jpe dec 2012 - sample issue

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December, 2012 Vol.12, No.4

Great SouthernPress ClarionTechnical Publishers

Journal of

Pipeline Engineeringincorporating

The Journal of Pipeline Integrity


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Journal of Pipeline Engineering

Editorial Board - 2012

Dr Husain Al-Muslim,Pipeline Engineer, Consulting Services Department, Saudi Aramco, Dhahran,

Saudi Arabia

Mohd Nazmi Ali Napiah, Pipeline Engineer, Petronas Gas, Segamat, Malaysia

Dr-Ing Michael Beller,Landolt Steuer & Unternehmensberatung AG, Luzern, Switzerland

Jorge Bonnetto, Operations Director TGS (retired), TGS, Buenos Aires, Argentina

Dr Andrew Cosham, Atkins Boreas, Newcastle upon Tyne, UK

Dr Sreekanta Das,Associate Professor, Department of Civil and Environmental Engineering, University

of Windsor, ON, Canada

Leigh Fletcher, Welding and Pipeline Integrity, Bright, Australia

Daniel Hamburger, Pipeline Maintenance Manager, Kinder Morgan, Birmingham, AL, USA

Dr Stijn Hertele, Universiteit Gent Laboratory Soete, Gent, Belgium

Prof. Phil Hopkins, Executive Director, Penspen Ltd, Newcastle upon Tyne, UK

Michael Istre, Chief Engineer, Project Consulting Services,

Houston, TX, USA

Dr Shawn Kenny, Memorial University of Newfoundland Faculty of Engineering and Applied

Science, St Johns, Canada

Dr Gerhard Knauf, Salzgitter Mannesmann Forschung GmbH, Duisburg, Germany

Prof. Andrew Palmer, Dept of Civil Engineering National University of Singapore, Singapore

Prof. Dimitri Pavlou, Professor of Mechanical Engineering,

Technological Institute of Halkida , Halkida, Greece

Dr Julia Race, School of Marine Sciences University of Newcastle,

Newcastle upon Tyne, UK

Dr John Smart, John Smart & Associates, Houston, TX, USA

Jan Spiekhout, Kema Gas Consulting & Services, Groningen, NetherlandsProf. Sviatoslav Timashev, Russian Academy of Sciences Science

& Engineering Centre, Ekaterinburg, Russia

Patrick Vieth, President, Dynamic Risk, The Woodlands, TX, USA

Dr Joe Zhou, Technology Leader, TransCanada PipeLines Ltd, Calgary, Canada

Dr Xian-Kui Zhu, Senior Research Scientist, Battelle Pipeline Technology Center, Columbus, OH,

USA


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4th Quarter, 2012 249

The Journal of

Pipeline Engineeringincorporating

The Journal of Pipeline Integrity

Volume 12, No 4 Fourth Quarter, 2012

ContentsStephen J Wuori ...........................................................................................................................................251

Pipelines for the 21st Century: safety, innovation, and technology

Dr Mo Mohitpour ........................................................................................................................................255

Obituary

Willard A Maxey ..........................................................................................................................................257

Obituary

Eric Jas, Dermot OBrien, Roland Fricke, Alan Gillen, Prof. Liang Cheng, Prof. David White,

and Prof. Andrew Palmer .............................................................................................................................259

Pipeline stability revisited

Prof. Andrew Palmer ....................................................................................................................................269

10-6and all that: what do failure probabilities mean?

Dr Filip Van den Abeele and Raphael Denis .................................................................................................273

Numerical modelling and analysis for offshore pipeline design, installation, and operation

Rob Bos, Suzanne Mooij, Leen Pronk, and Wessel Bergsma ..........................................................................287

Risk control at lower cost

Pipeline Pigging Conference in Houston: 25 years ........................................................................................305

OUR COVER PICTURE shows a graphic of typical loading pattern for a subsea

pipeline. The figure is taken from the paper on pages 273-286 which examines theissue of numerical modelling and analysis for the design, installation, and operation of

subsea pipelines.

The Journal of Pipeline Engineering

has been accepted by the ScopusContent Selection & Advisory Board

(CSAB) to be part of the SciVerse

Scopus database and index.


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This is a transcript of the Petrobras Luncheon keynote address given at the International Pipeline

Conference held in Calgary on 24-28 September, and organized by the Pipeline Systems Division ofthe ASME and the Canadian Energy Pipelines Association.

by Stephen J WuoriPresident, Liquids Pipelines and Major Projects, Enbridge Inc, Edmonton, AB, Canada

Pipelines for the 21st Century:safety, innovation, and technology

IWOULD LIKE to address four topics today: therole of pipelines, opposition to pipelines, technologydevelopments, and regulatory environment. But first I want

to lay out a challenge to you one that I will repeat at the

end of my speech: we must have unwavering dedication to

zero leaks and zero incidents. It must be our goal. Pipeline

companies in North America are being held to a standard

of perfection, and it is technology that will help us reach

that goal.

Role of pipelines

I see us focusing on two key frontiers regarding the

expectations of the public:

to find ever-smaller features in the pipelines; and

to find ever-smaller leaks in the pipelines.

Our industry is not perfect, but we must continue to progress

and be proud of the vital function we provide to society.

Let me congratulate all of you here in the audience for yourrelentless devotion to research and the development of newer

and better technologies to keep moving this industry forward

in continuing to deliver oil and natural gas in the safest, most

efficient and most economical way possible and to do it

with this one goal in mind: zero leaks. Without us providing

our services to tens of millions of people every day, the very

nature of peoples lives would be fundamentally altered.

People may read negative things about us these days in the

media and on line, and they may hear the negative stories on

the six-oclock news, but there is a more important element

of communication going on in nearly every aspect of theirregular lives that reinforces for them and for us that we

take our jobs very seriously and we do that job well.

Whenever they turn the heat on in their homes, or are

able to put gas in their cars or enjoy the wide variety of

fresh goods at their local grocery store, that is us doing

our job well.

Some things we need to keep in mind:

90% of all crude shipped in North America is

shipped by pipeline.

Canadians obtain 70% of our energy from

hydrocarbons and these hydrocarbons are delivered

by pipelines.

The oil and natural gas industry is the backbone

of the American economy supplying more than

60% of US energy.

Every year since 2007, Canadian pipelines have

delivered more than $100 billion worth of energy

to Canadian energy users and to export customers.

Canadas transmission pipeline companies, operate

approximately 109,000 km of pipeline in Canada

and the United States. These energy highwaysmoved approximately 1.2 billion barrels of liquid

petroleum products and 5.3 trillion cubic feet of

natural gas last year.

AOPL members carry nearly 85% of the crude oil

and refined petroleum products moved by pipelines

in the United States.

Although we are in the heart of the upstream producing

region for Canada, this is truly an international conference

with dozens of energy producing nations represented, so

it is important to note that these statistics represent only

Canada and the US. However, I think we can all agreethey demonstrate the crucial role our industry plays in

the global economy.

Guest Editorial


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The Journal of Pipeline Engineering252

represented, even in this very room, on the advancement

of technology in support of pipeline inspection and leak

detection. Without your research and the application of

new technologies, we would not be able to continue to work

towards our goal of zero leaks.

The safety of the communities and environment along our

rights-of-way and the integrity of our pipelines and facilities

are of the utmost importance for this industry and also

for the citizens of the countries in which we operate. As

an example, Enbridge itself, during the last decade alone,

transported nearly 12 billion barrels of crude oil with a safe

delivery record better than 99.999%. Our safety record as

a company is strong, but our goal is zero incidents, a goal

many other companies in North America and around the

world share, and we are relentlessly committed to continuing

to improve our processes, technology and vigilance so that

we can deliver on this goal.

To reflect on improvements made in recent years:

Spills along rights-of-way in the US have fallen from

2.0 incidents per thousand miles in the earliest

three-year period (1999-2001) to 0.8 incidents

per thousand miles in the latest three-year period

(2007-2009), a decline of 60%.

The volume released along rights-of-way has fallen

from just over 600 barrels per thousand miles in

the earliest period to less than 400 barrels in the

most recent period, a decline of 35%.

The liquid spilled from pipelines in Canada over

the past ten years is equivalent to three teaspoons

dripped out of a gasoline nozzle over the course of

50 fill-ups of 50 li each.

5.5 litres is the amount of liquid spilled per million

litres transported by pipeline in Canada between

2002 and 2011.

We have also found that there has been a significant reduction

in types of spill (corrosion, features, equipment failures) and

this is directly attributable to the advancements in technologyas a result of your considerable and diligent work.

Akin to the improvements in medical technology that

have progressed from x-rays to CAT scans to MRI,

pipeline integrity has seen dramatic advancements. In

fact we have employed much of this medical technology

for pipeline monitoring and maintenance. Ten years ago

in our industry we were able to detect dents; throughout

the 1990s and 2000s, however, a variety of inspection

techniques enabled us to identify corrosion features both

internally and externally on a pipe; and today we have

the ability to detect smaller and smaller features. In fact,Enbridge has used more crack-detection tools that the rest

of the world combined.

Opposition to pipelines

So whats the problem? Why all the scrutiny and opposition

and why now? To use a medical analogy I will compare

pipelines with the 100,000 or so miles of arteries and veins

in our bodies. As vitally important as they are to our health,

we typically only think about our arteries or veins if and

when something goes wrong with one of them. Similarly,

the public does not think about the thousands of miles

of pipelines as a vital function of society, but rather only

notices them when something goes wrong.

Environmental groups have made it clear that we are a

deliberate and strategic target for opposition to the oilsands

and, quite frankly, it makes sense. If you are opposed to the

oilsands, it doesnt make much sense to go after producers

who work in remote areas surrounded by industry workers

for a few YouTube hits.

They also know that they will only undermine their popular

support by attacking the end consumer. The mom and dad

with three kids putting gas in their SUV to drive around

the neighbourhood for groceries and to attend sporting

events are people you want as a grassroots advocates: they

are not a good target.

We, on the other hand, are the efficient target. We are

everywhere, in peoples back yards and communities. We

run across cultivated fields and along public and private

roads. Our terminals and pump stations can be in close

proximity to large populations and are clearly branded.

Get people upset about pipelines and you hit the oilsands

industry where it hurts. If we cant move the product to

market, we cant utilize or capitalize on it to its full potential

and that product cannot support growth, the economy or be

available for energy consumers growing needs. This frustrates

the market, our clients and the consumer.

Another exposure we manage daily in our industry is

the complexity of the terrain, geography, and diversity

of demographics along of a pipeline right of way. There

are endless opportunities to sow and cultivate opposition

on all kinds of issues and for all kinds of reasons, evenif many of those reasons are imagined or exaggerated.

Our focus has to be on what opportunities this scrutiny

provides for us, how it can make us better operators and

service providers. We all know that these opposition

strategies affect all of us who work in every part of this

industry. And one of the most important ways we can

address this is through our science, and our continually

advancing technology.

Technology development

As integral as our industry is to all developed and developing

economies worldwide, what is intriguing is the work


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Growth of construction features

Compressor and pump station

Measurement

Integrity management systems are being developed in

most companies throughout our industry that encompass

comprehensive analysis of pipelines and set prescriptive

performance-based regulations and standards intended to

meet the dynamic nature of pipeline operations.

Regulatory environment

We need to have and do have very strong regulators

in Canada and the United States. The public must have

confidence in them as it is the role of the regulators to be

the proxy of the people.

Conclusion: a call to action

We all agree that incidents, especially significant ones, are

unacceptable, but we are a good industry, we do an important

job well, and we need to continue to move forward. We have

too many important customers, both on the refining and

producer side, but also at the curbside, who are all relying

on us and help give us that motivation and momentum togo forward every-day.

We still have the confidence and imperative to grow our

business and our infrastructure to meet our ever-growing

need for energy.

With your on-going efforts and research we will reach this

goal. Your work is being noticed and appreciated. And

it is having an impact you heard the statistics on the

improvements in spill records since 1999.

Again, I think it is worth repeating, especially in these daysof increased media coverage and public scrutiny focused

largely on the oilsands and our pipeline industry, that we

Let me cite a few examples of what is going on in the industry:

As many of you know, CEPA recently announced

its Integrity First Programme an industry-wide

initiative that will improve pipeline safety as well

as environmental and social performance. This

programme is based on sharing best practices

and applying advanced technology throughout

the industry, and will focus on inline inspection

and leak detection in four key areas: prevention,

emergency response, reclamation, and education.

AOPL and API leadership teams: with a focus

on public awareness and damage prevention,

eight leadership teams on pipeline performance

improvement have been established whereby

executive leaders of pipeline operators join

operational personnel to focus on the following

specific pipeline safety improvement areas:

Research and development/enhanced technology

Leak detection

Enhanced data integration

Sharing safety practices and lessons learned

Damage prevention

External communications

Strategic planning; and Emergency response

An international organization with which you are no doubt

familiar, the Pipeline Research Council International

(PRCI), develops dynamic research programmes devoted to

identifying, prioritizing, and implementing the industrys

core research objectives. Examples of collaborative

technology developments providing a foundation for the

safe and reliable operation of the worldwide pipeline

infrastructure include such key areas as:

Corrosion: location and assessment Mechanical damage: location and assessment

ROW monitoring

Steve Wuori giving hisspeech at the IPC.


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So I would leave with you this thought: as you read the

paper, or watch the TV, or go online and see the stories

about pipelines, about our industry, I would urge you not

to be discouraged. Know that all of you here are part of

what makes these pipes flow every day and for that reason

we work in a good and vital industry and because of your

innovation and effort, it will continue to get better.

Lets take this time under the microscope to show the public

what we can do and what we know we can do better for the

future of this industry. Lets keep doing our jobs well and

continually look for ways to do our jobs better, and we will

get to that goal of zero spills.

We cant do it without the hard work and innovation of the

people in this room and for that I thank you.

do our job well. That every day we are delivering millions of

barrels of energy safely to many communities that wouldnt

be able to exist, function, or prosper without that pipeline

infrastructure and supply.

Your work is critical to the future of this industry and the safe

delivery of the products we transport. We will continually

work to advance pipeline safety and integrity with all the

players involved many of whom are with us today and

they include representatives from both corporate and

industry associations world-wide as well as regulators and

members of the community.

We are all working to make pipelines and pipeline

transportation of hydrocarbons safer today and are dedicated

to continuous future improvement.


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a smoking hot deal whether it was on Petaling Street in

KL, the flea markets of Beijing, or the stalls along

Copabacana. Quite how he intended transporting home

some of the huge objects he had purchased left him

characteristically unfazed.

Mo passed away after a short illness from congestive heart

failure leaving behind his beloved wife Carol, his son Bijan,

daughter Rachel, and the sunshine of his riper years four

grandchildren of whom he was immensely proud.

A moving and happy celebration of Mos life was held at

the International Pipeline Conference in Calgary on 27

September, attracting upwards of 200 of those attending

the conference to share stories and reminisce about this

remarkable man.

Dr Alan Murray

ASME conferred on Mo their Distinguished Service Award

and he was very proud of his election to the Fellowship grade

of the Institution of Mechanical Engineers, the ASME, and

the Engineering Institute of Canada.

When TransCanada merged with the Pipeline Division

of Nova in 1999, it shortly thereafter disengaged from

international work. Mo took this as an opportunity to branch

out on his own and he successfully set up Tempsys Pipeline

Solutions providing consultancy services and training to

many North American and international clients. In doing

so he added to an ever widening circle of friends: Mo had no

need for Facebook or Linkedin, he had long since perfected

the art of making friends and influencing people.

Travelling with Mo internationally was a never-to-be-

forgotten experience. His cultural background and natural

competitiveness meant he was always on the lookout for


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Willard A Maxey1930 2012

AT THE recent International Pipeline Conference,held in Calgary in September, a special session theWillard A (Bill) Maxey Distinguished Lecture Series was

inaugurated to celebrate the life and achievements of Bill

Maxey, who died last March.

Bill was truly a dominant figure in the technical development

of the pipeline industry. Throughout his 40-year career

serving the industry, he developed many concepts and

methods that are still at the core of linepipe specification

and integrity assessment practices. Many of the experimental

techniques that he developed for carrying out demanding

(and potentially dangerous) full-scale testing are in use today,

though digital techniques have greatly simplified the tasks

of data acquisition and reduction. Bill made continuous

technical contributions in a number of fields throughout hismore than 30 years with Battelles Columbus Laboratories

and subsequent time with Kiefner and Associates, but

is probably best remembered for his theoretical and

experimental work on fracture, which provided practical and

intuitive solutions that the industry could apply to fracture

control design and integrity management.

His massively-influential 1974 paper Fracture initiation,

propagation and arrest was re-presented, as part of the

Distinguished Lecture Series, by Dr Gery Wilkowski, a

former colleague at Battelle. Bill progressively updated the

work presented in this paper over subsequent years, to keep

pace with the developments taking place in the industry,

particularly in terms of the materials being used and the

fluids being transported. For example, he was one of the

earliest researchers to recognize, and show how to deal with,

the special problems presented by fracture control in carbon

dioxide pipelines. It is also noteworthy that Bills work on

the integrity of steel transmission pipelines was subsequently

extended to LNG piping; plastic gas distribution pipe; nuclear

pipe, pressure tubes, vessels, and steam generator tubing;

and chemical plant piping, involving a variety of materials

from aluminum to zircalloy. His work is the technical basis

of many codes and standards, not only in North America,

but around the world.

Apart from his technical and experimental skills, Bill Maxey

had an extraordinary ability to find solutions from outside

the main stream, and it was often this which allowed him

to by-pass the road-blocks encountered by more linear

thinking. Those who worked with him, whether as colleagues

or through organizations such as the Pipeline Research

Committee, benefited enormously from his guidance. He

is remembered by all who knew him as an extraordinary

researcher and a patient and effective communicator, whose

open and friendly manner invited dialogue. He will truly

be missed.

Dr Brian Rothwell

Dr Gery Wilkowski


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THE STABILITY assessment of the 40-in North Rankin A trunkline, operated by Woodside Energy Ltd,has provided better insight into uid-soil-pipe interactions during extreme storm events. The resultingconclusion of the work is that the trunkline, a major subsea natural gas artery in Australias Northwest Shelf

since its installation in 1982, can continue to be operated safely for the next 30 years from a hydrodynamic

stability point of view. This conclusion was reached after substantial study and physical model testing wasperformed considering the tripartite interaction between uid, seabed, and pipeline.

To provide vital information feeding into the stability analysis, a physical model testing programme was

developed, and a new world-class hydrodynamic testing facility designed, constructed, and commissioned

at the University of Western Australia. This facility allows the replication of near-seabed conditions during

tropical cyclones in controlled laboratory conditions, and observation of the interaction between ocean,

seabed, and pipeline. Tests were performed using a range of pipeline embedment proles, storm realizations,

and pipe xity conditions simultaneously to model hydrodynamic loading onto the pipeline and seabed

scour. This data were then used in the three-dimensional numerical modelling of pipeline response using

nite-element analyses, which included the effects of seabed instability.

*Corresponding authors contact details:

tel: +61 8 9322 7922

email: [email protected]

by Eric Jas*1, Dermot OBrien1, Roland Fricke2, Alan Gillen2, Prof. Liang

Cheng3, Prof. David White3, and Prof. Andrew Palmer4

Pipeline stability revisited

THE 134-km long, 40-in diameter, and 23.8-mm wallthickness North Rankin A (NRA) trunkline wasconstructed by Woodside Energy Ltd in 1982 based on

a design life of 30 years. The gas pipeline links the NRA

Platform to the North West Shelf Venture gas plant on the

Burrup Peninsula (Fig.1) Primary stabilization is provided

in the form of a concrete weight coating. In the area of

interest, the concrete weight coating is 64 mm thick and

has a density of 3,043 kg/m3, and the corrosion coating

comprises a 6-mm thick layer of asphalt enamel with a

density of 1,281 kg/m3; the contents density for stability

design purposes is 90 kg/m3, and consequently the specificgravity (SG) of the pipeline in this area is 1.23 relative to

seawater. The current practice, 30 years after the NRA

trunkline was installed, is for large-diameter pipelines to be

designed in similar water depths with a much thicker (and

sometimes much higher density) concrete weight coating,

with much higher SG values. This assists considerably in

achieving on-bottom stability without the need for applying

secondary-stabilization measures.

Along the first 22.8 km from shore, the trunkline is covered

with a minimum of 2.5 m of quarried rock to provide

protection from accidental external impacts. From KP 22.8 to

KP 123.8 the pipeline was post-trenched by ploughing in loose

and variably cemented carbonate marine sands and silts. The

plough formed an open V-shaped trench below the pipeline

with the intention that the depth of the trench would place

the top of the pipeline at or below the natural seabed level.

In April 1989, a severe Tropical Cyclone (TC) Orson

caused significant changes to the seabed bathymetry along

the trunkline, which resulted in the distinct V-shapedploughed trench shape disappearing. Consequently, the

required sheltering which the trench previously offered

was no longer present everywhere along the pipeline route.

Typical data from a survey undertaken after TC Orson in

1989 are shown in Fig.3, reconstructed to provide a three-

dimensional visualization of the degree of burial along a

typical length of the trunkline.

Upon discovering the changed bathymetry of the seabed in

relation to the trunkline, a remedial stabilization programme

was developed and implemented between 1990 and 1992.

This comprised rock dumping along selected sections ofthe pipeline, both to improve pipeline stability and to also

stabilize the seabed either side of the pipeline.

1 Atteris Pty Ltd, Perth, WA, Australia

2 Woodside Energy Ltd, Perth, WA, Australia

3 University of Western Australia, Crawley, WA, Australia

4 National University of Singapore, Singapore


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embedment levels, which are not compatible with the

existing codes and recommended practices.

At the time when the assessment commenced (2006) the

only available and reliable recommended practice was DNV

RP E305 [1], which:

does not provide guidance on pipe-seabed interaction

forces for pipelines on carbonate soils;

does not allow for the effect of pipeline embedment

on soil resistance and hydrodynamic loading; and

does not consider the effects of seabed instability

within the response of the pipeline during storm

loading.

The successor to DNV RP E305, published in 2007, is

DNV RP F109 [2]. This updated code does allow for

some effects of pipeline embedment; however it does not

consider asymmetrical embedment levels, and also does

not provide quantitative guidance for carbonate soils. Both

recommended practices focus on pipeline on-bottom stability

with relatively low embedment levels, and are not suited

to the assessment of highly embedded pipeline sections.

In addition, none of the existing codes and recommended

practices considers the effects of the changes in seabed

bathymetry and characteristics during a storm event

i.e. when subjected to the wave- and current-inducedhydrodynamic loads on pipeline stability. Such changes can

include sediment scour and deposition, excess pore pressure

build-up and dissipation and, sometimes, liquefaction.

The pipeline engineers who undertook the assessment

considered these effects of great importance. It has been

mentioned before that the seabed, if it comprises a fine- or

medium-sized uncemented material, will lose strength and

become mobile during the ramp-up period of a storm, long

before the pipeline becomes unstable [3], depending on

the SG of the pipeline. Whilst the recommended practices

and guidelines do not incorporate these effects within theirmethodologies, many pipelines that are in operation have a

track record of losing contact with the seabed over sometimes

The desire to extend the lifespan of the trunkline beyond

2012 triggered the need to undertake a rigorous engineering

assessment of this asset. It included a study of thehydrodynamic stability of the system for the next 30 years.

A screening process indicated that the critical area

that needed thorough review was the section along the

Continental Shelf, between the 26 m and 73 m water depth

contours, or between KP 22.8 and KP 116. The two adjacent

pipeline sections were either stabilized by quarry rock (the

shore approach) or stable under the pipelines own weight

(the platform approach).

The challenges faced by the pipeline engineers who carried

out the assessment were the following:

The trunkline comprises sections with highly variable

levels of embedment in sediments, ranging between

0% (of pipeline diameter) to 100% or more.

Along many of these areas, the level of embedment

either side of the trunkline is not the same; in some

areas there is as much as 100% embedment on one

side with little to no embedment on the other side.

At the commencement of the assessment there was

insufficient clarity as to the degree of instability of

the seabed in the immediate vicinity of the trunkline.

The SG of the trunkline (1.23 in seawater) along

the area of interest is relatively low. A review of existing 3D pipeline-stability software

packages indicated that they would be inadequate

to undertake the assessment accurately in the given

seabed conditions, unless considerable modifications

were made to the software to account for the effects of

seabed instability onto the pipeline-response model.

The Northwest Shelf of Western Australia comprises

carbonate soils, and international pipeline codes and

recommended practices are written on the basis of

any seabed sand being siliceous.

In summary, the assessment required an unconventionalmethodology in view of the nature of the sediments, the

potential effects of seabed instability, and the pipeline

Fig.1. The NRA trunkline.


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over more than two decades, including survey data from

annual and post-tropical cyclone inspections. Also, past

studies which had assessed the potential of the seabed

sediments along the pipeline route to liquefy and/or scour

were studied. Mechanical design properties of the trunkline

were also collated to create an overall picture of the asset

and its environment.

Table 1 presents the metocean data applicable to the section

of the pipeline route between KP 52 and KP 63. The seabed

along this section of the pipeline route comprises a 1 - 2

m thick layer of fine- to medium-carbonate sand overlyinga calcareous rock pavement. The sand has a D50 of 150 -

200 microns.

significant lengths in areas of loose sediments through the

forming of scour holes. The forming of scour holes along

a pipeline can sometimes be so extensive that the pipeline,

depending on its SG, experiences self-burial over time.

Methodology

As a consequence of the limitations in the design codes and

recommended practices, an unconventional methodology

was developed for this case (Fig.4). The aim of the stability

assessment was to develop an understanding of the processes

contributing to the stability (or instability) of the trunkline

and to assess whether satisfactory evidence can be gatheredto demonstrate that the risk of future trunkline instability

is sufficiently low.

The following main steps were identified when developing

the methodology of the stability assessment of the NRA

trunkline.

Overview of the assessment

Input data gathering

The NRA trunkline had previously been the subject ofmuch study, in particular in relation to its hydrodynamic

stability. A significant amount of data had been collected

Project Execution

Plan

Step 1

Input Data Gathering

Step 5

2D Physical Model

Step 6

3D Stability Assessment

Step 7

Remedial Stabilisation

Design

Step 8

Redefine Trunkline

Stability Criteria

Close-Out Report

Trunkline

Stable?

Step 2

Seabed Stability

Step 4

Rock Berm Stability

Assessment

Step 3

2D Pipeline On-Bottom

Stability

Fig.2. Typical as-built post-lay ploughed trench prole.

Fig.3. Various levels of pipeline embedment post-TropicalCyclone Orson.

Fig. 4. The assessment process.


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Excess pore pressure build-up will result in soil softening and

reduced soil passive resistance, and could result in partial

pipeline flotation where significant excess pore pressures

are generated. However, it is difficult to precisely correlatea decrease in lateral soil resistance to a value of excess pore

pressure. Although excess pore pressures generated in the

region where the pipeline stability has been analysed in detail

are not expected to have a significant effect, sensitivity load

cases have been performed in the 3D FEA analysis using

reduced soil passive-resistance values to assess the potential

effects of excess pore pressure build-up on pipeline stability.

A regional (free-field) scour analysis was also performed.

Two independent methods were used: the first used the

Soulsby method [5] to determine the volume of sediment

suspended in the water column, while the second assessed

the possibility of sheet flow, using the Flores and Sleath

method [6] to estimate the regional (free-field) scour depth.

The regional scour analysis indicated that over the long term,

the regional scour depth is likely to be limited to less than

0.1 m along the NRA trunkline route in the area of interest.

Local scour was assessed using the computational-fluid-

dynamic (CFD) package SCOUR-2D developed by the

Hydraulics Research Group, led by Professor Liang Cheng

at the University of Western Australia. It is believed that

local scour did occur during TC Orson in 1989, whereby

the spoil banks of the ploughed trench (and otherunconsolidated, cohesionless, fine-grained sediments) were

deposited on and around the pipeline inside the V-shaped

ploughed trench. Following the suspected liquefaction of

this material during the same and/or subsequent tropical

storms, and rise of the pipeline through this material,

further local scour of this seabed material is likely to have

occurred alongside the trunkline.

The results of the seabed-stability assessment indicate that

both liquefaction and scour have played a significant role

in the stability of the trunkline. Also, now that the pipeline

is exposed at the seabed, (predominantly) local scour islikely to have a significant influence on the stability of

this pipeline.

Seabed stability assessment

The pipeline engineers who undertook the assessment

recognized that the overall stability of the pipeline dependson the tripartite interaction between the hydrodynamic

loads induced by tropical cyclones, the seabed comprising

predominantly calcareous sediments and the trunkline.

Consequently, an in-depth study of these processes

was undertaken.

As a starting point, the interaction between the hydrodynamic

loads and the seabed was assessed. Specialists were engaged

to undertake seabed liquefaction and scour analyses.

The seabed liquefaction analysis performed for this project,

which used the methodology described by Bonjean et al.

[4] concluded that, although the large hydrostatic pressure

fluctuations caused by tropical-cyclone-induced waves do not

have the capacity to induce free-field seabed liquefaction,

it is likely that loose and fine sediments deposited within

the open trench would have liquefied during a significant

storm event (such as TC Orson in 1989). This would have

caused lift of the pipeline by several tens of centimetres. The

end result, after a significant storm, created a picture which

was perceived at the time by many as a general lowering of

the seabed (due to regional scour), while in reality it could

well have been the pipeline which had risen.

This trench backfill material liquefaction theory is consideredto be the most likely explanation for the observed change

in the burial of the NRA trunkline. It casts doubt onto the

validity of the broadly accepted regional-scour theory, or at

least the depth extent of such seabed erosion and its effect

on submarine structures such as pipelines in this region.

The seabed-liquefaction assessment also concluded that

where soils are classified as sand, excess pore pressure build-

up around the pipeline during an extreme load condition

is expected to be small (5 - 10%), which is not expected to

impact pipeline stability. However, where soils are classified

as silty sand, excess pore pressure build-up around the

pipeline can be much higher under extreme load conditions(60 - 70%) which may be expected to cause localized partial

flotation of the pipeline.

Description Symbol Value

Significant wave height Hs 12.94 m

Peak period Tp 14.76 s

Water depth d 55.8 m

Significant wave-induced current

(perpendicular to pipeline)

Us 1.62 m/s

Steady-state current (perpendicular to pipeline) VR 0.52 m/s

Maximum wave-induced current

(perpendicular to pipeline)

Umax 2.46 m/s

Table 1. NRA trunkline 100-year RP design metocean data (from KP 52 to KP 63).


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section was consequently used as the basis for the physical

model testing programme and subsequent 3D dynamic

stability assessment.

Rock-berm stability assessment

In parallel with the pipeline-stability assessment, the

stability of the rock berms (which were installed as part of

the remedial stabilization project in 1991) was re-analysed

using the industrys latest reliable software. The rock-berm

stability software package PROBED[7] was used to calculate

the minimum rock-armour layer D50values that would be

statically stable for the 100-year return period conditions.

The PROBEDsoftware package has been developed by Delft

Hydraulics and is based on tests performed on schematized

structures. It allows for the design of graded rock structures

that are subjected to a combination of steady-state currentsand oscillating currents induced by non-breaking waves. The

software uses empirical and semi-empirical design equations

2D pipeline-stability assessment

The limitations of the available pipeline-stability

recommended practices [1, 2] were assessed in great detail.

It was decided that, initially, a 2D analysis using a modified

RP F109 approach would be performed based on absolutestability criteria. In view of the relatively low pipeline

SG and the fact that the pipeline had been trenched

following installation, it was a safe assumption that should

pipeline break out occur, instability had been reached to

an unacceptable degree. It was recognized that this was a

conservative approach, with the aim of identifying which

areas of the trunkline needed further assessment. Thus, the

results from the 2D stability analysis would then be utilized

to prepare the scope of work for a more-realistic, but also

more-complex, 3D FEA analysis.

The results of the 2D stability assessment are summarized inFig.5. The region between KP 52 and KP 63 was identified

as the most likely to experience instability: this 11-km route

Fig.5. Pipeline signicant stability results between KP 28 and KP 124.

Fig.6. The O-tube hydraulic testing facility at UWA.


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to be incorporated in the design assessment, it was decided

that physical model testing would be performed to provide

additional information specific to the conditions relevant to

this pipeline. To achieve the required results the minimum

parameters of a testing facility were defined, which resulted

in the following main conclusions:

It was considered impractical to build a facility which

would enable testing a 40-in weight-coated pipeline at

the prototype scale. To practically model the tripartite

interaction between hydrodynamics, seabed soils,

and pipeline, scaling would need to be limited to a

maximum of 1:5 1:6; the quantification of scour

processes around pipelines become increasingly

difficult to model at smaller scales.

At such a scale, the use of an open wave and current

flume would be impractical, requiring a flume depth

of at least 10 m with the ability to concurrently

model wave-induced, as well as steady-state, currents.Existing conventional open flumes are plagued by

wave breaking and non-linear affects.

U-tubes, which have commonly been used in the past

for similar work, have significant limitations in relation

to varying the wave periods as well as including steady-

state currents. It is difficult to control the frequency

in a U-tube much away from its natural frequency.

After several meetings in which the physical model

testing aspects were discussed, the concept of the

O-tube [8] was developed by the University of Western

Australia specifically for this project. To obtain the

additional required funding to construct such an

ambitious facility, Woodside formed a collaboration

with Chevron Australia with the aim of undertaking

additional testing over and above that required for the

NRA trunklines stability assessment. In addition,

federal funding was successfully applied for through

the Australian Research Council.

A number of scaled physical model tests were performed in

the O-tube facility for various symmetric and asymmetric

initial embedment profiles. The tests were performed using

an appropriately scaled representation of a 100-year return

period irregular wave-induced and steady-state current storm

time series. Seven realizations of complete on-bottom wave andcurrent velocity storm time series were generated. The storm

realization with the largest peak flow velocity was selected as

the base-case flow velocity time series for use in the physical

model testing programme.

The following key data were measured from each test for use

in the subsequent 3D FEA stability model:

The lateral and vertical hydrodynamic loads were

measured during each test as a function of time.

In addition, the profile of the artificial seabed, created

using sediments sourced from the Northwest Shelf,was monitored and measured throughout each test

using a small echo-sounder.

based on wave height, critical Shields parameter, shear stress,

and other inputs to calculate a D50rock particle size. Some

engineering judgement is required as graded rock structures

are essentially a collection of graded rock particles, and

the ability of an individual rock particle to withstand the

design hydrodynamic load depends not just on the weight

and dimensions of the particle, but also on the level of

protrusion and interlocking with adjacent particles.

The analysis performed using PROBEDindicated that the

rock berm along the trunkline met the design functional

requirements.

Physical model testing

The 2D pipeline-stability analysis provided information as

to which sections of the pipeline along the area of interest

were critical from a hydrodynamic-stability point of view. It

was identified that within the limitations of existing design

codes it was not possible to demonstrate that the pipeline

satisfied on-bottom stability requirements. However, it wasrecognized that these limitations overlooked potentially

beneficial effects from seabed mobility. To allow such effects

Fig.7. Extract of test results (example).


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diameter subjected to a 100-year return period hydrodynamic

load in the form of a storm time-history with 3 hrs ramp-up,

3 hrs peak storm duration, and 3 hrs ramp-down (prototype

timescale), and with a scaled-down combination of irregular

wave-induced and steady-state currents.

It was noticed that seabed instability (local scour), for this

particular test, occurred before the peak of the storm, and

well before the model pipe became unstable. The model pipe

was displaced laterally after approximately 4 hrs (prototype)

from the start of the storm, upon which the model pipe

was restrained laterally (but not vertically i.e. constant

SG) to monitor the seabed response to hydrodynamic

loading in the event that adjacent sections of the pipeline

would be stable (due to more embedment and/or reduced

hydrodynamic loading).

The measured forces and seabed data of all testing performed

were subsequently analysed and captured in a numerical

model for input into the 3D finite-element analyses.

Soil lateral-resistance model

The development of a reliable and realistic pipe-soil resistance

model for the carbonate soil used here was a key element of

the trunkline stability assessment. Pipe pull-out tests were

performed in the O-tube using the model pipe embedded

into the seabed soils, for a range of embedment levels, to

define a pipe-soil resistance model specific for this pipeline

and soil combination.

DNV RP-F109 [2] recommends modelling soil resistance

using the Verley and Sotberg soil passive-resistance model[9] in combination with a Coulomb friction factor of 0.6.

According to this model, the Coulomb friction factor is

Ancillary tests were also performed to provide additional

information needed for the stability analysis, including:

Hydrodynamic loading of the test pipe on a rigid

seabed, to obtain lift and drag forces for a range of

KC numbers.

Pull-out tests to define a pipe-soil resistance model

specific for this pipeline and soil type.

It is recognized that the 1:5.8 scaling used for the testing

programme introduced several scaling issues, particularly

in view of the inability to use a similarly scaled soil for the

tests (the tests were performed using prototype soil). This

problem was addressed during the preparation phase of

the physical model testing campaign. An assessment was

therefore performed, in particular to quantify the level

of error associated with the onset and extent of scour

development in the physical model relative to what was

expected in the prototype.

Despite the inevitable scale of 1:5.8 used during the testingcampaign in the O-tube, it is noted that this is closer to full

scale compared to previous (similar) physical model testing.

To date, physical model tests involving subsea pipelines

have typically been performed at scales of 1:20 or smaller,

and this leads to scour-scaling distortions in the order of

100 to 200%. In comparison, the time required to reach

equilibrium scour in the O-tube has been assessed to be

approximately 35% relative to prototype scale. The soil-

particle size will not affect the equilibrium scour depth or

the initiation of local scour.

An example of the results of one test is provided in Figs 7 and8. This particular test was intended to simulate a 2D pipeline

slice embedded symmetrically to a level of 50% of its external

Fig.8. Extract of test results (example), lift force (top), and drag force (bottom).


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The analysis software was validated against the O-tube initial

embedment test results, which included scaling the test results

from the model scale to prototype scale. The following steps

were undertaken to define the sub-sections to be analysed:

A three-point moving average was applied to the

pipeline embedment data to define the initial

embedment profiles at 5-m intervals along the

pipeline.

The initial embedment level along the trunkline

varies, and eight such embedment cases were

modelled in the O-Tube (i.e. in terms of left:right

embedment level as a percentage of pipeline diameter

(D): 0D:0D, 0D:50D, 0D:75D, 50D:0D, 50D:50D,

50D:75D, 75D:75D, and 90D:90D). The initial

embedment profile (at 5-m intervals) was rounded

down to the nearest test profile. For instance, where

the survey data shows a section of the trunkline

route with an initial embedment of 0D:25D, the

initial embedment has been rounded down to the

nearest available test data, which is the 0D:0D initialembedment test data. This is considered to be a

conservative approach.

The extent of each analysis model has been

determined by identifying regions that will provide

highly stable, effectively fixed-end, conditions such

as long regions (50-100 m) of fully buried or rock

dumped pipeline.

The schematic presented in Fig.11 is a representation of the

loads applied to the beam in an analysis time increment.

Based on the 3D dynamic stability analysis results, itwas concluded that the most critical region of trunkline

(between KP 52 to KP 63) is not expected to break out of its

test data, and comparisons between the modified model and

the O-tube pull-out tests are shown in Fig10. This allows the

un-conservatism inherent in the Verley model when applied

to this particular carbonate soil type to be removed, while

maintaining the generic form of loading-unloading-break-

out behaviour described within the Verley model in the 3D

dynamic FEA modelling of the NRA trunkline.

3D nite-element analyses

A 3D finite-element pipeline stability analysis was performed

to translate the results from the 2D scaled physical model

testing to the 3D prototype 40-in diameter pipeline. This

dynamic on-bottom stability analysis was performed using

the CORUS-3Danalysis software [11]. The 11-km section

of trunkline route (KP 52 - KP 63) was partitioned into

ten sub-sections, varying in length from 220 m up to 1,830

m, with each sub-section analysed independently. The

assessment was performed by subjecting the trunkline, in its

current condition, to a 100-year return period hydrodynamic

loading condition.

The objective of using the CORUS-3Danalysis software over

and above the 2D physical model testing was to account

for the stabilizing effects of adjacent pipeline sections with

higher initial embedment levels on sections of pipeline

which would otherwise be considered unstable in 2D.

CORUS-3Dis also able to reduce the conservatism present

in the 2D modelling by simulating 3D wave loading.

This FEA model incorporated the interactions that exist

between a pipeline and the surrounding fluid (hydrodynamic

effects), a pipeline and the seabed (passive resistance) and,to a limited extent, the dynamic interaction between the

seabed and the fluid.

Fig.11. Schematic of the loads applied during an analysis increment.


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PIPELINE RELIABILITY analysis appears at rst sight to be related to the probability analysis to whicheveryone is accustomed. In reality, it is substantially different, and the numerical failure probabilities itarrives at are nominal and unrelated to real probabilities. This matters because it misleads the engineer

and the wider community, and because it may lead to an illusion of condence and safety that the analysisand the underlying data do not begin to justify. The paper discusses the problem, and how codes might be

better written.

Authors contact details:

tel: +65 6516 4601


by Prof. Andrew Palmer

Keppel Professor, Centre for Offshore Research and Engineering, National University of Singapore

10-6and all that: what do failure

probabilities mean?

THE NOTION OF probability was developed by gamblers.Cardano (1501-1576) asked how many times you wouldneed to throw two dice to have an even chance of two sixes.

He is described in one book [1] as physician, philosopher,

scientist, astrologer, religionist, gambler, murderer, and as

it happens he got the wrong answer. The ideas were picked

up and developed by many mathematicians, some of them

known in the context of pipelines, among them Pascal, De

Moivre, Bernoulli, Fermat, Gauss, and Poisson. If you throw

a fair dice, the probability of a six is 1/6. If you throw two

dice, the probability that the sum of the pips will be 10 is

3/36, and so on. We have a clear idea of what that means,

and we can use it to inform decisions.

Turning to pipelines, we frequently come across statements

such as:

the nominal target failure probability level shall be

based on the failure type and safety class as given in

Table 2-5[2, section 2 C 503]

and there follows a table which says that the nominal failure

probability per pipeline per year for safety class very high

shall be 10-6for ultimate, fatigue, and accidental limit states,

and 10-7to 10-8for pressure containment, and so on.

A reasonable question to ask is how those fai lure

probabilities relate to the customary understanding of

what probability means. Or does the weasel word nominal

take care of the question?

The point was made more forcefully by Richard Feynman

[3] in his account of his discussions with NASA following

the Challenger space shuttle disaster:

As range safety officer at Kennedy, Mr Ullian had to

decide whether to put destruct charges on the shuttle.

(If a rocket goes out of control, the destruct charges

enable it to be blown into small bits. Thats much less

perilous than a rocket flying around loose, ready to

explode when it hits the ground.)

Every unmanned rocket has these charges. Mr Ullian

told us that 5 out of 127 rockets that he had looked

at had failed a rate of about 4%. He took that 4%

and divided it by 4, because he assumed a manned

flight would be safer than an unmanned one. He came

out with about a 1% chance of failure, and that was

enough to warrant the destruct charges.

But NASA told Mr Ullian that the probability offailure was more like 1 in 105.

I tried to make sense of that number. Did you say

1 in 105?

Thats right: 1 in 100,000.

That means you could fly the shuttle every day for

an average of 300 years between accidents every day,

one flight, for 300 years which is obviously crazy

Yes, I know said Mr Ullian.


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Again there are standard responses. One is to assert that

the probability distribution of each of the relevant variables

is one of the standard distributions such as Gaussian or

log-normal applies, so that the distribution is completely

characterized by a mean and a variance. That assertion is

totally unjustified: there is no reason at all why a form that

describes the middle should also describe the distant tails. It

has been well said that mathematicians believe the Gaussian

distribution to be a law of physics, and physicists believe it

to be a law of mathematics, but of course neither is correct.

Some arguments appeal the Central Limit Theorem, but

that is unlikely to be applicable.

The other standard response is to revert to the argument

described earlier, and to say that the calculated probabilities

are only nominal.

The difficulty has of course been recognized. The DNVclassification note [8] has this to say:

The analysis models are usually imperfect, and the

information about loads and resistances is usually

incomplete. The reliability as assessed by reliability

methods is therefore generally not a purely physical

property of the structure in its environment of actions,

but rather a nominal measure of the safety of the

structure, given a certain analysis model and a certain

amount and quantity of information.

Correspondingly, also the estimated failure probability

is dependent on the analysis model and the level

of information, and it can therefore usually not be

interpreted as the frequency of occurrence of failure for

that particular type of structure. An ideal frequentistic

interpretation of the estimated failure probability would

require a large population of the particular type of

structure in conjunction with perfect analysis models

and full knowledge about the governing uncertainties.

This will practically never be fulfilled.

which acknowledges the difficulty. It might be thought better

to replace usually by always in the first paragraph, to delete

generally later in the same paragraph, to replace usuallynot by never in the second, and to delete practically in

the last sentence.

Does it matter? How to move on

Ideally, we would throw out all these spurious numbers. They

are not probabilities, and they do not help us to make decisions.

The reality is that ideas of this kind nowadays have so much

momentum, and have an industry of structural reliability

analysts in the background, that it is probably politically

unacceptable to dispense with them completely. Anotherpossibility is that we make a careful distinction between

the different ideas, retain the term probability for the

A conventional reply

This point has of course been made many times before

[4-6], and Goldberg [4] amusingly cites still more bizarre

examples, such as an electronic component with a failure

rate quoted as 5.93 10-92per hour.

The proponents of structural reliability analysis have a

ready-made response. You are being nave, they say, you

are not meant to treat those probabilities as it they were the

same kind of frequentist probability you are used to. That

is why we call them nominal. They express some kind of

confidence in the safety of the system.

The difficulty with that response is that it conflates two

quite different ideas.

When we attach a number to a failure probability, we are

consciously or semi-consciously exploiting the conventionalunderstanding of what that number signifies, and trying to

give ourselves or someone else confidence from it:

10-6per pipeline per year! That is pretty good, isnt

it, the public can sleep easily. A thousand pipelines

for a thousand years, and only one failure.

In reality, numbers like that neither say what they mean

nor mean what they say. They have no meaning that could

possibly be justified by any calculation or any available data.

If the number is taken seriously, and is not just some kind of

hollow public-relations exercise, the difficulty is that people

will come to take decisions based on the numbers, as the

NASA example demonstrates. In that instance, NASA made

a foolish statement, without any justification. Fortunately,

Ullian was more thoughtful, arrived at a rough-and-ready

but justifiable calculation, and based his decision on it.

Tail sensitivity

Much of the difficulty is that the data that ought to be used

to justify the numbers are not available (and can never be

available). That point too is obvious, and again has been

made many times. It is illustrated schematically in Fig.1 which

plots a probability density distribution for strength againsta strength parameter: it can of course trivially be extended

to multiple loads and multiple components of strength.

If the probability of failure is to be 10-6, the important part of

the probability density distribution is the extreme left-hand

tail. The data such as they are are all near the middle of

the probability density distribution, but the middle of the

distribution is of no importance to the failure probability. There

are no data from the left-hand end, but that is the only part

of the distribution that is important to the failure probability.

Tail sensitivity is the central problem of structural reliability,but theorists devote astonishingly little time to it. Melchers

book [7], for instance, gives to it half a page.


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MAY 1516 2013

MARRIOTT WESTCHASE HOTEL

HOUSTON, TX, USA

Exhibition and sponsorship

options available, visitwww.clarion.org


27/68


THE INCREASING demand for oil and gas, currently estimated at 135 million barrels of oil equivalentper day, keeps pushing the boundaries of offshore engineering into ever-deeper waters. For instance, inthe Gulf of Mexico, exploration and production activities are performed in water depths exceeding 3000

m. Such remote locations and challenging environments call for new procedures and solutions in the design

and installation of offshore pipelines.

In this paper, numerical modelling and analysis of offshore pipelines is reviewed and discussed. Finite-element

techniques to assist in pipeline design are introduced, and applied to pipeline routeing optimization. Special

emphasis is devoted to out-of-straightness and on-bottom stress analysis.

Contact algorithms allowing the simulation of pipelaying on an uneven seabed (using bathymetry) are

reviewed, and recent developments in modelling of pipe-soil interaction are highlighted. The importance

of free-span detection and evaluation is stressed. In addition, it is shown how nite-element analysis can

contribute to the prediction and mitigation of both upheaval and lateral buckling of subsea pipes. At the

end of this paper, pipeline walking on an inclined seabed is simulated, and the importance of seabed friction

on the walking rate is demonstrated.

*Corresponding authors contact details

tel: +32 497 548 916


by Dr Filip Van den Abeele* and Raphael DenisFugro GeoConsulting Belgium, Brussels, Belgium

Numerical modelling and analysis

for offshore pipeline design,

installation, and operation

OIL AND GAS exploration and production is embarkinginto ever greater water depths. Consequently, offshorepipeline engineering is continuously pushing the boundaries,

installing flowlines and export pipelines in water depths

exceeding 3000 m. The availability of high-performance

computing systems and dedicated software tools enable

pipeline engineers to cope with the challenges associated

with design of subsea completions.

In this paper, an overview is presented of numerical

modelling and analysis for offshore pipeline design,installation, and operation. SAGE Profile 3D[1-3] is used

to demonstrate the added value of numerical modelling as

a design aid and decision tool throughout the entire life of

an offshore pipeline, covering:

preliminary pipeline design

route selection and optimization

offshore pipeline installation

free-span assessment

on-bottom stress analysis

SAGE Profile 3Duses a transient dynamic explicit integration

kernel, which enables the efficient simulation of the

pipelaying process and the response of the subsea pipe

when subjected to hydrodynamic loading and operational

conditions (time-dependent pressure and temperature

profiles). In this paper, the numerical algorithms governing

pipeline laydown, pipe-soil interaction, and numerical

integration are briefly covered, and some examples on

free-span evaluation, lateral buckling, upheaval buckling,

and pipeline walking are highlighted to demonstrate the

versatility of finite-element methods as a powerful supporttool in offshore pipeline design.

Pipeline route selection andoptimization

One of the early tasks for the pipeline engineer is to determine

the preliminary route and evaluate the feasibility of the

selected pipeline corridor. An informed route selection

cannot be made without information on the seabed

topography and geotechnical data [4].

Performing an initial desk study before embarking onan extensive (and expensive) marine survey can save a

considerable amount of time and money [5]. In SAGE Profile,


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of seabed elevation versus KP is updated simultaneously,

which allows evaluating the on-bottom roughness of the

selected route already during pre-processing, without anyrequirement for computing power.

At the same time, the allowable bending radii can be quickly

screened. Each pipeline bend radius R should be large

enough to ensure that the bending stresses do not exceed

the allowable stress a:

! > !!!2!

!

(1)

where Eis the Youngs modulus of the pipeline steel and

Do is the outer diameter of the pipe. Moreover, the pipeline

requires sufficient frictional force to resist being dragged

over the seabed by the lay barge. Hence:

! > !!!

!

(2)

with the lateral friction factor,Tthe lay tension, and wsthe

submerged weight per unit length. In addition to bathymetric

considerations, selection of the optimum pipeline route also

depends on a broad spectrum of other factors, including:

politics and regulatory requirements

crossing of existing pipelines or submarine cables iceberg plough marks, pockmarks

areas of very soft or very hard seabed

boulder fields, rock outcrops

risk of anchor damage and trawling gear impact

proximity of other subsea installations

cost-efficiency of installation

environmental and ecological issues

The SAGE Profile pre-processor allows introduction of

different layers of information, by importing additional

information such as admiralty charts, test locations, existing

pipelines, and shipwrecks. In Fig.3, for instance, a proposedpipeline route is shown on a digital-terrain model and, in

addition, an overlay plot is made to display data associated

the seabed topology can easily be created or imported from

survey data, either as:

kilometre point (KP) versus seabed elevation

Easting-Northing-elevation (ENE) coordinates

full 3D digital-terrain model (DTM)

In Fig.1, two corridors imported from survey data are

compared. In the northern corridor, a curved pipeline

route has been drawn, whilst a straight pipeline section is

proposed for the southern part.

The pipeline route can be easily imported, or constructed

through a user-friendly and straightforward graphical

interface. This interface will convert the constructed route

automatically into a proprietary route format, with successive

sections of straight lines and circle arcs. The straight sections

(like the green route in Fig.1) are defined by a start and end

point, whereas the circular arcs (for example, the middle

section of the red route shown in Fig.1) are defined by the

tangent points and the centre of the circle linking these

tangent points.

As demonstrated in Fig.2, the user interface enables an earlyassessment of seabed topography and on-bottom roughness.

Whilst modifying the proposed pipeline route, the graph

Fig.2. Early assessment of seabed topography.

Fig.1. 3D digital-terrain model based on survey data.


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at either side by nodes. The distributed mass of the pipe

is lumped at these nodes. The finite-element kernel uses

an explicit solver, which computes the dynamic motion

of the pipe and is therefore ideally suited to simulate the

pipelaying process.

During this pipeline installation process, new pipe elements

are continuously created and the pipe is laid along the target

path defined on the seabed. The lay tension T, applied at

the barge, is used as an input and the unstressed length

L0of the last element is updated such that the axial force

corresponds to the applied lay tension:

! !!

!!

!" !! = !(3)

with Lthe original element length,

! = !4

!!

! !!

! (4)

the cross-sectional area of a circular pipe with inner diameter

Diand outer diameter Do, and

!! = 1 2! !!!! !!!! (5)

the pressure induced axial force component, accounting

for both the internal pressure pi and the (hydrostatic)

external pressure po. As a result, both empty and water-filled

installation can be simulated. In Equn 3, is the Poissons

coefficient of the pipeline steel, whereAiandAoare the surface

areas of the interior and exterior of the pipe respectively.

When the unstressed element length:

!! =!!!

! + !! + !! (6)

with the pockmarks. This layered presentation of information

offers the pipeline designer an intuitive dashboard with a

wealth of data to select the most appropriate pipeline route.

In addition to overlay plots, contour maps, and slope angles

can easily be visualized, which provides additional input to

assess potential geohazards.

Simulating pipe laydown andinstallation

Offshore pipeline installation is performed from a laybarge,

typically in S-lay configuration. For smaller diameters,

pipeline reeling can be the most cost efficient solution,

whereas J-lay is the only feasible approach in (ultra-) deep

water. Depending on the installation method, the pipeline

is subjected to different load patterns during installation,

including hydrostatic pressure, lay tension, and bending on

the stinger and in the sagbend. A comprehensive overview

on the mechanics of installation design can be found in [6].

The simulation of the pipelaying process is one of the most

challenging tasks once the optimum route has been selected.

Implementing pipeline installation in a general-purpose

finite-element package can be a time-consuming and tediousjob, in particular when importing vast amounts of seabed

data. Most often, advanced scripting techniques are required

to define the seabed profile, select the optimum pipeline

route, and simulate the laydown process. In addition, the

available constitutive models for pipe-soil interaction may

not comply with industry standards.

Finite element tools like SAGE Profilehave been tailored

to assist the pipeline engineer during offshore pipeline

design. Using an explicit integration algorithm, the actual

pipeline-installation process can be approximated. The pipe

is simulated by discretising the entire pipeline into sectionof finite length. These sections are represented by beam

elements with 12 degrees of freedom (DOF), bounded

Fig.3. Digital-terrain model with pockmark indications.


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where is the angle between the pipe and the target path,

and h is the height of the feeding point above the seabed.

Replacing the laybarge with a feeding point close to the

seabed allows for a significant reduction in calculation time,

without losing accuracy. Given the inherent complexity of

pipeline laying, an accurate and robust steering mechanism

of the feeding point is of paramount importance. In SAGE

Profile, this steering mechanism is governed by a proportional-

integrating-differentiating (PID) controller, providing a

smooth movement of the feeding point and ensuring that

the pipeline is installed on the pre-defined target path

(shown in red in Fig.5).

In addition to the concept of a feeding point, an efficient

element-killing procedure has been implemented to

control the computational effort during pipeline laydown.

Indeed, it would be too expensive to simulate the entire

length of the pipe from its starting point up to the feeding

point. In order to reduce the required calculation time,elements that are already lying on the seabed and are no

longer moving will be removed from the simulation. If the

magnitude of the velocity vector for a node is lower than

a predefined threshold, the associated element has little

or no contribution to the simulation results and can be

killed without losing accuracy. In Fig.5, the elements that

have been killed are also shown.

Evaluation of free-spanning pipelines

Accurate prediction of free spans (location, length, and

height) is an important prerequisite in offshore pipeline

design. Indeed, free-span lengths should be maintained

within an allowable range [7], which is determined during

the design phase. Pipelines installed on a very rough

seabed can cause a high number of free spans that can be

difficult to rectify. A judicious assessment of free spans

can dramatically reduce the costs associated with seabed

intervention (trenching, rock dumping, and span supports).

Figure 6 demonstrates that finite-element analyses enable

the simulation of pipeline installation on an uneven seabed,

and allow detection of free spans. The colour code on

Fig.6 reflects the local span height, i.e. the gap between

the pipeline and the seabed. After the pipelay simulationhas been completed, SAGE Profileautomatically detects

the spans over the entire pipeline route, and plots the

span location, length and height in comprehensive and

easy-to-read design charts, as shown in Fig.7.

Once a free span that is longer than the allowable span length

occurs, the span may suffer from vortex-induced vibrations

(VIV) which can induce fatigue damage in the pipe. It was

only recently that the commonly used pipeline design codes

allow free vibrating spans, as long as the structural integrity

of the pipeline system remains assured [8].

Span checks can be performed to assess whether an installed

pipe is compliant with the guidelines recommended in

becomes longer than twice the initial length, the element

is split in two new elements. An additional node is placed

along the last element such that the newly formed element

obtains the original unstressed length. This algorithm

accurately reflects the continuous supply of welded pipe

joints from a moving laybarge. Gravity, applied during the

pipelay simulation, will force the newly created pipe elements

into place; Fig.4 shows the typical catenary shape during

pipeline installation.

For long pipelines and significant water depths, simulating

the entire laydown process (from the barge down to the

seabed) tends to be time-consuming and is computationally

expensive. The sophisticated architecture of the currently

available numerical solvers allows for a significant reduction

in the resources required to simulate pipeline laydown. Bydefault, the laybarge and most of the free-hanging pipe is

replaced by a single feeding point in the water column moving

close to the seabed, as shown in Fig.5. This feeding point

acts as a submarine laybarge, generating new pipe joints

as it moves forward. The lay tension is now applied at the

feeding point, generating a residual on-bottom tension in

the laid pipe section.

Assuming a catenary shape [6], the lay tension at the feeding

point can be expressed in terms of the submerged weight

per unit length ws:

! = !!tan! !

1+ 1+ tan! ! (7)

Fig.4. Pipeline catenary shape during S-lay installation.

Fig.5. Denition of feeding point and target path.


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of the pipe-soil interaction, which is the most important

parameter governing the design. The elastoplasticconstitutive behaviour of the pipeline steel can be described

by the Ramberg-Osgood equation [11-12], connecting pipe

DNV-RP-F105 [9]. For each detected span, SAGE Profile

will calculate the associated reduced velocity:

!! = !! +!!!!!!

(8)

where Ucis the mean current velocity (normal to the pipe),

Uw the significant wave-induced flow velocity, and f1 an

approximation [9] for the lowest natural frequency given by:

!! 1+ !"# !"!

!!!

! 1+ !!

!!"

+ !! !!!

! (9)

with SCF the stiffening effect of the concrete coating, Lethe effective span length [10], methe effective mass, Fethe

effective axial force, the static deflection and C3the endboundary coefficient. The moment of inertia for the hollow

circular pipe is given by

! = !64

!!

! !!

! (10)

and the critical buckling load can be calculated as

!!"

= 1+ !"# !! !!!

!

!"(11)

where C2is an end boundary coefficient as well.

In addition to the reduced velocity (Equn 8), the software

calculates the stability parameter:

!! = 4! !!!!

!!

!!

!

(12)

for each span, where T is the total modal damping ratio,comprising structural damping, hydrodynamic damping and

soil damping. Based on the values of the reduced velocity from

Equn 8 and the corresponding stability parameter from Equn

12, the software will check whether the conditions for the

onset of in-line or cross-flow VIV are met in full compliance

with DNV-RP-F105. This powerful capability provides a quick

and easy tool to evaluate the severity of free spans for a givenpipeline route, and hence can save a tremendous amount of

time and money associated with seabed rectification.

In the next sections, some operational analyses are presented

to evaluate the susceptibility of high-temperature subsea

pipelines for buckling and walking. First, some details and

recent developments on numerical modelling of pipe-soil

interaction are reviewed.

Numerical modelling of pipe-soilinteraction

The key to a successful simulation of offshore pipeline

installation and operation is a profound understanding

Fig.6. Free-spanning pipeline on an uneven seabed.

Fig.7. Overview of span location, height, and length.


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Ramberg-Osgood formulation in SAGE Profile takes into

account the combined effects of plasticity, ovalization [14-

15], axial force, and hydrostatic pressure ([15-16].

The pipe is assumed to be in contact with the seabed when

the difference between the z-coordinate of a pipe node and

the corresponding seabed elevation at this (x,y) location

is less than the external pipe radius Ro. Once contact has

been detected, a soil response will be exerted depending on

the type of seabed soil. The soil response is captured by a

combination of vertical, axial and lateral springs.

The bearing capacity Qu is reflected by the vertical soil

reaction. For sands, DNV-RP-F105 recommends:

!!

!! =

!!!!2

!(!!)+ !!!!!! !(!!) (16)where sis the submerged unit weight,

!! = exp ! tan! tan! !4+

!

2

(17)

with the friction angle, and

!! =3

2 !! 1 tan! (18)

The bearing width Bdepends on the pipe penetration zp,

as is schematically shown in Fig.8, and can be calculated as:

! !! = 2 !! !! !! 0 !! !! 2!!

otherwise (19)For clays, DNV-RP-F105 recommends:

!!

!! = 5.14!! + !!!! ! !! (20)whereCuis the undrained shear strength. Figure 9 compares

the vertical soil-spring reaction forces for a medium-dense

sand (with a friction angle = 33 and a submerged unitweight s= 8.5 kN/m) with the soil reaction of a soft clay(with undrained shear strength Cu= 30 kPa and a submerged

unit weight s= 7.5 kN/m).

In addition to the vertical soil springs recommended by

DNV-RP-F105 [9], other soil models for both cohesive and

cohesionless materials are described in DNV- CN30.4 [17-

18]. For very soft clays (Cu< 20 kPa), a buoyancy formulation

could be used, assuming that the soil behaves like a liquid and

that the soil-induced buoyancy of the pipeline is equal to the

vertical soil reaction:

!!

!! = !!6!(!!) 3!!! + 4!!(!!) !! (21)

curvature with bending moment Mthrough:

!

!!

= !!!

+ ! !!!

!(13)

where the nominal curvature 0and bending moment M0are related by:

!!

!!

= !"(14)

and the parameters andare chosen to fit the moment-curvature relationship obtained by integrating the stresses

across the sectionAfor a given curvature:

! = !!!

! !"

!

(15a)

Hence, Equn 13 is equivalent to the well-known stress/

strain relationship [11, 13]:

! ! = !!+ ! !

!!

!(15b)

with ythe yield stress and {K,n} the parameters describingthe hardening behaviour of the steel grade. The enhanced

Fig.8. Bearing width as a function of pipe penetration.

Fig.9. Vertical soil reaction for sand and clay.


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The combination of a vertical, axial and lateral soil spring

fully defines the pipe-soil interaction. In addition to the

commonly used soil-spring models, presented here, SAGE

Profileoffers dedicated and more enhanced soil models todescribe complex soil behaviour such as berm formation,

buried pipes, and trenching operations [19]. Moreover, an

application programming interface (API) can be used to access

an advanced soil library based on the incremental plasticity

approach described by Zhang [20-21]. In this approach, the

load-displacement relationship for an elastoplastic soil model

is expressed in its incremental form:

!" = ! !" (24)where the vector of incremental loads {dF} is connected to the

resulting displacements {dU} by the compliance matrix [C].

In addition to an extensive library of predefined soil models,

user-defined constitutive laws can be implemented as well to

construct the compliance matrix.

Accurate pipe-soil interaction is a key requirement for the

reliable prediction of the on-bottom behaviour of offshore

pipelines. Significant development efforts are being conducted

to continuously improv

jpe dec 2012 - sample issue

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