Patent Publication Number: US-6210075-B1

Title: Spar system

Description:
CROSS REFERENCE 
     Applicant claims priority from U.S. provisional patent application 60/074,469 filed Feb. 12, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     Offshore hydrocarbon production systems generally include a plurality of wells extending to undersea deposits of oil, with trees located on the sea floor, wherein each tree includes a plurality of valves and pipe couplings. Risers extend up from the trees to apparatus floating at the sea surface that has oil handling equipment. One low-cost production apparatus comprises a spar buoy or spar in the form of a body having a height that is a plurality of times its average width, and usually at least 5 times as tall as wide. The small width of the spar results in only moderate drift in reaction to winds, currents, and waves, which results in only moderate bending of the risers and fluid-carrying pipes therein. To keep the spar upright, its upper portion is made highly buoyant while its lower portion contains considerable ballast to weight it and thereby lower its center of gravity. There are several occasions when it would be desirable to disconnect a spar buoy from the risers that extend down to the sea floor. Some of these include disconnection when icebergs approach, and disconnection to permit use of a workover vessel such as a semi-submersible platform that carries pipes that can extend to the tree to carry tools to clean out wax deposits. In deep seas, expensive workover vessels must be used, with conduits that can extend down to trees at the sea floor. An offshore hydrocarbon production system that facilitated installation of the spar and its disconnection, especially to enable a workover vessel to work on the risers, trees and undersea pipes, would be of value. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, an offshore installation is provided that uses a spar at the sea surface that is coupled through risers extending to the sea floor, which facilitates detachment of the spar in the event of an approaching iceberg or when conduits such as risers must be cleaned, and which simplified set-up of the system and ballasting of the lower end of the spar. The system includes a subsea buoy lying under the spar and attached to the spar. The subsea buoy can be made negatively buoyant to ballast the lower end of the spar and keep the spar upright. The subsea buoy can be made positively buoyant to hold up risers while the spar moves away from the vicinity of the installation. The upper ends of the risers can be attached to buoyancy cans that can slide vertically with respect to the subsea buoy to keep the risers taut, and with trees at the upper ends of the buoyancy cans. If a workover vessel is to be used, it can connect to the trees at the upper ends of the risers, without pipes from the workover vessel having to extend all the way down to the sea floor. 
     The subsea buoy can be hung from the lower end of the spar by a chain or other tension member which allows the spar to tilt by more than the subsea buoy, so as to minimize bending of the risers at the bottom of the subsea buoy. Flexible hoses extend from the upper end of the subsea buoy, as from the trees on the buoyancy cans, to the lower end of the spar, where the hoses connect to spar pipes extending up to handling equipment at the upper end of the spar. The use of a hanging ballast for a spar, can be used in any spar installation, where the hanging weight lies closer to the surface than the sea floor. 
     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 
     FIG. 1 is a side elevation view of an offshore installation constructed in accordance with one embodiment of the invention, in its usual configuration wherein the subsea buoy hangs from the spar. 
     FIG. 2 is a partial view of the system of FIG. 1, with the spar disconnected from the subsea buoy and with a workover vessel lying over the subsea buoy. 
     FIG. 3 is a partial sectional view of the system of FIG. 1, showing the subsea buoy, buoyancy cans attached to risers, and trees at the upper ends of the risers. 
     FIG. 4 is a sectional view taken on line  4 — 4  of FIG.  3 . 
     FIG. 5 is a view of one of the risers of FIG.  4 . 
     FIG. 6 is a partial sectional view of an installation of another embodiment of the invention, wherein the spar and subsea buoy are disconnectably fixed together, and showing, in phantom lines, the spar disconnected and moved away and a makeover vessel in its place. 
     FIG. 7 is a more detailed view of a portion of the system of FIG. 6, showing the connecting apparatus. 
     FIG. 8 is a partial sectional view of an offshore installation of another embodiment of the invention, wherein the buoyancy cans slide along a moonpool within the subsea buoy. 
     FIG. 9 is a sectional view taken on line  9 — 9  of FIG.  8 . 
     FIG. 10 is a partial sectional view of an offshore installation similar to that of FIG. 6, but with the buoyancy cans sliding within external I-tubes of the subsea buoy. 
     FIG. 11 is a side elevation view of a portion of the system of FIG. 1, with the spar having drifted, showing tilting of the various components. 
     FIG. 12 is a view similar to that of FIG. 11, but with a central hang-off line rather than a plurality of hang-off lines. 
     FIG. 13 is a partial sectional view of an offshore installation of another embodiment of the invention, wherein a subsea buoy part is permanently attached to a spar. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a hydrocarbon production system  10  of the present invention, which includes a sea floor base  12  and sea floor pipes  14  extending largely downwardly to reservoirs  16  in the seabed. Hydrocarbons from the reservoirs pass up through the pipes  14  and through one or more production risers  20  to trees  22 . The trees include valves and couplings. The production risers  20  are kept under tension by buoyancy cans  24  or other means that can slide up and down within a subsea well head buoy  26 . When valves on the trees  22  are open, the hydrocarbons pass up through flexible lines  30  to spar pipes  32  that lie within a spar  34  or on the outside of the spar hull. The spar pipes carry the hydrocarbons up to processing equipment  36  on a deck  40  of the spar. The processing equipment may remove sand and water. The processed hydrocarbons pass down through additional spar pipes  32  and additional flex lines  30 , and pass down along export risers  50  that carry the hydrocarbons to a remote location such as an onshore processing plant or a storage vessel located in the vicinity of the spar  34 , or a terminal. Although rudimentary valves may lie at the sea floor base  12 , the more sophisticated valves and couplings such as remotely hydraulically operated valves and couplings lie at the trees  22  which lie high above the sea floor  52 . 
     In one example, the underwater portion of the spar buoy has a height A of 292 feet, and the hangoff lines  60  have a height B of 164 feet. As a result, the top of the subsea buoy  26  lies about 450 feet below the sea surface  62 . The height C of the top of the subsea buoy  26  above the sea floor is a plurality of hundreds of feet and is generally greater than its height (A+B) below the sea surface. As a result, the subsea buoy  26  lies where wave and current forces are negligible, and lies under most icebergs. Also, the tree  22  lies less than about 600 feet below the sea surface so shallow water workover vessels can be used to dean the risers (after the spar  34  is removed) and to clean the subsea buoy at a depth where it is diver accessible. 
     FIG. 2 shows a situation where the wells are being worked over by a workover vessel  70  in the form of a semi-submersible platform, although other vessel shapes are possible. The spar  34  has been detached from the subsea buoy  26  and some water has been pumped into ballast chambers (not shown) of the spar to lower it somewhat in the water for stability. Water has been pumped out of the subsea buoy to make it (with loads on it) neutrally buoyant and the buoyancy cans are fixed to the subsea buoy, before disconnection from the spar. The workover vessel has lowered workover vessel pipes  72  by connecting pipe sections for a total length of about 400 feet, so they extend to and are connected to the trees  22 . Then cleaning equipment passes down through the pipes  72 , the production risers  20 , and the sea floor pipes  14  to clean out wax buildup. When workover is completed, the workover vessel  70  is disconnected and towed or sailed away, and the spar  34  is reconnected. The top of the subsea well head buoy  26  is preferably located more than 200 feet but less than 800 feet, such 500 feet, below the sea surface  62 , to isolate it from almost all wave action while making the trees reasonably accessible. In most cases the height C of the top of the well head buoy is at least 500 feet above the seafloor. 
     A spar normally includes air-filled tanks at its upper portion and ballast-filled containers (filled with high density material at its bottom to provide a large moment urging the spar to remain vertical and therefore to provide stability. Applicant constructs the subsea buoy  26  so that in the producing configuration of the system (FIG.  1 ), the subsea buoy  26  is negatively buoyant. This weight is applied to the bottom  76  of the spar  34  through the hangoff lines  60  that support the negatively buoyant subsea buoy  26 . Because of the large load applied by the subsea buoy  26 , the spar  34  does not need as much ballast at its lower end, and a smaller and lighter spar  34  can be used. 
     It is noted that it is known (e.g. U.S. Pat. No. 4,637,335) to use a weight hanging from the bottom of a tall transfer structure whose upper end moors a vessel that can drift far from its quiescent position (more than about 8% of the sea depth) in severe weather, unlike a spar, to obtain a “pendulum effect” that urges the structure and vessel back. Applicants tensioned risers  20  can accommodate only a moderate spar drift, which increases the distance between the seafloor base  12  and the subsea buoy  26  (e.g. no more than 10% of riser length by moving down the buoyant cans  24 ). Thus drift of the spar must be limited, by making the spar narrow and tall. 
     Applicant uses the subsea buoy, in addition to well supports, as an external spar ballast which lies below the spar and therefore which is effective in avoiding excessive tilt of the spar. The weight applied by the subsea buoy is applied to the extreme bottom of the spar where the weight is most effective in minimizing spar tilt. The separate subsea buoy and spar are easier to handle and transport than one massive spar. Although a massive spar can be moved in sections and welded at the site, the present system avoids the high cost of such welding. 
     The subsea buoy  26  is made positively buoyant before the spar  34  is to be disconnected from the subsea buoy  26  for the connection of the workover vessel, to avoid iceberg damage, or other reason. This is accomplished by pumping water out of tanks of the subsea buoy  26  until it is positively buoyant, so it can support itself and the weight of mooring chains and steel catenary risers (the risers  20  are kept taut by their own buoyancy cans). 
     The spar  34  is moored by a group of spar mooring chains  80  or other flexible lines that extend in catenary curves to the sea floor  52  and along the sea floor to anchors  82 . Retrieval lines  84  extend from couplings  86  lying along the spar mooring chains up to marker buoys  88 . When the spar is to be removed, each of the mooring chains  80  is separately disconnected from the spar and allowed to drop to the sea floor or be held suspended by small buoys. If a workover vessel  70  is to be used then it may pick up the marker buoys  88  and connect to the spar mooring chains  80 . The subsea buoy  26  may be moored by its own buoy mooring chains  90  although this is not necessary in many cases, so chains  90  are not necessarily required. 
     In systems of the type shown in FIGS. 1-5, tilt motions of the subsea buoy can be controlled by variation in the length of the hangoff lines  60 , and the amount of tension in the hangoff lines (variation in the weight of the subsea buoy). Tilt motions of the subsea buoy also can be controlled by choice of the radial distance between the axis  92  of the subsea buoy and locations where the hangoff lines are attached to the subsea buoy and the spars. 
     In FIGS. 1-5 and  11  the hangoff lines  60  are attached to the radial outside of the subsea buoy and to the radial outside of the spar, and the subsea buoy pitch will be very dose to that of the spar. FIG. 11 shows a situation where the spar has drifted with the spar axis  94  tilted by 11°, and the subsea buoy  26  has tilted by 9° from the vertical. This results in the risers  20  undergoing a bend of 5° at the bottom of the can floats  24 . This bending (bending about a small radius of curvature) would create high stress points and should be minimized to avoid design difficulties. FIG. 12 shows a situation where hangoff lines  60 A are attached close to the axes of the spar and subsea buoy, resulting in a smaller pitch angle of the subsea buoy. In FIG. 12, the spar has drifted and the spar axis has tilted by 12.5°, and the subsea buoy has tilted by 2° from the vertical. The risers  20  undergo a bend of 3° at the bottom of the subsea buoy. The hangoff chains can be placed so the subsea buoy and the risers tilt almost in unison for a certain range of spar drift, to minimize riser bending and high stress points. It should be noted that the hangoff lines are pivotally connected at their upper and lower ends to the spar and subsea buoy, respectively. While flexible chains or cables are desirable, it would be possible to use rigid rods whose ends are pivotally connected. 
     FIG. 3 illustrates some details of the subsea buoy  26 . The buoy includes a large tank  100 , that may include bulkheads to separate it into multiple chambers. When the subsea buoy is connected to the spar, the tank is filled with water, as to the level  102  so that the buoy  26  with its permanent high density ballast  106  applies a large weight to the spar and acts as an external ballast. However, when the buoy  26  must float to support itself, the export risers  50 , and the weight of any buoy mooring chains thereon, the water is pumped out as to the level  104 , to make the buoy  26  and loads thereon neutrally buoyant. It is noted that the subsea buoy holds high density material at  106 . 
     The buoy  26  includes I-tubes  110 ,  112 . Buoyancy cans such as  24 A,  24 B can slide vertically within the tubes which serve as vertical guideways. The buoy  26  is allowed to heave (move up and down) as the spar  34  of FIG. 1 moves up and down in the waves. The buoyancy cans  24  are rigidly attached to the production risers  20  which are attached to the well templates at the sea bed which anchor the risers to the sea floor. Accordingly, the tubes of the subsea buoy  26  move up and down around the buoyancy cans. If the spar drifts under the influence of wind, waves and currents, the buoy  26  will also drift and the can floats  24  will drift and move downward within the tubes since the risers  20  are of a fixed length. Thus, while horizontal translation motions of the spar and subsea buoy are coupled, the risers do not move up and down with the spar and subsea buoy. In summary, the translational motions are coupled while heave is uncoupled. 
     FIG. 4 is a sectional view of the subsea wellhead buoy  26  of FIG.  3 . It shows the tubes  110 ,  112  on opposite sides of the tank  100 . Mooring line connectors  120  connect to the hangoff lines  60 . Flexible riser couplings  122  connect to export/import risers. FIG. 5 is one example of a sectional view of a riser  20 . 
     FIG. 6 illustrates another system  150  which includes a spar  152  and a separate subsea buoy  154 . The spar  152  and buoy  154  are fixed together at a coupling  156 , instead of having the buoy  154  hang through hangoff lines from the bottom of the spar. FIG. 7 shows that the bottom of the spar  152  includes a groove  160  and that the coupling  156  includes hydraulic actuators  162  with pistons  164  that enter the groove  160  to lock the spar  152  to the buoy  154 . The subsea buoy  154  includes tubes  170 , with can floats  172  being vertically slidable within the tubes, and with risers  174  lying within the can floats. A tree  180  lies at the top of the uppermost can float  172  and has multiple remotely-operable valves  182  and pipe couplings  184 . Spar pipes  186  move up and down within the shell of the spar, and flexible couplings ( 187  in FIG. 6) are contained within the spar to accommodate such vertical movement. 
     When the spar  152  is connected to the subsea buoy  154  as shown in solid lines in FIG. 6, the subsea buoy  154  is made negatively buoyant, by flooding its tank with water. When the spar  152  is to be disconnected, as when a workover vessel  190  must be used, the spar is disconnected and floats to the position indicated at  152 A. 
     In a system of the type shown in FIG. 6 that applicant has designed, the top of the buoy  154  lay a distance E of 210 feet below the sea surface  62 . Wave action thereat is relatively low, and movement of the buoy  154  is minimized in rough weather by the fixing of the spar  152  to the buoy  154 . The buoy  154  had a height F of 263 feet. The risers  174  extended to the sea floor in the same manner as shown in FIG. 1, with export risers  50  similarly extending to the sea floor. Only a single chain table  200  and mooring chains or lines  202  are required since the spar and buoy are fixed together. 
     The systems can be constructed in different ways. As shown in FIGS. 8 and 9, the subsea buoy  200  can be provided with a moonpool  202 , with production risers  204  passing through buoyancy cans  206  passing through the subsea buoy. The subsea buoy can be used to support only flexible risers, with the well heads at the sea floor. FIG. 10 shows a subsea buoy  210  with a buoy part  212  damped to a spar  214 , where I-tubes  216  lie outside the buoy part. Risers  220  are fixed to buoyancy cans  222  that can slide within the I-tubes. 
     FIG. 13 shows a production system  230  with a permanently moored spar assembly  232  moored by lines  234  extending the sea floor. The spar assembly includes a spar part  236  and a subsea hung part  238  hung by hangoff lines  240  from the spar part. The lines  240  are preferably longer than the average width of the spar part  236  or hung part  238 . The subsea hung part  238  includes a quantity of high density material  242  (e.g. iron ore) and a tank  244  that is normally filled with water. The spar part  236  is easier to install, while the hung part is especially effective in keeping the spar part upright. The figure shows a production riser  246  for carrying hydrocarbons. The hung part  238  preferably lies a distance J below the sea surface, which is less than its height K above the sea floor. In one example, the height J is 100 meters while the height K is 200 meters. 
     Thus, the invention provides an offshore hydrocarbon production system of the type that includes a spar (a long thin buoyant body), that produces oil from undersea wells, which minimizes cost. A subsea buoy lies under the spar with trees on the subsea buoy connected through vertical risers to pipes that lie within the seabed. A spar of only moderate weight and cost is provided by fixing or hanging a separate weight from its lower end, where the weight is a negatively buoyant subsea buoy whose tank can be made positively buoyant or highly negatively buoyant. The subsea buoy normally is negatively buoyant to weight the bottom of the spar, but is converted to a positively buoyant state to support the risers, trees and mooring lines before the spar is disconnected. 
     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.