With hurricanes and loop currents in the Gulf of Mexico (GoM), semisubmersible rigs often must disconnect the riser from the blowout preventer (BOP) in a relatively short period of time. It is advantageous to drilling operations to ensure the riser is clean before it is disconnected. This minimizes drilling mud residue hazards on the deck and limits potential clean-up time before the riser can be reconnected.

Historically, there has been no simple solution to cost-effectively displace invert emulsion drilling fluids to seawater and quickly disconnect the riser. The new Riser Displacement Cleaning Tool (RDCT) now answers that need. The RDCT is designed to eliminate the risks associated with disconnecting and laying down portions of the riser or leaving the riser open-ended while waiting out the weather. It also allows for easy displacement without risking the contamination of costly drilling fluids or expensive completion fluids.

The RDCT is designed to mitigate the hazards associated with riser fluid displacement. (Images courtesy of M-I Swaco)

Design, application

The RDCT is designed to mitigate the hazards associated with riser fluid displacement and riser disconnect by acting as a mechanical barrier between fluids. It also acts as a cleanup tool, reducing the potential environmental impact of disconnecting the riser and allowing for simplified and more confident emergency disconnects for storms.

In a typical deepwater operation, high volumes of fluid interface of mud and seawater are generated each time the riser fluid is changed or the riser is disconnected. Because of the zero discharge requirements, this can lead to significant environmental cost to the operator if the entire volume must be treated or disposed.

When an operator displaces one fluid with another, contamination can occur, resulting in significant volumes of interface that must either be processed to recover the original fluid properties or disposed. As much as 6% seawater is intermingled with the fluid during conventional riser displacement.

The contaminated fluid volume at the interface between the fluids can amount to several hundred barrels. The potential volume increases significantly with water depth and riser length. An average semisubmersible rig has to disconnect from the BOP an average of eight times per year. The loss or contamination of fluid during these operations can cost tens to hundreds of thousands of dollars per disconnect. Numerous activities, such as displacing heavy brines in a storm or carrying out an indirect displacement of heavy mud, can quickly increase these costs. The RDCT reduces the total volume of contaminated fluid produced by acting as a physical barrier between fluids.

The RDCT is attached to a running tool in the drillstring and lowered down the riser to the BOP. The upper annular preventer closes to grip the RDCT, and the running tool is pulled free. The protective shroud around the RDCT is removed, and the wipers press against the interior wall of the riser.

The displacement fluid is pumped up the riser via external choke/kill lines, and the tool is transported to surface as a moving barrier between the drilling or completion fluid above and the new displacement fluid, typically seawater or brine, below.

As the RDCT rises, the displaced fluid is recovered, and the new fluid fills the riser. While it rises, the operation of the RDCT is similar to that of a pipeline pig, wiping the riser interior wall clean. This eliminates any potential for environmentally unfriendly fluids to be handled outside of the mud system on the rig.

The RDCT can be racked in the derrick along with other storm management equipment so it is available in the event of an approaching hurricane or other severe weather that requires the rig to disconnect the riser. When the decision to disconnect is made, the RDCT tool can be run quickly to displace the drilling or completion fluid with a brine or seawater. Total running time is short, around 40 minutes per 300 ft (92 m), allowing the operator to stay connected and operating longer.

RDCT in the field

A test was run on a well offshore Norway on a semisubmersible rig. The objective was to vet the procedures for running the tool as it was released in the subsea BOP, pumped to surface, and retrieved at the drill floor.

The RDCT was run to a water depth of approximately 330 ft (100 m) and set subsea by holding pressure on the upper annular preventer. Once the running tool was release

The RDCT can be racked in the derrick along with other storm management equipment so it is available in the event of an approaching hurricane or other severe weather that requires the rig to disconnect the riser.

d, pressure on the upper annular was released, freeing the tool and allowing it to move upward while fluid was pumped down the boost line. The RDCT was pumped up the riser at a rate of 792 gal/min (3,000 L/min) and arrived at surface in the diverter in 14 minutes.

Running time was not affected by using the RDCT on the test well, and tripping out with the running string took place as normal. The time savings achieved by eliminating the need to over-displace the riser to get a satisfactory fluid cleanliness is potentially greater than the time spent actually setting and running the RDCT.

Normal operation of the RDCT does not require additional rig time because the time needed to make up the tool is recovered by eliminating the need to over-displace the riser with clean fluid. It can be run in several configurations, for example, in conjunction with a storm packer or similar isolation tool or on its own.

The RDCT offers operators a cost effective technique to displace fluids when operational or environmental requirements necessitate that the riser be disconnected from the BOP without risking additional clean-up operations. Loss of valuable fluids to cross-contamination is eliminated, potentially saving hundreds of thousands of dollars.

The RDCT is attached to a running tool in the drillstring and lowered down the riser to the BOP.

The tool is expected to be especially useful in deepwater operations when using expensive or environmentally unfriendly fluids or when drilling in environmentally sensitive areas.

Several prototype tools are being constructed for conventional risers. Future testing with drilling fluids will be completed in the GoM by summer 2010.