DRILLING TECHNOLOGY Riserless drilling: circumventing the size/cost cycle in deepwater

Allen Gault Conoco Profile of riserless drilling concept Deepwater drilling dilemma. Seafloor detail of riserless configuration. Proposed versus conventional casing points and mud weight equivalents with riserless drilling Estimated pore pressures and fracture gradient plots typical of a US Gulf of Mexico well

Conoco, Hydril project seek enabling technologies
to drill in deepest water depths economically

Allen Gault
Conoco

Floating drilling operations in deepwater presently involve the use of a 21-in. marine riser. The capacity of this marine riser is about 400 bbl for every 1,000 ft of length. In deep water, the mud volume within the riser constitutes a majority of the total mud system and is of no benefit in the drilling process.

This long weighted mud column introduces hydrostatic pressures, which requires numerous casing points in areas with high pore pressures and low fracture gradients, conditions typically found in areas of rapid deposition (Gulf of Mexico). Numerous casing points require a larger subsea wellhead, which requires a larger marine riser, which means that a larger drilling rig is needed to support the riser and mud column. The cycle repeats itself as the water depth increases.

A means of breaking this cycle is needed. A key factor in the solution is reducing the hydrostatic pressure at the seafloor to near that of a column of seawater, thus tricking the well into believing the drilling rig is located on the seafloor. In order to achieve this feat, removing the riser as an annulus and replacing it with a mud return line at the wellhead is necessary.

Background

Schemes for drilling without a riser were developed in the 1960s and 1970s to reduce casing points and rig weight in deepwater floating drilling operations. The concept of riserless drilling was first promoted by Shell. Charles Peterman (Hydril) was with Shell and involved in the project at the time. Bruce Watkins (DrilQuip) was with Regan Offshore during this time period and patented a riserless drilling concept, which has since expired. Frank Williford (Sedco Forex) helped further the concept. Others also have seen merit in it.

However, these concepts were not advanced because the maximum drilling depth in the 1970's was 3,000 ft, requiring only an additional casing point or two, and the technology to implement riserless drilling was not available.

The solution at that time was to increase the size of both the marine riser and subsea wellhead. The first wellheads were 13-5/8 in. in diameter; the common size today is 18-3/4 in. Increasing the diameter of the wellhead and marine riser to drill in deeper water depths, however, greatly increased the weight and space requirements for floating drilling rigs.

For example, the weight attributed to the marine riser, mud volume, additional mud pumps, and solids separation for a rig drilling in 6,000 ft water depths today reportedly has been estimated at 4,000-5,000 tons (Frank Williford).

In addition to additional weight and space needed, a large diameter riser requires tight stationkeeping while drilling. A usual rule of thumb is 5% of water depth for the operating radius. However, operations without the riser are not as severely restricted and can maintain an operating radius of 15-20% of water depth.

The type of well drilled also has a great influence on the casing program. Exploration wells that are not to be completed or tested only require a hole size through the pay zone that can be logged. However, development wells require that the casing through the production zone be large enough in diameter to accommodate the completion. Most deepwater developments require high flow rates to justify the high cost. Production rates in excess of 10,000 b/d are common. Equivalent flow rates are required for gas developments.

Depth constraints

Today, the industry is contemplating drilling in water depths of 10,000 ft on the slope and more than one operator is planning on drilling the distal plain beyond the Sigsbee Escarpment in the US Gulf of Mexico at the turn of the century. The water depth there averages 12,000 ft.

However, current equipment cannot take the industry into such depths without changes. There are three problems with existing technology:

  • The 21-in. riser cannot be pushed much further.

  • Even if the rig could support the riser length, the riser cannot withstand the stresses.

  • Well control is marginal at best at maximum water depths now.

So, revisiting some of the earlier concepts proposed to eliminate the riser makes sense today. All too often, sound concepts were not implemented because of limits in technological capabilities at the time.

An example of such limits is horizontal drilling. The technology was first discussed and tried in the 1950s, but was limited by the technology of the time. Now horizontal drilling and completions have radically changed business is conducted. Riserless drilling falls into the same category and the industry needs to revisit the concept.

Various concepts put forth in the past have included the following:

  1. Diverting the flow at the sea floor to a return line

  2. Gas lifting, mud density reduction, or pumping the drilling riser/return line

  3. Isolating the riser from the wellhead and reducing the hydrostatic column within the riser

  4. Using a reduced diameter drilling riser.

Drilling systems that use several of these concepts have been proposed. Numerous configurations of the basic concepts can be derived, due to the number of variables or degrees of freedom. The number of concepts is a function of the number of variables (4) raised to that power (superscript 4), which equates to 256 variations. Some will be trivial solutions and others will incorporate several variations.

Riserless experience

The industry actually has riserless drilling experience, but that experience does not include returning the drilling fluid column back to the rig. Texas A&M operates a drillship in a riserless configuration to obtain deep ocean floor borings and has worked in 17,400 ft water depths and drilled/cased up to 8,200 ft below the mud line. As many as three strings of casing have been run and cemented using a guidlineless re-entry system.

Stationkeeping is limited by contact of the drillpipe with the side of the moonpool and is estimated to be 20% of water depth. Ocean depths of 3,000 ft or less are considered shallow water. Due to the reduction in weight and space, the drill ship operates for several months at a time without re-supply and is self sufficient.

Riserless drilling offers the capability to drill for hydrocarbons in water depths previously thought impossible. The development of a subsea BOP system capable of accommodating a riserless system would be one of the enabling technologies for drilling in very deep water.

Neil Hudson of Shell International reported it was Shell's position that riserless drilling is justified for water depths in excess of 7,000 ft and that 5th generation drilling rigs will adopt the method.

However, the concept is just as applicable for 2,500 ft water depths for the purposes of weight and space reduction, and elimination of casing points.

Benefits

There are many benefits to drilling without a riser:

  • Stationkeeping: Relaxation of the stationkeeping will reduce waiting-on-weather time or a less expensive mooring system could be deployed. In the case of a dynamically positioned rig, this would reduce the incidents of drive off and provide for fewer/smaller thrusters.

  • Wider well pattern: In development scenarios, a wide pattern of subsea wells could be drilled with a more flexible mooring system.

  • Smaller production structure: The reduction in weight and space would greatly reduce the cost of the floating production structure. Riserless drilling would provide for drilling and workover capability without a large weight and space penalty.

  • Rig upgrade: If the weight requirement for drilling in deepwater can be substantially reduced, then smaller semisubmersible drilling rigs can be used. In effect, a third generation rig could be configured to drill in 6,000-ft water depths. This step is important because all of the deepwater drilling rigs are presently in great demand.

  • Time and cost: Elimination of casing points will reduce the number of days required to drill the well, in addition to the average $1 million per casing point.

  • Alternative fluids: Use of a closed system to support riserless drilling will allow other drilling methods not normally considered in floating operations to be used. They include foam drilling fluids, air drilling, under-balanced drilling, and reverse circulation drilling.

  • Circulate out kicks: Another benefit to the use of a return line is the additional system to circulate out gas kicks, which expand rapidly above the blowout preventer (BOP) on the seabed. An 11-in. mud return line with a surface choke could handle up to 5,000 psi, while the conventional choke and kill lines would be available to handle higher pressures at the seabed.

Conventionally, with a 21-in. marine riser in deepwater, well control consists of "bullheading" an influx back into the formation. No attempt is made to circulate the kick in a normal manner. There is little or no kick tolerance in conventional deepwater drilling because the differential between mud weight and fracture gradient is about 0.3 lb/gallon. With this differential, predictive programs are not utilized since the analysis suggests the well not be drilled.

With a mud return line and surface choke in place, the well can be secured and the expanding gas handled separately from the well systems located on the seafloor. Kicks could be detected by an increase in pressure at the seafloor, minimizing the influx.

Rig upgrading

The current deepwater rig market is extremely tight. Fourth generation semisubmersibles and drillships are all under long term contract. Third and second generation semisubmersibles are being upgraded to extend their water depth capability.

The total number of semisubmersibles available today numbers about 130. The semisubmersible fleet is composed of the following:

4th generation units 14

3rd generation units 40

2nd generation units 76

The target group for rig conversion for riserless drilling would be the more numerous second generation rigs. The water depth limit would be determined by the mooring system. At least one deepwater drilling contractor confirms that were it not for the additional weight due to drilling gear, a second generation rig could be moored in 4,000 ft water depths.

The average age of the second generation fleet is 20-25 years, which is close to the projected useful life. This life can be extended another 10-15 years. However, to get the full benefit of a 25-year life, a purpose-built rig, costing around $100 million might make sense.

An evaluation of the prospects for upgrading second and third generation semisubmersibles using small riser or riserless drilling would be necessary. Integral to this review would be modeling hydraulics and well control situations. Technical input from operators, drilling contractors, and manufacturers would be necessary.

Joint industry project

Conoco and The Hydril Company have undertaken to determine if concepts can be progressed to reduce weight and space requirements for semisubmersibles and drillships. A secondary pursuit will be to reduce the casing points required to drill deepwater wells.

The casing point issue is related to rapid deposition and young sediments (Gulf of Mexico, Brazil, Nigeria) where the industry experiences high pore pressures and low fracture gradients. In other areas (North Sea, West of Shetlands) we experience low pore pressures and high fracture gradients. Therefore, casing points are not the issue, but weight and space for the rig are.

The initial phase will involve evaluating which methods have the best chance of succeeding. The goal is to have a commercial system developed in 3-5 years. The cost of the initial phase is estimated at $250,000-500,000. Bringing a concept to commercial development could conceivably cost $20-30 million. Conoco and Hydril have elected, contigent on additional industry participation, to go ahead with the initial phase and extend an invitation to interested operators and drilling contractors who wish to join in the joint industry project.

The Conoco/Hydril initiative could conceivably study as many as 20 concepts. Included in the concepts will be provisions for staged development, which would provide for an orderly implementation.

Technical objectives

The Conco/Hydril program's objective is to identify the most likely concept of riserless/return line drilling that can be implemented with existing technology. Prior concepts will be analyzed and input will be sought from participants for various system configurations. If required, outside services will be contracted.

Each configuration will be analyzed hydraulically and dynamically. Particular emphasis will be placed on transient conditions. Preliminary loads, forces, and energy requirements will be calculated for each system. From this analysis, a recommendation for the most likely system will be developed. Included will be a proposed workplan, cost estimate, and time schedule for the implementation of design and prototype testing for the system(s) identified as having the best potential for succeeding. Full commercial development would likely cost $20-30 million.

Conoco will offer the following concepts for the program:

1. Selective use of riserless drilling for the range of casing sizes between 20 in. down to 11-3/4 in. Reduction of hydrostatic by means of gas lift, pumping, or glass beads. Fluid returns would be via a return line and the drill string would not be contained within a riser. The return line could either be a dedicated line or the choke line.

2. With the present 21-in. riser, incorporate a subsea rotating head, bypass piping, and control system, and reduce the hydrostatic in the marine riser by use of glass beads. The application goal would be to reduce casing points and tensioning requirements. This would be a partial solution that could be addressed short term. The use of glass beads for underbalanced drilling has been utilized by the Russians.

3. Using a small diameter marine riser (13-5/8 in.), reduce the hydrostatic in the small riser through use of gas lift or glass beads. If enough casing points were eliminated, this smaller riser could be adapted for deepwater exploratory drilling for "throw away" wells, which would allow for a reasonable hole diameter (8-1/2 in.) for evaluation purposes. This system could be used as a staged development of concept.

4. With any sized riser, utilize a rotating head and control system in conjunction with a pump and bypass manifold to reduce the hydrostatic.

5. With a rotating head and subsea controls, examine reverse circulation using both a pressurized system with light mud and heavier mud with internal gas lift inside the drill string. Utilize the kill and choke lines as a means of directing flow to the annulus.

6. Use a subsea buoy to tension a return line, riser, or marine riser.

7. A staged development concept would be to isolate the current marine riser with a rotating BOP and reduce the density of the mud column by use of glass beads. Returns would be diverted around the rotating BOP back into the marine riser. The mud would mix with glass beads and carrier fluid (base mud) to lighten the mud column. A subsea choke would be used for well control. A differential pressure of zero psi would be maintained on the rotating BOP during normal drilling operations.

A high capacity centrifugal pump could be used to circulate a mixture of glass beads and clean mud to lighten the column in the marine riser. Separation of the glass beads will be done at the surface by use of settling pits. This would partially solve the problem by reducing the number of casing points and reducing weight, however it would not eliminate the large diameter drilling riser. However, the concept would provide for introducing equipment in stages to the deepwater drilling industry.

Workplan

The hydraulic behavior should be analyzed for each of the cases considered. Included in the hydraulic analysis would be the input requirements for pump or gas lift as required. Novel fluid density reduction concepts, such as foam, glass beads, and others, would be analyzed as required. The initial work will involve screening the various concepts. More detailed hydraulic work will follow for configurations with the most promise.

The configuration of the equipment will be dynamically analyzed as to practicality for use in floating drilling operations. The emphasis will be on interference between return lines, risers, and exposed drillstrings. Characteristic vessel motion will be included. More detailed dynamic behavior work will follow for configurations with the most promise.

The design concepts for those configurations with the most promise will be provided. A more detailed hydraulic and dynamic analysis will be performed for those concepts. Conceivably more than one concept could be selected.

The development of a deepwater system to reduce casing points, in conjunction with reducing drilling costs and making more drilling units available for deepwater work, are critical to working in depths beyond the 7,000-8,000 ft contour. Without it, development drilling will be enormously expensive and limit the development of discoveries with medium and small reserves.

Further, if the industry is to be able to drill, develop, and produce successfully in water depths beyond 10,000 ft, alternatives to conventional systems will be necessary.

AUTHOR'S NOTE:

Interested participants can contact Allen Gault at Conoco, Drilling Technology, Offshore, 1028, 600 North Dairy Ashford, P. O. Box 2197, Houston TX 77252-2197; Tel: US (713) 293-3338; Fax: US (713) 293-3424.

Copyright 1996 Offshore. All Rights Reserved.

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