Stuck pipe has traditionally been a challenge for the oil and gas industry, but during recent years it has become clear operators are even more determined to reduce the impacts associated with stuck-pipe issues.

Even with the best planning and practice, certain wells have a significant probability that some of the string will not come back out of the hole. While losing a bottomhole assembly is never ideal, far worse can be the time wasted trying to get it free and redrilling afterward. Churchill Drilling Tools decided to take a fresh look at the problem in 2014.

The results of collaboration with drilling crews in the North Sea and the Gulf of Mexico (GoM) and extensive R&D work resulted in the HyPR HoleSaver, the first hydraulic pipe-recovery system.

In March 2015 the system was first deployed by a supermajor to optimize stuck-pipe contingency in zones highly vulnerable to differential sticking. Since then, the tool has been selected by two other major operators and is being considered for deployment in a wide range of upcoming wells, in particular in the GoM.

The tool consists of a full-strength sub positioned in the drillstring, which can be severed in a couple of hours. A jetting dart is launched and lands inside the sub, jetting the internal API pin connection.

Before the concept reached the drawing board, a number of questions were considered. Could significant time savings be delivered from the point at which the decision is made to sever the string until sidetracking or fishing commences? With its expertise in dart-activated tools, Churchill decided to see whether applying those principles in a new approach could significantly cut wait times. If a new system were to help, what would its traits be?

Need for new method
A major delay in severance is often due to the mobilization of specialist equipment or personnel. This is exacerbated in remote locations or when moving restricted equipment such as explosives or hazardous chemicals. Ideally, any new method should be self-operated or at least allow cutting and retrieval operations to begin while third-party services are mobilized.

As strings become more sophisticated, each additional element has the potential to upset others. The sub must have full structural integrity and be totally benign as far as other tools are concerned (Figure 1). This means being wider in internal diameter (ID) than any lower component and having fully compliant and tapered lead-ins to ensure all activating devices and flow have an unrestricted path through the tool.

FIGURE 1. Full-strength (both in tension and compression) rotary API connections are put into the string, and there are no special weak points. Tool dormancy also is completely benign, with large IDs and fully compliant and tapered lead-ins. (Source: Churchill Drilling Tools)

The weakest point in every joint is normally the pin, but if that area was targeted with enough energy, then perhaps it could be sufficiently weakened so that parting could be achieved with just a small loading. Calculations confirmed that removing half the pin ID would put most connections in this breakable range. The logical solution for Churchill was to use a dart to focus energy from the mud pumps precisely on to the weak point.

R&D leads to proven performance
Establishing the feasibility of making deep cuts into connections using only hydraulics was the first objective. Initial testing confirmed the first few millimeters of steel could be claimed within 10 minutes. Tests were, however, short-lived as these highlighted the major design challenge for the system.

Depending on gap size, the energy dissipated and produced by the jets at 1,325 l/min (350 gal/min) is approximately 250 hydraulic horsepower. The dart assembly has to sustain very high loads as it deflects the flow at right angles and accelerates it up to 91.5 m/sec (300 ft/sec) through the nozzle. The jet and resulting eddies generate harmonic effects, amplifying the stresses on vulnerable areas in the assembly. Initial designs were not able to survive more than 10 minutes of flow.

In a secondary version, the seat was relocated from the tail to the nose of the dart. While this would add complexity in terms of the need for bypass channels around the seat area, it would greatly increase the stability by effectively removing an unsupported cantilever oscillating in the wake of the jet.

Priority was given to maximizing the safety factors in the fixings to remove the possibility of structural failure. By having seating below the jet, the dart can be left behind after cutting as it is no longer held in the top half of the sub (Figure 2). This enables the recovered
pipe to be clear after cutting for cementing a sidetrack plug immediately.

FIGURE 2. Above: The image shows a top sub with the pin cut and mud draining from the dart jet. The dart is unaffected despite the
4½-in. IF pin being completely severed. The process took just 112 minutes. Bottom: Clearly visible are the remains of the pin left
inside the bottom sub, which retains a completely undamaged and fishable box. (Source: Churchill Drilling Tools)

As the darts became stronger and longer tests became possible, patterns of washing began to emerge both on the target pin area and on the dart itself. There was a dramatic wearing just below the pin in the target area. However, a smaller secondary eddy also appeared further down the wake as well as a short eddy zone just in advance of the jet. While immaterial to the cut, this wear geometry had implications for the integrity of the dart.

High-velocity zones
It was clear the high-velocity zones on the steel dart were not being totally defended by their tungsten carbide coatings. The deflection, jet and eddy areas on the dart were all showing signs of washing in the coatings. The materials in these zones were therefore replaced with solid ceramic assemblies.

Once the system was working, attention switched to establishing the performance envelope. Variables such as mud type, particle size, jet size, flow rate and connection size all impact the load on the dart and the speed of the cut. The test program was performed on a full land-rig setup with twin 5,000-psi 12-in.-stroke triplex pumps and was designed to model a full range of scenarios from slimhole to deepwater.

The performance envelope is first bounded by the circulating system power and pressure capability; second by the particle size, which limits the tightness of the jet and therefore jetting velocity; and third by the power limits of the dart in terms of physical integrity (Figure 3). The data showed it was not necessary to push close to the limits of the envelope because the cutting is very fast, even at lower flow rates.

FIGURE 3. A high-velocity jet is produced from the dart by deflecting the flow through a tight ceramic nozzle at the pin’s weak spot. The pin weakens rapidly, and what was a full-strength API connection is now easily parted with a small loading from above. (Source: Churchill Drilling Tools)

The tool’s dart is designed for free fall and/or pumping into place with a pressure increase confirming landing. As expected, the tests showed the user would see falling pressure within a few minutes of landing as the ID of the pin started to wash. Once the initial cut is made, pressure falls become less indicative as to the progression of the cut, and there is not a specific pressure indication for the pin approaching its yield point (Figure 4). Applying a small amount of “pull” during cutting will part the pipe when the time comes, and the positive result will be self-evident.

FIGURE 4. This is a fluid velocity profile as the pin is jetted away. Velocities above 30.5 m/sec (100 ft/sec) cause wear, and the red zones illustrate rapid wear at more than 91.5 m/sec (300 ft/sec). A secondary eddy cutting also can be seen below the first cut. There was a strong correlation between finite element modeling and the live rig test results. (Source: Churchill Drilling Tools)

From weakness to strength
It took less than 12 months for progression from the identification of a major problem for the drilling industry to a potential solution and ultimately the creation of the tool. The HyPR HoleSaver is now being deployed to provide stuck-pipe insurance. While no operator wants to get stuck, it’s reassuring to know a rapid solution is at hand should it be needed.