offshore platforms_sanjay joshi

7/31/2019 OFFSHORE Platforms_Sanjay Joshi

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Consequences of Recent Environmental Disasters on Design Consideration of

Offshore Platforms

S. R. Joshi

Department of Petroleum Engineering

Maharashtra Institute of Technology, Pune.

This paper is an overall view of structural aspect of fixed offshore platforms used to drilland produce crude oil from beneath the sea. Offshore activity for drilling for crude oil has

usly shows the growth of an entire offshore industry.

The offshore oil industry began off the coast of California in the late 1890. After WorldWar II offshore industry booms in development. In 1946 Mangolia Oil Co. constructed a

platform in 14 ft of water and approximately 8 km offshore. Although it was still within

sight of land. In 1947 two platforms were constructed and became the design standard for

many years. In which six jackets or templates fabricated onshore and carried to the site bybarge. They were then lowered into the water, by a crane and fixed to the bottom using

steel piles driven through the jacket legs. In 1959 platform had been installed in the Gulf ofMexico in more than 200 ft (60 m) of water. Shell Oil Company installed a platform in 310

m of water on the continental slope. In India offshore activity started in 1974 in Bombay

High and now it is in East Coast deepwater offshore also.

Types of Platforms:

There are several types of offshore platforms used to host drilling and productionequipment. The include,

(1) Drilling / well protector platforms(2) Tender platforms

(3) Self-contained template platforms

(4) Self-contained tower platforms(5) Production platforms

(6) Quarters platforms

(7) Flare jacket and flare tower

(8) Auxiliary platforms

(1) Drilling / Well Protector Platform:

Platforms built to protect risers on producing wells in shallow water are called well

protectors or well jackets.

There are two types (1) Slip-over type (2) Development type. Both types protects

the well from ship collisions and environmental forces and serve as the support aids

to navigation devices, wireline units, helicopter pads flowline risers and conductor

tubes. Slip over jacket is used for exploratory wells in 50 to 100 ft water depth.


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The development well jacket may accommodate several wells depending on design

conditions. This type of jacket is installed prior to drilling.

Self-contained platform are large, usually multiple decked platform with adequate

strength and space to support entire drilling rig with its auxiliary equipments.

Production platforms support buildings, compressor, storage tanks, treating

equipments etc. Production platform is basically a platform for separating the oilgas water mixture of the produced crude petroleum and treating primarily prior to

transport the processing facilities on a particular treatment platform may very

depends on crude oil is to be off loaded into tankers or pumped to shore through a

pipeline. Sometimes there are facilities on the platform for injecting pressurizedwater into the oil-bearing strata within the earth by means of injection wells to

improve the production of crude oil form other wells. Platform contains two

pipelines one for crude oil and another for gas. If there is no gas pipeline natural

gas is usually burned off through a flare tower. A flair jacket or flare tower is atriangular shaped tubular steel truss structure. The flare jacket is usually constructed

with either k. or x bracing throughout most of its length. The top two or three levelsof bracing may be diagonals.

Auxiliary platforms may built adjacent to larger platforms to increase availablespace these two platform may join with catwalk (bridge). Warren bridge truss is

used for Catwalk. Catwalk also carries pipelines and wirelines.

Overview of engineering design of a fixed offshore structure shows followingstages:

1) Preliminary studies:a) Soils

b) Size selection for derrick and transportation barges

c) Condition to corrosion, ice, earthquakes etc.2) Design and preparation of engineering drawing

a) Foundation design

b) Structural design

3) Construction phase

a) Fabrication on shore

b) Loading and transportationc) Erection offshore

1) Placement of underwater components

2) Installation of pipe foundation3) Setting of above water components and equipments etc.


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Role of structural engineering in offshore platform design includes

Structural Engineering

1) Material selection and corrosion,

2) Stress Analysis,

3) Welding,4) Structural Analysis,

5) Design for Fabrication and installation.

Foundation design requires soil characteristics of ocean floor where the platform is to be

placed geologist and specialist in soil mechanics evaluate the data and informs the capacity

of soil to resist operational and environmental forces transmitted to it through the structure.Structural design must contain operational loads and environmental forces.

Jacket consist large diameter tubular legs framed together by a large number of smaller

tubular members called braces. The truss work between each pair of outside jacket legs isusually of the warren bridge type. Basic frame work needed for structural strength,

provisions for conductors and risers for the functional requirement for drilling andproduction.

The number of wells to be drilled is decided at the beginning of the project the wells arethrough conductor tubes (30-36 dia) positioned in an array which can be reached by

sliding the derrick form location to location across the deck.

Risers are various vertical tubes located within the jacket framework for pumping seawaterto the deck, for pipeline connections to other platforms or the shore and many other

processing functions. (14 to 16 dia).

Environmental conditions for design

(1) Water depth (300 ft generally)(2) Design wave Maximum hurricane wave with recurrence interval of 100 years

height of wave and period in seconds.

(3) Average Wave At normal sea conditions needed for fatigue load calculations.

(4) Ocean Current Measure at surface, middle, at bottom the current is assumed to beapproach the platform from any direction.

(5) Wind speed The maximum sustained used speed based on a 100 year storm

recurrence interval.(6) Geotechnical data Bottom of Gulf contains sand, silty sand, silty clay and clay

because of many rivers emptying into gulf. Geotechnical parameters differ from

one location to another. From soil data find out unconfined compressive strengthfor soft clay, silty sand, unconsolidated sand. Etc. There are several other factors

that may influence design criteria depending on local situation.


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The oceanographic and meteorological data needed for the design of a platform are storm

wave heights, storm wind speed, gust condition, tides, swells ocean bottom scours out of

this the most important are (1) storm wind and (2) storm wave loadings.

(1) Wind Forces:

Force of wind on a structure is a function of the wind velocity, orientation of astructure and aerodynamic characteristics of the structure and its members.

AV

CF

z

DD 2

2= (Force parallel to wind)

(Force perpendicular to wind)

CD = drag coefficient CL = Lift coefficient

= density of the air Vz = Wind velocity at height z.A = Area perpendicular to wind velocity, wind velocity is not constant because ofshear forces but it is zero at surface and increases exponentially to a limitingmaximum speed known as gradient speed. Wind speed at any elevations above a

water surface is given as

71

3030

=Z

VVZ

Where,

V30 = Wind speed at a height of 30 ft.

Z = desired elevation in ft.30 = reference height ft.

This equation is called 1/7+h power law in mehie units.

In metric units7

1

1010

=Z

VVZ

V10 = the wind speed at a height of 10 metersZ = desired elevation m

10 = reference height - m

Calculate wind effect on all parts of the structure while calculation of wind speed,

gust factor should consider. Gust factor is a multiplier and in the range of 1.35 to1.45. Gust factor with height is negligible. The fastest mile velocity is used for

designing (Gust factor x wind speed).

AV

CF

z

L

L 2

2=


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(2) Wave Forces: Wave forces are based on worst expected storm conditions with an

average expected recurrence interval of 100 years.

The horizontal force exerted by waves on a cylindrical member consists of a dragforce related to the kinetic energy of the water and an inertial force related to the

acceleration of the water particles. Total force on the member is the pressure of the

water multiplied by volume or area of the structural member perpendicular to thedirection of wave advance. The force per unit length on a member is computed by

morison equation. This equation represent force on a cylindrical tube in the form

two components (1) internal (2) drag.

F = Fi + FD

//4

2

uDxug

w

Z

Cd

dt

du

g

DWCF m +=

Cm =Mass coefficient Cd = drag coefficientW Weight density of a sea water N/m3

D = Dia of a cylinder m.

F = Wave force per unit length acting perpendicular to the member axis N/m.

FI = Inertial force per unit length along the member N/m.FD = Drag force per unit length along the member N/m.

U = Horizontal water particle velocity m/sec.

|u| = Absolute value of u m/sec.Du/dt = Horizontal water particle acceleration m/sec2.

g = Gravitational acceleration m/sec2

For morison expression Cd and Cm are generally used one 0.7 and 2.0 respectively.

Water particle velocity and acceleration are functions of wave height, wave period, water

depth, distance above bottom and time. Idea of wave profile can get form stokes equation.

(non-linear in case of storm conditions).

For computing bending moment in individual structural members either vertical or inclined

the full wave force (drag + inertial) applied as an uniform load perpendicular to themember. Maximum bending moment M is

M = WL2 / 10w = av. Load per unit length

L = member length

Current force: add current velocity vectorially to horizontal water particle velocity thencompute drag force.


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Case study:

Catrina / Rita hurricane in 2005 destroyed 113 offshore platforms, so challenges are howoffshore industry contends with hurricane forces by innovations in structural design.

Platforms should withstand both gale force wind and severe wave activity.

(1) For upcoming season, new clamps capable of wistanding 2 million psi have been

added to hold major components in place under Katrina like wind conditions.

(2) Some regulations to avoid such damages the platform deck must exceed theaverage height of hurricane driven swells, generally 80 ft (category 3, 4, & 5 storms

all generate waves of approximately the same height).

(3) For designing analyze new types of ocean data i.e. currents, wave patterns, wind

effect etc. is being generated.(4) Consider extra ballast water load to increase the stability of the structure while

considering axial load on the column and beams

Hurricane Catrina before it made land fall it had been a category 5, wind speed was 128mph and hurricane force winds extended over 120 miles from the center or the eye of the

storm. General practice for designing platform is to consider 100 years criteria but afterCatrina, this was thought inadequate 100 years criteria. Now a days hurricanes are

hyperactive than previous i.e. upto 1994. So we have to consider the latest data and look at

whether we need to update our standards. When Catrina passed over the Gulf of Maxico itwas at its peak category 5 storm wind of 270 km. It snapped mooring lines, wave toppled

steel structures. Hence the suggestion is to consider storms forward movement

(translation) with orbital (circular) wind velocity. As waves and swells move from deep

water into shallow water their wavelength shortens and the height increases causing thewave eventually brake. A wave is a traveling disturbance of the sea surface. Waves are

primarily caused by the action of wind on water. The total energy in a wave is proportional

to the square of height and energy of swell (waves that are transmitted beyond the windaffected zone are called swells) is proportional to its length. Wave height is governed by

wind speed, duration, and temperature of air relative to sea. Scale of hurricane losses led to

rapid re-valuation to the techniques used to design offshore platforms. Figure 1 shows theelements of structural design of a typical platform.CFD technique can be used instead of

physical prototype model for (CFD Computational Fluid Dynamics). CFD computer

model has the same dimensions as the actual production platform hence results can be

interpreted directly. The wave impact on a platform can be viewed from any angle and theinstantaneous forces acting on any part of the structure can be calculated. By considering

above points we can reduce the damage in any part of the system upstream, mid stream or

downstream.

Acknowledgement

The author is grateful to Prof. Vishwanath D. Karad, Director and Dr. L. K. Kshirsagar andDr. P.B. Jadhav for encouragement.

offshore platforms_sanjay joshi

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