Abstract:
A barge ( 14 ) that stores liquid hydrocarbon (oil) from a producing facility ( 12 ) and offloads it perhaps once a month to a shuttle taker ( 40 ), is constructed for unmanned operation except during the once-per-month unloading. The barge has a permanent nonadjustable ballast ( 53, 54, 55 ) and has a solar powered system for communication with a central center for normal operation and emergency shutdown. The barge avoids a “hotel system” for a permanent crew (up to 50 people) by avoiding seawater ballast tanks, ballast pumps and related systems. The only person-operated equipment is an engine-generator set ( 142 ) and pumps ( 44 ), to be operated only during offloading for perhaps 3 days every month. The barge is unpowered except by solar energy or batteries, and is left unmanned except during offloading, so only temporary crew quarters are provided. The barge has a tank assembly ( 57 ) with rows of tanks connected in series so oil can be loaded and unloaded from the frontmost tank in each row. The barge is ballasted so the bottom walls ( 130 ) of the tanks extend at a slight downward-forward tilt so the last amount of stored oil can flow downhill to the frontmost tank for offloading onto the shuttle tanker.

Description:
CROSS-REFERENCE 
     Applicant claims priority from provisional patent application No. 60/142,236 filed Jul. 2, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     A common hydrocarbon production system includes a production facility with seafloor wells, and a pipeline connecting the production facility to a storage barge that can store a large quantity of liquid hydrocarbons, or oil, such as 40 days of production. At intervals such as every month, an offloading tanker or shuttle tanker removes oil from the storage barge and carries it to an onshore processing facility. It requires perhaps one to three days to transfer the stored oil to the offloading tanker. It may be noted that the production facility may include a platform, spar, TLP (tension lag platform), etc. to enable initial drilling of wells and maintenance and workover of existing wells. 
     A crew is commonly left on the storage barge at all times. The permanent crew members operate cargo and ballast pumps, provide maintenance and monitor proper operation of equipment on the barge. A seawater ballast system operated by the crew, compensates for changes in weight distribution during loading and unloading to assure barge stability at all times and assure that the strength limits of the vessel are not exceeded. The cost of the permanent crew of perhaps 50 people is substantial and it would be desirable if the barge could operate without them. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a storage barge is provided for receiving a constant flow of hydrocarbons from an offshore production facility and storing the liquid hydrocarbons, or oil, and for occasionally offloading the oil onto a tanker, where the barge is constructed so it can operate without a permanent crew. Instead, a crew boards the barge only during offloading, which occurs about once a month and lasts perhaps one to three days. The barge has a plurality of rows of tanks, each row extending along the length of the barge, with the tanks in a row being connected in series. Where oil from each row is removed by pumping from one end of the barge such as the bow end, there will be a slow flow of the last amounts of oil to the frontmost or bow tank. To facilitate such flow, the permanent ballast causes the barge to assume an orientation wherein bottom walls of the tanks in a row are angled between 0.5° and 8° from the horizontal so oil flows downhill towards the frontmost tank when the tanks are almost empty. 
     The barge is designed to not have a permanent crew onboard, but to have only a small temporary crew during offloading. This is accomplished by eliminating powered systems such as a seawater ballast system and conventional hydrocarbon-fueled power center, and using automatic and remote controlled systems to perform functions with power obtained from solar cells and batteries. The barge has a permanent unpowered ballast system, which eliminates a major prior need for permanent crew members. 
     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 an isometric view of a hydrocarbon production system, with an offloading tanker connected to the barge. 
     FIG. 2 is an isometric view of the storage barge of FIG.  1 . 
     FIG. 3 is a side elevation view of the storage barge of FIG. 2, when it is almost empty. 
     FIG. 4 is a side elevation view of a storage barge of another embodiment of the invention, when it is almost empty. 
     FIG. 5 is a side elevation view of the barge of FIG. 2, showing additional equipment thereon. 
     FIG. 6 is a plan view of the barge of FIG.  5 . 
     FIG. 7 is a schematic plan view of the barge of FIG. 2, showing the tank and coupling arrangement thereon. 
     FIG. 8 is a partial sectional side view showing one fluid connection between adjacent tanks of a row of tanks of the barge of FIG.  7 . 
     FIG. 9 is a partial isometric view of the fluid connection of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a hydrocarbon production and transfer system  10  which includes a facility  12  for producing hydrocarbons from undersea wells  13 , such as a platform, Spar, TLP, etc.. The system also includes a storage barge  14  that is connected to the production facility  12 , as by a pipeline  16  that extends from the facility  12  and along the sea floor  20  up to the barge  14  (possibly through a subsea buoy), or via a mid-depth flowline. The barge  14  is permanently anchored (although it might be removed perhaps every few years) at the general location of the facility, by anchor chains  22  that extend in different compass directions to the sea floor and along the sea floor. Other anchor facilities such as a single anchor chain, a rigid member extended towards the sea floor, can be used. The chains and the barge end  24  of the pipeline, extend to a turret  26  that allows the hull  30  of the barge to weathervane about the vertical axis  32  of the turret. That is, as winds, waves, and currents change, the barge hull  30  can pivot about the axis  32  to any position about it, as well as drift. The turret is preferably beyond the bow end of the hull, but instead can lie in a moonpool. An alternate is a spread moored barge. 
     The turret  26  carries a fluid swivel  34  (or at least the non-rotating part), with an inlet port connected to the pipeline  16  and with an outlet port coupled to storage tanks in the barge hull. In some cases the well effluent is fully processed at the production facility  12 , as to remove almost all debris (e.g. sand) and separate out all liquid hydrocarbon (oil) from gas. However, such processing is not provided at this unmanned barge. 
     The barge can hold perhaps 40 days production from the facility  12 . At intervals such as every 30 days (possibly more if a storm is present in the vicinity) a shuttle tanker  40  moves close to the storage barge  14 . The tanker has a connection  45  that connects to a transfer line  42  extending to a coupling  46  on the storage barge. The coupling  46  is connected through a rigid pipe  43  that extends to pumps  44  at the bow of the storage barge, so hydrocarbons stored in the barge can be transferred to the tanker. Additional hydrocarbons can be pumped directly out of more rearward tanks for faster offloading. It requires perhaps 24 to 72 hours to transfer 30 days of production to the tanker. The tanker can be moored to the barge by mooring line  49 . 
     Prior barges of this type generally have a permanent crew on the storage barge. The permanent crew quarters include at least 400 ft 2  (40 m 2 ) per crewman, including toilet, storage for food, etc. and food preparation facilities. The crewmen adjusted ballasting as the barge filled with hydrocarbons, notified a central station in the event of damages and repaired minor damage, and performed operations during offloading including connecting pipes, adjusting ballast as oil was offloaded, etc. The present invention is directed to a storage barge that avoids the need for a crew to stay on the barge during the long periods while it is being filled, which minimizes crew requirements during offloading, and which minimizes maintenance. The temporary crew quarters are less than 400 ft 2  (40 meters 2 ) per crewman, and usually less than 200 ft 2  (20 m 2 ). Usually, ten temporary crew members are used for each offloading, during a maximum of 3 days, so little living space is required. The temporary crewmen can make any repairs, while a control system with monitoring equipment can alert a central station of any major leaks and remotely operated valves can shut off valves when necessary. 
     The barge is provided with an intelligent and autonomous working control system  190  in FIG. 1 which monitors, controls and looks after the barge in the unmanned loading situation, and which is remotely controlled via telemetry by onshore people or people on another platform, vessel or barge. It is energized by a solar powered system on the barge that includes solar cells and batteries  192 . The function of the control system is to monitor the loading of the barge and assure proper barge trim, and draft, and bending moments within safe limits. It manages the loading and safety procedures during the unmanned period and sends the information via a telemetric link to a manned control station  206 . It also assures that the barge remains safe in case the telemetry link to the manned control point is lost, and it can assure an emergency shut down whenever it is necessary. The control system can, whenever needed, close certain valves in the unmanned loading mode. The control system has 100% redundancy; therefore there are dual systems on the barge, like a double valve system for fluid connections between the tanks. The control system receives information from various sensors, sends necessary alarms to the manned control station, sends necessary information to the manned station for operational and safety decisions, computes parameters for safety and operational decisions, and functions autonomously when cut off from outside communication. The control system thereby assumes the role of crew members. 
     FIG. 1 shows a radio transceiver  200  on the barge, that is connected to sensors such as tilt sensors  202  that sense the pitch and roll of the barge, fill sensors  204  that sense how much hydrocarbons are in each tank, fire sensors, etc. The transceiver sends this information to the central station  206  located more than a kilometer away, and usually tens or hundreds of kilometers distant, on shore or on a platform. The transceiver receives radio signals (from a broadcast station, satellite, etc.) from the remote station, to operate equipment such as shut-off valves  210  (FIG.  8 ). 
     FIG. 2 shows that the storage barge  14  includes front and rear ends, or bow and stern  47 ,  48  that are separated along a longitudinal direction M that is perpendicular to a lateral direction L along which the port and starboard are spaced. Bow and stern regions  50 ,  52  are left empty to provide buoyancy there and to stabilize the vessel. Fixed ballasts lie at  53 ,  54  and  55 . The barge is devoid of an active ballast system, wherein seawater is pumped into or out of ballast tanks as the barge fills with hydrocarbons. Such active systems in prior barges were operated by crew members, using power from generators driven by hydrocarbon-energized engines. Applicant constructs the barge so incoming oil is evenly distributed, so an active ballast system is not necessary during loading. 
     The barge forms a tank assembly  57  divided into twelve cargo tanks  56 . In designating the tanks, “S” stands for starboard, “P” for port, and “C” for center. FIG. 7 shows that the twelve cargo tanks are arranged with four starboard cargo tanks labeled C 1 S, C 2 S, C 3 S, and C 4 S arranged along the starboard side of the barge. Four port tanks labeled C 1 P, C 2 P, C 3 P, and C 4 P are arranged along the port side of the barge. Four center cargo tanks labeled C 1 C, C 2 C, C 3 C, and C 4 C are arranged along the middle of the barge. Two bulkheads  60 ,  62  extending in a longitudinal direction M separate the center tanks from the starboard and port tanks. Three additional bulkheads  64 ,  66 ,  68  separate adjacent tanks that lie one behind the other. Short conduits  70 ,  72 ,  74  connect adjacent starboard tanks or adjacent center tanks, or adjacent port tanks. Two additional short conduits  76 ,  78  connect the other center tanks. Long conduits  80 ,  82  connect the fourth side tanks C 4 S, C 4 P to the second side tanks C 2 S, C 2 P, bypassing the third side tank C 3 S and C 3 P (which are buffer tanks). The barge hull is symmetric on laterally L opposite sides of a center plane  84 . 
     All fluid conduits extending between tanks are provided with two valves for redundancy. Under normal conditions the valves are open and the hydrocarbons can flow freely from one tank to another. In an emergency the valves can be closed by the solar powered control system. 
     Liquid hydrocarbons from the production facility are flowed into the frontmost tanks C 1 S, C 1 C, and C 1 P, to flow to all tanks behind the front ones, except for the third, or buffer tanks C 3 S and C 3 P. Inlet ports  101 - 103  are shown at the bow end, to which flowlines can be connected to fill the barge tanks. 
     When a shuttle, or offloading, tanker such as  40  in FIG. 1 is moored close to the storage barge, and the transfer line  42  is connected, the ends of the transfer line at the barge are connected to three pumps shown in FIG. 7 at  44 A,  44 B, and  44 C. Engines at  142  (FIG. 6) in an engine house are started by a temporary crew member. The engines drive a generator whose electrical output energizes motors on the pumps  44 . Selected valves are set so the pumps pump out oil from all tanks except for the third side tanks C 3 S, and C 3 P which are middle isolated buffer tanks. Oil from the production facility continues to flow to the barge during offloading, but such new oil is diverted, as through bypass pipes  118  (FIG. 7) to continue to fill the tanks C 3 S and C 3 P. This allows for a correct metering of the amount of oil that has been pumped from the barge to the tanker, as by a flow meter  104  connected in series with the pipe  43 . After all but the third side cargo tanks are empty, and the tanker is disconnected from the barge, oil is pumped via pumps  120 ,  122  and valves that can be opened and/or energized and closed from the third side tanks C 3 S and C 3 P into another tank. 
     If substantially all liquid hydrocarbons, or oil, in the barge (except in the third side tanks) is to be pumped out to the shuttle tanker, then substantially all of the oil has to be flowed to the pumps  44  that are close to the bow of the barge (except for oil directly pumped out of a rearward tank). Applicant assures that oil will flow towards the bow end through the locations of the pumps  44 , by assuring that the barge is tilted, or trimmed by the forward end, i.e. the bottom of the barge is tilted forward and downward. 
     FIG. 3 shows the bottom  130  of the barge, which is straight and extends at a downward D and forward F incline at an angle A where A is about 2°. The barge  14  automatically assumes this orientation when the barge is substantially empty (under 2% of maximum capacity, except for the third side tanks). The weight of the turret  26  and of chains of the anchor lines  22  weight the bow of the tanker. The distribution, shown in FIG. 2, of the empty spaces at  50  and  52  and the weight of the ballast  53 ,  54 ,  55  plus the placing of heavy equipment including engines for energizing the pumps, at the bow of the vessel, all result in the orientation of the barge bottom at  130 , at the downward-forward incline. The incline angle is preferably at least 0.5 degrees, but no more than 8 degrees. An incline angle of much less than about 0.5 degrees, results in oil at the bottom of the rearmost tanks moving very slowly or not at all, so that a substantial amount of such oil will not be pumped out to the tanker. An angle of more than 8 degrees can result in a tall bow and a barge that does not efficiently use the steel of its hull to store hydrocarbons, and that is unstable. The deck  58  is horizontal when the barge bottom is inclined at angle A. 
     FIG. 4 shows another barge  14 A, where the stern and bow of the barge are of about the same height, but the stern lies higher out of the water than the bow. (both when the barge is full of oil and when it is almost empty). This results in the straight bottom  132  extending at a forward downward incline B of about 2°. 
     FIG. 5 shows some of the equipment on the barge, including an accommodation module  140  for a temporary crew and an engine house  142  that protects the electricity (or hydraulic) generation engines. A helicopter landing pad  146  facilitates access to the barge. An inert gas vent mast  148  vents gas. Only the liquid with fine debris (e.g. sand) is flowed into the storage tanks. 
     FIGS. 8 and 9 shows a sump  160  that connects adjacent tanks such as C 1 P and C 2 P and that results in projections in the bottom of the hull. The sumps, that include valves, are the short conduits  79 - 82  shown in FIG. 7. A wall  166  separates the tanks, and a pipe  220  extends through the wall at a level within the sump. A pair of shut-off valves  222 ,  224  lie along the pipe to shut off the flow. The valves are remotely operated valves, which are well known. The valves  222 ,  224  can be operated by a person at the remote station  206  through the onboard transceiver  200 . For example, if the barge should tilt so the stern moves down more than normal, valves that connect to more forward tanks can be closed to prevent more oil from flowing under the force of gravity into the rearmost, or sternmost, tanks. Only a small mount of oil remains in the sump when the tanks are emptied. FIG. 9 shows that the bulkhead wall  166  has wall portions  230 ,  232  that converge to direct oil toward the sump as would be seen in a plan view. 
     The barge does not have propulsion or steering systems. During an unmanned period such as one month, the barge uses no conventional power generation, (i.e. no engine using hydrocarbon fuel). This is the safest solution from a fire aspect, and reduces operating costs. The battery and solar cell assemblies  192  (FIG. 1) on the barge supply energy to the control system, and the telemetry and navigation aids. The barge has an empty space  52  (FIG. 2) at the stern of the vessel, which serves as a crumple zone that takes up shock in the event that a tanker strikes the barge. The lack of tanker propulsion increases the value of the crumple zone. 
     In a barge that applicant has designed, the barge has an overall length, excluding the turret, of 172 meters, a beam width of 40 meters, and a depth of 20 meters. The draught of the vessel is about 15.5 meters. It is noted that at a 2° tilt, which the barge assumes when nearly empty, the bottom of a bow tank such as C 1 C, is about 6 meters below the bottom of a stern tank such as C 4 C. 
     It is noted that the engine house  142  and temporary crew quarter  140  are located forward of the cargo tanks. This increases safety, because in the event of a cargo fire, smoke and gas is blown rearwardly along the barge, due to the fact that the barge weathervanes itself so its bow is always directed upwind. With the pumps at the bow, the long rigid pipe  43  carries fluid to the rear of the barge to minimize the required length of the hose  42  extending to the offloading tanker. 
     Thus, the invention provides a hydrocarbon transfer system with a storage barge that is constructed to minimize the cost for barge construction, maintenance and operation. The barge has a tank assembly that includes rows of tanks lying one behind the other, so hydrocarbons can be passed through them in series. The barge is of the type that has a turret at one end that is anchored to the seafloor to allow the barge to weathervane, or is of the type that is spread moored. The barge is devoid of conventional powered (by hydrocarbons) systems that operate during loading, and can be remotely controlled via telemetry and a control system installed on the barge, to enable the barge to operate for long periods without a crew. The barge has a crumple zone at the rear. Produced oil is preferably offloaded from the bow end through pumps, using a rigid pipeline that extends along the length of the vessel, with the rear of the pipe having a coupling for a connection to a hose that leads to an offloading vessel. Middle tanks of the rows of tanks, are separated from the other tanks, and receive produced oil while oil from the other tanks is offloaded, to enable accurate counting of the amount of offloaded oil. 
     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.