PowerPoint PresentationFPSO Hull Structure,
Design and Maintenance
The Basic Principles
OUTLINE
Notes expand on issues covered by presentation
Content:
Introduction
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INTRODUCTION
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WHAT MAKES AN FPSO DIFFERENT FROM A TRADING TANKER?
Operations and safety requirements
Complexity of design, construction & commissioning
In-situ inspection / maintenance
Economic consequences of hull failure can be enormous for FPSO
This not the case for trading oil tanker
Hull typically of the order of 10% of project in terms of CAPEX
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TYPICAL FPSO PARAMETERS
New-build or Conversion
Topsides 2000 - 33000 tonnes
Air temp -20°C - > 30°C
Turret or Spread mooring arrangements
Permanent / Disconnectable Turret
Wave height is 100 years maximum value
Water depth limit is being constantly pushed deeper (may be greater than 2000m)
Note air temperature influences materials (steel grade)
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SOME TYPICAL ENVIRONMENTAL CRITERIA
Nova Scotia 16.0 14 / 18
West of Shetland 17.1 16.0
Hs is “average” measure of wave height (in metres)
100 year maximum “average” is shown
Maximum is about 1.8 times average value
Tp is measure of period (in seconds)
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Typical General Arrangements
Utilities
New build might be only option for very harsh locations
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MOORING ARRANGEMENTS
Turret (internal) involves significant modifications for conversions
External turret prefferential to internal (from structural perspective)
Internal necessary for harsh conditions
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TURRET MOORING SYSTEMS
Large Internal Turret
Small Internal Turret
Most systems are disconnectable to avoid environmental hazards, i.e iceberg, cyclone etc
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Turret & Swivel System
Concept study
Global hydrodynamic/structural analysis undertaken by LR
Low on-site motions + global loads
Reduced construction schedule due to offshore (on-site) mating
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Hull form gives lower motions
Concept study highlighted problems with transit condition/loads/configuration
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DEVELOPMENT ALTERNATIVES :
KEY CHOICE DRIVERS 1 OF 2
Project and client requirements
Schedule / time to first oil (new build: 2 to 3 years; conversion: 1 to 2 years)
Location / environment
Field life, CAPEX and OPEX
Early production or full field?
Conversions more suitable for benign locations
New build might be only option for very harsh locations
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KEY CHOICE DRIVERS 2 of 2
Ship yard schedule
Availability / capability / price
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NEW BUILD HULL - ADVANTAGES
Compliance with latest regulations
Integrated design
NEW BUILD – GRIFFIN VENTURE FPSO
90m water depth
Turret moored (external)
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NEW BUILD - TERRA NOVA FPSO
150,000 bobd
Hull is twice the strength of typical tanker
Capable of withstanding limited iceberg collision
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NEW BUILD – SHELL EA FPSO
100,000 bopd
250m long
NEW BUILD - SNEPCO BONGA FPSO
225,000 bopd
300m long
CONVERSION - VESSEL SELECTION ISSUES
hull global strength
remaining corrosion margins
remaining fatigue life
structural steelwork replacement
Must account for previous fatigue history
Additional strengthening / brackets possibly required
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CONVERSION MAJOR WORKS SUMMARY
CRANES
EXAMPLE OF CONVERSION
EXAMPLE OF CONVERSION
RULES AND REGULATIONS
DEFINITIONS
Class
to Class Society Rules
LR Rules for a Floating Offshore Installation at a Fixed Location (FOIFL Rules)
LR Rules for Ships
GOVERNING REGULATIONS
Classification Rules
Some authorities require identification of Safety Critical elements (ie. hull)
SCEs then require verification by third party against Performance Standard
Class rules provide appropriate Performance Standard
Flag state
Disconnectable FPSOs effectively become ships – flag becomes relevant
There is a great diversity of approach regarding NA (National Authority) regulations
Some are very prescriptive
Others are less so
None (to my knowledge) are so detailed as to specifically address structural details relating to ship type FPSOs
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SCOPE OF LR FPSO RULES
LR Rules cover the following aspects :
hull and marine systems
FOIFL Rules refer to Ship Rules – Oil Tankers
As of April 2006 new tanker rules for double sided vessels
Developed jointly by ABS, DNV and Lloyd’s Register
See www.jtprules.com
LR GUIDANCE NOTES SUPPORTING THE RULES
An Overview of Regulatory Issues
Conversion of Tankers for Floating Production Service
Ship Type Hull Structural Appraisal and supplement Additional Structural Aspects
Positional Mooring Systems and supplement Fibre Ropes for Offshore Moorings
Periodic Inspection
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HULL STRENGTH ISSUES
HULL GIRDER - WAVE BENDING
Ship rules are simple parametric
FPSO loads (as shown) are complex to evaluate, and require site metocean data
Note: diagram shows only WAVE loads
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HULL GIRDER - BENDING MOMENT CAPACITY
-800000
-600000
-400000
-200000
0
200000
400000
600000
800000
Bending due to cargo also shown here
Outcome of analysis: permissables assigned along length
Vessel requires a loading instrument
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HULL GIRDER - WAVE SHEAR
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GLOBAL HULL DESIGN – WAVE BM & SF
Still water loading conditions often more severe for FPSO
Wave loading depends upon site environmental criteria:
harsh environment wave BM and SF are close to and may exceed Ship Rule requirements
benign environment wave BM and SF are less than Ship Rules (but not to be taken less than 70% of unrestricted service Ship Rule requirement)
Multisite operation may require assessment
Towing, inspection and temporary conditions to be considered
FOIFL Rules require assessment of site specific wave loading
(100 year return values)
(20 year return values)
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Example Transit case (Bonga)
HULL STRENGTH / FATIGUE – FE MODELS
Cargo / ballast tank area
Riser porches and supporting structure
Topsides support structure and hull integration
Flare tower support structure
Offloading station support structure
Helideck structure
See also section on Fatigue
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CARGO TANK MODEL
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MOORING TURRET INTEGRATION
TOPSIDE SUPPORT STRUCTURE
High lever arm of topsides
Deck can be highly stressed already
Misalignment possibilities
Deck flexing can be an issue
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BOTTOM SLAMMING
Fore end draughts might be lower than for trading ship
Heading probabilities of FPSO different compared to a ship
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BOW AREA IMPACT
Rules contain requirements relating to draft, pressure, extent and scantlings
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GREEN SEAS – BOW AND MAIN DECK
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GREEN SEAS – BOW AND MAIN DECK
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GREEN SEAS – BOW AND MAIN DECK
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GREEN SEAS – BOW AND MAIN DECK
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GREEN SEAS – MAIN DECK
Wind
Current
Energy is concentrated
Wave height increases
deck in midship section
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GREEN SEAS
Rules for ships incorporate allowances within design heads for superstructures, deckhouses, bulwarks and decks.
Model testing advisable
SLOSHING IN CARGO OIL TANKS
Diagram shows cross section of cargo tank
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SLOSHING IN CARGO OIL TANKS
Sloshing is the dynamic magnification of pressures within cargo/ballast tanks
Partial fillings the norm for FPSOs (NOT so for trading oil tankers)
Influencing Factors:
Tank size
Hull form
Environment / Weathervaning
Tank fill levels
Sloshing analysis required (e.g. LR FLUIDS)
Analysis may result in an increase in scantlings at tank tops
Restrictions may be placed on fill levels in certain tanks
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ACCIDENTAL LOADS
Dropped object
Explosion / blast
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HULL FATIGUE DESIGN
FATIGUE – the problem
The effect on metal of repeated cycles of stress.
There is no obvious warning, a crack forms without appreciable deformation of structure making it difficult to detect the presence of growing cracks.
Fractures usually start from small nicks or scratches or fillets which cause a localised concentration of stress.
Failure can be influenced by a number of factors including size, shape and design of the component, condition of the surface or operating environment.
Fatigue life of structural details need to be assessed for FPSO
Fatigue occus mostly at welds and grows from there
Note also stress/corrosion problem
Level and approach to be adopted is set out in Rules
Fatigue is independent of strength of steel
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FATIGUE LIFE ASSESSMENT
Site specific analysis of dynamic loads
Previous service history to be assessed (for conversions)
Selection of S/N curves
Note high sensitivity to load
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EXAMPLE FEM MESH ARRANGEMENT TO
DETERMINE SCFS
Note we are trying to get an accurate assessment of the stress at the possible site of fatigue crack
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FATIGUE LIFE - FACTORS OF SAFETY
Substantial consequences of failure are defined as loss of life, uncontrolled outflow of hazardous products, collision or sinking and should include for progressive failure scenarios.
Note 1 : Includes external components which can be dry inspected / repaired.
Note 2 : Includes external components below the minimum operating draft which can only be inspected in water, but dry repairs could be carried out.
Inspectable / Repairable
Non - substantial
Substantial
Yes, dry (see note 1) Yes, wet (see note 2) No
1 2 3
2 4 10
TYPICAL CRITICAL AREAS
Note the wetted area fatigue due to high dynamic loads
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FABRICATION TOLERANCES AND WELDING
Tolerances should be in accordance with good shipbuilding practice, and agreed with Class Society
Special attention given to fatigue sensitive areas
Enhanced NDE levels will be necessary for identified fatigue sensitive areas
Assumptions used in analytical fatigue assessments to be consistent with construction
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FPSO FATIGUE ENHANCEMENT DETAILS CONVERSION
Transverse
frame
Longitudinal
Extent Item
Additional Lugs Additional Bkts
Bottom and side shell (up to limit of wetted area) Bilge area and side shell (up to limit of wetted area)
Additional
lug
FPSO IN-SERVICE SURVEY
IN SERVICE SURVEY PROGRAM
developed by the owner / operator against list of surveyable items
Survey plan to be approved by Class
To address :
Class requirements
Regulatory requirements
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STRUCTURAL INSPECTION
Internal survey considerations:
cleaning & gas freeing
IN WATER SURVEYS
Underwater marking
Venting/isolation arrangements for tank entry
Arrangements considered for survey/changeout of thrusters, sea chests, rudder bearing etc.
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Permanent Means of Access (PMA)
SOLAS requirements for newbuild vessels
All internal tanks
Access for close up examination
Utilise tanks structural members, or provide dedicated arrangements
Standards for ladders, handrails, etc
Portable equipment may be used for some
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SOLAS Permanent Means of Access (PMA)
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SOME LESSONS LEARNT and IN-SERVICE INCIDENTS
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GENERAL LIFE OF FIELD ISSUES
‘Holistic’ approach for good integrated solution
Offshore instead of Ship maintenance philosophy
Design for accidental loads, including wave impact
Compatibility of hull structural, systems and topsides – interface design
Fatigue Design (& life to date for conversions)
Production equipment for extreme weather conditions
Marine equipment robustness, particularly cargo/ballast systems
‘OIWS’ requirements and corrosion protection
Access (operability / maintainability / inspectability)
Provision of spare parts
FPSO INCIDENT HISTORY – Examples of problems
Hull Structure
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FPSO INCIDENT HISTORY - STATISTICS (REPORTED)
Ref : 22 vessels, approx 10 years of records
Vessel system
Hull Topsides Machinery Other
Structure failure in heavy weather Other structural failure Collision Structural failure in heavy weather Process equipment failure Fire Engine room fire Engine room power failure Main engine failure Rudder / steering gear failure Hydrocarbon release Explosions Loss of anchor (s)
1 3 6 3 2 0 2 1 1 3 2 0 1
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FATIGUE FAILURES
Longitudinal extent is approximately 150-200 metres
Vertical extent is approximately 10 metres
Transverse webs are not shown
Fatigue problems occurred at transverse webs/longitudinal stiffeners in wetted area (red dots)
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TYPICAL DAMAGE
EXAMPLES OF INTERNAL WELDED REPAIRS
Side shell
SCHIEHALLION FPSO BOW WAVE IMPACT DAMAGE
Similar incidents
FPSO RULE AMENDMENTS FOR FORE END STRUCTURES
Basis - Ship Rule requirements using min. service speed 12 - 15 knots
Site specific assessments required where expected to be more severe
Model test measurements of impact pressures recommended
Integrated structural design approach to be adopted using direct calculation methods (LR Rules for permissible stress requirements)
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For more information, please contact:
Lloyd’s Register Asia
Oil and Gas Services
T: +65 6273 3424
F: +65 6273 1662
Bending Moment
Shear Force
Deep Draft
100 Year
Light Draft