CIVIL ENGINEERING – Offshore Drilling and Production Equipment - S. Tanaka, Y. Okada, Y. Ichikawa
OFFSHORE DRILLING AND PRODUCTION EQUIPMENT
S. Tanaka
Professor Emeritus, The University of Tokyo, Tokyo, Japan
Y. Okada
General Manager, Japan Oil Engineering Co., Ltd., Tokyo, Japan
Y. Ichikawa
General Manager, Japan Drilling Co., Ltd., Tokyo, Japan
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Keywords: Offshore, rotary drilling, offshore drilling rig, jack-up, semisubmersible,
drillship, offshore oil and gas production, platform, FPSO, subsea production systems.
Contents
1. Introduction
2. Outline of Rotary Drilling Method
3. Offshore Drilling Structures and Equipments
4. Offshore Oil/Gas Production Systems
Glossary
Bibliography
Biographical Sketches
To cite this chapter
Summary
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World consumption of primary energy in 1999 was estimated at 8533.6 million tons oil
equivalent. Oil and natural gas accounted for about 65％ of the world energy supply.
Offshore areas produced 20-30 % of the oil and gas supply.
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Oil and natural gas are brought to the surface from underground reservoirs through wells
that have been drilled and completed to produce these fluids safely and economically.
Fundamental technology of drilling, completion and production of oil and gas is common
to onshore and offshore areas. But environmental conditions of a field affect facilities and
engineering works of the field.
This chapter covers the basics of rotary drilling technology, recent progress of drilling
engineering, characteristics of various offshore drilling rigs, and types of offshore
production systems. The offshore production system adopted to develop a field extends
its influence on the drilling and completion method of the field.
1. Introduction
The chapter describes mainly the present situation of offshore drilling and production of
oil and natural gas. The first section is devoted for an outline of the rotary drilling method,
as oil and gas wells onshore and offshore are drilled by the method.
A hole made by a drilling bit is called a well. The objective of making the well is to
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produce underground fluids such as fresh water, brine, crude oil, natural gas and
geothermal fluids, and to study properties of deeply situated formations.
The Middle East area and China are said to have had wells producing water or natural gas
even in the era before Christ. By the year 1200, wells 450 m deep were drilled in China by
a spring-pole drilling method. The principle of the method is to generate percussion by
dropping heavy tools on the bottom of the hole. The spring-pole drilling method was the
predecessor of a cable drilling method that had been used till after 1970s.
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Hand-powered rotary devices were introduced to make geothermal wells in Italy and
water wells in France in early 1800s. Machine-powered rotary devices and circulation
systems were introduced by the late 1850’s. In 1901, the Lucas gusher at Spindletop oil
field, the United States of America, was successfully drilled and completed by the rotary
drilling method with circulation of fluid that consisted of water and clay. The first
cementing job to shut off water was carried out in 1903. The use of bentonite as an
ingredient of drilling fluid began in 1935 and has contributed to improve mud properties.
Three-cutter rock bits equipped with jet nozzles were introduced to clean the bottom-hole
of cuttings around 1950.
The technology of directional drilling has made great progress to the level of extended
reach drilling (ERD) and horizontal wells through the development of down-hole mud
motors and measurement-while-drilling tools (MWD).
Over-water drilling from a pier extended from seashore was carried out in the late 1890’s.
Drilling and production of oil in the location where the land was out of sight was
accomplished offshore Louisiana, the United States of America, in 6 m of water in 1947.
The well was drilled from a tender-assisted platform system.
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Four basic types of mobile offshore drilling rigs were developed not long after drilling the
first offshore well: the submersible rig in 1949, the ship-shaped rig in 1953, the jack-up
rig in 1954, and the semisubmersible rig in 1962. Drill ships and semisubmersible rigs are
called floaters. These structures float during operations. Floaters are equipped with
unique facilities that are not used in onshore operations: the marine riser, the motion
compensation, and the stationkeeping system. The dynamic positioning system (DPS)
was introduced for deep-sea operations in 1961.
The scientific research well “SG-3” in Russia reached the depth of 12 263 m in 1988, and
has had the depth record ever since. The deepest exploration drilling for hydrocarbons
was carried out to the depth of 9583 m in the United States of America in 1974. As for
offshore wells, a hydrocarbon exploration well was drilled offshore Brazil in 2965 m of
water in 2001. A production well was completed with a subsea completion system
offshore Brazil in 1852 m of water in 1998. The offshore technology is steadily in
progress toward deeper and deeper seas to search and produce subsea resources for the
future welfare of the world.
2. Outline of Rotary Drilling Method
The rotary drilling method is usually applied to make deep wells. In the rotary drilling a
bit breaks down rocks at the bottom of the hole by scraping and crushing actions. The bit
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is rotated through a drill stem by a rotary table on a rig floor.
Figure 1: Diagrammatic View of Rotary Drilling Rig
(Modified from Rabia H. (1985). Oilwell Drilling Engineering,
Principles and Practice, Graham & Trotman)
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A diagrammatic view of the rotary drilling rig is shown in Figure 1. The left side of the
figure shows main surface equipment. The substructure indicated by 3 is constructed on
the ground as the foundation to support the derrick floor G on which the derrick 1, rotary
table H, and drawworks M are placed. The monkey board (or platform) 2 is a working
floor to handle pipes. The heavy materials such as the drill stem and casing are lowered
into or lifted up from the hole by a hoisting system composed of the drawworks M,
drilling line 4, crown block A, traveling block B, and the hook C.
A circulation system of drilling fluid consists of the suction pits P, pumps Q, surface
piping, standpipe, rotary hose (or kelly hose) F, and swivel D which is connected to the
kelly E, and directed lines show the flow path of the drilling fluid.
In the figure main power sources are the diesel engines N, and the power is transmitted to
the rotary table, drawworks and pumps by the main transmission system O. The rotary
table is driven by the drive J. Sometimes electric motors are used to drive the relevant
machines. A driller controls the machines from the console by the drawworks and
conducts the drilling operations.
In the right side of the Figure 1 showing the cross-section of the derrick floor R and the
hole, blowout preventers (BOPs) S and T are mounted on the top of the wellhead
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connected to the surface casing V. It is the primary function of the BOPs to safely confine
fluids suddenly entering into the hole out of formations , and to bleed them off from the
hole through the outlets U in a controlled manner.
The drill stem is composed of the kelly E, the drill pipe X, and drill collars Y. The bit Z is
attached at the lower end of the drill collars. The components of the drill stem are made of
steel pipes. The drilling fluid is circulated down to the bit through the drill stem, and up to
the surface through the annular space between the drill stem and the borehole or casing.
The drilling fluid returned to the surface flows into the return line L, and then to the shale
shaker K to separate cuttings and fluid. The fluid falls into the suction pit P to be
circulated again
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The casing consists of lengths of steel pipe being joined to another. A number of strings of
casing are set in the well. The purposes of casing are to protect fresh-water sands, to
prevent drilling problems such as heaving formations and high-pressure zones, and
finally to provide a means of production of oil and gas if the well is productive. The
annular space between the casing and the borehole should be filled with cement W to
support the casing and prevent the flow of underground fluids up to the surface and/or
into the fresh-water zones. Conductor casing is the largest-diameter casing used to protect
the surface soils. The next smaller-diameter casing is the surface casing V. Its main
function is to protect fresh-water zones. Intermediate strings of casing are set to case the
long open section of the hole or the zones causing trouble. The last string of casing is the
production casing that is set immediately above, or through, the production formations
The main functions of the rotary drilling rig are as follows:
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Penetrating operations. The bit breaks down rock at the bottom-hole by the
rotation under the weight. The rotating force of the rotary table is transmitted
through the drill stem to the bit. Some portion of the weight of drill collars is
applied to the bit as the bit weight to push the bit against the rock.
Hoisting operations. The drill stem with the bit is lowered and lifted by the
hoisting system. The casing is also handled by the hoisting system.
Conditioning and circulating the drilling fluids by the circulation system.
Preventing the formation fluids from entering into the wellbore and controlling
them.
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The International Association of Drilling Contractors (IADC) classifies the bits used in
the rotary drilling as follows:
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Roller bits (or roller-cone bits).
Steel tooth bits.
Insert bits (or tungsten carbide insert bits).
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(b) Fixed cutter drill bits.
PDC bits (PDC: polycrystalline diamond compacts)
TSP bits (TSP: thermally stable polycrystalline)
Natural diamond bits
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Three types of bits are shown in Figure 2.
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Figure 2: Three Types of Bits Used in Rotary Drilling
A: steel tooth bit, B: insert bit, and C: fishtail PDC bit.
(From Drill Bit Catalog © 1995 – Hughes Christensen
Reproduced Courtesy of Hughes Christensen Company)
The IADC bit classification system provides conventional methods for categorizing roller
bits by three digits, and fixed cutter drill bits by four characters. The choice of bits
depends upon properties of the formations, and drilling techniques (see Drilling
Machines).
The drilling fluids, conventionally simply called as muds, have lots of important
functions in the rotary drilling. Main functions are as follows:
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Removal of cuttings from the bottom of the hole to the surface. Cuttings are
separated from the mud at the shale shaker. These cuttings and samples of the mud
are analyzed to study geological properties of the rocks penetrated, and to find out
the indication of oil and gas in the formations.
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Controlling hydraulic pressure in the hole by adjusting the density of the mud to
prevent collapse of the wall of the borehole, and to contain formation fluids in the
formations.
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Cooling and lubricating the bit and the drill stem.
The drilling fluids are composed of base fluids, clay minerals, chemicals, and inert solids.
Their base fluids classify them as follows: water-base muds, oil-base muds, air or gas
drilling. Bentonite, a kind of clay, is preferred to make up water-base muds. A small
quantity of chemicals adds in the mud to control the viscosity and filtration properties.
Inert solids such as barite are mixed in the mud to adjust the density.
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In the conventional system of the rotary drilling, the rotary table rotates the drill stem, but
the down-hole mud motor and the top drive device are applied to rotate the bit in the
directional and horizontal well drilling, or to improve operations in the vertical well
drilling. The technical advancement of the measurement-while-drilling tools (MWD) and
the logging-while-drilling tools (LWD) has contributed to the almost real-time
acquisition of the down-hole information. Owing to these tools it has become easy to drill
directional and horizontal wells.
Directional wells with long horizontal departure are called extended-reach-drilling
(ERD) wells. The definition of an extended reach well is a well with a measured depth to
true vertical depth ratio greater than 2.0. An ERD well in the united Kingdom drilled in
1999 to access offshore reserves from onshore had a record of a departure of 10 728 m
with a measured depth of 11 287 m and approximately 1600 m true vertical depth. A
definition of a horizontal well is a well with a hole section exceeding an inclination of 85
degrees.
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The adoption of the directional wells, extended-reach wells, horizontal wells, and the
multilateral wells has a share in the economical development of oil and gas fields.
Figure3 is an example of horizontal wells in the North Sea. Figure 4 shows various types
of multilateral wells.
Figure 3: Examples of Horizontal Wells in the North Sea
(From Blikra H., Drevdal K.E., & Aarrestad T.V. (1994). Extended Reach, Horizontal and
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Complex Design Wells: Challenges, Achievements and Cost-benefits, Vol.2, Exploration,
Production, and Reserves, Proc. of the 14th World Petroleum Congress, John Wiley &
Sons.
Reproduced Courtesy of World Petroleum Congress)
Figure 4: Various Types of Advanced Wells
(From Renard G., & Delamaide E. (1998). Complex Well Architecture､ IOR and Heavy
Oils,
Vol. Production, Proc. of the 15th World Petroleum Congress, John Wiley & Sons.
Reproduced Courtesy of World Petroleum Congress)
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There are two basic types of down-hole mud motors; one is a turbine type (turbodrill), and
the other is a positive displacement type (PDM). Figure 5 is an illustration of a multi-lobe
(5/6) rotor/stator configuration type of the positive displacement motor. The motor is
designed primarily for the directional performance drilling motor, but can also be used for
straight-hole drilling.
The top drive drilling system is suspended from the swivel, moves up and down together
with it, and rotates directly the drill stem by electric or hydraulic motors.
In the MWD and LWD systems, sensors are set within the drill collars just above the bit.
In the MWD, the hole direction and inclination are measured, and the downhole weight
and torque on the bit are included in a modified type of the tool. In the LWD, the
formation resistivity and natural gamma ray are measured. The term MWD is often used
as a synonym for the term LWD. Data measured at the downhole are transmitted to the
surface in a real-time mode by coded mud pulses sending up inside of the drill stem or the
annular space. Some sophisticated data transmission systems consist of electromagnetic
wave propagation through the earth, or sonic wave propagation through the drill stem.
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CIVIL ENGINEERING – Offshore Drilling and Production Equipment - S. Tanaka, Y. Okada, Y. Ichikawa
Figure 5: Diagram of Multi-Lobe Mud Motor
(From the General catalog, Eastman Christensen TM
Reproduced Courtesy of Baker Hughes INTEQ)
Figure 6 is an illustration of a positive mud pulse system of the MWD system. In the right
side of Figure 6, a MWD tool A is enlarged. In the left side of the Figure 6, positive mud
pulses B move up through the drill stem to a pressure detector C at the surface. In the
MWD tool measured data are converted into binary signals by an electronics package.
The binary data control movement of a valve actuator to produce positive pulses of the
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mud in the drill stem.
Figure 6: Positive Mud Pulse System of MWD
(Modified from Busking B.E. (1979). Developments in Drilling Technology,
Vol. 3, Production, Proc. of 10th World Petroleum Congress, Heyden & Son Ltd.
Reproduced Courtesy of World Petroleum Congress)
3. Offshore Drilling Structures
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3.1. Technical Features of Offshore Drilling
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Offshore drilling needs a floating or bottom-supported rig. Offshore drilling rigs have
drilling equipment to conduct all the functions similar to the land drilling rigs and have
facilities peculiar to offshore operations. Because of the location remote from
infrastructure, offshore rigs also carry on board a number of service systems such as
cementing, geophysical logging, and so on. In addition, there are lots of specific services
on board such as ROV, divers, meteorological measurements, helicopter, etc.
Accommodations and catering for crews working for 24 hours are required on the rig. All
these factors make offshore rigs complex and sophisticated, and therefore offshore
drilling costs are higher than land drilling costs for similar depth wells.
There are two main categories of drilling rig structures used offshore:
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Mobile bottom- supported and floating rigs
Stationary production structures used exclusively for development wells
The first category of mobile structures includes the following rigs:
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Jack-up rigs
Submersible rigs (swamp barges)
Anchor-stationed or dynamically positioned semisubmersible rigs
Anchor-stationed or dynamically positioned drillships
Drilling structures used for developing offshore fields from stationary platforms are of
two types:
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Self-contained platforms
Tender or jack-up assisted platforms or well-protector jackets
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The large production platform equips a complete set of drilling equipment, and is called
as self-contained platform, which is described later in the Section 4 of the chapter. The
small platform has a space only to accommodate derrick and drawworks, so a kind of
tender assists the work, which is described later in the section.
There is a guideline to choose roughly the type of offshore drilling rigs according to water
depth and conditions of sea state and winds:
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Water depth less than 25 m: submersible rigs (swamp barges)
Water depth less than 50 m and calm sea: tender or jack-up assisted
platforms
Water depth less than 400 m and mild sea: self-contained platforms
Water depth from 15 m to 150 m: jack-up rigs
Water depth from 20 m to 2000 m: anchored drillships or semisubmersible
rigs
Water depth from 500 m to 3000 m: drillships or semisubmersible rigs
with dynamic positioning system
Isolated area with icebergs: drillships with dynamic positioning system
Severe sea conditions: semisubmersible rigs or new generation drillships
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3.2. Mobile Bottom-supported Rigs
3.2.1. Jack-up Drilling Rigs (Jack-up Rigs, Self-elevating Drilling Rigs)
Jack-up drilling rigs are used in water depth that typically ranges from 15 to 100 m with a
maximum depth of 150 m. A hull of the jack-up drilling rig is typically constructed in a
triangular shape with 3 legs, and in a few cases in rectangular or other shapes. A jack-up
rig is moderately stable when floating on its hull with its legs up. A jack-up rig is moved
by being towed by a tugboat or is transported by a heavy lift carrier from one drilling
location to another, and then jacked above the sea surface on tubular or derrick legs.
During towing, the legs are raised so that just a few feet protrude below the hull. When
moving a jack-up rig, it is necessary to ensure that weather conditions (sea states and
winds) do not exceed the allowable operating parameters established for each jack-up rig
design.
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Figure 7: Jack-Up Drilling Rig with Triangular Shape and 3 Legs (JDC Hakuryu 8)
(Reproduced Courtesy of Japan Drilling Co.)
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There are two basic leg configurations of jack-up rigs:
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Independent-leg type for relatively firm seabed: Each independent leg has a spud
can on the end. The leg penetrates soil below the mud line, i.e. the sea bottom. The
penetration depends on the composition of the soil and the shape of spud can.
Mat-supported type for soft seabed: Legs is connected with a mat. The mat rests
on the seabed to stably support the rig. The type is used on flat sea-bottom in
water depth of up to 50 m. The penetration is slight.
Legs of the independent-leg type of jack-up rigs are either vertical or tilted slightly
outward to provide stability when the hull is raised out of the water. Each leg typically
consists of three to six vertical members called chords. The vertical members (chords) are
attached by cross members (k-braces) and form interconnected truss members. Each of
the chords has a gear rack that runs the full length of the leg. The legs are raised and
lowered with electric motors mounted to the main deck of the hull, which drive a number
of pinion gears attached to each of the leg chord gear racks. Once the rig arrives at the
drilling location, the legs are lowered to the sea floor and the hull is raised (jacked) out of
the water. The hull is jacked up (elevated) at a speed that ranges from 0.3 to 1 m/min.
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Prior to raising the hull to the safe working height above the ocean waves, it is required to
preload the legs that penetrate the ocean sediments. It is necessary for the sediments that
the legs are pushing against to support the weight of the jack-up hull and also the
additional drilling equipment placed on the rig during drilling operations and certain
drilling loads. If the sediments are not dense enough to support these loads, one or more
of the jack-up legs may suddenly/rapidly push (punch) through the sea floor sediments. A
leg punch through can severely damage the legs and hull, thus jeopardizing the safety of
the rig.
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Once the preloading operation is complete, the hull can be raised to the desired height
above the ocean waves. The height of the hull above the sea surface is called the air gap.
The air gap depends on the expected height of the waves and also the height of the
production platform if drilling a well on a production platform. Depending on the design
of the rig, there is a slot either in the hull to allow the wellhead to be positioned under the
rig floor, or the rig floor and support structure (substructure) can be extended/slid-out
(cantilevered) from the side of the hull to the desired drilling position. Once the rig is
placed in operating position, drilling process is carried out in a way similar to land
operations after the outer casing and surface BOP is installed.
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3.2.2. Submersible Drilling Rigs (Submersible Rigs, Swamp Barges)
Figure 8: Submersible Drilling Rig (Noble FriRodli)
(Reproduced Courtesy of Noble Drilling Corporation)
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Submersible drilling rigs consist of upper and lower hulls connected by a network of
posts or beams. The drilling equipment and living quarters are installed on the upper hull
deck. The lower hull has the buoyancy capacity to float and support the upper hull and
equipment. When water is pumped into the lower hull, the rig submerges and rests on the
seabed to provide a working place for the drilling. Movement and drilling operations
proceed as that of the jack-up rig. Most submerged rigs are used only shallow waters of 8
to 10 meters.
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Ship-shaped submersible rigs are also used, which are called swamp barges. Swamp
barges have operated in swamp and marsh areas in Nigeria, Indonesia, and the southern
part of United States of America. An arctic mobile drilling unit having conical,
sloping-side structure belongs to the submersible type.
3.2.3. Tender-Assisted Platforms and Tenders
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In regions where the weather conditions are not harsh, it is possible to use lower cost
fixed platforms that are designed to support only the derrick and the drawworks. The
tender anchored alongside the platform contains drilling equipment such as pumps and
tubular goods, and accommodation for personnel. A catwalk connects the platform and
the tender.
Figure 9: Platform (left) and Semisubmersible Tender (right) (Atwood Oceanics
SEAHAWK)
(Reproduced Courtesy of Atwood Oceanics, Inc.)
If weather conditions (wind, swell, and current) become too harsh, the drilling operations
must be shut down due to excessive motion of the tender. The tender platforms are used in
Gulf of Guinea and the Persian Gulf waters where good weather conditions prevail,
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resulting in low downtime less than 2% of total operation time.
3.3. Floating Offshore Drilling Rigs (Floaters)
3.3.1. Technologies Required by Floaters
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In water depths greater than 100 m, floaters are commonly used. Drillships and
semisubmersible rigs are classified as floaters. Drilling operations with floaters require
peculiar technologies that are not used in the operations of mobile bottom-supported
drilling rigs. They are stationkeeping system, marine riser system, and drillstring motion
compensator. These systems are shown in Figure 10 in a case of semisubmersible rig with
an anchor-mooring system.
Figure 10: Outline of Drilling System of Semisubmersible Rig
(Modified from Sekiyukaihatsu Gijutsu no Shiori (1st edition).
Reproduced Courtesy of Japan Petroleum Development Association)
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Wind and current forces push a floater away from the location directly above the subsea
wellhead. Waves raise and fall the floater. The technology has developed to overcome
these effects caused by weather conditions.
An adequate stationkeeping system is necessary to keep a floater within acceptable limits
above the subsea wellhead. One of the methods is an anchor mooring system consisting
of mooring lines (chains, wire ropes or a combination of chains and wire ropes) and
anchors that is shown in Figure 10. It is important to arrange the mooring-system
according to environmental conditions prevailing at the location.
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The other is DPS, which is the acronym of dynamic positioning system. The vessel has
several thrusters under the bottom of it. Computers onboard manipulate the thrusters
automatically. Acoustic positioning beacons are located around the subsea wellhead, and
send the signals to the vessel. And/or the vessel receives position signals from satellites.
The computers analyze the signals, and command movement of the thrusters to keep the
vessel's position within acceptable limits.
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The marine riser system consists of riser pipe, riser tensioners, and ancillaries. The riser
pipe is connected to the top of subsea BOP, and is pulled up by the riser tensioner system
onboard to keep vertical configuration. The riser pipe serves as a conduit for returning
mud to the surface from the hole, and as a guide for running drill stem and casing from the
floater to the hole under the seafloor.
Figure 11: Crown Mounted Type of Heave Compensator
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(Reproduced Courtesy of National Oilwell - Kristiansand)
The motion compensator is a device to maintain constant weight on the bit during drilling
operation in spite of oscillation of the floater due to wave motion. One of the devices is a
bumper sub. The bumper sub is used as a component of a drill string, and is placed near
the top of the drill collars. A mandrel composing an upper portion of the bumper sub
slides in and out of a body of the bumper sub like a telescope in response to the heave of
the rig, and this telescopic action of the bumper sub keeps the bit stable on the
bottom-hole. The bumper sub is able to transmit the torque from the drill stem to the drill
collars to rotate the bit.
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Other device is a heave compensator. The heave compensator is placed in the derrick.
There are two types of the heave compensator. One is a crown mounted heave
compensator shown in Figure 11, and the other is an in-line compensator that is hung
below the traveling block in the derrick shown in Figure 10. Both systems use either
hydraulic or pneumatic cylinders that act as springs supporting the drill stem load, and
allow the top of the drill stem to remain stationary, as the rig heaves up and down.
3.3.2. Drillships
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The first drillship was introduced in offshore services in 1953, and the dynamic
positioning system was equipped in drillships in 1961. Drillships are shown in Figure 12.
Figure 12: The Larger is a Drillship with Dual-Activity Drilling System (TSF Discoverer
Enterprise), and the Smaller is a Previous Generation Drillship (TSF Discoverer 534)
Alongside with a Supply Boat; (Reproduced Courtesy of Transocean Inc.)
Drillships contain all of the equipment and material needed to drill and complete the well.
An opening called a moon pool is equipped in the center of the ship from the main deck to
the water. Drilling assembly, riser pipe, wellhead equipment, and so forth are lowered
through the moon pool to the sea floor.
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New generation drillships built in 1990s have the capacity to drill wells in the waters up to
3000 meters. These ships have production test facilities and oil storage tanks in addition
to the usual drilling equipment. To improve the efficiency of deepwater operations with
the aim of economic advantages, the PROPRIETARY dual-activity drilling system OF
TRANSOCEAN INC. is shown in the larger drillship in Figure 12. The dual-activity rigs
have two sets of drilling equipment such as mud pumps, drawworks, top drives, and mud
treatment system. When a hole is drilled by the first drilling system, the second drilling
system is used for making up casing and tubing strings in advance, and so on. Although
the dual-activity system in the deepwater drillship is in the early stage of deployment, it is
readily recognized that the system can save significant time and cost of drilling
operations.
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3.3.3. Semisubmersible Drilling Rig
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Semisubmersible drilling rigs, which are simply called as semisubmersibles, jointed in
the drilling fleet in 1962. Semisubmersibles have submerged pontoons (lower hulls) that
are interconnected to the drilling deck by vertical columns as shown in Figure 10 and
Figure 13.
Figure 13: Semisubmersible Drilling Rig (JDC Hakuryu 3)
(Reproduced Courtesy of Japan Drilling Co.)
The lower hulls provide improved stability for the vessel. Also, the open area between the
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vertical columns of semisubmersibles provides a reduced area on which the environment
can act. In drilling operations, the lower hulls are submerged in the water about
half-length of the column, but do not rest on the seabed. When a semisubmersible moves
to a new location, the lower hulls float on the sea surface. Semisubmersible rigs are towed
by boats, and some rigs have self-propelled capacity. On drilling site to keep the position,
the anchors usually moor semisubmersibles, but the dynamic positioning systems are
used by new generation semisubmersibles.
Drilling equipments, mud systems, living quarters and so forth are placed on the deck,
and ballast tanks, thrusters, sea water pumps are equipped in the lower hulls.
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Semisubmersibles have minimum structures exposed to wave actions. So
semisubmersibles provide more stable station for the drilling operations, and are able to
operate in harsher environmental conditions as compared with drillships. The main
disadvantage of semisubmersibles is that variable deck loading capacity required for
storing drilling materials is limited by the structure.
In the late 1970s, semisubmersibles had the capability to operate in 500 m water depth
using an anchor system. In the late 1990s, the drilling operation capability of
semisubmersibles increased to over 1800 meters water depth. Hoisting capacity and
variable deck loading capacity are factors restricting the water depth capability of
semisubmersibles.
3.4. Location Surveys for Offshore Drilling
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The offshore environment has a much more significant influence on drilling operations
than the onshore environment. It is necessary to carry out a suite of location surveys
before starting drilling operations in order to obtain data such as weather forecast during
drilling operations, bathymetric map around the location, current profiles, properties of
the sea bottom soil, topography of the sea bottom, and shallow geological hazards. The
surveys will cover at least area a kilometer square centered on the proposed location. The
survey companies analyze and evaluate the data to present reports that are used to prepare
the mooring plan and the casing design for a top hole, and so forth. The minimum
requirement of the survey includes following instruments: sparkers, sub-bottom profilers,
side-scan sonars, fathometers, and gravity corers. Wind and current measurements for
several months would be carried out at a proposed location about one year ago before
operations. In critical areas, surveys in consideration of the culture, the archaeological
and biological background are required.
4. Offshore Oil/Gas Production Systems
4.1. Brief History of Offshore Production Systems
History of offshore petroleum production dates back to the end of the 19th century, when
oil wells were drilled from piers constructed along the Californian coast of the United
States of America. Geologically continental shelf is considered to be the extension of
continental land and it was quite natural to pursue the extension of onshore oil
accumulation down to sea bottom. These Californian wells were the first attempt of this
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hunt.
If hunt for petroleum, i.e. oil and natural gas, was extended from onshore to offshore, so
was the use of equipment employed for its production. Petroleum production and field
processing equipment typically includes wells to safely bring oil and gas from
underground formation to surface, separation equipment to separate gas, oil and
associated water, some means of transporting the products to market such as pressure
booster (pump/compressor) and pipelines. In the majority of offshore fields these pieces
of equipment used are essentially the same as those used in onshore fields.
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In order to bring the equipment to offshore environment, some sort of supporting
structure is needed to keep them above water, namely, offshore platforms. The above
Californian piers were in this sense the first offshore platforms for petroleum production.
Bottom-supported platforms have been in use ever since, though material to build them
changed from wood to steel and concrete. These platforms, whilst good in that they can
provide working environment probably closest to that onshore, have problems of sharply
increasing cost with increasing water depth and long lead time for construction. To
counter these problems, the petroleum industry came up with floating platforms in the
1970s. These include semisubmersibles, a natural functional extension of their sisters in
the MODU fleet, ship-shaped floating production storage and offloading systems
(FPSOs) and tension leg platforms (TLPs). More variations were added in later years.
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If these platforms are one way of coping with the offshore environment, enabling
equipment intended for onshore application to be used there more or less as it is, the other
way is to make the equipment capable of functioning in underwater environment and put
it on the seabed, that is, a subsea production system. The first production equipment that
was put completely underwater was a well, a gas well completed on the bottom of Lake
Erie in United States of America in 1943. Although this well used completion equipment
for onshore use without modification, one designed specifically for subsea application
was subsequently developed. To date there have been more than 1000 wells worldwide
drilled and completed subsea. The advancement of subsea technology meanwhile has led
to development of other kinds of equipment tailored for subsea application, namely,
manifolds to collect/divert produced and service fluids to desired flow paths, multi-phase
pumps that can boost the pressure of the mixture of gas and liquid and gas/liquid
separators, all with associated controls equipment.
Today these platforms and subsea systems are applied in various combinations, each
aimed at best suiting the particular environment in which they operate. These are offering
the petroleum industry many options to choose from for production systems. However,
research and development still continue to tap oil and gas in still deeper water and still
harsher environment.
4.2. Various Types of Offshore Platforms
4.2.1. Bottom-supported Platforms
The most widely used platforms are so-called template platforms. This type of platform,
Figure14, usually consists of jacket, piles and deck. The jacket is fixed to sea bottom by
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means of piles and they together support the deck load. The deck is the topside structure
of the platform and houses most of the equipment. The jacket is fabricated from steel
tubulars at a yard, transported on and launched at the site from a barge, upended, lowered
to the sea bottom and piled. The deck is usually divided into several modules, which are
individually fabricated at a yard or shipyard, transported on a barge to the site where the
jacket is already installed, lifted and fixed onto the jacket.
Figure 14: Template Platform
(Reproduced Courtesy of Offshore Iwaki Petroleum Company, Ltd.)
Yard space and launch barge size available at the time of the project limit the jacket size.
Cognac platform jacket installed in 312 m of water in the Gulf of Mexico in 1977 to 1978
was fabricated in three pieces and they were sequentially lowered to and connected
together on the seabed, because there was no barge large enough to transport it in one
piece at that time. Size of the individual deck module is greatly influenced by the lifting
capacity of the crane vessel available. Attractiveness of less number of heavier lifts in
terms of cost and other aspects has encouraged construction of heavy lifting vessels and
those with twin cranes with several thousand ton lifting capacity are available. Function
of this type of platform can vary from simple well protection to combined drilling and
production with all the necessary equipment to drill wells and process and transport the
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produced fluid. Storage function is not normally provided. This type has found
application in water as deep as 411 m in the Gulf of Mexico.
As the water depth increases, maintaining stiffness to rigidly resist the overturning
moment as the template platforms do becomes prohibitively expensive. The alternative is
structures that have much longer sway period than that of high-energy storm waves,
avoiding resonance of structures with these waves. This type of platforms is called
compliant towers and has been applied in water depths in excess of 500 m.
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Another type of bottom-supported structure is the gravity platforms, Figure15. They
derive required stability from their own weight. The substructure is usually built from
concrete in deep, protected water near shore such as fjord and firth. Deck is usually built
in one piece, brought on a barge or barges over to and then mated with almost completely
submerged substructure. The completely assembled platform is then towed to the
installation site and ballasted down to seafloor. Because of this unique way of
construction, geographical locations to which this type is applicable are limited. Only a
few applications can be found in the world except in the North Sea, where Norwegian
fjords and Scottish firths provide ideal construction sites. This type is inevitably massive
and suitable for self-contained drilling and production role. Storage capability can be
easily incorporated, making this type suitable for situations where pipeline transportation
is not readily available.
Figure 15: Gravity Platform
(From Ager-Hanssen H., & Medley E.J (1979): Main Considerations in Development of
StatfJord Field, Volume of Production, Proc. of the 10th World Petroleum Congress.
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Reproduced Courtesy of World Petroleum Congress)
4.2.2. Floating Platforms
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Though the bottom-founded platforms provide stable working environment, they
typically have the drawbacks of long lead time and cost tendency quite sensitive to water
depth. A solution to these problems has been the floating platforms moored to the seafloor.
An additional advantage is seen in ease of relocating and reusing them after a field is
depleted. This type was pioneered by a semisubmersible platform converted from an
MODU in the mid 1970s. Quite a number of conversions were made particularly in the
early days of this type of platforms so that fields could be brought into production quickly,
improving project economics. Units intended for deep-water applications are mostly
purpose-built so that they can satisfy the demanding conditions of such applications.
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Mooring is usually by eight to twelve point catenaries. Motion of the platform does not
allow wells to be completed on the deck. So they are usually completed subsea and
produced fluid is brought to the processing equipment aboard the platform by means of
pipeline and riser, Figure 16. Riser is one of the technical focal points and flexible pipes
are widely used for this application. Disadvantages of this type can be summarized as
limited payload capacity and lack of storage capability.
Figure 16: Semisubmersible Platform
(Reproduced Courtesy of Norsk Hydro ASA)
Another type of floating platform having relatively long history of application is the
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floating production storage and offloading system (FPSO). They are ship-shaped
platform either with or without propulsion capability, Figure 17. First application was
made in the late 1970s and it was a converted ocean-going tanker. Converted or
purpose-built, FPSOs have a large payload and storage capacity making them suitable for
application in isolated locations where pipeline transportation cannot be an option. Single
point mooring is the most widely used station-keeping means, where the platform is
allowed to weathervane around the mooring mechanism. Multi-point moorings have been
applied in relatively benign environment such as West Africa and the Gulf of Suez. Like
semisubmersibles wells are completed at separate locations, either subsea or on separate
platforms.
Figure 17: Floating Production Storage and Offloading System
(Reproduced Courtesy of Norske Stats Oljeselskap a.s. (STATOIL))
One of the floating platforms specifically developed for deep-water application is the
tension leg platform (TLP), Figure 18. Except for the very first application that appeared
in the mid 1980s, platforms of this type are in water depths in excess of 500 m and the
deepest application is in more than 1200 m of water. TLP is essentially a semisubmersible
attached to the seabed by vertical members called tendons, which are usually made of
steel tubulars and tensioned by excess buoyancy of the platform hull. Tendons are pinned
to the seabed directly or indirectly by piles. Motion characteristics of the TLP allows
wells to be completed on its deck, a big advantage because wells are one of the most
important and expensive components of a petroleum production system and ease of
access to them is a matter of prime concern in field development planning.
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Figure 18: Tension Leg Platform
(Reproduced Courtesy of ConocoPhillips Norge)
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The most recent species of floating platform is the spar platform, or deep-draft caisson
vessel (DDCV). As its name indicates, it has a deeply submerged, spar-shaped hull and a
deck structure. The platform is moored to the seabed by means of catenary or taut
mooring. This type of structure was first applied as a storage and loading buoy in the
North Sea in the mid 1970s. Application for the drilling and production role came into
reality in the latter half of the 1990s in the Gulf of Mexico, and three spar production
systems are operated there as of 2000. A platform of this type has been built in 1463 m of
water. Like TLP it is possible to put Christmas trees of wells on the platform deck, and
like FPSO, oil storage capability can be incorporated in the hull, making this type
attractive at isolated deep water locations.
4.3. Subsea Production Systems
Subsea-completed wells were initially envisaged by many as an unavoidable evil for
deep-water production. As their reliability improved and confidence was gained on them,
they have found increasing use even in the water depth range of bottom-supported
platforms. Reservoirs discovered close to existing infrastructures or parts of producing
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reservoirs too far away to reach from existing platforms are typically developed utilizing
subsea wells tied back to the host platforms, providing economical means of field
development. Also exploratory wells, typically plugged and abandoned after a short
period of test not so long ago, are completed subsea and put into production for some
months using drilling vessels equipped with temporary production facilities or
purpose-built well test vessels, providing valuable reservoir data for subsequent field
development planning.
4.3.1. Subsea Christmas Trees
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Subsea trees are, like land trees, primary means of flow control for subsea wells and
consist of series of valves and sometimes a flow control device (choke) along the flow
path of produced fluid with associated controls equipment, Figure 19. Although trees in
early days of subsea development relied heavily on diver assistance in installation and
operation, the trend is toward less reliance and remotely operated connectors, valves and
chokes by means of hydraulics are used extensively. They are installed by drilling rigs
using guidelines established between a pre-installed guide base structure and a rig for
positioning and orientating. For deep waters where use of guidelines is not practical,
guidelineless system is available.
Figure19: Subsea Christmas Tree
(From API RP 17A “Recommended Practice for Design and Operation of Subsea
Production Systems”, Second Edition, 1996. Reproduced Courtesy of the American
Petroleum Institute.)
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Where it is desirable to drill a number of wells from one location, a well template is
sometimes used, Figure 20. A template is a steel structure that provides structural support
and appropriate spacing to wells. As the drilling rig can move from one well position to
another only by adjusting anchor lines, use of template can bring savings in drilling cost.
Figure 20: Well Template
(From API RP 17A “Recommended Practice for Design and Operation of Subsea
Production Systems”, Second Edition, 1996. Reproduced Courtesy of the American
Petroleum Institute.)
4.3.2. Subsea Manifolds
A manifold consists of appropriately arranged valves and pipings with associated controls
equipment and a structure to support these components. It allows produced fluid to be
commingled or diverted and injection fluids to be distributed to desired flow paths. With a
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subsea manifold, number of flowlines and injection lines between wells and host platform
can be reduced substantially, saving large amount of investment. Disadvantage is added
complexity in not-easily-accessible subsea environment with implications on
maintenance cost. A manifold can be a stand-alone component or built into a well
template, Figure 21.
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Figure 21: Subsea Manifold Mounted on Well Template
(From API RP 17A “Recommended Practice for Design and Operation of Subsea
Production Systems”, Second Edition, 1996. Reproduced Courtesy of the American
Petroleum Institute.)
4.3.3. Subsea Boosting and Processing
Wells, templates and manifolds have been successfully applied subsea. The next
candidates for subsea application are multi-phase pressure boosters and fluid separators.
The idea behind the multi-phase pressure booster, or multi-phase pump, is that if pressure
boosting can be done at the subsea wells, a platform long distance away can be their host,
widening the possible range of step-out development. More ambitiously, produced fluid
could be sent directly onshore, eliminating the need for host platform altogether.
Difficulties in boosting pressure of multi-phase fluid with widely fluctuating void
fraction have prompted some to take a different approach, that is, to separate produced
fluid into different phases and then boost the pressure. Another application of the subsea
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separation technology being considered is to separate produced water at the wells and
re-inject it underground for disposal, saving energy to transport unnecessary water to
surface.
The subsea boosting concept has been considered since the early 1980s and the subsea
separation concept a little later. Both technologies have come to the stage of field-testing
prototype equipment, but are yet to be applied on commercial basis.
4.3.4. Subsea Control System
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In order to ensure safe and efficient operation of the subsea production systems, their
various components such as valves, chokes and connectors must be properly controlled.
Also it is often desirable to have feedback from these components and other
instrumentation indicating various process parameters such as downhole/wellhead
pressures and temperatures and fluid flow rate and diagnostic parameters of the control
system itself. Control systems currently employed utilize hydraulics and often electronics
to differing degrees. Of these most commonly used is the electro-hydraulic multiplexed
system.
The system requires hydraulic power supplied from the host platform to actuate control
devices. Coded signals for operating these field devices and data signals from field
instruments to the host platform are transmitted through an electrical cable. Electric
power is usually supplied through a separate cable, but superposition of data signals on
the power cable has been successfully tried. Research is underway for developing an
all-electric control system, which will eliminate the need for hydraulic link between the
subsea system and host platform altogether, leading to further cost reduction.
4.4. Prospect of Offshore Production System
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The technology for offshore development has been commercial technology all along.
Every time oil was found in such an environment where the offshore petroleum industry
had had no previous experience, new technology was developed to cope with it and
commercially produce oil there. In pursuing this goal, the balance between safety,
reliability, economics and, increasingly, environmental considerations has never been
forgotten.
The North Sea developments of the 1970s and the deep-water developments in the Gulf
of Mexico and Brazil of the 1990s were probably the two epoch-making eras in the
history of this industry. The North Sea-type platforms with huge payload capacity to
accommodate all required functions on a single structure, sometimes with built-in or
separate storage capacity, made it possible to develop fields in remote, isolated areas.
Implications of the advancement of deep-water technology are obvious.
The twenty first century will see this process continuing, at probably a slow but steady
pace. The North Sea activities have already spread to the Atlantic Frontier (west of
Shetland) to the west and the Norwegian Sea to the north. The water depth record of both
subsea completion and platform installation has exceeded 1800 m and developments in
more than 2000 m water depths are being planned. Also somewhat specialty technology
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is enabling oil production in such ice-infected areas as offshore Newfoundland and
offshore Sakhalin Island.
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A major challenge may come, however, when the industry moves to tap methane hydrate
deposits under the ocean floor, generally in waters more than 500 m deep. Methane
hydrate is a solid compound of water and methane where methane molecules are taken in
to the cage-like structure formed by water molecules. Production of methane from
hydrate is considered to involve its dissociation. Although the biggest problem is seen in
maintaining the dissociation rate high enough to sustain commercially viable rate of gas
production, problems are also foreseen, for example, in maintaining integrity of wells and
preventing or coping with potential sea floor instability such as subsidence. The attempts
to meet the challenge have already begun. When it is successfully met, humans will have
obtained another important source of precious energy.
Glossary
Casing:
Christmas tree:
Drill stem:
Riser:
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Well completion:
API:
BOP:
DDCV:
DPS:
ERD:
FPSO:
IADC:
LWD:
MODU:
MWD:
PDC:
PDM:
ROV:
TLP:
Steel pipe lowered into a hole drilled and bonded to formation by
cement to keep the well safe.
An assemblage of valves installed at the top of a well to control
the flow of oil and gas after the well has been completion.
A drilling assemblage of tubular goods, to rotate a bit at the
bottom of the hole from the surface, which comprises of the kelly,
the drill pipe, and drill collars.
Any pipe with the fluid flow upward in it. In offshore drilling a
marine riser system is used to establish a connection between the
rig and the seabed. In offshore petroleum production, production
riser systems extend from the seabed to the deck of the production
platform.
A series of work to make a well ready for production after it has
been drilled and tested. Although there are wide variations, it
typically involves installing the production (deepest) casing,
perforating the casing and installing tubing (flow path for
produced fluid) and the Christmas tree. A subsea completion or
subsea-completed well is a well that sits entirely, that is, up to its
Christmas tree, on the seabed.
American Petroleum Institute
Blowout Preventer
Deep-Draft Caisson Vessel
Dynamic Positioning System
Extended Reach Drilling
Floating Production Storage and Offloading system
International Association of Drilling Contractors
Logging-While-Drilling tools
Mobile Offshore Drilling Unit
Measurement-While-Drilling tools
Polycrystalline Diamond Compacts
Positive Displacement Motor
Remotely Operated Vehicle
Tension Leg Platform
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Thermally Stable Polycrystalline
TSP:
Bibliography
Bourgoyne Jr. A.T., Millheim K.K., Chenevert M.E. and Young Jr. F.S. (1991). Applied Drilling
Engineering. Richardson, TX 75083-3836, USA: Society of Petroleum Engineers. [This is a good textbook
on rotary drilling engineering.]
Gerwick, Jr. B.C. (1986). Construction of Offshore Structures. Baffins Lane, Chichester, Sussex PO19 1UD,
UK: John Wiley & Sons. [This book gives details of how offshore structures, particularly the
bottom-supported platforms, are constructed.]
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Silcox W.H., Bodine J.A., Burns G.E., Reeds C.B., Wilson D.L. and Sauve E.R. (1989). Chapter 18
Offshore Operations. Petroleum Engineering Handbook (editor-in-chief H.B. Bladley), pp. 18-1 – 18-52.
Richardson, TX 75083-3836, USA: Society of Petroleum Engineers. [This work provides a good overview
of offshore drilling and production systems and operations.]
API Standards and Publications. 1220 L Street, Northwest Washington, D.C. 20005-4070, USA. American
Petroleum Institute. [The series contain detail standards and information on rotary drilling method of
onland and offshore operations.]
OTC Proceedings. Richardson, TX 75083-3836, USA. Society of Petroleum Engineers. [The proceedings
published yearly provide up-to-date information on offshore engineering and operations.]
Proceedings of SPE/IADC Drilling Conference. Richardson, TX 75083-3836, USA. Society of Petroleum
Engineers. [The proceedings published yearly provide up-to-date information on drilling engineering.]
Biographical Sketches
Shoichi Tanaka is professor emeritus of the University of Tokyo in Tokyo, Japan. He majors in drilling
engineering and petroleum engineering. He holds a Doctor of Engineering in mining engineering from the
University of Tokyo. He was with the University of Tokyo from 1960 to 1995.
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Yo Okada is general manager of Petroleum Engineering & Consulting Dept. with Japan Oil Engineering
Co. (JOE) in Tokyo and heads a pool of engineers and scientists, providing a range of technical services to
various clients in the petroleum industry, financial institutions and investors. He joined JOE in 1975 and has
worked in numerous projects in varying capacities since then. Typical projects include offshore field
construction and maintenance, field development project coordination and management, field facility
technical assessment, field development feasibility studies including economic evaluation, technology
surveys and technical seminars. He holds a BSc in mining engineering from the University of Tokyo and an
MA in economics from Vanderbilt University, Nashville, Tennessee.
Yuichiro Ichikawa is currently working concurrently as general manager of Methane Hydrate
Development Division of Tokyo head office of Japan Drilling Co. (JDC). He is responsible for drilling
operations, drilling engineering and offshore engineering services to the drilling industry and governmental
bodies. He joined JDC in 1977 and has worked in numerous projects in varying capacities since then. Areas
of engineering expertise - Deepwater Drilling Rig Design, Deepwater Location Surveys, Deepwater Well
Planning, Deepwater Subsea Well Control, Coring Technology, Downhole Tools Development, Hydrate
Drilling, Safety Management, Information Management, Logistics Support and Deepwater Offshore
Drilling. He holds a BSc in petroleum engineering from the University of Tokyo.
To cite this chapter
S. Tanaka, Y. Okada, Y. Ichikawa, (2005), OFFSHORE DRILLING AND PRODUCTION
EQUIPMENT, in Civil Engineering, [Eds. Kiyoshi Horikawa, and Qizhong Guo], in Encyclopedia of Life
Support Systems (EOLSS), Developed under the Auspices of the UNESCO, Eolss Publishers, Oxford ,UK,
[http://www.eolss.net]
©Encyclopedia of Life Support Systems (EOLSS)