CIVL4171 Offshore Structure

7/29/2019 CIVL4171 Offshore Structure

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CIVL 4171 Platform,Pipeline & Subsea Technology

Offshore Structures 8551

Lecture Plan & Assessment

Dr Andrew GrimeArup


Gravity base offshore structures

Characteristics & Types of GBSs

Key Issues in GBS Selection

Steel & Concrete GBSs

GBS Development

Design

Construction

Oils Storage Systems

Installation

Summary

The Future


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Characteristics of GBSs

Use the mass of the topsides, substructure andtrapped soilin skirts

(or solid ballast) to resist uplift.

Can be concrete or steel:

Traditionally concrete.

Steel examples becoming more common, particularly for self-installing

platforms

Well-suited to one or more of the following:

Storage (oil, condensate, LNG, LPG) / no pipelines.

Large topsides particularly concrete GBSs.

Remote locations for mobilisation of installation equipment.

Relatively weak soils.

Removable / relocatable (must be designed in).

Ice load resistance.


Types of GBSs

Concrete:

1st GenerationCondeeptype

Mid 70s to mid90s

2nd Generation dry-built CGSs (Concrete Gravity Substructures)

Late 80s onwards

Steel:

Suited to self-installing platforms - becoming more common

ArupACE

TalismanMOAB

Technip TPG500


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Key Drivers for Selecting a Substructure Type

CAPEX: Total installed substructure cost:

Onshore fabrication

Transport to site

Substructure installation

Follow-on works piles for jackets solid ballast & / or scourprotection for GBSs (if required)

Total installed topsides cost:

Onshore fabrication

Transport to site

Topsides installation

HUC

OPEX:

Seldom a key driver for fixed substructures.


Key Drivers for Selecting a Substructure Type

Function of offshore facility

Is infield storage needed ? or

New (or existing) pipelines instead ?

Physical drivers:

Water depth

Soils

Metocean (both operational and for installation)

Size of topsides

Offshore industry capacity: Currently becoming an issue

Fabricators

Marine contractors

Schedule drivers usually, but not always, its neededASAP


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Steel GBSs - Self-installing / Relocatable Platforms


Typical ACE Installation

Entire facility installed

as one unit

Removal is a reverse

of the installation

sequence


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Steel GBSs Other Examples


Concrete GBS Key Elements


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GBS Development Phase 1 70s & 80s

GBSs answered the development needs of the Northern North Sea in the1970s.

Gulf of Mexico technology OK for Southern North Sea (started in 60s):

Piled jackets

Subsea pipelines

GoM technology insufficient in northern North Sea:

Piles

No subsea hammers

Large fields = heavy topsides

High loads = long piles = long time

Limited offshore floating crane capacity

Adverse seastates

Cost of subsea pipelines Deep waters

Adverse installation seastates


GBS Development Phase 1 70s & 80s

Phase 1GBSs solved these issues:

Very large concrete substructures

Innovativewet dockconstruction (Norwegian fjords or Scottish sea lochs)

Most successful example, the NorwegianCondeep

Last Condeep,Troll, installed in 96

~20 such GBSs built, ~75% Condeeps

Condeeps provided:

Fast offshore installation (gravity base = no piles)

Well suited to harsh metocean (installation & operation)

Good topsides solution:Ability to carry large topsides

Deck-mating in inshore sheltered waters

Limited offshore HUC

On-board oil storage in base caisson:

Limiting need for subsea pipelines


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Condeep Type GBSs


Condeep Construction and Installation


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Became progressively more complex

to design & build:

Bigger, deeper

Based on shells structures

Very efficient for high hydrostatic

loads on elements

Particularly for base caisson

But this is hardest part to design &

build

Marrying shell elements increased

design & construction complexity Needed progressivelyhigher tech

concrete up to 85MPa cube

Condeep Construction and Installation


Meanwhile Other Technologies Improve

Piled jackets (in the 80s)

Improved installation barges

Pile hammers larger, & subsea

Heavy lift crane vessels for topsides installation

Floating platforms - particularly FPSOs (in the 90s)

FPSOs seen as the ideal solution

Cheap ship hulls

Apparently simple topsides

Good answer for deep waters

Simple infield storage Reusable

Many FPSOs did not achieve initial promise (substantial cost& schedule overruns)

but


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Meanwhile Other Technologies Improve

but Majority of these problems now solved

Subsea pipelines (on-going)

Improved installation technologies

More extensive (& virtually permanent) networks

So infield storage requirement diminished


GBS Development Phase 2 89 & onwards

Condeeps

Constrained by places to build

Wet dock construction expensive

No Condeep type substructures currently

planned worldwide

Phase 2 the dry-built CGS:

Built entirely in the dry

Ravenspurn North - the 1st

Made concrete competitive again

10 built so far

Advantages of concrete GBSs retained Storage

Large topsides

Good for remote locations

Good for relatively weak soils

Removable / relocatable

Ice load resistanceRavenspurn North, 1989


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Built entirely in the dry Most skilled civil contractors can build them.

Well suited to countries without large offshorefabricators:

Eg Australia & some SE Asia

Focus on simple construction & installation:

Flat slabs & straight walls easier than cylinders &domes.

Lower concrete strengths (50 60MPa cube).

All outfitting also in the dry.

Much simpler ballasting systems.

Well suited to offshore deck floatovers.

More applicable to shallower waters

Flat elements become inefficient with

high hydrostatic heads

But efficient designs for 160m

water depths have been developed.

WandooB

1996



CGS and CONDEEP comparison


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Design requirements

Stable under transportation and installation.

Carries topsides weight.

Resists hydrostatic pressure at installation & operation.

Transmits environmental loading to the underlying soils.

Accommodates oil temperature differences in operation.

Abandonable or Removable.


CGS Codes of Practice & Guidance Notes

Det norske Veritas -Rules for the Classification of Fixed

Offshore Installations

ISO 19903:2006 -Petroleum and natural gas industries -- Fixedconcrete offshore structures

BS8110 -British Standard for the Structural Use of Concrete

NS3473 -Norwegian Standard for Concrete Structures


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Foundation criteria


Foundation design issues

Surficial soils critical unlike piles different SI data & testingneeded.

Generally governed by sliding, overturning or bearing capacityfailure.

Skirts used to transfer loads to competent strata but they have to

penetrate to it too.

Sandy soils: weight x angle of shear resistance.

Silt and clay soils: base area x the shear strength.

Skirts also contribute.

Soil strength degredation in storms - build up of porewater pressure. Scour protection needed if competent strata is shallow.

Settlements generally higher than with piles

Potential for liquefaction under extreme seismic


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CGS Construction

Dock or casting basin facilityrequired.

Only one casting basin in

Australia at Bunbury.

Several European docks exist.

Potential sites in SE Asia.

Medium to large projects can

economically cover the cost of

casting basin construction.

Designing for easy construction

IS KEY!


CGS Construction


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CGS Construction

Suspended roof slab constructionneeds an economic methodof decking out to fixreinforcement:

Steel beams on metal decking isone solution (sacrificial).

No on-site welding is involved.

Use relatively low averagereinforcement density (300 350kg / m3) for CGS as awhole.

Higher densities uneconomic, &

excessively slow to build.


CGS Construction

Slipforming is the most commonconstruction method.

Form rises continually onhardened concrete.

Very successful method, ifcorrectly done.

Elements must be detailedcorrectly.

Must get concrete mix right too: Strength & durability. Workability. Varied setting times for

slipforming.


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CGS Outfitting

Usually critical path & timeconsuming.

Keep steel to concrete interfacesimple.

Modularise when possible, maximisingoff-site fabrication.

Minimise number of connection pointsto the concrete.

Maximise work at low level.

Provide good temporary man access,& weather protection.

Minimise work in shafts.

Dont do civil & mechanical work in

the same area at the sametime.


CGS Outfitting

Oil & condensate can be stored in

CGSs.

Three systems available:

Closed wet

Open wet

Dry (for condensate)

Simplicity is the key on modernsystems.

Cast-in pipework ensures durability &

robust, leak-tight pipeworksystems.


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Oil storage systems


Floatover Deck Installation

Malampaya shown 12,000tedeck

Naturally suits CGS layout &foundations.

Deck floatovers good solution tominimise HUC.

Wandoo was first open oceanapplication.

Economic where large heavy lift

crane vessels notavailable.

Only relatively small oceangoing crane vesselscurrently operate in theAsia Pacific (circa 2,000temax. offshore lift).


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Future uses for CGS


Future uses for CGS


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Gravity base offshore structures - Advantages

No crane vessel needed

Fast offshore installation

Good for weakish soils

Good for heavy topsides

Oil storage in base

Ideal for deck floatover

Durability / low maintenance

Fatigue resistant

Robust re ship impact

Appurtenance protection

Low skill labour to build

Local content

Fast HUC

New generation CGSsstraightforward to remove

Good option if fabricators arebusy


Gravity base offshore structures - Disadvantages

Best suited to remote offshore sites

Construction requires casting basin and deep channel for tow.

Less suited to well developed oil province with large numbersof existing pipelines (storage not required)

Likely to be less economic than than traditional piled jacketsif Advantages issues not key.

CIVL4171 Offshore Structure

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