Topic

Field Development Concept

PRODUCTION ENGINEERING II

PCB 3073

Semester: January 2015

1. Differentiate between various types of offshore structure.

2. Analyze the HSE requirement for oil and gas fields operation.

3. Propose the suitable processes and treatment equipment to

manage produced oil, gas and water.

4. Perform standard engineering calculations for the design of

separator, storage tanks and pipelines.

COURSE LEARNING OUTCOMES (CLO)

At the end of this lecture, students should be able to:

Theory and description of oil platform

Identify different types of oil platforms

Differentiate between various types of offshore structures

LECTURE OUTCOMES

• Platform size depends on facilities to be installed on top

side i.e. Oil rig, living quarters, Helipad etc.

• Classification of water depths are commonly divided as

follows:

< 350 m - Shallow water

< 1,500 m - Deep water

> 1,500 m - Ultra deep water

SELECTION OF DEEP-WATER

PRODUCTION CONCEPTS

BASIC TECHNICAL PRINCIPLES

Stability

The formula for the metacentric height GM (definition of static stability) is given as:

GM = CB + I/V - CG

where CB = Centre of buoyancy, I = area moment of inertia, V = volume and CG

= center of gravity.

Metacentric height (GM): The metacentric height is the distance between the center

of gravity of an offshore structure and its metacenter.

Metacenter: Metacenter, the theoretical point at which an imaginary vertical line

passing through the centre of buoyancy and centre of gravity intersects the imaginary

vertical line through a new centre of buoyancy created when the body is displaced,

or tipped, in the water, however little.

Centre of buoyancy (CB): The center of buoyancy is the center of mass of the

displaced water.

Center of gravity: The center of gravity is a geometric property of any object.

The center of gravity is the average location of the weight of an object.

Excitation forces

The drag forces due to wind and current will cause the column to move

laterally (giving an offset) while the inertia-dominated forces on the column due

to the wave particle accelerations will cause column dynamic excursions. Since

water particle accelerations and velocities rapidly decline with distance from the

water surface, these drag and inertia forces can be reduced by shifting the

structure displacement away from the water surface.

Response of the unit to Excitation forces

The physical properties that affect the response of a linearly elastic

structural system subjected to an external source of excitation or loading are its

mass, elastic properties, and energy-loss mechanism, or damping.

BASIC TECHNICAL PRINCIPLES

PILE SOIL INTERACTION Deck

Riser: Pipes used for production, drilling, and export of oil and

gas from seabed. Riser remains in tension due to self weight.

There are two types of riser – Rigid, and Flexible.

Mooring System: Mooring systems have been around just as

long as man has felt the need for anchoring an offshore structure

at sea. It is important to reduce the excursion of a floating

structure due to the action of the wind, current and waves and

keep it as close as possible to its required position without

creating high restraining forces in the mooring lines.

Payload: Payload is the carrying capacity of an offshore

structure, usually measured in terms of weight.

Hull: Columns and pontoons

SOME IMPORTANT DEFINITION

Conductor Pipe: The Conductor Pipe is a large diameter pipe

that is set into the ground to provide the initial stable structural

foundation for a borehole or oil well. It can also be referred to as

a drive pipe because it is often driven into the ground with a pile

driver.

It is typically set, on petroleum wells, before any drilling

operations are performed. It is usually set with special pile-

driving or spudder rigs, though the drilling rig is sometimes used

to save time and money.

SOME IMPORTANT DEFINITION

EXAMPLE

A floating offshore platform consist of a square deck and a cylindrical shell

column. Column contains ballast to give the structure sufficient stability. The

keel of the structure is 240ft below the water. Illustration of parameters

affecting the metacenter is given in TABLE 1. Calculate the metacentric

height of the structure using the following formula:

The formula for the metacentric height GM (definition of static stability) is

given as:

GM = CB + I/V - CG

where CB = Centre of buoyancy (ft)

I = area moment of inertia (ft4)

V = volume (ft3)and

CG = center of gravity (ft)

Description Variable Comment

Square Deck

1. Weight (tons) 4000

2. CG elevation (ft) 260 above keel

Cylindrical Column

1. Diameter (ft) 40.0

2. Length (ft) 250

3. Steel weight (tons) 4000 includes compartmentation

4. CG elevation (ft) 120 above keel

5. Ballast weight (tons) 8080 sea water of (heavy ballast)

6. CG elevation (ft) 50 above keel

Stability

1. I (ft4) 250,000

Table 1: Illustration of parameters affecting the metacenter

Offshore Structure

Fixed

Floating Bottom Supported

Compliant Neutrally Buoyant Positively Buoyant

Jacket Based

Gravity Based

Guyed Tower

Compliant Tower

Semi-submersed

based FPSs

Ship-shaped FPSOs

Mon-column Spars

Tension Leg

Platform (TLP)

Articulated Platforms Minimal Semi-submersed

Built on concrete or steel legs or both.

Anchored directly onto the seabed

Designed for long term use

Installed in water depths up to 1700ft

Handles production up to 50,000 bbl/d

FIXED PLATFORMS

MINIMAL PLATFORMS

For the marginal field development in shallow water, fixed

production platforms with a small deck are often used.

At a minimum these structures may support the following: (1) a

few wells typically less than 10: (2) a small deck with enough

space to handle a coil tubing or wireline unit; (3) a test separator

and a well header; (4) a small crane; (5) a boat landing; and (6) a

minimum helideck.

Chevron carried out a study to identify and select, among

existing production platform concepts, the ones that would

optimise the development of fields in 150 ft (46 m) and 200 ft

(61 m) of water, and for three different design return periods (25,

50 and 100 years).

Three types

Tripod (300-433 ft)

Caissons and Braced Caissons (90-240 ft)

Monotower (120-180 ft)

MONOPOD OIL PLATFORM

TRIPOD OIL PLATFORMS

• Space framed structure with tubular members supported on piled foundations.

• Used for moderate water depths up to 400 M.

• Jackets provides protective layer around the pipes.

• Typical offshore structure will have a deck structure containing a Main Deck, a

Cellar Deck, and a Helideck.

• The deck structure is supported by deck legs connected to the top of the piles. The

piles extend from above the Mean Low Water through the seabed and into the soil.

• Underwater, the piles are contained inside the legs of a “jacket” structure which

serves as bracing for the piles against lateral loads.

• The jacket also serves as a template for the initial driving of the piles. (The piles

are driven through the inside of the legs of the jacket structure).

• Fixed jacket structures consist of tubular members interconnected to form a three-

dimensional space frame. These structures usually have four to eight legs battered

to achieve stability against toppling in waves.

• These platforms generally support a superstructure having 2 or 3 decks with

drilling and production equipment and workover rigs.

JACKET STRUCTURES

FULMAR JACKET PLATFORM

2-3 Decks

4-8 Legs

Pile

Jacket Structure

Fixed-bottom structures made from concrete – Heavy and

remain in place on the seabed without the need for piles

Used for moderate water depths up to 300 M.

Part construction is made in a dry dock adjacent to the sea. The

structure is built from bottom up, like onshore structure.

At a certain point , dock is flooded and the partially built

structure floats. It is towed to deeper sheltered water where

remaining construction is completed.

After towing to field, base is filled with water to sink it on the

seabed.

Concrete gravity structures can handle production up to 200,000

bbl/D

GRAVITY BASE STRUCTURES

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.

The jackups are used for the exploratory drilling operation and, therefore, are

designed to move from site to site. The jack-up barges are towed while supported

by the buoyancy of their own hull. Sometimes, they are transported on top of

transport barges.

They are called jack-ups because once at the drilling site, the legs are set on the

ocean bottom and the deck is jacked up on these legs above the waterline. The

jack-up barges behave like the stationary platform during the drilling operation.

Typically used in water depths up to 500ft

JACK-UPS

COMPLIANT STRUCTURES

The definition of a compliant structure includes those

structures that extend to the ocean bottom and directly

anchored to the seafloor by piles and/or guidelines.

These structures are 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.

Three types:

Articulated Platforms

Compliant Tower

Guyed Tower

ARTICULATED PLATFORMS

• An articulated tower is an upright tower, which is hinged at its

base with a cardan joint and is free to oscillate 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).

• The articulated tower is used as a single-point mooring system

(SPM) to permanently moor storage and production tankers or is

utilised as a mooring and offloading medium for a shuttle tanker.

• The tower must survive its lifetime storm as well as the operating

sea when attached to the tanker. Fatigue is an important criterion

for this type of system.

o A compliant tower is similar to a traditional platform and

extends from surface to the sea bottom, and it is fairly

transparent to waves.

o However, unlike its predecessor, a compliant tower is

designed to flex with the forces of waves, wind and

current.

o It uses less steel than a conventional platform for the same

water depth.

o Typically used in water depths ranging from 1,500 to 3,000

feet

COMPLIANT 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

guylines.

A guyed tower may be applicable in

deep hostile waters where the loads on

the gravity base or jacket-type

structures from the environment are

prohibitively high.

The guylines typically have several

segments.

Depth: 1,200 - 3,000ft

GUYED TOWER

PROGRESSION OF FIXED PLATFORMS IN

THE GOM - DEPTHS IN METERS

(COURTESY SHELL)

SEMI-SUBMERSIBLES

They have hulls (columns and pontoons) of sufficient buoyancy

to cause the structure to flow, and sufficient weight to keep it

upright

Partially submerged and movable

Can be ballasted up or down (i.e. buoyancy tanks)

Anchored by combinations of chain, wire rope or polyester

rope, or both.

Stabilized by anchoring and ballasting

Can also be positioned by using dynamic positioning

Water Depth: 200 – 10,000ft

A more popular design for

semisubmersible rigs is the

column-stabilized semisub.

Here, two horizontal hulls

are connected via cylindrical

or rectangular columns to the

drilling deck above the

water.

Smaller diagonal columns

are used to support the

structure.

o Large ships of varying

offshore operations’

applications: Floating Storage

Unit (FSU), Floating

Production, Storage and

Offloading (FPSO), Floating

Storage and Off-loading (FSO)

o FPSOs consist of large

monohull structures equipped

with processing facilities

o They are moored to a location

for long duration

o Water depth: 650 – 6,500ft

FLOATING PRODUCTION SYSTEMS

FLOATING STORAGE UNIT (FSU)

The FPSO generally refers to ship-shaped structures with

several different mooring systems.

Early FPSOs in shallow waters and in mild environment had

spread mooring systems.

As more FPSOs were designed and constructed or converted

(from a tanker) for deep-water and harsh environments, new

more effective mooring systems were developed including

internal and external turrets.

Some turrets were also designed to be dis-connectable so that

the FPSO could be moved to a protective environment in the

event of a hurricane or typhoon.

FLOATING PRODUCTION, STORAGE

AND OFFLOADING (FPSO)

FPSO

FLOATING STORAGE AND

OFFLOADING (FSO)

Spar Platforms • A spar is a vessel with a circular cross-section that sits vertically in

the water and is supported by buoyancy chambers (hard tanks) at the

top, a flooded midsection structure hanging from the hard tanks, and

a stabilizing keel section at the bottom.

• Some unique features of a spar include favorable motion

characteristics compared with other floating systems, stability (the

center of buoyancy is above the center of gravity), cost insensitivity

to water depth, and water-depth capability up to 10,000 ft (3,048 m)

and beyond.

• Moored to the seabed like Tension Leg Platforms (TLPs)

• While a TLP has vertical tension tethers, a spar has more

conventional mooring lines

• More stable than the TLPs

• There are three (3) design configurations:

1. Conventional/ Classic spar

2. Truss spar

3. Cell spar

Cell SPAR

Tension Leg Platforms (TLPs) are floating facilities that are tied down to

the seabed by vertical steel tubes called tethers.

This characteristic makes the structure very rigid in the vertical direction

and very flexible in the horizontal plane. The vertical rigidity helps to tie

in wells for production, while, the horizontal compliance makes the

platform insensitive to the primary effect of waves.

Have large columns and Pontoons and a fairly deep draught.

TLP has excess buoyancy which keeps tethers in tension. Topside

facilities, no. of risers etc. have to fixed at predesign stage.

Used for deep water from 2000 ft to 5000 ft

It has no integral storage.

It is sensitive to topside load/draught variations as tether tensions are

affected.

The first TLP was installed in Hutton Field in about 148 m water depth

Three types

1. Seastar Mini TLP

2. Moses Mini TLP

3. Buoyant Leg Structure (BLS)

Tension Leg Platform (TLP)

• 4,300 ft. water depth

• 13,700 tons of topsides payload

• 400 MMscfd

• 120,000 BOPD

• 1,200 HP Work over Rig

• 50,000 BWPD

• 6 Dry Surface Trees

• 8 (28”) O.D. Tendons

SeaStar MiniTLP for Typhoon Field Moses MiniTLP for the Prince Field

SeaStar and Moses MiniTLP

Buoyant Leg Structure (BLS)

System design covers all aspects of:

•Topsides (structures and process)

•Hull

•Mooring system

•Riser system

•Subsea components

Expensive – anywhere from $300M to $2B

Design must cover all aspects of system life including

installation and decommissioning

Integrated Development Systems

Water depth

Payload

Production Characteristics – Well Access Requirements

Availability of Infrastructure & Market location

Platform drilling, predrilling vs. post drilling

Gas Disposal Requirements

Local Content Requirements

Field Life

Metocean (meteorology & oceanography) Conditions.

Primary Drivers for Deep-water FPUs

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