Model testing of offshore structures

Experimental Methods in Marine Hydrodynamics Lecture in week 39, 22nd September 2014 Presented by Chittiappa Muthanna, MARINTEK

Outline • Part I: Offshore Structures

– Typical test objectives, requirements and set-ups – Documentation tests – Modeling of risers and anchor lines – Importance of slow drift motions – Challenges in deep water exploration – and model testing!

• Minimization of models • Truncation of risers and mooring lines • Hybrid model testing

– Marine operations – challenges and typical model tests

• Part II: Vortex Induced Vibrations – Introduction – 2D tests of rigid cylinders – 3D tests of long elastic cylinders

Offshore structures • ”All other applications except ships in transit”

Examples: – Floating platforms and ships applied for production and/or

storage of oil and gas – Fixed structures – Risers – Mooring systems – Floating and submerged bridges – Fish farming structures – Offshore windmills (fixed & floating)

• Commonality: Hydrodynamic problems are important. In most cases are also surface waves involved

Typical Test Objectives 1. Feasibility studies

• Early stage of new concepts / new applications / new environments

2. Concept Verification studies • Apply design loads to a completely modeled structure to verify that it

satisfies requirements • Typically an oil installation (drilling rig, production or storage unit)

3. Operational limits studies

• Typically the limiting sea state of a demanding marine operation

4. Parts testing studies • Experiments with parts of a complex system • Determination of coefficients (drag , added mass, damping, RAOs …)

for input to numerical simulations

5. Validation and/or verification of software • For actual type of structure and loading condition

Typical Test Requirements For concept verifications and operational limits studies

• Zero (or very low) speed • High-accuracy modeling of complete environment

– Multi-directional and short-crested waves – Time-varying wind (correctly modeled gusts) – Depth- (and time-) varying current

• Correctly (Froude) scaled water depth – is often important

• Correctly (Froude) scaled risers and mooring lines • Low speed and requirement for high accuracy waves

implies a short but wide “tank” (=basin)

The Ocean Basin Laboratory

Length: 80 m - Width: 50 m - Depth: 0-10 m

TOW

ING

TA

NK

OCEAN BASIN

50 mMulti-flapwave maker

80 m

Cross-section of Ocean Basin

Double-flapwave maker

Multi-flapwave maker Double-flap

wave maker

Typical test set-up • Measurement of:

– 6 DoF motions by use of optical position meas. system

– Mooring line forces – Wave elevation close to

structure – Riser forces – Wave run-up/slamming forces

• Observation (by video): – Green water – Motions of mooring lines and

risers (by underwater video)

Scaling considerations

• Floater (ship or platform) built to geometrical scale – No particular scaling problems of motions and global loads

• Risers: – Correct drag coefficient of sections

• Scale effects – modify diameter to obtain correct forces – Froude-scaled bending stiffness – Correctly scaled weight in water

• Mooring lines – Axial stiffness might need to be modeled, but bending

stiffness might be neglected ⇒Solution:

Non-homogeneous models of risers and mooring lines

Drag coefficient vs. Reynolds Number in Steady Flow

Mooring lines Risers

Model scale Model scale Full scale

Rnms=Rnfsλ -3/2ν fs /νms

Non-homogeneous mooring lines

• Might be a combination of: – Thin rope (fishing line) – Wire – Chain – Springs – Discrete lead weights – Floats

Mooring line

Non-homogeneous models of risers

Example: Steel core gives correct bending stiffness

Diameter to give correct drag force

Material of outer pipe to give correct buoyancy force

Risers

Risers

Documentation tests • The tests are performed in order to document

that the models and test set-ups are functioning as intended

• They are important • Purposeless to continue if resonance periods are wrong • Avoid unwanted/unintended resonance period of the test

set-up was within the response period range

• Type of documentation tests • Decay tests • Pluck tests • Static pull-out tests • Test wave

Decay tests

Coupling to other degrees of freedom

Excited manually in order to create wake

• There is a lot information in a simple decay

• Resonance period • Damping • Added mass

• Important to start decay without exciting other motions

• Wake can be generated by carefully exciting the model

Resonance frequencies in model

Pluck tests – Pluck tests are performed

in order to document all resonance periods in the model and test set-up

– One single pluck excites a lot of frequencies because it is a kind of ”step function” containing all frequencies

– Cutting of wire – A plank may also work

Static pull-out tests

• For documentation of the mooring system

• We pull the model to various offsets by a rope with a force ring inserted

• Important that the lines don’t hook up in welding points on the floor, photo stands etc.

Static pull-out tests

• For documentation of the mooring system

• We pull the model to various offsets by a rope with a force ring inserted

• Important that the lines don’t hook up in welding points on the floor, photo stands etc.

BM2

F

Restoring force

Tension L4&L5

Test wave

• The test wave is run for verification that – the wave makers are working properly – the instrumentation is stable – the test set-up doesn’t change characteristics

during the test period (re-rigging, rotation of set up for new heading etc.)

• Using a standard test wave, but a wave condition specially designed for the project can also be used

Slowdrift (2nd order) forces • Very important for design of mooring and dynamic

positioning systems • Stiffness of mooring system is of vital importance

(provides the restoring force in the dynamic system)

Natural periods of deep water moored units

Unit Natural periods (s)

Surge Sway Heave Roll Pitch Yaw

FPSO >100 >100 5-12 5-30 5-12 >100

Semi >100 >100 20-50 30-60 30-60 >100

Spar >100 >100 20-50 50-100 50-100 >100

TLP >100 >100 <5 <5 <5 >100

Low Frequency Motions

• Dynamic system compromising – vessel mass and added mass – spring from station keeping system – damping from vessel and station keeping

• Normally in horizontal plane • Small forces excitation in resonance Damping important

Damping Spring Mass

Surge motion spectra from model test of a tanker

Frequency (Hz)

Low frequency motion

Wave frequency motion

LF Surge Damping of a Turret Moored Tanker

Aerodynamic damping

Wave drift damping

Viscous damping

Mooring damping

Total damping

Time series, spectra & probability distribution from model test of a turret moored FPSO

Wave

Tension

Surge

Semi-submersible Visund (GVA8000) (from VERIDEEP study): Drift coefficients,surge Empirical, in 4 sea states + potential theory assuming linear dynamical system (Deviations from standard potential theory: Viscous drift & current- wave interaction)

Background (1)

Movie Shown here

Deep water testing

• Deeper basin! • Ultra small scale model testing

(λ=1:>>100) • Passive equivalent mooring system or

truncated hybrid system • Solutions with active control systems • Outdoor testing

Exististing basins

Deep water

Mooring lines

Verification of Deepwater Systems by Physical Model Testing

Ultra small scale model testing • Model scale λ=1:>>100 • Challenges:

– Weight and accuracy of models ⇒ difficult to make – Viscous effects and surface tension might influence floater – Risers and mooring lines become extremely small

Ultra-small scale model testing: Comparing 3 scales

RAO & Phase Tension vs. fairlead motion

RAO WF motion vs. wave

Hybrid verification procedure

Stansberg et.al. 2002

Hybrid Testing: Passive equivalent mooring system • Mooring stiffness can be correctly

modeled • Mooring and riser dynamics usually not

correctly represented

Stansberg et.al. 2004

Trade-off between model size and use of hybrid techniques

Stansberg et.al. 2002

Solutions with active control system Exististing basins

Deep water

Mooring lines

Basin

Full water depth

Mooring lines

Mooring line servo units

Mooring line servo unit

Power

Controlsignals

Output motionsof mooring line endFloater motions

Mooring line tension

Control systemD/A

A/D

Outdoors testing • Verification tests with complete system cannot

be done outdoors, due to lack of control of environment

• Phenomenal studies can be done, provided measurement of both environment and responses

• Have been done for investigation of riser Vortex-Induced Vibrations & inteference – Hanøytangen – Skarnsundet

Testing of Marine Operations (1) • Testing of critical events of the marine

operation, e.g. an object in the splash zone, near sea bed, impact at guidepines

• Determination of environmental limits for specific operations (Wind, Waves & Current)

• Trying out of different procedures • Typical operations

– Heavy lift – Installation of bottom equipment – Pipe laying – Towing operations

Testing of Marine Operations (2)

• Determination hydrodynamic coefficients used as input to numerical studies – Free Oscillation Tests – Forced Motion tests

Installation of slender structures

• Dynamic forces during lowering – Wave forces in the

splash zone, incl. slamming

– Force contribution from crane motion

– Forces from waves and current towards the sea bed

• Identification of critical

steps • Recommendations to

limiting seastate

Photo: Halliburton

512384

TML Lift of Frigg jacket, model tests

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Marine operation – Hybrid Riser Tow Out

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Forced motion test: Detecting added mass and damping

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Example: Roof structure Forced motion mechanism

Forced motion test: Estimated and measured added mass (left) & damping (right)

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Example: Roof structure

Free Oscillation Test: Detecting added mass and damping

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0)()()()( =+++ tzktzctzam f

Example: Suction anchor

m = structure mass kf = spring stiffness a = added mass in vertical direction

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fkamT +

= π2

ca. 1

mFree Oscillation Test: Detecting added mass and damping

Challenges in Deep-water Marine Operations

• Time-variant current forces increasingly important – Large offsets – Positioning difficulties – Risk for line entanglement – Manoeuvring from the surface is difficult

• Vertical resonance, resulting in motion amplification and reduced limiting sea states

• Wire weight – Lifting gear capacity – New liftline materials

• Increased operation time (more uncertain weather forecast)

Software verification and validation studies – Examples:

• Green water on deck • Run-up and air-gap/deck slamming

SEMI

Wave runups and slamming

Extreme wavesand deck slamming

Bow slamming

FPSO

Green waterslamming on deck

Green waterslamming on deck

Extreme events:

Summary of Part I: Offshore Structures

• Typical test objectives, requirements and set-ups • Documentation tests • Modeling of risers and anchor lines • Importance of slow drift motions • Challenges in deep water exploration – and model testing!

– Minimization of models – Truncation of risers and mooring lines – Hybrid model testing

• Marine operations – challenges and typical model tests • Software verification and validation studies – extreme and

rarely occurring events – Green water on deck – Wave run-up on platform legs

Current

The cylinder starts to oscillate

Vortex Induced Vibrations

fst = St U/D, St ≅ 0.15 – 0.3

SPAR with D=30 m, U=1.5 m/s, fSt ≅ 0.01 Hz (TSt=100 s)

Riser with D=0.30 m, U=1.5 m/s, fSt ≅ 1 Hz

Vortex shedding

VIV problem areas Statens veivesen

More specifically…

SPAR: Increased global motions Increased drag (Off-set) Increased mooring line tensions (ULS &FLS)

Risers: Reduced fatigue life

Increased axial tension

Increased extreme loads

Increased drag

Increased axial tension

T

Increased “diameter” and

drag

2d

d

Riser eigenmodes To each mode, n, there corresponds an eigen-frequency, fn. The riser will oscillate when the Strouhal frequency is close to an eigenfrequency: fn ≈ fs = St⋅U/D

Hence, the speed of the current will determine which mode (n) will respond.

n: 1 2 3 4 5 6 7 ....

.....

f1 f2 f3 f4 f5 f6 f7 ....

Complex hydroelastic interactions for long risers in sheared flow

f1 f2 f3 f4 f5 f6

Strouhal Frequency fs = St U/d

Current profile, U Riser

Natural frequencies:

Competing modes

Varying current profile: Many possible frequencies of oscillation exist. ”Competition” between modes. Difficult to predict frequency.

Two Typical VIV test set-ups • 2D tests with rigid cylinder with various

geometrical shapes that are either elastic mounted, free to move or with forced motion and towed in still water

3D test with long elastic cylinder with varying geometries and boundary conditions, free to vibrate. Various flow condition and current profiles may be arranged

Typical Test Requirements • Well defined flow with small (neglectable) turbulence • Correct (e.g. Froude) scaled models • Accurate measurements of motion / deformation /

reaction forces • For free vibration test: low and documented

“material” damping • No unintentionally interference between VIV

response frequencies (≈Strouhal frequencies) and eigen frequencies of test rig

Strouhal Number vs. Reynolds Number

MS (& CFD)

FS

Rigid cylinder section (2D) tests (L/D = 10-25)

• Determine hydrodynamic coefficients Study in-line and/or cross-flow

oscillations Study effect of VIV suppression

devices

Cross-Flow VIV

Behaviour

Susan Swithenbank (post doc. at CeSOS)

Free oscillation of rigid cylinder; amplitude vs. Re Godvardsen and Williamson

Prototype Rigid Riser Section Test, Elastic Mounted

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VIV Behaviour of Drilling Riser with Protection Fins

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Bare Cylinder Tests – Sub-critical

and Critical Reynolds-Numbers

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Ur=U/(fn D) Rn=U D/ν

Principle sketch of one test apparatus for

2-D free oscillation tests

Triple-start strakes

P/D=13.2

H/D=0.25

Force gauge

Photo of test cylinder in test apparatus

Cross-flow displacements (rms value/ diameter). Bare riser and straked risers

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Vr [-]

rms

x/D

[-]

Bare pipe P/D=5, H/D=0.14 P/D=17.5, H/D=0.25

xRMS / D

Reduced velocity, Vr=U/fn D

Systematic study of triple-start straked risers

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-0.250 -0.217 -0.183 -0.150 -0.117 -0.083 -0.050 -0.017 0.017 0.050 0.083 0.117 0.150 0.183 0.217 0.250

Vector map: 3D vectors, 124×96 vectors (11904)Burst#; rec#: 1; 41 (6), Date: 09.02.2005, Time: 02:43:59:185

3D velocity vector plot based on the PIV measurements Arrows present velocity in the paper plane Colours the velocity normal to paper plane

Soft marine growth (slimy marine grass)

Soft marine growth on a real riser

Soft marine as a model

Hard marine growth (Shell, wart barnacle, etc.)

Hard marine growth on a real riser

Hard marine growth as modeled

Principle sketch of a pendulum test apparatus for 2-D free oscillation tests

Models of various cross sectional geometries

Pendulum test of in-line oscillations with horizontal pipe

Using transient for extracting lift coefficient

Cross-flow response w/ strakes

Amplitude

Time

Test set-up for forced motion test in Marine Cybernetics Laboratory (MCLab)

In-line VIV - important for free spanning pipelines

Lift Coefficient from Forced Motion 2D Test (Gopalkrishnan)

/A D

ˆ /oscf f D U=

Flexible beam (3D) tests

3-D Pipe & Umbilical Tests (L/D =50-150) – To study multimode oscillations of free

spanning pipelines in uniform current 3-D Riser Tests (L/D > 300)

– To study multimode oscillations in uniform and sheared current

Pure IL VIV: Free spanning pipelines

Current

Measured orbits From free spann model tests

Small amplitudes, but still more fatigue damage from pure IL than CF in many cases

Coupling between IL and CF oscillations; free spanning pipelines

Illustration of deep water part of the Ormen Lange pipelines

Example of pipe lay-out

Dredge

Installation rock

Ormen Lange Free Span Pipeline VIV Tests New version of DNV RP105 free spanning pipelines

Example from Ormen Lange 2003 tests, Multi-span: L1/L2=112d/135d

TEST SERIE 74xxCROSS-FLOW MODES

DNV-RP-F105 vs. MARINTEK Tests

0.00

0.25

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1.00

1.25

1.50

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Uprototype

A/D

CF - MODE1

CF - MODE2

CF - MODE3

CF - MODE4

MARINTEK-MODE1

MARINTEK-MODE2

MARINTEK-MODE3

MARINTEK-MODE4

NDP High Mode VIV Test in Ocean Basin

3D tests with 38m long riser model, Do=27mm

NDP High Mode VIV Test in Ocean Basin Test Set-up for Uniform and Sheared Flow

Riser model Clump weight

∇

riser

gondol

riser

gondol

CF and IL fatigue vs. tow speed for bare riser in uniform flow

Instability of Faired Riser, 3 D Test

Fairing

Riser

Summary of Part II: Vortex Induced Vibrations

• Phenomenon & problem areas • Objective of testing

– VIV behavior – Suppression devices – Determine empirical coefficients used as input to prediction codes – Establish experimental data used for validation of semi-empirical

codes and CFD codes • Typical test set-ups

– 2D tests – 3D tests