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CONSTRUCTION
TECHNOLOGY &

maintenance



CEM 417

Stages for construction

WEEK 4

1.
Building

2.
Retaining walls, Drainage

3.
Road, Highway, Bridges

4.
Airports, Offshore/Marine structure

AIRPORT/AIRFEILDS,
OFFSHORE/MARINE
STRUCTURE

WEEK
4

At the end of week 4 lectures, student will be
able to :

-
Identify the different types of airfields and
marine structures and their respective
functions. (CO1; CO3)

Reference:
-

http://www.globalsecurity.org/military/library/policy/army/fm/5
-
430
-
00
-
2/Ch11.htm

http://www.tpub.com/content/engineering/14071/css/14071_80.htm

AIRFIELDS



Road construction and airfield construction have
much

in

common,

such

as

construction

methods,
equipment

used,

and

sequence

of

operations.



Each

road or airfield requires a
subgrade
, base course,
and surface course.



The

methods

of

cutting

and

falling,

grading

and
compacting, and surfacing are all similar. As with roads,
the responsibility for designing and laying out lies with
the

same

person the

engineering

officer.



Again,

as
previously

said

for

roads,

you

can

expect

involveme
nt when airfield projects occur.


RUNWAY DESIGN CRITERIA

Runway location, length, and alignment are the foremost
design criteria in any airfield plan. The major factors that
influence these three criteria are
--


1.
Type of using aircraft.

2.
Local climate.

3.
Prevailing winds.

4.
Topography (drainage, earthwork, and clearing).


Location


Select the site using the runway as the feature foremost in
mind. Also consider topography, prevailing wind, type of
soil, drainage characteristics. and the amount of clearing
and earthwork necessary when selecting the site

AIRFIELD DESIGN STEPS

The following is a
procedural guide

to complete a
comprehensive airfield design. The concepts and required
information are discussed later in this chapter.

1.
Select the runway location.

2.
Determine the runway length and width.

3.
Calculate the approach zones.

4.
Determine the runway orientation based on the wind
rose.

5.
Plot the centerline on graph paper, design the vertical
alignment, and plot the newly designed airfield on the
plan and profile.

6.
Design transverse slopes.

7.
Design taxiways and aprons.

8.
Design required drainage structures.

9.
Select visual and
nonvisual

aids to navigation.

10.
Design logistical support facilities.

11.
Design aircraft protection facilities
.

Length

When determining the runway length required for any
aircraft, include the surface required for landing rolls or
takeoff runs and a reasonable allowance for variations in pilot
technique; psychological factors; wind, snow, or other
surface conditions; and unforeseen mechanical failure.
Determine runway length by applying several correction
factors and a factor of safety to the takeoff ground run (TGR)
established for the geographic and climatic conditions at the
installation. Air density, which is governed by temperature
and pressure at the site, greatly affects the ground run
required for any type aircraft. Increases in either temperature
or altitude reduce the density of air and increase the required
ground run. Therefore, the length of runway required for a
specific type of aircraft varies with the geographic location.
The length of every airfield must be computed based on the
average maximum temperature and the pressure altitude of
the site.

At the top is the

Surface Course

which is usually an asphalt or Portland
cement concrete material.


Bound surfaces such as these provide stability and
durability for year
-
round traffic operations.


Asphalt surfaces are from 5 to 10 cm
(2 to 4 inches) thick and concrete surfaces from 23 to 40 cm (9 to 16 inches)
thick.


The next layer is the

Base Course

-

a high quality crushed stone or gravel
material necessary to ensure stability under high aircraft tire pressures.


Bases
vary in thickness from 15 to 30 cm (6 to 12 inches).


The bottom layer is the

Subbase Course

which is constructed with non
-
frost
susceptible but lower quality granular aggregates.


Subbases increase the
pavement strength and reduce the effects of frost action on the
subgrade.


Subbase thicknesses are usually 30 cm (12 inches) or more.


These three (3) layers (Surface, Base and Subbase Courses) have a


combined thickness of 60 to 150 cm (2 to 5 feet) and are placed on the
subgrade
-

the pavement foundation.


The

Subgrade

is the natural in
-
situ soil material which has been cut to grade,
or in a fill section, is imported common material built up over the in
-
situ
material.


The subgrade must provide a stable and uniform support for the
overlying pavement structure.

PLANNING AN AIRFIELD

Planning

for

aviation

facilities

requires

special
consideration

of


1.
the

type

of

aircraft

to

be accommodated;

2.
physical

conditions

of

the

site, including weather
conditions, terrain, soil, and
availability

of

construction

materials;

3.
safety

factors, such as approach zone obstructions and
traffic control;

4.
the

provision

for

expansion;


5.
and

defense.


6.
Under wartime conditions, tactical considerations are also
required.


All

of

these

factors

affect

the

number, orientation, and
dimensions of runways, taxiways, aprons, hardstands,
hangars, and other facilities.

SUBBASE AND BASE COURSE


Pavements

(including

the

surface

and

underlying Courses)
may be divided into two classes

rigid and flexible.

The wearing surface of a rigid pavement is constructed of portland
cement concrete.

Its flexural strength enables it to act as abeam and allows it to
bridge over minor irregularities in the base or subgrade up on
which it rests.

All other pavements are classified as flexible.
Any

distortion

or

displacement

in

the

subgrade of a flexible
pavement is reflected in the base course and upward into the
surface course.

These

courses

tend

to conform to the same shape under traffic.

Flexible
pavements

are

used

almost

exclusively

in

the

operations for
road and airfield construction since they adapt to nearly all
situations and can be built by any construction battalion unit in the
Naval Construction Force

(NCF) ate.


FLEXIBLE PAVEMENT STRUCTURE



A typical flexible pavement is constructed as shown below,

which
also defines the parts or layers of pavement.

All layers shown in the figure are not presenting every flexible
pavement.

For example, a two
-
layer structure consists of a compacted
subgrade and a base course

only.

Figure shows

a

typical

flexible pavement using stabilized layers.
(The word

pavement,
when used by itself, refers only to the
leveling, binder, and surface course, whereas

flexible
pavement

refers to
the

entire

pavement

structure

from

the

subgrade

up.)

The

use

of

flexible

pavements

on

airfields

must

be limited to
paved areas not subjected to detrimental effects of jet fuel spillage
and jet blast. In fact, their use is prohibited in areas where these
effects are severe.


Flexible

pavements

are

generally

satisfactory

for runway interiors, taxiways, shoulders, and overruns.
Rigid pavements or special types of flexible pavement, such as tar rubber, should be specified in certain
critical operational

areas.

MATERIALS
Select

materials

will

normally

be

locally

available coarse
-
grained soils, although fine
-
grained soils may be used in certain cases. Lime rock, coral, shell, ashes,
cinders,

caliche,

disintegrated

granite,

and

other

such
materials

should

be

considered

when

they

are economical.

Subbase
Subbase

materials

may

consist

of

naturally occurring

coarse
-
grained

soils

or

blended

and

processed soils.

Materials,

such

as

lime

rock,

coral,

shell,

ashes,
cinders, caliche, and disintegrated granite, maybe used
as

subbases

when

they

meet

area

specifications

or project

specifications.

Materials

stabilized

with
commercial admixes may be economical as subbases in certain instances. Portland cement, cutback
asphalt,emulsified asphalt, and tar are commonly used for this purpose.

Base

Course
A wide variety of gravels, sands, gravelly and sandy soils, and other natural materials such as
lime rock, corals, shells, and some caliches can be used alone or
blended

to

provide

satisfactory

base

courses.

In

some instances, natural materials will require crushing
or removal of the oversize fraction to maintain gradation limits. Other natural materials may be controlled
by mixing

crushed

and

pit
-
run

materials

to

form

a satisfactory

base

course

material.
Many

natural

deposits

of

sandy

and

gravelly materials

also

make

satisfactory

base

materials.

Gravel
deposits

vary

widely

in

the

relative

proportions

of coarse and fine material and in the character of the
rock fragments. Satisfactory base materials often can be
produced

by

blending

materials

from

two

or

more deposits. Abase course made from sandy and
gravelly material has a high
-
bearing value and can be used to
support

heavy

loads.

However,

uncrushed,

clean
washed

gravel

is

not

satisfactory

for

a

base

course because the fine material, which acts as the binder
and fills

the

void

between

coarser

aggregate,

has

been washed away. Sand and clay in a natural
mixture maybe found in alluvial deposits varying in thickness from 1 to 20 feet.
Often

there

are

great

variations

in

the

proportions

of
sand

and

clay

from

the

top

to

the

bottom

of

a

pit


Deposits of partially disintegrated rock consisting of fragments of rock, clay, and mica flakes should not be
confused

with

sand
-
clay

soil.

Mistaking

such

material for sand
-
clay is often a cause of base course failure
because of reduced stability caused by the mica content.
With

proper

proportioning

and

construction

methods, satisfactory

results

can

be

obtained

with

sand
-
clay

soil. It is excellent in construction where a higher type of surface is to be added later. Processed
materials are prepared by crushing and screening

rock,

gravel,

or

slag.

A

properly

graded crushed
-
rock

base

produced

from

sound,

durable

rock particles makes the highest quality of any base material.
Crushed rock may be produced from almost any type of rock that is hard enough to require drilling, blasting,
and crushing. Existing quarries, ledge rock, cobbles and
gravel,

talus

deposits,

coarse

mine

tailings,

and

similar hard, durable rock fragments are the usual
sources of processed materials. Materials that crumble on exposure to air or water should not be used. Nor
should processed materials be used when gravel or sand
-
clay is available, except when studies show that the
use of processed materials will save time and effort when they are made necessary by project requirements.
Bases made from processed

materials

can

be

divided

into

three

general types
-
stabilized,

coarse

graded,

and

macadam.

A stabilized base is one in which all material ranging from coarse
to fine is intimately mixed either before or as the material is laid into place. A coarse
-
graded base is
composed of crushed rock, gravel, or slag. This base may

be

used

to

advantage

when

it

is

necessary

to
produce crushed rock, gravel, or slag on site or when commercial aggregates are available. A macadam base
is one where a coarse, crushed aggregate is placed in a relatively thin layer and rolled into place; then fine
aggregate or screenings are placed on the surface of the coarse
-
aggregate

layer

and

rolled

and

broomed

into

the coarse rock until it is thoroughly keyed in place. Water
may be used in the compacting and keying process. When water is used, the base is a water
-
bound
macadam. The

crushed

rock

used

for

macadam

bases

should consist of clean, angular, durable particles
free of clay, organic

matter,

and

other

objectional

material

or
coating.

Any

hard,

durable

crushed

aggregate

can

be used, provided the coarse aggregate is primarily
one size and the fine aggregate will key into the coarse aggregate


Definition of Airport Categories


1.
Commercial Service Airports

are publicly owned airports that have at
least 2,500 passenger
boardings

each calendar year and receive
scheduled passenger service.

2.
Nonprimary

Commercial Service Airports

are Commercial Service
Airports that have at least 2,500 and no more than 10,000 passenger
boardings

each year.

3.
Primary Airports

are Commercial Service Airports that have more than
10,000 passenger
boardings

each year.

4.
Cargo Service Airports

are airports that, in addition to any other air
transportation services that may be available, are served by aircraft
providing air transportation of only cargo with a total annual landed weight
of more than 100 million pounds.

5.
Reliever Airports

are airports designated by the FAA to relieve congestion
at Commercial Service Airports and to provide improved general aviation
access to the overall community. These may be publicly or privately
-
owned.
commonly described as

General Aviation Airports
.

http://www.faa.gov/airports/planning_capacity/passenger_allcargo_stats/categories/

TYPE OFFSHORE
STRUCTURE

TYPE OFFSHORE
STRUCTURE

TYPE OFFSHORE
STRUCTURE

TYPE OFFSHORE
STRUCTURE

TYPE OFFSHORE
STRUCTURE

OFFSHORE
PLATFORM
DESIGN

Offshore platforms are used for
exploration of Oil and Gas from
under Seabed and processing.


The First Offshore platform was
installed in 1947 off the coast of
Louisiana in 6M depth of water.


Today there are over 7,000
Offshore platforms around the
world in water depths up to
1,850M

OVERVIEW

Platform size depends on facilities to be
installed on top side eg. Oil rig, living
quarters, Helipad etc.


Classification of water depths:

< 350 M
-

Shallow water

< 1500 M
-

Deep water

> 1500 M
-

Ultra deep water


US Mineral Management Service (MMS)
classifies water depths greater than 1,300
ft as deepwater, and greater than 5,000 ft
as ultra
-
deepwater.

Offshore platforms can broadly categorized
in two types.


Fixed structures that extend to the Seabed.

Steel Jacket

Concrete gravity Structure

Compliant Tower


Structures that float near the water
surface
-

Recent development

Tension Leg platforms

Semi Submersible

Spar

Ship shaped vessel (FPSO)


OVERVIEW


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.

TYPE OF PLATFORMS (FIXED)



JACKETED PLATFORM


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).


Natural period (usually 2.5 second)
is kept below wave period (14 to 20
seconds) to avoid amplification of
wave loads.


95% of offshore platforms around
the world are Jacket supported.


Narrow, flexible framed structures
supported by piled foundations.


Has no oil storage capacity.
Production is through tensioned
rigid risers and export by flexible
or catenary steel pipe.


Undergo large lateral deflections
(up to 10 ft) under wave loading.
Used for moderate water depths
up to 600 M.


Natural period (usually 30
second) is kept above wave
period (14 to 20 seconds) to
avoid amplification of wave loads.

TYPE OF PLATFORMS (FIXED)



COMPLIANT TOWER


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.


Advantage
-

Less maintenance

TYPE OF PLATFORMS (FIXED)



CONCRETE GRAVITY STRUCTURES


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.

TYPE OF PLATFORMS (FLOATER)



Tension Leg Platform (TLP)


TLP has excess buoyancy which
keeps tethers in tension. Topside
facilities , no. of risers etc. have to
fixed at pre
-
design stage.


Used for deep water up to 1200 M


It has no integral storage.


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

TYPE OF PLATFORMS (FLOATER)



SEMISUB PLATFORM


Due to small water plane area ,
they are weight sensitive. Flood
warning systems are required to
be in
-
place.


Topside facilities , no. of risers etc.
have to fixed at pre
-
design stage.


Used for Ultra deep water.


Semi
-
submersibles are held in
place by anchors connected to a
catenary
mooring system.


Column pontoon junctions and
bracing attract large loads.


Due to possibility of fatigue
cracking of braces , periodic
inspection/ maintenance is
prerequisite


Concept of a large diameter single
vertical cylinder supporting deck.


These are a very new and
emerging concept: the first spar
platform, Neptune , was installed
off the USA coast in 1997 .


Spar platforms have taut catenary
moorings and deep draught, hence
heave natural period is about 30
seconds.


Used for Ultra deep water depth of
2300 M.


The center of buoyancy is
considerably above center of
gravity , making Spar quite stable.


Due to space restrictions in the
core, number of risers has to be
predetermined.

TYPE OF PLATFORMS (FLOATER)



SPAR


Ship
-
shape platforms are called
Floating Production, Storage and
Offloading (FPSO) facilities.


FPSOs have integral oil storage
capability inside their hull. This
avoids a long and expensive
pipeline to shore.


Can explore in remote and deep
water and also in marginal wells,
where building fixed platform and
piping is technically and
economically not feasible


FPSOs are held in position over
the reservoir at a Single Point
Mooring (SPM). The vessel is
able to weathervane around the
mooring point so that it always
faces into the prevailing weather.

TYPE OF PLATFORMS (FLOATER)



SHIP SHAPED VESSEL (FPSO)


Facilities are tailored to achieve
weight and space saving


Incorporates process and utility
equipment

1.
Drilling Rig

2.
Injection Compressors

3.
Gas Compressors

4.
Gas Turbine Generators

5.
Piping

6.
HVAC

7.
Instrumentation


Accommodation for operating
personnel.


Crane for equipment handling


Helipad

PLATFORM PARTS



TOPSIDE

Used to tie platform in place

Material

1.
Steel chain

2.
Steel wire rope

a)
Catenary shape due to
heavy weight.

b)
Length of rope is more

3.
Synthetic fiber rope

a)
Taut shape due to
substantial less weight
than steel ropes.

b)
Less rope length required

c)
Corrosion free

PLATFORM PARTS



MOORINGS & ANCHORS


Pipes used for production,
drilling, and export of Oil and Gas
from Seabed.


Riser system is a key component
for offshore drilling or floating
production projects.


The cost and technical
challenges of the riser system
increase significantly with water
depth.


Design of riser system depends
on filed layout, vessel interfaces,
fluid properties and
environmental condition.

PLATFORM PARTS



RISER


Remains in tension due to self
weight


Profiles are designed to reduce load
on topside. Types of risers

1.
Rigid

2.
Flexible
-

Allows vessel motion
due to wave loading and
compensates heave motion


Simple Catenary risers:
Flexible pipe is freely
suspended between
surface vessel and the
seabed.


Other catenary variants
possible

Various methods are deployed based
on availability of resources and size
of structure.


Barge Crane


Flat over
-

Top side is
installed on jackets. Ballasting
of barge


Smaller jackets can be
installed by lifting them off
barge using a floating vessel
with cranes .

Large 400’ x 100’ deck barges
capable of carrying up to 12,000 tons
are available

PLATFORM
INSTALLATION



BARGE LOADOUT


The usual form of corrosion
protection of the underwater part
of the jacket as well as the upper
part of the piles in soil is by
cathodic protection using
sacrificial anodes.


A sacrificial anode consists of a
zinc/aluminium bar cast about a
steel tube and welded on to the
structures. Typically approximately
5% of the jacket weight is applied
as anodes.


The steelwork in the splash zone
is usually protected by a sacrificial
wall thickness of 12 mm to the
members.

CORROSION PROTECTION


The loads generated by
environmental conditions plus by
onboard equipment must be
resisted by the piles at the seabed
and below.


The soil investigation is vital to the
design of any offshore structure.
Geotech report is developed by
doing soil borings at the desired
location, and performing in
-
situ
and laboratory tests.


Pile penetrations depends on
platform size and loads, and soil
characteristics, but normally range
from 30 meters to about 100
meters.

PLATFORM
FOUNDATION



FOUNDATION


Stability is resistance to capsizing


Center of Buoyancy is located at
center of mass of the displaced
water.


Under no external forces, the
center of gravity and center of
buoyancy are in same vertical
plane.


Upward force of water equals to
the weight of floating vessel and
this weight is equal to weight of
displaced water


Under wind load vessel heels, and
thus CoB moves to provide
righting (stabilizing) moment.


Vertical line through new center of
buoyancy will intersect CoG at
point M called as Metacenter

NAVAL ARCHITECTURE
HYDROSTATICS AND STABILITY


Intact stability requires righting moment
adequate to withstand wind moments.


Damage stability requires vessel
withstands flooding of designated volume
with wind moments.


CoG of partially filled vessel changes,
due to heeling. This results in reduction
in stability. This phenomena is called
Free surface correction (FSC).


HYDRODYNAMIC RESPONSE:


Rigid body response


There are six rigid body motions:

1.
Translational
-

Surge, sway and
heave

2.
Rotational
-

Roll, pitch and yaw


Structural response
-

Involving structural
deformations

Loads:

Offshore structure shall be designed
for following types of loads:

1.
Permanent (dead) loads.

2.
Operating (live) loads.

3.
Environmental loads

a)
Wind load

b)
Wave load

c)
Earthquake load

4.
Construction
-

installation
loads.

5.
Accidental loads.


The design of offshore structures is
dominated by environmental loads,
especially wave load

STRUCTURAL DESIGN

Permanent Loads:

Weight of the structure in air,
including the weight of ballast.

1.
Weights of equipment,
and associated
structures permanently
mounted on the
platform.


2.
Hydrostatic forces on
the members below the
waterline. These forces
include buoyancy and
hydrostatic pressures.

STRUCTURAL DESIGN

Operating (Live) Loads:

Operating loads include the weight of
all non
-
permanent equipment or
material, as well as forces generated
during operation of equipment.

1.
The weight of drilling,
production facilities, living
quarters, furniture, life support
systems, heliport, consumable
supplies, liquids, etc.

2.
Forces generated during
operations, e.g. drilling, vessel
mooring, helicopter landing,
crane operations.

3.
Following Live load values are
recommended in BS6235:

4.
Crew quarters and passage
ways: 3.2 KN/m 2

5.
Working areas: 8,5 KN/m 2

STRUCTURAL DESIGN

Wind Loads:


Wind load act on portion of platform
above the water level as well as on any
equipment, housing, derrick, etc.


For combination with wave loads, codes
recommend the most unfavorable of the
following two loadings:


1 minute sustained wind speeds
combined with extreme waves.


3 second gusts .


When, the ratio of height to the least
horizontal dimension of structure is
greater than 5, then API
-
RP2A requires
the dynamic effects of the wind to be
taken into account and the flow induced
cyclic wind loads due to vortex shedding
must be investigated.

STRUCTURAL DESIGN

Wave load :


The wave loading of an offshore structure is usually the most important
of all environmental loadings.


The forces on the structure are caused by the motion of the water due to
the waves


Determination of wave forces requires the solution of ,

a)
Sea state using an idealization of the wave surface profile and the
wave kinematics by wave theory.

b)
Computation of the wave forces on individual members and on the
total structure, from the fluid motion.

Design wave concept is used, where a regular wave of given height and period
is defined and the forces due to this wave are calculated using a high
-
order
wave theory.

Usually the maximum wave with a return period of 100 years, is chosen. No
dynamic behavior of the structure is considered. This static analysis is
appropriate when the dominant wave periods are well above the period of the
structure. This is the case of extreme storm waves acting on shallow water
structures.

STRUCTURAL DESIGN

Wave Load: (Contd.)

Wave theories

Wave theories describe the
kinematics of waves of water. They
serve to calculate the particle
velocities and accelerations and the
dynamic pressure as functions of
the surface elevation of the waves.
The waves are assumed to be long
-
crested, i.e. they can be described
by a two
-
dimensional flow field, and
are characterized by the
parameters: wave height (H), period
(T) and water depth (d).

STRUCTURAL DESIGN

Wave theories: (Contd.)

Wave forces on structural members


Structures exposed to waves experience forces much higher than wind
loadings. The forces result from the dynamic pressure and the water
particle motions. Two different cases can be distinguished:


Large volume bodies, termed hydrodynamic compact structures, influence
the wave field by diffraction and reflection. The forces on these bodies
have to be determined by calculations based on diffraction theory.


Slender, hydro
-
dynamically transparent structures have no significant
influence on the wave field. The forces can be calculated in a straight
-
forward manner with Morison's equation. The steel jackets of offshore
structures can usually be regarded as hydro
-
dynamically transparent


As a rule, Morison's equation may be applied when D/L < 0.2, where D is
the member diameter and L is the wave length.


Morison's equation expresses the wave force as the sum of,


An inertia force proportional to the particle acceleration


A non
-
linear drag force proportional to the square of the particle
velocity.

STRUCTURAL DESIGN

Earthquake load:


Offshore structures are designed for two
levels of earthquake intensity.


Strength level :Earthquake, defined
as having a &
quot
; reasonable
likelihood of not being exceeded
during the platform's life &
quot
;
(mean recurrence interval ~ 200
-

500 years), the structure is
designed to respond elastically.


Ductility level : Earthquake, defined
as close to the &
quot
; maximum
credible earthquake &
quot
; at the
site, the structure is designed for
inelastic response and to have
adequate reserve strength to avoid
collapse.

STRUCTURAL DESIGN

Ice and Snow Loads:

Ice is a primary problem for marine structures in the arctic and sub
-
arctic zones.
Ice formation and expansion can generate large pressures that give rise to
horizontal as well as vertical forces. In addition, large blocks of ice driven by
current, winds and waves with speeds up to 0,5 to 1,0 m/s, may hit the structure
and produce impact loads. Temperature Load: Temperature gradients produce
thermal stresses. To cater such stresses, extreme values of sea and air
temperatures which are likely to occur during the life of the structure shall be
estimated. In addition to the environmental sources , accidental release of
cryogenic material can result in temperature increase, which must be taken into
account as accidental loads. The temperature of the oil and gas produced must
also be considered. Marine Growth: Marine growth is accumulated on
submerged members. Its main effect is to increase the wave forces on the
members by increasing exposed areas and drag coefficient due to higher
surface roughness. It is accounted for in design through appropriate increases
in the diameters and masses of the submerged members.

STRUCTURAL DESIGN

Installation Load :

These are temporary loads and arise during fabrication and installation of the
platform or its components. During fabrication, erection lifts of various
structural components generate lifting forces, while in the installation phase
forces are generated during platform load out, transportation to the site,
launching and upending, as well as during lifts related to installation. All
members and connections of a lifted component must be designed for the
forces resulting from static equilibrium of the lifted weight and the sling
tensions. Load out forces are generated when the jacket is loaded from the
fabrication yard onto the barge. Depends on friction co
-
efficient

STRUCTURAL DESIGN

Accidental Load :

According to the DNV rules , accidental loads are loads, which may occur as a
result of accident or exceptional circumstances.

Examples of accidental loads are, collision with vessels, fire or explosion,
dropped objects, and unintended flooding of buoyancy tanks.

Special measures are normally taken to reduce the risk from accidental loads.

STRUCTURAL DESIGN

Load Combinations :

The load combinations depend upon the design method used, i.e. whether limit
state or allowable stress design is employed.

The load combinations recommended for use with allowable stress procedures
are:

Normal operations

Dead loads plus operating environmental loads plus maximum live loads .
Dead loads plus operating environmental loads plus minimum live loads .

Extreme operations

Dead loads plus extreme environmental loads plus maximum live loads.
Dead loads plus extreme environmental loads plus minimum live loads

Environmental loads,should be combined in a manner consistent with their joint
probability of occurrence.

Earthquake loads, are to be imposed as a separate environmental load, i.e.,
not to be combined with waves, wind, etc.

STRUCTURAL DESIGN

The analytical models used in offshore
engineering are similar to other types of on
shore steel structures

The same model is used throughout the
analysis except supports locations.

Stick models are used extensively for
tubular structures (jackets, bridges, flare
booms) and lattice trusses (modules,
decks).

Each member is normally rigidly fixed at its
ends to other elements in the model.

In addition to its geometrical and material
properties, each member is characterized
by hydrodynamic coefficients, e.g. relating
to drag, inertia, and marine growth, to allow
wave forces to be automatically generated.

STRUCTURAL ANALYSIS

ANALYSIS MODEL

Integrated decks and hulls of floating platforms
involving large bulkheads are described by plate
elements.

Deck shall be able to resist crane’s maximum
overturning moments coupled with corresponding
maximum thrust loads for at least 8 positions of the
crane boom around a full 360
°

path.

The structural analysis will be a static linear analysis
of the structure above the seabed combined with a
static non
-
linear analysis of the soil with the piles.

Transportation and installation of the structure may
require additional analyses

Detailed fatigue analysis should be performed to
assess cumulative fatigue damage

The offshore platform designs normally use pipe or
wide flange beams for all primary structural
members.

STRUCTURAL ANALYSIS

ANALYSIS MODEL


The verification of an element consists of
comparing its characteristic resistance(s) to a
design force or stress. It includes:


a strength check, where the characteristic
resistance is related to the yield strength of the
element,


a stability check for elements in compression
related to the buckling limit of the element.


An element is checked at typical sections (at least
both ends and mid span) against resistance and
buckling.


Tubular joints are checked against punching.
These checks may indicate the need for local
reinforcement of the chord using larger thickness
or internal ring
-
stiffeners.


Elements should also be verified against fatigue,
corrosion, temperature or durability wherever
relevant.

Acceptance Criteria

Design Conditions:

Operation

Survival

Transit.


The design criteria for strength should
relate to both intact and damaged
conditions.


Damaged conditions to be considered may
be like 1 bracing or connection made
ineffective, primary girder in deck made
ineffective, heeled condition due to loss of
buoyancy etc.

STRUCTURAL DESIGN

Offshore Standards (OS):

Provides technical requirements and
acceptance criteria for general
application by the offshore industry
eg.DNV
-
OS
-
C101


Recommended Practices(RP): Provides
proven technology and sound
engineering practice as well as guidance
for the higher level publications eg. API
-
RP
-
WSD


BS 6235: Code of practice for fixed
offshore structures.

British Standards Institution 1982.

Mainly for the British offshore sector.

CODES


W.J. Graff: Introduction to offshore
structures.


Gulf Publishing Company, Houston
1981.


Good general introduction to
offshore structures.



B.C.
Gerwick
: Construction of offshore
structures.


John Wiley & Sons, New York 1986.


Up to date presentation of offshore
design and construction.



Patel M H: Dynamics of offshore
structures

Butterworth & Co., London.

http://www.slideshare.net/surya3303/offshore
-
structures
-
presentation

REFERENCES

Q & A

THANK
YOU