NEW TOOLS AFFORD MORE SECURITY FROM SHALLOW GAS

Joseph R. Roche
Hydril Co.
Houston

A variety of new tools for handling shallow gas is available for matching the diverter to the application and affording backup security for lifesaving systems.

Shallow gas has been, and will continue to be, a major hazard during drilling operations.

A risk assessment of a typical diverter system' reveals numerous accident possibilities, any one of which could become a catastrophe. Case histories show that erosive cutout, valve malfunction, and system complexity make quick failure of the diverter system the rule rather than the exception. Many times, diverter systems have been shown to be too fragile, too complex or too poorly designed to do the job.

Indeed, designing a dependable and durable diverter system offers formidable challenges for physical and philosophical reasons, including these:

The volumetric rate of flow from a shallow formation can be tremendous. The accompanying forces acting on the rig can be terribly destructive.
Shallow gas is often entrained with sand, rendering it an excellent metal-cutting medium.
Equipment budgets are extremely tight. For the most part, the bottom line rules.
Space for diverter equipment is limited. A lot of other things get in the way of the vent lines, so the vent lines often have bends in them.
Lifting and storage capacities under the rig floor are generally at a premium.
Cheap valves and thin-walled pipes are readily available, fit the budget and, therefore, are often used for vent line components.
Complex diverter equipment arrangements require complex sequenced control systems.
People believe that they are too smart or too careful to have an accident.
People believe that shallow gas events are rare and predictable.
Shallow gas can burn.

NEW STANDARDS

The U.S. Department of Interior's Minerals Management Service has mandated safer diverter systems by requiring larger bore vent lines and minimizing of bends in the overboard piping.

The U.K. Department of Energy and Norway's det Norske Veritas are planning similar restrictions.

The API is culminating 9 years of subcommittee work with the planned publication of a comprehensive Recommended Practice on Diverter Systems Equipment and Operation.

ADVANCED TECHNOLOGY

The economic depression that has beset the offshore drilling business has severely retarded the spread of improved diverter technology. Even today, outmoded equipment can be bought and installed just to meet regulatory agency requirements as economically as possible.

Where diverter technological advancement has taken place, it has commonly been of necessity, e.g., in reaction to a disaster (West Vanguard blowout, Explorer drillship sinking) or to meet the demands of a new application such as tension leg platforms (TLP's).

Surface diverter systems have been strengthened and simplified, while concepts for subsea diverting are being promoted. Since 1984 there have been several new developments.

MATERIAL IMPROVEMENTS

Erosion is the most troublesome enemy of the diverter system designer. Although combatting erosion is still largely a black art, advances are being made in extending the life of materials subjected to the flow of abrasive media.

One Norwegian operator is examining ceramic coatings which are applied inside a diverter spool to resist erosive cutout where flow is redirected from vertical to horizontal.

Another is constructing a spool with hardfacing applied to the critical internal surfaces.

Hydraulically operated two-position targetS2 used on numerous rigs worldwide have a hardfacing applied to the exposed target face, while the alloy steel target itself is massive in solid volume, for longer endurance.

SIMPLIFICATION

Big bottom-founded rigs such as the Rowan Gorilla III and IV jack ups, Transworld's Mister Mac jack up, and Total's Alwyn platform in the U.K. North Sea, are using strengthened and streamlined diverter units with an integral venting feature.

Pemex's Holkan jack up is switching from a cartridge-type diverter to a unitized high-strength diverter for a rebuild following a blowout and fire that severely damaged the rig.

Diverters similarly designed for strength and simplicity are in service aboard world-class floating rigs such as the Sonat Goodrich and the Reading & Bates Zane Barnes.

BETTER TELESCOPIC-JOINT PACKER

One of the apparent factors reported during the West Vanguard accident investigation was the inability of the telescopic-joint packer to contain internal pressure while diverting.

Typically, as the well fluid initially unloads through the riser, a bladder-type packer is subjected to a pressure spike.

The marine drilling risers on the Zane Barnes and the West Stadrill have been equipped with improved piston-activated, annular-type packing elements (Fig. 1) developed expressly to deal with the occasional pressure surge, as well as everyday sealing against a minimal head of drilling mud.

This annular packing element provides better regulated control of the sealing pressure, feedable rubber for longer service life, and elevated-pressure sealing capability. The capability of this design to maintain a seal against 500-psi gas pressure, while recirculating, has been proven.

SUBSEA DIVERTING

The most noteworthy development in diverter technology has been the concept of diverting at the wellhead rather than at the rig substructure. To accomplish this, a diverter spool is positioned just above the sea floor. it may be equipped with an annular piston-operated integral vent, or it can be fitted with external hydraulically operated ball valves.

The annulus closing device (annular BOP) can be either positioned atop the spool (subsea) or mounted under the rotary table (surface). To start diverting, the vent outlets are opened and the annular BOP is closed, thus directing the shallow gas flow into the sea.

The destructive potential of the flowing gas/sand is largely dissipated by discharging against the backpressure of the sea. The expelled gas rises to the surface, being dispersed as it goes, first by the ocean currents and then by winds. The sand settles harmlessly to the sea floor.

The advantages of this arrangement are numerous and significant:

The hazardous highthrust flow is kept away from rig personnel.
The cutting potential of the sand particles is effectively neutralized.
The diverter system presents minimal obstruction to the flow.
The danger of a fire is greatly reduced. The combustible gas is cooled and diluted.
The ocean serves as a vast heat sink, so that frictional heating is counteracted by conductive cooling.
The hydrostatic head of ocean water present at the vent outlet impedes the flow, reducing erosiveness and thrust load applied to the diverter equipment.
The cost of a subsea diverter stack can be less than that of a complete surface diverter installation.

A number of subsea shallow gas diverter stacks have seen service and several more are in the works.

TREASURE SAGA

The stack shown in Figs. 2, 3, and 4 has been in service since the spring of 1984 drilling approximately 19 wells from Saga's semisubmersible in the North Sea.

When using the seabed diverter (SBD), the normal setting depth of 30-in. conductors is 100-200 m. This provides extra support around the 30-in. shoe to withstand the forces caused by shutting in the seabed diverter (SBD).

While drilling through the 30 in., if the well starts to flow, Saga's procedure is to close the SBD and vent into the sea. The surface diverter system is activated before operating the SBD to avoid any gas in the riser escaping through rotary table.

Depending on the volume of gas that is percolating through the sea and the environmental conditions around the rig, several options are possible:

Pump "kill mud" and try to reestablish well control.
Open the 3-in. choke and kill lines, close the 8-in. vent valve, and bring the influx through the rig's well-control equipment. Wellhead pressure can be monitored, most likely indicating that formation integrity below the 30-in. conductor shoe is exceeded.
Move the rig off location. This is facilitated by disconnecting one of the two hydraulic latches.
Using a seabed diverter system while flow checking is safer, easier, and faster than with a conventional diverter system. This is because the mud column in the riser is kept on standby for the duration of the flow check.

A measurement-while drilling (MWD) tool is used on all pilot holes drilled by Saga. Evaluating the MWD information relative to the drilling data (penetration rates, gas readings, cuttings) permits determination of the correct mud weight for flow checking against seawater column.

Prior to disconnecting the riser, the hydrostatic head in the well bore must exceed the hydrostatic head that was present during the initial flow check.

The procedure is as follows:

Close the SBD annular preventer on the drill pipe, keeping the bit close to bottom.
Open the 8-in. vent valve and allow the fluid level in the drill pipe to equalize with the seawater hydrostatic head.
Observe the well for flow for 30 min by monitoring the open vent valve with subsea television.
If the well is stable, close the vent valve and open the annular preventer. Circulate bottoms up and monitor for possible increased gas concentration in the mud.
If no indications of instability are observed, pull the drillstring out and open the hole to 26 in. using an hydraulic underreamer.
If flow is observed, close the vent valve and open the annular preventer. The well bore is immediately exposed to the full hydrostatic head of the mud column in the riser. Heavier mud can be circulated into the well and the flow check procedure repeated.

In the event of diverting subsea, the hazards surrounding the rig could be severe if the gas plume is close to the rig. Depending on the force and direction (velocities) of the wind and current, it may become necessary to move the rig off location.

In that event, a disconnect with either venting or shut-in can be executed by one of these alternatives:

Disconnect the upper hydraulic latch and leave the SBD with open vent valve on the 30-in. housing and winch the rig off location using the anchor windlasses.
Disconnect the lower latch from the 30-in. housing and winch the rig off location.
If disconnect cannot be achieved, close in the well and winch the rig off location.

STACK WITH SHEAR RAM

The West Vanguard shallow gas blowout, offshore Norway in October 1985, inflicted serious fire damage to the rig and cost one life.' The tremendous flow quickly caused erosive cutout of vent piping, and leakage from the telescopic-joint packer was observed. Following this calamity, the operator, Statoil, made the decision to equip the new West Vision semisubmersible with a subsea diverter.

After having run the shallow gas stack on several wells offshore Norway, the West Vision semisubmersible was converted to a production platform and renamed Veslefrikk B. This shallow gas stack has not been used since.

CONTROL ALTERNATIVE

In a dire emergency, when the surface diverter system is overloaded, the subsea diverter stack can provide a swift, decisive control alternative.

Use of a marine riser and subsea diverter stack may be preferred to drilling riserless because of the many extra control options afforded, such as drilling with treated mud, ability to circulate out a kick, or shearing pipe for moving off the hole.

SEDCO 600

Indonesia has been the site of at least two drillship sinking by shallow gas. The Petromar V was lost in 1983, and in October 1988 the Viking Explorer went down.

Following this most recent loss, the operator, Total Indonesia, resolved to employ a subsea diverter stack to help prevent a recurrence. The Sedco 600 semisubmersible will be equipped with the stack pictured in Fig. 5. The closing of the annular BOP is interlocked with the opening of the ball valves.

The Sedco 600 is equipped with emergency windlass release to provide quick abandonment of drilling location.

From bottom to top, the stack consists of:

A 30-in. external latch, 1,000-psi working pressure, hydraulic-actuated with mechanical override.
Ported diverter spool with two each 1,000-psi hydraulic-actuated, double-acting ball valves 12-in. nominal bore.
Shear rams, 21 1/4-in. x 2,000 psi, fitted with shear booster pistons.
Crossover double studded adapter 21 1/4-in., 2,000 psi to 18 3/4-in., 10,000 psi.
Mandrel, 10,000 psi to fit LMRP connector.
Subsea diverter-handling frame with retractable beams to fit Sedco 600 handling system and lower female pod receptacles to provide redundant controls to connector, rams, and vent valves.
Frame complete with guide columns to fit permanent guide base posts.
LMRP from the main BOP stack with hydraulic latch, annualr BOP, flex joint, riser adapter, accumulators, and pods latched to the top of the subsea diverter. The control system to the pods is not significantly changed from the normal BOP control system.

TENSION LEG PLATFORMS

Designers of tension leg platforms for the North Sea and the Gulf of Mexico are planning to use subsea driverters while drilling through the 30-in. marine conductor pipe.

In greater water depths, where TLP's are practical, the rising gas plume has a very high probability of being swept away by currents and, consequently, surfacing away from the platform.

BOTTOM-SUPPORTED RIG

A jack up or platform rig cannot be moved off location while diverting. It must sit in place and take whatever the well dishes out. Hence, the matter of handling shallow gas is the highest priority.

We know that some shallow-gas blowouts are powerful enough to destroy even the stoutest surface installation. Some jack ups have severe space and weight limitations in the rig substructure. It would be desirable to have an alternate system to stand behind the primary diverter; one that is totally independent in functional elements and control, one that does not consume much space, and one that is not prohibitively expensive. A driveable, expendable subsea diverter spool to be installed in the 30-in. casing could offer such options.

Fig. 6 shows an arrangement employing an expendable spool with the vent mechanism built into an annular piston. This conceptual design is still being evaluated along with other considerations such as the chances of gas plume emergence beneath the rig and its affects on attempts to launch lifeboats.

CONTINUING STUDY

Performance of the well and the diverter system during well fluid flow are the subject of continuing study at Louisiana State University's MMS research well facility in Baton Rouge. This work includes:

The development of improved methods for predicting pressures, velocities, and erosion rates within diverter systems, and
The development of a diverter-exit monitor for measuring gas rate and erosion rate.

REFERENCES

Roche, Joseph, "Better diverter system design stressed," OGJ, Mar. 7, 1989, pp. 37-40.
Roche, Joseph, "Diverter improvements help handle shallow gas kicks safely," OGJ, Feb. 10, 1986.
Roche, J.R., and Andersen, J.0., "Subsea diverting is new option for shallow gas, "OGJ, May 5, 1986, pp. 94-96.

BIBLIOGRAPHY

Vatn, Terje, "Diverter Systems on the Sea Bed, Possibilities and Limitations," Norwegian Petroleum Directorate Shallow Gas Seminar, Aug. 27-28, 1987.
Uncontrolled blowout on the West Vanguard Platform, West Vanguard Report NOU 1986:16.
"Shallow Gas Events, Gulf of Mexico OCS Region," OCS Report MMS 84-0029, U.S. Dept. of the Interior/Minerals Management Service.
Nokleberg, L., Schuller, R.B., and Sontvedt, T., "Shallow Gas Kicks-Safety Aspects Related to Divarier System," A.S. Veritas Research NIF-Course 7137-June 1986.
Blount, Elmo, M. "Shallow Gas Well Control: SPE/API meeting Jan. 13, 1988, Lake Charles, La.
Bourgoyne, Adam J. Jr., "The Development of Improved Blowout Prevention Systems for Offshore Drilling Operations, Part 1 - Shallow Gas Hazards," Final Report MMS Contract 14-12-0001-30274 June 12, 1989, Louisiana State University, Petroleum Engineering Department.
Engineering Times, March 1983.