introduction to subsea systems networks part4
TRANSCRIPT
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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
o 2000 - Removed to shore
Frigg
o
2003 Removal to shore
Ardmore
o 2005 - Removed to shore
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