Introduction to offshore structure

INTRODUCTION TO OFFSHORE STRUCTUREINTRODUCTION TO OFFSHORE STRUCTURE

Searching for oil over water using seismology

ANATOMY OF OIL RIG

MAJOR SYSTEMS OF LAND OIL RIG

Power system

electrical generators 

large diesel engines 

Mechanical system

  hoisting system 

turntable 

Rotating equipment 

Hoisting System: The hoisting system is used to raise and lower pipe in and out of the hole and to support the drill string to control the weight on the drill bit during drilling. It also acts as an elaborate pulley to lift the travelling block and remove the drill pipe.

Rotating equipment - used for rotary drilling

swivel - large handle that holds the weight of the drill string; allows the string to rotate and makes a pressure-tight seal on the hole

drill string - consists of drill pipe (connected sections of about 30 feet (10 meters) and drill collars (larger diameter, heavier pipe that fits around the drill pipe and places weight on the drill bit)

turntable or rotary table - drives the rotating motion using power from electric motors

drill bit(s) - end of the drill that actually cuts up the rock; comes in many shapes and materials (tungsten carbide steel, diamond) that are specialized for various drilling tasks and rock

Casing - large-diameter concrete pipe that lines the drill hole, prevents the hole from collapsing, and allows drilling mud to circulate

Drilling mud:

used to stabilize the excess pressure created due to fluid in the

formation

used to cool drill bit

remove rock chips

Derrick - support structure that holds the drilling apparatus; tall enough to allow new sections of drill pipe to be added to the drilling apparatus as drilling progresses

Blowout preventer - high-pressure valves (located under the land rig or on the sea floor) that seal the high-pressure drill lines and relieve pressure when necessary to prevent a blowout (uncontrolled gush of gas or oil to the surface, often associated with fire)

DRILLING PLATFORMS

Types of offshore drilling platforms:

Movable drilling rig-(MODU-mobile offshore drilling unit)

exploration purposes

Eg, drilling barge, jack-up barges, ,submersibles ,semi-submersibles ,FPSO (floating, production, storage and offloading systems), drill ships

Permanent drilling rig-

extraction and production

Eg, jacket platform, gravity platform

Complaint platform:

tension leg platform

guy towers

articulated towers

Questions

1.Petroleum and natural gas deposits are found in -------- rocks in sea bed.

2.Oil and gas are made up of --------.

3.When was 1st offshore drilling began and where.

4Name one active location where many platforms exist

5.What is derrick?

6.List some functions of drilling mud.

7.Write few movable drilling platform names and draw free sketch of one

GRAVITY PLATFORM:Hibernia oil platform

GRAVITY PLATFORMS

The Hibernia Gravity Base Structure is the world's largest offshore oil platform

Caisson: void spaces used as storage compartment for crude oil or filled

with permanant Fe ore ballast

importance:

to provide buoyancy during construction/towing and later serve as oil storage space

Skirts:

improve foundation stability

act erosion resistance feature

provides transverse resistance to platform sliding

Troll A platform (condeep )

GRAVITY PLATFORM

GBS: gravity based structure-concrete, steel

Condeep (concrete deep water structure) :gravity base structure for oil platforms developed

and fabricated by Norwegian Contractors in Stavanger, Norway .

The Condeep is used for a series of production platforms introduced for crude oil and natural gas production in the North Sea and Norwegian continental shelf

Design: production

: large reinforced concrete bottom mounted structure

:uses its weight to resist environmental loads. If it fails ,it fails severely

:generally built in soft soil like Gulf of Mexico

Depth : medium--- up to 350m

Advantages

Support large deck loads

Possible reuse

Construction and testing may be completed before floating and towing to site

Large field , long term production

May have large storage capacity

More tolerant to overloading and sea water exposure .

Disadvantages

Cost increases exponentially with depth

Foundation settlement (sea-bed profile increases due to geological disturbances)

Subject to seafloor scour

May require more steel than steel jacket structure

Questions

1.Draw a neat sketch of one of gravity platform and mark the parts

2.What are CONDEEP.

3.What are essential features of gravity platform.

4.Give one field example of gravity platform.

5.What is the key word of gravity platform.

6.What is a caisson.

7.What is the use of skirts.

8.Give essential features of gravity platforms.

9.What are exclusive advantage of gravity platform.

10.State few demerits.

11.Show in sketch possible failure modes of foundation of gravity platforms

FIXED PLATFORM

Two main components:

substructure

superstructure (topside)

JACKET PLATFORM

Sketch of jacket platform

26

Superstructure:

supported on jacket structure

consist of drilling equipment, production equipment, gas turbine ,generating sets, gas flare stations

it weighs up to 40,000 tonnes

Substructure:

Jacket-tubular platform support structure

- It serves as guides for driving of piles –called template structure

- It supports and protects well conductors,

pumps, risers etc

- Suitable for water depths:150-250m

- Soil condition: clay-penetrable- because of piles

Most common: steel jacket platform

Substructure and jacket is fabricated from steel welded pipes and is pinned through to sea floor with steel piles which are driven through pile guides on outer members of jacket

The piles are thick steel pipes of 1 to 2m diameter and can penetrate as much as 100m into sea-bed

The jacket can weigh up to 20,000 tonnes

PARTS OF JACKET PLATFORM

Transition piece: connects topsides and jacket structure

Conductors: long hollow straight or curved tubes that embed into sea- bed and through which drilling for oil, gas or both is performed

Risers: long slender tubes that carry the crude or partially processed oil/gas to another location for further processing either to a land based facility or to another platform

Boat landing, Barge bumper and riser guard:

-for berthing of supply vessels

-for smooth berthing to facilitate a reduced vessel impact on the structure

Riser guard:Another protective structure that protects the oil/gas carrying risers.

They are designed for accidental vessel impacts

Launch truss:are provided on one side of jacket to facilitate the loading out on to the barge

Mud mat: bottom most framing of platform that helps resist natural forces of nature towards stability of platform before piles are driven through legs

ADVANTAGES

Support large deck loadsConstruction and testing may be

completed before floating and towing to site

Large field , long term productionPiles result in good stabilityLittle effect from sea floor scour

DISADVANTAGES

Cost increase exponentially with depth High initial and maintenance cost

Not reusable

Steel structure member subject to corrosion

Corrosion severe near the SWL

Questions

1.List the purpose for building an offshore structure2.What are the two main components of a fixed

platform3.Describe important features of jacket platform4.Draw a line diagram of jacket platform and mark

the parts5.What do you understand by the term template

structure6.What is the most suitable soil condition for a jacket

platform 7.What is the typical weight of a jacket platform in

tonnes8.Write a brief note on flare boom

QUESTIONS

1.Give advantages of jacket platform.

2.Give some demerits of jacket platform

3.Generally top-sides are connected to jackets by-----

4.Which component of the platform helps them from skidding from barge

5.What do you understand by mud mats.

6.With neat sketches ,show installation steps of jackets.

7.Jacket structure have high maintenance cost .Why?