lecture 4.pdf

Dr Sourav Poddar
Department of Chemical Engineering
National Institute of Technology Warangal
T.S., India

FIGURE 1 Schematic of conventional turboexpander process with no recycle to
demethanizer. Note that the one heat exchanger represents a network of exchangers.
(Adapted from Engineering Data Book, 2004e.)

FIGURE 2 Cold-residue recycle process for maximizing ethane recovery.

Three basic methods are used for removal of nitrogen
from natural gas:
✓ Cryogenic distillation
✓ Adsorption
✓ Membrane separation

TABLE 1 : Comparison of Nitrogen Removal Processes

FIGURE 3 NRU by use of two-column cryogenic distillation. Valves are J-
T valves.

FIGURE 5 Separating N2 from natural gas by use of membranes.

FIGURE 6 Schematic of an enhanced oil recovery (EOR) system.

NATURAL GAS COMPOSITION
COMPONENT VOL.%
• METHANE C1 82.75
• ETHANE C2 5.74
• PROPANE C3 3.82
• BUTANES C4 1.40
• GASOLINE C5+ 1.00
• CO2 & N2 5.00
• H2S 0.04

COMPARISION OF RECOVERY FROM
• ABSORPTION
REFRIGERATION
Recovery.
• ETHANE. 20-40%
• PROPANE 70-90%
• BUTANE 100%
• NATURAL
• GASOLINE 100%
• CRYOGRNIC
REFRIGERATION
Recovery.
• 60-90%
• 90-98%
• 100%
• 100%

CONCLUSION
• You can see from the comparision that the main
difference is in the ethane recovery.
• Ethane is a useful feedstock in petrochemical plants.
• With methane,it releases 75% more heat than
methane when used as fuel gas. But ethane is worth
more as petrochemical feed stock.

FUNCTIONAL SECTIONS OF A
CRYOGENIC PLANT
• Cryogenic plant has four functional sections.
• Dehydration to remove moisture from gas.
• Chilling and expansion to liquefy part of the gas.
• Demethanizing to remove methane which liquefies.

CRYOGENIC OPERATIONS
• Compression to boost pressure to sales gas pipeline pressure.
• Root the liquid to light end fractionating tower t1 to remove C1 and
C2.
• Bottoms of t1 to LPG tower to separate LPG and natural
gasoline(NGL).
• LPG to spheres and NGL to tanks for storage.

DEHYDRATION UNIT
• Gas contains moisture and all of it is to be removed
prior to its temperature reduction to sub zero.
Moisture is removed in a dry desiccant adsorption
system which is a fixed bed tower with desiccant
such as silica-gel, molecular sieves, etc.

DEHYDRATION UNIT
• Conventionally, in all gas processing industries
world wide molecular sieves are being extensively
used today for its higher selectivity in the order of
H2O, H2S and other heavy hydrocarbons but not
CO2.

MOLECULAR SIEVES
ADSORPTION(FLUID-SOLID)
• Gas flows from top to bottom in the fixed bed
vertical dryer where moisture gets adsorbed by
the molecular sieves which are nothing but
alumina silicates in pellets form and reduces the
moisture conc. to less than 4ppm.This in turn,
suppresses the moisture dew point and
maintains it around –700C.

ADSORPTION CYCLE
• After an optimum time cycle, a reversal in dew
point is observed, i.e. as the partial pressure of the
moisture content in the gas increases due to
saturation, the dew point starts increasing towards
positive side which means the dew formation will
take place at much higher temperature when gas
is chilled to –500C leading to ice formation choking
the pipe line.

REGENERATION OF THE DESICCANT
• After an optimum time period of about 10 hrs the
tower is changed to stand by one and the former
will be regenerated by hot fuel gas and cooled to
normal temp. This cycle takes about 10 hrs.

EXPANSION-COMPRESSION PROCESS
• The turbo expander-compressor is a devise for
chilling the inlet gas and re-compressing the outlet
gas. The expander is simply a gas turbine. Gas
enters at a high pressure and drives the expander
with its own kinetic energy, expands to lower
pressure and thus gets chilled to as low as –700C
resulting in condensation of the gas partially.

• The compressor is a conventional centrifugal type
that is on the same shaft as the expander. The
energy required to drive the compressor is the
same as the energy given up by the gas passing
through the expander.

THEORY OF GAS EXPANSION
• Gas at a temperature above absolute 0 and a
pressure about absolute 0 contains internal energy.
We cant see the energy but we know it exists
because it produces work. Energy is contained in
the gas in the form of heat, pressure ,or velocity.

• One of the basic laws of science(first law of
thermodynamics) is that energy contained in a
fluid cannot be either created or destroyed. Thus,
if energy is added to or removed from a gas, there
must be a change in heat content(temperature) or
pressure or velocity.

JOULE-THOMPSON EFFECT
• Gas in a cryogenic plant has been cooled to a
lower temp. of about –400C by reducing the
pressure from say 60kg/cm2 to 30kg/cm2 by using
a choke or a pressure reduction valve.
The internal energy of the gas does not change
when its pressured is lowered because no heat is
entering or leaving the

• Pressure reduction valve, and no work is being
done to turn a turbine or other devise, so the
internal energy of the gas has to remain the same.
• The temperature drop which occurs when the
pressure is reduced is called the joulethompson (j-
t) effect.
Note:
In a typical expansion-compression process, of the
total heat removed from the gas, 10-15% occurs in
the expander and for the balance removal takes
place in the heat exchangers.

• If the latent heat to be further removed we have to
go for additional refrigeration in addition to exp-
comp.
• The liquid which condenses when the inlet gas is
chilled is a mixture of methane, ethane, propane,
butane and gasoline(C5+). The methane whose
volume is maximum, is to be removed from the
others for them to be of commercial value.

LIGHT END FRACTIONATION
TOWER.
• In this fractionation tower methane and ethane
are boiled out and goes out as top product at a set
temp. and pressure while the remaining goes out
as bottom product or feed stream to next
fractionation tower. Efficient operation ensures
minimum loss of propane from the top. Heat duty
to boil out the top streams is supplied by medium
pressure steam in the reboiler.

PARAMETERS AND CONDITIONS
PREVAILING IN LEF TOWER:
• TOP PRESSURE: 27.0 KG/CM2
• TOP TEMP. : 0-1.0 0C
• BOTTOM TEMP : 100-105 0C
• NO. OF TRAYS : 35 BUBBLE CAP
TOP PRODUCT ANALYSIS (BY VOL.)
METHANE 65%
ETHANE 25%
PROPANE 1.5-2.0%
REMAINING: 7.0-8.0% CO2.

• Over head condenser is a shell and tube partial
con-denser to condense only heaviers and use
propane as the chilling medium.
• Reboiler is of kettle type which maintains liquid
level on a level control valve allowing the excess
liquid to the next tower which is called the LPG
fractionator. M. P. steam is used as the heating
medium.

LPG FRACTIONATION TOWER.
• In this section by setting appropriate temp. and
pressure at the top and bottom of the tower
recovery of maximum propane and butane mixture
is facilitated. The by-product from the bottom is
called natural gasoline or NGL.
• The heating media in the reboiler is M. P. steam
and the reboiler type is thermo syphon where all
the liquid is vaporised and put back into the
column.

PARAMETERS IN THE LPG TOWER
TOP PRESSURE: 11.0 KG/CM2
TOP TEMP. : 50.00C
BOTTOM TEMP.: 150.00C
NO.OF TRAYS : 35(BUBBLE CAP)
ANALYSIS OF THE PRODUCTS
LPG PRODUCT(TOP): C3- 55%
C4- 44.55%
BOTTOM PRODUCT(NGL): C5+
C6,C7,…

PRODUCT STORAGE
• LPG storage: spheres since it is stored in vapor-
liquid form the stresses exerted by the fluid will be
distributed uniformly radially all along the
spherical surface.
storage pressure: 10.5kg/cm2
temperature : 30.00C
Storage of NGL: floating roof tanks at 1.0 kg/cm2
and 30.00C

ADDITIVES: MERCAPTANS
• Since LPG is odourless some additive is to be
administered for any leakage.
• Any sulphurous compound in controlled quantities
is added such as: RSH where ‘R’ is any methyl
group, ‘S’ is sulphur and ‘H’ is hydrogen. qty:
approx 30 ppm

CRITICAL COMMERCIAL
PROPERTIES OF C3-C4
PROPERTY PROPANE BUTANE LPG
(50%MIX)
SP.GR-LIQ 0.504 0.582 0.543
AT 15o C
SP.GR-VAP 1.5 2.01 1.75
VAP.PR(38DEG) 13.8 2.60 8.00
BOILING.PT(DEG) -42.0 0-9.0 -25
(AT ATM.PR)
AUTO IGN.TEMP. 495-605 480-535 480-605
LAT.HEAT.VAP(BTU/LB) 184 167 175

REID VAPOR PRESSURE OF LPG
• Vap.pr. of LPG is important and is found by Reid’s
VAP. pressure apparatus.
RVP(reid vap.pr.) of LPG:
at 650C :10-26 kg/cm2
at 380C : 8.0 kg/cm2
Volatility :this is also called weathering and done at
95% evaporation of a sample of LPG and it should
fall in the range of –2.0 to +2.00C for optimum
evaporation.
Calorific value of LPG:12000kcal/SM3.

LPG FROM NATURAL GAS-
ITS CHALLENGES
• The production and consumption of natural gas is
on the rise world wide as a result of its wide
availability, ease of transportation and use and
clean burning charecteristics. The emerging
commodity nature of natural gas has created
tighter competition among the natural gas
processors .

• Processing rights, resulting in increasingly narrow
operating margins between the processing costs
and the market price for which the recovered
liquids can be sold.
• Carbon dioxide(CO2 ) is a particularly trouble some
contaminant found in natural gas. Many NGL
recovery processes require CO2 removal to avoid
solid ice formation in the cold sections of the
processing plant.

• Instead of going for the CO2 removal equipment,
which will increase the equipment cost and the
operating cost, CO2 tolerant process are actively
envisaged in the NGL and LPG recovery facility.

• Within the liquids recovery facility, there are both
operating costs and operating flexibility issues that
directly impact the processing costs.
• While it is easily recognized that the efficiency of
the selected liquids recovery process is an
important factor in the processing cost.

• Process to either recover or reject ethane
without sacrificing the efficiency or propane
recovery is often the critical factor in determning
the profitability of a gas processing plant.
• With the historically cyclic nature of ethane values
as a petrochemical feed stock, it is absolutely vital
for the gas processors to be able to quickly
respond to the changing market conditions.

• To maximise profits. When the value of the ethane
as a liquid is high, maximum ethane recovery for
the gas processor gives more income. When the
value of ethane as a liquid is low, selling the
ethane in the residue gas for its BTU value gives
higher income, so efficient rejection of ethane
with out sacrificing propane recovery is the key to
plant profitability when the liquids market is
depressed.

NATURAL GAS LIQUIDS RECOVERY
• Most natural gas is processed to remove the heavier hydrocarbon
liquids from the natural gas stream.
• These heavier hydrocarbon liquids, commonly referred to as
natural gas liquids (NGLs), include ethane, propane, butanes,
and natural gasoline (condensate).
INTRODUCTION
• Recovery of NGLcomponents in gas for dew point control &
yields a source of revenue.
• Lighter NGL fractions, such as ethane, propane,and butanes, can
be sold as fuel or feedstock to refineries and petrochemical
plants, while the heavier portion can be used as gasoline-
blending stock.

Gas Condensate Reservoirs
• Gas condensate reservoirs have been defined as those hydrocarbon
reservoirs that yield gas condensate liquid in the surface separator(s).
• A retrograde gas condensate reservoir is one whose temperature is below
the cricondentherm (the maximum temperature at which liquid and vapor
phases can coexist in equilibrium for a constant-composition multicomponent
system).
• As pressure decreases below the dew point due to production, a liquid
phase develops within the reservoir, which process is called retro grade
condensation.

Figure 1: shows a pressure-temperature phase
diagram.

Options of Phase Change
• To recover and separate NGL from a bulk of gas stream, a
change in phase has to take place. In other words, a new phase
has to be developed for separation to occur. Two distinctive
options are in practice depending on the use of ESA or MSA.
Energy Separating Agent
Mass Separating Agent
• To separate NGL, a new phase is developed by using either a
solid material in contact with the gas stream (adsorption) or a
liquid in contact with the gas (absorption).
(Refrigeration)
(Distillation)

Parameters Controlling NGL Separation
• Operating pressure, P
• Operating temperature, T
• System composition or concentration, x and y
To obtain the right quantities of specific NGL constituents, a
control of the relevant parameters has to be carried out:
1. For separation using ESA, pressure is maintained by direct control.
Temperature, on the other hand, is reduced by refrigeration using
one of the following techniques:
(a) Compression refrigeration
(b) Cryogenic separation; expansion across a turbine
(c) Cryogenic separation; expansion across a valve

2. For separation using MSA, a control in the composition or the
concentration of the hydrocarbons to be recovered (NGL); y and x is
obtained by using adsorption or absorption methods.
• The efficiency of condensation, hence NGL recovery, is a function of
P, T, gas and oil flow rates, and contact time. Again, absorption
could be coupled with refrigeration to enhance condensation.
• A proper design of a system implies the use of the optimum levels of
all operating factors plus the availability of sufficient area of contact
for mass and heat transfer between phases.
In Summary

Figure 2:Thermodynamic pathways of different NGL recovery technologies.

Mechanical Refrigeration
Mechanical refrigeration is the simplest and most direct process
for NGL recovery. of condensate are expected. This process may
also lead to the recovery of liquified petroleum gas, where for
LPG recovery up to 90%, a simple propane refrigeration system
provides refrigeration at temperatures to −40o F.
Flow sheet of a mechanical refrigeration process

Salient Features
• propane as the refrigerant
• gas-to-gas heat exchanger recovers additional refrigeration
• The temperature of the cold gas stream leaving this exchanger
“approaches” that of the warm inlet gas
• The chiller in is typically a shell and tube, kettle-type unit
• The refrigerant (often propane) boils off and leaves the
chiller vapor space essentially as a saturated vapor.
• The thermodynamic path followed by the gas in an external
refrigeration process is shown as line ABC in Figure 2. From A
to B indicates gas-to-gas heat exchange; from B to C, chilling.
• Hydrate formation is prevented either by dehydration of the
gas or by injection of a hydrate inhibitor.

Choice of Refrigerant
• Any material could be used as a refrigerant. The ideal refrigerant is
nontoxic, non-corrosive, has Pressure-Volume-Temperature (PVT) and
physical properties compatible with the system needs, and has a high
latent heat of vaporization.
• The practical choice reduces to one, which has desirable physical
properties and will vaporize and condense at reasonable pressures at the
temperature levels desired.
Cascade Refrigeration
• Cascade refrigeration refers to two refrigeration circuits thermally
connected by a cascade condenser, which is the condenser of the low-
temperature circuit and the evaporator of the high-temperature circuit.
• A cascade system utilizes one refrigerant to condense the other primary
refrigerant, which is operating at the desired evaporator temperature.
This approach is usually used for temperature levels below −90◦F, when
light hydrocarbon gases or other low boiling gases and vapors are being
cooled.

Mixed Refrigerants
• An alternative to cascade refrigeration is to use a mixed refrigerant. Mixed
refrigerants are a mixture of two or more components. The light components
lower the evaporation temperature, and the heavier components allow
condensation at ambient temperature.
• The evaporation process takes place over a temperature range rather than
at a constant temperature as with pure component refrigerants. The mixed
refrigerant is blended so that its evaporation curve matches the cooling
curve for the process fluid.
• Heat transfer occurs in a countercurrent exchanger, probably an aluminum
plate fin, rather than a kettle-type chiller. Mixed refrigerants have the
advantage of better thermal efficiency because refrigeration is always being
provided at the warmest possible temperature.

Self-Refrigeration
Flow sheet of a self-refrigeration system
In this process, the nonideal behavior of the inlet gas causes the
gas temperature to fall with the pressure reduction, as shown by
line ABC’ in Figure2. The temperature change depends primarily
on the pressure drop.

• If the objective is to recover ethane or more propane than
obtainable by mechanical refrigeration, a good process can be
self-refrigeration, which is particularly applicable for smaller
gas volumes of 5 to 10 MMCFD.
• The self-refrigeration process is attractive if the inlet gas pressure
is very high. It is important that the reservoir pressure remain high
for the intended life of the plant.
• Low-pressure inlet gas favors a cryogenic refrigeration plant
or straight refrigeration process

Cryogenic Refrigeration
• Cryogenic refrigeration processes traditionally have been used
for NGL recovery.
• These plants have a higher capital cost but a lower operational
cost.
• In the cryogenic or turboexpander plant, the chiller or Joule–Thomson
(JT) valve used in two previous processes is replaced by an expansion
turbine.
• The expansion process is indicated as line ABC” in Figure 2.
• The turbine can be connected to a compressor, which recompresses
the gas with only a small loss in overall pressure.

Typical flow sheet of a cryogenic refrigeration plant

Schematic of Ortloff gas subcooled process

Schematic of Ortloff residue split-vapor process

Simplified flow diagram of an oil absorption plant

Schematic of a solid bed adsorption plant

NATURAL GAS: OFFSHORE
PRODUCTION & HANDLING

Process of Offshore Oil and Gas Developments
The process of developing offshore oil and gas reserves can be divided into the
following major
steps:
1. Exploration
2. Exploratory drilling
3. Development drilling
4. Production
5. Storage and offloading
6. Transportation

FACTORS DRIVING DEEPWATER RUSH
1. Growing global demand for energy.
2. Traditional fields fast exhausting.
3. Declining production & reserves.
4. Pressure to diversify supply.
5. Oil supply jitters.
6. Energy economics.
7. Technological advent.

DEEP WATER TECHNOLOGIES
In order to meet the current demand for hydrocarbon based fuel,
the scout for it is widespread
with demanding impetus on technological innovations.
Problems associated with Offshore (deep water) areas are:
1. Reservoir characterization.
2. Reservoir management.
3. Source- rock prediction.
4. Formation water properties.
5. Granite reservoir characterization.
6. Pore pressure & temperature prediction.
7. Tidal waves
8. Corrosion
9. Wind
10. Fatigue
11. Salinity
12. Thermal shock (steep gradient, seasonal
change, fluid injection)

Factors affecting field services in deep water on a macro-basis can be
given as:
1. Unconventional oil (tar sands) vs. deepwater.
2. Novel Deepwater technology trends.
3. Drilling technologies.
4. Subsea technologies.
5. Forecast for deepwater oilfield services.
6. Hydrate formation.
7. High temperature High Pressure.

Classification of problems encountered in general:
1. Deepwater projects take up to 10 years from discovery to first production.
2. Geology not cooperating (Like finding 100MMbl pockets when we used to find 500MMbl to
1bln barrel fields).
3. Cluster developments are expensive (five (100MMbl) fields do not equal one (500MMbl) field).
4. Escalating rig rates were a leading indicator for the cost increases across the sector for deepwater
developments.

To solve the deepwater issues it requires blend of many
technologies
like:
1. Reservoir geophysics
2. Seismic imaging
3. Seismic signal processing
4. 3D seismic characterization of reservoirs
5. Multi-component seismology
6. Time-lapse seismic
7. Seismic litho stratiography
8. Imaging while drilling
9. Ocean sensor arrays

Technologies for deepwater drilling.
Advances in a
number of key
technologies,
including 3D
seismic; directional
drilling, LWD and
MWD for extended
reach and
horizontal wells;
early, floating and
subsea production
systems; and subsea
completions
are providing the
tools necessary for
the industry to
explore in ever
deeper waters.

Location Surveys for Offshore Drilling
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 minimum requirement of the survey includes following instruments:
1. sparkers
2. sub-bottom profilers
3. side-scan sonar
4. fathometers
5. gravity corers
Wind and current measurements for several months would be carried out at a
proposed
location about one year ago before operations.

History of Offshore
1. 1st offshore well was drilled in 1947 in 15 feet of water in (Louisiana, USA).
2. 30 years ago, a deepwater operation implies exploring water depths up to 500 feet.
3. Today, deepwater refers to a well in up to 5,000 feet (1524m) of water .
4. Ultra-deepwater exploratory drilling now occurring in water depths over 5000 ft to 10,000
feet. i.e.,( 1524m to 3048m)
5. The challenges in ultra deep reserves are more complicated than exploring space.

Classification of water depths
❖Shallow water generally refers to a depth less than 1000ft (304.8m).
❖Deep water refers to a depth greater than 1000ft (304.8m) and less than
5000ft (1524m).
❖Ultra-deepwater refers to a depth greater than 5000ft (1524m).

Record depths achieved in Onshore/Offshore
Onshore
1. The scientific research well “SG-3” in Russia reached the depth of 12,263 m in
1988, has had the depth record ever since.
2. The deepest exploration drilling for hydrocarbons was carried out to the depth of
9583 m in the United States of America in 1974.
Offshore
1. A hydrocarbon exploration well was drilled offshore Brazil in 2965 m of water
in 2001.
2. 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 towards deeper and deeper seas to
search and produce subsea resources for the future welfare of the world.

As per SPE publication:
“Since 1947, the offshore industry has moved from the first platform out of
sight of land to safely producing in 7,000 feet (2,100 meters) of water and
safely drilling in 10,000 feet (3,050 meters) of water.”
The industry is still learning, and there is more to come…

Offshore Drilling Structures
All these factors make offshore rigs complex and sophisticated, and therefore offshore drilling
costs are higher than land drilling costs for similar depth wells. Technical Features of Offshore
Drilling
1. 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.
2. In addition, there are lots of specific services on board such as ROV, divers, meteorological
measurements, helicopter, etc.
3. Accommodations and catering for crews working for 24 hours are requiredon 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 basic types of offshore drilling rigs:
1. Rigs that can be moved from place to place, allowing for drilling in
multiple locations.
2. Rigs that are permanently placed.
Moveable rigs are often used for exploratory purposes because they are much cheaper to
use than permanent platforms.
Once large deposits of hydrocarbons have been found, a permanent platform is built to
allow their extraction.

Different types of moveable offshore platforms:
Rigs that can be moved from place to place, allowing for
drilling in multiple locations (Mobile bottom- supported
and floating rigs).
1. Drilling Barges.
2. Jack-Up Rigs.
3. Submersible Rigs (swamp barges).
4. Semisubmersible Rigs (Anchor-stationed or dynamically positioned).
5. Drillships (Anchor-stationed or dynamically positioned ).

Drilling structures used for developing offshore fields from
stationary platforms are of two types:
Rigs that are permanently placed.
1. Self-contained platforms: (The large production platform equips a
complete set of drilling equipment, and is called as self-contained
platform)
2. Tender or jack-up assisted platforms or well-protector jackets :The small
platform has a space only to accommodate derrick and drawworks, so a
kind of tender assists the work)

Guidelines
To choose roughly the type of offshore drilling rigs according to water depth and conditions of sea state and
winds:
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 .

Mobile Bottom-supported Structures
1. Jack-up Drilling Rigs (Jack-up Rigs, Self-elevating Drilling Rigs)
2. Submersible Drilling Rigs (Submersible Rigs, Swamp Barges)
3. Tender-Assisted Platforms and Tenders

Floating Offshore Structures (Floaters)
Neutrally buoyant structures which are dynamically unrestrained and are allowed to have 6 degrees of
freedom (heave, surge, sway, pitch, roll and yaw) are:
1. Drillships.
2. Semisubmersible Drilling Rig (centre of buoyancy is typically above the centre of gravity).
3. Spars (centre of gravity is greater than its centre of buoyancy, hence it is intrinsically stable).
Positively buoyant structures which are tethered to the seabed
and are heave-restrained are:
1. Tension Leg Platforms (TLPs)
2. Tethered Buoyant Towers (TBTs)
3. Buoyant Leg Structures (BLS)

❖For drilling as well as production these units are modified for dual function. (Excluding TLP and SPAR, because of
limited motions these are suitable for surface-completed wells only)
❖Example for Drilling, production & Storage in 1 unit is FPDSO (Floating Production Drilling Storage & Offloading)
(vessel motions is the only hesitation for its development)

General Classification of Structures:
(I) Bottom-Supported Structures
(II) Compliant Structures
(III) Floating Structures

4. Jack-ups:
The jack-up barges are typically three-legged structures having a deck supported on
their legs. The legs are made of tubular truss members. The deck is typically buoyant.
5. Subsea Templates:
Subsea technology covers a wide range of offshore activities. Examples are subsea
Xmas trees, manifolds, templates, flowlines and risers, control systems, well fluid
boosters, multiphase pumping and metering, water separation, water injection, remote
and diverless connections, guideline-free installations, seabed electrical power
distribution systems, interventions, etc.
6. Subsea Pipelines:
Subsea pipelines are used to transfer oil from the production platforms to storage
facilities or to the shore

There are two basic leg configurations of jack-up rigs:
1. 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.
2. 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 seabottom in water depth of up to 50 m. The
penetration is slight.

7. Submersible Drilling Rigs (Submersible Rigs, Swamp Barges)
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.
Ship-shaped submersible rigs are also used, which are called swamp barges.

8. Tender-Assisted Platforms and Tenders
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.
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, resulting in low downtime less than 2% of total operation time.

Compliant structure by definition includes those structures that extend to the ocean bottom and directly
anchored to the seafloor by piles and/or guidelines.
Typically designed to have their lowest modal frequency to be below the wave energy, as opposed to the
fixed structures, which have a first modal frequency greater than the frequency of wave energy.
1. Articulated Platforms:
One of the earliest compliant structures that started in relatively shallow waters and slowly moved into deep
water.
“Artculated tower is an upright towr, which is hinged at its base
with a cardan joint and is free to oscilate about this joint due to
the environment”
The base below the universal joint on the seabed may be a gravity base or may be piled.
The tower is ballasted near the universal joint and has a large enough buoyancy tank at the free surface to
provide large restoring force (moment).

2. Compliant Tower:
A compliant tower is similar to a traditional platform and extends from surface to the sea bottom, and it is fairly
transparent to waves.
Compliant tower is designed to flex with the forces of waves, wind and current. It uses less steel than a
conventional platform for the same water depth.
3. Guyed Tower:
A guyed tower is a slender structure made up of truss members, which rests on the ocean floor and is held in
place by a symmetric array of catenary guidelines.

Technologies Required by Floaters:
Outline of Drilling System of
Semisubmersible Rig (Modified from
Sekiyukaihatsu Gijutsu no Shiori (1st
edition). Reproduced Courtesy of
Japan Petroleum Development
Association)

Technologies Required by Floaters:
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.
Crown Mounted Type of Heave Compensator (Reproduced
Courtesy of National Oilwell - Kristiansand)

Production Units (FPSO and FPS)
Most floating production units are neutrally buoyant structures (which allows six-degrees
of freedom) which are intended to cost-effectively produce and export oil and gas.
1. FPSO:
The FPSO generally refers to ship-shaped structures with several different mooring
systems.
2. FPS:
FPS refers to Floating Production systems which are finding application in marginal
and deepwater field development.

Drillships
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

3. Semi-Submersible Platform:
Semi-submersibles are
multi-legged floating
structures with a large
deck. These legs are
interconnected at the
bottom underwater
with horizontal
buoyant members
called pontoons.
Semisubmersible Platform

A computer graphic of a semisubmersible installation.
A computer graphic of a
semisubmersible
installation.

The advantages of semisubmersibles include the following:
1. Semisubmersibles can achieve good (small) motion response and, therefore, can be more easily positioned over a
well template for drilling.
2. Semisubmersibles allow for a large number of flexible risers because there is no weathervaning system.
Disadvantages of semisubmersibles:
1. Pipeline infrastructure or other means is required to export produced oil.
2. Only a limited number of (rigid) risers can be supported because of the bulk of the tensioning systems required.
3. Considering that most semisubmersible production systems are converted from drilling rigs, the topsides weight
capacity of a converted semisubmersible is usually limited.
4. Building schedules for semisubmersibles are usually longer than those for shipshaped offshore structures.

Semisubmersible (As Drilling Rig)
Semi-submersibles are multi-legged floating structures with a large deck. These legs are interconnected at
the bottom underwater with horizontal buoyant members called pontoons.
Semisubmersibles have submerged pontoons (lower hulls) that are interconnected to the drilling deck by
vertical columns
The lower hulls provide improved stability for the vessel. Also, the open area between the 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.

Semisubmersible Drilling Rig
Semisubmersible Drilling Rig (JDC Hakuryu 3) (Reproduced
Courtesy of Japan Drilling Co.)

4. Spar:
The Spar concept is a large deep draft, cylindrical floating Caisson designed to support drilling and production
operations. Its buoyancy is used to support facilities above the water surface.
It is, generally, anchored to the seafloor with multiple taut mooring lines.
Because of the reduced heave motion, the use of rigid risers (instead of flexible risers), which are self-buoyant, is
easier.
Types of Spars:
1. Classic spar
2. Truss spar
3. Cell spar
(Cell spar is3rd generation spar. The hull consists of multiple ring-stiffened tubes, or “cells”,
which are connected by horizontal and vertical plates. This method of construction is
cheaper than the traditional plate and frame methods.)

5. Tension Leg Platform:
A Tension Leg Platform (TLP) is a vertically moored compliant platform. The floating platform
with its excess buoyancy is vertically moored by taut mooring lines called tendons (or tethers).
The structure is vertically restrained precluding motions vertically (heave) and rotationally (pitch and roll).
It is compliant in the horizontal direction permitting lateral motions (surge and sway).

A computer graphic of a tension leg platform (TLP) installation

5.1. MiniTLPs: SeaStar and Moses
SeaStar is a deepwater production and utility mini-platform.
SeaStar is a small TLP with a single surface-piercing column.
It borrows from the concept of the tension leg platform and provides a cost-effective
marginal field application.
Moses MiniTLP appears to be a miniaturized TLP as the deck structure is supported by four columns and
the columns are connected by pontoons.
Motion characteristics of Moses is similar to that of SeaStar and, unlike the standard TLPs, miniTLPs
need to dedicate a large percentage of their displacement (35 - 45%) for pretension.

Worldwide offshore rigs and offshore production growth.
Worldwide offshore rigs
and offshore production
growth. For more than
20 years, there has been
a direct relationship
between offshore
production and the
number of development
drilling rigs operating, a
trend that is expected to
continue well into the
21st century.

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