wellhead fatigue operators perspective - ptil.no 2012/subsea... · conservative loads at wellhead....

WELLHEAD FATIGUE CONFERENCE

WELLHEAD FATIGUE

Operators Perspective

Copyright of NORSKE SHELL CONFIDENTIAL 6 Dec 2012 1

Operators Perspective

6th December, Oslo

Use this area for cover image(height 6.5cm, width 8cm)

Peter Ostrowski

Snr Subsea Hardware Engineer

� Project Summary

� Well Design

� Fatigue Assessment Work Flow

� Analysis assumptions (to be done)

Copyright of Norske Shell CONFIDENTIAL 03 December 2012 2

� Analysis assumptions (to be done)

� Initial Fatigue Assessment

� Proposed Improvements

� Results from Improvements

� Lessons Learned

East Browse Basin

Objectives and Drivers

Objectives

� Develop Prelude Swan resevoir to

facilitate production 700MMScf/d of

gas from 7 subsea producer wells to

FLNG by 2016

� Deliver within budget

Project Value Drivers

� Robustness

� Max NPV – Minimize Wells CAPEX &

� Project Delivery – wells off crtical path

� Repeatabilty – use on other fields

Repeatabilty – use on other fields

� Flexibility – use on other fields

� Max well availability high MTBF + No

Interventions

� CAPEX Certainty

Fatigue Analysis Workflow

Drilling Riser Configuration

� The drilling riser consists of

� 5 x slick joints

� 2 x 2,000’ rating buoyed joints

� 2 x slick joints

� 1 x telescope joint

� 12 x 160 kips tensioning system

Riser computer model input parameters

� Riser Joints:

� Riser joint weight = 100% design weight

� LMRP/BOP/Tree/THS weight = 100% design weight

� Buoyancy module net lift = 100% design lift

� Boundary condition:

� top fixed at drill floor with rotational stiffness;

Copyright of Norske Shell CONFIDENTIAL

� top fixed at drill floor with rotational stiffness;

� conductor connected with soil springs and fixed at bottom.

� Soil spring stiffness: calculated using P/Y data and effective OD

based on 42-inch (use of hole/grout OD, rather than the conductor

� Bare & Buoyed joints: CD = 1.0; Cam = 1.0, drag diameter =

adjusted for auxiliary lines

� Tensioner stiffness: (Tension change 0.75%) /(ft stroke) 03 December 2012 7

Operating Conditions

� Prelude wells will b e executed over an estimated duration of 2-3yrs

with Noble clyde Boudreaux Semi sub rig

� All wells will be drilled from one drill center

� Rig will be moored

� Climate is Monsoonal

Winter: April to September

Summer: October to March

Tropical cyclones typically Dec -Apr

� Riser will be disconnected during cyclones

03 December 2012 8

Operating Conditions

Example of riser load bins for case 1

ASSUMPTIONS

� 9 Riser Load bin

� 10 year wave

dominated non

cyclonic conditions

Weather from worst

� Weather from worst

direction relative to

the rig

03 December 2012 10

Prelude Fatigue Load Cases

Time on well

� Operational Time Distribution Cases (days)

� 153 WH / 132 THS

� 198 WH / 92 THS

� 153 WH / 118 THS / 14 XT

� 198 WH / 80 THS / 14 XT

� 198 WH / 80 THS / 28 XT

� 198 WH / 132 THS / 28 XT

03 December 2012 12

Wellhead Local Model

Analysis assumptions

� Perfect alignment and seating during installtion

� Contact friction of 0.1 for vertical faces 0.15 for landing shoulders

� Connection between RLSA threaded adjustment ring and body was

modelled with tie constraint

� Cement was was accounted for by constraining the in-plane DOF of

adjacent casing.

� Preload between 36” housing and wellhead was included.

� Fatigue loading was applied at top of WH housing using kinematic

coupling. Stress transfer functions applied independently.

� H4 profile not modelled. SAF=3 was assumed to determine fatigue

03 December 2012 14

Key Assumptions for Initial Fatigue Analysis

� Analysis performed to SEPCO Fatigue Service Wellhead Standard

� Minimum Service Life = 1year

� Minimum un-factored fatigue life =1x100=100years (SF=100)

� Fatigue damage calculation with S-N curve methodology

� DEn-B Curve for base material – WHH, Connectors

� DEn-E Curve for ground welds – connector welds

� DEn-F2 Curve for un-ground welds –bulls eye weld plate

� Operating weather condition- upto 10year Non cyclonic

03 December 2012 15

Initial Fatigue Analysis

� Several features did not meet the prelude requirements

�All welds

�H4 profile

� Base material locations

Initial Fatigue Analysis Results – 36” Housing

Initial Fatigue Analysis Results – 26” Housing

Initial Fatigue Analysis Results – 18 3/4” Housing

Analsysis Refinements

� Wellhead geometry modification

� Eliminate unnecessary features e.g. PGB groove

� Include stress relief grooves (scoop cut) at corners

� Reduce the RTT

� Review Metocean data & apply directionality

� Review actual material properties

� Review SF against industry – API 2A & API 2RD

� Review soil stiffness and apply location specific data

03 December 2012 20

WH Geomtry modification – 36” LPWH

WH Geomtry modification -26” LPWH

WH Geomtry modification 18 ¾ HPWH

Modified Design fit assessment -Running the 18 ¾ WH Housing /RLSA

Key Assumptions for Updated Fatigue Analysis

� Analysis done as per SEPCO Fatigue Service Wellhead standard

� Minimum Service Life = 1year

� Minimum un-factored fatigue life = 1 x 10 = 10 years (SF=10)

� Fatigue damage calculation with the same S-N Curve methodology

� Operating Weather condition – Site specific data based on data

Operating Weather condition – Site specific data based on data

gathered between 2007 & 2010

� Directionality (8 directions) taken into account with 338 load bins for

each direction.

03 December 2012 25

Prelude Updated Fatigue Load Cases

Results (198 WHD / 132 THS / 28 XT) ; RTT => 500 kips

� 36” Conductor Housing

�All locations meet Prelude minimum

requirements.

� Location A_01 has a Fatigue life = 294 years

� 36_Weld has a Fatigue life = 625 years

�All other locations > 1000 years

03 December 2012 27

� 26” Conductor Housing

�All locations meet Prelude min

requirement

� 26_Weld Fatigue life = 370 years

�All other locations have > 1000 years

� 18.3/4” HP Wellhead Housing

� Location G_01 = 256 years

� Location G_04 = 625 years

�H4 Profile = 455 years

�All other locations > 1000 years

Case 1: With top two joints being 18 m

� 36” Welds and Connectors

� Uppermost splice weld on Jt#5

Fatigue life = 170 years

� Uppermost connector

� Max SAF = 19 (10yr fatigue)

� Deeper connectors

�No issues or concerns

� Viper connectors for Prelude min

03 December 2012 30

Case 1: With top two joints being 18 m

� 26” Welds and Connectors

� Uppermost splice weld on Jt#2

Fatigue life = 440 years

�Max SAF = 33 (10yr fatigue)

� Deeper connectors

�No issues or concerns

Lessons learned

� The common hypothesis in the world of riser design is that “using

fixed boundary conditions at the mudline location will give the

conservative loads at wellhead. Not always the case with weak soils

and heavy lower stack.

� Omni directionality leads to over conservative results

� Resonance of lower stack is very sensitive to damping (riser and soil

damping). Significant reduction in response was observed with

damping which the analysis did not include.

03 December 2012 32

Lessons Learned

� Fatigue analysis needs to be incorporated into the Subsea wells design

workflow to gain assurance that well integrity will be maintained over the

lifecycle.

� Fatigue analysis involves multiple disciplines Start early and engage the

right people

� Realistic Site specific input data facilitates optimal engineering solution for

fatigue issues.

� Improvements on weld quality, thickness transitions are cost effective and

improve fatigue lives.

� Good coordination between engineering and manufacturing is required

to ensure that proposed modifications are feasible

� Specify that forging QTS’s be obtained from prolongations only and that

it is sized to facilitate a retest of all mechanical and chemical properties.

03 December 2012 33

Lessons learned

� Get adequate quantity of QTS’s especially if you need to qualify weld

procedures.

� The fatigue friendly design can only be improved through extensive

cooperation with the suppliers.

� There is potentially large conservatism in conductor fatigue estimates

due to use of SN curves, safety factors and ignoring compression

benefits.

03 December 2012 34

wellhead fatigue operators perspective - ptil.no 2012/subsea... · conservative loads at wellhead....

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