Mitigating Drilling Riser and Conductor Fatigue...Mitigating Drilling Riser and Conductor Fatigue Dr...

Mitigating Drilling Riser and Conductor Fatigue

Dr Hugh Howells2H Offshore Engineering Limited

30th October 2013IBC Energy, 2nd Annual Drillships, Seoul, Korea

Learn more at www.2hoffshore.com

Agenda

Sources of fatigueRiser, wellhead and conductor fatigue hotspotsFatigue analysisWhat factors contribute to high fatigueWhy drilling riser fatigue is an increasing concernSteps to minimise riser and wellhead system fatigue damageDrilling riser maintenance and fatigue management


Riser Loading


Wellhead System Loading

BOP

LMRP

Flex-joint

Riser

36” x 1.5” seam welded conductor pipe

36” x 1” seam welded conductor pipe

3.5m 1.5m

6.2m

13m

19m

-10.5m Weld-on connector

Housing extension weld

Tree

HP and LP housing

Riser tension Cyclic loading

Snag Load


Vortex Induced Vibration


Wellhead System VIV Fatigue


Wave Induced Motion – Flowbase


Wave Induced Motion - BOP


Riser Fatigue Hotspots

Pipe to pipe weldsOne or two per joint

Pipe to coupling welds

Riser couplingsTransitions in sectionLoad shouldersBolts

Along entire riser length

10


WellheadSystemFatigue Hotspots

Conductor to Wellhead Girth WeldE-Class, SCF=1.3

Conductor to Compression Ring Girth Welds

E-Class, SCF=1.3

Extension Girth WeldE-Class, SCF=1.5

Conductor CouplingB-Class, SCF=5.0

Conductor to Coupling WeldE-Class, SCF=1.3

ConductorSeam Weld

C-Class, SCF=1.0

LMRP / BOPNo fatigue check

performed

Riser pipeE-Class, SCF=1.3

(Not Typical)


Conductor Fatigue

0.01

0.1

1

10

100

1000

10000

100000

-16 -14 -12 -10 -8 -6 -4 -2 0 2 4

Fati

gue

Life

(Y

ears

)

Elevation Above Sea Bed (m)

UNFACTORED FIRST ORDER FATIGUE LIVES ALONG CONDUCTORRigid Lockdown WH

Lower Bound Upper BoundLearn more at www.2hoffshore.com

Wellhead Fatigue Hotspots

Woodside Xeres and Laverda BOP Oscillation AssessmentUNFACTORED FATIGUE LIFE

Xeres Well, BH1 Soil Data, Hs=5.1m, Tp=7s

-15

-10

-5

0

5

1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04

Fatigue Life (Years)

Elev

atio

n A

bove

Mu

dlin

e (m

)

Conductor Casing

LPWH

36x1.5"XO Jt

36x2"UpperHAC

36x1.5"LowerHAC

HPWH

20"Jt

13-3/8"Pup Jt

13-3/8" Jt

Unavoidable hotspot atLP housing weld

Connectors < 10m belowmudline impair OVERALL

fatigue resistance

Peak bending loads at3-5m below mudline


VIV and First Order FatigueFATIGUE LIFE ALONG LENGTH OF DRILLING RISER

485.6ft Water Depth, Water Filled, Dual Annular BOP Model, E-Class Curve

0.001

0.01

0.1

1

10

100

1000

10000

100000

1000000

-120 -60 0 60 120 180 240 300 360 420 480 540

Elevation (ft)

Fatig

ue L

ife (Y

ears

)

Slick Joint

Mandrel

Buoyant Joint

UFJDual Annular BOP

ConductorWellhead

RKB

Outer and Inner Barrel

TreeLower

LMRP Connector

LFJ


VIV Fatigue Damage Build-Up


Wellhead VIV Variation with Location

WELLHEAD VIV FATIGUE LIVES WORLDWIDE ( Low Pressure Housing Weld)

1.0

10.0

100.0

1000.0

10000.0

0 200 400 600 800 1000 1200 1400Water Depth (m)

Fatig

ue L

ife (D

ays)

WoS - 2 WoS - 3 Asia Pacific North of Shetland West of HebridesAtlantic Margin Norwegian Sea Malaysia Egypt West of Shetland**

** Calibrated software and safety factors - x3 improvement

Envi

ronm

enta

l Har

shne

ss

Scatter due to different BOP heights and

weights


BOP Stack VIV

• BOP stack natural frequency excitation hasbeen observed in loop currents and underwave loading

• This can result in high accumulated fatiguedamage in the wellhead and conductorsystem below the mudline

17

Seabed

BOP

LMRP

Wellhead

36” / 20" pipe

38” / 36"pipe

Conductor_____Connector

Drilling Riser BOP Stack Excitation


BOP ResonanceBOP RESONANCE

Effect Of BOP Stack on Curvature at LP Housing Weld

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Mode Number

Cu

rvat

ure

(1/

m)

Increasing BOP Height and Weight (addition of tree)

Load Case 3

Load Case 12

Load Case 13

Load Case 15

Load Case 16

Curvature Factor 2.6

Curvature Factor = 2.6

Damage Factor (2.6)3 = 19

New vessels with large BOP’s give more severe fatigue damageLearn more at www.2hoffshore.com

19

Just how bad can it be?

Summary Acceleration Spectra for 10 min Window

GoM BOP stack VIV at 0.16Hz (6.25 sec stack natural period), Total fatigue consumption at the first connector is 40% in 12 days

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Frequency (Hz)

Acc

eler

atio

nA

mpl

itude

(m/s

2 )

BP Ocean Confidence Event 112 - Magnitude of Peak Response vs. Frequency

Z Axis SpectraMaxima Spectra

1st Harmonic

2nd Harmonic

3rd Harmonic

BP Ocean Confidence Riser VIV Monitoring CUMULATIVE FATIGUE DAMAGE AT CONDUCTOR CONNECTOR

DnV B-Class Curve, SCF = 8.0

0

5

10

15

20

25

30

35

40

45

50

8/3/05 8/8/05 8/13/05 8/18/05 8/23/05

Date

Cum

ulat

ive

Fati

gue

Dam

age

(%)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Cur

ren

t Sp

eed

(kn

ots)

Cumulative Fatigue Damage, SCF=8.0 Max Current Speed

Event 1338th Aug,05 00:00




What We Don’t Want to Happen

West of Shetland RegionDiscoverer534 DP drillship440 meter water depth42in air cans on riserPeriodic cycle 5-12 sec2 degree angular motion at riser baseFailed in 29 daysReference DOT paper 1983, C. Hopper, Britoil

One (the only) well documented fatigue failureLearn more at www.2hoffshore.com

Fatigue Drivers

Deep waterHigh VIV fatigue damageCurrents greater than 1m/s (2 kts)Generally low FOF damage

Shallower waterWave generally drives fatigueRiser vibration frequencies may not correspond with current excitation (VIV) frequencies

Large BOP and LMRP’s enable wellhead excitation at higher periods (lower frequencies)

Results in greater wave and VIV induced fatigue


Fatigue Damage Mitigation –Things to Consider/Avoid

Extension joint length/connector locationCasing swedges (20 to 13-3/8in)“Add-ons” that do not consider fatigueNon-rigid-lockdown wellheadsFatigue details/weld qualityVIV suppression


Cement Top-Up System


Guidance System Below Wellhead


Anti-RotationFins


BullseyeMountings


Non-Rigid Lockdown Wellhead

Resting on Landing Shoulder

Lateral Gap Closed

Vertical Gap

Lateral Gap


Non-Rigid Lockdown Wellhead -Conductor and Casing Stresses

Maximum Stress in Casing, Conductor & Wellhead0.0m Cement Height

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-12 -9 -6 -3 0 3 6 9 12Vessel Offset (% Water Depth)

Ben

ding

Str

ess

/ Yie

ld S

tres

s

Casing Stress - Light Casing Stress - Heavy Conductor Stress - LightConductor Stress - Heavy Wellhead Stress - Light Wellhead Stress - Heavy


Fatigue Improvement –VIV Suppression Devices


Strakes – Below Choke and Kill Lines

OMAE2008-57046


Fins –(Langhorst)


Riser Fabrication Considerations

Need to achieve high quality

Welding to get good quality fatigue details with high grade steels is not simple

How does C-class quality need to be proved?

Effort spent on qualifying and obtaining good quality fabrication is generally good value

Pipe dimensional control, welding, coating

Casing

Slick andBuoyant Joints

BOP

LMRP

Lower Flex-Joint

Telescopic

Joint

Tension Ring

Upper Flex-Joint

Drill Floor

Conductor


Fatigue Integrity Management

Record riser joint usage and times on wellSchedule and implement regular joint inspection

Typical - every 5 years in service Brazil - every 2 years offshore

InspectionLimited ability to inspect fitted riser jointsStrip down of auxiliary lines and buoyancy neededInspection of fatigue hotspots needs good access requiring strip down of joints

Use extended monitoring where needed to measure riser and wellhead system fatigue

Calibrate analysis software and assumptions – reduce conservatismVerify design data – soils


Example Wellhead Monitoring

3 axis accelerometerDual axis angular rateLoggers located on

BOPWellheadConductorTemplate

Magnetic holdersROV Installed


Stress and Motion Monitoring - Steps

Monitoring system designOffshore installationSignal processing of measured data

1 LOGGER ON STACK BELOW LOWER FLEX JOINT

5LOGGERS SPACED APPROX .1 EVERY 2 JOINTS

1 LOGGER ON VESSEL

5 LOGGERS SPACED APPROX. 1 EVERY 2 JOINTS


Current / Wave / Tension / Mud weight monitoring

Ways to Manage Fatigue - MonitoringSoftware and Model Calibration

36

On Rig:

Indicates VIV


VIV Fatigue Analysis Calibration


Future Current Concerns

Increased recovery targetsLonger times on the well

Higher pressure (deeper) wellsLonger drilling durations

Post-Macondo design requirementsLarger BOP’s, capping stacks

Use of newer vessels on older wells


What Works Where?

Wellhead and Conductor Configuration

Low Fatigue Resistance High Fatigue Resistance

3rd and 4th Generation Vessels

5th and 6th Generation Vessels

Wat

er D

epth

Shal

low

Dee

p

Influence of Soil Strength


Developments

Greater emphasis on fatigue specification of wellhead systems

Longer wellhead extension joints (18m)

Braced wellheads

Wellhead caissons

Larger diameter (42in) conductors


Summary

Fatigue wasn’t a major design challenge

Vessels, risers and BOP’s are changing to provide greater capability and operating conditions are more diverse

The wellheads are lagging behind

Greater care is required when developing new wells or working on old wells (with new equipment) to ensure fatigue fitness-for-purpose


Thank you for your time.

Questions……

Further information:

2H Offshore Engineeringwww.2hoffshore.com+44 1483 774900


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