7/29/2019 CIVL4171 Offshore Structure
1/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Offshore Structures 8551
Lecture Plan & Assessment
Dr Andrew GrimeArup
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
2/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
3/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
4/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Steel GBSs - Self-installing / Relocatable Platforms
CIVL 4171 Platform,Pipeline & Subsea Technology
Typical ACE Installation
Entire facility installed
as one unit
Removal is a reverse
of the installation
sequence
7/29/2019 CIVL4171 Offshore Structure
5/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Steel GBSs Other Examples
CIVL 4171 Platform,Pipeline & Subsea Technology
Concrete GBS Key Elements
7/29/2019 CIVL4171 Offshore Structure
6/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
7/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Condeep Type GBSs
CIVL 4171 Platform,Pipeline & Subsea Technology
Condeep Construction and Installation
7/29/2019 CIVL4171 Offshore Structure
8/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
9/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
10/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Gravity base offshore structures
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
CIVL 4171 Platform,Pipeline & Subsea Technology
Gravity base offshore structures
CGS and CONDEEP comparison
7/29/2019 CIVL4171 Offshore Structure
11/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
12/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Foundation criteria
CIVL 4171 Platform,Pipeline & Subsea Technology
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
7/29/2019 CIVL4171 Offshore Structure
13/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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!
CIVL 4171 Platform,Pipeline & Subsea Technology
CGS Construction
7/29/2019 CIVL4171 Offshore Structure
14/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
7/29/2019 CIVL4171 Offshore Structure
15/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
7/29/2019 CIVL4171 Offshore Structure
16/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Oil storage systems
CIVL 4171 Platform,Pipeline & Subsea Technology
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).
7/29/2019 CIVL4171 Offshore Structure
17/18
CIVL 4171 Platform,Pipeline & Subsea Technology
Future uses for CGS
CIVL 4171 Platform,Pipeline & Subsea Technology
Future uses for CGS
7/29/2019 CIVL4171 Offshore Structure
18/18
CIVL 4171 Platform,Pipeline & Subsea Technology
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
CIVL 4171 Platform,Pipeline & Subsea Technology
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.
Top Related