Emergency Severance of Casing for Deepwater Operations: A Design Build and Test Programme of Novel Technologies N. Richmond, N.J.S. Bamford A.S. Bamford, SPE, Geoprober Drilling

Copyright 2011, SPE/IADC Drilling Conference and Exhibition This paper was prepared for presentation at the SPE/IADC Drilling Conference and Exhibition held in Amsterdam, The Netherlands, 13 March 2011. This paper was selected for presentation by an SPE/IADC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers or the International Association of Drilling Contractors, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE/IADC copyright.

SPE/IADC 139493

Abstract

In deep-water, emergency disconnection while running casing, is the action of last resort. It requires the casing to be sheared

and the well bore sealed. In an 18-3/4" BOP cutting casing sizes of 9-5/8" and above can be difficult and may impinge on the

bore of the BOP.

An Annular Cutting Tool (ACT) has been developed that can be implemented inside an API housing, spool piece or an

extension to a BOP ram body. The ACT has been designed to work with conventional drill pipe blind rams. The cutting

mechanism is swarf free and produces a good quality cut for later re-entry to the well.

After engineering design, a prototype tool to cut 7-5/8" casing was developed and tested. The system in its current state of

design, uses a rotating plenum driven by a sealed bevel gear cartridge, with six elliptical pistons each incorporating a cutting

wheel. The paper will describe the selection and testing of bearing and seals and how the drive and control system was

optimised to cut the casing in less than 45 seconds required by API RP 53. The effects will be illustrated of varying system

rotational speed and applied pressure to the cutting wheels.

Plans for future testing are described including; the effects of varying pipe grade, wall thickness, and casing load conditions

consisting of different combinations of tension and bending.

The ACT design has to ensure that there are two physical blocks between the wellbore pressure and the external seawater

environment; each of these blocks is independently tested.

Introduction

When drilling from a dynamically positioned vessel in deepwater, emergency disconnection

of the well requires shutting in the well and disconnecting the riser. API RP 53 requires that

this is carried out within 45 seconds.

With drill pipe or casing in the wellbore, this requires blind shear rams or casing rams to be

positioned in the subsea BOP stack. Figure 1 shows a typical deepwater subsea BOP stack

arrangement showing the position of these rams.

For large diameter casing, where there is a small clearance between the bore diameter of the

BOP stack and the casing this is a particular problem as ram type shear blades cannot operate

very effectively. For example shearing a 16 casing inside an 18-3/4 subsea BOP leaves

virtually no space for the shear blade to operate since the elongated sheared pipe cannot fit

inside the bore of the BOP. Generally, a tubular will flatten during shearing to a width equal

to about 1.51 times its diameter (experimentally derived) (West Engineering Services 2004).

If this is wider than the blade, shearing is jeopardized.

The Annular Cutting Tool can be implemented inside a subsea BOP to sever the casing with

no distortion. It can be placed in a housing that is an extension of the shear ram BOP and

replace casing shear rams; this reduces the BOP stack up height. Before severing the casing in

Figure 1: Typical deepwater subsea BOP stack showing possible location of ACT

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an emergency, it should be spaced out so that the casing collar or connector is landed out on the pipe rams and adequate

tension applied to the string at the surface to ensure that the cut is always made in tension.

The current technology development programme is part of a project sponsored by Statoil to develop a Drill-in Wellhead

System (the GEO-DWS). This is an exploration only wellhead system that allows a well to be established in a single trip. The

surface casing is drilled in and cut at the seafloor using the ACT and then sealed to the wellhead. This system will prove the

technology before implementation in a deepwater casing cutting tool.

System Overview

The ACT is a rotary pipe cutting tool which can be implemented inside a high-pressure housing. It consists of eight

subassemblies; the plenum, the cartridge and six identical elliptical pistons with cutting wheels. The prototype tool is

illustrated in figure 1. The housing for the tool could be configured as part of a subsea BOP stack up (see Figure 1).

Plenum

The plenum consists of a large thick walled ring. It contains six precision machined elliptical slots to fit the elliptical pistons.

On the bottom face of the plenum a bevel crown gear is held on by a series of pins and threaded studs. This gear meshes with

the bevel pinion of the cartridge assembly and transmits torque from the motor to the plenum.

Elliptical Pistons

Located within the wall of the plenum are the six elliptical pistons. Each piston includes a cut out, within which sits a cutting

wheel. Each wheel (or blade) is held in place via a pin and is therefore free to rotate around its own central axis. The pistons

travel along the radius of the plenum. Therefore extruding the pistons indents the cutting wheels radially into the casing (see

figure 3).

Cartridge The cartridge assembly consists of an integrated pinion gear and shaft and a housing cartridge. The shaft is coupled to a

hydraulic motor and the whole assembly is located within the wall of the housing. The pinion meshes with the crown gear of

the plenum assembly. This transmits torque to the plenum, causing the wheels to orbit the casing.

Figure 2: 3D images of; the prototype tool assembly within sample test housing (left) and plenum assembly, including elliptical pistons (right)

Figure 3: Elliptical piston assembly including cutting wheel (left), view through bore of tool post-disconnection (middle) and cartridge assembly with attached motor (right)

SPE/IADC 139493 3

Cutting Mechanism

The tool operates by indenting a series of hardened steel cutting wheels radially into the outer surface of the casing. This

deformation can be likened to the brinell hardness test, in which a ball bearing is pressed into a piece of steel under a

controlled load; the hardness of the test piece is then found by evaluating the contact area between the ball and the indented

profile at the point which prohibits further plastic deformation. In the case of the ACT, the contact area between each cutting

wheel and the casing is kept below this critical level. Therefore the wheels are able to indent the casing to a depth greater than

the wall thickness of the test piece, thereby completing a successful severance.

A purely linear penetration of the cutting blades radially into the casing would result in an increasing contact area based on

their respective geometries and the depth of penetration. However, during operation, the wheels are also forced to orbit the

casing via a bevel gear arrangement and hydraulic motor. Combining these two directions of travel of the blades causes them

to follow a spiral trajectory through the casing wall. This rotation ensures the indented profile is steadily progressed around

the entire circumference of the casing, thereby optimizing the contact area to intensify the plastic deformation.

This method of cutting implies that work hardening of the material occurs at the cut surface; therefore the rate of penetration

may decrease towards the end of the cut.

The principal feature of the cutting tool is that no cuttings are created during its operation. This is a result of the indentation

method of cutting. Emitting no swarf during severance procedures ensures that the wellbore environment is not contaminated

post-disconnection.

The second advantage of the cutting tool over other disconnection methods is the resulting cut profile. This has been observed

to have very little deformation to the inner and outer diameter of the casing. This allows for simple re-entry to the well, and

eliminates the need for any time consuming and costly milling procedures that are commonplace when removing the crimped

stub created when disconnection is carried out using shear rams.

Figure 4a: Close up view of upper cut surface Figure 4b: Bore of Tool post-disconnection

Figure 4c: Top and bottom surfaces placed side by side

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Wellbore Sealing/Well Integrity

The tool requires torque to be transmitted to the plenum through the high pressure housing. A solution that was explored

which could potentially have allowed us to avoid a penetration is a magnetic coupling. This system does have a service history

within the industry, however preliminary design work showed that currently available magnetic couplings are not a feasible

solution. In order to meet space requirements significant material would have to be removed from the outside of the housing

causing areas of stress intensity which would reduce structural strength and fatigue performance.

One of the requirements of the ACT is that it must seal wellbore pressure. However this is only after a cut has been

completed; therefore there is no situation in which it must seal full wellbore pressure and cut simultaneously. This statement

has led us to a sealing concept in which the seals must be capable of holding hydraulic system pressure whilst rotating and

subsequently hold wellbore pressure statically. From this the cartridge concept was developed:

This system allows torque to be transferred along the shaft of the pinion. The pressure envelope integrity is maintained using

rotary seals between the shaft and the cartridge, and a static seal between the cartridge and the wellhead. The cartridge

assembly is secured to the wellhead with a bolted flange. The pinion shaft is kept concentric by back-to-back tapered roller

bearings, preloaded with a nut which is prevented from backing off using an anti-rotation washer.

In addition to the rotation a hydraulic force must be applied to the back of the pistons in order to drive the blades into the

casing to perform the cut. This requires two further sealing systems: rotary seals on the plenum and sliding seals on the

pistons; both must hold hydraulic system pressure from the inside whilst cutting and wellbore working pressure from the other

direction whilst static.

In order to be considered a true wellbore barrier by Norsok D-010 and industry standards, a second block element in the form

of an isolation valve is added to the two motor hydraulic lines and the single plenum pressure line. These are attached to the

outside of the housing using a standard flange and gasket arrangement. This isolation valve can be a commercially available

fail closed gate or ball valve; for the wellhead version of the annular cutting tool development work is currently being

undertaken to source an isolation valve that minimizes the footprint of the tool.

APPENDIX-A shows the well integrity schematic for the wellhead version of the Annular Cutting Tool, it can be clearly seen

that two wellbore sealing requirements are through the plenum pressure gallery and the motor. Well integrity is implemented

in the first instance by the non-metal seals which are backed up by wellbore isolation valves.

Figure 5: Section view of cartridge assembly with mounted motor

SPE/IADC 139493 5

Design and Development

The annular cutting tool has been developed over a number of years. A logical progression has been followed from proof of

concept to the cutting tests. The various issues encountered throughout the development have been addressed.

Proof of Concept

A requirement to be able to sever casing without deforming the casing dimensions was identified during the concept design of

the Geoprober Deepwater Exploration Drilling System. Extensive research into non-mechanical methods of cutting pipe was

carried out. These methods involved using a chemical based cutting tool or even an arrangement of thermal torches. Although

testing of these methods was carried out, it soon became evident that due to the nature of the propellants used in both methods,

a mechanical solution was more appropriate.

The initial design concept was heavily based on a conventional plumbers pipe cutting tool with cutting wheels rotating around

the pipe under load. This concept requires power to drive the cutting wheels round and into the casing material.

To prove the concept that drill casing could be cut using a series of cutting wheels a manual test was carried out. This test

involved hiring a hinged pipe cutter and using it to cut a piece of 7-5/8 (33.7 lbs/ft) L80 API casing manually. This test was

successful, the final cut profile was clean and no cuttings were emitted during the process. However it was found that the

torque required to drive the tool was excessive.

The torque required to cut was crudely quantified at 590Nm for a four blade tool. Although the proof of concept test was

successful, cutting the pipe took over an hour. Attempts to speed this up failed as the increased loads damaged the cutting

wheels rendering them useless.

It was then decided to follow up this proof of concept test, with the design and build of an automated prototype tool. The

initial concept involved using a hydraulic motor via a straight bevel gear to rotate the cutting wheels around the pipe and

hydraulically energized pistons, to drive the cutting wheels against the pipe.

Blade Selection

In attempts to speed up the manual pipe cutting tests the original cutting wheels were found to be unsuitable and could not

withstand a load above 6.867kN. Although a cut was successful using these blades, it was necessary to reduce the total time to

cut pipe. To do this it would be required to increase the load on blades, therefore new blades had to be sourced.

A manufacturer of standard cutting wheels was contacted and it was learned that a stronger blade designed for cutting heavy

wall steel, was readily available. Some basic tests (same as proof of concept) using these blades showed that they were

suitable to cut the casing. Attempts to further speed up the cut were carried out and the blades were found to be capable of

withstanding much larger loads than the original blades provided.

Therefore it was decided to carry on the design using the stronger blades.

Future development of the tool will involve up scaling to cut larger diameter casing. To do this wheels with larger diameters

will be required. The manufacturer has been consulted on this and currently there are no off the shelf products available.

Therefore potential development could be carried out in designing custom cutting wheels.

Piston Design

To house the cutting wheels a simple bucket style piston was designed (see Figure 3). The wheels were held in place by a

retaining pin, allowing them to rotate around their central axis. This pin and the hollow which encased half of the wheel were

designed based on the dimensions of the off-the-shelf blades.

Figure 6: Hinged pipe cutting tool

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The working area of these pistons was sized based on a rough measurement of normal load applied to the blades. This was

measured by constructing a bracket for the hinged pipe cutter to hold a compressive load cell. The pipe cutter handle was then

turned until the torque required to cut (590Nm) could be applied by hand. The load cell then showed that the pipe cutter was

applying a normal load of 1400kg to the blades.

In line with sealing capabilities it was deemed appropriate to maintain a relatively low pressure to provide the normal load.

This was because, while this load was being applied, the tool and therefore the seals would be rotating; hence a lower pressure

would simplify the sealing process. The piston working area and hydraulic pressure were then optimized to give the required

load. The final piston working area was chosen at 2940mm2.

These pistons were housed within a thick walled ring (see figure 2); the thickness of the housing was dependent on the

required through bore of the tool and the necessary travel of the pistons to complete a cut.

It was important to maintain the correct alignment of the cutting blades within the cut groove. Therefore a circular shaped

piston was unsuitable, as it may twist. In line with basic sealing requirements no sharp edges could be included on the piston,

as this would damage the seal. These constraints led to an elliptical shaped design (see figure 3) to ensure the piston could not

rotate and to provide the minimum bend radius required when installing seals.

Machining these pistons was a difficult procedure due to the complicated tolerances involved with the sealing requirements of

the piston. Therefore all pistons and slots within the housing were machined at a specialist machine shop. This meant they

could be matched to fit before being sent back. For the first cutting tests three pistons were machined which meant three blades

were used to cut. After the cutting tests it was decided to increase this to six pistons this would enable further testing to be

carried out on the effect on cutting time by increasing the number of blades.

Seal Selection

Since it was decided that the tool would be hydraulically powered it was then necessary to design a sealing solution. Due to the

disconnection procedure philosophy, it was clear that the seals would have to statically withstand wellbore pressure once a cut

was complete. The seals must also be able to withstand a lower pressure during the tools operation (i.e. while rotating or

sliding respectively). Due to the proposed deployment of the tool all seals had to be rated for sour service.

Six piston seals were required for the elliptical pistons. For these circular piston seals were sourced with the same

circumference as the elliptical shape, it was then possible to stretch the seals over the piston. It was also recognised that two

rotary seals would be required to seal the annulus behind the elliptical pistons. These were sourced from the same company as

the piston seals.

Once the test setup was machined and the seals installed, a pressure test was carried out to prove that the seals could hold 1.5

times wellbore pressure. All seals passed this test. A second pressure test was carried out to prove the rotary seals could

withstand operating pressure while rotating. This test was also passed.

In the early stages of the tools design the main sealing concerns were as detailed above. However as the design progressed, a

sealed cartridge was required to allow torque to be transmitted to the plenum while maintaining a pressure envelope. This

required a small diameter rotary seal between a rotating shaft and the bore of the static cartridge and a slightly larger static

shaft seal between the cartridge and the test housing. These seals were again tested to 1.5 times wellbore pressure this was

repeated ten times. Between hold periods the shaft was spun for one minute at 150RPM. No significant drop in pressure was

observed across any of the hold periods of the tool therefore the seals selected were labeled fit for purpose.

Bearing Selection

Since the tool contains a steel plenum rotating within a steel housing, a reliable bearing solution was required to prevent

galling. The first method used was a set of PTFE bearing strips. These were located within grooves above and below the

seals.

Figure 7: Hinged pipe cutting tool with attached load cell

SPE/IADC 139493 7

During installation it was found that these grooves were unsuitable and that locating the

bearing strips while installing the plenum within the test housing was difficult. This

bearing solution was used for the cutting tests, the strips were glued in place to stop

them falling out. The cutting tests were successful and the bearing strips successfully

prevented galling. However it was recognised that further work was required to

simplify the bearing installation.

The second bearing solution explored used a solid brass bearing ring press fit onto a

shoulder on the plenum. Installation of this bearing was relatively simple. However

once the plenum was installed within the test housing it was found that the plenum

could not rotate due to the friction between the bearing and the test housing. Rather than

persevere with the solid bearing ring it was decided to upgrade the bearing to roller

bearings.

To reduce friction and ensure correct alignment of the plenum within the test housing a set of deep groove ball bearings were

selected. These were again press fit onto the plenum but used a sliding fit on the test housing. Installation of these bearings

was more complicated than the solid bearing ring as they had to be installed in the correct orientation. However once the

plenum was installed within the test housing, it was found that it could be rotated by hand. Therefore this solution was

selected as the most appropriate for the tool.

Drive System

To drive the plenum a motor and gear design was required. It was estimated, using similar methods as the proof of concept,

that a torque of 1050Nm was required to drive the plenum. This included cutting torque and a conservative estimate of all

resistive friction involved in turning the plenum.

To achieve this torque a straight bevel gear arrangement was decided upon to

allow rotation to be transferred through 90. This allowed the motor to be

mounted to the outside of the test housing. The arrangement would consist of a

crown-mounted gear on the plenum and an integral pinion and shaft coupled to

the rod end of a hydraulic motor. The gears used for the cutting tests had a ratio

of 4:1 between the pinion and crown. The safety margins on these gears were

relaxed due to space requirements within the tool. The gears were manufactured

and used during the cutting tests, no problems were encountered that could be

accredited to the gears.

During further development of the tool involving its placement within a

wellhead, the decision was taken to revisit the design of the gears particularly the

crown gears mounting onto the plenum. By changing the gear to a thicker

section and including a mounting procedure using threaded studs and spring pins

in the back face of the crown. It was possible to increase the face width of the

gears; this allowed the safety margin to be restored in their design. The new gears

have a ratio of 40:7.

This wellhead arrangement also included overhanging the crown gear onto the pinion to allow the tools protrusion through the

wellhead to be closer to the seabed. Therefore to set the mounting distance from the back of the crown to the centre of the

pinion, a series of tight tolerances were required as the plenum could not be shimmed back. The mounting distance from the

back of the pinion to the centre of the crown could be set by shimming.

Selecting a gear ratio meant that a torque rating could be chosen for a drive motor. It was decided to allow a range of torques

to be tested to see what could be used to perform the fastest cut. After consultation with a manufacturer a low speed/high

torque orbital motor was acquired. This included a 25mm straight keyed shaft, 1/2 BSP

hydraulic ports and a two bolt mounting arrangement. This then allowed a motor

mounting bracket and a shaft coupling mechanism to be designed.

Shock Absorber

During system shakedown of the tool it was noticed that once the blades had broken

through the inner diameter of the casing the pressure sensors were reading aggressive

fluctuations in plenum pressure. It was observed that as the breakthrough point

propagated circumferentially around the casing, any blades passing the broken through

section would be forced into their fully extended position. This caused a sudden

expansion of hydraulic volume (pressure drop). Once the blade had passed the broken

through section it was forced by the casing groove to retract causing a sudden contraction

of hydraulic volume causing a pressure spike in the region of 10bar.

Figure 8: Flat hacksaw blade bridging between the bearing strips

Figure 9: Crown gear design as used in cutting tests

Figure 10: Image of blades protruding through casing

8 SPE/IADC 139493

These fluctuations meant that the operator could not accurately monitor the plenum pressure, and the system was prone to

stalling. To counter this issue a 320ml diaphragm shock absorber pre-charged to 15bar was introduced to the system. The

flexibility of the diaphragm absorbed the pressure spikes and damped the fluctuations to amplitudes of below 5 bar (see Figure

10) making the control system easier to work with and prevented the system from stalling.

Once the cutting tests had been completed further design work was carried out on modifying the tool for subsea use. This

involved ensuring all components could function at operational water depth. As a result of this it was recognised that the

current shock absorber was not suitable for offshore deployment, as it could not be pressure compensated.

A custom shock absorber suitable for subsea use was designed in-house. This used the same principle of the fluctuating

volume being absorbed by the flexibility of the shock absorber. This flexibility was provided via a spring loaded piston open

to the plenum line-in. When the plenum pressure spiked, a load was exerted on this spring via the piston, this load caused the

spring to retract, allowing the volume of the piston to expand. Thereby absorbing the pressure spikes created during the tools

operation. This design allowed the back of the absorber to be open to the environment and was therefore naturally pressure

compensated.

Cutting Tests

Programme

A test procedure was devised to deduce the relationship between various system parameters and the total time to cut pipe.

These included the plenum pressure and system rotational speed. Other parameters that were considered were the number of

blades used to cut and the deterioration of the blades after they complete a series of cuts.

It was considered a prerequisite for the other tests to find how the number of cuts previously performed by a blade affected the

cut performance. A blade register could then be kept to allow a correction factor to be included with further test results based

on the number of cuts previously performed by the blades.

It was decided to test the relationship between the plenum pressure and the cut time. The system speed would be set at a

relatively low value (15RPM) and the pressure would be increased up to the maximum decided test value (55bar). From the

pressure value the load on blades normal to the casing can be found using the working area of a single piston.

It was also decided to complete a series of cuts with varying rotational speed whilst maintaining a constant plenum pressure.

Due to limitations with the hydraulic power unit it was necessary to maintain a lower plenum pressure (30bar) to allow the

velocity to reach its maximum test value (35 RPM). It should be noted that the system speed is representative of the load on

blades tangential to the casing. This load could be evaluated by measuring the pressure drop over the hydraulic motor and the

motors rotational speed; these could be used to find the motor torque, which could then be combined with the gear ratio to give

the plenum torque. The plenum torque could then be combined with the elliptical pistons stroke length at any point during

operation to give the total load on all blades. This could then be divided by the number of blades to give the load on a single

blade tangential to its orbit.

Figure 11: Damped plenum pressure readings for a six blade cut

SPE/IADC 139493 9

One idea for the tools further development was to show the effect of increasing the number of blades used to cut from three to

six. A simple method of measuring this relationship was to carry out tests using one to three blades, while keeping all other

parameters constant. The relationship could then be plotted graphically and the effect of six blades could be extrapolated from

the results.

One issue with carrying out the number of blades test was that the tool requires a symmetrical orientation of blades. This is

necessary to ensure the radial loads exerted outwards on the plenum by the contact with the casing are balanced. Therefore it

was not possible to simply remove cutting wheels from the pistons; instead a pair of rollers were designed and manufactured to

maintain the load balance when cutting with less than three blades.

These rollers were designed to fit into the same hollow within the elliptical pistons as the cutting wheels. They also featured a

groove which ensured no contact would be made with the cut edge of the pipe. The contact area between the roller and the

casing was evaluated using an approximation of the hertzian contact stress between two cylinders (Shigley 2008). The load on

blades was then combined with this area to ensure no plastic deformation occurred to the casing as a result of the presence of

the rollers.

Once the structured test programme was complete it was decided to attempt to achieve a cut in less than 45 seconds. This

would involve increasing both the system speed and plenum pressure to the maximum that could be provided from the

hydraulic power unit.

To set the system parameters to their maximum, the tool was first set to rotate with no contact between the blades and the

casing. The flow restricting valve controlling the system RPM was opened to full and the maximum speed was recorded at

around 45RPM. Since both parameters were powered from the same hydraulic power unit, increasing the plenum pressure

whilst the RPM was at its maximum resulted in a drop in speed. Therefore it was not possible to test cuts across the total

range of system parameters

Equipment

The main test equipment as used during the cutting tests are as follows; the prototype tool including a steel test housing, a

7kW hydraulic power unit (HPU) connected to a set of hydraulic valves, a set of pressure sensors to monitor the pressure in

and out of the motor as well as within the plenum, a magnetic gear tooth sensor to monitor the system RPM, and a desktop

computer with graphical programming environment software.

The data acquisition system allows the Graphical User Interface to be adapted quickly by the operator in accordance with

changes in test procedures, equipment and measurement priorities. Additional tests were carried out to prove the seals on the

cartridge; this required a dedicated pressure testing can.

Figure 12: Elliptical piston assembly with roller insert

Figure 13: Prototype tool being installed in test housing Figure 14: Cartridge pressure test can

10 SPE/IADC 139493

Results

Bluntness of Blades

To test this it was decided to repeat a cut with the same parameters six

times. The time to break through the casing and the total time to cut

were recorded and plotted using spreadsheet software. The plenum

pressure was kept constant across all six cuts at 30bar, and the system

speed was kept at 20 RPM. The same blades were used for all tests.

The results of the bluntness test series are shown in figure 15. No

significant variation in the cutting performance of the blade could be

found after six cuts. It was therefore decided that further test results

need not be corrected for the effect of blade bluntness. However it was

also decided that after six cuts all blades would be replaced to ensure

that none were used excessively.

Plenum Pressure

For this test the system speed was set at 15 RPM. The cutting wheels

were replaced and used throughout the entire test series. Six cuts were

made with increasing pressure from 30 to 55bar in increments of 5bar.

Figure 16 displays the results for the breakthrough and complete

cutting time of the plenum pressure test series. A significant drop in

both breakthrough and cutting time is seen for increasing pressure up to

45bar. For pressures higher than 45bar this decrease flattens off.

System speed

For these tests the plenum pressure was set to a constant 30bar, and the

system speed was increased from 5 to 35RPM in increments of 5 RPM.

Figure 17 depicts the results of both the breakthrough and complete

cutting time of the RPM test series. A decreasing slope in both can be

seen. After 20 RPM this slope flattens out and becomes more or less

constant. This shows that after an initial increase of RPM the gain in

cutting time becomes insignificant. It is noted that the missing data

points for complete cutting time at 5 and 10 RPM are due to the fact

that it was not possible to complete a cut at these settings. During these

tests the motor kept stalling and the tests were aborted after 15 minutes.

Number of Blades

For these tests the plenum pressure was kept at a constant 30bar

throughout the tests, and the system rotational velocity was kept at a

constant 20RPM. The cutting wheels were replaced with rollers and

cuts were made with 1-3 blades.

As expected it was found that increasing the number of blades, reduced

the total cut time (see figure 18).

Optimisation

A series of optimisation cuts were attempted by increasing the system

parameters to the maximum limit of the hydraulic power unit. The first

cut of this test series was successful and the casing was severed in 42

seconds. The cut was performed at 45bar plenum pressure and 35RPM

Further optimisation cuts were carried out with varied system

parameters, a 3D graph of breakthrough and cut times for all accepted

tests can be seen in APPENDIX-B.

Increasing the system speed above the plenum pressure meant that the

parameters were restricted to 35bar and 40RPM. This gave a

successful cut in 75 seconds. A cut was attempted with 50bar and

30RPM. This could not be completed at these settings as the motor

repeatedly stalled.

Figure 15: Results from bluntness tests

Figure 16: Results from Pressure Tests

Figure 17: Results from system speed tests

Figure 18: Results from No. Blades tests

Figure 19: Data for a 42 second cut

SPE/IADC 139493 11

However initial break-through of the casing was achieved in 32 seconds. It was then decided to increase the plenum pressure

further to 70bar and let the speed fall to 20RPM; again a cut could not be achieved due to motor stalling however the initial

break-through occurred in 22 seconds.

Discussion Pressure

The plenum pressure test series results can be explained by the development of the contact area between the blades and the

casing at a certain depth of penetration. Limiting the rotational velocity to 15RPM means that the blades were allowed to

penetrate further into the casing wall per degree of rotation. The contact area was therefore increased and further

improvements on cut time were not possible beyond 45 bar.

Due to the pre-decided test values no cuts above 55bar were undertaken. Testing beyond this pressure would allow the depth

versus rotation theory to be further tested, as at some depth of penetration at a low RPM, the indenting contact area may reach

the critical level at which plastic deformation can no longer occur. This would be recognizable by the motor stalling

RPM

The rotational speed test series results confirm the theory on the cutting mechanism. As it shows that above a certain ratio of

load on blade to system speed the system stalls. This is due to the depth of blade penetration per degree of rotation allowing

the contact area to reach the critical level at which plastic deformation cannot occur. It also shows that below a certain ratio

the cut time does not drop significantly. Therefore at these system speeds the cut time can only be reduced by further

increasing the plenum pressure, as the contact area is kept at its minimum.

Due to the pre-decided test values no cuts above 35RPM were undertaken. It would be beneficial to perform further RPM

tests to see if the cut time stayed at this constant, this would further prove the cutting mechanism theory. Also it may be

possible to perform a series of very low RPM and very low pressure tests, to prove that a cut is possible and that the motor

does not stall as long as the required ratio between parameters is maintained.

No. Blades

The results found were not considered sufficient to predict the cut time using more than three blades, as there were not enough

data points for an accurate linear approximation. Therefore it was deemed necessary to carry out further tests using up to 6

blades. For the next version of the tool it was decided to increase the number of blades to six to allow further testing of this

relationship to be carried out.

Optimisation

The first optimisation cut achieved complete casing severance in 42 seconds.

Setting the maximum possible system parameters for this cut highlighted the limitations of the test setup. During all previous

tests the hydraulic flow to the motor was restricted and therefore altering the plenum pressure had little effect on the system

speed. Once the flow restrictor was fully opened, the motor line-in was susceptible to the alterations made to the plenum

pressure. Also the nature of the tool requires that the system be set to rotate before the plenum pressure is applied. This meant

that achieving maximum system speed and load on blades was a delicate process.

As the first optimisation test cut in less than 45 seconds it was decided to carry on optimizing the cut performance instead of

proving the 42 second cut could be repeated. This involved experimenting with different system parameters. Cuts were

attempted using a lower plenum pressure to allow the system speed to be higher than 35RPM. Cuts were also attempted using

an increased plenum pressure while allowing the system speed to drop below 35RPM. Although not all tests yielded a

successful cut, these results further prove the cutting mechanism theory. When the load on blades was significantly higher

than the system speed the motor stalled. When the system speed was higher than the load on blades the cut time was affected

purely by the plenum pressure.

Conclusions and Recommendations

The tests showed that a complete severance of 7-5/8 casing could be completed in under 45 seconds with the tool. They also

suggest that this could be further improved, however in doing so the power requirements may increase excessively. Therefore

optimisation of the cut performance should be carried out in line with an assessment of the various loads exerted on the tool.

The results heavily support the cutting mechanism theory. Further tests should be performed to confirm the theory and define

the optimum ratio between the two loads acting on the blades. To carry out these tests it would be necessary to upgrade the

current test setup. The main upgrade would be to acquire a second hydraulic power unit. This would allow the system speed

and plenum pressure to be independent of each other and a wider range of parameters could then be tested.

To understand the cutting mechanism theory from first principles a method of finding the actual tangential load on the blades

should be found. The main problem to overcome with this is measuring the piston stroke throughout operation. A software

program could then be written which allows the user to maintain the appropriate ratio between load on blades normal to the

casing and tangential to the casing.

12 SPE/IADC 139493

Future Development

Dual Cylinder Arrangement

During the cutting tests it was found that the setup procedure between tests was laborious and time consuming. This was

mainly due to the fact that it was required to drive the elliptical pistons back by hand. During subsea operation the pistons

must be capable of automatic retraction to allow re-entry to the well. Therefore it was practical to design a system which

could be implemented to both the workshop test rig and the final offshore version of the tool.

A coupled dual cylinder arrangement was devised to be implemented outside the pressure housing as part of the control

system. This included two equally sized double acting cylinders with a coupling between their rod ends. The cylinders were

restrained within a u-shaped channel.

When hydraulic pressure is applied to the rod end of cylinder A the mechanical link between the rods causes the rod of

cylinder B to extend. The plenum pressure line is connected to the cylinder end of cylinder B. This means that a negative

pressure is established within the well causing the blades to be retracted. The system is designed to operate in at least 100m of

water where the hydrostatic pressure assists in retraction of the blades; however during workshop tests the blades were shown

to retract without this pressure.

To understand the cutting mechanism theory from first principles a method of finding the actual tangential load (caused by the

motor) on the blades should be found. The main problem to overcome with this is measuring the stroke of the elliptical

pistons throughout operation. This stroke could be combined with a cutting torque to find the tangential load on blades.

This dual cylinder arrangement allowed for a simple method of monitoring the penetration rate of the blades. The stroke

length of the elliptical pistons is proportional to the stroke length of the dual cylinder arrangement. The ratio between the

working area of one of the dual cylinders and the total area of all the elliptical pistons can be used to convert between the two.

A stroke sensor can then be mounted on the dual cylinder arrangement to monitor the extension of the rod end of cylinder A.

Further Testing

A second test program will be devised and undertaken. The scope of this will be to prove the reliability and the repeatability

of a cut performance of less than 45 seconds. This testing will also be more focused on proving the cutting mechanism theory.

Testing the full range of system speeds and plenum pressures will enable us to find the optimum ratio between the loads on

blades that will enable the fastest cut to be performed.

Testing into the effects of various casing conditions will also be undertaken. For differing hardness values of drill casing the

contact area which prevents plastic deformation due to a controlled load will change and therefore the ratio of loads on blade

will need to be adjusted accordingly.

A B

Figure 20: Dual Cylinder Arrangement

Figure 21: Plenum pressure readings during blade retraction process

SPE/IADC 139493 13

In order to be adapted to cut different casing strings within the BOP stack several elements of the annular cutting tool design

must be improved and modified. First, adapted pistons must be designed capable of much further radial travel, this requires

either a thicker housing, which may be acceptable as part of a BOP stack, or the use of telescopic pistons. The cutting blades

currently being used are a commercially available product but the standard sizes are limited. Future work will concentrate on

Geoprober designing custom blades capable of cutting greater wall thicknesses; this will lead to optimisation of shape,

material selection and heat treatment specific to the application to provide the maximum reliability and performance.

For the operational load conditions on casing, it is currently believed that tension will assist the cutting mechanism and

therefore could result in a lower cut time. However the effects of this tension on the final cut profile are of interest to

Geoprober, and work on adapting the current test setup to accommodate tension cuts is ongoing. Once a range of tension cuts

have been performed a new test rig will be devised which will allow all casing offset variables to be tested. For example

fluctuating casing tension, bending moment and pipe offset. These are the main load conditions expected during offshore

operation.

Increased No. Blades Since the completion of the cutting tests a basic comparison between six blades and three blades has been carried out to prove

the linear relationship between cut time and number of blades. All other system parameters were kept reasonably constant

during this test. Although a difference in plenum pressures was recorded, these tests were considered comparable

Figure 22: Graph with plenum pressure readings for 3 blade and 6 blade cuts superimposed on one graph

The result of increasing the number of blades from three to six is evidently linear. A total time to cut for 3 blades was

approximately 140 seconds and for 6 blades was approximately 70 seconds. Therefore it is believed that in further six blade

tests when the plenum pressure and system speed are replicated for the 42 second cut, a 21 second cut should be achievable.

Conclusions

The prototype tool has clearly shown that mechanically severing casing in less than 45 seconds is possible. Therefore using the tool as a primary means of disconnection is feasible.

The results from the cutting tests support the cutting mechanism theory. This means more focused testing can be carried out in the future to further prove the theory and to define the optimum blade loads ratio which maximizes the

intensity of the plastic deformation.

Optimum cutting parameters can then be chosen based on the limitations of other components. Providing the fastest most reliable cut.

The fastest cutting time could be improved beyond 42 seconds, potentially by simply increasing the number of blades.

Future development into testing under tension and bending is required to gain a full understanding of the cutting process in real life conditions

14 SPE/IADC 139493

Unit conversion

Symbol When you know Multiply By To find symbol

mm millimetres 0.039 inches in

mm2 square millimetres 0.0016 square inches in

2

mL millilitres 0.034 Fluid ounces fl oz

kg kilograms 2.202 pounds lb

N newtons 0.225 pound force lbf

bar bar 14.5 Pounds per square inch PSI Table 1: Unit conversion factors

Acknowledgements

GDL would like to thank Statoil for their support in the development of the Annular Cutting Tool. We especially thank Scott

Kerr and Glenn Gabrielsen for their involvement with the project.

GDL would also like to thank Neptune Deeptech, for their services during the cutting tests and the ongoing work on the

prototype tool.

References

Shigley. J.E., Budynas. R.G., Nisbett. J.K., Shigleys Mechanical Engineering Design Eight Edition, McGraw-Hill, NY, 2008, Section 3-19, page 119.

Nelemans, A. M. B., GDL09-01-0001-REP-ACT A.C.T. Phase 1B Test Results June 2009.

Verhoven. J. D., Steel Metallurgy for the Non-Metallurgist, The Materials Information Society, OH, U.S.A. , 200.7

Wasilewski, R. F., Power Transmission Design, How to Install Bevel Gears for Peak Performance March 1994.

West Engineering Services, Shear Ram Capabilities Study For U.S Minerals Management Service, Sept 2004.

SPE/IADC 139493 15

APPENDICES

APPENDIX-A GEO-DWS Well Integrity Schematic

16 SPE/IADC 139493

APPENDIX-B Cutting Test Results