AIR-LIFT PUMP CLEANS AROUND SUBSEA WELLHEAD

Ken Vargas
Suncor Oil Sands Group Inc.
Fort McMurray, Alta.

A relatively simple and inexpensive air-lift system moved cement spoils from around a subsea wellhead to the moon pool of a rig, allowing the crew to monitor the cement returns without the drilling riser in place.

This equipment setup also removed cuttings from around the blowout preventer (BOP) stack to keep the wellhead area free of debris.

Air-lift pumps have typically been used to transfer liquids and slurries from mine shafts and oil from dead wells. These pumps have proven applicable in field operations because they have:

Good reliability (minimal equipment needed - only a dependable air compressor)
A low level of component maintenance
The ability to handle hazardous materials safely.

For offshore operations, air-lift pumps can remove material on the sea bottom, help set a BOP or wellhead, or help monitor cementing operations. Cementing operations are monitored by a confirmation of cement flow at the surface while the production or drilling riser is not in use.

Fig. 1 shows a basic air-lift system, which consists of a vertical tube partially submerged in liquid and an air pipe connected near or at the bottom of the vertical tube. (This pipe supplies air from the surface to the air/tube connection.)

The air mixes with the liquid to reduce the fluid density. The lower density of the mixture provides a buoyant force which lifts the fluid. The mixture ideally moves in slug flow and discharges above the surface of the liquid.

ALP-LIFT PUMPING

Numerous technical papers cover the theory and application of the use of air to pump liquids and solids through a vertical pipe. Clark and Dabolt give the most accurate equations for the design described here. These equations account for multiphase flow in the vertical pipe of the air lift.

For the following application, however, comprehensive, theoretically correct equations were not used. The major consideration was to move large volumes of water and solids from the sea floor around the wellhead. In general, this approach is valid where the following apply:

The lift height (HL) above the water surface is much smaller than (Hs) the submergence vertical distance (Hs/HL + Hs 0.7).
The diameter of the pipe used for the lift is greater than 2 in.
The overall height (HL + Hs) is much less than 1,000 ft.
The overall density of the fluid is similar to that of seawater (specific gravity 1.03). The lifted material here was water with mud or cement.
An accurate flow prediction of the water and spoils lifted is not critical.

Maximum pump performance occurs with the fluid moving in slug flow. Fig. 2 shows the results of Stenning and Martin's analysis for the air-to-water lift ratio vs. the liquid velocity.

In slug flow for an air lift, large air bubbles are surrounded by an annular film of liquid in contact with the pipe wall. The preferred slug flow regime is to the left of Y. = 2.0 because less air is required to lift the water. An accompanying box presents the air-lift design equations.

WELLHEAD CLEANING

A drilling rig operated in ice-infested waters. The wellhead BOP assembly sat on the ocean floor and needed protection from incoming ice. A "glory hole" or excavation in the sea bed was necessary to contain the wellhead and protect it from ice scour if the rig would have to move off location.

The air-lift system was installed on the rig to keep the glory hole clean and to help in the surface monitoring of cementing operations. The rig needed a system that was light and easy to put together and take down.

Fig. 3 shows the general arrangement of the air-lift design for the rig. The fiberglass lifting pipe was designed for quick connection in the moon pool by means of air tuggers. A frame placed over the moon pool supported the vertical pipe during operation.

According to the design calculations for this well, the air-lift system needed 6-in. fiberglass composite piping with an air line banded on the side. The air compressor needed a rating of at least 343 hp and the capability of putting out 1,360 scfm of air at 100 psia. (See the box of calculations for air-lift cleaning around a wellhead.)

These design calculations were verified on Neotechnology's two-phase well flow computer program, Well-flow 6.00. The program predicted 63 psi at the surface for these input parameters, indicating the assumptions were reasonable.

The compressor design calculations are important - an adequately sized compressor must be used to ensure enough discharge pressure to overcome the water column head.

The air was sent down through a 1 1/2-in. pipe banded to the fiberglass pipe and connected by a transition hose at the pipe joints.

The buoyancy and flexibility of the fiberglass piping presented the only problem in the design and hookup. Waves tended to move the pipe as if it were wet spaghetti until the pipe was fixed to the bottom and supported adequately.

The bottom transition piece was then secured firmly in the glory hole to prevent waves or currents from flapping the pipe about. For long runs of pipe, an alternate means of support with cables or more rigid piping is recommended.

Table 1 lists the costs for this system with suitable detail to indicate the complexity of the equipment.

The air-lift components were delivered to the rig and assembled on location, and the compressor was hooked up temporarily on the rig deck. All of the fiberglass piping was assembled easily in the moon pool and then lowered and located near the wellhead.

The actual effluent fluid flow rate was never measured because the design did not require a predetermined fixed flow rate of mud or cement to be lifted from the bottom. The goal was merely to clean around the wellhead. The air-lift equipment worked well - large quantities of cuttings and debris were pumped from around the wellhead.

BIBLIOGRAPHY

Clark, N.N., and Dabolt, R.J., "A General Design Equation For Air Lift Pump Operating in Slug Flow," AIChE journal, Vol. 32, No. 1, January 1986.
Gibbs, C.W., Compressed Air and Gas Data, Ingersoll Rand Co., New York, 1971.
Allen, J.H., "Review of Reverse Circulation Air Lift Methods for Big Hole Drilling," Society of Mining Engineers, AIME Transactions, Vol. 262, June 1977.
Odrowaz-Pieniazek, S., "Solids Handling Pumps-A Guide to Selection," The Chemical Engineer, February 1979.
Stenning, A.H., and Martin, C.B., "An Analytical and Experimental Study of Air-Lift Pump Performance," Transactions of ASME, April 1968.