Defining Structural Monitoring · Defining Structural Monitoring Requirements for Slugging in Rigid...

Defining Structural Monitoring Requirements for Slugging in Rigid Jumpers and Pipeline Spans

Ryan Koska, Tze King Lim, Hugh Howells

AOG 2014

Learn more at www.2hoffshore.com

Agenda

Introduction

Finite element analysis and typical response

Monitoring system definition

FEA model calibration and integrity management

Conclusions


Why Monitor?

Cyclic stresses from weight changes and inertial loading due to slugging can cause significant fatigue loading

Uncertainties in slugging behavior can lead to lack of confidence in fatigue performance

In-service structural response measurements can validate design analysis and be incorporated into a integrity management program

Careful consideration of expected response and monitoring systems specifications required to get useful data


Monitoring System Specification

Analysis Input Data

Evaluate Structural Response to Identify Parameters and

Locations of Interest

Finite Element Analysis

Sensor Specifications

Define Preliminary Monitoring System

Compare Analysis Response to Sensor Specifications

Sensors Adequate to Capture Response?

Reposition Sensors or Evaluate Sensors with Different

Performance Specs

No

Yes Proceed with Procurement


Slugging Structural Response Analysis

Key analysis inputs:

Jumper/pipeline span configuration

Pipe properties (OD, WT, coating properties)

Slugging regime properties (slug and bubble density, slug length, and flow velocity)

Methodology:

Time-domain dynamic analysis

Weight and inertial loads on structure calculated based on position of slugs

Loads applied to a pipe element model in ANSYS

Structural response (e.g. – displacement, angles, etc.) extracted at key locations


Flowline Span Example

Pipeline sleeper crossing:

Sleeper height: 1m

Span length: 50m (both sides)

Slugging regime:

Slug length: 40m

Density change: 400 kg/m3


Flowline Span Example Mid-Span Vertical Motion Timetrace

Negligible high frequency response

0.35

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0.80

250 500 750 1,000 1,250 1,500 1,750 2,000 2,250

Ele

va

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bo

ve

Mu

dli

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Time (s)

MID-SPAN ELEVATION TIMETRACE1m Sleeper Height, 50m Span on Both Sides, 40m Long Slugs

Flow Direction

Mid-Span

Slug Entering Span

Entire Slug in Left Side Span

Slug Moving into Right Side Span

Entire Slug in Right Side Span

Slug Exiting Span


Flowline Span Example Motion Envelope

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0.1

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1.0

1.1

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Ele

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bo

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(m

)

Distance from Sleeper (m)

PIPELINE SLEEPER CROSSING DISPLACEMENTS DUE TO SLUGGING1m High Sleeper, 50m Free Span on Both Ends

Static Position Motion Range

Sleeper

Touchdown Region

Touchdown Region


Flowline Span Example Vertical Motion Range

Maximum vertical motion range occurs mid span

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-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Ele

va

tio

n R

an

ge

(m

)


MAXIMUM ELEVATION RANGE DUE TO SLUGGING1m Sleeper Height, 50m Free Span on Both Ends

SleeperTouchdown Region

Touchdown Region


Flowline Span Example Angular Motion Range

Maximum angular motion occurs at sleeper

0.0

0.1

0.2

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0.9

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

An

gle

Ra

ng

e (

de

g)


MAXIMUM ANGLE RANGE DUE TO SLUGGING1m Sleeper Height, 50m Free Span on Both Ends

Sleeper

Touchdown Region

Touchdown Region


Monitoring System Options

Standalone:

Individual data loggers fully contained with battery supply and memory card

Loggers periodically retrieved to download data and replace batteries

Acoustic:

Batteries contained within loggers

Periodic transmission of data via acoustic modem


Structural Monitoring Sensor Options

Accelerometers:

Can be compared directly to analysis or double integrated to obtain displacements

Inclinometers and angular rate sensors:

Used in conjunction with accelerometers to obtain 6 DoF motions

Strain gauges:

Direct stress/strain measurement at a given location

Pressure sensors:

Measurement of low frequency vertical motion


Flowline Span Monitoring System

Pressure sensors and inclinometers capture low frequency response

Could be supplemented with accelerometers and angular rate sensors if high frequency loading is a concern

Pressure Sensor

Inclinometer


Key Sensor Performance Specifications

Range

Accuracy

Resolution

Noise

Frequency Response

Temperature Learn more at www.2hoffshore.com

Sensor Performance Pressure Sensor Example

EXAMPLE ACTUAL AND MEASURED VALUESPressure Sensor

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-10

-5

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20

Time (s)

Dis

pla

ce

me

nt

(mm

)

Actual Values Actual + Systemic Error Actual + Systemic Error + Noise Recorded ValuesEffect of Accuracy Effect of Accuracy + Noise Actual Values


0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Acce

lera

tio

n A

mp

litu

de

(m

/s

2)

Frequency (Hz)

ACCELERATION SPECTRARMS Noise = 0.0035m/s2

Actual Response Recorded Values Response after Noise Filtered Out Noise Cutoff Threshold

Example of Low Signal to Noise Response

Low frequency response buried in noise

Sensor noise typically present across entire frequency range

Noise from other sources such as waves can be present in certain frequency ranges

Can result in large uncertainty in measurements


Example of High Signal to Noise Response

Noise has negligible effect

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Acce

lera

tio

n A

mp

litu

de

(m

/s

2)

Frequency (Hz)

ACCELERATION SPECTRARMS Noise = 0.00035m/s2

Actual Response Recorded Values Response after Noise Filtered Out Noise Cutoff Threshold


Flowline Span Example Vertical Motion Range

Maximum vertical motion range well above sensor uncertainty level

0.00

0.02

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0.12

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0.20

0.22

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

Ele

va

tio

n R

an

ge

(m

)


MAXIMUM ELEVATION RANGE DUE TO SLUGGING1m Sleeper Height, 50m Free Span on Both Ends

Elevation Range Sensor Uncertainty

SleeperTouchdown Region

Touchdown Region


Flowline Span Example Angular Motion Range

Maximum angular motion well above sensor uncertainty level

0.0

0.1

0.2

0.3

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0.5

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0.8

0.9

-70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

An

gle

Ra

ng

e (

de

g)


MAXIMUM ANGLE RANGE DUE TO SLUGGING1m Sleeper Height, 50m Free Span on Both Ends

Elevation Range Sensor Uncertainty

Sleeper

Touchdown Region

Touchdown Region


Rigid Jumper Slugging Example

40m

15m

Slug length: 60m

Slug vs. bubble density difference: 750 kg/m3

Flow speed: 5.5m/s

10m


Rigid Jumper Example Global Response


Rigid Jumper Example Connector Stress Timetrace

Large amount of high frequency response due to inertial effects

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0 5 10 15 20 25 30 35 40

Str

ess (

MP

a)

Time (s)

Jumper Slugging ResponseTOTAL STRESS TIMETRACE

Stress at Flowline Connection

Weight Effects Only Weight and Inertia Effects

Lead edge of 1st slugimpacting 1st jumper bend

Slug filling up jumper's length

Tail end of 1st slugexiting 1st bend

1st slug exiting jumper

Lead edge of 2nd slugimpacting 1st bend

Flow direction


Rigid Jumper Example Mid-Span Vertical Motion

High frequency, low amplitude motions present – difficult for pressure sensors to accurately capture

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pla

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(mm

)

Time (s)

MID-SPAN VERTICAL MOTION TIMETRACE

Flow direction

Slug filling up jumper's length

1st slug exiting jumper

Response Location


Rigid Jumper Example Lateral Motion Timetrace

Displacements can be obtained by double integrating accelerometer measurements

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pla

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(mm

)

Time (s)

LATERAL DISPLACEMENT TIMETRACE

Flow direction1st slug exiting bend 3

1st slugimpacting bend 3


Response Location


Rigid Jumper Example Angular Motion Timetrace

Low frequency angle component obtained from inclinometer High frequency component obtained by integrating angular rate sensor measurements

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An

gle

wit

h R

esp

ect

to V

ert

ica

l (d

eg

)

Time (s)

ANGULAR MOTION TIMETRACE

Flow direction

1st slug exiting bend 3


1st slug impacting bend 3

Response Location


Rigid Jumper Example Preliminary Monitoring System

Accelerometers, angle rate sensors and inclinometers

Strain gauges

Accelerometers and angle rate sensors capture high frequency response

Inclinometers capture low frequency response

Option for direct strain measurement just above connectors


Slug Response Model Calibration

Structural monitoring can help verify:

Slugging frequency

Slug properties at subsea equipment

Direct monitoring for slugs can be used to calibrate flow assurance models

Model calibration can help identify:

Differences in structural response from design

Changes in span configuration over time (burial or trenching)

Learnings can improve understanding for future designs

1000 1010 1020 1030 1040 1050 1060 1070 1080 1090 1100-0.08

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0.01Fixed233 y Plot 20: Combined Displacement Timetrace

Time (s)

Dis

pla

cem

ent (m

)

FLEXCOMProcessed


Input to Integrity Management

Monitoring allows for the tracking of key fatigue parameters over time:

Slugging frequency

Flow speed

Accelerations/displacements

Measured data used as part of an IM program to:

Determine performance of as-built system

Identify the need for additional analysis or inspection of at-risk components

Confirm on-going fitness for service and potential for expanding system capability

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3-Aug 8-Aug 13-Aug 18-Aug 23-Aug

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Date

ACCUMULATED FATIGUE DAMAGE


Conclusions

Uncertainties in slugging behavior can lead to lack of confidence in fatigue performance

A well-specified monitoring system can validate design assumptions

Analysis is essential for defining monitoring system requirements

Long term monitoring can provide valuable data used to confirm integrity of structure



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Defining Structural Monitoring · Defining Structural Monitoring Requirements for Slugging in Rigid...

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