SHEAR RAMS HOLD 10,000 PSI AFTER CUTTING PIPE

Einar Framnes
Saga Petroleum a.s.
Forus, Norway

Joseph R. Roche
Hydril Co.
Houston

The purpose and effectiveness of a ram blowout preventer (BOP) in a worst case situation was illustrated in related incidents offshore Norway.

A ram BOP is of course designed as the last line of defense when a well starts to flow and control is lost. When all else fails, the blind/shear ram BOP must be relied upon to protect the rig, the environment, and especially the crew.

When the BOP is activated, powerful hydraulic cylinders drive the two rams together to close the well bore. Hardened and specially honed steel blades cleanly sever the pipe in the hole. As the rams contact each other, the fluid pressure in the well is contained below by elastomeric seals that fill the spaces between the two rams and between each ram and the BOP body.

It is a rare occasion when a critical component is subjected to a worst-case service environment in the field. A worst-case scenario would demand that a shear ram quickly and cleanly cut pipe in the hole and seal highpressure, high-temperature well bore fluid for an extended period.

The presence of coiled tubing inside the drill pipe and the necessity to open, close, and seal the well a second time would compound the demands on a shear ram.

Such an extraordinary performance was exhibited by an 18 3/4-in., 15,000-psi shear ram BOP on the Saga Petroleum a.s. North Sea Well 2/4-14 offshore Norway during most of 1989.

AVERTING A SURFACE BLOWOUT

In January 1989, the Wilrig a.s. semisubmersible Treasure Saga was drilling Well 2/4-14 in 68 m (223 ft) of water near the Ekofisk area in the Norwegian sector of the North Sea when circulation problems occurred.1 At 4,733 m from the rotary kelly bushing (RKB) a drilling break occurred, and large pit gains were observed. The Hydril GL 18 3/4-in., 5,000-psi upper annular BOP was closed on the drill pipe, and well control procedures were initiated.

After 10 days of attempting to get control of the well, the bottom hole assembly (BHA) became stuck in the hole at 4,700 m RKB. A cement plug was set and coiled tubing was run in to ascertain the stuck depth.

During this operation, the well began to flow inside the drill pipe and control of the well was lost.1

On Jan. 20, 1989, the Hydril 18 3/4-in., 15,000-psi shear ram BOP was activated to shear the 5-in. drill pipe with the 1 1/2-in. coiled tubing inside. The rams sealed the well bore to stop the hazardous, uncontrolled flow.

Pressure inside the well reached 703 bar (10,200 psi) at the wellhead. While preparations were being made to bullhead down the kill line, the 15,000-psi drape hose on the telescopic joint burst at the goose neck, allowing the well to blow out.

The fail-safe kill valves on the BOP stack were closed after about 1 min of well flow. The riser was disconnected at the lower marine riser package (LMRP), permitting the floating rig to abandon .the site while leaving the BOP stack, with its closed shear rams securely holding back the well fluids (Fig. 1).1 The subsea BOP stack remained in a vertical position.

To install a backup seal of the well bore and to ascertain if the shear ram was leaking, the Treasure Saga was moved back over the wellhead on Jan. 31, 1989. A mandrel with a flared bottom was lowered on a wire line from the semisubmersible rig to a level below the Hydril GX 18 3/4-in., 10,000-psi lower annular BOP in the previously abandoned BOP stack.

A minipod was specially designed and built to latch onto one of the female receptacles on the BOP stack. This minipod and a charged bank of accumulator bottles were lowered on wire line to the sea floor.

A remotely operated vehicle latched the pod to the BOP stack and connected a jumper hose from the manifold to the pod. This control system then closed the GX around the mandrel.

The mandrel had been marked with circumferential black and white stripes. Thus, if the shear ram BOP were leaking, the increasing pressure would push the mandrel up, and the movement could be observed by the divers or subsea television cameras. The Treasure Saga was then moved off location to be fitted with another BOP stack for drilling the relief well.

With the well successfully shut in, equipment was prepared to snub in through the BOP stack.

Seal integrity of the shear ram BOP was verified by lack of movement of the mandrel and by no change in pressure in the space between the shear ram and the GX BOP. The pressure did not increase between this time and the time when the BOP stack was re-entered in May 1989.

To permit snubbing in at the original wellhead, the Neddrill Trigon jack up rig was moved into position over the well (Fig. 2). A custom-built, 11-in., 10,000-psi BOP stack having one annular BOP and five snubbing ram BOPS, with an 18 3/4-in. hydraulic connector on the bottom, was lowered and latched to the mandrel at the top of the 18 3/4-in. BOP stack on the original location. A special, high-pressure riser designed to accommodate a snubbing unit was used.

Subsequent to re-entry, on May 1, 1989, it was learned that an underground blowout was in progress from about 4,700 m depth to about 900 m through the drill pipe, the broached 13 3/8-in. casing and 9 5/8-in. casing, and around the 20-in. casing shoe.1 The circulation within the well had caused the pressure at the wellhead to drop to 200 bar (3,000 psi).

The flowing fluid was a light oil or gas condensate with a high gas/oil ratio (GOR) and a reservoir pressure estimated at 980 bar (14,200 psi). The static bottom hole temperature was 167 C. (335 F.).

SECOND BLOWOUT STOPPED

The Trigon was equipped to fully control all components in both the 11-in. and the 18 3/4-in. BOP stacks. Using the 11 in. snubbing BOP stack to contain well bore pressure, the coiled tubing and drill pipe were fished out of the well over about 4 1/2 months. In October 1989, a milling operation removed scale inside the drillstring. This caused the drillstring to part, and once again the well blew out.

On Oct. 20, 1989, the 18 3/4-in. Hydril shear ram BOP was used to stop the blowout. The drill pipe was sheared by the same rams that sheared the pipe during the first blowout and held well pressure for more than three months. Safe conditions were restored for both the rig and crew.

Meanwhile, the Treasure Saga, outfitted with a rented BOP stack (having four Hydril 18 3/4-in., 15,000-psi rams and two Hydril GX 18 3/4-in., 10,000-psi annular BOPS), was drilling a relief well. On Dec. 12, 1989, the directional hole drilled from the Treasure Saga penetrated Well 2/4-14. Well control procedures were successfully implemented and the well was killed.

After retrieval from the sea floor, the BOP stack was disassembled and inspected. Dimensional checks, magnetic particle inspection, and hardness tests showed no significant damage or deterioration to the shear rams pictured in Fig. 3. The blades and rubber components were replaced and the rams returned to service.

Dealing with the blowout of Well 2/4-14 was an example of dependable performance of the drilling rigs and BOPs to facilitate killing a treacherous well. By twice shearing pipe in critical situations and holding back high-pressure reservoir fluids, the shear rams played a major role in averting disaster by allowing time to think and plan options to stop the blowout. Thus, no life was lost and there was minimal damage to equipment.

REFERENCE

Leraand, F., Wright, J.W., Zachery, M., and Thompson, B., "Relief well planning and drilling for a North Sea underground blowout," SPE 20420, presented at the Society of Petroleum Engineers 65th Annual Technical Conference and Exhibition, New Orleans, La., Sept. 23-26, 1990.