Control over a catastrophic blowout can be gained by any technique that blocks the escaping reservoir fluid either in the wellbore or in the formation. The method most frequently used is wellbore blockage, i.e., the recapping of a wild well. Relief well techniques, although more expensive to implement, have popular support because water pollution can be minimized by allowing the flowing wells to burn.
Relief wells are sometimes drilled to establish direct connection with the wild-well bore hole. Then heavy mud is injected at a rate greater than the lifting capacity of the blowing well, until the well is brought under control. If the bottom hole pressure of the reservoir exceeds a pressure that can be balanced by feasible mud injection rates, the relief well may be deliberately aimed off the wild well landing point so that mud can be injected into the reservoir to plug the passageways open to the escaping reservoir fluids.
After a wild-well is brought under control with a relief well, final plugging operations are still necessary to gain permanent control over the well. These surface operations involve clearing away rig and/or platform wreckage around the wells, setting temporary wellheads, and killing the wells from the surface. With control of the wells assured, cleanup can continue by removing all of the wreckage before the platform and wells are completely restored. Usually the wells are restored by installing new conductor pipe and relanding the casing strings. Of course, some wells may be cemented in and abandoned.
One of the industry's most complex and precisely engineered directional drilling programs was successfully executed to control eleven wild wells which blew out on Shell Oil Company's Platform B in Bay Marchand, offshore Louisiana, in December 1970. A total of five rigs drilled 126,925 feet of hole1 in ten relief wells to control the blowouts with 17 lbs,/gal. mud. One relief well was used to kill two wild wells. It took 136 days to bring the wells under control at a total cost of about $28 million, including pollution control measures. Several million dollars of oil and gas was also burned.2 The cost of restoration operations was estimated at $2.8 million in addition to the $1,8 million for a new platform (the old platform was lost at $5.2 million).
Four drilling rigs took 47 days to drill four relief wells to Amoco Production Company's wild wells that blew out on Platform B in Eugene Island, offshore Louisiana, in October 1971. It cost about $9 million to drill the relief wells, fight fires, and control pollution.3 The cost of restoration was estimated at $5.4 million.
In order to recap a burning wild well from the surface, the fire must first be extinguished. With the explosive snuffing technique the blast from a dynamite capsule positioned over the fire uses the oxygen necessary for conflagration and the fire is snuffed out.
As seen in Figure 3-1, the burning wells and rig or platform must be sprayed with a battery of jet nozzles to cool the metal so the fire will not reignite after it is snuffed (also to prevent deterioration of the wellheads and structure). Jet barges, equipped with fire-fighting pumps, hoses and nozzles, and capable of pumping up to 15,000 gpm of seawater on the wells and structure, are available in some drilling areas. A work platform (Figure 3-1) is usually set to provide spray capability and a stable work surface for the positioning of the explosive charge and for subsequent surface operations.
After the fire is extinguished, the spewing wellheads and conductor pipes must be cut clean with shaped charges so the wild wells can be capped. A long casing cap is lowered over the spewing conductors to direct the flow of oil and gas, and water is injected through the casing cap while the shaped charges are ignited to minimize the threat of reflash of the fire. Temporary wellheads are then installed on the conductor pipes to bring surface control to the wild wells.
The explosive snuffing technique has a major disadvantage in the potential oil spill with resulting offshore pollution. The fire
shown in Figure 3-1 burned for four weeks while Chevron Oil Company amassed the largest collection of pollution control equipment ever marshalled for offshore purposes, as they prepared to snuff the blaze and cap the flowing wells.4 5
A crew of wild well control specialists was used to cap six wells after the fire was snuffed out with explosives on Chevron's Platform C in Main Pass, offshore Louisiana, in February 1970. Two additional wells were brought under control by injecting mud and cement into relief wells. Because the wells were allowed to flow for three weeks after the fire was snuffed out,4 oil pollution damaged wildlife and two islands, drawing close public attention to the catastrophe, and causing postponement of an offshore lease sale, Louisiana oyster fishermen filed a $31,5 million federal pollution suit against Chevron, and the shrimp fishermen filed a $70 million suit.5'6 Chevron was fined $1 million for the oil spill by a U.S. District Court in the first prosecution under the 1953 Outer Continental Shelf Lands Act.7
A well killing technique has been developed by Tenneco Oil Company and Brown Oil Tools, Inc.8 Rather than fighting a fire at the surface of the sea where both wind and waves add to the difficulty and danger of the task, crews approach the well beneath the water surface where there is no danger of fire or explosion. In this relatively safe environment, divers can cut into the casing strings of a wild well, and then tap into and plug off the tubing that sustains the fire. This technique has the advantage of allowing oil and gas produced from wild wells to burn, minimizing offshore pollution during the killing operations.
Before the well plugging method can be utilized, access to the tubing strings that are blowing must be effected. This involves two general operations: (1) access through the platform/well debris to the well casing; (2) access through the casing program to the tubing strings. After diver reconnaissance of the general condition of the platform structure and wellheads, the decision can be made to begin access operations.
To gain access to the well casing does not require full scale salvage of the platform and wellheads (which may be done after the wells are brought under control). Instead, the divers must clear only sufficient space to allow travel to and from the well casings for the diver and equipment and provide work room around the well casings of interest. Temperature and sonic measurements on the well casings can be used to locate the casing strings containing blowing wells.
To provide working space, much of the tangle of debris can be moved aside with air tuggers and winches controlled from a work barge. If debris cannot be moved aside to provide access to the well casings, standard explosive devices can be used to sever the obstructing members. Straight, circular, and structural beam explosive-shaped charge cutters are widely used to produce slot-type cuts to any desired length, depth, and configuration.
Once access to the well casings is obtained the divers can begin access operations to the tubing strings. For cutting windows in casing a "picture-frame" cutter was developed.8 It uses a combination of linear and circular shaped charges, and the cutter is designed so there will be no damage to a casing or tubing string inside the casing being cut even though the inside string may be laying against the wall in the line of the cut. Windows are cut in the successive casing strings until access to the tubing strings is obtained.
Before proceeding with the window cutting operations on a casing string (and almost certainly on the production casing string) the divers may hot-tap the annulus to check for pressure if communication with the well is suspected or is remotely possible. If the annular area contains pressure, this must be bled off or killed with mud or cement. It if is not possible to kill the annulus, the well must be shut in with alternate techniques.
Where water depth permits, a window is cut in the casing strings near the sea floor, and another window is cut at diver height above this window. If the water is not deep enough to allow two windows or if it is more expedient, one elongated opening may be used as seen in Figure 3-2.
A hot-tap device is attached to the tubing in the lower window, and an entry hole is cut into the string or strings which are out of control. In the upper window a crimping device is applied to the tubing to form a stricture (Figure 3-2). Sealer balls are
introduced into the flow stream from a canister attached to the hot-tap device. The fluid somce for injection is a flexible hose leading to the surface. The flow inside the tubing lifts the sealer balls until they are trapped by the crimp. The sealer balls are followed by a finer lost circulation material until an impermeable plug has been formed at the crimp, as shown in Figure 3-3.
Prior to and during the plugging operations, the flowing oil and gas burns instead of spilling in the water. With the plug in place, flow to the surface terminates, the fire dies, and the well is ready to be killed by pumping mud and/or cement through the hot-tap hole.
Hurtsteel Products, Ltd. of Canada has proposed a similar diver activated well killing technique called the SNUFF System,9 With this system access to the inner casing string and/or tubing strings is obtained by the techniques described above. However, the wild well is plugged by injecting CO2 into the tubing, or casing annulus, to form an ice plug until the well is brought under control. The wellhead equipment could also be frozen with C02 injections.
Neither diveT activated well killing technique has been used to kill an actual wild well. However, the Tenneco/Brown technique has been completely developed and successfully tested on a simulated blowout at an offshore location.10 This technique is ready for commercial application with a complete set of the required tools and equipment assembled for emergency use.
Obviously, the diver activated well killing techniques cannot be used to control every catastrophic blowout (as mentioned above, the casing annuli must be killed). If debris from the burning wells and rig/platform structure are falling, the decision may be made to delay access operations until the situation stabilizes, construct a structural umbrella to protect the divers, or use alternate techniques to control the blowout. It should be emphasized that divers cannot be paid enough to commit suicide, so they won't go under a burning structure if the situation is one of high risk. Also, equipment and structural members do not fall instantaneously—the diving master has time to warn the divers to safety.
Figure 3-4 shows a situation suitable to diver activated tech-
niques. This blowout occurred offshore Libya in 85 feet of water in August 1986. When the rig blowout control equipment failed during a kick, the drillship was moved off location ripping the riser, kill and choke lines, and hose bundle from the wellhead.
The blowout and fire shown in Figure 3-5 occurred offshore Australia in August 1969, The semisubmersible rig, drilling in 330 feet of water, could not be pulled off location after a tool joint ripped the packing element of the annular preventer during heave following a kick. A flash fire gutted the rig and living quarters, but the situation was ideal for diver activated well killing techniques. The wild well was killed with a relief well 15 months after the blowout.
Blowout equipment is much like insurance; the greater the risk to life and property, the greater the cost of offsetting the risk. Despite equipment improvements and increased reliability, catastrophic surface blowouts will occur. Whatever the
cause, each situation must be met with an intelligent approach that will lead to the logical control of the well. A prepared plan of action for foreseeable eventualities should be available. This plan, along with the necessary equipment, must be ready before the blowout occurs.
In this section some emergency procedures in the event of offshore blowouts and fires are discussed, and some techniques are described that can be used to bring catastrophic blowouts under control. As operating companies move into deeper waters, the tremendous financial burden and unfavorable public opinion associated with major blowouts should assure that adequate equipment, proper training of operating and contract personnel, and continued testing of equipment and personnel are provided for offshore drilling operations.
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