ENHANCED FLOATING POWER GENERATION SYSTEM

BACKGROUND OF THE INVENTION

Hydrocarbons can be produced from undersea reservoirs that lie in the vicinity (e.g. within 50 kilometers) of a consumer such as a community that could use electricity. Some of the produced hydrocarbons can be used to generate electricity that can be delivered to such community. However, there may be much more produced hydrocarbons than are needed to supply the demand for electricity in the vicinity. Where the produced hydrocarbons include considerably quantities of liquid and gas, and the gaseous hydrocarbons include a variety of different molecules with different numbers of carbon atoms per molecule, it would be desirable to use the hydrocarbons so the greatest economic value can be derived from the production.

Liquid hydrocarbons are usually the most valuable, in part because they are economical to store and transport since they remain liquid over a wide range of environmental temperatures. Gaseous hydrocarbons, which are gaseous at environmental temperatures such as 1O ⁰ C and at temperatures often encountered in a sun-heated tank such as 40 ⁰ C, are less valuable because they are more difficult to store and transport. For example, methane, which is often the most common gaseous hydrocarbon, must be cooled to -162 ⁰ C to liquify it and transport it economically over a long distance. The most economical use of gaseous hydrocarbons from an undersea reservoir is highly desirable.

SUMMARY OF THE INVENTION In accordance with one embodiment of the present invention, applicant provides a method and system that economically uses a variety of hydrocarbons that are produced from an undersea reservoir that lies in the vicinity of a consumer such as a community while supplying electricity to the consumer. Hydrocarbons that are produced from a floating structure are separated into liquid and gaseous hydrocarbons, and the gaseous hydrocarbons are separated into heavy and light gaseous hydrocarbons. The liquid hydrocarbons are stored for offloading to a

-2- 05/149 tanker. The light gaseous hydrocarbons are immediately used to fuel an electricity generator unit. The heavy gaseous hydrocarbons are stored preferably after they are liquefied, and are used to fuel the electricity generator unit only when sufficient light gaseous hydrocarbons are not available. If excess heavy gaseous hydrocarbons are available, they are shipped to a distant consumer.

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 system for the production of hydrocarbons and the generation of electricity, of one embodiment of the present invention.

Fig. 2 is a sectional side view of another system which is similar to that of Fig. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 illustrates a system 10 which includes a floating structure in the form of a vessel 12 with a hull 16 that floats at the sea surface and with a turret 20 that is moored by a mooring system that includes catenary lines 32 that extend to the sea floor and along it. Risers 34 extend from a fluid swivel 36 on the turret to a sea floor platform 40, and from there extend to a subsea hydrocarbon reservoir 64. The turret allows the vessel to weathervane, that is to head in different directions with changing winds and waves, while the catenary lines allow the vessel to drift but only a limited distance, from a location 44 over the sea floor platform. Other mooring systems that can be used include spread mooring systems, disconnectable buoy- turret systems, etc.

The vessel carries an electricity generating unit 42 that uses hydrocarbons as a fuel to generate electricity. A preferred unit is a turbine-generator set wherein the turbine is powered ,or fueled, by hydrocarbons and the turbine spins a rotor of an electric generator. Such turbine-generator set is of light weight in proportion to the electrical power it generates. The system includes a power cable 50 that

-3- 05/149 extends from the vessel and that has a major portion 56 that extends along the sea floor to an onshore distribution facility 52. The facility distributes electricity to consumers such as residential, factory and office structures.

Hydrocarbons produced from the subsea reservoir 64 commonly includes liquid hydrocarbons, which have at least five to eight carbon atoms per hydrocarbon molecule, and gaseous hydrocarbons, which have one to four, or one to seven carbon atoms per molecule. Gaseous molecules are in a gaseous state at temperatures commonly reached during storage and transport. There is an overlap (of molecules with five to seven carbon atoms per molecule) between liquid and gaseous hydrocarbons. At the production system, liquid hydrocarbons are more valuable than gaseous hydrocarbons. This is largely due to the fact that liquid hydrocarbons are much more easily transported long distances because they do not have to be cooled to liquify them and do not later have to be heated to regas them. Where substantial amounts of gas and liquid hydrocarbons are produced from the reservoir 64, applicant uses the gas to fuel the electricity generator unit 42. Gas is less valuable if it is to be stored or transported, but it produces less polluting gasses and is easier to use in turbines. Applicant stores all liquid hydrocarbons that are not needed to produce electricity, which may be all of the produced liquid hydrocarbons. The stored liquid hydrocarbons are offloaded to a tanker 56, such as at predetermined intervals when it is expected that the liquid hydrocarbon tanks will be largely full. In Fig. 1 , the tanker is shown moored through a capstan 66 to the vessel 12 during unloading, with the liquid hydrocarbons transferred though a transfer conduit 68. It would be possible to use all of the natural gas produced from the undersea reservoir to fuel the electricity generator unit 42. However, applicant uses the produced hydrocarbons more economically by separating, by processor 60, the produced gaseous hydrocarbons into light gaseous hydrocarbons, which are gaseous hydrocarbons with a minimum number of carbon atoms per molecule, and heavy gaseous hydrocarbons, which have a greater number of carbon atoms per molecule. Light gaseous hydrocarbons include methane (CH ₄ ). Heavy gaseous

-4- 05/149 hydrocarbons include propane (C ₃ H ₈ ) and butane (C ₄ H ₁₀ ). Ethane (C ₂ H ₆ ) can be placed in either category. Applicant first uses light gaseous hydrocarbons to fuel the electricity generator unit, and stores the heavy gaseous hydrocarbons. Only when there is an insufficient supply of light gaseous hydrocarbons to provide the amount of required fuel, does applicant use the stored heavy gaseous hydrocarbons (often referred to as LPG, or liquid petroleum gas).

Fig. 2 illustrates a system 170 with a floating structure 172 in the form of a vessel similar to that of Fig. 1 but with the vessel being spread moored. The vessel carries an electrically-energized processor 182 that receives effluent from a reservoir 174. The effluent passes along a riser 176 and pipe 180 to the processor which removes sand, stones, etc. The processor includes a separator that separates liquid hydrocarbons from gaseous hydrocarbons. The liquid hydrocarbons are stored in a liquid oil tank structure 183 that includes several tanks 184. The processor also separates light gaseous hydrocarbons (e.g. methane) from heavy gaseous hydrocarbons (e.g. including propane and butane, and possibly also ethane, pentane and hexane). The light gaseous hydrocarbons, or light gas (which includes at least methane), passes into an accumulator 190 that holds the gas under a variable pressure to thereby store a limited amount of the light gas. The light gas is delivered to a gas turbine-generator unit 192 that uses gas to generate electricity.

Heavy gaseous hydrocarbon, or heavy gas (which includes at least propane and butane), passes to a storage tank 212. The tank 212 usually is cooled by an electrically-energized refrigerator unit 213 to liquify the heavy gases for more efficient storage. About 200 times more gas can be stored in a given volume as cold liquified gas than in a gaseous state at atmospheric pressure. Since the amount of light gas stored in accumulator 190 is small, there is a possibility that the supply of light gas to the generator unit 192 will not be sufficient to meet requirements. This can occur if there is an interruption in the flow of effluent from the reservoir , or if the reservoir effluent temporarily contains less light gas than usual, or if the demand for electricity temporarily increases beyond the capacity of the processor to supply light gas. In any of these cases, liquified heavy gas from the

-5- 05/149 storage tank 212 is heated by a heating station 215 to regas it, and the gaseous heavy hydrocarbons are delivered through a conduit structure 214 that also carries light gasses, to the electricity generator unit 192. In a situation where the heavy gas storage tank is repeatedly full so additional heavy gas would have to be flared, the liquified heavy gas can be offloaded to a refrigerated tank 217 (Fig. 1) that is provided on the tanker.

The different gaseous hydrocarbons have widely different boiling, or vaporization temperatures. Methane (CH ₄ ) has a boiling point temperature of - 162°C, and considerable refrigeration is required to liquify it for storage. Ethane (C ₂ H ₆ ) has a boiling point temperature of -89°C which is moderately difficult to liquify. Propane (C ₃ H ₆ ) and butane (C ₄ H ₁₀ ) have boiling point temperatures of -42 ⁰ C and -0.5 ⁰ C, respectively, which are much easier to achieve. By using only the light gas (primarily methane) to fuel the electricity generator unit when sufficient light gas is available, while efficiently storing the heavy gas as cooled liquified gas to be used only when there is insufficient light gas, applicant provides a more reliable source of fuel in an economical manner. Excess light gas can be flared (burned) or released in an unburned condition into the atmosphere if there are winds to widely disperse it.

Some of the electricity produced by the electricity generator unit 192 (Fig. 2) is used to energize electrically-energized equipment on the vessel, such as the processor 182. Most of the produced electricity is delivered though a cable 194 that extends in the sea 200 to an onshore distribution facility 202. The output of the generator is AC (alternating current). An AC-DC converter 204 is on the vessel, and a DC-AC converter 206 is at the onshore facility. This allows electricity to be sent though the in-sea cable 194 at e.g. 150 kV (kilovolts) for smaller losses. If AC current were sent though the same cable, it would have to have an equivalent voltage of 106 RMS to assure the peak voltages were no more than 15OkV, resulting in 30% greater current and therefore 30% greater losses at the same power levels.

Waste heat from the gas powered electricity turbine unit 192 is delivered to a steam generator 196 where the steam output drives a steam turbine-generator set

198. In one example, eight gas turbine generator units 192 are provided that each

-6- 05/149 has an output of 40 megawatts, while three steam turbine generator units are provided that each has an output of 35 megawatts. Not all units are operating at one time, and the generating capacity is 400 megawatts. Steam from the steam turbine generator units pass into condensers 201 after passing though the turbines. Applicant locates the condensers below the water line 202. This reduces the static head to a minimum to improve efficiency. Water for cooling the condensers is preferably drawn though free hanging, deep suction hoses 205 that carry cold water.

Thus, the invention provides a system for producing hydrocarbons from a subsea reservoir and using some of the produced hydrocarbons to generate electricity for local consumption. The produced hydrocarbons are efficiently used by separating the liquid hydrocarbons from the gaseous ones, with the liquid hydrocarbons usually stored and transported to a market. The gaseous hydrocarbons are separated into light gas which includes methane and possibly ethane, and into heavy gasses which include propane and butane and possibly ethane. The light gases are immediately used, with only short term storage of light gas in gaseous form, to fuel an electricity generator unit. The heavy gases are usually refrigerated to liquify them for storage, and kept in reserve for fueling the electricity generator in the event there is temporarily insufficient light gases. If excess heavy gases are accumulated, they also can be shipped to a market. 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.