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Transcript of [LECTURE] Design of Fixed Offshore Structures
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Lecture 15A.1
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Offshore Structures: General Introduction
OBJECTIVE/SCOPE
To identify the basic vocabulary, to introduce the major concepts for offshore platform
structures, and to explain where the basic structural requirements for design are generated.
PREREQUISITES
None.
SUMMARY
The lecture starts with a presentation of the importance of offshore hydro-carbon exploitation,the basic steps in the development process (from seismic exploration to platform removal)
and the introduction of the major structural concepts (jacket-based, GBS-based, TLP,
floating). The major codes are identified.
For the fixed platform concepts (jacket and GBS), the different execution phases are briefly
explained: design, fabrication and installation. Special attention is given to some principles of
topside design.
A basic introduction to cost aspects is presented.
Finally terms are introduced through a glossary.
1. INTRODUCTION
Offshore platforms are constructed to produce the hydrocarbons oil and gas. The contribution
of offshore oil production in the year 1988 to the world energy consumption was 9% and isestimated to be 24% in 2000.
The investment (CAPEX) required at present to produce one barrel of oil per day ($/B/D) and
the production costs (OPEX) per barrel are depicted in the table below.
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Condition CAPEX $/B/D OPEX $/B
Conventional
Average 4000 - 8000 5
Middle East 500 - 3000 1
Non-Opec 3000 - 12000 8
Offshore
North Sea 10000 - 25000 5 - 10
Deepwater 15000 - 35000 10 - 15
World oil production in 1988 was 63 million barrel/day. These figures clearly indicate the
challenge for the offshore designer: a growing contribution is required from offshore
exploitation, a very capital intensive activity.
Figure 1 shows the distribution of the oil and gas fields in the North Sea, a major contribution
to the world offshore hydrocarbons. It also indicates the onshore fields in England, theNetherlands and Germany.
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2. OFFSHORE PLATFORMS
2.1 Introduction of Basic Types
The overwhelming majority of platforms are piled-jacket with deck structures, all built in steel(see Slides 1 and 2).
Slide 1: Jacket based platform - Southern sector North Sea
Slide 2: Jacket based platform - Northern sector North Sea
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Slide 4 shows an integrated deck (though excluding the living quarters and helideck) being
moved from its assembly building.
Slide 4 : Integrated topside during load out
5.2.2 Structural Design for Integrated Topsides
For the smaller decks, up to approximately 100 MN weight, the support structure consists of
trusses or portal frames with deletion of diagonals.
The moderate vertical load and shear per column allows the topside to be supported by
vertical columns (deck legs) only, down to the top of the piles (situated at approximately +4 m
to +6 m L.A.T. (Low Astronomic Tide).
5.2.3 Structural Design for Modularized Jacket-based Topsides
A major modularized topside weighs 200 to 400 MN. In this case the MSF is a heavy tubular
structure (Figure 4), with lateral bracing down to the top of jacket.
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5.3 Structural Design for Modularized Gravity-based Topsides
The topsides to be supported by a gravity-based substructure (see Figure 2) are in a weight
range of 200 MN up to 500 MN.
The backbone of the structure is a system of heavy box-girders with a height of approximately
10 m and a width of approximately 12 - 15 m (see Figure 5).
The substructure of the deck is rigidly connected to the concrete column and acts as a beam
supporting the deck modules. This connection introduces wave-induced fatigue in the deck
structure. A recent development, foreseen for the Norwegian Troll platform, is to provide a
flexible connection between the deck and concrete column, thus eliminating fatigue in the
deck [10].
6. EQUIPMENT AND LIVING QUARTER MODULES
Equipment modules (20-75 MN) have the form of rectangular boxes with one or two
intermediate floors.
The floors are steel plate (6, 8 or 10 mm thick) for roof and lower floor, and grating forintermediate floors.
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In living quarter modules (5-25 MN) all sleeping rooms require windows and several doors
must be provided in the outer walls. This requirement can interfere seriously with truss
arrangements. Floors are flat or stiffened plate.
Three types of structural concepts, all avoiding interior columns, can be distinguished:
conventional trusses in the walls.
stiffened plate walls (so called stressed skin or deck house type).
heavy base frame (with wind bracings in the walls).
7. CONSTRUCTION
7.1 Introduction
The design of offshore structures has to consider various requirements of construction
relating to:
1. fabrication.
2. weight.
3. load-out.
4. sea transport.
5. offshore installation.
6.
module installation.7. hook-up.
8. commissioning.
A documented construction strategy should be available during all phases of the design and
the actual design development should be monitored against the construction strategy.
Construction is illustrated below by four examples.
7.2 Construction of Jackets and Topsides
7.2.1 Lift Installed Jackets
The jacket is built in the vertical (smaller jackets) or horizontal position (bigger jackets) on a
quay of a fabrication site.
The jacket is loaded-out and seafastened aboard a barge. At the offshore location the barge
is moored alongside an offshore crane vessel.
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The jacket is lifted off the barge, upended from the horizontal, and carefully set down onto the
seabed.
After setting down the jacket, the piles are installed into the sleeves and, driven into the
seabed. Fixing the piles to the jacket completes the installation.
7.2.2 Launch Installed Jackets
The jacket is built in horizontal position.
For load-out to the transport barge, the jacket is put on skids sliding on a straight track of steel
beams, and pulled onto the barge (Slide 5).
Slide 5 : Jacket being loaded onto barge by skidding
At the offshore location the jacket is slid off the barge. It immerses deeply into the water and
assumes a floating position afterwards (see Figure 6).
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Two parallel heavy vertical trusses in the jacket structure are required, capable of taking the
support reactions during launching. To reduce forces and moments in the jacket, rocker arms
are attached to the stern of the barge.
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The next phase is to upright the jacket by means of controlled flooding of the buoyancy tanks
and then set down onto the seabed. Self-upending jackets obtain a vertical position after the
launch on their own. Piling and pile/jacket fixing completes the installation.
7.2.3 Topsides for a Gravity-Based Structure GBS)
The topside is assembled above the sea on a temporary support near a yard. It is then taken
by a barge of such dimensions as to fit between the columns of the temporary support and
between the columns of the GBS. The GBS is brought in a deep floating condition in a
sheltered site, e.g. a Norwegian fjord. The barge is positioned between the columns and the
GBS is then deballasted to mate with and to take over the deck from the barge. The floating
GBS with deck is then towed to the offshore site and set down onto the seabed.
7.2.4 Jacket Topsides
For topsides up to approximately 120 MN, the topside may be installed in one lift. Slide 6
shows a 60 MN topside being installed by floating cranes.
Slide 6 : Installation of 60MN K12-BP topside by floating crane
For the modularized topside, first the MSF will be installed, immediately followed by the
modules.
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7.3 Offshore Lifting
Lifting of heavy loads from barges (Slide 6) is one of the very important and spectacular
construction activities requiring a focus on the problem when concepts are developed.
Weather windows, i.e. periods of suitable weather conditions, are required for these
operations.
7.3.1 Crane Vessel
Lifting of heavy loads offshore requires use of specialized crane vessels. Figure 7 provides
information on a typical big, dual crane vessel. Table 1 (page 16) lists some of the major
offshore crane vessels.
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7.3.2 Sling-arrangement, Slings and Shackles
For lifting, steel wire ropes in a four-sling arrangement are used which directly rest in the four-
point hook of the crane vessel, (see Figure 8). The heaviest sling available now has a
diameter of approximately 350 mm, a breaking load of approximately 48 MN, and a safe
working load (SWL) of 16 MN. Shackles are available up to 10 MN SWL to connect the
padeyes installed at the module's columns. Due to the space required, connecting more than
one shackle to the same column is not very attractive. So when the sling load exceeds 10
MN, padears become an option.
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Table 1 Major Offshore Crane Vessels
Operator Name Mode Type Lifting capacity (Tonnes)
Fix 2720
Thor MonohullRev 1820
Fix 2720Odin Monohull
Rev 2450
Fix 4536 + 3628 = 8164Hermod Semisub
Rev 3630 + 2720 = 6350
Fix 3630 + 2720 = 6350
Heerema
Balder SemisubRev 3000 + 2000 = 5000
Fix 4000DB50 Monohull
Rev 3800
Fix 1820DB100 Semisub
Rev 1450
Fix 3360
DB101 SemisubRev 2450
McDermott
DB102 Semisub Rev 6000 + 6000 = 12000
Micoperi M7000 Semisub Rev 7000 + 7000 = 14000
ETPM DLB1601 Monohull Rev. 1600
Notes:
1. Rated lifting capacity in metric tonnes.
2. When the crane vessels are provided with two cranes, these cranes are situated at
the vessels stern or bow at approximately 60 m distance c.t.c.
1. 3. Rev = Load capability with fully revolving crane.
Fix = Load capability with crane fixed.
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7.4 Sea Transport and Sea Fastening
Transportation is performed aboard a flat-top barge or, if possible, on the deck of the crane
vessel.
The module requires fixing to the barge (see Figure 9) to withstand barge motions in rough
seas. The sea fastening concept is determined by the positions of the framing in the module
as well as of the "hard points" in the barge.
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7.5 Load-out
7.5.1 Introduction
For load-out three basic methods are applied:
skidding
platform trailers
shearlegs.
7.5.2 Skidding
Skidding is a method feasible for items of any weight. The system consists of a series of steel
beams, acting as track, on which a group of skids with each approximately 6 MN load
capacity is arranged. Each skid is provided with a hydraulic jack to control the reaction.
7.5.3 Platform Trailers
Specialized trailer units (see Figure 10) can be combined to act as one unit for loads up to 60
- 75 MN. The wheels are individually suspended and integrated jacks allow adjustment up to
300 mm.
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The load capacity over the projected ground area varies from approximately 55 to 85
kN/sq.m.
The units can drive in all directions and negotiate curves.
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7.5.4 Shearlegs
Load-out by shearlegs is attractive for small jackets built on the quay. Smaller decks (up to 10
- 12 MN) can be loaded out on the decklegs pre-positioned on the barge, thus allowing deck
and deckleg to be installed in one lift offshore.
7.6 Platform Removal
In recent years platform removal has become common. The mode of removal depends
strongly on the regulations of the local authorities. Provision for removal should be considered
in the design phase.
8. STRUCTURAL ANALYSIS
8.1 Introduction
The majority of structural analyses are based on the linear theory of elasticity for total system
behaviour. Dynamic analysis is performed for the system behaviour under wave-attack if the
natural period exceeds 3 seconds. Many elements can exhibit local dynamic behaviour, e.g.
compressor foundations, flare-stacks, crane-pedestals, slender jacket members, conductors.
8.2 In-place Phase
Three types of analysis are performed:
Survival state, under wave/current/wind attack with 50 or 100 years recurrence
period.
Operational state, under wave/current/wind attack with 1 or 5 years recurrence
period, under full operation.
Fatigue assessment.
Accidental.
All these analyses are performed on the complete and intact structure. Assessments at
damaged structures, e.g. with one member deleted, and assessments of collision situations
are occasionally performed.
8.3 Construction Phase
The major phases of construction when structural integrity may be endangered are:
Load-out
Sea transport
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11. CONCLUDING SUMMARY
The lecture starts with the presentation of the importance of offshore hydro-carbon
exploitation, the basic steps in the development process (from seismic exploration to
platform removal) and the introduction of the major structural concepts (jacket-based,
GBS-based, TLP, floating). The major codes are identified.
For the fixed platform concepts (jacket and GBS), the different execution phases are
briefly explained: design, fabrication and installation. Special attention is given to the
principles of topside design.
A basic introduction to cost aspects is presented.
Finally terms are introduced within a glossary.
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12. GLOSSARY OF TERMS
AIR GAP Clearance between the top of maximum wave and underside of the topside.
CAISSONS See SUMPS
CONDUCTORS The tubular protecting and guiding the drill string from the topside down to 40
to 100m under the sea bottom. After drilling it protects the well casing.
G.B.S. Gravity based structure, sitting flatly on the sea bottom, stable through its weight.
HOOK-UP Connecting components or systems, after installation offshore.
JACKET Tubular sub-structure under a topside, standing in the water and pile founded.
LOAD-OUT The operation of bringing the object (module, jacket, deck) from the quay onto the
transportation barge.
PADEARS (TRUNNIONS) Thick-walled tubular stubs, directly receiving slings and
transversely welded to the main structure.
PADEYES Thick-walled plate with hole, receiving the pin of the shackle, welded to the main
structure.
PIPELINE RISER The piping section which rises from the sea bed to topside level.
SEA-FASTENING The structure to keep the object rigidly connected to the barge during
transport.
SHACKLES Connecting element (bow + pin) between slings and padeyes.
SLINGS Cables with spliced eyed at both ends, for offshore lifting, the upper end resting in
the crane hook.
SPREADER Tubular frame, used in lifting operation.
SUBSEA TEMPLATE Structure at seabottom, to guide conductors prior to jacket installation.
SUMPS Vertical pipes from topside down to 5-10 m below water level for intake or discharge.
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TOPSIDE Topside, the compact offshore process plant, with all auxiliaries, positioned above
the waves.
UP ENDING Bringing the jacket in vertical position, prior to set down on the sea bottom.
WEATHER WINDOW
A period of calm weather, defined on basis of operational limits for the offshore marine
operation.
WELLHEAD AREA Area in topside where the wellheads are positioned including the valves
mounted on its top.
13. REFERENCES
[1] API-RP2A: Recommended practice for planning, designing and constructing fixed offshore
platforms.
American Petroleum Institute 18th ed. 1989.
The structural offshore code, governs the majority of platforms.
[2] LRS Code for offshore platforms.
Lloyds Register of Shipping.
London (UK) 1988.
Regulations of a major certifying authority.
[3] DnV: Rules for the classification of fixed offshore installations.
Det Norske Veritas 1989.
Important set of rules.
[4] AISC: Specification for the design, fabrication and erection of structural steel for buildings.
American Institute of Steel Construction 1989.
Widely used structural code for topsides.
[5] AWS D1.1-90: Structural Welding Code - Steel.
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American Welding Society 1990.
The structural offshore welding code.
[6] DnV/Marine Operations: Standard for insurance warranty surveys in marine operations.
Det norske Veritas June 1985.
Regulations of a major certifying authority.
[7] ABS: Rules for building and classing offshore installations, Part 1 Structures.
American Bureau of Shipping 1983.
Regulations of a major certifying authority.
[8] BV: Rules and regulations for the construction and classification of offshore platforms.
Bureau Veritas, Paris 1975.
Regulations of a major certifying authority.
[9] ANON: A primer of offshore operations.
Petex Publ. Austin U.S.A 2nd ed. 1985.
Fundamental information about offshore oil and gas operations.
[10] AGJ Berkelder et al: Flexible deck joints.
ASME/OMAE-conference The Hague 1989 Vol.II pp. 753-760.
Presents interesting new concept in GBS design.
14. ADDITIONAL READING
1. BS 6235: Code of practice for fixed offshore structures.
British Standards Institution 1982.
Important code, mainly for the British offshore sector.
2. DoE Offshore installations: Guidance on design and construction, U.K. Department of
Energy 1990.
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Governmental regulations for British offshore sector only.
3. UEG: Design of tubular joints (3 volumes).
UEG Offshore Research Publ. U.R.33 1985.
Important theoretical and practical book.
4. J. Wardenier: Hollow section joints.
Delft University Press 1981.
Theoretical publication on tubular design including practical design formulae.
5.
ARSEM: Design guides for offshore structures welded tubular joints.
Edition Technip, Paris (France), 1987.
Important theoretical and practical book.
6. D. Johnston: Field development options.
Oil & Gas Journal, May 5 1986, pp 132 - 142.
Good presentation on development options.
7. G. I. Claum et al: Offshore Structures: Vol 1: Conceptual Design and Hydri-
mechanics; Vol 2 - Strength and Safety for Structural design.
Springer Verlag, London 1992.
Fundamental publication on structural behaviour.
8.
W.J. Graff: Introduction to offshore structures.
Gulf Publishing Company, Houston 1981.
Good general introduction to offshore structures.
9. B.C. Gerwick: Construction of offshore structures.
John Wiley & Sons, New York 1986.
Up to date presentation of offshore design and construction.
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10.T.A. Doody et al: Important considerations for successful fabrication of offshore
structures.
OTC paper 5348, Houston 1986, pp 531-539.
Valuable paper on fabrication aspects.
11.D.I. Karsan et al: An economic study on parameters influencing the cost of fixed
platforms.
OTC paper 5301, Houston 1986, pp 79-93.
Good presentation on offshore CAPEX assessment.
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Page 5 of 16
depth and deep water conditions. Corresponding particle paths are illustrated in Figures 3 and
4. Note the strong influence of the water depth on the wave kinematics. Results from high-
order wave theories can be found in the literature, e.g. see [9].
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2.2.2 Wave Statistics
In reality waves do not occur as regular waves, but as irregular sea states. The irregular
appearance results from the linear superposition of an infinite number of regular waves with
varying frequency (Figure 5). The best means to describe a random sea state is using the
wave energy density spectrum S(f), usually called the wave spectrum for simplicity. It is
formulated as a function of the wave frequency f using the parameters: significant wave
height Hs(i.e. the mean of the highest third of all waves present in a wave train) and mean
wave period (zero-upcrossing period) To. As an additional parameter the spectral width can
be taken into account.
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Designs for ductility level earthquakes will normally require inelastic analyses for which the
seismic input must be specified by sets of 3-component accelerograms, real or artificial,
representative of the extreme ground motions that could shake the platform site. The
characteristics of such motions, however, may still be prescribed by means of design spectra,
Lecture 15A.2
which are usually the result of a site specific seismotectonic study. More detail of the analysis
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of earthquakes is given in the Lectures 17: Seismic Design.
2.5 Ice and Snow Loads
Ice is a primary problem for marine structures in the arctic and sub-arctic zones. Ice formationand 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 that
can approach 0,5 to 1,0 m/s, may hit the structure and produce impact loads.
As a first approximation, statically applied, horizontal ice forces may be estimated as follows:
Fi= CifcA ......................................... (7)
Where,
A is the exposed area of structure,
fcis the compressive strength of ice,
Ciis the coefficient accounting for shape, rate of load application and other factors, with usual
values between 0,3 and 0,7.
Generally, detailed studies based on field measurements, laboratory tests and analytical work
are required to develop reliable design ice forces for a given geographical location.
In addition to these forces, ice formation and snow accumulations increase gravity and wind
loads, the latter by increasing areas exposed to the action of wind. More detailed information
on snow loads may be found in Eurocode 1 [8].
2.6 Loads due to Temperature Variations
Offshore structures can be subjected to temperature gradients which produce thermal
stresses. To take account of such stresses, extreme values of sea and air temperatures
which are likely to occur during the life of the structure must be estimated. Relevant data for
the North Sea are given in BS6235 [6]. In addition to the environmental sources, human
factors can also generate thermal loads, e.g. through accidental release of cryogenic material,
which must be taken into account in design as accidental loads. The temperature of the oil
and gas produced must also be considered.
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[8] Eurocode 1: "Basis of Design and Actions on Structures", CEN (in preparation).
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[9] Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design and Hydromechanics",
Springer, London 1992.
[10] Anagnostopoulos, S.A., "Dynamic Response of Offshore Structures to Extreme Wavesincluding Fluid - Structure Interaction", Engr. Structures, Vol. 4, pp.179-185, 1982.
[11] Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.
[12] Graff, W.J., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.
[13] Gerwick, B.C. Jr., "Construction of Offshore Structures", John Wiley, New York, 1986.
Table 1 Results of Linear Airy Theory [11]
Phase = kx - t
Relative water depth d/L
Deep water
d/L 0,5
Finite water depth
d/L < 0,5
Velocity potential
Surface elevation z
Dynamic pressure pdyn=
acos
gaekzcos
acos
Water particle velocities
horizontal u =
vertical w =
aekzcos
aekzsin
Water particle accelerations
horizontal u' =
vertical w' =
a2ekzsin
-a2ekzcos
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Wave celerity c =
Group velocity cgr=
Circular frequency =
Wave length L =
Wave number k =
co=
cgr=
=
Lo=
ko=
c =
cgr=
=
L =
kd tanh kd =
Water particle displacements
horizontal
vertical
Particle trajectories
-aekzsin
aekzcos
Circular orbits
Elliptical orbits
Where a=
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Lecture 15A.3
conditions can be taken as static. Typical values of friction coefficients for calculation of
skidding forces are the following:
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steel on steel without lubrication..................................... 0,25
steel on steel with lubrication...........................................0,15
steel on teflon.................................................................. 0,10 teflon on teflon................................................................. 0,08
3.3 Transportation Forces
These forces are generated when platform components (jacket, deck) are transported
offshore on barges or self-floating. They depend upon the weight, geometry and support
Lecture 15A.3
conditions of the structure (by barge or by buoyancy) and also on the environmental
conditions (waves, winds and currents) that are encountered during transportation. The types
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of motion that a floating structure may experience are shown schematically in Figure 3.
In order to minimize the associated risks and secure safe transport from the fabrication yard
to the platform site, it is important to plan the operation carefully by considering, according to
API-RP2A [3], the following:
1.
Previous experience along the tow route
2. Exposure time and reliability of predicted "weather windows"
3. Accessibility of safe havens
4. Seasonal weather system
5. Appropriate return period for determining design wind, wave and current conditions,
taking into account characteristics of the tow such as size, structure, sensitivity and
cost.
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Lecture 15A.3
Table Characteristic Lo
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LIMIT STATES FOR TEMPORARY
PHASES
Progressive Collapse
Serviceability
LOAD
TYPE
Service
ability
Fatigue
Ultimate
Abnormal
effects
Damage
condition
DEAD EXP
LIVE SPE
DEFORM
ATIONEXPECTE
ENVIRON
MENTAL
Depen
dent onoperati
onal
require
ments
Expect
ed load
history
Value
dependent on
measures taken
Dependent on operational requirements
ACCIDEN
TAL NOT APPLICABLE
Dependent on
operational
requirements
NOT APPLIC
ads according to NPD [4]
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Page 13 of 13
LIMIT STATES FOR NORMAL OPERATIONS
Progressive Collapse
Fatigue Ultimate
Abnormal
effectsDamage condition
ECTED VALUE
CIFIED VALUE
D EXTREME VALUE
Expect
ed load
history
Annual
exceedance
probability 10-2
Annual
exceedanc
e
probability
10-4
Annual exceedance probability 10-2
ABLE
Annual
exceedanc
eprobability
10-4
NOT APPLICABLE
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Lecture 15A.4
P are permanent loads (structural weight, dry equipments, ballast, hydrostatic pressure).
L are live loads (storage, personnel, liquids).
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Page 5 of 14
D are deformations (out-of-level supports, subsidence).
E are environmental loads (wave, current, wind, earthquake).
A are accidental loads (dropped object, ship impact, blast, fire).
3.3.2 Material factors
The material partial factors for steel is normally taken equal to 1,15 for ULS and 1,00 for PLS
and SLS design.
3.3.3 Classification of Design Conditions
Guidance for classifying typical conditions into typical limit states is given in the following
table:
Loadingsondition
P/L E D A
Design
Criterion
Construction P ULS,SLS
Load-Out P reduced wind support
disp
ULS
Transport P transport wind
and wave
ULS
Tow-out
(accidental)
P flooded compart PLS
Launch P ULS
Lifting P ULS
In-Place
(normal)
P + L wind, wave &
snow
actual ULS,SLS
Lecture 15A.4
In-Place
(extreme)
P + L wind & 100
year wave
actual ULS
SLS
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Page 6 of 14
In-Place
(exceptional)
P + L wind & 10000
year wave
actual PLS
Earthquake P + L 10-2quake ULS
Rare
Earthquake
P + L 10-4quake PLS
Explosion P + L blast PLS
Fire P + L fire PLS
Dropped
Object
P + L drill collar PLS
Boat
Collision
P + L boat impact PLS
Damaged
Structure
P + reduced L reduced wave
& wind
PLS
4. PRELIMINARY MEMBER SIZING
The analysis of a structure is an iterative process which requires progressive adjustment of
the member sizes with respect to the forces they transmit, until a safe and economical design
is achieved.
It is therefore of the utmost importance to start the main analysis from a model which is close
to the final optimized one.
The simple rules given below provide an easy way of selecting realistic sizes for the main
elements of offshore structures in moderate water depth (up to 80m) where dynamic effects
are negligible.
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Lecture 15A.4
p(t) =
The plot of the amplitudes pjversus the circular frequencies jis called the amplitude power
spectra of the loading Usually significant values of p j only occur within a narrow range of
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Page 13 of 14
spectra of the loading. Usually, significant values of pjonly occur within a narrow range of
frequencies and the analysis can be restricted to it.
The relationship between response and force vectors is expressed by the transfer matrix H,
such as:
H = [-M 2+ i x C + K]
the elements of which represent:
Hj,k=
The spectral density of response in freedom j versus force is then:
The fast Fourier transform (FFT) is the most efficient algorithm associated with this kind of
analysis.
6.4.2 Time Domain Analysis
The response of the i-th mode may alternatively be determined by resorting to Duhamel's
integral:
Xj(t) =
The overall response is then obtained by summing at each time step the individual responsesover all significant modes.
6.5 Direct Integration Methods
Direct step-by-step integration of the equations of motion is the most general method and is
applicable to:
non-linear problems involving special forms of damping and response-dependent
loadings.
Lecture 15A.4
responses involving many vibration modes to be determined over a short time
interval.
The dynamic equilibrium at an instant is governed by the same type of equations, where all
matrices (mass, damping, stiffness, load) are simultaneously dependent on the time and
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Page 14 of 14
structural response as well.
All available integration techniques are characterized by their stability (i.e. the tendency for
uncontrolled divergence of amplitude to occur with increasing time steps). Unconditionally
stable methods are always to be preferred (for instance Newmark-beta with = 1/4 or Wilson-
theta with = 1,4).
7. CONCLUDING SUMMARY
The analysis of offshore structures is an extensive task.
The analytical models used in offshore engineering are in some respects similar to
those used for other types of steel structures. The same model is used throughout the
analysis process.
The verification of an element consists of comparing its characteristic resistance(s) to
a design force or stress. Several methods are available.
Simple rules are available for preliminary member sizing.
Static in-plane analysis is always carried out at the early stage of a project to size the
main elements of the structure. A dynamic analysis is normally mandatory for every
offshore structure.
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Lecture 15A.5
[9] UEG, Node Flexibility and its Effect on Jacket Structures/CIRIA Report UR22, 1984.
[10] Hallam M.G., Heaf N.J. & Wootton L.R., Dynamics of Marine Structures/ CIRIA Report
UR8 (2nd edition), October 1978.
[11] Wilson J.F., Dynamics of Offshore Structures/Wiley Interscience, 1984.
[12] Clough R.W. & Penzien J., Dynamics of Structures/McGraw-Hill, New York, 1975.
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Page 15 of 16
[ ] g , y , ,
[13] Newland D.E., Random Vibrations and Spectral Analysis/Longman Scientific (2nd
edition), 1984.
[14] Zienkiewicz O.C., Lewis R.W. & Stagg K.G., Numerical Methods in Offshore
Engineering/Wiley Interscience, 1978.
[15] Davenport A.G., The Response of Slender Line-Like Structures to a Gusty Wind/ICE
Vol.23, 1962.
[16] Williams A.K. & Rhinne J.E., Fatigue Analysis of Steel Offshore Structures/ICE Vol.60,
November 1976.
[17] Anagnostopoulos S.A., Wave and Earthquake Response of Offshore Structures:
Evaluation of Modal Solutions/ASCE J. of the Structural Div., vol. 108, No ST10, October
1982.
[18] Chianis J.W. & Mangiavacchi A., A Critical Review of Transportation Analysis
Procedures/OTC paper 4617, May1983.
[19] Kaplan P. Jiang C.W. & Bentson J, Hydrodynamic Analysis of Barge-Platform Systems in
Waves/Royal Inst. of Naval Architects, London, April 1982.
[20] Hambro L., Jacket Launching Simulation by Differentiation of Constraints/ Applied Ocean
Research, Vol.4 No.3, 1982.
[21] Bunce J.W. & Wyatt T.A., Development of Unified Design Criteria for Heavy Lift
Operations Offshore/OTC paper 4192, May 1982.
[22] Walker A.C. & Davies P., A Design Basis for the J-Tube Method of Riser Installation/J. of
Energy Resources Technology, pp. 263-270, September 1983.
Lecture 15A.5
[23] Stahl B. & Baur M.P., Design Methodology for Offshore Platform Conductors/J. of
Petroleum Technology, November 1983.
[24] DnV - Rules for the Classification of Steel Ships, January 1989.
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Lecture 15A.6
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Page 3 of 25
2.1 Design Loads
These loads are those transferred from the jacket to the foundation. They are calculated at
the mudline.
2.1.1 Gravity loads
Gravity loads (platform dead load and live loads) are distributed as axial compression forces
on the piles depending upon their respective eccentricity.
Lecture 15A.6
2.1.2 Environmental loads
Environmental loads due to waves, current, wind, earthquake, etc. are basically horizontal.
Their resultant at mudline consists of:
shear distributed as horizontal forces on the piles.
overturning moment on the jacket, equilibrated by axial tension/ compression insymmetrically disposed piles (upstream/downstream).
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Page 4 of 25
y y p p ( p )
2.1.3 Load combinations
The basic gravity and environmental loads multiplied by relevant load factors are combined in
order to produce the most severe effect(s) at mudline, resulting in:
vertical compression or pullout force, and
lateral shear force plus bending.
2.2 Static Axial Pile Resistance
The overall resistance of the pile against axial force is the sum of shaft friction and end
bearing.
2.2.1 Lateral friction along the shaft shaft friction)
Skin friction is mobilized along the shaft of the tubular pile (and possibly also along the inner
wall when the soil plug is not removed).
The unit shaft friction:
for sands: is proportional to the overburden pressure,
for clays: is calculated by the "alpha" or "lambda" method and is a constant equal to
the shear strength Cuat great depth.
Lateral friction is integrated along the whole penetration of the pile.
2.2.2 End bearing
End bearing is the resultant of bearing pressure over the gross end area of the pile, i.e. with
or without the area of plug if relevant.
The bearing pressure:
for clays: is equal to 9 Cu.
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Lecture 15A.6
The energy of the ram hitting the top of the pile generates a stress wave in the pile, which
dissipates progressively by friction between the pile and the soil and by reflection at the
extremities of the pile.
The plastic displacement of the tip relative to the soil is the set achieved by the blow. Curves
can be drawn to represent the number of blows per unit length required to drive the pile at
different penetrations.
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Page 8 of 25
The wave equation, though representing the most rigorous assessment to date of the driving
process, still suffers a lack of accuracy, mostly caused by the inaccuracies in the soil model.
3. DIFFERENT KINDS OF PILES
Driven piles are the most popular and cost-efficient type of foundation for offshore structures.
As shown in Figure 2, the following alternatives may be chosen when driving proves
impractical:
insert piles.
drilled and grouted piles.
belled piles.
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Lecture 15A.6
account the changes in load direction during lifting). Padeyes are then carefully cut before
lowering the next pile section.
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Page 13 of 25
Lecture 15A.6
Sketch E shows the different steps for the positioning of pile sections:
pile or add-on lifted from the barge deck.
rotation of the crane to position add-on.
installing and lowering of the pile add-on.
4.4.2 Pile connections
Different solutions for connecting pile segments back-to-back are used:
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Page 14 of 25
either by welding, Shielded Metal Arc Welding (SMAW) or flux-cored, segments held
temporarily by internal or external stabbing guides as shown in Figure 4. Welding time
depends upon:
- pile wall thickness: 3 hours for 1in. thick (25,4mm); 16 hours for 3in. thick, (76,2mm)
(typical).
- number and qualification of the welders.
- environmental conditions.
or by mechanical connectors (as shown in Figure 4):
- breech block (twisting method).
- lug type (hydraulic method).
4.4.3 Hammer placement
Figure 5 shows the different steps of this routine operation:
Lecture 15A.6
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Page 15 of 25
lifting from the barge deck.
positioning over pile by booming out or in (the bell of the hammer acts as a stabling
guide... very helpful in rough weather).
alignment of the pile cap.
lowering leads after hammer positioning.
Each add-on should be designed to prevent bending or buckling failure during installation and
in-place conditions.
Lecture 15A.6
4.4.4 Driving
Some penetration under the self weight of the pile is normal. For soft soil conditions, particular
measures are taken to avoid an uncontrolled run.
Piles are then driven or drilled until pile refusal.
Pile refusal is defined as the minimum rate of penetration beyond which further advancement
of the pile is no longer achievable because of the time required and the possible damage to
the pile or to the hammer A widely accepted rate for defining refusal is 300 blows/feet (980
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Page 16 of 25
the pile or to the hammer. A widely accepted rate for defining refusal is 300 blows/feet (980
blows/meters).
4.5 Pile-to-Jacket Connections
4.5.1 Welded shims
The shims are inserted at the top of the pile within the annulus between the pile and jacket leg
(see Figure 6) and welded afterwards.
Lecture 15A.6
4.5.2 Mechanical locking system
This metal-to-metal connection is achieved by a hydraulic swaging tool lowered inside the pile
and expanding it into machined grooves provided in the sleeves at two or three elevations as
shown on Figure 7.
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Lecture 15A.6
This type of connection is most popular for subsea templates. It offers immediate strength and
the possibility to re-enter the connection should swaging prove incomplete.
4.5.3 Grouting
This hybrid connection is the most commonly used for connecting piles to the main structure
(in the mudline area). Forces are transmitted by shear through the grout.
Figure 8 shows the two types of packers commonly used. The expansive, non-shrinking grout
must fill completely the annulus between the pile and leg (or sleeve).
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Page 18 of 25
must fill completely the annulus between the pile and leg (or sleeve).
Lecture 15A.6
Bonding should be excellent; it is improved by shear connectors (shear keys, strips or weld
beads disposed on the surface of the sleeve and pile in contact with the grout).
The width of the annulus between pile and sleeve should be maintained constant by use of
centralizers and be limited to:
1,5in. minimum, (38,1mm)
about 4in. (101,6mm) maximum (to avoid destruction of the tensile strength of the
grout by internal microcracking).
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Packers are used to confine the grout and prevent it from escaping at the base of the sleeve.
Packers are often damaged during piling and are therefore:
installed in a double set.
attached to the base of the sleeve to protect them during pile entry and driving.
Thorough filling should be checked by suitable devices, e.g. electrical resistance gauges,
radioactive tracers, well-logging devices or overflow pipes checked by divers.
4.6 Quality Control
Quality control shall:
confirm the adequacy of the foundation with respect to the design.
provide a record of pile installation for reference to subsequent driving of nearby piles
and future modifications to the platform.
The installation report shall mention:
pile identification (diameter and thickness).
measured lengths of add-ons and cut-offs.
self penetration of pile (under its own weight and under static weight of the hammer).
blowcount throughout driving with identification of hammer used and energy, as
shown in Figure 9.
record of incidents and abnormalities:
- unexpected behaviour of the pile and/or hammer.
- interruptions of driving (with set-up time and blowcount subsequently required to break the
pile loose).
- pile damage if any.
elevations of soil plug and internal water surface after driving.
Lecture 15A.6
information about the pile/structure connection:
- equipment and procedure employed.
- overall volume of grout and quality.
- record of interruptions and delays.
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Lecture 15A.6
TABLE 2 Large pile driving hammers
A. Air/Steam Hammers
Make Model
Rated
Energy
(ft lb )
Ram
Weight
(ki )
Max.
Stroke
( )
Std.
Pilecap
Weight
Typical
Hammer
Weight
( /l d )
Rated
Operating
Pressure
Steam
Consumption
(lb ht)
Air
Consumption
(lb ht)
Hose
ST/F
Rated
BPM
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(ft-lbs) (kips) (m)(kips)
(w/leads)
(kips)(psi)
(lbs ht) (lbs ht) .....
Conmaco
6850
5650
5300
300
200
510.000
325.000
150.000
90.000
60.000
85
65
30
30
20
72
60
60
36
36
57,5
59,0
12,7
12,7
12,7
312
262
92
86
74
180
160
160
150
120
31.500
8.064
6.944
5.563
7.500
1.711
1.471
1.195
2 @
4
3 @
4
4
3
3
40
45
46
54
59
Menck
(MRBS)
12500
8800
8000
7000
5000
4600
3000
1800
850
1.582.220
954.750
867.960
632.885
542.470
499.070
325.480
189.850
93.340
275,58
194,01
176,37
154
110,23
101,41
66,14
38,58
18,96
69
59
59
49
59
59
59
59
50
154,32
103,62
85,98
92,4
66,14
52,91
33,07
22,05
11,5
853
600
564
583
335
313
205
125
64
171
150
142
156
150
142
142
142
142
53.910
32.400
30.860
30.800
20.940
19.840
12.130
7.060
3.530
26.500
16.700
15.900
14.830
10.400
9.900
6.000
3.700
1.950
2 @
6
8
8
4 @
4
6
6
5
4
3
36
36
38
35
40
42
42
44
45
MKT
OS-60
OS-40
OS-20
18.000
120.000
60.000
60
40
20
36
36
36 38,65 150 3 60
Lecture 15A.6
C. Hydraulic Hammers
Make Model
Rated Energy
(ft-lb)
Ram Weight
(kips)
Standard
Pilecap
Weight
(kips)
Hammer
Weight
(kips)
Typical
Operating
Pressure
(psi)
Rated
Oil Flow
(gal. min)
Rated
BPM
4000
3000A
1.200.000
800.000
205
152
490
414
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Page 24 of 25
Note 1: With the heavier hammers in the range given, the wall thicknesses must be near the
upper range of those listed in order to prevent overstress (yielding) in the pile under hard
driving.
HMB
3000
1500
900
500
725.000
290.000
170.000
72.000
139
55
30,8
9,5
33
17,6
1,1
172
88
27,5
40-70
Menck
MRBU
MHU
1700
MHU
900
MH
195
MH
165
MH
145
MH
120
MH 96
MH 80
760.000
1.230.000
650.000
141.000
119.000
105.000
87.000
69.000
58.000
132
207
110
22,0
19,0
16,5
13,9
11,0
9,3
84
77
6,0
6,0
6,0
6,0
1,9
1,9
415
617
386
59
51
46
40
27
24
3400
3400
3100
3550
3190
2755
2320
2830
2465
845
845
580
98
103
102
103
75
75
50-80
32-65
48-65
38
42
42
44
48
48
Lecture 15A.6
Note 2: With diesel hammers, the effective hammer energy is from one-half to two-thirds the
values generally listed by the manufacturers and the above table must be adjusted
accordingly. Diesel hammers would normally only be used on 36-in. or less diameter piles.
Note 3: Hydraulic hammers have a more sustained blow, and hence the above table can be
modified to fit the stress wave pattern.
TABLE 3 Typical values of pile sizes, wall thickness and hammer energies
Pile Outer Diameter Wall Thickness Hammer Energy
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(in.) (mm) (in.) (mm) (ft-lb) (kN-m)
24
30
36
42
48
60
72
84
96
108
120
600
750
900
1.050
1.200
1.500
1.800
2.100
2.400
2.700
3.000
5/8 - 7/8
7/8 - 1
1 - 1
17- 1
17 - 1
1 - 2
1 - 2
1 - 2
1 - 2
1 - 2
15-21
19
21-25
25-32
28-44
28-44
32-50
32-50
32-50
37-62
37-62
50.000 - 120.000
50.000 - 120.000
50.000 - 180.000
60.000 - 300.000
90.000 - 500.000
90.000 - 500.000
120.000 - 700.000
180.000 - 1.000.000
180.000 - 1.000.000
300.000 - 1.000.000
300.000 - 1.000.000
70 - 168
70 - 168
70 - 252
84 - 120
126 - 700
126 - 700
168 - 980
252 - 1.400
252 - 1.400
420 - 1.400
420 - 1.400
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Lecture 15A.7
Tubular Joints in Offshore Structures
OBJECTIVE/SCOPE
To present methods for the design of large tubular joints typically found on offshore
structures.
PREREQUISITES
Lecture 15A.1: Offshore Structures: General Introduction
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Page 1 of 23
RELATED LECTURES
Lecture 15A.8: Fabrication
Lecture 15A.12: Connections in Offshore Deck Structures
SUMMARY
The lecture defines the principle terms and ratios used in tubular joint design. It presents the
classifications for T, Y, X, N, K and KT joints and discusses the significance of gaps, overlaps,
multiplanar joints and the details of joint arrangements. It describes design methods for static
and fatigue strength, presenting some detailed information on stress concentration factors.
1. INTRODUCTION
The main structure of topside consists of either an integrated deck or a module support frame
and modules. Commonly tubular lattice frames are present, however a significant amount of
rolled and built up sections are also used.
This lecture refers to the design of tubular joints. These are used extensively offshore,
particularly for jacket structures. Connections of I-shape sections or boxed beams whether
rolled or built up, are basically similar to those used for onshore structures. Refer to the
corresponding lectures for appropriate design guidance.
Two main calculations need to be performed in order to adequately design a tubular joint.
These are:
1. Static strength considerations
2. Fatigue behaviour considerations
Lecture 15A.7
The question of fatigue behaviour always has to be addressed, even where simple
assessment of fatigue behaviour shows this will not be a problem. The joint designer must
therefore always be "fatigue minded".
2. DEFINITIONS
The following definitions are universally acknowledged [1]: (refer to Figure 1 for clarification):
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The CHORD is the main member, receiving the other components. It is necessarily a through
member. The other tubulars are welded to it, without piercing through the chord at the
intersection.
Other tubulars belonging to the joint assembly may be as large as the chord, but they can
never be larger.
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Lecture 15A.7
2.2 Geometrical ratios
= Can slenderness ratio
= Brace to chord diameter ratio (always 1)
= Chord slenderness ratio
= Brace to chord thickness ratio
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= Relative gap
These are non-dimensional variables for use in parametrical equations.
3. CLASSIFICATION
Load paths within a joint are very different, according to the joint geometry. The following
classification is used, see Figure 2.
Lecture 15A.7
3.1 T and Y Joints
These are joints made up of a single brace, perpendicular to the chord (T joint) or inclined to it
(Y joints).
In a T joint, the axial force acting in the brace is reacted by bending in the chord.
In a Y joint, the axial force is reacted by bending and axial force in the chord.
3.2 X Joints
X joints include two coaxial braces on either side of the chord
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X joints include two coaxial braces on either side of the chord.
Axial forces are balanced in the braces, which in an ideal X joint have the same diameter and
thickness. In fact, other considerations such as brace length, which can be very different on
each side of the chord, may lead to two slightly different braces. Angles may be slightly
different as well.
The important point to note is the balance of forces in the braces. If the axial force in one
brace is far higher than the one in the other brace, the joint may be classified as a Y (or a T)
joint rather than an X joint.
3.3 N and K Joints
These joints include two braces. One of them may be perpendicular to the chord (N joint) or
both inclined (K joint).
The ideal load pattern of these joints is reached when axial forces are balanced in the braces,
i.e. net force into chord member is low.
3.4 KT Joints
These joints include three braces.
The load pattern for these joints is more complex. Ideally axial forces should be balanced
within the braces, i.e. net force into chord member is low.
3.5 Limitations
For a joint to be able to be fabricated and to be effective, the geometrical ratios given in
Section 2.2 have limitations. Table 3.1 shows these limits and their typical ranges.
Lecture 15A.7
Table 3.1 Geometrical Limits and Typical Ranges
LimitationsParameter Typical range
min max
0,4 - 0,8 0,2 1
12 - 20 10 30
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0,3 - 0,7 0,2 1 (2)
40- 90 30(3) 90(1)
(1) Physical limitation
(2) Brace shall be less or equal to chord thickness (see punching shear)
(3) Angle limitation to get a correct workmanship of welds.
3.6 How to classify a joint
This classification deals only with braces located in one plane.
It must always be remembered that this classification is based on load pattern as well as the
geometry. Engineering judgement must therefore be used to classify a joint. For example a
geometrical K joint may be classified as.
a K joint when forces are balanced within braces.
a Y joint when the force in one brace is reacted predominantly by the chord, rather
than by the second brace.
Lecture 15A.7
4. GAP AND OVERLAP
4.1 Definitions
The GAP is the distance along the chord between the weld toes of the braces (Figure 3).
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Page 7 of 23
The theoretical gap is the shortest distance between the outer surfaces of two braces,
measured on the line where they cross the chord outer surface. The real gap is the onemeasured at the corresponding location, between actual weld toes.
A brace OVERLAPS another brace when one brace is welded to the other brace.
The overlapping brace is always the thinner brace.
The overlapped brace is always completely welded to the chord.
Lecture 15A.7
4.2 Limitations
The minimum gap allowed is 50mm. This limitation is set to avoid two welds clashing. This is
important because the gap is a highly stressed zone.
4.3 Multiplanar Joints
The same definitions and limitations apply to multiplanar joints.
5. JOINT ARRANGEMENT
As a rule, welds in a joint have to be kept away from zones of high stress concentration.
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Page 8 of 23
The following practice, see Figure 4, should be followed:
1. The chord circumferential welds are to be located at either 300mm or a quarter of the
chord diameter, whichever is the greater, from the nearest point of a brace-chord
connection.
2. The brace circumferential welds are to be located at either 600mm or a brace
diameter, whichever is the greatest, from the nearest point of the brace-chord
connection.
3. The actual gap shall not be less than 50mm. To achieve this, most designers use a
70 or 75mm theoretical gap.
4. Eccentricity and offset are to be kept within a quarter of the chord diameter. When
higher values can not be avoided, secondary moments have to be introduced in the
structural analysis by introducing extra nodes.
5. Thickness transitions are smoothed to a 1 in 4 slope, by tapering the thicker wall.
Lecture 15A.7
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Lecture 15A.7
6. STATIC STRENGTH
6.1 Loads taken into account
The loads considered in a joint static strength design are the axial force, the in-plane bending
moment and the out-of-plane bending moment for each brace.
The other components (transverse shear and brace torsion moment) are usually neglectedsince unlike the preceding loads, these loads do not induce bending in the chord wall.
Nevertheless, their presence must never be forgotten and in some specific cases, their effects
must be assessed. The axial load, in-plane and out-of-plane bending moments are normally
the dimensioning criterion for tubular joints.
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6.2 Punching shear
6.2.1 Acting punching shear
The acting punching shear is the shear stress developed in the chord by the brace load.
The acting punching stress vpis written as:
vp= f sin
where f is the nominal axial, in-plane bending or out-of-plane bending stress in the brace
(punching shear for each kept separate), see Figure 5.
Lecture 15A.7
6.2.2 Allowable punching shear
Allowable punching shear values in the chord wall are determined from test results carried out
on full scale or on reduced scale models.
Tests are performed on experimental rigs such as the one shown in Figure 6. They are
performed for a single load-case (axial force, in-plane bending, or out-of-plane bending).
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Page 11 of 23
The ultimate static strength obtained through these tests can then be expressed in terms of
punching shear, as defined above.
Statistical treatments of results allow formulae to be defined for the allowable punching shear
stress.
Lecture 15A.7
6.2.3 The API method
Several offshore design regulations are based on the punching shear concept [1,2]. The
following method is presented in API RP2A [2]:
A. Principle
This method applies to a single brace without overlap, for a non-stiffened joint. Whenthe joint includes several braces, each brace connection is checked independently.
Punching shear for each load component (axial force, in-plane bending, and out of
plane bending) is calculated and compared to the allowable punching shear stress for
the appropriate load and geometry.
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Interaction formulae are given for combined loading, combining the three punching
shear ratio calculated for each component.
B. Allowable punching shear stress
The allowable punching shear stress for each load component is:
Vpa= QqQf
where: Fycis the yield strength of the chord member
Qqis to account for the effects of type of loading and geometry, see Table 6.1.
Qfis a factor to account for the nominal longitudinal stress in the chord
Qf= 1 -
fAX, fIPB, fOPBare the nominal axial, in-plane bending and out of plane bending stresses in the
chord
Lecture 15A.7
Value for and Qqare given in Table 6.1
Table 6.1 Values of Q
q
for allowable punching shear stress f rom APIRP2A
Load component Axial load In-plane bending Out of plane bending
Stress in brace fax fby fbz
Acting punching shear Vpx= faxsin Vp= fbysin Vp= fbzsin
K joints
T & Y Joints
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T & Y Joints
Qqw/o diaphragm
X
w diaphragm
Tension Compression
0,030 0,045 0,021
Qg= 1,8 - 0,1 for 20
Qg= 1,4 - 4 g/D for > 20
but Qgmust be 1,0
Q= for > 0,6
QB= 1,0 for 0,6
Lecture 15A.7
C. Loading Combination
For combined loadings involving more than one load component, the following equations shall
be satisfied:
where: IPB refers to in-plane bending component
OPB refers to out-of-plane bending component
AX f t i l f t
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AX refers to axial force component
and
ax
where: arc sin term is in radians.
6.3 Overlapping joints
The parametric formulae discussed in Section 6.2 were specifically established for non-
overlapping joints with no internal reinforcement. These formulae cannot be used foroverlapping joints.
In an overlapping joint, part of the load is transferred directly from one brace to the other
through the overlapping section, without that part of the load transferring through the chord.
The static strength of an overlapping joint is higher than a similar joint without an overlap.
API RP2A, [2] allows the static shear strength of the overlapping weld section to be added to
the punching shear capacity of the brace-chord connection, see Figure 7.
Lecture 15A.7
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The allowable axial load component perpendicular to the chord, P(in Newtons) should be
taken to be:
P= (vpaT l1) + (2vwatwl2)
where:
vpais the allowable punching shear stress (MPa) for axial stress.
l1is the circumference for that portion of the brace which contacts the chord (mm), see Figure
7.
vwais the allowable shear stress for weld between braces (MPa).
twis the lesser of the weld throat thickness or the thickness t of the inner brace (mm).
l2is the projected chord length (one side) of the overlapping weld, measured perpendicular to
the chord (mm), see Figure 7.
6.4 Reinforced joints
6.4.1 Definition
Large chord wall thickness may be reduced by stiffening the chord. The most usual
reinforcement consists of ring stiffening inside the chord.
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Lecture 15A.7
In-plane bending- as for K joint
Validity range
The above equation for T/Y, K and KT joints are generally valid for joint parameters within the
following limits:
8,333 33,3
0,20 0,8
0,3 0,8 unless stated otherwise
6 667 40 unless stated otherwise
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6,667 40 unless stated otherwise
0 90unless stated otherwise.
8. FATIGUE ANALYSIS
A fatigue analysis of a joint consists of the following steps:
1. Calculation of nominal stress ranges in the brace and the chords
2. Calculation of hot-spot stress range
3. Calculation of joint fatigue lives using S-N curves for tubular members at joints.
8.1 Nominal stress range
Nominal stress ranges in braces and chords are calculated by a global stress analyses.
Lecture 15A.7
8.1.1 Wave histogram
A wave histogram has to be obtained for each direction around the platform. A simple form of
a wave histogram is as follows:
Wave height metres) Average number per year
0-1,5
1,5-3
3-4,5
3 100 000
410 000
730 000
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4,5-6
6-8
8-10
5 000
800
20
8.2.2 Nominal stress ranges
Nominal stress ranges can be calculated by following the steps below:
1. Wave heights are grouped in "blocks", for which just one stress range will be
calculated. Different wave directions need to be considered with a minimum of three
"blocks" per wave direction.
2. For each block one representative wave is chosen, whose action is supposed to
represent the action of the whole block. The highest wave of the block is normally
chosen.
3. Nominal stresses for each joint component are then calculated for different phase
angles of the chosen wave, for one complete cycle (360 ). The nominal stress range
for the joint component is defined as the difference between the highest and the
lowest stress obtained for a full wave cycle. Four to twelve phase angles per wave
are usually considered.
8.2 Hot spot stress ranges
Hot spot stress ranges are then evaluated for each chosen joint location by applying
parametric formulae [4] (or by applying the SCF calculated from a detailed analysis).
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Lecture 15A.7
8.4 Cumulative Fatigue Damage Ratio
The stress responses should be combined into the long term stress distribution, which should
then be used to calculate the cumulative fatigue damage ratio, D, given by:
D =
Where,
n is the number of cycles applied at a given stress range
N is the number of cycles to cause failure for the given stress range (obtained from
appropriate S N curve)
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appropriate S-N curve).
In general the design fatigue life of each joint and member should be at least twice the
intended service life of the structure, i.e. a safety factor of 2,0.
For critical elements whose sole failure would be catastrophic, use of a larger safety factor
should be considered.
9. CONCLUDING SUMMARY
Terminology, geometric ratios and joint classifications are now standardised for
tubular joints.
The presence of gaps and overlaps significantly influence joint behaviour.
Determination of static strength is generally based on the concept of punching shear,
with the allowance of overlapping joints.
Special analysis are required for reinforced joints.
Stress concentration factors (SCF) are defined for most commonly occurring joints.
Determination of fatigue strength is based on nominal stress range multiplied by
appropriate SCF.
Lecture 15A.7
10. REFERENCES
[1] Offshore Installations: Guidance on Design, Construction and Certification. Fourth Edition,
HMSO, 1990.
[2] Recommended Practice for Planning, Designing and Constructing Fixed Offshore
Platforms, API RP2A Nineteenth Edition.
[3] Young, Warren C, Roark's Formulae for Stress and Strain. Sixth Edition, McGraw-Hill.
[4] Stress Concentration Factors for Simple Tubular Joints, 1989, Volumes 1 to 5, Lloyds
Register of Shipping-Offshore Division.
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Lecture 15A.8
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Lecture 15A.9
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Lecture 15A.9
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Jackets destined for deeper water are heavier and are usually erected on their side and
launched from a barge (Figure 2). This method of construction is currently applicable for
jackets up to 25000 tonnes. A launched jacket usually requires additional buoyancy tanks with
extensive pipework and valving to enable the legs and tanks to be flooded in order to ballast
the jacket into the vertical position on site. For instance, in the case of the Brae 'B' jacket (alarge 19000 tonne jacket installed in 100m water depth in the North Sea) it was necessary to
provide 11000 tonnes of additional buoyancy. This buoyancy was primarily to limit the jacket
trajectory through launch (i.e. to stop it hitting the sea bed) but was also essential for
maintaining bottom clearance during up-ending. The additional buoyancy took the form of two
'saddle' tanks, two pairs of twin 'piggy-bank tanks' and twelve 'cigar' tubes installed down the
pile guides (Figure 3). Altogether the auxiliary buoyancy added about 3,300 tonnes additional
weight to the jacket.
Lecture 15A.9
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Lecture 15A.9
Very large jackets, in excess of launch capacity, have been constructed as self-floaters in a
graving dock, towed offshore subsequent to flooding the dock, and installed on location by
means of controlled flooding of the legs (see Figure 4).
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1.3 Installation Planning
The installation of a jacket consists of loading out, seafastening and transporting the structure
to the installation site, positioning the jacket on the site and achieving a stable structure in
accordance with the design drawings and specifications, in anticipation of installation of the
platform topsides.
An important aspect is the avoidance of unacceptable risk during offshore activities from
loadout through to platform completion. It is recognised that the potential cost to the projectassociated with failure to successfully execute marine activities is particularly high. Normally
therefore the contractor is obliged to produce procedures for these activities which
demonstrate that the risk of failure has been reduced to acceptable levels. He is also required
to demonstrate that, prior to the commencement of an activity, all the necessary preparations
have been completed.
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Lecture 15A.9
5. Lloyds Register of Shipping, Rules and Regulations for the Classification of Fixed
Offshore Installations, 1989. Based on Lloyd's experience from certification of over
500 platforms world-wide.
Table 1 Major Offshore Crane Vessels
Operator Name Type Mode Lifting Capacity
Thor Monohull
Fix
Rev
2720
1820
Odin Monohull
Fix
Rev
2720
2450
Hermod Semisub
Fix
Rev
4536 + 3628 = 8164
3630 + 2720 = 6350
Heerema
Fix 3630 + 2720 = 6350
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Balder Semisub
Fix
Rev
3630 + 2720 = 6350
3000 + 2000 = 5000
DB50 Monohull
Fix
Rev
4000
3800
DB100 Semisub
Fix
Rev
1820
1450
DB101 Semisub
Fix
Rev
3360
2450
McDermott
DB102 Semisub Rev 6000 + 6000 = 12000
Micoperi M7000 Semisub Rev 7000 + 7000 = 14000
Notes:
1. Rated lifting capacity in metric tonnes
2. When the crane vessels are provided with two cranes, these are situated at the
vessels stern at approximately 60m distance etc.
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Lecture 15A.10
Superstructures
I
OBJECTIVE/SCOPE
To introduce the functional requirements; to identify major interfaces with the process,
equipment, logistics, and safety; to introduce the structural concepts for jacket and gravity
based structure (GBS) topsides; to elaborate on structural design for deck floors.
PREREQUISITES
Lectures 1A& 1B: Steel Construction
Lecture 2.4: Steel Grades and Qualities
Lecture 2.5: Selection of Steel Quality
Lectures 3.1: General Fabrication of Steel Structures
Lecture 6.3: Elastic Instability Modes
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Page 1 of 21
Lecture 7.6: Built-up Columns
Lectures 8.4: Plate Girder Behaviour & Design
Lectures 11.2: Welded Connections
Lecture 12.2: Advanced Introduction to Fatigue
Lectures 15A: Structural Systems - Offshore
SUMMARY
The topside lay-out is discussed, referring to API-RP2G [1], and to general aspects of
interface control and weight control.
The different types of topside structures (relevant to the type of substructure, jacket or GBS)
are introduced and described. These types are:
1. integrated deck.
2. module support frame.
3. modules.
Floor concepts are presented and several aspects of the plate floor design are addressed.
Lecture 15A.10
1. INTRODUCTION
This lecture deals with the overall aspects of the design of offshore topsides.
The topside of an offshore structure accommodates the equipment and supports modules and
accessories such as living quarters, helideck, flare stack or flare boom, microwave tower, and
crane pedestals.
The structural concept for the deck is influenced greatly by the type of substructure (jacket or
GBS) and the method of construction, see Figures 1 and 2.
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Lecture 15A.10
Heavy decks, over 10,000 tons, are provided with a module support frame onto which a
number of modules are placed. Smaller decks, such as those locat