  to a winching device such as a winch of the J-lay tower or a separate winch, and to the near end  56  of the pipe string, and operates the winching device to lower the near end of the pipe string a plurality of meters (a depth of more than 10 meters) below the FPU to position  56 A. After the far end of the pipe string has been pulled further forward, applicant operates the winch device to raise the pipe string near end  56 A to the deck of the FPU and connects the pipe string near end to the hydrocarbon-receiving port  42  ( FIG. 1 ). The raising of the near end of the pipe string so the near end portion lies at  62 A, increases the near end angle  70  ( FIG. 5 ) from about 6° to about 20° (15° to 28°). With the near end portion at  62 A of the pipe string which is now serving as a portion of the riser, extending at such a large angle, tension in the riser near end part  62 A greatly increases. This higher tension avoids buckling of the near end part  62 A of the riser from compression in the event of storms that produce large waves.  
         [0018]      FIG. 1  shows that the FPU has many ducts that each form connectors  100  that each connects a pipe string in its primarily downward extension into the sea. A separate connection is used for each pipe string, or riser to prevent risers from rubbing against each other. Once in a while, it is desired to extend a pipe string  44 A ( FIG. 2 ) to a well head such as  46 A where the only guide available  100 A is positioned so any pipe string extending from guide  100 A must cross another riser  44 B (as seen in the plan view of  FIG. 2 ). This can be done in the normal way, with the tug pulling the new pipe string  44 A under the existing riser  44 B that it must cross. The near end of the new pipe string  44 A is maintained at a near end angle of 3° to 12° during installation. At the end of installation, the near end portion of the new riser is pulled to a connector  100  that avoids riser crossings and the riser extends at the same near end angle such as 20° from the vertical as the other risers it crosses.  
         [0019]     Thus, the invention provides a method and apparatus for economically installing risers in extension between a FPU (floating production unit) and well heads. The method includes using a tug boat (any vessel that can pull is considered a tug boat) to pull a pipe string (that will become a riser when installed) as the pipe string is lengthened by connecting additional pipe sections to the near end of the pipe string. Bending of the pipe string is controlled by pulling the far end of the pipe string so the near end extends at a near end angle to the vertical where the angle is maintained within a predetermined range that is preferably between 3° and 15°. The tug initially extends a tug line rearwardly long enough so it extends in a double catenary curve to the far end of the short pipe string. The tug maintains a constant position and the tug line is pulled forward into the tug while the pipe string lengthens, until the pipe string extends in a double catenary curve and preferably extends along most of the distance along the forward portion of the double catenary curve. Thereafter, the tug moves forward in steps and stops between steps, and additional pipe sections are added while the tug is stopped. Movement of the tug and adding of pipe sections is controlled so the near end angle of the pipe string remains within the predetermined range. Finally, the near end of the pipe string is lowered into the sea below the FPU, the far end is pulled, and the near end is raised, to increase the near end angle.  
         [0020]     Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

Summary:
A method for installing risers so they extend from a FPU (floating production unit) to multiple subsea hydrocarbon wells of an oil field, which avoids damage to the pipe sections and that is economical. As pipe sections are connected to a near end ( 56 ) of a pipe string portion ( 68 B- 68 D) that lies at the FPU, a tug boat ( 30 ) pulls the far end ( 65 ) of the pipe string toward a well head. Initially, a long pull line ( 34 ) extends from a winch ( 32 ) on the tug boat, in a double catenary curve to the far end of the pipe string. The winch is operated to shorten the pull line until the pipe string extends in a double catenary curve ( 60 ). Tension in the pipe string is maintained at a proper level during installation by maintaining the near end portion of the pipe string so it extends at a near end angle ( 70 ) to the vertical that continually lies within a predetermined range, such as from 3° to 12°.