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CONSTRUCTION TECHNOLOGY

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Title: CONSTRUCTION TECHNOLOGY


1
  • CONSTRUCTION TECHNOLOGY
  • maintenance

CEM 417
2
WEEK 4
  • Stages for construction
  1. Building
  2. Retaining walls, Drainage
  3. Road, Highway, Bridges
  4. Airports, Offshore/Marine structure

3
AIRPORT/AIRFEILDS, OFFSHORE/MARINE STRUCTURE
4
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
5
  • 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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11
  • 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

12
  • 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.

13
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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15
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.
16
  • 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.

17
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.
18
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.
19
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20
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
21
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
22
  • 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/
23
TYPE OFFSHORE STRUCTURE
24
TYPE OFFSHORE STRUCTURE
25
TYPE OFFSHORE STRUCTURE
26
TYPE OFFSHORE STRUCTURE
27
TYPE OFFSHORE STRUCTURE
28
OFFSHORE PLATFORM DESIGN
29
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.
30
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)
31
  • 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.

32
  • 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.

33
  • 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

34
  • 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.

35
  • 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

36
  • 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.

37
  • 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.

38
  • 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

39
  • 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

40
  • 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

41
  • 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

42
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.

43
  • 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.

44
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

45
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

46
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.

47
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

48
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.

49
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.

50
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).
51
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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