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Offshore Petroleum Platforms

what are offshore platforms?

offshore platformOffshore structures are used worldwide for a variety of functions and in a variety of water depths, and environments. Since right selection of equipment, types of platforms and method of drilling and also right planning, design, fabrication, transportation, installation and commissioning of petroleum platforms, considering the water depth and environment conditions are very important, this post will present a general overview of these aspects. This post reviews the fundamentals behind all types of offshore structures (fixed or floating) and, in the case of fixed platforms, will cover applications of these principles. The overall objective is to provide a general understanding of different stages of design, construction, loadout, transportation and installation of offshore platforms.

Offshore platforms have many uses including oil exploration and production, navigation, ship loading and unloading, and to support bridges and causeways.
Offshore oil production is one of the most visible of these applications and represents a significant challenge to the design engineer. These offshore structures must function safely for design lifetimes of twenty-five years or more and are subject to very harsh marine environments. Some important design considerations are peak loads created by hurricane wind and waves, fatigue loads generated by waves over the platform lifetime and the motion of the platform. The platforms are sometimes subjected to strong currents which create loads on the mooring system
and can induce vortex shedding.

see our Offshore Books section

Offshore platforms are huge steel or concrete structures used for the exploration and extraction of oil and gas from the earth’s crust. Offshore structures are designed for installation in the open sea, lakes, gulfs, etc., many kilometers from shorelines. These structures may be made of steel, reinforced concrete or a combination of both. The offshore oil and gas platforms are generally made of various grades of steel, from mild steel to high-strength steel, although some of the older structures were made of reinforced concrete.
Within the category of steel platforms, there are various types of structures, depending on their use and primarily on the water depth in which they will work.
Offshore platforms are very heavy and are among the tallest manmade structures on the earth. The oil and gas are separated at the platform and transported through pipelines or by tankers to shore.

offshore platforms

read also Offshore Drilling Rigs

Design of offshore fixed platforms

The most commonly used offshore platforms in the Gulf of Mexico, Nigeria, California shorelines and the Persian Gulf are template type platforms made of steel, and used for oil/gas exploration and production (Sadeghi 1989, 2001).
The design and analyses of these offshore structures must be made in accordance with recommendations published by the American Petroleum Institute (API).
The design and analysis of offshore platforms must be done taking into consideration many factors, including the following important parameters:
• Environmental (initial transportation, and in-place 100-year storm conditions)
• Soil characteristics
• Code requirements (e.g. American Institute of Steel Construction “AISC” codes)
• Intensity level of consequences of failure.
The entire design, installation, and operation must be approved by the client.

Environmental parameters

The design and analysis of fixed offshore platforms may be conducted in accordance with the API’s “Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms – Working Stress Design (API-RP-2AWSD)”.
The latest revision of API-RP-2A-WSD is the 21st edition dated December 2000. The API specifies minimum design criteria for a 100-year design storm.
Helicopter landing pads/decks on offshore platforms must conform to API RP-2L (latest edition being the 4th edition, dated May 1996).
Normally, for the analysis of offshore platforms, the environmental parameters include wave heights of as much as 21 meters (depending on the water depth) and wind velocities of 170 km/hr for Gulf of Mexico, coupled with tides of up to 4 m in shallow waters. The wave heights up to 12.2 meters and wind velocities up to 130 km/hr for the Persian Gulf, coupled with tides up to 3 m are considered in design of platforms (Sadeghi 2001).
The design wave height in the Southern Caspian Sea is about 19 m for a return period of 100 years, and for the North Sea is over 32 m depending on the location.
The API RP-2A also specifies that the lowest deck must maintain a minimum of 1.5 m air gap between the bottom of the deck beams and the wave crest during the maximum expected level of water considering the combination of wave height and tides.
The platform should resist the loads generated by the environmental conditions and loadout, transportation and installation loads plus other loads generated by onboard equipment.

A typical offshore structure supported by piles normally has a deck structure containing a Main Deck, a Cellar Deck, Sub-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 mudline 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 may also serve as a template for the initial driving of the through leg piles (The piles may be driven through the inside of the legs of the jacket structure). In the case of using skirt piles.
the piles may be driven from outside of the legs of the jacket structure. The structural model file consists of:
• The type of analysis, the mudline elevation and water depth.
• Member sizes
• Joints definition.
• Soil data (i.e. mudmat bearing capacity, pile groups, T-Z, P-Y, Q-Z curve points).
• Plate groups.
• Joint coordinates.
• Marine growth input.
• Inertia and mass coefficients (CD and CM) input.
• Distributed load surface areas.
• Wind areas.
• Anode weights and locations.
• Appurtenances weights and locations
• Conductors and piles weight and location
• Grouting weight and locations
• Load cases include dead, live and environmental loading, crane loads, etc.

Any analysis of offshore platforms must also include the equipment weights and a maximum deck live loading (distributed area loading), dead loads in addition to the environmental loads mentioned above, and wind loads. Underwater, the analysis must also include marine growth as a natural means of enlargement of underwater
projected areas subject to wave and current forces.
The structural analysis will be a static linear analysis of the structure above the mudline combined with a static non-linear analysis of the soil with the piles.
Additionally, checks will be made for all tubular joint connections to analyze the strength of tubular joints against punching. The punching shear analysis is colloquially referred to as “joint can analysis”. The Unity Checks must not exceed 1.0.
All structural members will be chosen based on the results of the computer-aided in-place and the other above-mentioned analyses. The offshore platform designs normally use pipe or wide flange beams for all primary structural members.
Concurrently with the structural analysis the design team will start the development of construction drawings, which will incorporate all the dimensions and sizes optimized by the analyses and will also add construction details for the field erection, transportation, and installation of the structure.
The platforms must be capable of withstanding the most severe design loads and also of surviving a design lifetime of fatigue loading. The fatigue analysis is developed with input from a wave scatter diagram and from the natural dynamic response of the platform, and the stiffness of the pile caps at the mudline by applying Palmgeren-Miner formula (Sadeghi 2001). A detailed fatigue analysis should be performed to assess cumulative fatigue damage. The analysis required is a “spectral fatigue analysis” or simplified fatigue analysis according to API.
API allows a simplified fatigue analysis if the platform (API 1996):
• Is in less than 122 m (400 ft) water depth.
• Is constructed of ductile steel.
• Has redundant framing.
• Has natural periods less than 3 seconds.

References:
1. Offshore Platform Design.
2. Installation of Petroleum Offshore Platform.

Offshore Books

offshore platform

a collection of books about:
Offshore oil and gas production – offshore drilling – deepwater drilling – subsea and others.
all you have to do is to choose the required book then press on Download Links under its name. enjoy!

Subsea Structural engineering Handbook
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Subsea Engineering Handbook
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 Subsea well Control
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a Primer Offshore Operation
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Validation of Blowout Rate Calculations for Subsea Wells
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Offshore Basins in General 56 MB
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Deepwater Cementing
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  Offshore Pipeline Laying
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Drilling and Producing Offshore 38 MB
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Offshore Construction Part.1Subsea Pipeline Engineering
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Offshore Construction Part.2 Subsea Pipeline Engineering
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  Offshore Drilling
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  Offshore Drilling PowerPoint
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Marine and Offshore Engineering
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   Dynamics of Offshore Structures
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  Global offshore oil drilling prospects
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  the Technology of Offshore Drilling, Completion & Production
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  Handbook of Offshore Engineering Part.1        Download

  Handbook of Offshore Engineering Part.2        Download


  Offshore Mooring and  Buoy Systems
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  Introduction to Offshore Platform
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  Offshore Diving and Submersibles
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   Offshore Structures
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  Offshore Support Systems
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  Oil & Gas Offshore Production
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   Oil & Gas Pipelines Repair in Offshore Platforms
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   Petroleum offshore Structures – Fixed Platforms
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  Offshore Platform Removal
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    Offshore Drilling Waste Treatments & Risk Management Plan
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     Offshore Asset Integrity
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   Offshore Drilling
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   Offshore Pipeline
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  Offshore Energy Capabilities
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  Offshore Safety Management
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    Jack Up Rig Operational Aspects Offshore
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   Recovery Optimization in a Multi-Reservoir Offshore Gas Field
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  Offshore Blowouts Causes and Control
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Construction of Marine and Offshore Structures
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  Offshore Piping Systems
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   Offshore Structures
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  Australian Offshore Petroleum Safety Regulations
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  Deep Water – the disaster of Golf of Mexico and the future of Offshore Drilling
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  Offshore Well Drilling
   Download Link


  a Brief History of Offshore Well Drilling
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   Introduction to Offshore Structures
Download Link 1      Download Link 2


  Offshore Oil Well Drilling
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  Offshore Platform Design
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  Offshore pile design International practice
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Offshore Access-primer
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Offshore and Onshore Oil and Gas Exploration, Development and Production
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Offshore Drilling Presentation 458 MB

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Offshore Movies

offshore platformOffshore Movies

in this section you will find very useful movies about offshore platforms – drilling – deepwater – drill rig – drill ship and other offshore platforms.

Offshore Platform
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Offshore Platform Installation
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Offshore Drilling Animation
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How a Deepwater Well is Drilled?
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Scientific Deep Sea Drilling and Coring Technology
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Offshore Platform Construction With Subsea Drilling System
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Deepwater Drilling
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Offshore Installations and Drilling Rigs in Norway
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Offshore Rig Operations
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DrillShip Specification
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Deepwater Production Jumper to produce Oil and Natural Gas
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Overview on Deepwater Drilling
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Offshore Oil Drilling Industry – Full Documentary
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Oil Rig – Offshore Platform (Perdido construction)
Download


Offshore Platform Construction  Arabic
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Offshore Deepwater Drilling Process 
Download Link 1        Download Link 2

Offshore Drilling Rigs

types of drilling rigs
types of drilling rigs

The sequence of operations is as follows when a land well is drilled:

–  Prepare location before rig arrives.
– Dig cellar.
– Install conductor pipe.
– Prepare support pad for rig, camp, etc
– Build roads, fencing, dig pits.
– Sometimes drill water well.
– Move rig on to location, rig up and prepare to start drilling.

Offshore Drilling Rigs:
     Two main types: floating and bottom-supported unit.

   Floating unit include: semi submersible (bottle-type, column stabilized), barge rig and drill ship.
   Bottom-supported unit include: submersible (posted barges, bottle-type submersibles, arctic submersibles),
jackups and platforms.

(1). Semi Submersible

Semi Submersible offshore drilling rig
Semi Submersible offshore drilling rig

     This floating drilling unit has columns when flooded with seawater, cause the structure submerge to a
predetermined depth.
Although it is moved by wave action, it sits low with a large part of its structure under water combined with
eight huge mooring anchors, make it a very stable installation.
This type of rig drills a hole in the seabed then it moves to the next location. With advancing technology
some semi submersibles can drill in water depths over five thousand feet.

(2). Platform
This immobile structure can be built from concrete or steel and rests on the seabed. When oil or gas is
located a platform may be constructed to drill further wells at that site and also to produce the hydrocarbon.

steel jacket platform
steel jacket platform

Steel Jacket Platform
Most common type of platform consist of the jacket, a tall vertical section made of tubular steel members.  Supported by piles driven into the seabed.
Additional sections on top of the jacket provide space for drilling rig, crew quarters, and other equipment.

   

Concrete Platform

Concrete Gravity Platform
Build from steel reinforced concrete Tall caissons, or column are the dominant feature of this platform. Sometime, special concrete cylinder are fixed at the base of the
caissons on the sea floor to store crude oil.

see our Offshore Movies section

Steel-Caisson Platform
Specifically for use in cold area – where fast-moving tidal currents carry pack of ice that can destroy steel-jacket.  The caissons are made of two layers of thick steel to
prevent ice damage.

    Compliant Platform
  Using rigid platform in water much over 1000 feet depth is not practical – very much expensive to build. In deep water, most companies use compliant platform, which
contain fewer steel parts and are lighter than rigid steel-jacket.  Guyed-tower platform and tension-leg platform.

(3). Jack up
This is a mobile drilling rig, different from the semi submersible. Instead of floating over its drilling location the Jackup has long leg structures, which it lowers to and into the seabed raising the rig out of the water.  The obvious limitation with this type of installation is the depth of water it can operate in. The maximum being five hundred feet.

    (4). Drill Ship

drill ship
drill ship

   As the name suggests this is a ship shaped drilling vessel. Unlike the semi submersible and the Jackup, it does not require tugboats to tow it to location.  Although they are not as stable as semi submersibles they also drill in very deep waters.

read more about Drilling Bits

Rotary Drilling
Rotary drilling uses a sharp, rotating drill bit to dig down through the Earth’s crust. The spinning of the drill bit allows for penetration of even the hardest rock.
The actual mechanics of modern rigs are quite complicated. In addition, technology advances so rapidly that new innovations are being introduced constantly.
A rotary drilling rig with some of its major components identified is illustrated in the next figure.

The basic rotary drilling system consists of four groups of components:
Prime movers – Hoisting equipment  –  Rotating equipment –   Circulating equipment

Prime Movers
The prime movers in a rotary drilling rig are those pieces of equipment that provide the power to the entire rig. Recently, while diesel engines still compose the majority
of power sources on rotary rigs, other types of engines are also in use. Some rotary rigs may use electricity directly from power lines. Most rotary rigs these days
require 1,000 to 3,000 horsepower, while shallow drilling rigs may require as little as 500 horsepower.
The energy from these prime movers is used to power the rotary equipment, the hoisting equipment, and the circulating equipment.

Hoisting Equipment
The hoisting equipment on a rotary rig consists of the tools used to raise and lower whatever other equipment may go into or come out of the well.
The most visible part of the hoisting equipment is the derrick, the tall tower-like structure that extends vertically from the well hole.
The hoisting system is made up of the drawworks, derrick, crown block, traveling block, hook and wire rope.
If a drill bit needs to be changed, either due to tear or a change in the subsurface rock, the whole string of pipe must be raised to the surface. The hoisting equipment is
used to raise all of this equipment to the surface so that the drill bit may be replaced.

   Whenever the drillstem is suspended by the traveling block and drill line, the entire load rests on the derrick. The standard pyramid derrick is a structure with four
supporting legs resting on a square base.
In comparison, a mast is much more slender and may be thought of as sitting on one side of the rig floor or work space. The derrick is erected on a substructure which
supports the rig floor and rotary table and provides work space for the equipment on the rig floor.

  The derrick and its substructure support the weight of the drillstem at all times, whenever it is suspended from the crown block or resting in the rotary table. The height of the derrick does not affect its load-bearing capacity, but it is a factor in the length of the sections of drill pipe that can be removed.

Hoisting Equipment
Traveling Block, Crown Block, Drill Line & Hook
Use to connect the supporting derrick with the load of drill pipe to be lowered into or withdrawn from the borehole. During drilling operations, this load usually consists of
the weight of the drill pipe, drill collars and drill bit. The drill line passes from the drawworks to the top of the derrick. From here is sheaved between the crown block and traveling block to give an eight, ten or twelve-line suspension. It is then clamped to the rig floor by the deadline anchor. Suspended from the traveling block, on standard drilling systems, is the hook which when drilling carries the swivel and kelly and when tripping it lifts the drill string.

The Drawworks
The drawworks is a mechanism commonly known as a hoist. The main purpose of the drawworks is to lift the drill string out of and to lower it back into the borehole.
The drill line is reeled (spooled) on a drum in the drawworks.  When engaged, the drum turns and either reels in the drill line to raise the traveling block, or lets out the drill line to lower it. Because the drillstem is attached to the block, it is raised or lowered. One outstanding feature of the drawworks is the brake system, which enables the driller to easily control a load of thousands of pounds of drill pipe or casing.
An integral part of the drawworks is the gear (transmission) system. This gives the driller a wide choice of speeds for hoisting the drill string.

The drawworks also has a drive sprocket that drives the rotary table by means of a heavy-duty chain. In some cases, however, the rotary table is driven by an independent engine or electric motor.  Another feature of the drawworks are the two catheads. The make-up cathead, on the drillers side, is used to spin up and tighten the drill pipe joints. The other, located opposite the driller’s position on the drawworks is the breakout cathead. It is used to loosen the drill pipe when the drill pipe is withdrawn from the borehole The rotating equipment consists of components that actually serve to rotate the drill bit.  Rotating equipment from top to bottom consists of swivel, a short piece of pipe called the kelly, rotary table/topdrive, drill string and bit. A component called the swivel, which is attached to the hoisting equipment, carries the entire weight of the drill string, but allows it to rotate freely. The drill bit is located at the bottom end of the drill string, and is responsible for actually making contact with the subsurface layers, and drilling through them. There are four main types of drill bits, each suited for particular conditions:
–  Steel Tooth Rotary Bits (most basic type).
–  Insert Bits (tungsten carbide inserts).
– Polycrystalline Diamond Compact Bits (diamond inserts).
– Diamond Bits (diamonds implanted in them).

   Diamond bits are forty to fifty times harder than traditional steel bits.                                           

 References:
1. Drilling Equipment and Operation.
2. drilling Operation
3. Offshore Drilling Engineering.