deepwater drilling challenges - 2hoffshore.com · introduction present the challenges of moving to...

Deepwater Drilling Riser Technical Challenges

Ben Middleditch

October 2011

Learn more at www.2hoffshore.com

2H - an ACTEON company


Introduction

Present the challenges of moving to deepwaterDefine the operational requirements of a deepwater drilling riserWhich of these become more difficult with deeper water

Discuss options for overcoming these challengesModifications to conventional systemsOptimised well designAlternative solutions

5Learn more at www.2hoffshore.com

Contents

Drilling Riser System OverviewDrilling Riser OperabilityChallenges of Scaling to Deeper WaterConventional SolutionsAlternative Technologies


ConventionalDeepwater LP Drilling Riser

Derrick

Drill Floor

Riser Tensioners

Telescopic Joint

Upper Flex Joint

Riser Joints

Lower Flex Joint

LMRP

BOP


Conventional LP Drilling Risers

Typically 21” OD marine riser80ksi steelRange of Connector TypesExternal Auxiliary LinesBuoyancy Modules to 10,000ft18-3/4” Subsea BOP system


Loading


BOP

Conductor

Lower Flex Joint

Riser

Flex Joint Rotation

Drilling Riser Systems:Static Loading

10

Drift-offDrive-off

current

On Station


Drilling Riser Systems:Operating Criteria

Flexible Joint Rotation Limits<2° for drilling

11

FLEX JOINT ROTATIONS

0

1

2

3

4

5

6

7

8

9

10

-20% -15% -10% -5% 0% 5% 10% 15% 20%

Vessel Offset (% Water Depth)

Fle

x Jo

int

Ro

tati

on

(D

egre

es)

LFJ 1% Exceedeence Current UFJ 1% Exceedence Current

current


Drilling Riser Systems:Operating Criteria

Flexible Joint Rotation Limits (<2° for drilling)Increasing top tensions increase operability

12

TENSION OPTIMISATION - LOWER FLEX JOINT RESPONSE0% Vessel Offset, 1-Year Return Current

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

90 190 290 390 490 590 690Base Tension (Te)

Fle

x Jo

int

rota

tio

n (

deg

)

354 454 554 654 754 854

Top Tension (Te)

1 Year Return Period 0.1% Exceedence 0.5% Exceedence1% Exceedence 5% Exceedence 10% Exceedence


Drilling Riser Systems:Operating Envelopes

Flex jointrotation

Conductorstress

Tensionerstroke out

RISER OPERATING ENVELOPE WITH CURRENT ONLY400kips Base Tension - Mud Filled 1.35SG - Soft Soil Profile

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-20 -15 -10 -5 0 5 10 15 20

Vessel Offset (% of water depth)

Surf

ace

Cu

rren

t V

eloc

ity

(m/s

)

SurvivalFJ Rotation < 90% maxVM / Yield Stress < 1.0

Connected (Not Drilling)FJ Rotation < 90% maxVM / Yield Stress < 0.8

DrillingFJ Rotation < 2deg (mean)FJ Rotation < 4deg (max)VM / Yield Stress < 0.67

Stroke Limit1 Year Current1% Current


Moving to Deeper Water


Scaling to Deeper Water:Top Tension Capacity

Increased Riser Top TensionIncreased Vessel Tensioner Requirement

15

Longer riser string

Heavier riser string

Increased volume of drilling mud

Increased internal burst pressure (HP reservoirs)

Increased external hydrostatic collapse pressure

Increased riser pipe wall thickness

Higher axial stress

Limited by minimum ID for drill bits & casing hangers


Scaling to Deeper Water:Drilling Riser Buoyancy

16

External buoyancy attached to riser joints reduce the in water mass of the riser string

Typically made from syntactic foamGlass or ceramic hollow micro spheresSuspended in a polymer matrix

External durable coating


Scaling to Deeper Water:Buoyancy Impacts

17

Greater deck space usageHandling difficultiesIncreased riser dragReduced buoyancy effectiveness at greater depthsRotary table opening size limit


Scaling to Deeper Water:Buoyancy Impacts

18

Reduced buoyancy effectiveness at greater depths

REQUIRED VESSEL TENSION CAPACITY WITH INCREASING WATER DEPTH21" OD x 7/8" wt Riser, 16ppg Mud Weight, 1000kips Overpull

0

200

400

600

800

1000

1200

1400

1600

600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200

Water Depth (m)

Req

uir

ed V

esse

l Ten

sio

n C

apac

ity

(Te)

0

500

1000

1500

2000

2500

3000

1970 2626 3282 3938 4594 5250 5906 6562 7218 7874 8530 9186 9842 10498

Water Depth (ft)

Req

uir

ed V

esse

l Ten

sio

n C

apac

ity

(kip

s)


Scaling to Deeper Water:Vortex Induced Vibration Fatigue

All bodies in current flows experience forcesWhen an in-line current causes the bodies to vibrate at a natural frequency in the cross-flow direction this is called VIVConstant vibration of the riser causes stress cycling, which causes fatigue damage on the riser, which can cause the riser to fail

Vortex


Scaling to Deeper Water:Vortex Induced Vibration Fatigue

Longer riser strings have lower natural frequenciesSo slow currents which may not excite a shallow water riser can excite the same diameter riser in deeper waterFor the same current speed higher frequencies can be excitedThese higher frequencies have greater curvatures and larger bending stress cycles


Conventional LP Drilling Risers

Deepwater IssuesHigh tension requirements (2500kips+ for 10,000ft)Buoyancy becomes less efficientHighly susceptible to VIV (Vortex Induced Vibration) Complex Choke / Kill line interaction and sealing due to large riser deflectionWorldwide lack of deepwater rigsHigh rig costRig availability will dictate development schedule


Solutions


Conventional Solutions

New larger capacity rigsGreater deck capacityLarger tensioner systemsLarger rotary tables

Long term rig contracts

VIV suppressionStrakesDistributed buoyancyTension variation

Optimised well design23


Well Construction:Design for Fatigue Resistance

Optimise the choice of well components:Conductor sizeConductor joint lengthWeld quality

Construction variables:Wellhead stick-upCement shortfall

Impact of riser design:Operation modeSubsea component size


Conductor to WellheadGirth Weld

Extension Girth Weld

Conductor Coupling

Conductor to Coupling Weld

Conductor

LMRP /BOP

Riser pipe

Well Construction:Design for Fatigue Resistance

Critical welds


Lateral Load from Drilling Riser

26

Woodside - Omar 1 Drilling Riser and Conductor AnalysisBENDING MOMENT DISTRIBUTION ALONG CONDUCTOR

300kips Base Tension - 380 W.D. - 10 Year Current - Upper Bound Soil

-50

-40

-30

-20

-10

0

10

-12000 -8000 -4000 0 4000 8000 12000 16000 20000

Bending Moment (kNm)

Elev

atio

n a

bove

Sea

bed

(m)

-20% Offset -18% Offset -16% Offset -14% Offset

-12% Offset -10% Offset -8% Offset -4% Offset

0% Offset +4% Offset +8% Offset +10% Offset

+12% Offset +14% Offset +16% Offset +18% Offset

+20% Offset HDH4 Connector HAC D90 Connector

HAC Upper Joint HAC Lower Joint 36" x 1.5", 52ksi Conductor

36" x 1.5" Conductor

(52ksi)

BOPHDH4

LMRP

HAC Upper Joint HAC

D90

Wellhead

HAC Lower Joint


Effects of Conductor Sizes

30”x1.5” 36”x1.5” 36”x2.0” 38”x2.0”

27

Fatigue life increased by a factor of 5 with an upgrade from 30” OD to 36” OD


Effects of Weld Quality

28

Increase in fatigue life by a factor of 10 whenachieving a C class weld compared to an E class

MINIMUM UNFACTORED FATIGUE LIFE

1

10

100

1000

10000

C Class,SCF 1.1

C Class,SCF 1.3

D Class,SCF 1.1

D Class,SCF 1.3

E Class,SCF 1.1

E Class,SCF 1.3

F1 Class,SCF 1.1

F1 Class,SCF 1.3

Weld Class

Fati

gu

e Li

fe (

day

s)


Effects of Wellhead Stick-up

2m Stickup 40% - 50% better than 3m Stick-up


Effects of Cement Shortfall

No cement shortfall 50% better than 2m cement shortfall


Effects of Operating Mode

Drilling Mode(no Subsea tree)

Completion Mode(with Subsea tree)


Effects of Operating Mode

Completion Mode 3 times more damaging than Drilling Mode


Well Construction Findings

Larger LRMP & BOP stacks typical on deepwater drilling rigs have a detrimental affect on a wellhead fatigue performanceThis can be offset by:

Incorporating a larger diameter conductorChoosing conductor joint lengths based on peak lateral loadSpecifying better weld detailsUsing a rigid lock down wellheadEnsuring a good conductor cement jobEnsuring a low wellhead stick-up


Alternative Solutions

Steel replacementsHP surface BOP drilling riserNear Surface BOP SystemsFree standing drilling riser


Steel Replacements

Drilling Riser Joints including Choke & Kill LinesCompositeAluminiumTitanium

IssuesLower stiffness induces greater riser deflectionsLower wear resistance (composite & aluminium)Lower corrosion resistance (aluminium)Higher production costs


HP Surface BOPDrilling Riser

Subsea ESD

Subsea Wellhead

13-3/8” Casing (typ.)

Surface BOP

Stress Joints

Telescopic Joint

Buoyancy Modules (Optional)

Flex-Joint

Tie-back connector


HP Surface BOP Drilling Riser

13-5/8” 15ksi Surface BOPTypically 13-3/8” or 16” casingSubsea ESD for emergency disconnectionUse of additional buoyancy dependent on vessel tension capacity and water depthTop configuration to suit vessel (modifications may be required)Casing can be re-used or run downhole on next wellAllows lower spec / cost rigs to be used in deepwater applications


Near Surface BOP Systems

Elevates BOP to near surface region (100m – 250m water depth)

Buoyancy tank used as ‘artificial seabed’

Standard rig marine riser and BOP used to surface

Large diameter tie-back casing (21”+) used to seabed

System can freestand with BOP in event of disconnect

Allows use of 3rd generation rigs for deepwater wells


FreestandingDrillingRisers BOP

Upper disconnect package

LMRP and BOP

Aircans to support lower riser

Conventional lower riser

Conventional upper riser

LMRP

Aircan to support casing and BOP

Well casing

Tie-Back Connector


Summary

Scalability of conventional marine risers becomes increasing difficult with increasing water depthsNew purpose built rigs overcome the majority of these challenges, but their availability is limitedNew technologies are being championed which can allow the upgrading of older rigs for use in deeper waterProblems still exist such as VIV and other forms of fatigue that need to be considered in well planning


Thank you for your time

Questions?

Further information:

2H Offshore Engineeringwww.2hoffshore.comABERDEEN +44 1224 452 380LONDON +44 1483 774 900


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