Mud in Your Eye

By Centek Ltd

A common problem for rig drilling teams is spillages of drilling fluid onto the drilling rig floor when sections of drilling pipe are removed from the drill string.

When sections of drilling pipe are removed or "tripped out" from the drill string during drilling operations, the column of drilling fluid contained in the pipe section spills onto the drilling rig floor. The spilled fluids are not only a Health and Safety hazard for the workers on the rig site, they can be an environmental hazard depending on the composition of the fluids themselves. They also represent expensive downtime.

Casing installed in subsea completions is positioned within the well with a landing string, typically a drill string, which has a smaller internal diameter than that of the casing. Unless the pipe is lowered very slowly, a reverse flow of drilling fluids is induced through the smaller diameter drill string being used to install the casing.

"Everyone knows that filling a drill pipe can be a nightmare, it's time consuming, messy and if you damage the thread on the top drive you are in deep trouble," said Peter Aird, a drilling superintendent in the North Sea.

Wells are drilled in stands of three joints with an overall length of approximately 95 feet. Every 10 stands (or around 1,000 feet of drilling) the drill pipe must be filled with fluid to equalize the pressure inside and outside the pipe to stop it collapsing and to balance the well. Drilling rigs equipped with top drives make up the threaded end of the top drive into each stand of drill pipe as the pipe is being run into the well.  It is necessary to repeatedly make up and disengage the top drive threads with a succession of stands. Top drive filling is the most frequently used option and here savings can be made because the top drive is a closed system, however when the top drive is disconnected drilling fluid bubbles up out of the drill pipe, and because there is air in it, it spurts on to the underside of the bell housing creating a mess on the rig floor that has the drillers slipping and sliding so it has to be cleaned up before work can start again. In volume terms you are talking as much as 600 gallons a minute.

Centek Limited has introduced the Itsfu Internal Drill String Fill Sub designed to provide controlled drill string filling and prevent spillage of well drilling fluids onto the rig floor with the consequent downtime.

The Itsfu tool screws quickly and directly onto the drill string, it is still a closed system for filling but with a telltale that ensures there is no over displacement or pressurisation of the string fluid and no overspill. Importantly, the Itsfu also allows the driller to pump at a higher controlled speed.

The Itsfu can rapidly pay for itself. With rig-rates ranging from $200-$500/minute, and filling pipe every 1,000ft in a 20,000ft trip, then a saving of as little as 1 1/2 minutes per fill will save 30 minutes of overall rig time, reducing operating costs by $6,000-$15,000 per trip; at the same time producing a safer, speedier and more efficient operation. In practice if there is a spillage it can take 5-6 minutes to clean up and this is downtime with no pipe is being run through the rotary table. Over a year the Itsfu should save 3-4 days of rig time, or at a rig day rate of $250,000, around $750,000-$1,000,000. Centek claims that these time savings are the minimum rig teams can expect, and that in most situations the Itsfu will deliver substantial savings when compared to other systems in use.

Installation and use of the Itsfu is straightforward. A 2-inch supply hose is connected to a lo-torque valve which is then attached to the Itsfu's 1502 Weco connection. The supply hose is connected to the standpipe or fill line. An air winch line is fastened to the lift-ring so that the Itsfu can be lifted from the storage area and positioned over the drill pipe box. The Itsfu is stabbed into the DP box, with a swivel joint allowing the lower part of the Itsfu to be rotated by hand so the threaded end of the Itsfu engages with the DP box threads. Hammer-lugs are built onto the Itsfu for rapid tightening or slackening of the connection.

When the driller starts the charge pumps, and as fill-up begins the strokes are counted. At around 5% from full, the pumps are stopped. The Itsfu has a built in telltale to indicate when the string is full which also allows air in the system to escape without spurting. Once the pipe is full, a hose is attached to the telltale and excess fluid is drained off in a controlled manner. When the process is complete the Itsfu is simply racked back and is then ready for the next operation. 

"With the Itsfu we believe we have a simple, reliable and cost-saving solution to filling the string and the common, time wasting and potentially dangerous problem of drill pipe spillage," said Cliff Berry, Sales and Marketing Manager at Centek Limited. "It gets its name from the 'it's fu' cry of the North Sea drillers as the drill pipe overflows."

The Itsfu is a simple, low-maintenance answer to the problems of filling drill strings, including spillage, clean up time, safety and volumetric loss control. In addition, the top drive saver sub threads are protected and need only be exposed for drilling. The top drive is also freed for picking up and making ready the next stand.

A cement application adaptor is available, which incorporates a foam ball or dart in a specially modified housing that allows for positive displacement when carrying out an inner string cement job.                          

The Itsfu is pressure tested to 5,000 psi and offers a flow rate of 600-1,000 gpm, with an internal bore of 3.75 in and a 4.5 in. thread connection. It has a height of 40ins and weighs in at 250lbs.

The Itsfu is available now for purchase or rental, with pricing and detailed application information available from Centek. www.centekltd.co.uk