Fishing in the subsea BOP: DP has its limits. A disconnect was required that resulted in a small fish and a unique way to solve the problem

World Oil, July, 2006 by Tim Burns, John Shaughnessy, Brett Cocales

BP recently had to overcome a unique subsea fishing problem in 6,838 ft of water in the Gulf of Mexico. While drilling with a dynamically positioned (DP) semi-submersible drilling rig, the operator used available technology to recover a fish (piece of drill pipe) that, in years' past, would have not been recoverable. While the operation was time consuming and costly, attempting to move forward without recovery could have resulted in the well being lost or significant subsea equipment damage.

BACKGROUND

A DP rig (Fig. 1) stays on location with computer controlled thrusters that keep it over the well. A subsea BOP is connected to the well with a hydraulically controlled clamp, Fig. 2. The BOP has six rams. Three are pipe rams that are designed to close around the drill pipe to prevent 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.

[FIGURES 1-2 OMITTED]

When a DP rig is overpowered by the weather (or loses power), it is necessary to disconnect the lower marine riser package (LMRP) that attaches the marine drilling riser to the BOP. If the rig does not disconnect during excessive weather, the riser, BOP and, subsequently, the wellbore, can be damaged or lost. If the rig is drilling when an emergency disconnect is necessary, the automatic procedure is:

1. Pick up drill pipe and hang off a tool joint on a closed BOP ram (to secure the wellbore).

2. Activate the BOP shear rams to cut the drill pipe.

3. Close valves to secure and protect the wellbore.

4. Release the LMRP (so as not to stress the subsea system).

5. Allow environmental forces to move the rig away from the wellbore.

The above sequence is designed to be done in about one minute. The subsea BOP and riser could otherwise be subjected to the weather load and damaged if the rig cannot resist the wind, waves and currents. The need for an emergency disconnect is rare. In most cases, the recovery 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 deal with loop currents, which are associated with the Gulf Stream between the Yucatan Peninsula, Florida and Cuba. Loop currents can result in water flow up to 4 knots. Eddies are high-current areas that can rotate for months after disconnecting from the main loop 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.

[FIGURE 3 OMITTED]

THE DP INCIDENT

On BP's operation, at a critical time, the wind was co-linear with the eddy current, with both coming from the south. The combination of wind and current resulted in greater than 3 knots of effective current 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 absolutely necessary, the pipe was pulled into the casing, hung off on the BOP rams and the hole secured. Next, the drilling mud was circulated out of the riser. The next step taken was to cut the drill pipe with the Lower blind/shear rams (second ram from the top in the subsea BOP (Fig. 2).

About an hour later, the rig could no longer resist the weather forces, and the decision was made to disconnect the drilling riser. One button is pushed to execute the 36 steps of the emergency disconnect procedure.

By taking the earlier steps to prepare for the disconnect, two potentially significant problems were avoided. Pulling the bit and drill pipe into the casing eliminated the potential of stuck pipe 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.

THE PROBLEM

Within 18 hours of disconnect, the rig was ready to resume productive work. The LMRP was re-attached to the BOP. An overshot (designed to latch onto the cut drill pipe) was run to where the pipe was cut. However, the pipe was encountered 4 1/2-ft deeper 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 determined that the Casing shear rams (third cavity, Fig. 2) functioned when the emergency disconnect sequence was made. Thus, a 4 1/2-ft piece of drill pipe was cut off the top of 10 ft of drill 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).