Stages for construction
Retaining walls, Drainage
Road, Highway, Bridges
Airports, Offshore/Marine structure
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)
Road construction and airfield construction have
road or airfield requires a
, base course,
and surface course.
compacting, and surfacing are all similar. As with roads,
the responsibility for designing and laying out lies with
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
Type of using aircraft.
Topography (drainage, earthwork, and clearing).
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
to complete a
comprehensive airfield design. The concepts and required
information are discussed later in this chapter.
Select the runway location.
Determine the runway length and width.
Calculate the approach zones.
Determine the runway orientation based on the wind
Plot the centerline on graph paper, design the vertical
alignment, and plot the newly designed airfield on the
plan and profile.
Design transverse slopes.
Design taxiways and aprons.
Design required drainage structures.
Select visual and
aids to navigation.
Design logistical support facilities.
Design aircraft protection facilities
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
At the top is the
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)
The next layer is the
a high quality crushed stone or gravel
material necessary to ensure stability under high aircraft tire pressures.
vary in thickness from 15 to 30 cm (6 to 12 inches).
The bottom layer is the
which is constructed with non
susceptible but lower quality granular aggregates.
Subbases increase the
pavement strength and reduce the effects of frost action on the
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
the pavement foundation.
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
The subgrade must provide a stable and uniform support for the
overlying pavement structure.
PLANNING AN AIRFIELD
site, including weather
conditions, terrain, soil, and
factors, such as approach zone obstructions and
Under wartime conditions, tactical considerations are also
number, orientation, and
dimensions of runways, taxiways, aprons, hardstands,
hangars, and other facilities.
SUBBASE AND BASE COURSE
may be divided into two classes
rigid and flexible.
The wearing surface of a rigid pavement is constructed of portland
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.
subgrade of a flexible
pavement is reflected in the base course and upward into the
to conform to the same shape under traffic.
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
FLEXIBLE PAVEMENT STRUCTURE
A typical flexible pavement is constructed as shown below,
also defines the parts or layers of pavement.
All layers shown in the figure are not presenting every flexible
For example, a two
layer structure consists of a compacted
subgrade and a base course
flexible pavement using stabilized layers.
when used by itself, refers only to the
leveling, binder, and surface course, whereas
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.
for runway interiors, taxiways, shoulders, and overruns.
Rigid pavements or special types of flexible pavement, such as tar rubber, should be specified in certain
grained soils, although fine
grained soils may be used in certain cases. Lime rock, coral, shell, ashes,
cinders, caliche, and disintegrated granite, maybe used
commercial admixes may be economical as subbases in certain instances. Portland cement, cutback
asphalt,emulsified asphalt, and tar are commonly used for this purpose.
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
some instances, natural materials will require crushing
or removal of the oversize fraction to maintain gradation limits. Other natural materials may be controlled
of coarse and fine material and in the character of the
rock fragments. Satisfactory base materials often can be
more deposits. Abase course made from sandy and
gravelly material has a high
bearing value and can be used to
course because the fine material, which acts as the binder
been washed away. Sand and clay in a natural
mixture maybe found in alluvial deposits varying in thickness from 1 to 20 feet.
Deposits of partially disintegrated rock consisting of fragments of rock, clay, and mica flakes should not be
material for sand
clay is often a cause of base course failure
because of reduced stability caused by the mica content.
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 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
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
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
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
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
should consist of clean, angular, durable particles
free of clay, organic
be used, provided the coarse aggregate is primarily
one size and the fine aggregate will key into the coarse aggregate
Definition of Airport Categories
Commercial Service Airports
are publicly owned airports that have at
least 2,500 passenger
each calendar year and receive
scheduled passenger service.
Commercial Service Airports
are Commercial Service
Airports that have at least 2,500 and no more than 10,000 passenger
are Commercial Service Airports that have more than
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.
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
commonly described as
General Aviation Airports
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
Platform size depends on facilities to be
installed on top side eg. Oil rig, living
quarters, Helipad etc.
Classification of water depths:
< 350 M
< 1500 M
> 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
Offshore platforms can broadly categorized
in two types.
Fixed structures that extend to the Seabed.
Concrete gravity Structure
Structures that float near the water
Tension Leg platforms
Ship shaped vessel (FPSO)
Space framed structure with
tubular members supported on
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)
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
Natural period (usually 2.5 second)
is kept below wave period (14 to 20
seconds) to avoid amplification of
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)
bottom structures made from
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.
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
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
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)
Due to small water plane area ,
they are weight sensitive. Flood
warning systems are required to
Topside facilities , no. of risers etc.
have to fixed at pre
Used for Ultra deep water.
submersibles are held in
place by anchors connected to a
Column pontoon junctions and
bracing attract large loads.
Due to possibility of fatigue
cracking of braces , periodic
inspection/ maintenance is
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
Used for Ultra deep water depth of
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
TYPE OF PLATFORMS (FLOATER)
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
Gas Turbine Generators
Accommodation for operating
Crane for equipment handling
Used to tie platform in place
Steel wire rope
Catenary shape due to
Length of rope is more
Synthetic fiber rope
Taut shape due to
substantial less weight
than steel ropes.
Less rope length required
MOORINGS & ANCHORS
Pipes used for production,
drilling, and export of Oil and Gas
Riser system is a key component
for offshore drilling or floating
The cost and technical
challenges of the riser system
increase significantly with water
Design of riser system depends
on filed layout, vessel interfaces,
fluid properties and
Remains in tension due to self
Profiles are designed to reduce load
on topside. Types of risers
Allows vessel motion
due to wave loading and
compensates heave motion
Simple Catenary risers:
Flexible pipe is freely
surface vessel and the
Other catenary variants
Various methods are deployed based
on availability of resources and size
Top side is
installed on jackets. Ballasting
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
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
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
The steelwork in the splash zone
is usually protected by a sacrificial
wall thickness of 12 mm to the
The loads generated by
environmental conditions plus by
onboard equipment must be
resisted by the piles at the seabed
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
and laboratory tests.
Pile penetrations depends on
platform size and loads, and soil
characteristics, but normally range
from 30 meters to about 100
Stability is resistance to capsizing
Center of Buoyancy is located at
center of mass of the displaced
Under no external forces, the
center of gravity and center of
buoyancy are in same vertical
Upward force of water equals to
the weight of floating vessel and
this weight is equal to weight of
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
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).
Rigid body response
There are six rigid body motions:
Surge, sway and
Roll, pitch and yaw
Offshore structure shall be designed
for following types of loads:
Permanent (dead) loads.
Operating (live) loads.
The design of offshore structures is
dominated by environmental loads,
especially wave load
Weight of the structure in air,
including the weight of ballast.
Weights of equipment,
mounted on the
Hydrostatic forces on
the members below the
waterline. These forces
include buoyancy and
Operating (Live) Loads:
Operating loads include the weight of
permanent equipment or
material, as well as forces generated
during operation of equipment.
The weight of drilling,
production facilities, living
quarters, furniture, life support
systems, heliport, consumable
supplies, liquids, etc.
Forces generated during
operations, e.g. drilling, vessel
mooring, helicopter landing,
Following Live load values are
recommended in BS6235:
Crew quarters and passage
ways: 3.2 KN/m 2
Working areas: 8,5 KN/m 2
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
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.
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
Determination of wave forces requires the solution of ,
Sea state using an idealization of the wave surface profile and the
wave kinematics by wave theory.
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
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
Wave Load: (Contd.)
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).
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.
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
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
linear drag force proportional to the square of the particle
Offshore structures are designed for two
levels of earthquake intensity.
Strength level :Earthquake, defined
as having a &
likelihood of not being exceeded
during the platform's life &
(mean recurrence interval ~ 200
500 years), the structure is
designed to respond elastically.
Ductility level : Earthquake, defined
as close to the &
credible earthquake &
; at the
site, the structure is designed for
inelastic response and to have
adequate reserve strength to avoid
Ice and Snow Loads:
Ice is a primary problem for marine structures in the arctic and sub
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.
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
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.
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
Dead loads plus operating environmental loads plus maximum live loads .
Dead loads plus operating environmental loads plus minimum live loads .
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.
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,
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.
Integrated decks and hulls of floating platforms
involving large bulkheads are described by plate
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
The structural analysis will be a static linear analysis
of the structure above the seabed combined with a
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
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
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
Tubular joints are checked against punching.
These checks may indicate the need for local
reinforcement of the chord using larger thickness
or internal ring
Elements should also be verified against fatigue,
corrosion, temperature or durability wherever
The design criteria for strength should
relate to both intact and damaged
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
Offshore Standards (OS):
Provides technical requirements and
acceptance criteria for general
application by the offshore industry
Recommended Practices(RP): Provides
proven technology and sound
engineering practice as well as guidance
for the higher level publications eg. API
BS 6235: Code of practice for fixed
British Standards Institution 1982.
Mainly for the British offshore sector.
W.J. Graff: Introduction to offshore
Gulf Publishing Company, Houston
Good general introduction to
: Construction of offshore
John Wiley & Sons, New York 1986.
Up to date presentation of offshore
design and construction.
Patel M H: Dynamics of offshore
Butterworth & Co., London.
Q & A