offshore structures - dnv gl floating structures-day1... · wikipedia: a vertically moored floating...

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31 October 2016 SAFER, SMARTER, GREENERDNV GL © 2015

31 October 2016

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OIL & GAS

1

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

15

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


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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


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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

64

Journal of Offshore Mechanics and Arctic Engineering

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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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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

offshore structures - dnv gl floating structures-day1... · wikipedia: a vertically moored floating...

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