Welding enclosure eliminates platform shut-in

Dan Parascin
Hot-Hed Inc.
Houston

Welding habitats placed on offshore platforms have eliminated the need to shut in wells.

Without the habitats, welding operations in well bays on offshore platforms typically require all other wells to be shut in to prevent the heat source from igniting produced oil or separated gas.

In one example, Shell Offshore Inc. on its Gulf of Mexico Cognac platform, Block Mississippi Canyon 194, employed a welding habitat to enable it to replace the casinghead of one well without shutting in other producing wells on the platform.

Governing regulations would have required all producing wells to be shut in if welding is performed within 10 ft of the well bay.

Shell used a heat-resistant welding dome (Fig. 1 [8,555 bytes]) to isolate the welding area, containing the heat source.^{1 2}

Shell obtained a variance from the U.S. Mineral Management Service's (MMS) requirements and used the habitat while replacing a 133/8-in. slip-in casinghead with a 16-in. slip-on casinghead.

The Cognac platform has 40 active wells producing 10,000 bo/d and 100 MMcfd of gas.

The habitat unit isolated the welding area during the operation while maintaining platform production, saving Shell 12 hr of deferred production time and eliminating the risk of losing well productivity after a shut-in.

The casinghead replacement operation involved washing out the old weld of the existing head and then welding on the new casinghead.

Welding

The conventional method of welding on a platform is to shut in production on the platform to prevent the hot gases produced by the welding process from igniting flammable materials. Welding operations typically cause production deferment during the welding process.

However, shutting in production can lower flow rates in older wells when production resumes.

The API recommends that the casing be preheated to 400-600° F. before welding to produce a metallurgically correct bond at the weld.³ The preheating temperature must be maintained during the welding process.

Hot slag and fumes circulating around the welding area create a potential hazard for producing wells and personnel.

The welding habitat isolates the weld area (Fig. 2 [50,746 bytes]), preventing hot exhaust from reaching producing wells by venting the fumes to a nonflammable location. Heat-resistant fabric contains the sparks and welding slag.

The habitat allows the operator to isolate and repair a single well without shutting down all nearby wells.

Habitat specifications The habitat's floor and walls are made of heat-resistant fabric. Fig. 2 shows how the habitat floor sleeve slips over and seals around the conductor pipe.

The habitat body is placed over the area to be welded and connects to the floor. Water bags on the bottom of the body walls seal the dome interior.

The unit is inflated with a 4-hp blower (Fig. 3 [8,337 bytes]). The walls expand to fit the available space on the platform (Fig. 4 [12,375 bytes]).

An additional exhaust blower provides positive pressure and air circulation inside the dome. Table 1 [19,618 bytes] lists the technical specifications of the blower.

Fresh, clean air is pumped in by the blower, on the left in Fig. 2, and exits through the hose to the right.

The welding process produces welding slag, sparks, and heated smoke and fumes. The habitat forces the fumes and hot exhaust out of the unit to a safe, nonflammable location on the platform.

The habitat body is composed of an aramid-silicone-fiber glass fabric that is heat-resistant up to 1,400° F. An air-lock door seals the interior with positive internal pressure, protecting the welding process from the weather.

Other uses

The welding habitat has been used in fields around the world. The unit was used on a platform in the Gulf of Mexico to help reconstruct an existing oil surge tank into a flotation cell unit. The welding habitat isolated the unit during welding operations, saving the operator more than 120 hr of production time that would have been lost if the platform was shut in.

The unit is used regularly in Mexico to install casingheads on new wells. The other producing wells on the platforms do not have to be shut in and continue to produce.

In the Mexico case, two welders working simultaneously finished an installation in little more than 2 hr, saving the operator a minimum of 7 hr of well production.

In the South China Sea, two platform operators used the welding habitat to install a total of 28 wellheads. The average amount of time saved per job ranged from 8 to 10 hr. Use of the unit saved the operator about 270 hr of production. The platforms produce from 75 to 100,000 bo/d.

Acknowledgments

The author thanks the management of Shell Offshore Inc. for permission to publish this article and to John Cook, Shell Offshore Inc., for his contributions to its preparation.

References

1. Wardlaw, Louis J. III, "Subterranean well welding habitat," U.S. Patent 5,018,321, May 28, 1991.
2. Wardlaw, Louis J. III, "Subterranean well welding habitat," U.S. Patent 5,101,604, Apr. 7, 1992.
3. API Recommended Practice 5C1, 16th Edition, Care and Use of Casing and Tubing.

The Author