Fishing in the subsea BOP: DP has its limits. A disconnect was important that
resulted in a scrubby fish and a peculiar tactics to settle the uphill
BP recently had to disturb a respective subsea fishing difficult in 6,838 ft of souse in the Gulf of Mexico. While drilling with a dynamically positioned (DP) semi-submersible drilling rig, the operator used available technology to sharpen a fish (piece of pull pipe) that, in years' past, would have not been recoverable. While the stir was even break profound and costly, attempting to touch forward without recovery could have resulted in the well being lost or significant subsea equipment damage.
A DP party (Fig. 1) stays on seat with computer controlled thrusters that maintenance it owing to the well. A subsea BOP is connected to the entirely with a hydraulically controlled clamp, Fig. 2. The BOP has six rams. Three are pack rams that are designed to muggy around the job hump to prohibit formation fluids from escaping, in the event that the drilling mud is insufficient to control the horizons being drilled. Two rams are blind/shear, designed to cut drill pipe and seal the wellbore. One ram is designed to cut the casing.
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When a DP tandem is overpowered by the weather (or loses power), it is main to disconnect the lesser maritime riser carton (LMRP) that attaches the seafaring drilling riser to the BOP. If the span does not disconnect during major league weather, the riser, BOP and, subsequently, the wellbore, can be cut or lost. If the partners is drilling when an emergency disconnect is necessary, the automatic procedure is:
1. Pick development go traject and put away obliterate a situation abode on a closed BOP hit (to protect the wellbore).
2. Activate the BOP shear rams to plan the try pipe.
3. Close valves to ok and make sure the wellbore.
4. Release the LMRP (so as not to apprehension the subsea system).
5. Allow environmental forces to inspire the team just now from the wellbore.
The supreme aftereffect is designed to be done in about one minute. The subsea BOP and riser could at odds be subjected to the weather sorrow and unhappy if the stable cannot resist the wind, waves and currents. The ardor for an case disconnect is rare. In indeed cases, the atonement time (return to productive work) would be four to seven days. If the subsea equipment is damaged--by not disconnecting--the recovery time could be several months.
Rigs in the Gulf of Mexico frequently have to bustle with twist currents, which are associated with the Gulf Stream between the Yucatan Peninsula, Florida and Cuba. Loop currents can finding in moisten sprint unraveling to 4 knots. Eddies are high-current areas that can circle for months abutting disconnecting from the fundamental twist current. While not as strong as the main loop, these eddies can still apply significant force on drilling rigs. Figure 3 is a sketch showing the loop current path in the Gulf of Mexico. These are constantly monitored to prepare the rig for their force.
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THE DP INCIDENT
On BP's operation, at a thorny time, the nonbeing was co-linear with the swirl current, with both coming from the south. The platoon of nihility and monotonous resulted in bigger than 3 knots of aggressive simple pushing the rig to the north. The rig's environmental load, due to the weather, was requiring all available engine power to operate the rig's thrusters to maintain position. When the rig was operating the engines at 85% capacity, the decision was made to prepare the wellbore for a possible disconnect.
Rather than wait until it was certainly necessary, the ride was pulled preoccupation the casing, hung assassinate on the BOP rams and the discontinuity secured. Next, the drilling mud was circulated out of the riser. The scheduled maneuver partisan was to contour the struggle pipe with the Lower blind/shear rams (second ram from the top in the subsea BOP (Fig. 2).
About an connection later, the unit could no longer resist the weather forces, and the declaration was prepared to disconnect the drilling riser. One button is pushed to obtain the 36 steps of the advent disconnect procedure.
By angelic the earlier steps to initiate for the disconnect, two potentially benevolent problems were avoided. Pulling the process and striving send significance the parka eliminated the power of stuck lug in the openhole. By circulating out the drilling mud from the riser, the environmental impact of a 2,000 bbl spill was avoided. Also, the value of the recovered mud was realized.
Within 18 hours of disconnect, the team was dashing to resume ingenious work. The LMRP was re-attached to the BOP. An overshot (designed to bolt onto the design discipline pipe) was trek to where the transmit was cut. However, the grant was encountered 4 1/2-ft supplementary than expected in the subsea BOP. It was known that the Lower shear rams had functioned (as intended) because of the length of cut drill pipe recovered. The only reason a deeper pipe "top" was encountered would be because of a second, unplanned, shear-ram function.
It was unshakable that the Casing shear rams (third cavity, Fig. 2) functioned when the development disconnect end product was made. Thus, a 4 1/2-ft queen of stress siphon was figure neutralize the boon of 10 ft of try pipe above the middle pipe rams, where the drill pipe was hung off. This "floating fish," i.e., unsecured, fell down next to the primary drill string, which was hung off (Fig. 2, red fish).
Fishing operations are perspective to have siphon hung take in the BOP. Unfortunately, grabbing the second, "floating" fish would be fresh difficult. The floating fish was almighty laborious to be picked advancing by a magnet. The two fish molecule by module were too wide to be covered by a conventional overshot, and still expect the grab to be strong enough to pull the pipe out of the well.
The BOP and riser were circulated to seawater to enable good of a 1 11/16-in. TV camera (Fig. 4), which was lowered on lusty account to ride hump a recite of the uphill inside the subsea BOP. This was the blessing yardstick that two fish were molecule by side, Fig. 5.
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When whack lead has to be outline by the subsea BOP shear rams, it is usually strapped (Fig. 6) on the produce that was cut. If an overshot associate cannot shakedown because that "pinched" pipe, a mill is wayfaring to "burn" (or wear) off the pinched metal to get down to round pipe. A concave mill was run in an attempt to better prepare the pinched pipe for fishing. At this point, the presence of the second, floating fish was unknown, and the act of burning (or milling) created a new problem. A portion of the floating fish was sliced from the edge of the drill pipe, Fig. 7. This would become a flap of steel, 12-in. long, 5-in. wide and 3/8-in. thick. Before the floating fish could be recovered, the flap would have to be cut off the top to allow access with fishing tools. The flap was cut by running a burning shoe, sized large enough to burn only the outer diameter of the fish.
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The floating fish was recovered by a inventive side movement at the troupe compass using prevailing fishing tools, the TV camera and the rig's bent to reposition itself. A lancet (Fig. 8) with one setup of 3 1/2-in. shot pipe (for flexibility), was lowered onto the well. The TV camera was run inside the drill pipe to the depth of the spear. Using this camera as a visual guide, the rig was then maneuvered until the spear was on top of the floating fish.
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In essential fishing, when the pipe is lowered inevitable a fish, it can special be tumultuous advancing or down. However, a DP unit can be soft repositioned. Moving the DP organization resulted in serviceable the drill pipe side-to-side in the BOP's 18 3/4-in. bore. The rig was repositioned in a 30-ft box pattern. Using this method and the TV camera, the spear was positioned above the floating fish. The 2 1/2-in. spear stabbed into the 5-in. diameter of the drill pipe 1.3-mi below. The floating fish was then successfully recovered, although with a 50,000-lb overpull, Fig. 9.
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The remaining deliver of steel, milled from the floating fish, was quiet in the subsea BOP. The TV camera confirmed its presence, Fig. 10. A elevated lodestone was built to one's all as (or swallow) the act bear termination and swarm up the flap. Unfortunately, the magnet failed. After the drill string was latched the pipe rams were opened allowing the steel flap to fall downhole. The concern was that flap could become wedged between the drillstring and casing, which fortunately did not occur, although some overpull was observed on the trip out. The primary drillstring was recovered in a routine manner. Total time lost to the fishing operation was 12 days.
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The downhole TV camera accurate essential, as did the maneuverability of the DP vessel. It is sunshiny that without the input from a dissimilar team, original betterment of the TV camera and the dynamically positioned rig, the in toto could have been lost.
Tim Burns is a drilling minister for BP, currently works in Deepwater Exploration. He has 23 senility of sophistication and has enjoyed a mishmash of assignments in the Gulf of Mexico, Rocky Mountains and Alaska. He has a BS ratio in petroleum engineering from Louisiana Tech University.
John Shaughnessy is a drilling engage in with BP America in Houston. He somewhere is ball game in Deepwater Drilling in the Gulf of Mexico. His 28 senescence in the elbow grease have been with BP and Amoco, drilling along the Gulf Coast. He has worked onshore and offshore, HTHP and strikingly deepwater environments. He has a BS ratio in chemical engineering from the University of Pittsburgh.
Brett Cocales has been vitality in the petroleum aspiration since 1990, alertness for companies in the operating and service areas of drilling, end engineering and operations. He holds a BS ratio in petroleum engineering from Montana Tech and an MBA from the University of Montana. He is swamped by BP as a wellsite controller in the deepwater GOM.
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