energies
Review
An Overview of Flow Assurance Heat Management Systems in
Subsea Flowlines
Nsidibe Sunday
, Abdelhakim Settar * , Khaled Chetehouna and Nicolas Gascoin
INSA Centre Val de Loire, Université Orléans, PRISME EA 4229, F-18020 Bourges, France;
nsidibe.sunday@insa-cvl.fr (N.S.); khaled.chetehouna@insa-cvl.fr (K.C.); nicolas.gascoin@insa-cvl.fr (N.G.)
* Correspondence: abdelhakim.settar@insa-cvl.fr
Abstract: The enormous cost of handling the challenges of flow assurance in subsea wells, flowlines,
and risers, especially in deepwater applications, has necessitated a proactive approach to prevent
their risk of occurrence. To ensure that transportation of the hydrocarbon is economical and efficient
from the subsea wellhead to the processing units, a flow assurance heat management system is
relevant in the design and planning of a fluid transport system. Consequently, the advancement
of new technologies to serve the increasing need by exploring the technologically challenging and
hostile subsea fields is of great importance. A comparative study on heat management systems in
flowlines was conducted from the top five publishers (Elsevier, Springer, Taylor & Francis, Wiley,
and Sage) based on the number of publications to determine the level of work done by researchers
in the last decade, the figures from the study showed the need for scientific research in the field
of active heating. Additionally, a review was implemented to ascertain the likely advantages and
drawbacks of each technique, its limitations concerning field applications and then recommend
suitable cost-effective technique(s). The active heating system gives the most cost-effective solution
for subsea deepwater fields.
Keywords: heat management; passive insulation; active heating; flowline; flow assurance; deepwater
Citation: Sunday, N.; Settar, A.;
Chetehouna, K.; Gascoin, N. An
Overview of Flow Assurance Heat
Management Systems in Subsea
Flowlines. Energies 2021, 14, 458.
https://doi.org/10.3390/
en14020458
Received: 30 November 2020
Accepted: 6 January 2021
Published: 16 January 2021
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1. Introduction
Petroleum industry investments are capital intensive and can only be justified by
the optimal exploitation of oil and gas at a minimum cost. The industry is driven by
technological innovations with enormous resources committed to discovering ways of mitigating production problems. Best practices regarding reservoir management, production
surveillance, environmental management, and operations safety are evolving, all driven by
the quest for competitive advantage and profit maximization [1]. However, production
problems present enormous challenges for field operators owing to the prohibitive cost of
mitigation, opportunity loss cost for downtime, and startup problems. Routine wax cutting,
scale treatment, depressurization for hydrate removal, foam, and emulsion treatment are
disincentives to any production system. In subsea operations where process conditions
are unfavorable, flow assurance challenges have become a global issue. There is a need
for a proactive production surveillance system for monitoring and timely detection of
production problems and also for problem characterization [2,3].
Effective production surveillance system requires knowledge of changes in basic
production parameters like temperature and pressure evolution, flow rate, Gas–Oil Ratio
(GOR), fluid characteristics, Basic Sediment and Water (BS & W), sand production, etc.
Among these variables, temperature and pressure evolution conveys a lot of information
on what is happening down-hole or along flowlines. This information if duly analyzed
can provide timely insight on developing oilfield problems for control purposes. However,
accurate prediction of temperature and pressure is difficult owing to the complexity of
multiphase flow typical of oilfields operations.
4.0/).
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Energies 2021, 14, 458
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1.1. Gathering Systems
Gathering systems consist primarily of flowlines, valves, and fittings necessary to
convey well effluent from the wellhead to the separating facilities at the flow station [4,5].
Depending on the distance of distribution, the system may contain one or more lines or a
separate line to each well. Accessory equipment may include gross production metering
devices, corrosion inhibitors, and chemical injectors connected automatically to the valves
and chokes. Free gas may be removed from the gathering manifold or casing head into a
field gas gathering system. The gathering manifold can be more complex if a fluid injection
is done in the field. From the gathering manifold, fluid may be routed to a High-Pressure
(HP) or Low-Pressure (LP) test header to separators or a common well test line. In terms
of design, there is no difference in the calculations of a pipeline or a flowline and the two
terms are synonymous. In the industry, it is commonly understood that a flowline refers
to an inter- or intra-field multiphase line transporting oil, gas, and water coming from
production wellheads to the manifold [6,7]. Flowlines do also carry gas for gas-lift, gas for
disposal, or injection water. The diameters of the flowline vary from 2” up to 12” to 14”,
whereas a pipeline means a pipe connecting the offshore field(s) to onshore facilities for oil
or gas export, and with larger-diameter pipes, carrying single-phase hydrocarbon fluids.
1.2. Flow Assurance
Flow assurance is the capacity to generate multiphase fluids from the reservoirs to
processing plants in a technically and economically viable way throughout the field’s life [8];
the production goal seeks to guarantee the most favorable flow rates at an anticipatable
condition in all production equipment carrying produced fluids beginning at the reservoir–
wellbore to the treatment/refinery facility [9]. In recent years, flow assurance is seen as
the most critical task in the subsea energy production and systems operation [10–12] that
causes financial losses based on production interference and destruction in flowline or
surface facilities due to solid deposits [13,14]. It is employed to the petroleum flow path
during all stages of production, including system selection, surveillance, detailed design,
operation troubleshooting, increased late-life recovery, etc. [15,16]. In subsea systems,
the flowline usually comes before a riser that connects from the sea bottom to the production
topsides [11]. The production riser system becomes longer as the water depth increases
resulting in a higher operating flowline pressure because of the hydrostatic riser’s head [11].
Therefore, it is easier for hydrates to be formed in fluids with the same temperature and a
higher pressure [11,17].
The flow assurance targets or areas of interest in industrial oil production include
wax (paraffinic deposition), asphaltene deposition, hydrate formation, erosion, emulsion,
heat issue, multiphase flow, corrosion, and sand ingress. These occur as a result of temperature and pressure drop [18] and they tend to block/foul off production equipment during
normal hydrocarbon production or transportation. At reservoir conditions, involving high
temperatures (70–150 ◦ C) and high pressures (50–100 MPa), paraffin solubility in oil is
sufficiently high to completely dissolve the wax molecules in the oil, and the crude oil
with low viscosity act as a Newtonian fluid [19,20]. As the crude oil production progresses,
the oil leaving the reservoir is transported through subsea tubing-flowlines having cold
surfaces, heat is swiftly lost to water if the pipe wall is not protected with a good insulation
layer [11]. Additionally, when production fluid temperature inside the flowline gets too
low because of loss of heat, hydrocarbon, and water could form hydrates to plug fluid
flow. In addition, wax begins to precipitate and settles on the flowline wall if the fluid
temperature becomes too low [11,21,22]. In the following, some of the major flow assurance
issues and solutions are explained.
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1.2.1. Waxes
Wax is a higher molecular weight and saturated organic substance [23], which is
moderately soluble in the condensate and black oils liquid phase [24]. Wax mainly consists
of hydrocarbon normally between C18 H38 and C70 H142 [25–27]. When the waxy crude oil
temperature decreases to below its solubility limit in oil, the Wax Appearance Temperature (WAT) or cloud point might be reached [28,29]. Pour point is simply the minimum
temperature that the fluid stops to flow and the crude oil becomes “frozen solid” [17,30],
and the waxy crude oil properties increase with the wax content [31]. Oil in the flowline
cools quiescently during unplanned and planned shutdown operations to the ambient
temperature which is less than the pour point [31].
Based on this, the waxy crude oil undergoes a transformation of phase from a liquid
state exhibiting Newtonian behavior to a gel-like shape showing non-Newtonian flow
behavior [32–35]. The notable wax prevention and remediation techniques are active
heating, passive insulation, wax inhibitors, Pour Point Depressants (PPDs), dispersants,
cold flow, wax eater, choke cooling, and mechanical method.
1.2.2. Hydrates
Hydrates are ice-like crystals that consist of light hydrocarbons (methane, ethane,
propane, carbon dioxide, hydrogen sulfide, nitrogen, etc.) and water [11,36–38]. Hydrate is
formed from hydrocarbon fluids and water that can plug the flowline if untreated. The formation of hydrates is at a relatively low temperature and high pressure and the hydrates’
physical properties are close to those of ice [39–41]. When a plug is created, urgent action
is needed to remedy the situation that may visibly cause production downtime. Therefore,
it is significant to operate and design a subsea flowline system to successfully manage
the risk of hydrate [42–44]. The common solutions are heat management (active heating,
passive insulation), Thermodynamic Hydrates Inhibitor-THI.
(Mono Ethylene Glycol—MEG, Methanol—MeOH), Low Dosage Hydrate InhibitorLDHI (Kinetic Hydrates Inhibitor—KHI, Anti-Agglomerants—AAs), mechanical method,
dead oil circulation, cold flow, and depressurization.
1.2.3. Asphaltene
Asphaltene is a class of compound in crude oil that is soluble in light aromatic but
are insoluble in n-heptane [25,45,46]. Asphaltene contains the majority of the inorganic
component of hydrocarbon fluid [25,26], which includes nitrogen and sulfur, and metals
such as vanadium and nickel [25,26]. All hydrocarbon crude contains a certain level of
asphaltene and it only becomes an issue during production when they are not stable [25,26].
The stability of Asphaltene is dependent on the ratio of asphaltene to stabilizing factors like
resins and aromatics in the crude oil [25]. Pressure has the biggest impact on asphaltene
stability [25] and Asphaltene Onset Pressure (AOP) is the pressure at which asphaltene
begins to settle out in the flowline [46]. Asphaltene can be unsettled by the introduction of
some kind of acid or completion fluids and by elevated temperature observed during the
refining activities of crude oil [25,26].
In summary, the glaring asphaltene issues mostly happen downstream because of high
heat or blending [25,26]; their solutions are chemical injection and mechanical methods.
1.2.4. Scales
Scales are solid mineral deposit normally form from produced saltwater. The inorganic
scale is a stick, hard mineral precipitate from a brine solution [23]. Scales are also crystals
deposited from the available concentration of brine in the reservoir due to pressure, pH,
and temperature variation in the fluid production system [23]. There exists a large variety
of these solids, which may form anywhere from the reservoir to the production system [23].
Methods for scales prevention are chemical inhibition squeeze treatment and Downhole
Chemical Injection (DHCI); other methods of injection is by Nanoparticles [47,48].
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Figure 1 summarizes the flow assurance’s solution flowchart for wax, hydrates, and asphaltene.
Figure 1. Flow assurance solutions for subsea flowlines.
1.3. Heat Management Systems
Heat management systems is a practice of maintaining the fluid temperature inside
the flowline well above the Hydrates Formation Temperature and WAT [49,50]. The heat
management system is regarded as very valuable in flow assurance solutions due to its
cost-effectiveness [50]. As the oil and gas fields progress into deep water [51], there is
an increasing demand for a heat management system to stop the formation of hydrates
and wax in the subsea systems [51]. Therefore, a good heat management plan is selected
based on the needed cooldown time, water depth, U-value, and temperature range [51].
The heat management systems consist of active heating and passive insulation. The active
heating system uses external heat sources like hot water, direct electrical, and electrically
heat-traced flowline to warm the produced fluid [49] while the passive insulation system
utilizes material like Mineral wool and Polyurethane with a low thermal conductivity to
lower heat loss to the environment [49]. A heat management system flowchart is seen in
Figure 2 below.
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Figure 2. The heat management system in the subsea flowlines.
In the last ten years, the design and operation of flowline heat management systems
are increasing in preventing impediments caused by hydrates and wax deposition [49].
The flow efficiency in flowline insulation is affected by the distinctive issue of heat transfer [52]. Consequently, the hydraulic behavior of the subsea flowline fluid controls the
thermal or heat performance of the production system [51,53]. Conversely, it indirectly
influences the hydraulic design through the effect of temperature on the properties of the
fluid such as viscosity, GOR, and density [51,53,54]. One of the most significant aspects
of flowline design is in the prediction of temperature profile along the flowline during
the thermal design process [51,55,56]. This is because temperature details are needed for
flowline analyses including corrosion protection, lateral or upheaval buckling, expansion
analysis, wax deposition, and hydrate prediction analysis [51,57–61]. Often, the solids
deposit (wax, asphaltene, hydrate, and scales) determines the conditions of thermal and
hydraulic designs [62–64]. To preserve a minimal fluid temperature above the wax and
hydrates depositional temperatures in the flowline, layers of insulation are included in the
flowline [11,51,53]. The thermal design comprises of both transient state and steady-state
transfer analyses [51]. Therefore, the temperature profile in the subsea flowline system
must be higher than the conditions for HFT and WAT [51]. To confirm the effectiveness of
the active heating device and insulation coating that suits all operational scenarios, it is
very important to study both transient state and steady-state analyses [51,53,65].
1.4. Methodology
In light of the above, the objective is to conduct a detailed study to discover the level
of work done by researchers globally on heat management systems in flowlines from the
last decade and to also review the heat management techniques in the subsea flowline.
Reviewing the heat management technique is to compare their advantages and drawbacks,
identifies their limitations to field applications, i.e., water depth, and then recommends
suitable cost-effective techniques.
The study on heat management systems was done to evaluate and validate the popularity of the subject by researchers, data were collected in the last ten years (2010–2020)
from the databases of the top five publishers (Elsevier, Springer, Taylor & Francis, Wiley,
Sage) of the largest toll-access publishers [59,66] using two keywords (Heat AND Flowline,
Thermal AND Flowline). The top five publishers in the largest toll-access publishers were
assessed based on their number of publications. After data collection using keywords,
screening, and elimination of duplicates were done to determine the relevant articles, and
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subsequently, one hundred and twenty-five (125) relevant articles were finally screened,
selected, and classified into passive insulation and active heating as seen in the workflow
of Figure 3a. A comparison of the heat management systems from Figure 3b below shows
that approximately 42% (52 articles) of the total work done by researchers was in active
heating while 88% (110 articles) in passive insulation. This number is low for active heating
when compared with passive insulation. Thus, the need for more technical efforts and
scientific research in the area of active heating as the quest for hydrocarbon move into
deeper water.
Figure 3. (a) Workflow for screened papers. (b) Number of publications for heat management in
flowline from the top five publishers.
2. Passive Insulation
The new boundaries for subsea petroleum exploitation are the ultra-deepwater (about
3000 m) [53]. This demands pipes in the end (around 25 years) that can withstand the
hostile environmental and mechanical request [53]. One important ingredient for passive
insulation is to evade the hydrates and wax deposition in flowlines [53]. Hydrocarbon
fluid flow preservation and the capacity to start over the production system during subsea
deepwater oil and gas production is a major concern [53,67]. Heat management of the
hydrocarbon effluent has been observed in the past 5–10 years as the most accustomed tool
for curbing obstruction in the flowlines [53].
Passive insulation in the operation and design of subsea flowlines is very important because of the unfavorable low temperature and high-pressure conditions; such an
environment creates stringent needs for optimal insulation [68–71]. The passive insulation system of heat management is flowline insulation with materials of low thermal
conductivity like polypropylene, polyethylene, polyurethane, extruded polystyrene, fiberglass, mineral wool, rubber, glass-reinforced plastic, Vacuum Insulation Panels (VIPs),
and aerogel [50,72–74], to control and reduce the loss of heat from the crude oil to the
surrounding [50,75,76]. Some insulation types are dry and wet insulations, flowline burial,
Pipe-In-Pipe (PIP), and others [50,77,78] (Figure 4a). With separated flowline operating
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conditions, aside from installation approaches and cost, the option of insulation materials
is reliant on the material properties like density, mechanical strength, thermal conductivity, water absorption, flexibility, aging, and corrosion resistance [65,79–82]. The usual
thermal insulation is mostly manufactured in few format types like composite syntactic,
brown foam, and syntactic [81,83–86].
Figure 4. (a) Passive insulation system in the subsea flowline. (b) Number of publications for passive insulation from the
top five publishers.
Results from the study on passive insulation systems for the past ten years compared
the different insulation systems as seen in Figure 4b. It showed 58 % (66 articles) effort
for wet and dry insulation, 19% (20 articles) for flowline burial, and 23% (24 articles)
for others that included pipe in pipe, bundle, vacuum systems, phase-change materials,
and multilayered insulation.
2.1. Wet Insulation
The insulation of subsea flowline with wet insulating material does not require external protection to stop the entering of water or water entrance is very small and so cannot
affect the properties of insulation [82]. Wet insulation employs materials such as polypropylene, syntactic polyurethane, polyurethane, multilayered, and syntactic polypropylene
with low heat transfer coefficients of about 2 W/m2 K [82,87]. Deepwater insulation
mostly uses polypropylene and polyurethane materials [82]. Additionally, plastic or glass
matrix enhances the insulation properties of syntactic materials for appreciable depth
capacities [88,89]. Finally, it is appropriate for coating insulation material with additional
materials [9,82,90,91]. Table 1 lists the properties of wet insulations.
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Table 1. Summary of wet insulation properties.
Insulation Material
U-Value W/m2 K
Density (kg/ m3 )
Polyurethane (PU)—Solid.
0.20
1153.33
Polypropylene (PP)—Solid
0.22
897.03
2743.20
143.30
Syntactic PU
0.12–0.15
608.70–848.98
91.40–2743.20
55–115.60
Syntactic PP
0.13–0.22
592.68–800.92
Water Depth (m)
Service Temperature (◦ C)
Citations
115.60
115.60
Syntactic Phenolic
0.08
496.57
Syntactic Epoxy
0.10–0.14
592.68–720.83
1828.80–2743.20
71.11–100
Polypropylene (PP)—Solid
0.22
900
3000
145
Polypropylene (SPP)—syntactic PPF (e.g., carizite)
0.13–0.22
600–800
-
115
Polypropylene
(RPPF)—ReinforcedFoam combination
0.16–0.18
600–800
600–3000
115–140
Polyurethane (PU)—Solid
0.19–0.20
1150
[82]
200
115
◦C
Polyurethane (SPU)—Syntactic (plastic beads)
0.12–0.15
750–780
100 at 115
300 at 90 ◦ C
70–115
Polyurethane (GSPU)-Glass Syntactic
0.12–0.17
610–830
2000–3000
55–90
Polyurethane(RPUF)—Reinforced
Foamcombination
0.08
448
-
-
Phenolic Syntactic
0.08
500
-
200
Epoxy Syntactic
0.10–0.14
590–720
2000–3000
75–100
Epoxy Syntactic with
Mini Spheres
0.12
540
-
75
[51,92]
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2.2. Dry Insulation
Dry insulation materials are polyurethane foam and mineral with better heat transfer
coefficients of around 1 W/ m2 K are widely deployed for onshore and shallow water
applications [50,87,93]. With dry insulation, the loss of heat to the environment is low and
the hydrocarbon temperature is adequately maintained to reduce depositional issues [49].
Nevertheless, water ingress worsens the dry insulation performance [49] or degrades the
insulation properties, and hence the need for a PIP system (Figure 5).
Figure 5. Schematic view of a dry insulation system.
2.3. Flowline Burial
Flowline burial is simply a heat insulation system where the flowline is buried or
having rocks, grits, or seabed material placed over them below the mudline [50,51]. Flowlines are usually buried in subsea deepwater fields for on-bottom stability, thermal insulation, and also safeguarding from trawling and dropped objects [49]. In flow assurance
heat management systems, flowline burial is considered one of the best cost-effective heat
management techniques for onshore, shallow and deep water [50,51,87]. Flowline burial is
particularly selected for a project and is dependent on soil properties, geological hazards,
Pressure Volume and Temperature (PVT) characteristics, and basic structures [49,94,95].
Even if subsea soil provides good insulation, the porous materials like grits, seabed materials, and rocks only give little or no insulation due to water ingress into the spaces thereby
allowing heat transfer by convection to the surroundings [51]. The subsea flowline burial
is either fully or partially buried [51,96]. Comparing both methods, a partially buried
flowline offers less insulation effect to the fully buried flowline [51,97,98]. Additionally,
the partially buried flowline (trenched flowline) encounters lower heat loss than the bared
flowline but greater than the buried flowline [51,99,100].
Flowline burial has the edge of utilizing the soil good heat capacity that acts as both
heat storage and sinks [50]. Therefore, a buried insulated flowline has a greater heat capacity than a PIP system with extended cooldown time [49] while the installation cost of a
buried insulated flowline is roughly 35–50% higher than that of a PIP technique [49,101,102].
In the course of the shutdown time before the advent of hydrates nucleation or gelling
of wax, a buried flowline has four times higher retention capacity for heat than a PIP
system [50,103,104]. This can be an important reduction in the overall project for long
flowline systems [49]. Some other merits of flowlines burial system include more option on
installation vessels and contractors, more options on the vendor for the fabrication of flowline, when compared to PIP system gives a slower cooldown time during shutdown and a
likelihood of single flowline repair [49], and also provide a reduced time and enhanced
schedule for first oil [49,105,106]. Oh et al. [107] carried out laboratory experiments to
ratify some numerical and analytical models [50,108–111]. Figure 6 shows the comparison
between burial depth and U-value for both bared and a coated polypropylene foam flowline [51]. It is also important that challenges like upheaval buckling and seafloor buckling
are examined during the design of a good scouring depth for flowline burial [51,112,113].
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2.4. Others
The other insulation systems being deployed for insulation or currently undergoing
development are:
Figure 6. Compares flowline U-values for coated two-inch polypropylene foam against burial
depth [114].
2.4.1. PIP Insulation
PIP insulation is a single-insulated inner flowline centrally located inside an outer
protective pipe [49], between the protective outer pipe and inner pipe, there is an annular
space for insulation to prevent heat losses from the flowing fluid [115]. Different insulation
materials like aerogel, mineral wool, vacuum or inert gas, and Polyurethane Foam (PUF),
etc. occupy the annulus connecting the pipes [82], as seen in Figure 7. The PIP insulation is
an upgrade of the dry insulation to stop the ingress of water into the insulation layer for
superior performance [49,50,116,117].
The PIP systems are well-known solutions to flow assurance issues [49]; for this reason, several works are described on the different outlook of bundled flowlines and PIP
systems [49,118]. The need for thermal insulation and mechanical load is important as the
depth of water increases [49]. Consequently, the demand for thicker insulating material
to ensure the integrity and durability of the flowline due to the hostile environmental
conditions [49]. PIP insulation system is a technique of achieving a U-value of 1 W/m2 K
or less [51,72] and hence partial vacuum is added to decrease the heat transfer coefficients
to around 0.5 W/m2 K [50,87,119], producing an effective system for fluid flow management [50,120]. Flowlines in PIP configurations were essentially constructed in the Gulf of
Mexico to obtain high heat insulation for flow assurance purposes [72]. Having the internal
fluid warm helps to reduce paraffin deposition and stop the formation of a hydrate plug,
which can compel the production flow. Table 2 gives the materials’ insulation for the PIP
system, its U-values, insulation thickness, and properties [72].
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Figure 7. Pipe-in-pipe (PIP)-insulated systems [121].
2.4.2. Bundle Insulation
Bundles are mainly applied for combined flowlines installation inside a carrier pipe
or outer jacket [51]. The bundle systems are akin to the PIP systems other than that bundles contain several pipes [85,122–124]. Figure 8 shows the flowline bundle configuration.
Bundle systems give appealing solutions to a broad scope of the issues of flow assurance
by presenting a cost-effective technique and also the likelihood of circulating a medium
for heating [51]. The additional benefits are as follows: it allows for multiple flowlines
installation into a single pipe design [51]; the outer pipe is utilized to withstand subsea hydrostatic pressure in deepwater [51]; finally, bundles permit the introduction of monitoring
devices and heated pipes [72]. The bundles’ drawbacks are its length limit (shallow water
installations) and the demand for a comfortable launch and fabrication location [72].
Figure 8. Flowline bundle configuration (adapted from Bai et al. [72]).
The first known installation of the PIP technique was by the Indonesian Pertamina
offshore in 1973 [72]. Until 2000, 103 miles of PIPs and bundles were built in the North Sea,
which is about 1% of the total flowline population; The Gulf of Mexico has so far utilized
64 miles of bundles and PIPs [72,125]. Nearly 50 flowline bundles are installed in the North
Sea by a Controlled Depth Tow Method (CDTM) [72], and the earliest installation was at
Murchison field in 1980 [72].
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Table 2. Thermal Properties of Pipe-In-Pipe Insulation Materials.
Insulation
Material
Density
(Kg/m3 )
Thermal Conductivity
(W/m K)
U-Value
(W/m2 K)
Annulus Gap
(if any)
Maximum
Temperature (◦ C)
Comments
Mineral Wool
140
0.037
1.60
Clearance
700
Rockwood or Glava, always
combine with reflective Mylar film.
Aluminum Silicate
Microsphere
390–420
0.1
3.90
None
1000+
Commonly known as fly ash,
introduce to charge the annulus.
Thermal Cement
900–1200
0.26
-
None
200
Presently been studied by a Joint
Industry Project, to have collapse
resistance with a decrease in pipe
wall thickness.
LD PU Foam
60
0.030
0.76
None
147
Prefabricated as a double-jointed or
single system for the replacement
project of Erskine.
HS PU foam
150
0.04
1.20
None
147
Microporous Silica
Blanket
200–400
0.02
0.40
Clearance
900
Cotton blanket, glass,
titanium fibers and calcium-based
powder.
Closed-Cell
Polyurethane Foam
(CCPUF)
48.06–96.11
-
-
-
-
-
Open-Cell
Polyurethane Foam
(OCPUF)
32.03–64.07
-
-
-
-
-
Polyisocyanurate
Foam (PIF)
28.83–32.03
-
1.08
-
−182.78–148.89
-
Extruded
Polystyrene
96.11
-
1.47
-
−182.78–73.88
-
Fiberglass
56.06–88.10
-
1.36
-
−17.78–454.44
-
Mineral Wool
139.36
-
1.42
-
700
-
Citations
[51,72,92]
[82]
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2.4.3. Vacuum Systems
Vacuum Insulated Tubing (VIT) was initiated in the search to enhance better heat
retention [50]. The VIT or Vacuum Insulated Panels systems are upgrades of the PIP
methods [50], having a slim space kept at a vacuum state to decrease the transfer of
heat from the hydrocarbon to the surroundings [50,126]. The VIT is identified with a
low U-value of 0.008 to 0.36 W/m2 K [50,126,127]. VITs were successfully used in the
Gulf of Mexico deepwater fields to prevent hydrate plugging and wax deposition and
to manage annular pressure buildup during the crude oil transport process [127,128].
The Norman and Alaska production fields deployed the VIT techniques to control the
issues of flow assurance [50,126,129–131]. They are seen as expensive when compared to
other passive insulation techniques [50,126,131–133] but might be favorable in areas like
the Gulf of Mexico with a high cloud-point temperature [50,127]. Table 3 provides the
vacuum insulation properties.
Table 3. Thermal properties of vacuum systems.
Insulation
Material
Density
(Kg/m3 )
Thermal
Conductivity
(W/m K)
U-Value
(W/m2 K)
Annulus
Gap (if any)
Maximum
Temperature
(◦ C)
Comments
Citations
[51,72,92]
[82]
Vacuum
Insulation
Panels (VIPs)
60–15
0.016–0.008
0.26
Clearance
160
The foam of the shells
is made with
aluminum foil under
the condition of
vacuum and utilizes
gas absorbing “getter”
pills for free gas.
Vacuum
Insulation
Panels (VIPs)
59.27–
144.17
-
0.20–0.31
-
160
-
2.4.4. Insulation Modules
Insulation modules are made up of manufactured segments of insulations that are
connected to the insulation structure without being joined to it directly [85]. The composite
syntactic and syntactic foams are the most commonly applied materials for the insulation
technique [85,134,135]. One advantage of this technique for riser application is geometry
modifications to suit both riser and additional lines [85]. Insulation is commonly fastened
by straps or bolts and can be disconnected during the life span [85]. Insulation is normally
joined by constructing various molded segments of materials all over the structures for
most riser tower subsea applications [85,136,137].
2.4.5. Multilayer Insulation
The multilayer insulation system is a sub-units of the integral insulation technique [85]
and comprises various insulation layers [85,138,139]. The usual designs have syntactic
foam layer, blown foam layer, and solid layer [76], and individual layer conduct a particular
role [85,140].
The layer straight above the corrosion protective layer is comprised of the solid layers
that resist temperature [85], syntactic layers supply fine insulation properties and hydrostatic support [85,141,142], and the blown foam layers are the outside layers with the
finest properties of insulation [85]. The blown foam is usually exposed to a pronounced
lesser temperature than the internal layers due to thermal gradient across the pipes [85].
The other types of layers frequently added in the multilayer insulation system is the
adhesive layer that assists in joining and strengthening layers which preserve the outside layer from destruction [85,143–145]. The advantage of multilayer insulation is that
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the density and thickness of the individual layer may be adjusted to match the specific
application [85,146–148].
2.4.6. Phase Change Material Systems
Phase Change Material (PCM) is a material that delivers latent heat during a phase
change transformation [149]. The PCM is seen between the insulation material and flowline,
to ensure it is above the temperature of phase change during system operation [149].
Material external insulation thickness gives the system’s Overall Heat Transfer Coefficient
(OHTC) [149,150], and for this reason, a high insulation material performance must be
selected to restrict the thickness of the layers to reduce the increase of the outer pipe
diameter [149,150]. During a shutdown operation, the latent heat release will increase the
normal cooldown period from 3–5 days [149]. Nonetheless, where there is a very long
period of shutdown above the cooldown time performance, a flowline restart may result in
a big problem [149]. Since the system is capable of storing and releasing a large quantity
of heat, the system would as well demand to recover the same energy quantity to reach
steady-state conditions [149,151,152].
To improve PCM systems, PCM multilayer insulation was used and it is the addition
of PCM– or PCM–matrix composite material in multilayered flowline [8,81]. PCM can keep
a large quantity of latent heat at a liquid state and liberate it during a phase change process [81]. By this special quality, it has captivated further concerns in the evolution of novel
insulation solution [81]. Alawadhi [153,154] discovered that the heat loss in the flowline can
be notably decreased for an insulation system with PCM because of the latent heat released
during phase change [81]. An integration of convention insulating layers and PCM can give
the best design [155,156]. Despite showing better insulation features, PCM application is
limited by a likely liquid phase escape to the surroundings [81]. This gives difficulty during
installation and to subdue these problems [81], a structured plan centered on PCM–matrix
composite material was suggested [81]. The PCM–matrix composite comprises a matrix
of polymer material and Microencapsulated PCM (MicroPCM) fragments distributed in
spheres’ form in the matrix [81,157]. It is also described as a unique porous media with
composite heat transfer properties [81].
Figure 9 shows the application of passive insulation systems for PIP, wet, flexible,
and bundles. The towed bundles are constructed in water depth below 500 m [72] whereas
the PIP and wet insulation can be used for water depths of up to 2000 m [72]; the U-value of
the PIP flowline is relevant to most required U-values [72]. Table 4 compares the different
passive insulation systems.
Figure 9. Passive insulation system applicability (adapted from Watson et al. [158]).
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Table 4. Comparison of the different passive insulation systems.
Heat Management Technique
Advantages
−
Acceptable for a complete design. A high-resistance
temperature.
Multilayer technique could modify the design to attain
preferred properties.
Limitless depth of water for a solid system.
−
−
−
−
−
The housing of additional lines.
Could be removed in the course of service.
Cheaper than the PIP system.
Easy implementation on flowline.
Lower depth of water than PIP.
−
External Integral Insulation
(Multilayer Insulation included)
Insulation Bundle/
Module Systems
PIP Insulation
Vacuum Systems
Burial Trenching
−
−
−
−
−
−
−
Finest properties of insulation excluding vacuum.
Depth of water limit based on vessel installation
capability.
Drawbacks
U-Value
(W/m2 K)
−
−
−
−
−
Hard to detach.
There is a limit on the insulation level.
There is no buoyancy witha solid system.
Demand for a set time.
Restriction on thickness.
1.7–5.0
−
−
The module gaps may cause convection current.
A huge material quantity is needed, which can be
evaded.
Geometry could haveinsulation difficulty.
2.0–8.00
−
Citations
[51,85]
−
−
Costly fabrication andinstallation.
Impossible to reel a few PIP systems.
−
Once the external pipe is ruptured, the pipe section
losses insulation properties.
The effort is needed to conserve vacuum.
The vacuum level in the annulus is continually
observed.
<0.01
Impractical to bore trench for burial.
Employed only for lying pipes in the mudline.
Location affects the thermal conductivity of the soil.
Unpredictable thermal performance due to changes
in the properties of the soil and troubles predicting
or measuring them.
Insulation layers +
soil insulation
Best properties of insulation.
Transfer of heat is only by radiation.
−
−
Neighboring soil supplies part of the insulation.
Supplies on-bottom stability for flowlines.
−
−
−
−
0.3–1.5
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Table 4. Cont.
Heat Management Technique
Thin Film/Multilayer[159]
Conventional Insulation
Advantages
Phase Change Material (PCM)
Applied in Shell
Oregano and
Serrano, BP Marlin.
It gives good thermal resistance with a VIT combination.
The layer total thickness is below 3 mm.
−
Poor thermal storage creates quick cooldown,
desirable for the manifold, wellbore tubing,
Christmas trees, flowlines, and jumpers.
−
−
−
−
It has simple constructions.
Construction costs are low.
Life cycle costs low.
Has higher inspection reliability.
−
May not able to sustain the product’s temperature
above WAT for long flowline.
Once there is a product leak, it leaks into the
environment.
-
−
−
−
It has maintenance and operational flexibility
Suitable containment of a product from leaking.
Environmental safety due to management of oil in the
occurrence of an inner flowline failure.
Monitoring of the annulus for confirmation of a leak or
PIP degradation is easy.
Low chances of breakdown or leakage in both pipes.
Lower chance of product being discharged to the
surrounding than a solitary wall pipe.
Design and construction are more difficult than
insulated single wall pipe.
It has monitoring issues linked to bulkheads,
spacers, or shear rings.
Restricted capacity to check and monitor
conditions of outer protective pipe.
The repair costs of PIP for a total containment
failure are about 25% higher than a similar case of
insulated single wall pipe.
The VIT-PIP is expensive and not appropriate
where other PIP can be used as a replacement.
-
Once there is a product leak, this leaks into the
environment and cause degradation.
When the wax is formed, chemical inhibitors or
external heat are needed.
-
−
−
−
Flowline Burial
U-Value
(W/m2 K)
−
−
−
−
PIP Insulation
Drawbacks
−
−
−
−
−
−
−
−
The period of heat containment is high.
The construction and laying are simple.
Low costs of construction.
Life cycle costs are low.
−
−
During shutdown operation, latent heat release will
increase cooldown times by 3–5 days.
Heat loss in flowline can be noticeably reduced for
insulation systems containing PCM [153,154].
PCM–matrix composite material improves installation
problems because of phase change property [81]
−
−
−
−
−
−
Long shutdown period over cooldown time
performance.
A flowline restart can become a big problem.
Direct use of PCM insulation layers is restricted to
likely escape of its liquid-phase to the surrounding
temperature [81].
Citations
[50]
[149]
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3. Active Heating
Oil and gas production is inherently a risky business and how this risk is managed
will effectively determine its success. Flow assurance issues are part of these risks and with
most of the conventional fields already being produced, operators are facing in the past
15 years increasing issues in developing fields that have changed from stand-alone host
structures and largely concentrated resources to scattered ones with tie architectures to
existing facilities or shores [160]. In the last 10 years [161], the numbers of long tie-backs
doubled [162]. Truly, long tie-backs to existing infrastructures (higher than 10 miles for
oil fields and 30 miles for gas fields) are planned, from a cost perspective, as they give
entry to far reserves circumventing new topside installation [161]. By venturing further
and into deeper subsea, together with growing flow assurance issues associated with fluid
characteristics (hydrates, wax, high viscosity, pour point) as well as flowing conditions
(temperature, pressure, flow rate), have expanded conventional flow assurance methods
wax and hydrate prevention which included passive insulation to its limits, forcing the use
of conventional or hybrid loop architecture (e.g., depressurization, dead oil circulations,
and chemical injection to its limits) at elevated costs [160].
In this situation, active heating looks good enough as a cost-effective alternative for
both subsea in-field and long tie-back developments [160]. From a technical viewpoint,
the use of active heating permits transient and steady-state operations simplification of
highly complex fluids by managing wax, hydrates, high viscosity and pour point issues
with continuous or discontinuous heating [160]. The conservation and line start over by
active heating method permit the change to a single line framework that presents significant cost gains through a reduction in the number of flowlines from a conventional circuit
system with chemical injection and dead oil circulation [160,163–165]. The active heating
system involves the external addition of heat to the production flowline by an electrical
heat source or circulated hot water [166,167]. The active heating technique is an attractive,
economical, and effective solution for flowline heat management systems particularly in
subsea deepwater fields where flowline pressure and temperature conditions are unfavorable [49,50]. For most fields, the goal is to stay above the region of 15 to 40 ◦ C [161].
Therefore, the active heating method is used for many situations in flowlines [49,76], such
as the prevention of cooldown and warming up of non-flowing systems like (e.g., during
shutdown or startup) [49,168], sustaining and increasing temperature while flowing [49];
decreasing the need for chemical injection [49,76]
Compared to passive insulation, active heating can sustain the hydrocarbon fluid temperature beyond the hydrates and wax formations temperatures over long periods [49,76].
Three heating systems exist; Hot Water Circulation (HWC), Electrically Heat-Traced Flowline (EHTF), and Direct Electrical Heating (DEH) as shown in Figure 10a. A study on active
heating systems in the last decade compared the different heating systems as shown in
Figure 10b. There is 48% (25 articles) research work on Hot Water Circulation (HWC)-PIP
and Bundle, 46% (24 articles) for Direct Electrical Heating (DEH)-Wet DEH and DEH PIP
and 6% (three articles) for Electrically Heat-Traced Flowline (EHTF)-EHTF Bundle (IPB)
and EHTF PIP.
3.1. Wet Insulated Hot Water Pipe in Pipe
The Hot water circulation technique is a field-proven technology for flowline heating
mostly employed for deepwater operations [49]. For this type of system, the production
fluid becomes heated by the hot water that goes round in a little heating pipe through
a conduction and convection process [49,50]. The heating technology depends on the
circulation of hot water (fluid) in a close region to the production flowline in the annular
space (Hot Water PIP) [160]. Hot water circulation systems usually operate in a loop as the
water needs to be recirculated back to the topsides [160]. Generally, the hot water technique
uses heat exchangers with an extended secured tube having production fluid on the
inner part and the heating medium on the outer part of the tube [48,49,160,169] (Figure 11);
also the water may be injected into the well at the extremity of the system but creates a
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constraining coupling between the production and injection systems [170]. The heater
located near the subsea wellhead or over the processing facility on the surface heats the
hot water [49].
Figure 10. (a) Active heating system for subsea flowline. (b) Number of publications for active
heating systems from the top five publishers.
Figure 11. Wet insulated Hot Water PIP (adapted from Basilio et al. [171]).
3.2. Hot Water Bundle (Direct or Indirect)
A hot water bundle is a carrier pipe within which any combination of individual
flowlines and umbilical components are carried and there is hot water distribution in close
vicinity to the production flowline in an assigned line [160]. Bundles with dry insulation
having U-values of 0.5 W/m2 K are highly efficient insulation systems [161]. Two types of
Bundle HWC used in shallow waters are direct heating by annulus circulation and indirect
heating circulation through dedicated hot water lines (Figure 12a,b).
The direct HWC systems incorporate the production flowline contain in an insulated
sleeve pipe with hot water flowing in the annulus [161]. The hot water can either be injected
into water injection wells or returned to the topsides through a separate return line [161].
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The indirect HWC system operates through the use of heat supplied by dedicated hot water
supply and return flowlines to maintain the temperature in the production flowlines [161].
The production and hot water lines are inside the insulation layer that is filled with a
low-pressure gas such as nitrogen [161,172].
Figure 12. Hot water insulated flowline bundle system with (a) direct heating (b) indirect heating (adapted from
Zhang et al. [173]).
Comparing both solutions, direct hot water heating is more efficient in terms of
heat transfer [161]. Nonetheless, other criteria are less favorable [161]. Among others,
the volume of water is greater with direct heating and thus tank size required at the topside
in addition to pumping requirement [161]. The indirect hot water-heated bundle also has
the multipurpose heating lines as an advantage that can be used for water injection or
produced water re-injection [161].
Hot water circulation technology was used in Conoco Phillips’ Britannia fields and
Statoil’s Gullfak [50,167]. Brown et al. [174] narrated the design and execution of the UK
North Sea, Britannia 15 km subsea hot water heated bundle project [49]; regarded as the
finest solution for preventing the formation of hydrates and wax [49]. Zhang et al. [166]
discussed the problems associated with hot water heated production flowline bundle
designs [49], by two case studies of offshore West African developments [49]. The authors
analyzed and compared two key design possibilities such as direct and indirect heating [49].
It was concluded that direct heating gives a better heat transfer than indirect heating while
indirect heating provides reduced heat loss to the surrounding [49]. In terms of cost, the indirect heating bundle has the edge of a smaller bundle size and lower cost [59]. To reduce
the flow rate limit because of a high-pressure drop, the flowline size has the flexibility to
be increased for indirect heating than for direct heating [49]. Girassol field development
was another West Africa installation of production bundle located offshore Angola in
1400m of water reported by [49,161,175], and stringent operating requirements observed
because of the need for adaptability and issues of flow assurance [50]. The advantages
and drawbacks of hot water circulation systems for wet insulated and bundle systems is
highlighted in Table 5.
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Table 5. Advantages and drawbacks of Wet and Bundle HWC.
Advantages
−
Design
−
−
Fabrication
−
Installation
HWC- bundle can integrate insulation with
good thermal performance with U-value
from 0.6–6 W/m2 K.
HWC system is used to adequately warm
the production fluid during steady-state or
restart a line with issues of pour point.
HWC-Bundle structure presents the
advantage of limited thermal expansion
and low risk of lateral buckling, as well as
good resistance to accidental impacts
(dropped objects, trawl gear impact)
HWC-PIP diameter and water depth are
only limited by installation
vessel capabilities.
Drawbacks
−
−
HWC-PIP is prone to thermal expansion
and lateral buckling due to hot water
circulation in the annulus.
−
HWC-PIP can be installed by S-lay and
J-lay. Reel lay for small diameters only
would require specific development.
HWC-bundle construction necessitates a
dedicated bundle assembly plant (long
with specific equipment) and can only be
installed by towing up to 500 m
water depth
−
−
−
System
Efficiency/
Reliability
Possible combination with hot fluid from
more process units on the topside. This is
advantageous when the hot water is
produced from a heat recovery system
using production fluid heat or from fire
heaters fed with production gas.
Conventional HWC-PIP has relatively low
wet insulation performance, U-values
varies from 3–6 W/m2 K.
Only used during steady-state production
in practice. High power requirements for
preservation, and therefore conventional
operations are usually preferred.
−
−
−
Citations
[160]
A large footprint is required on the topside
for heat recovery and water treatment
systems.
The injected water needs to be treated to
avoid corrosion issues inside the
circulation path. For both pipe-in-pipe and
bundle, the material selection is also driven
by any long-term corrosion concerns in the
annulus.
No possibility for redundancy of HWC-PIP.
The heating apparatus being the pipe itself
has flowline integrity as the main design
criteria. Redundancy can be envisaged for
HWC bundles but would require an
additional circulation line in the
bundle structure.
3.3. Direct Electrical Heating-Wet Insulated Flowline (Wet-DEH)
DEH is based on the principle of direct circulation of single-phase alternating current,
at a typical frequency of 50–60 Hz, in limited flowlines wall thickness due to skin effect and
returned through a piggyback cable [100,160]. With the DEH system, the heated flowline
in a single-phase Alternating Current (AC) loop is the active conductor and with a forward
conductor powered cable [176], placed side by side near the pipe being heated as shown
in Figure 13 below [176–178]. The system of heating is provided from the power supply
located at the topsides [176]. Electrical heating does not require storage facilities like the
hot water circulation heating systems and applies to small size flowline such as PIP or
wet insulation flowlines [71,149]. In general, heat is generated by the Joule effect from the
current circulating in the flowline wall [49,179,180].
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Figure 13. Configuration of Wet-DEH system.
The DEH circuits are normally designed to work as an open system to mitigate AC
corrosion risk, therefore a significant part of the injected current flows through seawater
instead of the steel pipe [160,176].
The DEH system was initially established in 1996–1997 by a Joint Industry Project,
JIP (Sintef Energy Research and Nexans Norway) found on the patent of Statoil [176].
To sustain the needed temperature of the production fluid, DEH up until now has been employed in almost twenty different projects in the Norwegian North Sea [160,176]. The technique had recorded successes with validated laboratory tests and simulated results [176].
Also successfully installed the Chevron Lianzi project on a 43 km flowline in 1070 m water
depth [161]. The DEH wet insulated system advantages and drawbacks for the flowline
are explained in Table 6.
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Table 6. DEH wet insulated flowline advantages and drawbacks.
Advantages
Drawbacks
Design
−
−
−
−
Steel flowline direct heating effect of Joule
Small U-value of 2.5 W/m2 K
Extended heated circuits of approximately 50 km.
Large flowlines diameter of up to 30” inner diameter.
−
−
−
−
High linear power, voltages, and currents (1500 A).
The demand for 3-1 phase adaption.
Big cathodic system protection is needed.
The flowline corrosion design requires an electric field
occurrence.
Fabrication
−
It is field proven.
−
−
Line pipes sorting by magnetic permeability.
Piggyback cable protection and strapping.
Installation
−
J-Lay, S-Lay and Reel Lay.
−
Application limited to deep water (up to 1000 m water depth).
System Efficiency
−
It is field proven.
−
−
−
Low electrical efficiency of 30–60%.
The low heating efficiency of 50–75%.
It is not efficient for continuous heating.
Reliability/
Reparability
−
The cord and its components are possibly retrofitted/ restored.
−
There is no redundancy.
−
−
Heating by joule effect directly in flowline steel
It can be used for both preservation and remediation if designed
for such an objective [161].
Low U-value (≥2 W/m2 K) [161].
−
Low thermal performances (U-value of 3 to 6 W/m2 K) due to
flowline wet insulation.
High electrical power requirement (50–150 W/m) [161].
Design
−
−
−
Fabrication
−
Field proven [161].
−
In a significantly inductive system, apparent power is high
compared to active power.
Water depth is limited by piggyback cable qualified to date for
1100 m maximum.
Citations
[176]
[160]
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Table 6. Cont.
Advantages
Installation
System Efficiency
Reliability/Reparability
−
−
−
Wet-DEH can be installed either by S-Lay, J-Lay, or by Reel Lay.
Installation of wet-DEH is either by Reel or S-Lay [161].
Drawbacks
−
Non-homogeneous longitudinal and circumferential heating,
depending on steel material electromagnetic properties (pipe
sorting required) and coupling with piggyback cable.
−
Low efficiency: Heating efficiency is 50–70% due to low thermal
performances and Electrical is 30–60% due to the open system
electrical circuit, power flowing through seawater,
electromagnetic coupling, and inductive effects.
Low operability: Wet-DEH calibration and control is challenging.
No accurate adjustment of the power alimentation and
temperature control of the line is achievable.
Field proven [161].
−
−
DEH components can be replaced or retrofitted on already
installed wet insulated pipes if anticipated at the design stage.
−
−
No possibility for redundancy of the DEH system.
A/C corrosion risk, complex open-loop design with anodes.
Regular inspection and maintenance are required.
Citations
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Lastly, the heating efficiency of the wet-DEH system is small due to the small electrical
efficiency of the pipe (energy lost mostly due to power lost in seawater and Joule effect in
the DEH piggyback cable) and low thermal insulation capacity which is calculated to be
30–60% [160,176]. Such systems for uninterrupted heating solutions are not the best bet
for a large amount of power required from the topside [176]. But the idea of the system is
robust, field-proven, and is used for lengthy and big diameter flowline installation [176].
3.4. Direct Electrical Heating-Pipe-In-Pipe (DEH-PIP)
DEH-PIP works on the same principle as Wet-DEH except that current is supplied at
the middle of the flowline and returned through the carrier pipe instead of a piggyback
cable [176] (Figure 14). The DEH for PIP was initially developed for hydrates remediation
means nonetheless the tool is as well used for flowline heating in the course of fields’
development, i.e., continuous heating and/or field production shutdown [172,176,181].
Figure 14. Subsea DEH-PIP system (adapted from Candelier et al. [176]).
When compared to the Wet-DEH insulation system, this solution gives a higher power
efficiency due to [176], no links to the seawater, credit for skin effects and proximity;
PIP insulation decreases thermal losses.
Nonetheless, specially designed parts like non-metallic bulkheads are vital for the
system and this looks confined to only average lengths because extensive lengths result
in larger annulus width with higher voltage [176,182]. Table 7 compares the advantages
and drawbacks.
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Table 7. Advantages and drawbacks of DEH-PIP compared to Wet-DEH.
Advantages
Drawbacks
Design
−
−
The size of the voltage is 3 kV.
It has a good thermal performance of 2–0.5 W/m2 K.
−
−
−
High linear power and currents (1500 A)
Small flowline lengths’
Lengthy power cord needed for mid flowline electricity
provision site.
Fabrication
−
It is field-proven.
−
Required for special bulkheads and water stops.
Installation
−
−
J-Lay and Reel Lay is preferred.
For deepwater application.
−
S-Lay is not preferred.
System Efficiency
−
The high heating efficiency of up to 95%.
−
−
Comparatively small electrical efficiency of about 70%.
It does not appeal to continual heating.
Reliability/
Reparability
−
The cord and its components are possibly retrofitted/restored.
−
There is no redundancy
−
Design
Fabrication/
Installation
System
Efficiency/Reliability
[176]
−
High thermal performances (U-value of 0.6 to 2 W/m2 K)
provided by dry insulation in PIP annulus.
−
Low thermal performances (U-value of 3 to 6 W/m2 K) due to
flowline wet insulation.
Limited maximum heated length with a single power supply
which needs to be located at the mid-point of the line due to risk
of electrical arcing between the flowline and carrier pipe.
−
PIP arrangement mechanical robustness (resistance to accidental
loads, thermal expansion, buckling).
−
-
−
Low operability: DEH-PIP is not qualified for continuous
operation during production.
Electrical efficiency remains relatively low in practice (50–70%)
and long static power umbilicals are required for power supply
at the mid-point of the flowline.
−
High efficiency: Heating efficiency is 95–100% due to better
thermal insulation.
Citations
−
[160]
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3.5. Electrically Heat-Traced Flowline-Pipe-In-Pipe (EHTF-PIP)
The EHTF- PIP technique was developed in the last decade. The inner pipe in the PIP
assembly is installed with three-phase insulated heat-traced wires and it terminates in a
star end socket [176], consequently avoiding the desire to return the current back to the
topside power generation equipment [160]. The production/inner flowline receives heat
by conduction through the Joule resistive effect from the three-phase cord [176].
EHTF-PIP combines the heat trace cable’s higher efficiency with the reeled subsea
PIP’s higher thermal performance for both preservation and continuous heating during
production [71,160], as seen in Figure 15.
Figure 15. EHTF-PIP subsea systems.
Heat tracing cables are directly laid on the flowline, below the insulation layer, providing a very high heating efficiency. Fiber Optic Cable (FOC) is also incorporated in the
system to permanently monitor the internal fluid and cables temperature all along the
flowline through a Distributed Temperature Sensing (DTS) system [160]. The application
of EHTF-PIP for maximum size flowlines (ID 12 inch) is currently restricted to just a small
distance (about 25 km) and low-medium voltage (1–3.6 kV) used cords [176]. The EHTF-PIP
is presently qualified for only installation requiring towing and reel lay technique [176].
The installation method is very important for the technology used during heating as the
amount of field wire slices is reduced [176]. However, the reel lay is permitted for only
restricted pipe inner diameters of about 12”, therefore bigger production or inner flowlines
installation will demand other installation techniques [176].
The world’s first EHTF-PIP was installed in 2012 for the Islay development in the
North Sea, after a formal technology evaluation and qualification process completed with
TOTAL in 2009 [160]. Table 8 gives the advantages and drawbacks of EHTF-PIP.
In conclusion, the design of the EHTF-PIP is controlled by the need for use in distinct
operating conditions [176]. The power provided to EHT-PIP is produced by the essential
generator [176].
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Table 8. Advantages and drawbacks of EHTF-PIP.
Advantages
−
Design
−
−
Low linear power and currents. Low to medium voltages of 1–3
kV.
A good thermal performance of 2–0.5 W/m2 K.
FOC system application.
Drawbacks
−
−
−
Fabrication
−
Option for thorough electrical testing before installation–Test is
carried out onshore.
Installation
−
The EHTF system is preinstalled onshore in spool base
System Efficiency
−
−
−
−
A great heating efficiency of about 95%.
Good electrical efficiency of 90%.
It is used for continual heating.
FOC allows for effective heating system control.
Reliability/
Reparability
−
Good reliability is ensured by a redundancy of up to 300%.
−
Dry insulation in the annulus of the PIP provides good thermal
performance of U-value from 0.6–2 W/m2 K.
Dry insulation with a partial vacuum in the annulus of the PIP
gives a good thermal performance of U-value less than 0.5
W/m2 K [161].
Limited flowline length/ little heating loops of approximately 25
km.
The flowline inner diameter restricted to about 12 inches.
The amount of FOC and wire splices is important for every pipe
stalk.
The EHTF technique need unique installation equipment.
Unique waterstops/bulkheads is needed.
−
Installation method for Reel Lay limited-it is not feasible for
S-Lay or J-Lay
−
There are no significant drawbacks
−
The EHTF technique is subjected to varying loads in the course
of the flowline life span.
It is not repairable after installation.
−
−
Design
−
−
−
Citations
Flowline diameter is limited to 12” ID by reel lay vessel
capabilities.
[176]
[160]
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Table 8. Cont.
Advantages
−
Fabrication/
Installation
−
−
−
−
−
System Efficiency
−
−
−
−
−
Reliability/Reparability −
−
Homogenous longitudinal and circumferential heating of the
fluid
PIP arrangement and mechanical robustness (resistance to
accidental loads, thermal expansion, buckling)
Reel Lay is favored [161]
It is field-proven [161].
High heating efficiency: 90–100% tracing cables are in direct
contact with the flowline, and therefore the production fluid is
below the high-performance thermal insulation.
Electrical system is primarily resistive (minimum capacitive and
inductive effects): no specific apparent power requirement.
Hence, electrical efficiency of the system is high (around 90%)
Accurate fluid temperature monitoring all along the flowline
through the optical fibers [183].
Heated length up to 50 km with a single power supply at one
end of the flowline.
Low power consumption (below 50 W/m) [161].
Better operability: The heating power is correctly controlled and
exactly adjusted.
High redundancy: typically up to 300%, with one cable out of the
four installed within the system is sufficient to heat the flowline.
Possibility to mitigate cold spot issues with local trace heating of
critical components (jumpers, in-line tees, PLETs)
High redundancy and flexibility with separate activation and
online temperature monitoring [161].
Drawbacks
−
PIP is heavy and consequently, maximum water depth is limited
depending on pipe size and reel lay vessels capabilities
–
−
Repair not possible with tracing wires once installed subsea.
However, components are well protected within the carrier pipe
and a high level of redundancy is provided.
Citations
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3.6. Electrically Heat-Traced Flowline-Bundle or Integrated Production Bundle (IPB)
The IPB is a certified adaptable assemblage of risers that depends on the Integrated
Service Umbilical (ISU) field-proven technology and further technologies patent [149].
IPB combines passive insulation, gas lift hoses, and active heating [149]. It is a concept
based on elements construction with different roles all over a big production bore at the
center that is a usual ultra-deepwater adaptable riser structure [149], as seen in Figure 16.
IPB is employed in adaptable pipes for static and dynamic riser flowline application to give
solutions to the flow assurance issues [160]. Syntactic polypropylene foam is the insulation
material [149]. The layers of insulation are either composed of spiraled strips or thick
fillers joined all over the core structure in S–Z [149]. The S–Z insulation technique is a
field-proven technology [149]. IPB has been used with steel tubes to provide hot gas lift
injection at the riser base, and electrical tracing cables to heat the internal fluid and optical
fibers for temperature monitoring through DTS [160].
Figure 16. A schematic view of IPB Structure.
Two IPB installation projects for TOTAL West Africa were successfully delivered by
Technip [160]. Another IPB deepwater Papa Terra project in Brazil has been manufactured
and is currently being installed and commissioned [160]. Ansart et al. [160] shows the
advantages and drawbacks which are summarized in Table 9.
Table 9. Advantages and drawbacks of IPB.
Advantages
Drawbacks
−
−
Design/Fabrication/Installation
System Efficiency
Reliability/Reparability
Active heating for dynamic risers,
faced with a major flow assurance
issue: Joule-Thomson effect and
for static flowline applications.
−
High electrical efficiency of
tracing cables (around 90%)
−
High operability with
temperature monitoring by
optical fibers.
Redundancy of 25 to 100% for
tracing cables in the bundle
arrangement.
−
−
Citations
Low thermal performances
(U-value of 3 to 6 W/m2 K)
similar to wet insulation.
IPB internal diameter limited to
approximately 11–12”, depending
on insulation requirements, due
to present overall diameter
manufacturing limitations
−
Heating efficiency remains
around 40 to 60%.
−
Tracing cables cannot be repaired
or replaced subsea.
[160]
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IPB gives an efficient solution for active heating using electrical tracing, extending
over most of the application envelope of flexible pipe for infield production lines in both
flowlines and risers application [160].
4. Discussion
There is a need for heat management systems as hydrocarbon exploitation approaches
the next frontiers. Solutions to paraffin and hydrates are not limited to flowlines, the whole
system is considered for heating requirement [161]. Active heating or passive insulation system is needed to evade paraffin plug and hydrates formation inside the flowline.
The preservation of fluid flow during the steady and transient state is the main worry
during the subsea hydrocarbon production system operation.
A general study on heat management systems in subsea flowlines from the top five
publishers based on numbers of publications in the last decade was done. The result
showed that 42 % (52) of the total papers published by researchers were in active heating
while 88% (110) in passive insulation. This figure indicates a low research effort in active
heating when compared with passive insulation.
The review of the heat management systems for the conventional passive insulation
showed that wet insulation has U-values of 2 W/m2 K [49,82,87]. The dry insulation gives a
superior U-value of 1 W/m2 K [49,50,87,93], but water ingress deteriorates its performance
and degrades insulation properties, and hence the need for a PIP system. Due to the limitations of the use of dry insulation in subsea flowlines, PIP systems are deployed to stop water
from entering the insulation for a better insulation performance [50]. The flowlines burial
is regarded as one of the best heat management cost-effective technologies in solutions for
issues of flow assurance [50,87] but is less effective down deepwater [50]. The buried flowlines have a better heat capacity than a PIP system with an extended cooldown time [49]
while the installation cost for buried insulated flowlines is roughly 35–50% higher than
that of a PIP technique [49,101,102]. Throughout shutdown time, buried flowlines provide
four times higher retention capacity for heat than the PIP systems [50,103,104]. Some other
benefits of flowlines burial system include more option on installation vessels and contractors, more options on the vendor for the fabrication of flowline, when compared to PIP
system gives a slower cooldown time during shutdown and a likelihood of single flowline
repair [49], and also provide a reduced time and enhanced schedule for first oil [49].
Bundle systems give an appealing solution to a large span of the challenges of flow
assurance by supplying a cost-effective technique [51], and also its ability to circulate a
medium for heating [51]. Further benefits are; allowing for multiple flowlines installation
in a single pipe design [51], and the outer pipe is used to withstand subsea hydrostatic
pressure in deep water [51], and bundles allow the introduction of monitoring devices and
heated pipes [72]. The bundle’s drawbacks its length limit (shallow water) and the desire
for a comfortable launch and fabrication location [72].
In the search to enhance heat retention, VIT is established [50]. VIT is a remodeled
PIP design system with an annulus at vacuum conditions that reduce the transfer of heat
from the crude oil to the environment [50,126]. The VITs are regarded as expensive when
compared to other passive insulation systems [50,126,131–133] but are favorable in regions
where crude oil has a high cloud point [50]. The advantage of the insulation modules for
flow line applications is geometry modifications to suit both riser and additional lines [85].
Multilayer systems have the advantage that the density and thickness of individual layers
may be adjusted to fit the specific purpose [85]. For PCM systems, during shutdown
operation, latent heat release will increase cooldown times by 3 to 5 days [149]. However,
where there is a very long period of shutdown above the cooldown time performance, a
flowline start over may result in a big problem [149].
For active heating, technologies are done with HWC and DEH flowline systems,
benefiting from substantial track record and maturity [160]. The diameter and water depth
of HWC-PIP are limited by installation vessel capabilities and are also prone to thermal
expansion and lateral buckling due to hot water circulation in the annulus [160]. HWC-
Energies 2021, 14, 458
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bundle combines high-performance thermal insulation and limited thermal expansion
with low risk of lateral buckling as well as good resistance to accidental impacts [160].
Comparing HWC-bundle solutions, Direct HWC-bundle is more efficient than the indirect
HWC-bundle in terms of heat transfer [161], but other conditions are less favorable [161]
such as the volume of water that is higher with direct heating and therefore the size of
the needed tanks at topsides level as well as pumping requirement [161]. Additionally,
the indirect hot water heated bundle has the multipurpose heating lines as an edge which
is used for water injection or produced water re-injection [161]. For Wet-DEH system, it has
low heating efficiency due to the pipe’s low electrical efficiency and low thermal insulation
capacity [160,176]. Therefore, the system is not the better choice for continual heating
solutions in high power demands on the topside [176]. The DEH-PIP was developed
for hydrated remediation, but the technology is also used for flowline heating during
field development [172,176,181]. DEH-PIP compared to Wet-DEH insulation system gives
a higher power efficiency [176] because there is no link with the seawater and the PIP
insulation decreases thermal losses. However, DEH-PIP requires designed components like
bulkheads that are limited to medium lengths because longer lengths result in large annulus
width and higher voltage for electrical isolation reasons [176,182]. New technologies were
evolved on heat tracings components for rigid EHTF-PIP or flexible EHTF-Bundle (IPB)
flowlines active heating based on operational experience from HWC and DEH systems and
feed-back to enhance overall heating performances of the active system [160].
This improved efficiency and the fact that the tracing cables heating system is disconnected from the flowline itself contrary to the DEH system, allows better control and
adjustment of the power supplied to the fluid and hence greatly clarifies operations and
reduces related production costs [160]. Additionally, EHTF-PIP and IPB technologies can be
integrated to efficiently heat the fluid from the subsea wellheads to topsides, thereby solving likely cold spot issues [170]. The likelihood of continuous heating the production fluid
all along its path with EHTF systems and a low power consumption gives the flexibility
and chance to choose different field architecture and operating philosophy of single line
architecture rather than the conventional loop systems [160].
This single line architecture lowers chemical injection and power requirements, causing
a cost reduction in the overall operating and capital expenditures of the flowlines [160,176].
Disconnection of the heating apparatus and the production flowline gives a high level of
robustness and redundancy to the system [160]. The reliability of the system is additionally
increased by the chance of constantly monitoring the temperature of the fluid through an
optical fiber, both during transient and steady-state conditions [160].
In closing, conventional passive insulation systems are not the most suitable costeffective techniques for subsea deepwater long-distance transportation of crude oil and
it would demand additional supporting methods to convey hydrocarbon at short downtime [50]. Active heating systems are the game-changer within the subsea deepwater
industry that has opened doors to various field architectures and operating philosophies
leading to a global cost reduction [161].
5. Conclusions
Subsea deepwater and ultra-deepwater hydrocarbon field development have increased several flow assurance problems. The extension of the operating procedures and
application is practically not suitable from subsea shallow to ultra-deep water. Good flow
assurance technique is necessary during the designing and development phase of the subsea field projects to mitigate and manage hydrates and wax issues in the flowline systems.
The trend from the study showed the need for technical efforts and scientific research in
the field of active heating as the production exploration of the petroleum energy fields
advances into deep water.
Based on the review, some key conclusions were highlighted:
•
Both passive insulation and active heating systems are field-proven technologies to
sustain fluid temperature well above the HFT and WAT of the hydrocarbon.
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•
•
•
There is no single technology or architecture that suits all fields, the best solution is
based on data analysis and a specific company’s objectives, constraints, and philosophy. Therefore, it is necessary to have an established fit for purpose heat management
systems.
The passive insulation systems though not most suitable for long-distance deepwater
operations are always combined with the active heating system to prevent excessive
heat loss. However, combining two or more heat management technologies is found
to be very effective for flow assurance management in the industry.
From a cost perspective, active heating systems are the most cost-effective for subsea deepwater fields because it can cause a cost reduction in capital and operating
expenditures due to the use of a single line architecture instead of a conventional loop
which consequently lowers chemical injection and power requirement.
Finally, there is an ongoing effort to develop a model for thermo-hydraulic analysis in
the flowline. This would give its user the choice of multiphase models to use for pressure
and temperature distribution predictions and the accuracy of the predicted data generated
is subject to the built PVT models. In the end, will aid field operators in carrying out
proactive production surveillance that will save the company considerable cost.
Author Contributions: Conceptualization, K.C. and N.G.; Methodology, K.C. and N.G; Software,
N.S.; Investigation, K.C., N.S. and A.S. Resources, N.S. Writing-Original Draft Preparation, N.S.;
Writing-Review & Editing, N.S., K.C., N.G. and A.S.; Supervision, K.C.; N.G. and A.S. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Informed Consent Statement: Not applicable.
Acknowledgments: The authors express deep appreciation to the Petroleum Technology Development Fund (PTDF) for funding the Ph.D. thesis at PRISME Laboratory INSA Centre Val de Loire,
Bourges—France.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
DEH
DTS
EHTF
FOC
GOR
HFT
HWC
IPB
KHI
LDH
MEG
MeOH
OHTC
OPEX
PCM
PIP
PPD
PPF
PUF
PVT
THI
VIT
WAT
Direct Electrical Heating
Distributed Temperature Sensing
Electrically Heat Traced Flowline
Fiber Optic Cable
Gas–Oil Ratio
Hydrate Formation Temperature
Hot Water Circulation
Integrated Production Bundle
Kinetic Hydrates Inhibitor
Low-Dosage Hydrate Inhibitor
Mono Ethylene Glycol
Methanol
Overall Heat Transfer Coefficient
Operating Expenditure
Phase Change Material
Pipe In Pipe
Pour Point Depressant
Polypropylene Foam
Polyurethane Foam
Pressure Volume Temperature
Thermodynamic Hydrates Inhibitors
Vacuum insulation Tubing
Wax Appearance Temperature
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