Introduction to Subsea Systems Networks Part4

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MSC SUBSEA ENGINEERING INTRODUCTION TO SUBSEA SYSTEMS & NETWORKS

Jee Limited 2008

For distribution under licence by the University of Aberdeen to registered students for the purpose of

educational purposes only. All rights reserved.

No part of this publication may be reproduced or transmitted in any form or by any means whether

electronic, mechanical, photographic or otherwise, or stored in any retrieval system of any nature

without the written permission of the copyright holder. Copyright of this book remains the sole property

of Jee Limited, Hildenbrook House, The Slade, Tonbridge, Kent TN9 1HR

All information contained in this document has been prepared solely to illustrate engineering principles

for educational purposes, and is not suitable for use for engineering purposes. Use for any other

purpose constitutes infringement of copyright and is strictly prohibited. No liability will be accepted for

any loss or damage of whatever nature, for whatever reason, arising from use of this information other

than for education purposes.


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Table of Contents

1.0

OVERVIEW OF FIELD DEVELOPMENT OPTIONS 1

1.1

INTRODUCTION 1

1.1.1

THE HISTORY OF SUBSEA PRODUCTION 1

1.1.2 HISTORICAL OIL PRICES 2

1.1.3

SUMMARY 3

1.2

ONSHORE DEVELOPMENT 4

1.2.1

THE FIRST OIL WELL 4

1.2.2

EARLY OIL DEVELOPMENT 5

1.3 OFFSHORE DEVELOPMENT 5

1.3.1

SUBMERSIBLE 7

1.3.2

JACKUP 8

1.3.3 THE NORTH SEA 8

1.3.4

FIXED PLATFORMS 9

1.3.5

COMPLIANT TOWERS 10

1.3.6

GRAVITY BASE STRUCTURES 10

1.3.7

SPAR 11

1.3.8

TENSION LEG PLATFORM 11

1.3.9

SEMI-SUBMERSIBLE FLOATING PRODUCTION SYSTEM 12

1.3.10

OFFSHORE DRILLING UNITS 13

1.3.11

SUMMARY 14

1.4

SUBSEA DEVELOPMENT 15

1.4.1

INTRODUCTION 15

1.4.2

MONOHULL FPSO 15

1.4.3

SUBSEA FLOWLINES 16

1.4.4 EXPORT OR TRUNKLINES 17

1.4.5

BUNDLES 17

1.4.6

OTHER IN-FIELD LINES AND CABLES 17

1.4.7

RISERS 19

1.4.8

SUBSEA PRODUCTION TREES 20

1.4.9 MANIFOLDS 21

1.4.10

TEMPLATES 21

1.4.11

PLETS AND PLEMS 22

1.4.12

SPMAND SALM 24

1.4.13

CLUSTER AND SATELLITE WELLS 24


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1.4.14

TIE-BACKS 25

1.4.15

DIVERLESS INTERVENTION 25

1.5

FUTURE DEVELOPMENT 26

1.5.1

INCREASED OIL RECOVERY 26

1.5.2

RISERLESS WELL INTERVENTION 26

1.5.3

SUBSEA PROCESSING AND PUMPING 27

1.5.4

ALL-ELECTRIC CONTROL SYSTEM 27

1.5.5 VIDEOS 28

1.5.6

FUTURE DEVELOPMENTSSUMMARY 28

1.6

OVERVIEW OF FIELD DEVELOPMENT OPTIONS -SUMMARY 28

2.0

OWNERSHIP, INTERFACES AND OFFSHORE LEGISLATION 30

2.1

INTRODUCTION 30

2.2

INFRASTRUCTURE OWNERSHIP 30

2.2.1

CASE STUDY SAKHALIN PHASE II 32

2.3

STAKEHOLDER INTERFACES 35

2.3.1

WHAT IS A STAKEHOLDER? 35

2.3.2

WHO ARE TYPICAL STAKEHOLDERS? 35

2.3.3 IMPORTANCE OF STAKEHOLDERS 37

2.4

LEGISLATION 38

2.4.1

UKACTS 38

2.4.2

UKREGULATIONS 39

2.4.3

UKREGULATORYAUTHORITIES 40

2.4.4 US-ACTS 40

2.4.5

CODE OF FEDERAL REGULATIONS (CFR) 41

2.4.6

USREGULATORYAUTHORITIES 41

2.4.7

LAWWEST OFAFRICA (WOA) 42

2.4.8

LAWINTERNATIONAL 43

2.5

MARITIMEAUTHORITIES 44

2.6

LICENCES &LEASES 45

2.6.1

LICENCES -UK 45

2.6.2

LEASES -USA 46

2.7

PERMITS/CONSENTS 46

2.8

SUMMARY 48


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4.0

SYSTEM CONFIGURATIONS 75

4.1 INTRODUCTION 75

4.2

TEMPLATE CONFIGURATION 75

4.3

CLUSTER CONFIGURATION 77

4.3.1

ADVANTAGES 78

4.3.2

DISADVANTAGES 79

4.3.3 SUMMARY 79

4.4

DAISY CHAIN CONFIGURATION 80

4.4.1

ADVANTAGES &DISADVANTAGES 81

4.5

HYBRID CONFIGURATION 82

4.5.1

ADVANTAGES &DISADVANTAGES 83

4.5.2

SUMMARY 83

4.6

SATELLITE CONFIGURATION 84

4.6.1

SATELLITE WELL 84

4.7

SUBSEA SYSTEM CONFIGURATIONSUMMARY 85

5.0

SUBSEA PRODUCTION CONTROL 86

5.1

INTRODUCTION 86

5.1.2

COMPONENTS OF SUBSEA PRODUCTION SYSTEMS 88

5.1.3

SYSTEM INTERFACES 88

5.1.4 SUBSEA PRODUCTION STANDARDS 89

5.1.5

SUMMARY 90

5.2

SUBSEA TREES 91

5.2.1

INTRODUCTION 91

5.2.2

DUAL BORE CONVENTIONAL PRODUCTION TREES 92

5.2.3 HORIZONTAL PRODUCTION TREES 94

5.2.4

THROUGH FLOWLINE TREES 95

5.2.5

GUIDELINE OR GUIDELINELESS 96

5.2.6

GAS LIFT 97

5.2.7

CHEMICAL INJECTION 98

5.2.8 WATER AND GAS INJECTION TREES 99

5.2.9

TREE GATEVALVES 99

5.2.10

VALVEACTUATORS 100

5.2.11

ROVINTERFACES 101

5.2.12

CHOKEVALVES 101


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5.2.13

CHOKE TRIMS 102

5.2.14

TREE FABRICATION 103

5.2.15

TREE FABRICATORS 104

5.2.16

SUMMARY 104

5.3

SUBSEA CONTROL SYSTEMS 105

5.3.1

TYPES OF SUBSEA CONTROL SYSTEMS 105

5.3.2

VALVE POSITION FEEDBACK. 110

5.3.3 TOPSIDE EQUIPMENT 110

5.3.4

SUBSEA CONTROL MODULE 111

5.3.5

SUMMARY 112

5.4 SUBSEA MANIFOLDS 112

5.4.1

SUBSEA MANIFOLD OPTIONS 112

5.4.2

SCHIEHALLION MANIFOLD 113

5.4.3

MANIFOLD FABRICATION 117

5.4.4

LEAKING MANIFOLD 118

5.4.5

SUBSEA MANIFOLDS -SUMMARY 119

5.5

SUBSEA PRODUCTION SYSTEMSSUMMARY 119

6.0

SUBSEA PROCESSING 120

6.1

INTRODUCTION 120

6.2

SUBSEA MULTIPHASE FLOW METERS 120

6.2.1

INTRODUCTION 120

6.2.2

FRAMO MULTIPHASE METER 121

6.2.3 ROXAR MULTIPHASE METER 122

6.2.4

SOLARTRON MULTIPHASE METER 122

6.2.5

SUBSEA MULTIPHASE FLOW METERSSUMMARY 123

6.3

SUBSEA PUMPS AND COMPRESSORS 123

6.3.1

INTRODUCTION 123

6.3.2

HELICO-AXIAL SUBSEA MULTIPHASE PUMPS 124

6.3.3

TWIN SCREW SUBSEA MULTIPHASE PUMPS 126

6.3.4

SUBSEA COMPRESSORS 127

6.3.5

ORMEN LANGE SUBSEA COMPRESSION PILOT 128

6.3.6

SUBSEA PUMPS AND COMPRESSORSSUMMARY 128


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6.4

SUBSEA SEPARATION AND WATER REINJECTION 129

6.4.1

INTRODUCTION 129

6.4.2

TROLL PILOT PROJECT 130

6.4.3

TORDIS PROJECT 131

6.4.4

SUBSEA SEPARATION AND WATER REINJECTIONSUMMARY 133

6.5

SUBSEA HIPPS 134

6.5.1

INTRODUCTION 134

6.5.2 SUBSEA HIPPSCONFIGURATION 135

6.5.3

SUBSEA HIPPSAPPLICATIONS 136

6.5.4

SUBSEA HIPPSSUMMARY 136

6.6 SUBSEA PROCESSINGSUMMARY 137

7.0

STRUCTURAL DESIGN 138

7.1

INTRODUCTION 138

7.2

TEMPLATE DESIGN 138

7.2.1

TEMPLATE REQUIREMENTS 138

7.2.2

EQUIPMENT ON THE TEMPLATE 139

7.2.3

LOADS ON THE TEMPLATE 140

7.2.4 PROTECTION FROM IMPACT 140

7.2.5

ACCESS FOR MAINTENANCE AND RETRIEVAL 142

7.2.6

PREVENTING CORROSION AND CRACKING 143

7.2.7

COST EFFECTIVE INSTALLATION 144

7.2.8

TEMPLATE DESIGNSUMMARY 145

7.3 SEABED INTERFACE 146

7.3.1

SEABED DATA 146

7.3.2

MUD MAT 146

7.3.3

SKIRT 147

7.3.4

CONVENTIONAL PILE 148

7.3.5

SUCTION PILE 149

7.3.6

SEABED INTERFACE -SUMMARY 150

7.4

FABRICATION AND TESTING 151

7.4.1

CONSTRUCTION MATERIALS 151

7.4.2

COMMON LIFTING FEATURES 152

7.4.3

COATINGS AND CP 153

7.4.4

SUBSEA PROTECTIVE STRUCTURES 153

7.4.5

TESTING 154


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7.4.6

FABRICATION AND TESTINGSUMMARY 155

7.5

CASE STUDIES 156

7.5.1

TROIKA TEMPLATE 156

7.5.2

SHELL MENSA 157

7.5.3

VIDEOSUBSEA DEVELOPMENT 159

7.5.4

CASE STUDIESSUMMARY 159

7.6

STRUCTURAL DESIGNSUMMARY 160

8.0

INSTALLATION AND COMMISSIONING 161

8.1

INTRODUCTION 161

8.2

INSTALLATION ISSUES 161

8.2.1

INTRODUCTION 161

8.2.2

VESSEL COSTS AND CAPABILITY 161

8.2.3

INSTALLATIONVESSEL COST 162

8.2.4

SIZE AND WEIGHT OF SUBSEA EQUIPMENT 163

8.2.5

WEIGHT OF WIRE ROPE 164

8.2.6

METOCEAN ISSUES 165

8.2.7

DYNAMICAMPLIFICATION OF A LOAD DURING INSTALLATION 166

8.2.8 VESSEL STABILITY 167

8.2.9

INSTALLATION ISSUESSUMMARY 168

8.3

INSTALLATION METHODS 168

8.3.1

INSTALLATION ON WIRES 168

8.3.2

DRUM WINCHES 170

8.3.3 FOUR-POINT LIFT OF SUBSEA STRUCTURE 170

8.3.4

HEAVE COMPENSATION 173

8.3.5

SHEAVE INSTALLATION METHOD 175

8.3.6

PENCIL-BUOY METHOD 176

8.3.7

INSTALLATION ON A TUBULAR 177

8.3.8

INSTALLATION METHODSSUMMARY 179

8.4

AT THE SEABED 179

8.4.1

SEABED PREPARATION 179

8.4.2

INSTALLING PILES 180

8.4.3

LATCH-LOK-OIL STATES INDUSTRIES 182

8.4.4

HYDRA-LOKPILE SWAGING -OIL STATES INDUSTRIES 183

8.4.5

TEMPLATE LEVELLING ANDATTACHING TO PILES 184

8.4.6

GUIDELINES 185


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8.4.7

HOT INSTALLATION 185

8.4.8

AT THE SEABEDSUMMARY 186

8.5

NEW TECHNOLOGY 187

8.5.1

PENDULAR INSTALLATION 187

8.5.2

DECOUPLEDAIRVEHICLE INSTALLATION TOOL (DAVIT) 188

8.5.3

SYNTHETIC ROPES 189

8.5.4

NEW TECHNOLOGYSUMMARY 190

8.6 INSTALLATION AND COMMISSIONINGSUMMARY 190

9.0

WORKOVER 191

9.1

INTRODUCTION 191

9.2

WHY WORKOVER A WELL? 191

9.3

WORKOVER EQUIPMENT ANDVESSELS 192

9.3.1

WIRELINE 192

9.3.2

RISERLESS WELL INTERVENTION 194

9.3.3

COILED TUBING 195

9.3.4

DRILLSTRING WORKOVER 198

9.3.5

WORKOVER EQUIPMENT ANDVESSELSSUMMARY 199

9.4 MINOR WORKOVER OPERATIONS 200

9.4.1

WHAT IS A MINOR WORKOVER? 200

9.4.2

SAND REMOVAL 200

9.4.3

SAND PACKING 200

9.4.4

DEPOSITION 201

9.4.5 FRACTURING 202

9.4.6

ACID JOB 203

9.4.7

PRODUCTION TUBING REMEDIATION &MAINTENANCE 204

9.4.8

NEW PRODUCTION ZONE 204

9.4.9

RESERVOIR REMEDIATION 205

9.4.10

COILED TUBING DRILLING 206

9.4.11

FISHING 206

9.4.12

JARRING 208

9.4.13

SUMMARY 208

9.5

MAJOR WORKOVER OPERATIONS 209

9.5.1

WHAT IS A MAJOR WORKOVER? 209

9.5.2

CASING FAILURE 209

9.5.3

CASING REPAIR 210


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9.5.4

SIDETRACKING 211

9.5.5

COMPONENT REPLACEMENT 211

9.5.6

SUMMARY 212

9.6

WORKOVER SUMMARY 212

10.0ABANDONMENT OF SUBSEA DEVELOPMENTS 213

10.1

INTRODUCTION 213

10.2 SUBSEAABANDONMENT REGULATIONS 213

10.2.1

INTERNATIONAL REGULATIONS 213

10.2.2

UK/EUABANDONMENT REGULATIONS 214

10.2.3

USABANDONMENT REGULATIONS 215

10.2.4

SUBSEAABANDONMENT REGULATIONSSUMMARY 216

10.3

HISTORY AND FUTURE OF SUBSEAABANDONMENT 216

10.3.1

PLATFORMABANDONMENT IN NORTH SEA 216

10.3.2

HISTORY AND FUTURE OFABANDONMENT IN UK 218

10.3.3

SUMMARY 218

10.4

ABANDONMENT OF SUBSEA WELLS 219

10.4.1

SUMMARY 223

10.5 ABANDONMENT OF SUBSEA DEVELOPMENTSSUMMARY 223


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9.3 Workover Equipment and Vessels

9.3.1 Wireline

The use of wireline tools is the simplest method of well

intervention. The main advantage is that a tool can be lowered

through the production tubing without the need to halt

production.

Wireline tools can be used to clean the inside of the production

tubing, perform work such as acid jobs to stimulate production,

and remove junk and fish from the well bore.

The wire is wound from a drum on the deck of the vessel, so no

drilling derrick is required.

Wirelines are relatively weak, making them suitable for only light-weight well interventions.

Simplest method of intervention

Cable of diameter 2.7 mm to 3.2 mm (0.108in to 0.125in)

Wound from 1 m to 3 m (3 ft to 9 ft) diameter drum No derrick required

Wireline is relatively weak

E-Line is a wireline containing an insulated electrical conduit, which can be used to operate the tool

downhole.

Before a wireline is lowered from the vessel, a package is attached to the top of the subsea Christmas

tree. The package comprises a BOP at the bottom, with a lubricator and then a stuffing box attached to

it. The lubricator contains grease at a higher pressure than the well, and the stuffing box has rubber

seals which surround the wireline, preventing entry of seawater or loss of lubricator fluid.

A heave compensator in the vessel allows the tool to be lowered steadily despite vessel motion due to

swell.

Wireline tools courtesy of Weltec


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Well bores which have been directionally drilled may

require that a tractor tool be attached to the tool at the

end of the wireline. The tractor drives the tool deeper into

the well when gravity alone is not enough to overcome

friction in the tubing.

A wireline must enter the well through a BOP, lubricator

and stuffing box as the well is not killed for the workover.

The for a wireline is shown below, courtesy of

leespecialities.com

The lubricator (right) is shown courtesy of Schlumberger.

The lubricator is for a wireline intervention with a riser,

where the lubricator and stuffing box are located at the

topsides and the BOP is located subsea.

Lubricator and stuffing box

Courtesy of Schlumberger

Reel Drum for a Wireline - courtesy of leespecialities.com


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9.3.2 Riserless Well Intervention

Riserless well intervention does not require a riser to be lowered from a drilling vessel. A much lighter

vessel may be used for the workover saving money.

The Island Frontier is a vessel built by FMC and Aker Kvaerner for riserless light well intervention

(RLWI). It is a monohull dynamically

positioned (DP) vessel with a 70

tonne (77 US ton) handling tower

and a 130 tonne (143 US ton) crane.

It is capable of performing wireless

intervention and workover in water

depths up to 500 m (1640 ft) and

waves up to 5 m (16 ft) in height.

The vessel uses its own ROV to guide a BOP and lubricator

package on guidelines from the handling tower and through the

moonpool to the subsea Christmas tree. It is capable of logging,

perforation and equipment replacement.

The diagram to the left is courtesy of Lewis (Lightweight

Economical Well Intervention Systems)

Island Frontier - Courtesy of FMC, Aker Kvaerner


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9.3.3 Coiled Tubing

Coiled tubing is a metal tube, which can be wound onto a drum, and unwound into the well bore for

performing workover jobs. It has a few advantages over wireline intervention:

Hydraulic fluid may be transmitted through the tubing.

Coiled tubing can push as well as pull objects inside the well.

Coiled tubing is run through a coiled tubing riser (a type of

specialist workover riser) or a standard workover riser.

The continuous tubing is plastically deformed onto a spool

for transportation. On site the coil is unwound and

straightened before being fed into the well. Once installed

fluids can be pumped down the coiled tubing to undertake

a multitude of activities.

The photograph to the right shows Schlumbergers

compact coiled tubing unit which has been specially

designed to minimise the number of lifting operations

required for installation on the vessel. The straightener can be seen at the top of the photograph.

Small diameter, thick wall continuous tubing

o 25 mm to 114 mm (1in to 4in) diameter

o 610 m to 4570 m (2000 ft to 15 000 ft) long

Compressive strength

Fluid pumped through tubing and back to surface through production tubing

Multitude of uses

Log onto the modules WebCT site and watch the video entitled Coiled Tubing Manufactureat > Resources > Video Files > Coiled Tubing Manufacture

This video (courtesy of Precision Tubing) shows how coiled tubing is manufactured from steelstrip using high frequency induction welding and wound onto spools. Once fabricated theinternal diameter of the coiled tubing is validated by blowing a steel gauge ball through thetubing. The tubing is then hydrotested to 90% of specified minimum yield strength andpurged with nitrogen to remove water vapour.


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9.3.3.1 Coiled Tubing Deployment

During deployment the coiled tubing is plastically straightened

through a series of rollers, this plastic straining can lead to

hardening of the steel and loss of ductility, making the tubing

prone to fatigue crack growth. In order to detect these cracks

the coiled tubing is inspected after the straightening process

using an automated ultrasonic system. The tubing is also

inspected after it is plastically deformed back onto the reel.

The steels used for coiled tubing are selected for their ductility

and resistance to strain hardening in order to extend the life of

the tubing.

Tubing plastically deformed from reel to

straight

Strict quality control system after

straighteners to check thickness and

integrity

Careful spooling of tubing back onto reel

and further quality control

9.3.3.2 Running Coiled Tubing

Coiled tubing can be run through a top tension riser, a conductor or a workover riser. The coiled tubing

unit is usually supplied as a compact unit with fully integrated services in order to save time.

Shown in the picture below is a GeoFlo flow remediation tool. It is attached to coiled tubing and

inserted down a flowline to clear blockages. It is very similar to a jet wash tool, which is used with

coiled tubing to clean well bores.


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Need BOP rated to well shut-in pressure (SIP)

CT units provided as compact skid mounted units

In-line sensors provide

o

Temperature

o Pressure

o Loading

Swivels, centralisers and connectors used to run

CT

9.3.3.3 Coiled Tubing Inspection

Coiled tubing is inspected after each deployment as it is spooled back onto the reel. The inspection

techniques need to determine if the coiled tubing has elongated or become distorted during the

unreeling process. It is also important to inspect for gouges and other marks on the coiled tubing

surface where the tubing has been in contact with the production tubing, since these marks can result in

crack growth.

The inspection results are stored on a computer and compared with the previous inspection, based on a

tube position reference, to identify any deterioration in defects.

Tubing is inspected as it is reeled back onto spool

o Ultrasonic array

Wall thickness reduction

Cracking

o Diameter gauges

Ovality

o Visual inspection (camera)

Gouges, flats, scuffs

Inspection data over entire length retained for comparison over life of tubing


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9.3.4 Drillstring Workover

For major jobs such as replacing large pieces of equipment, or the deeper drilling of a well, it may be

necessary to use a drillstring workover. This is because a drillstring can carry more weight, and transmit

more torque to the tool. Using a drillstring is the most expensive form of well workover.

A Mobile Offshore Drilling Unit (MODU) is required for this job, as the production tubing must be

removed, and a riser installed. A MODU has a derrick, which is used to lower the workover riser in

single, double or triple joints, depending upon its height. It is worth noting that a workover riser does

not need to be as wide as a drilling riser, as it is not required to accommodate casing.

If the BOP is installed at the topsides then a high pressure workover riser must be used. The use of a

riser and a drillstring allows full communication with the well, as fluids may be pumped through the

drillstring and the annulus.

An advantage of drillstring workover is that in the case that the intervention fails, the MODU is already

in place to facilitate further drilling.

Major workover

Well must be killed

Mobile offshore drilling unit

Requires BOP and workover riser

o BOP can be topside or subsea

If topside requires high pressure

riser

o Full communication with well

o Option of drilling if intervention fails

Jackup MODU


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9.3.5 Workover Equipment and Vessels Summary

Wireline is lowered from a reel on a small vessel. It may contain an electrical conduit for powering

equipment (E-line). Wireline may be used in riserless well intervention.

Coiled tubing is also deployed from a reel onboard a small vessel. It is more versatile than wireline for

two reasons:

Fluids may be pumped through it.

It has compressive strength.

Drillstring workover requires the use of a drillship, which is expensive. This kind of workover is

necessary for replacing some subsea equipment, and sometimes the deeper drilling of a well.


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9.4 Minor Workover Operations

9.4.1 What is a Minor Workover?

A minor workover is performed using wireline or coiled tubing on a live well. It may be done with or

without a workover riser and can be performed using a light intervention vessel, which saves money.

9.4.2 Sand Removal

In some formations sand enters the production tubing and

impedes the flow of oil. Coiled tubing can inject water at high

pressure, which flushes out the sand from the production tubing.

Coiled tubing

o High pressure water

o Sand transported up production tubing

9.4.3 Sand Packing

A weak formation with crumbling sand may impede production. One solution is to pump resin slurry

(sand and resin) through the coiled tubing. The resin hardens to form a matrix which oil and gas, but

not sand can pass through. Once the resin has hardened it is common to drill a small pilot hole using amud motor on the end of the coiled tubing. This eases the flow of hydrocarbons.

Gravel packing is another, similar method used to achieve this, whereby gravel is used instead of sand.

Watch an animated flash movie of this process on themodules WebCT site. You can find the file at> Resources > Lecture Note Animations > SandRemoval


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Resin slurry

o Sand and resin

Hardens

o Forms porous matrix

Drilled

o Mud motor and CT

9.4.4 Deposition

Paraffins and asphaltenes are long-chain polymers which are known to solidify in the production tubing,

reducing the possible production flowrates.

A paraffin scratcher may be lowered on a wireline to remove the deposits mechanically. This requires

production to be temporarily stopped.

A permanent solution to deposition is a magnetic fluid conditioner (MFC), which can be installed

downhole. High-energy, permanent magnets alter the cloud point, viscosity, pour point, surface tension

and deposition temperatures of the oil, and make paraffin and asphaltene deposition less likely.


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9.4.5 Fracturing

Low permeability in the formation can be

counteracted by fracturing. Originally fracturing was

performed using explosives but it is now more

common to use high pressure liquids, which can be

water, oil or gas-based. It is also possible to use

acids to eat-away and enlarge cracks, thereby

improving production.

The region where the cracks have been enlarged by

water pressure can be seen in the diagram. The high

pressure liquid is pumped down coiled tubing, and

enters the formation through the holes in the casing

walls. A high-permeability formation will allow only

short cracks to form before the fluid is lost, and a

low-permeability formation will allow longer cracks to

form.

A proppant is a solid particle, such as a sand grain or glass bead, which is used to hold the cracks open

after fracturing. The proppant must be hard enough to hold the crack open once the pressure is

reduced, but not so hard that it fractures the formation and allows the crack to close around it.

9.4.5.1 Fracturing Procedures

When performing a fracturing operation the well bore must be first cleaned so that debris does not clog

the fissures which are supposed to be widened. Packers can be inserted above and (if necessary) below

the perforated zone. The pressure is monitored during the process, to determine when the job iscomplete.

Well bore must be clean

Packers inserted (straddle packer)

Pressure monitored


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9.4.6 Acid Job

An acid job is similar to hydrostatic pressure fracturing, in that a fluid is pumped down coiled tubing to

increase the permeability of the formation. Special equipment must be used, such as lined pipes and

processing equipment, as acid corrodes metal.

Hydrochloric acid is cheap and suited to dissolving limestone or dolomite formations. Hydrofluoric acid

is more expensive but necessary to dissolve the quartz that is the main constituent of sandstone. There

are associated health hazards because acid is harmful to the skin and respiratory system.

Surfactants are injected with the acid to stop it forming an emulsion with crude oil, and sequestering

agents prevent precipitation of minerals from the acid solution. As the acid eats away rather than props

open the cracks, proppants are not required.

Limestone or dolomite

o Hydrochloric acid

Sandstone

o Hydrofluoric acid

Health hazard

Additives

o

Corrosion inhibitor

o Surfactants

o Sequestering agents

o Proppants not required


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9.4.7 Production Tubing Remediation & Maintenance

Coiled tubing can be used to clean the production tubing and perforations

using high pressure water or solvents. Coiled tubing can also be used to

insert and remove inflatable packers and in-line valves to isolate sections of

the production tubing.

Tubing washing & cleaning tools

o Wire scratchers

o Jet washers

Setting & adjusting plugs & valves

o Inflatable packers

o

In-line valves

9.4.8 New Production Zone

After a well has become depleted it may be necessary to close off production and begin extracting from

another layer. The new zone may be higher or lower than the original.

Squeeze cementing is used to fill up the

perforations. The cement is pumped from the

surface at high pressure, and it fills up the

perforations in the old production zone. The

zone may be depleted as water has risen to a

higher level, in which case the casing could be

perforated directly above the old production

zone. There may be another production zone

much lower in the formation, in which case the

cement would be drilled through and the bore

extended further downwards.


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Sidetracking involves isolating the old zone, and then drilling off at an angle from the well bore at a

higher point.

If further drilling is necessary then a full workover using drillstring would be necessary. Moving the

production zone upwards can be done using only coiled tubing.

New zone may be deeper or shallower than original

Isolate original zone

o Squeeze cementing

o Sidetracking

Perforate new zone

9.4.9 Reservoir Remediation

Coiled tubing can be used to transport a perforation gun through

the production tubing to fracture a new pay-zone along the well.

Once perforation has occurred a slug of acid is usually required

to dissolve the copper lining around the perforations to allow the

hydrocarbons to enter the tubing. The diagram shows fracturing

and acidising assemblies courtesy of Schlumberger.

Fracturing

o Perforation guns

Explosive shaped-charges

fracture formation and maximise

contact area of well bore with

formation

Acidisingo Slugs of acid transported between plugs

Acid used to dissolve metal

(copper) used in shaped charge

Perforating tool


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9.4.10 Coiled Tubing Drilling

Drilling is carried out by a slimline bit which is driven by the flow of mud through the

coiled tubing or an electric motor, supplied from an internal electrical cable. Material

removed during drilling, fines, is returned to the surface with the drilling mud through the

annulus. The mud is then filtered to remove the fines before being pumped down the

coiled tubing again. The diagram shows a coiled tubing drilling assembly courtesy of

Schlumberger.

Motorhead unit at end of CT

o Driven by flow through CT or electric motor

Drill bit

Slimline bit

o Mud pumped down CT

Mud & fines returned up annulus

Good for drilling deviated wells

Motorhead Unit

9.4.11 Fishing

Junk is classed as small objects which are stuck down a well,

such as a broken part of a drill bit or a hand-tool which has been

dropped from the surface. Fish are large pieces of equipment

which have become stuck such as a length of drill pipe or a

collar. A spear can be lowered using coiled tubing or drillstring,and activated by rotation once inside the fish. During activation

the spear clamps itself inside the fish using slips and the spear

and fish may be lifted out of the hole together. An overshot has

slips on the inside, and is used to clamp around a cylinder with a

small external diameter.


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Coiled tubing can be used to identify and remove objects which

have fallen into the well or have broken off from the

completion. The geometry of a fish can be identified using an

impression block made of a soft metal such as lead, or a fibre-

optic video camera. Once the fish shape has been identified

the correct tool can be used for retrieval. For awkward metallic

components a magnet can be used to retrieve the fish.

The picture directly below shows the top of a fish which was

stuck down a well. The image was taken using a fibre-optic

video camera.

Watch an animated flash movie of this process on the modules WebCT site. You can find the

file at > Resources > Lecture Note Animations > Fishing


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9.4.12 Jarring

When the derrick or surface machinery does not have enough

power to simply lift a fish out of the well bore, then a hammer

blow must be delivered to it. This can be an upward or

downward blow, which is performed by attaching a jarring tool

to the drillstring above the fishing tool. For extra power a

number of drilling collars may be attached as well.

A jar may be attached to the top of an overshot or a spear. It

jars the equipment with a large bang either upwards or

downwards to free the stuck fish.

A rotary jar uses torque applied to the drillstring to provide the

jolt, and a hydraulic system relies upon the release of hydraulic

pressure contained within the jarring tool.

9.4.13 Summary

Wireline may be used to perform the following jobs:

Further fracturing of the formation to stimulate production.

Fishing for downhole junk and fish.

Coiled tubing is required for the following:

Removal of sand from the wellbore, and sand packing to increase production rates.

Removal of scale and deposits from the inside of the production tubing.

Widening of the production fissures using acid to stimulate production.

To drill new production zones.


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9.5 Major Workover Operations

9.5.1 What is a Major Workover?

A major workover requires that production be stopped and the well killed. The well is killed by pumping

heavy mud into the annulus. The production tubing must be lifted. This is more easily done in a

horizontal tree, which can remain in place during removal. If a vertical tree is in place over the well

then the tree must be removed before the production tubing can be lifted. A BOP is installed either

subsea or at the surface. If a surface BOP is used then a high pressure riser is necessary. A subsea

BOP does not require the use of a riser at all.

A drilling rig is necessary for a major workover, which is more costly than a light intervention vessel.

Requires tubing to be pulled

Trees

o Horizontal

Pull the tubing without tree

o Vertical

Pull the tree and tubing

BOP

o

Surface or subsea Drill rig

9.5.2 Casing Failure

A failure in the casing may cause fluids to leak into the formation or vice versa. Casing failures may be

caused by corrosion of the metal by acids, water or carbon dioxide which may all be present in the

drilling mud and the produced oil. Acid jobs may have caused failure of the casing through corrosion.

Abrasion and erosion from produced fluids and gas lift may cause wear and rupture the casing.

A collapse of the formation around the casing may cause it to rupture or buckle inwards.

Corrosion

o H2O, H2S, CO2

Abrasion

Mechanical collapse


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9.5.3 Casing Repair

When the casing has been damaged one of the following courses of action must be taken:

A new liner can be added which further reduces the

diameter of the casing.

A liner patch may be inserted which reduces the casing

diameter over a short distance. The patch is crimped so

that it may be inserted down the pipe. The outer surface

of the patch is coated with epoxy, and then the patch is

expanded mechanically to cover the damaged section.

The entire casing may be removed and replaced.

If the casing has collapsed then a casing roller using

offset rollers may be able to open up the kinked section.

If it is not possible to repair the casing it may be necessary to plug the

well above the damaged section and then sidetrack.

Patch Plan View


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Some wells are fitted with a gas lift, whereby gas is injected via a valve from the annulus to the

production tubing. The gas decreases the density of the well fluids, thus increasing the production

rates. It is necessary to periodically replace the gas-lift valves, and this may be done during a minor

workover, but sometimes requires a major workover.

9.5.6 Summary

Major workover is necessary if the casing becomes damaged, or some components need replacing. It is

often necessary when a wireline or coiled tubing workover has failed to do the job.

Casing failure

Component replacement

9.6 Workover Summary

In this module we first looked at the reasons for performing a workover on a well:

To stimulate production by clearing blockages and widening fissures.

To replace any broken equipment or recover junk and fish.

To drill to another production zone.

We considered minor workover operations using wireline and coiled tubing. Then we looked at

drillstring workover which requires a MODU and that production is stopped.

We went on to learn about the specifics of various workover jobs.

Reasons for workover

o Stimulate production

o Remediation

o Produce from another zone

Workover operations

Wireline, coiled tubing or drillstring workover


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10.0 Abandonment of Subsea Developments

10.1 Introduction

This module explains the requirements for decommissioning and abandoning of subsea systems. It

focuses primarily on the abandonment of wells since this is a complex operation compared to the

retrieval of subsea equipment.

Understand the international and national requirements for decommissioning subsea

structures

Understand how subsea structures are decommissioned

10.2 Subsea Abandonment Regulations

10.2.1 International Regulations

Article 5 of the Geneva convention was the first international regulation for removal of marine

structures. At this time no one has envisaged the requirement for deep sea structures. In 1982 article

5 of the Geneva convention was superceded by the United Nations convention on the law of the seas

(UNCLOS), article 60(3) of which permitted partial removal of offshore structures provided the

International Maritime Organisation (IMO) criteria were met. This convention entered into force in 1994

and was ratified in the UK in 1997.

The IMO published its first guidance on decommissioning in 1989 which required the complete removal

of offshore structures in water less than 75 m (246 ft). This was increased to 100 m (328 ft) in 1998.


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10.2.2 UK/EU Abandonment Regulations

OSPAR is the name given to the Oslo and Paris convention for the protection of the marine environmentof the north-east Atlantic. It comprises the following members:

Belgium

Denmark

Finland

France

Germany

Iceland

Ireland

Netherlands

Norway

Portugal

Spain

Sweden

United Kingdom

European Union

Luxembourg

Switzerland

UK Petroleum Act 1998

o Implementation of European OSPAR decision 98/3

o Reaction to public dissatisfaction over Brent Spar disposal in 1995


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Piper Alpha

o 1988 - Toppled

Brent Spar

o

1995 to 1999 Reused in quay Maureen

o 2001 to 2002 Reused in quay

The first North Sea subsea development to be abandoned was the Crawford field in 1991. This was

removed using a heavy lift vessel and returned to shore to be scrapped. The Blair field is the only field

to date where the subsea equipment has been re-used.

Crawford

o 1991 Removed to shore

Blair

o 1992 - Reused

Staffa

o 1996 - Removed to shore

Durward and Dauntless


Frigg

o

2003 Removal to shore

Ardmore


142 wells abandoned to date

Brent Spar Maureen platform


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10.3.2 History and Future of Abandonment in UK

The graph below is taken from the UK Department of Industry website and represents the historical and

predicted number of abandonments in the North Sea up to 2030. It can be seen that the number of

subsea fields which require abandonment is increasing and will peak at about 2015.

10.3.3 Summary

International and national regulations have been developed for abandoning offshore structures. In the

UK and EU these typically require the removal of all subsea structures, in the USA subsea structures can

be left on the seabed if they are in a water depth of 800 m (2624 ft) or more.


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10.4 Abandonment of Subsea Wells

Abandonment can be carried out by a drilling vessel or

workover vessel. Drilling vessels are expensive, especially if

the field is remote from the drilling vessel base, however, they

can usually abandon a well fairly quickly and most of the

subsea equipment can also be retrieved on the drill string.

Multi-service vessels are much cheaper than drilling vessels

and recent developments now allow complete abandonment to

be undertaken from them using a combination of coiled tubing

and wireline.

The process of abandoning the well is similar for both types of

vessel and is described in the following slides.

Vessels

o Drilling rig

Expensive

Quick

Use drill string to kill and plug wells and to remove subsea equipment

o Multi-service vessel (MSV)

Cheaper

Use coiled tubing to kill well

Use wireline to set plugs

Use A-frame to retrieve subsea equipment

The first stages of abandonment are:

Shut-in production at the tree. The production to the flowline must be isolated before

disconnection.

Depressurise flowline. The flowline is blow down to remove as much of the contents as

possible.

Drilling Rig

Multi-Service Vessel (MSV)


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Disconnect flowline from tree. The flowline is

removed by reversing the installation process.

Connect coiled tubing or drill string to tree. Coiled

tubing or a drill string is attached to the tree,

depending on the vessel used to abandon the well.

Kill well using heavy mud. Heavy mud is pumped

into the well to kill it; the hydrostatic pressure of the

mud is greater than the formation pressure at the

perforations, therefore stopping further production.

The next stages in the abandonment process are:

Pump cement into producing zone. The cement is

pumped down the coiled tubing or drill string and

pushed through the perforations.

Hydrotest plug. Once set the cement plug is

pressure tested to ensure that it has set correctly.

Install wireline lubricator. This is used to run the

perforator guns.


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The next stages in the abandonment process are:

Cut tubing below SCSSV. This is done with a

perforating gun or special cutting tool.

Release tubing hanger and tubing. The

tubing is now retrieved on the wireline to the

vessel.

Perforate upper casings. This ensures that

any annulus pressure is bled.

Set packers and plugs. This seals the

perforations and provides a secondary

barrier.

Remove wireline lubricator. The wireline operations are now complete and the

lubricator can be removed.

The final stages in the abandonment process are:

Unlatch and retrieve tree. The tree is

now removed on a drill string or using

an A-frame on the MSV.

Cut casings 4.5 m (15 ft) below mud

line. This allows the casing to be

retrieved.

Retrieve wellhead and casing stump.

This complies with the regulations to

remove equipment which could interact

with fishing gear.


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Remove manifold and template. These structures are now removed or made fisher

friendly, depending on their size and construction.

Abandonment complete. This process is now complete.

Log onto the modules WebCT site and watch the video entitled Abandonment at> Resources > Video Files > Abandonment

The video is courtesy of Norse Cutting & Abandonment. It shows how subsea casing isretrieved to the surface during the decommissioning of a well.

The casing and concrete is cut using a high-pressure water-cutter run in on coiled tubing(shown below):

The picture below shows the horizontally-mounted drill, which is used to bore through thepipe so that a pin may be inserted. Once the pin is in place, a cutter slices the pipe so thatthe upper section may be lifted out and removed.

Watch an animated flash movie of this process on the modules WebCT site. You can find thefile at > Resources > Lecture Note Animations > Abandonment of Subsea Wells


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The picture below shows lengths of casing that have been removed using this method. Thecement between the conductor and the casing can be seen in the cross section, as well asthe pin which was used to hold the inner tubing in place.

10.4.1 Summary

Abandonment of subsea wells is a complex operation that is traditionally carried out by drilling vessels,

however, new methods have been developed which use multi-service vessels at much cheaper day

rates.

Complex operation

Can be carried out using multi-service vessels

o Wireline and coiled tubing

10.5 Abandonment of Subsea Developments Summary

The abandonment of subsea wells and structures is now a mature technology. Given the extent of

facilities to be abandoned over the coming years a significant effort has been made to reduce the cost

of abandonment and decommissioning operations, with some success.

6 subsea developments with 142 wells abandoned in UK North Sea to date

149 subsea developments left in North Sea

o Large requirement for abandonment over next 20 years

Decommissioning technology mature

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