CONSTRUCTION TECHNOLOGY

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• CONSTRUCTION TECHNOLOGY
• maintenance

CEM 417
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WEEK 4

• Stages for construction

1. Building
2. Retaining walls, Drainage
3. Road, Highway, Bridges
4. Airports, Offshore/Marine structure

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AIRPORT/AIRFEILDS, OFFSHORE/MARINE STRUCTURE
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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
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• 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  involvement when airfield projects
  occur.

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• 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.
• Local climate.
• Prevailing winds.
• 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

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• 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.
• Select the runway location.
• Determine the runway length and width.
• Calculate the approach zones.
• Determine the runway orientation based on the
  wind rose.
• 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 nonvisual aids to navigation.
• Design logistical support facilities.
• Design aircraft protection facilities.

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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.
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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.
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• PLANNING AN AIRFIELD
• Planning  for  aviation  facilities  requires
   special consideration of
• the   type   of   aircraft   to   be
  accommodated
• physical   conditions   of   the   site,
  including weather conditions, terrain, soil, and
  availability  of  construction  materials
• safety  factors, such as approach zone
  obstructions and traffic control
• the  provision  for  expansion  
• and  defense.  
• 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.

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SUBBASE AND BASE COURSE Pavements  (including
 the  surface  and  underlying Courses) may be
divided into two classesrigid 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.
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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.
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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  CourseA 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
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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
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• Definition of Airport Categories
• Commercial Service Airports are publicly owned
  airports that have at least 2,500 passenger
  boardings each calendar year and receive
  scheduled passenger service.
• Nonprimary Commercial Service Airports are
  Commercial Service Airports that have at least
  2,500 and no more than 10,000 passenger boardings
  each year.
• Primary Airports are Commercial Service Airports
  that have more than 10,000 passenger boardings
  each year.
• 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.
• 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/pass
enger_allcargo_stats/categories/
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TYPE OFFSHORE STRUCTURE
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TYPE OFFSHORE STRUCTURE
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TYPE OFFSHORE STRUCTURE
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TYPE OFFSHORE STRUCTURE
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TYPE OFFSHORE STRUCTURE
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OFFSHORE PLATFORM DESIGN
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OVERVIEW
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
Platform size depends on facilities to be
installed on top side eg. Oil rig, living
quarters, Helipad etc. Classification of water
depths lt 350 M- Shallow water lt 1500 M - Deep
water gt 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.
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OVERVIEW
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)
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• TYPE OF PLATFORMS (FIXED)
• JACKETED PLATFORM

• 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.

• 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.

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• TYPE OF PLATFORMS (FIXED)
• COMPLIANT TOWER

• 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.

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• TYPE OF PLATFORMS (FIXED)
• CONCRETE GRAVITY STRUCTURES

• 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

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• TYPE OF PLATFORMS (FLOATER)
• Tension Leg Platform (TLP)

• 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.

• 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.

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• 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

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• TYPE OF PLATFORMS (FLOATER)
• SPAR

• 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.

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• TYPE OF PLATFORMS (FLOATER)
• SHIP SHAPED VESSEL (FPSO)

• 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.

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• PLATFORM PARTS
• TOPSIDE

• Facilities are tailored to achieve weight and
  space saving
• Incorporates process and utility equipment
• Drilling Rig
• Injection Compressors
• Gas Compressors
• Gas Turbine Generators
• Piping
• HVAC
• Instrumentation
• Accommodation for operating personnel.
• Crane for equipment handling
• Helipad

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• PLATFORM PARTS
• MOORINGS ANCHORS

• Used to tie platform in place
• Material
• Steel chain
• Steel wire rope
• Catenary shape due to heavy weight.
• Length of rope is more
• Synthetic fiber rope
• Taut shape due to substantial less weight than
  steel ropes.
• Less rope length required
• Corrosion free

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• PLATFORM PARTS
• RISER

• 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.

• Remains in tension due to self weight
• Profiles are designed to reduce load on topside.
  Types of risers
• Rigid
• 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

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• PLATFORM INSTALLATION
• BARGE LOADOUT

• 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

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CORROSION PROTECTION

• 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.

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• PLATFORM FOUNDATION
• FOUNDATION

• 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.

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NAVAL ARCHITECTURE HYDROSTATICS AND STABILITY

• 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

• 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
• Translational - Surge, sway and heave
• Rotational - Roll, pitch and yaw
• Structural response - Involving structural
  deformations

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STRUCTURAL DESIGN

• Loads
• Offshore structure shall be designed for
  following types of loads
• Permanent (dead) loads.
• Operating (live) loads.
• Environmental loads
• Wind load
• Wave load
• Earthquake load
• Construction - installation loads.
• Accidental loads.
• The design of offshore structures is dominated by
  environmental loads, especially wave load

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STRUCTURAL DESIGN

• Permanent Loads
• Weight of the structure in air, including the
  weight of ballast.
• Weights of equipment, and associated structures
  permanently mounted on the platform.
• Hydrostatic forces on the members below the
  waterline. These forces include buoyancy and
  hydrostatic pressures.

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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.
• 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,
  crane operations.
• 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

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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.

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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 ,
• 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-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.

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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).
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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 lt 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.

52
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.

53
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.
54
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
55
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.
56
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.
57
STRUCTURAL ANALYSIS ANALYSIS MODEL
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.
58
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 cranes
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.
59
Acceptance Criteria

• 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.

60
STRUCTURAL DESIGN
Design Conditions Operation Survival Transit. Th
e 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.
61
CODES
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.
62
REFERENCES

• 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.

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