NOVEL DRILLING EQUIPMENT ALLOWS DOWNHOLE FLEXIBILITY

David P. Huey
Stress Engineering Services Inc.
Houston

Michael A. Storms
Ocean Drilling Program
Texas A&M University
College Station, Tex.

The Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) have developed numerous downhole tools and systems that have already found or might soon find applications in oil and gas exploration.

Other potential applications for these coring tools, drillstring designs, and bottom hole assemblies (BHAs) include offshore mineral and resource evaluation, offshore geotechnical engineering, environmental monitoring, geothermal energy research, hazardous waste disposal/site qualification, and general geoscience research.

Since 1972, engineers for DSDP at Scripps Institution of Oceanography and, since 1984, for ODP at Texas A&M, have actively developed new tools and systems to support drilling, coring, sampling, and taking downhole measurements in the deep oceans. These engineers have successfully designed and deployed an array of tools and devices required for marine geology research conducted from the dynamically positioned drill ships Glomar Challenger and joides Resolution (Sedco/BP 471).

Both programs have been financed by the U.S. National Science Foundation plus large contributions from foreign partnerships. The primary mission of the projects has been basic marine geoscience research on a worldwide scale. A smaller, but still significant, goal has been the engineering development of new equipment to pursue previously unattempted scientific objectives.

This new technology is expected eventually to become used by industry and benefit all participants. To date, little of the anticipated technological spin-off has occurred.

Both DSDP and ODP have had limited formal technological reporting, and their engineers have not worked under a publish-or-perish mandate.

ODP is scheduled to continue research coring operations using the Joides Resolution through 1998, with possible options to extend to 2003. ODP's development engineering group will continue to address problems of coring and sampling technology in the deep ocean and produce equipment to satisfy those needs.

DRILLSHIP

The Glomar Challenger was built in 1966 as a scientific drillship, although it was originally planned for Global Marine Drilling Co.'s fleet of oil exploration drillships. The Sedco/BP 471, built in 1978, was converted in 1984 to become the joides Resolution scientific drillship after it received the multiyear ODP contract. The primary change on the joides Resolution was to remove the marine riser, blowout preventer (BOP), and mud-return systems.

ODP originally planned on 34 years of riserless drilling (called Challenger-type drilling) to pursue deep ocean science objectives beyond the depth of current riser technology. Then, riser capabilities were to be reintroduced to explore the continental shelves and rises where hydrocarbon accumulations might occur.

Later analysis of riser drilling options and overall scientific objectives led ODP to cancel future plans for conversion to riser drilling.

Typical water depths for sites cored during Challenger-type drilling are 3,000-15,000 ft. Hole penetrations below sea floor typically range 500-5,000 ft.

Removal of the riser, BOP, and related equipment opened up considerable space on the vessel for a modern and relatively roomy seagoing geologic laboratory. Enhancements to the drilling system included an in-line 400-ton heave compensator, a top drive, derrick upgrades to handle the added traveling iron, and a 30,000-ft tapered drillstring. The draw works received additional power for hoisting, and an additional electromagnetic brake was installed.

The horizontal pipe handling system was expanded from two bays holding 5-in. pipe to include an additional bay for 5/2-in. pipe. The drill floor received a dual elevator system and iron roughneck. In the moonpool, a retractable guidehorn structure with a 350-ft radius was built to control bending of the drillstring for ship roll/pitch angles up to 9.

The drillstring and topsides were designed to enhance wire line coring and open hole logging operations. All drillstring, top drive, heave compensator, and traveling block components had an internal drift large enough to pass 4-in. diameter core barrels and logging tools. A custom-built, dual-drum, high-speed sandline winch was added for deploying and retrieving core barrels. A 30,000-ft dedicated logging winch with wire line heave compensator was added to the fantail, where it interfaced with a commercial logging unit.

DRILLING EQUIPMENT

Both DSDP and ODP performed drilling operations exclusively with a top drive. Both programs also required a drillstring element at the top capable of handling the high tensile loads plus the cyclic bending caused by vessel roll and pitch combined with pipe rotation. Standard hex kellys with the minimum required 4/8-in. drift were analyzed and found to have insufficient fatigue resistance.

Where the drillstring passes through the rig floor and moonpool, special fatigue-resistant, knobby drilling joints are used in place of a hex kelly. Additional drill pipe is added below these joints as penetration increases. The conventional practice in ODP operations is to use a 20-ft plus a 30-ft knobby joint as an effective 50-ft drilling joint.

This technique ensures that individual joints of drill pipe will encounter very little repeated cycling because of rotational bending. The 50-ft drilling joints span most of the guidehorn from rig floor to keel during the majority of the rotating hours. The 50-ft effective drilling joint also offers significant flexibility in choosing "drill-to" points, drilling off fill, fighting poor hole conditions, overpulling against stuck pipe or core barrels, and advancing in nonstandard pipe length increments for special downhole measurements.

The knobby joints were originally specified for DSDP by Arthur Lubinski in 1975 and have been modified slightly over the years. The knobby joints are made from 120-ksi-yield drill collar stock with a 4 1/8-in. nominal bore, 3/4-in. wall in the tube sections, 8-in. OD at the tool joints and integral hubs, and 5 1/2-in. internal flush (IF) connections.

The integral hubs spaced at 4/2-ft intervals simulate the length and diameter of made-up pin and box tool joints. These hubs contact the guidehorn during ship roll. The short spacing between the hubs and tool joints creates relatively smooth bending in the adjacent, more flexible tube sections. This smooth bending prevents kink conditions that lead to high bending stress concentrations and short fatigue life.

The knobby joints are routinely used as top-of-the-string drilling joints in all drilling/coring operations. The use of these joints is mandatory across the entire guidehorn (rig floor to keel) in cases of high tensile load combined with low penetration rate where fatigue exposure to a particular joint of drill pipe would be unacceptably high. The knobby joints have serial numbers and are individually tracked for rotating hours. After 400 hr, a new joint has its connections recut, and it is permanently retired after another 400 hr of service.

STRESS ANALYSIS SOFTWARE

ODP uses a tapered drillstring of 5-in. and 5/2-in. S-140 pipe to reach a maximum depth of 9,150 m (30,000 ft). The relative length of each pipe size varies depending on anticipated loads. Determining the optimum drillstring configuration to minimize stresses and fatigue while optimizing payload and overpull capacity requires a complex calculation. Determining probable worst case stresses for a given drillstring configuration includes calculation of the following:

Simple pressure/area tensile loads
Bending effects on pipe sections forced against the guidehorn as the ship rolls
Dynamic loads caused by heave accelerations and drillstring dynamic response (which can exhibit near-resonance effects in some cases)
Hoop stress from internal pressure
Tensile loads because of up-heave stretch against stuck pipe or core barrels
Drilling torque loads
Overpull tension allowance for working stuck pipe.

The optimum drillstring configuration has the lowest stresses in both the uppermost joint of 5-in. and 5 1/2-in. pipe, the lowest fatigue factor, the least hazard of combined stress overload while tripping the string, the lowest derrick loading, the greatest allowance for overpull, and the highest available payload (for example, casing or a weighted guide-base).

All of these ideal characteristics cannot be achieved in one drillstring configuration. An iterative calculation is used to find all these loads for each candidate configuration until the optimum is achieved. This process is conducted in the advance planning of drilling operations and during operations when unexpected factors arise, such as stuck pipe or rough sea conditions.

To replace rule-of-thumb methods, traditional hand calculations, and on-the-fly decisions, ODP developed custom software made expressly to solve the drillstring configuration problem for any ODP site (OGJ, Apr. 4, 1994, pp. 43-49). The program runs quickly on a personal computer and can calculate peak drillstring stresses and derrick loading for a given string configuration or determine the optimum drillstring for a particular operation. The user can specify optimum as either the highest stress safety factors or maximum capacity (payload, overpull, or up-heave load tolerance for stuck pipe).

MULTIPURPOSE BHA

DSDP began operations with a single wire-line-retrievable core barrel built with a Hycalog design. The standard core bit was a four-cone, tungsten-carbide-insert roller cone bit with the cones moved radially outward to leave a nominal 2 1/2-in. core (Fig. 1).

The passive rotary core barrel (RCB) is latched and landed in a simple bottom hole assembly (BHA) consisting of the following (bottom to top):

A bit-sub housing a lower support bearing where the core barrel lands and a flapper-type float valve to inhibit backflow from swabbing when the core barrel is retrieved
An outer core barrel made from a standard 8 1/4-in. OD x 4 1/8 in. ID drill collar, with length held to 30 ft
A top sub with an enlarged bore to allow the core barrel latch fingers to spread out and latch under the fixed latch sleeve
A head sub with an internal thread to install the fixed latch sleeve to hold the core barrel down against the lifting force of the incoming core.

The BHAs have become more sophisticated to accommodate open hole logging and the new features of the coring instruments developed. The first significant change came with the hydraulic piston corer. The hydraulic piston corer (HPC) forced its 30-ft long coring section out through the opening between the roller cones which had to be enlarged to 3.80 in. ID, producing a bit OD Of 11 7/16 in. (Fig. 2).

Later came the extended core barrel (XCB), pressure core sampler (PCS), and motor-driven core barrel (MDCB), all of which operate in the same BHA, so that any coring tool could be deployed for any given cored interval. In addition, numerous specialty instruments were occasionally deployed by mating with the standard BHA (advanced piston corer/extended core barrel BHA). These tools included an in situ pore fluid sampler, temperature measuring tools, and in situ stress-state measuring instruments.

Ultimately, two standard lower BHA, configurations evolved (Fig. 3) (10428 bytes): one for the rotary core barrel (RCB) and another for the large diameter, through-the-bit coring tools and other downhole instruments. In the latter, all of the various core barrels and instruments land on a sub at the top of the seal bore drill collar, an 8 1/4-in. x 30-ft drill collar with a honed ID held to 3.800-3.840 in.

This smooth bore serves as the seal cylinder for the piston action of the advanced piston corer (APC). All the core barrels are designed with careful axial spacing to match the fixed length of the multipurpose advanced piston corer/extended core barrel BHA.

The 9 7/8-in. rotary core barrel (RCB) roller cone bit with its 2.32-in. diameter core throat inhibits passage of most logging tools. However, the drillstring must remain in the hole to convey the logging tools because no riser is used. Consequently, the ability to log holes drilled by this technique requires either reentry with open ended pipe (or a dummy logging bit) or release of the worn rotary core barrel (RCB) bit and bit sub with the pipe still in the hole using a mechanical bit release mechanism.

The extended core barrel (XCB) roller cone bit with its 3.80-in. ID throat allows access to the open hole for logging without releasing the bit if a lockable float valve is installed in the BHA.

DRILL-IN CASING

The majority of DSDP and ODP holes have been drilled as single-bit holes for one entry only. One or more casing strings are installed for hole stability only occasionally when more than one bit will be required to reach the target depth or when other downhole experiments call for reentry into a hole that must remain open for some time. In some cases, penetration into marine sediments with minimal drilling mud circulation produces hole conditions so unstable that the hole will not remain open long enough to install casing by conventional methods of drilling, tripping the bit out, and then reentering with casing.

The drill-in casing system was developed to enable placing 11 3/4-in. casing across a short unstable zone during the first pass of the drillstring. The system consists of an expendable roller cone casing bit, the desired length of 11 3/4-in. casing, and the drive head (Fig. 4) (9173 bytes). This assembly is driven by, and ultimately released from, a drive bushing made up as part of the rotary coring system BHA. After the casing and BHA have been drilled in to the desired depth, the casing is released using a wire line shifting tool. The casing remains in place while the coring system is able to continue penetration to total depth. The casing string is permanently left uphole, straddling the unstable zone and preventing hole collapse.

The drilled-in casing cannot be cemented in place, and reentering the casing is not considered feasible. Thus, this casing system is used only as a means of last resort to achieve certain difficult downhole scientific objectives.

Later modifications include the addition of an 8-ft diameter reentry funnel to the top of the drill-in casing system. This reentry funnel can be used for installing the 11 3/4-in, casing across unstable sands or other sediments at or near the sea floor. With the funnel, the hole can later be reentered for coring or logging operations.

The drill-in casing system was used once in DSDP and twice in ODP operations. For the DSDP installation, a 57-m (187 ft) section of 11 3/4-in. casing was successfully drilled to a casing bit depth of 323.5 m, but the releasing mechanism failed. Both ODP installations were successful-in one case 87 m of 11 3/4-in. casing was drilled to a casing bit depth of 105 m below the seafloor, and for the other, a 12-m joint of 11 3/4-in. casing with a reentry funnel was placed at the mudline to restrain flowing sands.

REENTRY SYSTEM

DSDP demonstrated the feasibility of deepwater borehole reentry from a dynamically positioned drillship in 1970. A primary requirement at that time was a large (14-ft diameter) reentry cone structure capable of being easily identified by a sonar system deployed through the drillstring. Since then, the resolution and efficiency of dynamic positioning systems have improved, reentry sonar has been largely replaced by subsea television cameras, and the size of the reentry funnels has decreased.

The latest ODP standard reentry cone structure uses a 14-ft cone mounted on a flat base with a variable footprint (60-120 sq ft) sized according to the bearing capacity of the seafloor sediment. A conductor casing (16 or 20 in.) is attached to the reentry cone and jetted without rotation into the seafloor. Bearing support at the base plate and reconsolidation around the 30-80 m long conductor string are required to support the weight of the next casing string installed conventionally and then cemented. Up to three additional strings after the conductor can be installed and landed in casing hangers in the cone structure. The smallest casing that can be run is 10 3/4-in. flush joint.

The maximum allowable length of a string is determined to a great extent by payload capacity of the primary drillstring during casing installation and rough sea conditions (Fig. 5) (10471 bytes). ODP has successfully installed a dual casing assembly with an 11 3/4-in. inner casing string about 1,000-m long in the Pacific Ocean in almost 5,000 m of water.

FREEFALL REENTRY CONE

Reentry into ODP boreholes is either planned well in advance (initial installation of a standard reentry cone assembly) or is not expected at all. In some cases, however, reentry is necessary in holes planned as single bit, single entry operations. Reasons for unexpected reentries include fishing for lost tools, a plugged bottom hole assembly (BHA), or premature bit failure.

Reentering a 10-12 in. hole in the seafloor in 1,000-7,000 m of water is nearly impossible unless the hole is marked by a reentry cone. To make this reentry possible, ODP developed a mini-reentry cone that could be dropped, freefall, to the seafloor before the drillstring is pulled, The 8-ft diameter mini-cone, or freefall funnel, has a split construction for assembly around the drill pipe at the moonpool level of the ship. A short section of split 13 3/8-in. casing acts as a stinger. Once assembled around pipe, the freefall funnel is launched and allowed to fall to the seafloor before the pipe is pulled clear of the bottom. The freefall funnel is generally modified on the rig to include a small mudskirt or extra legs to accommodate the particular seafloor terrain. Reentry later using subsea television is virtually routine.

HARD ROCK GUIDEBASE

When a borehole must be spudded on a hard formation outcrop, a special seafloor structure is deployed to confine the drill bit, provide some lateral support to the BHA, and mark the hole for later reentry. The hard rock guidebase concept has gone through several design changes as the required amount of mass, means of footing on the uneven rocky surface, and type of ballast have been determined by trial and error. The latest base design weighs more than 40,000 lb unballasted. The 1-m legs are standard to straddle rough seafloor relief.

A hard rock coring assignment will require many bits, and multiple reentries are anticipated from the start. The base is designed to accommodate landing at a 20 angle from the horizontal. Therefore, a small reentry cone is mounted in the hard rock guidebase with a gimbal to allow the cone and its internal casing hangers to find true vertical, despite the angle of the main base section.

When the drillstring is first pulled free in preparation for reentry with the first casing string, a counter weight forces the reentry cone into a vertical position (Fig. 6) (12625 bytes). Acoustic tilt beacons and visual bull's-eyes report base angle information and are monitored continuously in case the base shifts during drilling. ODP has developed several variations of the hard rock guidebase with different features for use either in conventional hard rock coring or as the seafloor termination of the diamond coring system riser.

BIBLIOGRAPHY

Storms, M.A., "Design and Operation of the Hydraulic Piston Corer," DSDP Technical Report No. 12, Scripps Institution of Oceanography, UCSD, May 1983.
Storms, M.A., Nugent, W., and Cameron, D.H., "Hydraulic Piston Coring-A new Era in Ocean Research," Paper 4622, presented at 15th Offshore Technology Conference, Houston, May 1983.
Cameron, D.H., "Design and Operation of a Wireline Pressure Core Barrel," DSDP Technical Report No. 16, Scripps Institution of Oceanography, UCSD, March 1984.
Huey, D.P., "Design and Operation of an Advanced Hydraulic Piston Corer," DSDP Technical Report No. 21, Scripps Institution of Oceanography, UCSD, July 1984.
Cameron, D.H., "Design and Operation of an Extended Core Barrel," DSDP Technical Report No. 20, Scripps Institution of Oceanography, UCSD, August 1984.
Serocki, S.T., and McLerran, A.R., "The Ocean Drilling Pro-gram: A Technical Overview," World Oil, August 1984.
Moore, W.D., "Ocean drilling program to generate new tools, technology as spinoffs to the industry," OGJ, Dec. 17, 1994.
Huey, D.P., and Storms, M.A., "Deep Water Coring Technology: Past, Present and Future," paper presented at Marine Technology Conference (Oceans '85), San Diego, November 1985.
Foss, G.N., "Joides Resolution: Scientific Drillship of the '80's," paper presented at Marine Technology Conference (Oceans '85), San Diego, November 1985.
Howard, S.P., Serocki, S.T., and Brittenham, T., "Development of a Scientific Drilling and Coring System for the Mid-Atlantic Ridge," American Society of Mechanical Engineers Conference, New Orleans, February 1986.
Storms, M.A., "Ocean Drilling Program Deep Sea Coring Techniques," Marine Geophysical Researches, Vol. 12, pg. 109-130, 1990.
Storms, M.A., et at., "A Slimhole Coring System for the Deep Oceans," Paper 21907, Society of Petroleum Engineers/International Association of Drilling Contractors Annual Drilling Conference, Amsterdam, March 1991.
Miller, J.E., and Huey D.P., "Development of a Mud-Motor-Powered Coring Tool," Paper 6865 presented at the 24th annual Offshore Technology Conference, Houston, May 1992.
Pettigrew, T.L., "Design and Operation of a Wireline Pressure Core Sampler (PCS)," Ocean Drilling Program Technical Note No. 17, Texas A&M University, August 1992.
Pettigrew, T.L., "Design and Operation of a Drill-In-Casing System (DIC)," Ocean Drilling Program Technical Note No. 21, Texas A&M University, September 1993.
Storms, M.A., Huey, D.P., et al., "Advanced Drilling Technology for Scientific Ocean Drilling," paper presented at the Offshore Australia Conference, Melbourne, Australia, November 1993.
"25 Years of Ocean Drilling," Oceanus, Vol. 36, No. 4, Winter 1993/94, Woods Hole Oceanographic Institution, Woods Hole, Mass.
Stahl, M.J., "Automated Stress Analysis and Design of Drill Strings for Riserless Offshore Coring Operations," OGJ, April 4, 1994, pp. 43-48.

THE AUTHORS

David Huey is a staff consultant for Stress Engineering Services Inc., a general consulting firm in Houston. Previously, he was the supervisor of development engineering for the Ocean Drilling Program. Huey was part of the small engineering group that transferred the ODP program to Texas A&M University in 1984 from its roots in the Deep Sea Drilling Project at Scripps Institution of Oceanography in California.

Huey worked as a staff development engineer for DSDP for more than 4 years. In his 13 years with these marine geology research drilling programs, Huey has been heavily involved in the development of new drilling and coring equipment and in directing coring operations at sea around the globe. He received a BS in mechanical engineering from California Polytechnic State University-San Luis Obispo in 1975 and an MS in mechanical engineering from Texas A&M University in 1989.

Michael A. Storms is operations superintendent for the Ocean Drilling Program at Texas A&M University in College Station, Tex. Formerly, he worked 14 years for the Deep Sea Drilling Project. Storms has served ODP in many capacities, including supervisor of drilling operations. He has been involved with the development of several optimized wire line coring and drilling tools. His current responsibilities involve the technical planning and execution of scientific drilling research expeditions worldwide.

Storms received a BS in geological engineering from the University of Arizona in 1970, at which he began his 24-year career in scientific ocean drilling.