IE45766B1 - Gas flare system for use in a body of water - Google Patents

Abstract

In a system for offshore waste gas disposal, a platform structure and a remote buoyant flare are interconnected by a flexible gasconveying conduit, which hangs as a catenary in the water below wave and navigation depth but within reach of ordinary diving operations for maintenance. The flexible conduit also serves as a load-carrying structural member acting in conjunction with an anchor chain or cable to hold the flare buoy in position.

Description

This, invention relates to an apparatus for disposal of waste gas produced at an offshore well and, more particularly, for flaring thegas at a location remote from the well using a buoyant flare.

Flare systems are required in conjunction with oil or gas production operations to' (1) provide emergency flaring capabilities ia the event of equipment failure or (?.) flare gas that cannot be economically utilized. In offshore areas, gas is flared some distance away from the main drllllng/productlon platform in order to minimize the fire hazard. Conventionally, this has been accomplished by piping the rss to the sea floor and along the floor to the anchor base of the flare. In deep water this arrangement necessitates a substantial and costly flare structure. Deep- diving operations are usually required to install and service tha flare pipes and connections. Also, water temperatures near the sea bottom can be low enough to cause liquid formation in the gaa lines unless dehydration or insulating measures are taken. These problems can be obviated by using a buoyant flare rather than having a supporting structure resting on the sea floor, with s gas supply line at sn intermediate depth gather than on the sea bottom. Such systems sre described in U.S. Patents 3,372,410, Hindman and Hues; and 3,666,395, Kubasta, Hindman ’410 uses three hawsers to connect a submerged ball joint to the flare buoy, the platform, and tha saa floor. The hawser between the flare and ball joint ia the only member connected to the flare to hold it in place. The gas supply line ls in two pieces, both aupportlngly attached along the hawsers and connected at tha ball joint. Applicant has disclosed a system wherein the hawsers and ball joint ar* not necessary. Kubasta '395 describee a eyetern wharein the submerged gaa aupply line conaists of a plurality of flexible hose sections.

Haights ara attached to the hose to give it a negative buoyancy. Tha flare buoy la held in place solely by a cable anchored to the sea floor. -243766 This presents a problem of predicting the stresses imparted to the gas supply line when wave or wind action dislocates the flare buoy from its neutral position. Castela et. al., U.S. Patent 3,902,378 discloses a system with a gas supply conduit that is in tension and serves to anchor a buoyant flare. But the conduit is arranged in an essentially vertical position so that the required separation from the well is attained in the conventional manner, running the pipes along tile sea floor. This introduces the problems of deepwater operation mentioned above.

We have now developed improved apparatus for disposing of waste gas produced at an offshore well by burning the gas in a location remote from the well and which utilizes a gas supply conduit that is always in tension so that stress calculations and fatigue life evaluations can be made.

According to the present invention there is provided apparatus for disposing of waste gas produced at an offshore well located in a body of water, said apparatus comprising: production facilities supported by a platform structure attached to the floor of said body of water and extended above the surface of the water, said production facilities being associated with said offshore well and having a waste gas outlet; buoyant flare means located at a distance from said production facilities permitting safe flaring of said gas; flexible anchoring means connecting said buoyant flare means to a location on the floor of said body of water and maintaining said buoyant flare means at a position remote from said platform structure; flexible anchoring gas conveying conduit extending substantially in the form of a catenary between said platform structure and said buoyant flare means, said conduit being -3under sufficient tension to provide partial anchorage of said buoyant flare means to said platform structure to maintain at all times said flare means at a position toward said platform structure and away from a position directly above the location on the floor of said body of water to which said buoyant flare f means is connected and; connecting means for providing fluid communication between said waste gas outlet and said gas-conveying conduit.

Fluid communication from the production facilities may be provided on the platform by a downcomer which usually extends to a point below the water surface. Located some distance from the platform is generally a flare whose tip is floatingly supported above the water surface by a buoy. A flexible, gas-conveying conduit under loading stress can then connect the downcomer and the flare, the conduit residing in the water at an intermediate depth between the sea floor and water surface. The conduit generally should have sufficient strength to transmit environmental loads on the buoy to the platform, yet be flexible so as to allow local motion of the buoy. The buoy mooring system can include a chain or cable which anchors the buoy to the The / sea floor anchorage location may be chosen and the gas-conveying 'Λ , X conduit and anchor cable lengths designed so as to maintain the buoy at such distance from the platform that the gas-conveying conduit will always be in tension. Thus, both the anchor cable and gas-conveying conduit are active in the mooring of the flare buoy and the stresses in these members can be determined.

The invention will now be described in more detail by way of example with particular reference to the accompanying drawings in which Figure 1 is an elevation view of the flare system, Figure 2 -is a plan view of the possible motion of the flare means, and Figure 3 is an elevation view of an alternative embodiment of 35 the flare system. -445766 Referring now to Figure 1, a preferred embodiment of the present invention is shown. A platform structure 20 is shown attached to floor 10 of a body of water 12 and extending above the water surface 14. Production equipment 22 is schematically shown supported by the platform structure. Such equipment Is provided to handle oil and pas as it is produced from the well. In oil production, the primary function of the production equipment is to separate pas and water from the oil. If sufficient quantities of gas are produced, It is, of course, transported to shore for use as fuel. Otherwise, the gas may be reinjected into an underground reservoir or it may be disposed of by flaring. In any case, the production equipment must be provided with emergency flaring capability so in the event of equipment failure gas can be immediately removed from the platform area and safely disposed of. For this purpose a gae-conveylng conduit called a downcomer 24 is connected to the gas outlet of the production equipment and extends to a point near the water surface 14. The downcomer 24 is securely fastened to the platform structure and is connected at 26 to a flexible gas-conveying conduit 50. Usually the downcomer is arrayed alongside a leg of the platform structure 20, although, for clarity, the figure shows It at the center of the platform structure. The downcomer is rigidly supported by the platform structure at connection 26 so that stresses Imparted by the gas-conveying » f - 5 457G6 conduit 50 are transferred to the platform structure. The connection 26 itself can be flexible or pivoted so as to reduce local stresses caused by deflections o.f the conduit. As shown in this embodiment, the gaeconveying conduit hangs in the water as a catenary between the platform structure 20 and tbe buoyant flare means 30. The flare means includes a buoyancy chamber 32 designed so that tha top of the chamber remains submerged at a depth of about 10-50 feet where it will be protected from normal wave activity. Attached to the buoyancy chamber 32 la a flare stack 34 which extends above water surface 14 and ia in fluid communlca10 tion with the gas-conveying conduit 50. Flare stack 34 ia provided with suitable ignition means for flaring waste gas. The details*of fluid and structural mechanics design for the buoy, which are required to attain proper operation of the flare means, can be calculated by known methods.

An anchor chain or cable 40 connects the buoyancy chamber 32 to tbe sea floor 10 at anchorage 42. This anchorage can be a pile driven into the sea floor, an explosive embedment type anchor, a gravity foundation base or any other means suitable for securely anchoring the cable 40. Fivot connections et the cable ends can be used to reduce stress concentrations in the cable. The cable operatea in conjunction with gas-conveying conduit 50 to comprise the gas-conveying mooring means which maintains tbe buoyant flare means 30 in an essentially fixed areal location. In tbe present configuration, the gas-conveying conduit 50 ia designed to have a negative buoyancy so it will bang aa a eatenary in tbe water at an intermediate depth. The conduit 50 has sufficient strength to transmit to the platform structure 20 loads imparted by motion of tbe buoyant flare means 30. Tat, the gas-conveying conduit 50 is also flexible so that the curvature of the catenary may vary aa tha buoyant flare means drifts locally. However, the lengths of the conduit 50 and the cable 40 and the location of anchorage 42 are designed ao aa to maintain at all timaa sufficient distance between platform structure - 6 43766 and buoyant flare means 30 so that the gas-conveying conduit 50 ia always in tension. In other words, cable 40 orevents buoyant flare means 30 from drifting to a point where the conduit 50 would go slack.

In this manner it can be seen that the pas-conveying conduit 50 and the cable 40 act in tandem to form the pas-conveying mooring means which restricta the flare means to local motion only. Generally, conduit 50 prevents motion away from the platform structure 20, cable 40 arreeta excessive drifting toward the platform structure, and both serve to restrict motion from side to side. Because the elements of the gasconveying mooring means are flexible, they need not develop large stresses to resist wave, current, and wind forces. Instead, the entire system is free to move with these forces.

Figure 2 shows a sketch of possible motion of the buoyant flare means 30. The design and layout of the gae-coaveying mooring means will take into account such factors as normal currant load and expected storm direction and severity. Thus, under normal sea conditions flare means 30 will assume a neutral position near the middle of the envelope 60. When disturbing forces are encountered, the flexibility of the gaa-conveying mooring means allows the flare means to move about within the envelope. Under severe storm conditions, the flare means could be forced to a position at the limit of the envelope, but this would be an extreme event and the probability of its occurrence would be very small.

A flexible or universal flow-through joint could be provided at location 36 to allow relative motion between the buoyancy chamber 32 and the gas-conveying mooring means. Flare stack 34 could be articulated by a universal Joint at 38 to improve the motion characteristics of the buoyant flare means.

The gas flaring system of this embodiment allows conventional diving operations to be used in installation and maintenance of the gaa- 7 45766 conveying conduit, ^regardless of the water depth. The gas-conveying conduit 50 is submerged oo as to minimise effects of waves and preclude collision with surface vessels. Since the conduit does not lie on the sea floor, it will not be subjected to very cold water temperatures and, therefore, liquid formation in the conduit will not be a problem in most cases. The mooring system employed utilizes the gas-conveying conduit as a structural member, thereby enabling tbe calculation of stresses in the conduit and eliminating the need for separate means to support the gas lines and hold the flare in place. The relevant calculations can be made using techniques such as those outlined in Fart II, Mechanics r of Mooring Lines, of Buoy Engineering by H. 0. Berteaux, published by John Wiley & Sons, New York, 1976, Another embodiment of the present Invention le shown in Figure 3. All components of this configuration are the same as in Figure 1 except the downcomer 24' and gas-conveying conduit 50’. Downcomer 24* extends to a precalculated point 26' at some distance above the sea floor 10 where it connects with conduit 50’. Gas-conveying conduit 50' is designed to have positive buoyancy so the conduit is supported in the water in a convex-upward, reverse catenary configuration. Again the cable 40 keeps the flare means 30 pulled away from the platform structure so that the gas-conveying conduit 50' is always in tension. The curvature of the reverse catenary may varv as the flare means' position changes. Both the gas-conveying conduit 50' and cable 40 serve to keep the flare means in place. In very cold areas where liquid formation ia likely to be a problem, this configuration has the advantage that any condensation in the conduit will run back to the bottom of the downcomer. Suitable liquid collection and handling means can then be provided to deal with the condensate problem.

In operation of either embodiment, waste gas is pumped from production equipment 22 through downcomer 24 and the gas-conveying - 8 45766 conduit 50 to the flare stack 14 where It la burned. Suitable valves, fittiapa, ami i»’tni?OntntIou would bo install·»'1 for pros or operation control and monitorino. - 9 . 4576®

Claims (6)

1. An apparatus for disposing of waste gas produced at an offshore well located in a body of water,, said apparatus comprising: production facilities supported by a platform structure 5 attached to the floor of said body of water and extended above the surface of the water, said production facilities being associated with said offshore well and having a waste gas outlet; buoyant flare means located at a distance from said 10 production facilities permitting safe flaring of said gas; flexible anchoring means connecting said buoyant flare means to a location on the floor of said body of water and maintaining said buoyant flare means at a position remote from said platform structure; 15 flexible anchoring gas conveying conduit extending substantially in the form of a catenary between said platform structure and said buoyant flare means, said conduit being under sufficient tension to provide partial anchorage of said buoyant flare means to said platform structure to 20 maintain at all' times said flare means at a position toward said platform structure and away from a position directly above the location on the floor of said body of water to whioh said buoyant flare means is connected;and connecting means for providing fluid communication between 25 said waste gas outlet and said gas-conveying conduit.

2. Apparatus according to claim 1 wherein said connecting means includes a gas-conveying downcomer connected to said waste gas outlet and extending downwardly to a point at or adjacent the surface of said body of water where it is connected to said gas-conveying conduit -1043706 5· Apparatus according to claim 2, wherein said downcomer extends to a depth of 0-40 feet and. in which said gas-conveying conduit inter-connects said downcomer and said buoyant flare means and is of such sise and weight so as to have negative buoyancy so that said conduit hangs as a catenary at an intermediate depth between the floor of said body of water and the surface thereof.

3. 4. Apparatus according to olaim 1 wherein said connecting means includes a gas-convQf ing downcomer extending downwardly to a predetermined distance above the floor of said body of water and said gas-conveying conduit is of such size and weight so as to have positive buoyancy so that said gas-conveying conduit is supported as a convex-upward, reverse catenary between said buoyant flare means and said platform structure.

4. 5. Apparatus according to any preceding claim, wherein said buoyant flare means comprises: a buoyancy chamber held in a submerged position between 10 and 4θ feet deep by said gas-conveying conduit and said flexible anchoring means; a flare stack attached to said buoyancy chamber and extending above the water surface through which said waste gas is flared; and a conduit providing fluid communication between said flare stack and said gas-conveying conduit.

5.

6. Apparatus according to claim 1 for disposing of waste gas substantially as hereinbefore described with particular reference to Figures 1 and 2 or to Figure J of the accompanying drawings. Dated this 9th day of August 1977. BY: rpHKHIS & CO. Appliihnts' Agents, (Signed) DUBLIN 6. -11STANDARD OIL COMPANY.