Bundles and towed installation
Bundles and towed installation
Subsea engineering and training experts
Enhanced reference notes
2
Bundles and towed installation
All information contained in this document has been prepared solely to illustrate engineering principles for a training course, and is not suitable for use for engineering purposes. Use for any purpose other than general engineering design training constitutes infringement of copyright and is strictly forbidden. No liability can be accepted for any loss or damage of whatever nature, for whatever reason, arising from use of this information for purposes other than general engineering design training. 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 England © Jee Limited 2018 Notes created: October 2018
Table of Contents BUNDLES AND TOWED INSTALLATION
5
Expectation
7 8
What are bundles?
Bundle design
11 20 23 28 36 43 48 51
Bundle fabrication Towhead structures Towing methods
Insulation and heating systems
Deep-water bundles
Pros and cons of bundles
Surface tow
ACRONYMS AND ABBREVIATIONS
57 71
ACKNOWLEDGEMENTS AND REFERENCES
Bundles and towed installation
Bundles and towed installation
7
EXPECTATION
EXPECTATION
EXPECTATION
Track record for bundles Bundle design and fabrication techniques Typical towhead structures Towing methods Insulation and heating systems Deep-water bundles Advantages and disadvantages
We address the following: What bundles are and their track record Considerations in bundle design The two alternative procedures for the beach-located fabrication of the bundles The primary methods used to position and install the bundles offshore Insulation and heating systems for bundles and the use of pipe-in-pipe Deep-water bundles Why bundles are selected for some projects The use of flat-pack bundles and other towed installation methods
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Bundles and towed installation
WHAT ARE BUNDLES?
WHATDOWEMEANBYBUNDLESANDTOWEDINSTALLATIONS?
WHAT DO WE MEAN BY BUNDLES AND TOWED INSTALLATIONS? Flowlines and carrier strings welded ashore Held together with bolted-on spacers Filled with pressurised nitrogen Support for carrier against later hydrostatic loading Chains added to balance buoyancy
‘Trimmed’ in sheltered bay nearshore Towed out to field – up to 4 days
DP tugs allow to settle within lay corridor Treated seawater displaces N2 in annulus Provides permanent stability on seabed Towed installation – flowlines with no carrier
Bundles are commonly used in the North Sea and West Africa (Girassol). They have been used elsewhere such as the Gulf of Mexico, Middle East as well as in the Artic and Australia in order to avoid the need for a laybarge. The assembly of flowlines is welded onshore and fitted within a thin-walled carrier pipe. This is not designed to resist the final water pressure at the seabed. Instead, the annulus between the flowlines and the carrier are filled onshore with pressurised nitrogen. The whole assembly including tow and trail heads is launched into an adjacent sheltered shallow bay where the weight of the bundle and chains is ‘trimmed’ or balanced to ensure the correct amount of buoyancy. Some short lengths of chain are attached to the bottom of some of the main chains using wire. Divers can cut through this and release some weight. Large tugs then tow the bundle to the field – usually at mid-depth so that it is beneath the effects of waves and yet not at risk of touching the seabed. This may take up to four days. If the tow stops, the bundle gently sinks to the seabed. At the field, the tugs manoeuvre the bundle within the lay corridor using their thrusters and DP. A survey vessel monitors the position. The annulus (and possibly the flowlines) are flooded with treated seawater, displacing the nitrogen. The additional weight – together with the chains – provides permanent stability to the bundle against environmental forces (current and wave action).
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Where the water depth is too great to safely allow this use of pressurised nitrogen, the flowlines may be towed without the use of a carrier. This is known as a ‘flat-pack bundle’.
TYPICALBUNDLECONTENTS
TYPICAL BUNDLE CONTENTS
Carrier
Thin wall – pressurised Production/ flowline Co-mingled flow Test line/pigging loop Methanol, glycol and chemical injection Water or gas injection Gas lift at well
Hot water – constant or following shutdown Umbilical cables – control lines to valves
The carrier provides the buoyancy for the bundle, but also some protection against dropped objects and trawling activities. Nevertheless, although of a large diameter – around 1 m (40 in), it is a thin wall – typically 10 mm (0.5 in). It requires nitrogen pressure within to prevent collapse when submerged. The other lines which may be carried are listed above. These are kept some 70 mm (3 in) from the wall of the carrier to allow some deformation of its wall (should it suffer impact from dropped objects) and provide space to fit the internal clamps. Production lines or flowlines carry the product (oil/gas mix) as co-mingled flow from a number of wells – typical size 323.8 mm (12¾ in). A smaller diameter test line of around 168.3 mm (6⅝ in) can be used to prove individual down hole conditions. Sometimes the test line is sized to enable round trip pigging. Some flows need injection from the platform to the well in order either to improve flow or prevent corrosion. Some fields require water to be injected to improve the recovery of oil and gas from the reservoir. This is injected through a separate well from the recovery well, driving the product ahead. Similarly, gas may be abstracted and re-injected into an oil reservoir to aid recovery. Or gas may be needed just at the well to aid lift heavy oil up through the production tubing. With sluggish oil flows, we may use the bundle to maintain heat or recover from a shut-down situation. This can be done by including a hot water line perhaps in combination with warm water returning through the annulus of the carrier. Control of the valves and manifolds at the field is usually accomplished with a separate umbilical cable laid adjacent to flowlines. With bundles, these can be incorporated within the carrier. They do not require the heavy armour normally used for cables. The advantage of bundles is that all these can be incorporated within a single flowline corridor.
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Bundles and towed installation
SELECTED PIPELINE BUNDLE PROJECTS
6,520 m (16,626ft) 3,810 m (12,571 ft) 6,800 m (22,440 ft) 7,449 m (24,583 ft) 1,775 m (5,855 ft) 1328 m (4,382 ft) 6,753 m (22,286ft) 7,203 m (23,770 ft) 2,200 m (7,260 ft) 4,100 m (13,532 ft) 3,600 m (11,811 ft) 5,000 m (16,404 ft) 238 m (786 ft)
BP
1995 Cyrus
711 mm (28 in)
116 m (381ft)
Statoil
1997 Åsgard
1080 mm (42.5 in) 320 m (1050 ft)
BHP
2000 Keith
622 mm (24.5 in)
124 m (407 ft)
ExxonMobil
2003 Skene NBR
1156 mm (45.5 in)
118 m (387 ft)
Total
2004 MCP01
1041 mm (41 in)
94 m (308 ft)
BP
2008 Machar
775 mm (30.5 in)
85 m (279 ft)
BP Norge
2009 Valhall
1270 mm (50 in)
85 m (279 ft)
Apache
2011 Bacchus
1028 mm (40.5 in)
105 m (344 ft)
BP
2011 Andrew
881 mm (34.7 in)
116 m (381 ft)
Hess
2012 South Arne
912 mm (35.9 in)
60 m (197 ft)
ConocoPhillips 2012 Jasmine
889 mm (35 in)
82 m (269 ft)
940 mm (37 in)
90 m (295 ft)
Premier
2016 Catcher
813 mm (32 in)
91 m (299 ft)
Repsol Sinopec 2016 Montrose
Summary A summary of bundles installed using the controlled-depth tow (CDT) method: 77 bundles installed in the North Sea upto 2013; Longest to date in a single tow is 7.2 km (4.4 miles); Costs can vary from $6M to $14M per km ($10M to $22M per mile), depending upon the complexity and scope of work to be undertaken. It should be noted that this cost is more than S-lay or J-lay for a single flowline – but the lengths are short, so the mobilisation and demobilisation costs for a laybarge are high also. The towed installation method relies on savings from simultaneous installation of multiple lines combined with the tow/trailhead structures (which double as manifolds or other infrastructure) using relatively lower-cost standard tugs. It should be noted that the table is restricted to the CDT method only. Single on- or near- bottom tows in the Gulf of Mexico have been longer. Flat-pack bundles in Angola have been installed in much deeper water – over 1000 m (3300 ft). The largest diameter carrier pipe to date is 1422 mm (56 in) on the West Franklin bundle that was installed in 2013 Total Jura is worth a special mention due to the largest ever tow head at 500 tonnes.
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BUNDLE DESIGN
CONSIDERATIONS
CONSIDERATIONS
Flowlines diameter and wall material and thickness
CP protection Sizing and spacing of anodes
Carrier wall thickness handling, towout (fatigue), expansion and pullout forces internal N 2 pressure balance Carrier diameter fit all flowline components C of G below C of B
Tow and trailheads fit valves and pipework withstand towout forces stability during tow protection against fishing stability on seabed (piles) connection to bulkhead
Ballast chains centres number of links 30 N/m (2 lbf/ft) net negative buoyancy
Bulkheads forces
Thermal design insulation active heating (continuous and from startup)
Spacers support rollers flowline buckling
thermal leakage
Bundle design considerations are: 1. Flowline pipes are designed as normally : The pipeline bore is sized to suit the throughput.
Material selection is based on product flows throughout the design life. The wall thickness is sized to suit internal pressure requirements. Insulation is provided to suit thermal and hydrostatic pressure. 2. Spacer units : These are designed to suit the specific configuration of flowlines and umbilicals. They need to provide sufficiently frequent support to prevent bending for all the sizes of contents. The spacing may be closer at ends in order to prevent buckling under thermal loading. Some spacers incorporate rollers to enable the flowlines be drawn within the carrier.
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Bundles and towed installation
3. Carrier pipe design : The diameter must comply with the geometric restraints of the flow lines and umbilical components. The wall thickness must suit the internal N 2 pressure onshore (but necessary to provide balanced internal pressure so avoiding hydrostatic collapse during tow-out). The wall thickness must be sufficient to accommodate pull forces during the beach launch and the tow offshore to the site location. It may be thickened slightly towards the ends to withstand the bending stresses resulting from the tow-out. Where the carrier is to be used for heating either by hot water flowing through the annulus or heating pipes, a thermal coating may be needed on the carrier. 4. Ballast chains : The centres and number of links are determined to provide near-neutral buoyancy of the bundle and towhead assemblies in seawater prior to and during the tow (minor adjustments using trimming links are made by divers prior to towing). This iteration can only be undertaken when allowance for all other weights have been made. It may involve changing the carrier diameter. Or some of the flowlines may be towed out flooded. The total submerged weight of all pipes, nitrogen, chains etc is just slightly heavy – with the bottom links of the chains only partly supported on the seabed. 5. Bulkheads : The thickness is sized to suit the forces due to internal N 2 pressure combined with the thermal expansion loads (for a number of case combinations), in order to avoid product leakage. Radiuses are used to reduce any stress concentrations. 6. Tow heads : Separately from the above, the tow and trail heads are designed to be near-neutrally buoyant during tow out. They must be stable during towout and transmit all forces into the bundle. They must accommodate and support all the valve and pipework within the structure. In areas of high fishing effort, they must provide protection against impact. Once in place, they should remain stable against environmental forces. This may require the incorporation into the structure of guides for piles. 7. CP: Both the carrier and towheads require anodes to provide external corrosion protection.
BUNDLECARRIERPIPE
BUNDLE CARRIER PIPE
Sized to contain flowline pipes and umbilicals Allowance for assembly Provides buoyancy C of G below C of B Wall thickness sizing To withstand assembly, tug pullout forces and expansion loads Safely contain internal pressure of N 2 on
Carrier
Roller on main spacer
construction site, suitable for maximum water depth
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13
The carrier must accommodate the flowlines and other items in a suitably spaced arrangement. It must be possible to assemble the different elements so no two flowlines are too close together, preventing the spacers from being fitted. The spacers are bolted on rather than welded (because this would cause stress raisers on the flowline). The carrier should provide around 70 mm (3 in) clearance to allow for trawl gear impact dents without the inside face touching the flowlines. The wall thickness must withstand all the stresses during towing and operation.
BUNDLECARRIERPIPE
BUNDLE CARRIER PIPE
What happens if centre of gravity is not directly below centre of buoyancy? Carrier rotates Towheads rotate Problems with laydown in the field
C of B
C of G
The centroid of the contents must be on the central axis to prevent rotation during tow. Here, we have a heavy-walled pipe shifting the centre of gravity to the right.
BALLASTCHAINS
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Bundles and towed installation
BALLAST CHAINS
Near-neutrally buoyant bundle for towout (flooded annulus once field is reached)
8 m to 10 m (26 ft to 33 ft)
Webbing strap
2 m to 3 m (6 to 10 ft) of 76 mm (3 in) scrap chain
Carrier sits ~5 m (16 ft) above seabed during trim
Fuse wire
Odd N° of links of chain
‘Extra’ links on seabed for stability to bundle during trimming for tow and at field location
Bundle design – ballast chains Centres are determined so as to provide neutral buoyancy of the bundle and pullhead assembly in the sea prior to and during the tow (adjustment made by divers using trimming links) A target net downward force on the sea bed of approximately 30 N/m (2 lb/ft) is achieved Stud link anchor chains are typically strung at between 8 m and 10 m (26 ft to 33 ft) centres. They normally have an odd number of links to help stability during tow. Every fifth or seventh chain has the fuse wire and three additional links.
TOWINGCONFIGURATION
TOWING CONFIGURATION
Controlled-depth tow (CDT) Also known as mid-depth tow Powerful tugs control depth of towheads
Up to 7½ km (5 miles) long
Trail tug
Tow tug
Transponders
Trailhead
Towhead
Ballast chains
Hydrodynamic lift on ballast chains causing bundle to rise in water column.
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CONTROLLED-DEPTH TOW – TOW-OUT FORCES
‘Lift’
Drag
Tow speed
Weight
Weight
At trimming station
During towout
At field
Flooded for stability
All the chain weight acts vertically at the trimming station. As the tow gets underway, the chains adopt an inclined angle as they suffer from drag through the water. The associated hydrodynamic lift on the ballast chains reduces the net weight, causing bundle to rise in the water column. Initially, the front towhead lifts off and gradually, more and more of the bundle ‘unglues’ itself from the seabed. Eventually, the trailhead also lifts off and the whole bundle ‘flies’ through the middle of the water column. It returns to the seabed once towing stops at the field. At this stage, the nitrogen is replaced by flooding with treated seawater.
ORCAFLEXANIMATION CONTROLLEDDEPTHTOW
ORCAFLEX ANIMATION – CONTROLLED DEPTH TOW
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Bundles and towed installation
This Orcaflex animation shows how the bundle ‘flies’ through the water column due to the effect of the drag chains. Orcaflex has a special feature which allows the modelling of bundle chains as attachments to the line (the bundle). In this model, the tow and trail heads are modelled as 3D buoys. The first animation shows the overall arrangement.
ORCAFLEXANIMATION CONTROLLEDDEPTHTOW
ORCAFLEX ANIMATION– CONTROLLED DEPTH TOW
The second animation shows a close up of the bundle as it moves off the seabed, flies along and then lies back on the seabed as the tugs slow down.
BULKHEADS
BULKHEADS
Thickness sized to:
Cold Restrained Hot
Accommodate thermal expansion seen in service
Calculate hydrostatic end forces Need to consider many cases Line 1 hot with lines 2 and 3 cold Startup from cold Ensure freedom from leakage ~300 mm (12 in) thick Usually machined from forged castings
Hot
FE stress in Britannia bulkhead
It is necessary to undertake an FEA of the bulkhead to check for regions of high stress, particularly where the flowlines pierce the plate. Here, the flowline flares out into a radiussed chamfer in the same manner as a flange.
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All possible different cases are analysed, with different combinations of flowlines running hot or remaining at the ambient seabed temperature. The outer carrier is fully constrained by the cold seawater, so the bundle does not expand and contract like a normal pipeline on the seabed near the spoolpiece. That means spools do not have to accommodate a lot of bending moment. Near the tow and trail heads the last 50 m (160 ft) or so of the carrier wall is generally thicker – at around 20 mm (¾ in) – than the rest of the bundle. This is to accommodate the stresses during tow-out and beach handling. The intersection between the wall and the bulkhead may also be radiussed. In order to accommodate the flaring, radii and resulting stresses, it may be necessary to rearrange the spatial arrangement of the flowlines for the end section of the bundle.
SPACERS
SPACERS
Maintain optimum stability configuration Flowline clamped support prevents bending and buckling No welding to lines Rollers attached to assist in bundle assembly
Typical centres 10 m to 15 m (30 ft to 45 ft) Partial intermediates for smaller diameter lines Tray for umbilical tubing
Subsea 7 Leadon field bundle
The flowlines are held in position by spacers. These are bolted on in separate stages during the assembly process, as the flowlines are brought together from their linear alignment (side- by-side) in the welding stations to the relationship they adopt within the bundle. Some of the spacer units have rollers to enable them to slide along the bottom of the carrier pipe, which is assembled separately. Small diameter lines may require support and fixity against buckling at more frequent spacing than shown above. These intermediate spacers may be bolted to a larger pipe. Control tubing elements may need a continuous tray, since the support normally provided by the umbilical armour layer is not present.
BPKINNOULL BUNDLE LAYOUT
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Bundles and towed installation
BP KINNOULL – BUNDLE LAYOUT
Production pipe = 14 in
Gaslift pipe = 6 in Methanol injection pipe = 3 in
Kinnoull drill centre
Intermediate towheads Bundle 4
Carrier pipe = 34.5 in
Insulation
Bundle 3
Andrew platform
Intermediate towheads
Bundle 2 Intermediate towheads
Bundle 1
PLEM towhead
This field layout represents one of the most complicated field developments using multiple bundles to date. It is the longest ever bundle tie back at 27.1 km (16.8 miles) utilising four bundles interconnected to BP’s Andrew Field. The longest individual bundle was 7.2 km (4.4 miles)
Carrier Pipe = 876 mm (34.5 in) Sleeve Pipe = 508 mm (20 in) Production Pipe = 356 mm (14 in) Gas Lift Pipe = 152 mm (6 in) Methanol Injection Pipe = 76 mm (3 in)
ADDITIONALFEATURESNEEDEDFORRE-USABLEBUNDLES
ADDITIONAL FEATURES NEEDED FOR RE-USABLE BUNDLES Marginal, short-term field developments One-off bundles may not be economically viable Requirements Ability to decommission and reposition Options Additional piggable ballast water line ‘Flat-pack’ with piggable core line Economics can be attractive to the operator Slightly more expensive for first installation Reduced or no construction costs for second field Issues of straps and refloating of tow/trailheads Cassandra N ˚ 2
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On marginal, short-term field developments, conventional one-off bundles may not be economically viable. There are two concepts to enable the reuse of bundles. The advantages and disadvantages are discussed: 1. A conventional bundle with a ballast water line could be flooded for long-term stability and emptied during repositioning This has more steel than conventional bundles Pig launchers and receivers for ballast pipe must be built into the towhead 2. A flat-pack bundle (used for deep-water bundles and single lines) with the central core flooded for stability and emptied during repositioning Suitability of the bundle would depend on the particular flowline configuration Future tie-ins to flowlines could be made easier Simple pig launchers and receivers are needed at each end of a central carrier pipe This provides only limited protection to flowlines Re-usable bundles are in the region of 5% to 10% more expensive than conventional bundles, but the cost of repositioning in a similar field is 40% to 50% of the original investment. They can be recovered to an onshore facility for refurbishment and length adjustment, or for decommissioning at the end of their useful life. Major problems are the long-term integrity of the ballast chains and, more particularly, their fabric strap connections, and the re-floating of the towhead structures. Cassandra N˚2 was installed in the North Sea 2007 on a field with a known short life with the intention of future reuse. However, the second use field is yet to be decided. Components of a bundle that require designing Flowline pipes Carrier pipe Ballast chains Bulkheads Spacer units Tow/trail heads Added equipment for reuse Any questions? BUNDLEDESIGNSUMMARY BUNDLE DESIGN – SUMMARY
Bundles are constructed from the components listed above. Each of these components has unique considerations for design. The work is generally undertaken by the installation company.
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Bundles and towed installation
BUNDLE FABRICATION
FABRICATIONPROCEDURE
FABRICATION PROCEDURE
Pre-assemble carrier and pull down site Weld up flowlines and draw into carrier
Carrier
Winch for pulling carrier down site and then drawing flowline bundle into carrier
Spacer units added
Umbilical components
Fabrication building
Pulling head
Conveyors
NDT and joint coating shed
Or pre-assemble flowlines Slip on sections of carrier (field welds) Pipeline welding building
There are two alternatives to the fabrication procedure: Either Fabricate the carrier pipe first Pull the flowlines from the fabrication building into the carrier pipe using a winch Or Fabricate the flowlines first Sleeve the carrier pipe sections over the flowlines and complete the field welds These are followed by: Attachment of the towhead structures at each end of the bundle Hydrotesting and pre-commissioning of the completed bundle onshore, prior to launching
BUNDLEASSEMBLYATTAIN
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21
BUNDLE ASSEMBLY AT TAIN
Bundle being
assembled
Fabrication
NDT
Field joints
Spacer attachment
The above slide depicts a bundle being assembled at Smit Land and Marine facility in Tain and shows: The fabrication building The NDT area The building for the completion of the field joints The building for the attachment of the first part of the spacer units There are currently two permanent bundle construction sites in Scotland, one in West Africa and one in Brazil. There are temporary sites in Australia and the Arctic.
SUBSEA7’SÅSGARDBUNDLERAIL-LAUNCHEDTRAILHEAD
SUBSEA 7’S ÅSGARD BUNDLE RAIL-LAUNCHED TRAIL HEAD
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Bundles and towed installation
Rockwater (now Subsea 7) installed two pipe bundles, 3.8 km (2.4 mile) in length with 1117.6 mm (44 in) carrier pipe, for Statoil’s Åsgard development, during 1997. The preferred launch method at their Wick site is to mount the bundle on rail bogies, which are released by passing into a dip just below water level. Lateral rails are required to support the skids fitted to the wider tow and trail heads.
BUNDLEFABRICATIONSUMMARY
BUNDLE FABRICATION – SUMMARY
Two methods Assemble whole carrier first from shop Install flowlines from welding shop All welding completed in controlled environment Sets of rollers and support plinths Assemble flowlines first
Sleeve on 200 m (650 ft) sections of carrier with field welding Assembly on rail track for ease of launch Needs long site with access to sheltered bay
Assembly shop – industrial unit Pipe storage and pulling winches Any questions?
The two methods used for fabricating a bundle are outlined above. Once fabricated, the towheads are attached and then the bundle is hydrotested, pre-commissioned and launched. We describe these subsequent activities in more detail in the following slides. The field welds are protected from weather by temporary shelters called howdahs.
Bundles and towed installation
23
TOWHEAD STRUCTURES
TOWHEADSTRUCTURES
TOWHEAD STRUCTURES
Britannia bundle towhead Incorporates two subsea isolation valves (SSIVs) ~170 tonnes Temporary buoyancy tanks Overtrawlable?
If bundles can incorporate other subsea structures or equipment, it can make them more cost- effective. The above picture shows the Britannia towhead containing two subsea isolation valves (SSIVs). It is a gravity-based structure weighing 166 tonnes (183 US tons). It is located near the Britannia platform with a separate umbilical to operate the valves. Although protected by a frame with this intention when launched, the structure really cannot be regarded as being overtrawlable.
TOWHEADMANIFOLD
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Bundles and towed installation
TOWHEAD MANIFOLD
The overhead shows a model of a typical integrated towhead manifold that can be towed and installed in the field with the bundles. The pipework and valving for each well within the manifold is a series of repeated modules for water injection or production. The towhead manifold can incorporate pigging loops and electric hydraulic control systems for the manifold valves and associated trees. A typical structure will be 30 m (98 ft) long, 4 m (13 ft), wide and 3 m (10 ft) high with a weight in the region of 100 tonnes (110 US tons). It can be designed either as a gravity-based structure or (as shown above) piled in position. Overhead protection can be provided by hinged grillages or separate protection structures as required. As mentioned previously the towhead on the Jura Project installed in 2007 weighed approximately 500 tonnes.
INTERMEDIATESTRUCTURES
Bundles and towed installation
25
INTERMEDIATE STRUCTURES
Intermediate structures can be incorporated within a bundle for future tie-ins. The above slide shows the intermediate structure in the Britannia bundle during launching with its integrated protection structure in place.
INTEGRATEDDYNAMICRISERS
INTEGRATED DYNAMIC RISERS
FPSO turret
Mid-water arch buoyancy units
Risers
Tether
Elevation – installed risers
Towhead structure
Buoyancy units
45 m (150 ft)
45 m (150 ft)
50 m (165 ft)
Plan – tow-out storage position
By connecting flexible dynamic risers to flowline bundle towhead structures prior to transportation to the field, significant cost savings can be achieved. Riser connections can be made onshore, eliminating the need for subsea activity and DSVs Steep riser configurations can be used if the towhead structure is utilised as a riser/tether gravity base, resulting in reduced lengths of flexible risers
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Bundles and towed installation
Initial studies have demonstrated that this is a viable alternative to conventional riser installation techniques.
BUNDLECONNECTIONATFIELD
BUNDLE CONNECTION AT FIELD
Simple method – normal insulation needs Flexible jumpers or rigid spools
50 m
(150 ft)
Direct connection – high thermal insulation Pre-installed flanged spools – no metrology Docking probes
It is possible to connect two or more bundles in the field, each up to 7.5 km (5 miles) long, enabling longer step-outs to be constructed. The simpler connection method is to lay down the tow and trail heads side-by-side at the ends, so that they can be connected with flexible jumpers or rigid spoolpieces. Sometimes, however, the risk of thermal losses means that only short spools can be used. In this case, the tow and trail heads are docked into each other, already fitted with the short, well-insulated pipe stub ends. Rigid flanged connections are then made up between the flowline ends by divers. There is no need for underwater metrology.
TOWHEADSTRUCTURESSUMMARY
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27
TOWHEAD STRUCTURES – SUMMARY
Contain manifold to tie-in bundle to other pipelines Flowlines and water injection lines Towhead may contain ancillary equipment Well control valves, sub-sea isolation valves (SSIVs), pig launchers, pigging loop, wellhead template Gravity-based structure or piled in position Can pre-install flexible risers
Any questions?
A towhead structure is fitted to each end of the bundle. It contains the tie-in points (such as flanges or clamps) and equipment that enable the lines within the bundle to be connected to other lines when located on the seabed.
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Bundles and towed installation
TOWING METHODS
TOWINGMETHODS
TOWING METHODS
Primary methods of towing single pipelines or bundles to field: Bottom tow Off-bottom or near-bottom tow Controlled-depth tow (CDT) Surface tow Controlled-depth surface tow (CDST)
There are five primary methods of towing single pipelines or bundles to their required location. These are described in the following slides.
BOTTOMTOW
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29
BOTTOM TOW
Bundle pulled along the seabed, usually with some buoyancy tanks attached Used in Gulf of Mexico, where lengths up to 16.1 km (10 miles) long have been installed Seabed must be flat with no obstructions Accurate surveying needed along whole route Any pipeline crossing must be protected
Tandem tugs
Bottom tow: The bundle is pulled along the seabed, usually with some buoyancy tanks attached, by one or more towing vessels This method has been primarily used in Gulf of Mexico There is a need for an accurately-surveyed tow corridor with no obstructions It is suitable for deep-water applications when a minimal bundle weight is required Crossing of third party pipelines can be a major problem Typical size of tugs might have a 250 tonnes to 300 tonnes (275 US tons to 330 US tons) bollard pull, towing in tandem at 8 knots (4.1 m/s). Perhaps two or three tugs may be required to overcome the seabed friction. It is unsafe to cross existing live gas lines using this method. Extensive mattress protection and ramps would be needed to lift the new line over. This approach does not have a good track record. In the Gulf of Mexico, there was a vertical step of rock some 3 m (10 ft) high, which had been missed by the survey team. During the tow-out, the front end of a pipeline was stopped by this feature but the back end continued under its own inertia. The pipeline was a complete write-off.
EXAMPLESOFBOTTOM-TOWFLOWLINEANDBUNDLEPROJECTS
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Bundles and towed installation
EXAMPLES OF BOTTOM-TOW FLOWLINE AND BUNDLE PROJECTS
Flowline and control installed beneath ice Multiple bundled lines Multiple bundled lines
1,000 m (3,180ft)
55 m (381 ft)
Panarctic 1977 Drake 76
730 m (2,400 ft) 470 m (1,550 ft) 840 m (2,750 ft) 1,350 m (4,430 ft)
Placid 1986 Green Canyon 16,100 m (52,820 ft)
9,660 m (31,700 ft) 11,300 m (36,980 ft) 2,900 m (9,510 ft)
Enserch 1992 Mississippi Canyon 441
Twin P-in-P flowlines
BP
1997 Troika
Twin flowlines in insulated carrier
Total
2001 Girassol
This table includes examples of bundles and flowlines installed by the bottom-tow method. This is not a comprehensive list but includes some notable examples including the longest installed length by this method of 16.1 km (10 miles).
OFF-BOTTOMORNEAR-BOTTOMTOW
OFF-BOTTOM OR NEAR-BOTTOM TOW
Suitable when the seabed is relatively even and there are only a few crossings required Reduction in tow forces Accommodates unevenness of seabed
Single tug
Buoyancy with chain ballast
Transponder
Simple towhead structure
Off-bottom tow: The bundle is 2 m to 3 m (6 ft to 10 ft) clear of the seabed with ballast chains maintaining contact with the seabed; It is suitable when the seabed is relatively even and there are only a few crossings required; Towing is by one or two vessels. A trail tug (not shown) is only required for final positioning; It results in a slower installation than controlled-depth tow (CDT) and the bundle is usually designed to be heavier to provide stability;
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Bundles can be positioned in curve by placing concrete blocks. Because the load is lower, a single tug can be used.
CONTROLLED-DEPTHTOW
CONTROLLED-DEPTH TOW
Traditional method used in the North Sea
Powerful lead tug
Trailing tug
Chain ballast
Large towhead
Controlled-depth tow: This is as for the off-bottom tow, but the tension in the bundle is increased by using a tow and trail tug. As the speed of the tow increases, the hydroplane effect of the chains causes the bundle to rise in the water column and ‘fly’; This is the traditional method used in the North Sea; In deeper water, the bundle adopts a snaking shape, reflecting previous lifting and lowering of the towhead as the tug speeds up or slows down. Towheads are typically 20 m to 30 m (60 ft to 100 ft) below the surface and lowest point of the bundle 20 m to 50 m (60 ft to 160 ft) above the seabed; Most third party pipelines operators request 15 to 20 m (50 ft to 60 ft) of vertical clearance when the bundle is being towed over their line; The average speed of the tow is about 4 knots (2 m/s); The bundle starts the tow in off-bottom mode and is returned to this configuration, by reducing speed and tension, for the final positioning in the field.
TOWCONFIGURATIONFORCDT
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Bundles and towed installation
TOW CONFIGURATION FOR CDT
Escort vessel
Command and survey vessel
Tow tug
Trail tug
Transponders
Large trailhead
Large towhead
Ballast chains
A typical configuration during controlled-depth towing (CDT) The vessels involved are: Tow tug – typically, this is an anchor-handling vessel, with a bollard pull of up to 200 tonnes and a towing winch with a load monitoring system; Trail tug – typically this requires a smaller anchor-handling vessel, with a bollard pull of about 60 to 100 tonnes and with a towing winch with load monitoring system that is rigged over the bow of the vessel; Escort vessel (warning vessel) – usually, a fishing boat is employed, acting as a warning vessel during transportation of the bundle to the field; Command vessel – usually, this is a DP vessel equipped with ROV facilities, deck crane, deck space for anchor blocks and flooding spread. Survey equipment will include surface positioning package, acoustic positioning package and telemetry links.
SURFACE TOW
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SURFACE TOW
Not normally used in open sea conditions Dependent upon environmental conditions Fatigue in heavy seas Careful control needed whilst lowering bundle to the seabed at the end of the tow Release of buoys by ROV or divers Safety issue
Tug
Surface tow: The bundle is towed on the surface of the water with or without additional temporary buoyancy tanks; This is not normally used in open sea conditions; It is very dependent upon the environmental conditions; Assessment of fatigue is required in the tail end of the pipeline for long tows through waves; Careful survey/tug control is involved in lowering the bundle to the seabed at the end of the tow. This system was used in West Africa to load out a rigid steel W riser to an SPM.
CONTROLLED-DEPTHSURFACE ORNEAR-SURFACETOW
CONTROLLED-DEPTH SURFACE OR NEAR-SURFACE TOW
Tow direction
Tubular buoyancy attached to lines
DSV tug
Riser top assembly (buoyant unit)
Riser bottom assembly
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Bundles and towed installation
Controlled-depth surface tow can use either fixed buoys or tethered tubular buoys distributed along the length of the bundle as shown above. Early methods made use of fixed buoys, but these accentuate the flexing of the pipeline in the waves. The resulting fatigue can cause a significant loss of strength during the tow-out – generally at the trail end. The newer development, near-surface tow makes use of tubular buoys fixed to lines which trail back at an angle, losing some of their buoyancy. At low speeds, the buoys float at the surface, allowing tow-out to proceed through the peaks and troughs of the waves. The tethers reduce some of the flexing due to wave action and the resulting fatigue found at the trailing end of the bundle on true surface tow. As the tow speeds up, the buoys change their attitude as drag increases. They thus lose some of their buoyancy, allowing the bundle to sink beneath the wave action completely. (This is analogous to the chains losing some of their weight for CDT, allowing the bundle to ‘fly’.) When the speed is reduced, the bundle will float back to the surface again. Tugs need not be as powerful as with CDT. Procedures are required to permanently sink the bundle within the lay corridor in a controlled manner, and to remove the buoys. The method has been used to install the rigid section of hybrid risers. In this instance, the bundle is flooded after the buoyancy is removed, and the tail end was pulled down to the seabed anchorage. At present, the lengths towed by this method are much shorter than other methods. However, the buoys are recovered and reusable – in contrast with the chains with CDT which are left on the seabed.
APACHEBACCHUSBUNDLESVIDEO
APACHE BACCHUS BUNDLES – VIDEO
This video shows the use of a bundle to develop the Apache Energy Bacchus field in the North Sea in 2012. Bacchus is tied back to the Forties field, one of the largest developments in the North Sea. The bundle was fabricated by Subsea 7 at their Wick facility in the north of Scotland. The towhead and trailhead, each weighing over 200 tonnes were fabricated by sub- contractors and after transportation to Wick attached to the ends of the assembled bundle. The 42.5 in carrier pipe for the bundle contains two 6 in production lines and two 4 in heating lines inside a 20 in sleeve pipe.
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The completed bundle system was launched into the North Sea during a suitable weather window and then towed 190 km (120 miles) to its subsea location in a 36 hour period. During the tow, the bundle crossed over existing pipelines in controlled depth tow configuration with the positioning data being transmitted to the towmaster on the control vessel. Once the bundle reached its final location the carrier pipe was flooded to stabilise the bundle on the seabed. The tie in spool pieces were then installed prior to final commissioning of the system
TOWINGMETHODSSUMMARY
TOWING METHODS – SUMMARY
Tow method
Advantages
Disadvantages
■ Minimum bundle weight ■ No additional chains or buoys needed ■ Can be installed in gentle curve ■ No issues with crossings or surveys ■ Bundle towed below wave-affected zone ■ DP tugs can install within curved corridor
■ Accurate seabed surveys required ■ Safety issues at crossings ■ High bollard pull (twin tugs) ■ Accurate seabed surveys ■ Permission and protection for crossings ■ Requires accurate control of tension ■ Close tolerance to ensure buoyant weight ■ Large tugs for depth control ■ Requires sheltered trimming bay adjacent to a long construction site ■ High risk of fatigue at trailing end ■ Requires calm conditions or short tows ■ Removal of any buoys (not Flowlay) ■ Requires separate sinking operation ■ Removal of buoys
Bottom tow
Off- or near- bottom tow
Controlled-depth tow
■ Simplest towing configuration
Surface tow
Controlled-depth surface tow
■ Reduces wave fatigue risk of pure surface tow
Any questions?
The advantages and disadvantages of the five methods of towing bundles are shown in the above slide.
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Bundles and towed installation
INSULATION AND HEATING SYSTEMS
THERMAL INSULATIONLIMITATIONS
THERMAL INSULATION LIMITATIONS
Thermal conductivity K-values – 0.025 W/m/K (0.015 BTU/hr/ft/F) U-values – 1 to 5 W/m²/K (0.2 to 0.9 BTU/hr/ft²/F) Operating temperature Above 100 °C (212 °F) Hydrostatic resistance Deeper than 100 m (330 ft) Crushing and creep during lifetime – use of P-I-P External impact resistance Handling or installation Thickness and density – net buoyancy Costs
Insulation applied to bundles, pipelines and risers has a number of competing limitations. Thermal conductivity can be quoted as a U-value, or, more usefully for a pipeline or riser, as a k-value. The former is based on area so requires a reference diameter for tubulars. This is usually the outside diameter of the steel, but can be the inside diameter or occasionally the outside of the insulation. The more logical k values are linear, defined along the length of the riser or pipeline. The best insulation can often achieve the values shown above. However, the other factors need to be taken into consideration. Lower U-values indicate better insulation. However, lightweight foams have very good thermal properties but are very poor at resisting collapse from the external hydrostatic pressure. They are also poor at resisting high operating temperatures. They require a pipe-in-pipe solution. It is possible to withstand operating temperatures greater than 100 q C (212 q F) but solid PU or PP, denser foam or syntactics may be needed for this. These materials have poorer insulation qualities.
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Similarly, once we get into deeper water than around 100 m (330 ft), we need stronger insulation or need to protect with pipe-in-pipe. Again, syntactic products may be needed to achieve the crushing resistance. All foams will tend to crush and creep with time. Any collapse of foam will degrade the thermal behaviour. It may be necessary to design the thermal behaviour for conditions at the end of life. For drilling risers, or pipelines installed using reel laybarges, the strength of the insulation may be important. Damage can occur either from impact or abrasion at rollers. Any water able to penetrate into the insulation will degrade its thermal properties. Better thermal behaviour may be achieved by applying thicker layers. However, the thickness and density of the insulation may be important when assessing the net buoyancy in seawater. Although not usually a deciding factor, it needs to be allowed for during the design. Risers may need additional buoyancy or weighting to help response in seas. Pipelines may need trenching or burial for stability. It is common for insulation and buoyancy to be manufactured from similar material so the solution can be combined in a single layer. The over-riding limitation is the total cost of procurement and installation. Some syntactic products are extremely costly.
INSULATIONSOLUTIONS
INSULATION SOLUTIONS
Use of multiple layers Inner layer to withstand high temperatures Middle layer provides insulation properties Outer layers for impact and handling Extreme depths Dense syntactic foams Use of pipe-in-pipe – insulation driven Presents significant engineering issues High operating temperatures Installation of heat exchanger to cool product
It is common to manufacture insulation in multiple layers. Each layer is optimised to resist the design condition. At extreme depths, syntactic foams are usually adequate. Normal design caters adequately for collapse resistance under combined external pressure, tension and bending loads. Where severe thermal problems demand a very efficient system, pipe-in-pipe may be required to resist the hydrostatic collapse pressures, protecting an efficient but structurally-weak foam insulant. However, this presents significant engineering issues for risers. It may even be cost-effective to cool product down at the wellhead to limit the line temperature or install subsea cooling loops.
INSULATIONSYSTEMSUSEDFORBUNDLES
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Bundles and towed installation
INSULATION SYSTEMS USED FOR BUNDLES
High temperatures and low OHTCs required U better (less) than 1 W/m²/K (0.176 BTU/ft²/hr/F°) Possible systems are: Low density polyurethane foam Microporous silica Mineral or glass wool Syntactic polyurethane (SPU) Polypropylene (PP) Silica aerogels P-I-P with air gap Avoids half-shells Aerogel (solid smoke) FBE coated carrier pipe There are a number of possible solutions available using bundles to achieve very high insulation values. The following slides give typical properties: k values are based on a typical 219.1 mm (8 in) pipe. Even for a single pipe-in-pipe, the bundle option may be selected. However, if the insulation is not injected into the annulus of the P-I-P, and the outer sleeve can move, it is possible to avoid the use of half-shell welding at each of the 12 m (40 ft) joints. This is the case for all but the injected foamed products listed above. Foam can be applied in preformed cast sections or helically wrapped on. Where: OHTC = overall heat transfer coefficient LDPUF insulated flowline in P-I-P sleeve pipe
COMPARISONOFMATERIALS
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COMPARISON OF MATERIALS
Material
Maximum temper- ature
K-value W/m/K (BTU in/hr/ft/F°) 0.021 to 0.024 (0.147 to 0.168)
Needs P-I-P
Installation method and comments
Low density polyurethane foam (LDPUF)
160 °C (320 °F)
Yes Injected
Compliant
Microporous silica
1000 °C (1832 °F)
0.019 to 0.033 (0. 133 to 0.231)
Yes Requires spacers
Alumina silicate microspheres
Mineral or glass wool (Fiberglass)
400 °C (752 °F)
0.040 (0.280)
Yes High thermal conductivity Must be kept dry Requires spacers
Syntactic polyurethene (SPU) and polypropylene (PP)
150 °C (302 °F)
0.200 to 0.400 (1.400 to 2.800)
No Water resistant
Strong but high cost | £2000/m³ ($100/ft³)
Silica aerogel
500 °C (932 °F)
0.012 (0.084)
Yes Novel at present – LNG and LPG lines Very good insulation
LDPUF is injected into the annulus between the flowline and the steel sleeve pipe and is able to transfer thermal expansion loading from flowline to sleeve pipe (compliant structure). Microporous silica is laid around the flowline in moulded tile or fabric quilt form and encased in steel sleeve pipe (pipe-in-pipe). Concentric spacers are required to accommodate the loose- fitting flowline within a sleeve pipe. To reduce cost, it is used in conjunction with alumina silicate microspheres to give a compliant structure. Mineral or glass wool is applied to flowlines in half shell assemblies and then encased in steel sleeve pipe (pipe-in-pipe). Its thermal conductivity is higher than LDPUF and it must be kept dry in a sleeve pipe with concentric spacers. SPU and PP are water-resistant materials that can resist high compressive loads (induced by the hydrostatic head) with minimal creep. They can be used in deep-water applications without a sleeve pipe but have a high material cost. Aerogels are currently being evaluated for cryogenic LNG and LPG lines. They have extremely good insulation properties (0.012 W/m/K (0.084 BTU in/hr/ft/ q F) and can be applied either as a helically-wound strip (see www.cabot-corp.com) or half-shells (see www.aerogel.com). Aspen aerogel, ‘Spaceloft AR5103’ is extremely hydrophobic, and will not absorb water. Since it is not brittle, water paths will not open up through the material. It maintains its flexibility, even at cryogenic temperatures, and lines do not become rigid.
ACTIVEHEATINGSYSTEMS
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