Offshore Structures - DNV GL Floating Structures-Day1... · Wikipedia: a vertically moored floating...
Transcript of Offshore Structures - DNV GL Floating Structures-Day1... · Wikipedia: a vertically moored floating...
DNV GL © 2015
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31 October 2016 SAFER, SMARTER, GREENERDNV GL © 2015
31 October 2016
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OIL & GAS
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Introduction - TLP
Offshore Structures
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What is a TLP
DNV-OS-C105: “A Tension Leg Platform (TLP) is defined as a buoyant unit
connected to a fixed foundation (or piles) by pre-tensioned tendons. The tendons
are normally parallel, near vertical elements, acting in tension, which usually
restrain the motions of the TLP in heave, roll and pitch. The platform is usually
compliant in surge, sway and yaw.”
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What is a TLP (cont’d)
Wikipedia: a vertically moored floating structure normally used for the offshore
production of oil or gas, and is particularly suited for water depths greater than
300 meters (about 1000 ft) and less than 1500 meters (about 4900 ft).
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Evolution of TLP
1st TLP in the world – Hutton for ConocoPhillips
– 6 column TLP
Conventional 4 Columns - Snorre
The deepest water depths installed to date:
– 4,674 ft (1,425 m) Magnolia ETLP
– 4,300 ft (1,300 m) Marco Polo TLP
– 4,250 ft (1,300 m) Neptune TLP
– 3,863 ft (1,177 m) Kizomba A
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TLP Worldwide
Offshore Magazine, 2010
25 installed (2010)
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TLP Types
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TLP Types
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MOSES TLP SeaStar TLP
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Worldwide Statistics
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How Does TLP Compare with Other Structures
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Courtesy of HOE
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TLP & SEMI
Differences and commonalities
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TLP MOTIONS AND HYDRODYNAMIC LOADS
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Motion Basics - Pretension
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Weight
Riser
Tension
Buoyancy
Tendon
Tension
Tendon
Tension
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Motion Basics – Offset & Setdown
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θ
Responses of the TLP to Steady Wind and Current are:
• Offset (lateral)
• Setdown (vertical)
Setdown
(Offset & Setdown are shown highly exaggerated)
Wind
Current
Offset
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TLP Global Stiffness
How do you Calculate the Stiffness of a TLP Tendon System?
Horizontal Stiffness:
KH = f (Tendon Pretension and Length) = P/L
Vertical Stiffnesses :
KV = f (Area, Material and Length) = AE / L
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TLP Global Stiffness
Assume:
• P = 1500 kips
• L = 3000 ft
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• A =100 in2 (per tendon)
• 8 Tendons
KH = P/L = 8 x 1,500/3,000
KH = 4 kips/ft
KV = AE / L = 8 x 100 x 30,000/3,000
KV = 8,000 kips/ft
Orders of
Magnitude
Difference!
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Motion Basics – Offset & Setdown
How to Calculate Offset?
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water depth (m) 300 600 1200
Displacement (tonnes) 47 000 52 000 63 000
Total tension (tonnes) 12 000 17 000 24 000
Draught (m) 31.0 32.0 34.0
Column Diameter 16.4 17.0 18.0
Tether Diameter (m) 0.26 0.30 0.31
Axial stiffness, one tether (kN/m) 34 080 20 000 11 300
Heave eigen period (s) 2.0 2.2 3.3
Horizontal eigen period (s) 85 107 140
Example values
k
mT 2
k=stiffness (water plane + tethers)
m = mstructure+m (added mass)
16 tendons
TLP Natural Period - Heave
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sec53.2/320000
52000**2
mkN
tT
mkNsmmkgkAwgk tethertot /000,201617/81.9/1025 223 TLP – example
9125kN/m +320,000 kN/m
Heave: cont’
= 320,000 kN/m
Water plane
stiffness
Tendon
stiffness
TLP Natural Period - Heave
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sec15/9125
52000**2
mkN
tT
SEMI – example
Heave: cont’
mkNsmmkgkAwgk tethertot /200001617/81.9/1025 223 9125kN/m +320 000kN/m
= 320,000 kN/m
Water plane
stiffness
Tendon
stiffness
TLP Natural Period - Heave
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Motion Characteristics
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TLP Stability
TLP stability is controlled by min/max
tendon tension
Allowable weight and COG shift envelope to
be established
Damage extent as per MODU:
– 1) Any one tendon compartment
– 2) All compartments that could be
flooded as a result of damages min 1.5 m
deep and 3.0 m high at any level
between 5.0 m above and 3.0 m below
SWL
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SWL should account for any possible draft, i.e.
vertical extent account for tide, storm surge,
setdown, seabed subsidence
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Airgap Requirement
Airgap Same as API-RP-2T
– 5’ for 100-year event
– Positive for 1000-year event
Relative motion between structure
& wave
Disturbed wave shall be used
Wave asymmetry factor of 1.1
(with 90% fractile response, or
1.2 for most probable max)
Local structure can be reinforced
against wave slamming, if
necessary
Should be checked at early design
phase
Upwelling
Run-up
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Complex response to wind, waves and current
– Wave frequency (WF) response due to wave loading on the floater.
– Low frequency response (LF) due to dynamic excitation from wind- and 2nd order wave forces.
– Horizontal LF is motion governed by resonance dynamics of the riser/mooring/floater system. Damping is essential for prediction of LF motions.
– Mean offset governed by mean environmental loading and restoring characteristics of the riser/mooring/floater system.
Su
rge
mo
tio
n
timeMean +LF+WF motion components
mean
LFWF
WF – and LF Floater Motion Characteristics
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Springing
– Continuous high-frequency response
– Low to moderate sea states
– Low energy level. High number of cycles.
– Important for tether fatigue (FLS)
Ringing
– High-frequency transient(impulsive) response
– High sea states. Steep individual waves.
– Few events
– Important for extreme tether tension (ULS)
Springing/Ringing
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Structural design
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General column :
-As for "General pontoon"
General pontoon :
Plate butts: Primary area, IC II
Girders/Frames: Secondary, IC III
Stiffeners: Secondary, IC III
IC x - Inspection Category x
Pontoon deck
Z-quality
Section A-A
A
A
A
A
Cast Node:
Special area, IC I
Primary area: IC I
Special Area, IC I
Radius=1m
1)
This detail is normally fatigue critical, and hence the inspection category is
increased from II to I.
1)
Structural Category Based on OS-C105
Structural Category:
- Special
- Primary
- Secondary
Special:
- Key Connections
- High load concentration areas
- Tendon interface with foundation and hull
- Tendon and tendon connectors
- Highly utilized areas supporting crane pedestals, flare boom etc
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Load and Load Conditions
Urbanist
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Design loads
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Wind loads
topside loads
Tendon
& Riser
loads
Tank
Loads &
Ballast
Wave &
current loads
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ALS Conditions
In general, most relevant
accidental events for hull and
deck are:
– dropped objects
– unintended flooding
– abnormal wave events
– explosion
– collision
– fire
Structural design to consider:
– resistance to a relevant
accidental event
– capacity after an accidental
event
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Oil Rig Photos
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Design Considerations
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Global Structure: Design wave approach or Stochastic approach, 3D FEA analysis
Local structure: Local effects, e.g. wave slamming, VIV etc.
Special attention: connections (pontoon to column, pontoon to deck, tendon porch
etc.)
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Design Wave Approach - Global Characteristic Responses
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7) Vertical bending moment for pontoons
Horizontal squeeze-pry loads
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Squeeze-Pry Loads
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FLS
Fatigue Critical Connections:
– Tendon and tendon components
– Tendon porch
– Pontoon to column connection
– Column to deck connection
– Heavily loaded foundations, e.g. riser
porch, crane pedestals etc.
Design Fatigue Factors:
– Hull: 1~3 based on accessibility for repair
and inspection
– Tendon and tendon components: min. 10,
typically operators specify much higher
factors for tendon components.
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Tendon system design
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Design conditions
Design approach (DeepC)
Redundancy
Tendon connectors
Stability
TLP Tendon Design Outline
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TLP Tendon System
TLP hull
Tendon pipes
Piles
Top connectors
Intermediate connectors
Bottom connectors
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Oil State Merlin connectors
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General Design Principle
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To be capable of being inspected, maintained, repaired or replaced
Tendon components designed to be ‘fail safe’
Unproven tendon components design requires Technology Qualification
Consequence of possible sudden rupture of tendon to be considered
Facture control strategy to ensure largest undetected flaw will not grow to a fatal sizeD
esig
n P
rin
cip
le
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Typical Parameters To Be Considered:
Different TLP drafts
Wave conditions and headings
Tidal effects, storm surges, set down,
foundation settlement, subsidence,
mispositioning, tolerances
Tendon flooding, tendon removal and
hull compartment(s) flooding.
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Typical Parameters To Be Considered: (cont’d)
Possible variations in VCG
– change in operation
mode (e.g.
drilling/production)
– changes in topside
weights (e.g. future
modules)
– tendon system changes
(altered utilization)
– changes in ballast
weights or distributions
– deviations from weight
estimate
– riser phasing scenarios
– drill rig position
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Various Design Phases To Be Considered:
Transportation
Pre-installation (free floating)
Mating
In-Place Intact
In-place Accidental
Decommissioning
Design Conditions
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OffshoreTechnology.com
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Design Conditions
Various Design Phases To Be Considered
– In-Place Intact
– In-place Accidental
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• Multi-body? (if TAD is used)
• Max offsets
• Max rotations
• Max accelerations
• Max tendon tensions
• Min tendon tensions
• Min airgaps
• No. of risers
Damage extent per MODU:• Any one tendon
compartment• All compartments that
could be flooded as a result of damages of: minimum 1.5 m deep and 3.0 m high at any level between 5.0 m above and 3.0 m below still waterline
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Tension Components
pretension (static tension)
tide (tidal effects)
storm surge (positive and negative values)
tendon weight (submerged weight)
overturning (due to current, mean wind or
drift load)
set down (due to current, mean wind or drift
load)
WF tension (wave frequency component)
LF tension (wind gust and slowly varying drift)
ringing (HF response)
hull VIM influence on tendon responses
tendon VIV induced loads
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Design Approach
Typically fully coupled time-domain
analysis is required
Wave frequency, low frequency,
high frequency
Various environmental effects, e.g.
wind sea, main swell, 2nd swell,
squall, current etc.
Model tests used for validation
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BMT Fluid Mechanics
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Tendon Design Criteria
Code check: API RP-2T
DFF for Tendon and Tendon Components: min 10
For tendon receptacles and other components connected to the pile while it is
driving, fatigue damage due to pile driving shall also be taken into account
In case of composite tendons, refer to DNV-OS-C501
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Tendon Design Criteria (cont’d)
Fracture Toughness
Fracture mechanics assessment in accordance with BS7910
Max allowable flaws under extreme design loads shall not grow to a critical size
causing unstable crack growth in 5 x tendon design life or tendon inspection
period (whichever is less).
Preferred critical flaw is a through-thickness fatigue crack
Proper inspection method
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TWI Ltd
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Tendon Accidental Design
Most relevant accidental events:
– flooding of hull compartment(s)
– missing tendon
– tendon flooding
– dropped objects
Tendon removal condition to be
combined with environmental loads
of 100-yr return probability
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Rigzone
The effect on the surrounding structure
due to possible accidental tendon
rupture and consequential release of
elastic energy stored in the tendon shall
be considered
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Tendon Design
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Guidance based on Lessons Learnt
• Single Event Fatigue – D (fatigue damage) over the duration of a single event based on 100-yr extreme storm, including ramp-up and ramp down < 0.01
• Robustness check- Under survival condition (S), no catastrophic failure - Non-brittle failure modes- E.g. tendon pipe should have ductile failure modes- E.g. no potential unlatching in case of down stroke of a tendon.
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Tendon Porch
Withstand breaking strength of
the tendon itself – could be overly
conservative
Underlying basis: tendon porch to
be stronger than the tendon itself
Designer to ensure that the
design is sufficiently robust that
catastrophic failure of a tendon
porch is highly unlikely.
Using breaking strength of tendon
pipe for porch design is one way
of achieving sufficient robustness.
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DNV-OS-C105 & API-RP2T
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General
DNV-OS: class scope, focus mostly on safety in in-place condition
API-RP-2T:
– Covers full design/fabrication/transportation/installation/operation cycle
– More prescriptive, including lots of guidance for designers
– Does not cover scantling requirements
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Safety Categories
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Category A – Operational Conditions
Category B – Extreme Conditions
Category S – Survival Conditions
Category C – Fatigue ConditionsSafe
ty C
ate
gories
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Design Criteria
Design Environment
Extreme Environment – 100yr
Normal Environment – operating
Reduced Extreme Environment – typically used for damaged condition, 10yr
Survival Environment – 1000yr
Calm Environment
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Design Conditions and Acceptance Criteria
API-RP-2T DNVDesign
Load Case
Safety
Category Project Phase
Platform
Configuration
Design
Environment
Annual
Probability of Exceedance
Category Conditions Environ-
ments
Safety
Factor
1 A Construction Various
2 A Load out Intact Calm
3 B Hull/Deck Mating Intact Site Specific
4 B Tow/Transportation Intact / Damaged Route Varies
5 A Installation Intact Installation Varies
6 A In-Place Intact 1-year Normal ≤1 a Static 0 0.6
7 B In-Place Intact 100-year
Extreme 0.01
b Combined
100-
year 0.8
8 S In-Place Intact 1000-year
Extreme 0.001
c Airgap/min
tension only
1000-
year 1.0
9 B In-Place Damaged – No Compensation
1-year Normal ≤0.01(a) c
Accidental 1-year 1.0
10 S(b,c) In-Place Damaged – No
Compensation
10-year Reduced
Extreme ≤0.001(a)
11 B In-Place Damaged -
Compensation 10- year Reduced
Extreme ≤0.01(a)
12 S(b,c) In-Place Damaged –
Compensation
100-year
Extreme ≤0.001(a)
13 B In-Place Tendon Removed
(planned)
10-year Reduced
Extreme
≤0.01(a)
14 S(b,c) In-Place Tendon Removed
(planned) 100-year Extreme
≤0.001(a) c Tendon
Removed 100-year
1.0
15 C In-Place Intact Annual Scatter
Diagram 1
Fatigue
Annual
Scatter Diagram
Lower
DDF
16 SLE(d) In-Place Intact SLE Seismic Varies
17 DLE(d) In-Place Intact DLE Seismic Varies
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Design Conditions and Acceptance Criteria
Safety Category Safety factors Load category Safety factors
A Operating 0.6 a Functional 0.6
B Extreme 0.8 b Combined 0.8
S Survival 1.0 c/d/e Accidental/
redundancy
1.0
C Fatigue DFF= min 3 for hull
DFF= 10 for
tendon/tendon porch
Fatigue DFF=1~3 for hull
DFF=10 for
tendon/tendon
porch
API-RP-2T DNV
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