active heating for life of field flow assurance (otc-25107) (paper)

8/11/2019 Active Heating for Life of Field Flow Assurance (OTC-25107) (Paper)

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Active Heating for Life of Field Flow AssurancePaul McDermott & Ratnam Sathananthan, Subsea7

Copyright 2014, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 58 May 2014.

This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not beenreviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect any position of the Offshore Technology Conference, itsofficers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Offshore Technology Conference i s prohibited. Permission toreproduce in print is restricted to an abstract of not more t han 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

Abst ract

Development of thermal management strategies in the design of subsea oil and gas production systems is critical forprevention of solids deposition such as hydrates and wax formation during turndown and shutdown scenarios. Passive

insulation provides no control of fluid temperature and forces the operator to depressurise the pipeline following a shutdown

and no touch period i.e. time after which the fluid falls below the hydrate temperature. For deep water and ultra-deep water

applications this approach would be ineffective as the large riser static head would prevent the pipeline pressure being

reduced below the hydrate formation pressure. In these cases there is a requirement for active heating or dead oil flushing.

Active heating provides the capability to monitor and control the fluid temperature during start up, shutdown and turndown

operations. Several electric and hot fluid circulation active heating systems are in operation as part of bundle or pipe in pipe

systems.

This paper will review current active heating system designs in pipe in pipe (PiP) and bundle systems. In light of recent

projects that have employed hot fluid circulation systems, the flow assurance challenges and subsequent design to ensure

complete system functionality across the life of field will also be discussed.

Introduction

Flow Assurance is an integral part of subsea oil and gas production system design and operation defining the structured

engineering approach to economic fluids production, from the reservoir to a production facility, over the life of the field and

in any environment. As is well reported in other literature, two of the main challenges with respect to flow assurance are

hydrate formation and wax deposition. Solids management in the system requires a holistic approach with a combination of

thermal, chemical, hydraulic and mechanical methods all applied in conjunction. In this article the focus is on the thermal

management aspects and primarily Active Heating systems in PiP and Bundle systems.

Pipeline active heating technologies have been developed to supplement the thermal performance of systems that pose the

risk of wax and hydrates particularly in shutdown, start-ups and in some case normal operations. To understand the need for

their inclusion in areas with more challenging subsea conditions (long distance deep water tiebacks, arctic region) it is

important to understand the limitations of well-developed industry techniques for the purpose of solids mitigation and inparticular hydrates.

General hydrate management strategies across the life of field are based on a combination of the following approaches:

Ensuring the system conditions are outside the hydrate region (e.g. passive insulation for normal operation)

Moving the system conditions outside the hydrate region (e.g. flowline depressurization followed by low-pressure

restart- commonly used in shallow water developments)

Removal of gas components and/or the water from the system (e.g. displacement of live oil using dead oil after shut-

down - commonly used in deeper water developments)

Inhibitor injection into water phase to change the hydrate equilibrium conditions (e.g. continuous chemical injection

into a wet gas flowline)


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As subsea oil and gas systems can be categorised into three groups; oil, gas condensate and gas export and injection systems,

there are varying approaches for each to hydrate management. In gas export and injection systems, dehydration is used to dry

the gas to such an extent that there is no risk of hydrate formation. In gas condensate and black oil fluid systems, the hydrate

management requirements vary at different stages of operation and involve a combination of techniques including chemical

inhibitor injection and those discussed briefly in the following.

During normal operation, the primary thermal management strategy is providing sufficient insulation on the flowlines and

risers to ensure the arrival temperatures are above the hydrate dissociation temperature. During shutdown the operator has no

control over the temperature and so must implement mitigation methods in a cooldown time (CDT) where the fluid cools tothe temperature conditions on the hydrate phase boundary. Within this period the system insulation is required to keep the

fluid temperature out with the hydrate zone during the investigation process and identifying mitigation measures where no

action is taken. The duration of this process is commonly known as the no touch time (NTT).

Following the NTT it is common practice to depressurize the pipeline system contents, moving the fluid conditions outside

the hydrate formation zone. In some deepwater production systems depressurisation to prevent hydrate formation will not be

possible due to the inherent large riser static head of the residual liquid I the system after a shutdown. To overcome this, the

common philosophy has been to replace the live crude in the pipeline system during a shutdown period with dead oil crude or

diesel from the topsides - commonly referred to as dead oil flushing. A looped dual flowline system is used, where the dead

crude/diesel is pumped into the system and the live crude is processed at the topsides. As the time taking for operations of

this type must be lower than the difference between the CDT and the NDT, the applicability of the dead oil flushing

technique is limited by length of the flow loop and topside pumping constraints.

As with the hydrates, passive insulation is used to ensure that the fluid arrives at the processing facilities above the wax

appearance temperature (WAT) at even the lowest the turndown rates. Periodic pigging is also used in severe cases where the

fluid temperature will regularly drop below the WAT during normal operation. During shutdown and start up, wax

deposition is not generally considered to be an issue due to the limited amount that can deposit during these transient phases

of operation.

It is therefore apparent that with congenital strategies of hydrate/wax management the operator is constrained to the

procedures they can implement and may require excessively long cooldown times to conduct and thus the more challenging

tieback developments generally of greater offset length that are becoming more prevalent within the industry will not be

possible with such conventional solutions. The application of several pipeline active heating systems in various subsea fields

allows control of system temperature at all stages of operation which is critical in ensuring production integrity and

represents a technological solution to develop the next generation of subsea tiebacks. An outline of the various thermal

management methods are shown in a Figure 1.

Figure 1 - Summary of Thermal management methods in Subsea pipeline systems


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Operating Philosophy of Act ive Heating Systems

The selection and design of an active heating system will be governed by its requirement for use in the following operating

scenarios: temperature maintenance during normal flowing conditions or a planned/unplanned shutdown, and fluid warm up

from ambient during restart.

Hydrate Mitigation: The heating system can be employed continuously in low flowing/turndown scenarios to maintain the

fluid temperature above either the WAT or hydrate formation temperature. Systems operating in turndown conditions withlong tie back distance and/or fluids with a high WAT in nature will normally require flowline heating. In the event of a

system shutdown the heating system can be brought online prior to the onset of solids formation to maintain the fluid

temperature. After an extended shutdown the fluid will cool to ambient conditions. To achieve the desired flowline

temperature the fluids must be warmed up prior to start up or restart. The required time to reheat the system is a critical

parameter in design which may dictate either the hot water circulation rates or maximum power requirements of the electrical

heating systems.

Hydrate Remediation: In the event of a hydrate plug forming the inherent safety issues related to its remediation means that

the use of pipeline heating for such a scenario should be carefully considered, particularly the heating rate. Raising the

flowline temperature above the hydrate formation temperature at such a rate that the gas is released locally in a rapid in a

fashion may cause a pressure build up leading to pipeline rupture. In addition to this incorrect application of the heating

system could dislodge the hydrate plug where a large pressure drop exists across it causing it to act like a bullet within the

pipeline. However production loses due to hydrate blockage is a problem that has plagued the industry and resulted in

significant financial penalties as developments progress into deeper water where remediation is takes longer and is thus more

costly. The prize of using heating systems for remediation is a therefore a significant one and so many operators who

previously had a philosophy of not using heating systems for hydrate remediation are now reconsidering that stance and are

participating in various on-going JIPs/field tests. These will investigate the possibility of safely using this technique for

remediation and is thus an area of on-going industry development within the discipline of flow assurance.

Wax Mitigation/Remediation: Active heating systems can be used to mitigate against wax deposition by maintaining the

temperature above the WAT. However in fields where the WAT of fluid is higher than normal (30 - 40 oC), the use of the

active heating system to melt deposited wax during lower flow scenarios can frequently become the controlling case for

power input (greater than power requirements to support hydrate mitigation) as the fluid temperature must be raised above

the wax melt temperature, normally ~10oC above the WAT. The use of an active heating system in such cases will allow the

selection of a single pipeline system instead of a dual looped flowline system, removing the need for periodic pigging thus

increasing operational efficiency.

To ensure the heating system is functioning, it is favourable to have it running continuously at a minimum heat load to

provide confidence in its ability to work as and when required. The required operating scenario will govern the design of

either the hot water circulation or electrical heating system and will determine important parameters such as power

requirements and circulation rates.

Life of Field/Life of Design

Flow assurance will become a critical discipline in the selection of active heating systems for field developments. It has a

large involment in the project design phase where concept selection is developed. The flow assurance engineer has an

important interface role:

Firstly to determine if active heating is a firstly required for any development,

If active heating will be effective and what active heating system will be most effective for the proposed system

based on required power requirements Finally the flow assurance engineer provides critical input to other engineering disciplines:

o Pipeline/flowline engineer - System thermal behaviour to determine the extra compressive loads associated

with heated systems

o Corrosion engineer - Define the most aggressive system operating conditions so that a corrosion

management strategy can be developed e.g. enhanced CP design

o Topside/Facilities - Determine power requirements to see if such a system can be accommodated within

the facilities weight/space/power generation limitations

o Subsea Hardware Engineer - Aid in the system selection, materials of construction

o Operational Staff - Development of operating procedures associated with the active heating system


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In the section of active heating as a field development solution the flow assurance engineer has to be actively aware that the

system has to perform over the life of field and in many cases the real benefit of such a system will only really be realized in

later field life when the rates have declined and the operation of the field becomes more problematic.

In doing this it is also important to realise that in many instances the controlling case from a heating/power requirement

perspective may be near the end of field life. It is at this stage that:

Operating temperatures and cooldown times are lower meaning the system reaction time needs to be faster,

Water cuts and liquid hold ups are higher due to lower flowrates resulting in a higher thermal mass to be heated isgreater and so more power is required

Operational flexibility is lowest, and so the greatest benefit of active heating systems can be seen

Reduction in chemical injection cost reductions that can contribute significantly overall project costs can be greatest

Although the purpose of the active heating system is for the field flow assurance strategy it is critical that there is efficient

and continuous interfacing between the various disciplines to ensure that it will function as and when is necessary across the

design life of the field.

Hot Fluid Circulation

Hot Fluid heated pipeline systems have been in use since the early 1990s and work on the same principle as industrial heat

exchangers where the production fluids are warmed by heat exchange with the counter current flow of a heating medium such

as water or glycol in either a PiP or bundled system across the pipeline profile. The required heating medium thermal energyis normally provided via a standalone heater at the topside host facility or by a waste heat recovery system (i.e. turbine

exhaust gases) where available. The method of heating can be defined into two categories; direct and indirect where

representative examples are provided in Figure 2.

Figure 2 - Outline of direct and indirect hot fluid circulation systems

With the direct heating approach the heating medium is supplied through the annulus of the PiP or bundled pipeline system,

heating the production fluids and returning via the annulus space of sleeve pipe or via a dedicated return line. Unlike

conventional PiP systems the insulation is bonded to the outer pipeline. This type of active heating has been implemented in

two fields; the King Oil field in the Gulf of Mexico and Britannia Gas Condensate field in the North Sea. The King field

utilises waste heat from the turbine exhaust gases to heat inhibited water that is circulated continuously in the annulus of the

closed loop 54km dual PiP flowline system for the purpose of hydrate inhibition (Harrison et al, 2000). It was reported that

operating experience from this field was good with the system working well. The Britannia field which was a world first for

this type of technology circulates the heating medium (corrosion inhibited potable water) in either direction between the

annulus and return line within the two 7.5km bundles depending on the operation (Clapham et al, 1999).

In contrast the more common approach in such systems is to install dedicated supply and return lines for the heating medium

which will indirectly heat the production pipeline within the same bundle sleeve pipe. Until recently the heating medium was


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solely used for this purpose and would flow in closed loop between the bundle and the topsides. In the case of, a recent North

Sea oil field development, hot produced water from the topsides process is used to both heat the production fluids and

provide the water injection service. A summary of systems where hot fluid circulation is applied is provided in Table 1.

System Field Area

Indirect Heating

Bacchus North Sea

Skene North Sea

Asgard North Sea

Gulfaks North Sea

DirectBrittania North Sea

King GOM

Table 1 - Summary of Hot Fluid Circulation Systems

The selection, design and operation of hot fluid circulation systems is governed and constrained by a number of factors which

include;

Heading medium supply temperature Available topsides heating and pumping capacity

Allowable pressure drop in either annulus or supply and return lines - therefore length of heating system

Provision of heating medium handling equipment on topsides.

Comparing both types of heating systems and considering all design factors above, the direct approach affords a more

efficient heating mechanism, the heating medium inventory will however be much higher as the annulus space provides a

much larger volume than a dedicated line. Thermal expansion of the heating medium will be much higher and will therefore

require much larger handling equipment on the topsides. Flow through the annulus and the PiP spacers creates a large

pressure drop and as a result significantly increases the topsides pumping requirements and will therefore limit the maximum

length of a direct heating system. With over 30 years of experience in using bundled pipeline technology, indirectly heated

bundled systems are generally the favoured technology for this type of heating system. The lower pressure drop and heating

medium inventory volume will reduce the capacity of heating medium handling and processing topsides equipment.

In terms of experience with bundle design, installation and operation, currently the deepest water depth in which one has

been installed and operating is 410m in the Norwegian Sector of the North Sea. Further development of the bundle

mechanical design is on-going to allow its application in deepwater oil and gas fields in areas including the Gulf of Mexico.

Pipeline Electrical Heating Systems

Electrically heated flowline systems work on the basis of utilizing the heat generated by the electrical resistance of a

conducting material when using an alternating current. Fluids can be heated directly via current flow within the steel pipe and

indirectly via a network of cables that run along the outside of the flowline that is commonly referred to as trace heating. In

such cases electrical power can be supplied from purpose built generators or spare capacity in those used to supply topsides

pumps and compressors. Both approaches will be discussed with a review of the current technologies and associated

applications.

Direct Electrical Heating (DEH) Solution - Systems Description

In DEH systems the pipe wall is used as the conductor of electric current which directly heats the pipeline. This method has

been applied in the form of open loop (wet insulated) and PiP (dry insulated) systems which are well documented in

literature. Two effects exist as a result of electrical current flow in these systems; skin effect where AC has a tendency to

flow to the outer surface of the conductor and proximity effects where current moves in close proximity to current flowing in

the opposite direction in coaxial cables. Both play an important role in the design of DEH systems.

Open Loop DEH systems carry the current in the cable piggybacked to the pipeline from topsides to the far end, the flowline

completes the circuit allowing return flow of current. At the far end of the pipeline the cable is electrically connected to the

seawater (grounding it to earth) through several anodes. Although this eliminates the risk of short circuit between the pipe

and seawater, the return current is split between the flowline and seawater reducing the efficiency of the system. It is


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System Field

Open Loop (Wet Insulated)Asgard

Huldra

Kristin

Urd

Tyrihans

Morvin

Alve

Skuld

Ormen Lange

Skarv

Continuous Flowing DEH Olowi

PiP End Fed Serrano and Oregano

PiP Centre Fed Habanero

Table 2 - Summary of DEH Systems

Trace Heated PiP Solution

In more recent years significant R&D and qualification programmes have been conducted to develop Trace Heated PiP

(ETH-PiP) systems. These indirectly heat the pipeline and its contents through the joule resistive effect of a series of cables

attached to the main flowline. It is a combination of a high performance PiP insulation and trace heating technology that is

widely used on topside installations for frost protection and winterization. The other significant difference with DEH systems

is that there is no requirement for a return line to complete the electrical circuit due to three-phase circuit configuration used

where cables are joined at the far end in a star configuration. An outline of the trace heated PiP solution is provided in Figure

4.

Figure 4 - Outline of Trace Heated PiP system

In principle the configuration of an ETH PiP system will allow higher efficiencies to be achieved in comparison with theDEH systems and more even distribution of temperature. The electrical heating cables are fitted between the high

performance dry insulation and production pipeline ensuring most of the heat generated is delivered to the pipeline and its

contents, therefore minimizing losses to the environment and topsides power generation. As the performance of the ETH PiP

system relies on uninterrupted electrical cabling along the pipeline profile, fabrication must be carried out onshore. This

means that in installation phase of the ETH PiP system only two methods may be considered: reeling or towing.

After a depth qualification process was conducted, this technology is now being piloted in the Islay North Sea field (De

Nerve et al, 2011) as the backup hydrate prevention strategy. In this case the ETH-PiP technology incorporates a fibre optic

temperature monitoring system installed to process and report the temperature of pipeline during operation (Decrin et al

2013). This will allow the detection of liquid accumulations left after shutdown which will be important towards the end of

field life. Detailed design work is currently being conducted for the application of ETH PiP technology for a deepwater field


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development in West Africa. This process will also encompass the discrete qualification of the traced heating system project

specific elements.

Passive Insulation

Passive insulation is an essential component in optimising the thermal performance of the pipeline system as a standalone

flow assurance strategy and with active heating systems where the requirement has been identified. Current high performance

insulation packages used in industry can be defined into two categories; wet and dry insulation. Wet insulation polymer basedmaterials are externally coated to the pipeline and as such will be subject to large hydrostatic loads and water ingress that will

over time limit its thermal performance. Dry insulation is placed in the annulus of a PiP or bundle configuration and

therefore protected from the external conditions using the outer pipe. The annulus space is a dry, low pressure environment

allowing the placement of high thermal performance (Nielsen et al, 2003). A summary of both wet and dry insulations is

provided in Table 3.

Wet insulation Dry insulation

U value U > 2 W/m2/K U < 2 W/m2/K

Thermal

properties*

- Thermal conductivity : 0.1 to 0.4

W/m/K

- Thermal conductivity : very low (0.015 to 0.04

W/m/K)

Mechanical

properties

- Designed to be water resistant

- Designed to withstand hydrostatic loads

- No resistance to water, pressure etc.

- Requires a protective carrier pipe (PiP/bundle)

Examples

- Multi-layer PP

- PU, neoprene, elastomer

- Syntactic materials

- Mineral wool, PU foam

- Izoflex

Pipelines

Type

- Rigid Pipeline

- Flexible

- PiP

- Bundle

Notes Wet insulation can provide buoyancyReduction of annulus pressure can improve thermal

performances

Table 1 - Summary of Passive Insulation methods

The comparison of the thermal performance of DEH and ETH PiP systems demonstrates the importance of passive

insulation. The basis for combining trace heated technology with a pipe-in-pipe configuration is to utilise the highperformance thermal insulation to minimise the electrical heating requirements. In contrast the setup of open loop direct

electrical heating systems is such that only wet insulations with their limited thermal performance may be used resulting in

significantly higher electrical heating requirements.

Case Study - Application o f Hot Water Circu lation, DEH and ETH-PiP

The relative merits of each active heating system discussed are compared in a case study of flowline warm up from ambient

conditions to above the hydrate formation temperature i.e. 25oC. Each system was modelled with following basic parameters;

9.3km, 14 production flowline

Flowline warm up from initial ambient condition of 4oC

The U values of the Production Flowline modelled for each system are detailed below;

Hot Water Circulation 0.9 W/m2/K

Open Loop DEH system 3.4 W/m /K

Traced Heated PiP system 0.5 W/m /K

The flowline warm up was modelled for varying parameters using an industry standard multiphase flow simulator. The Hot

Water Circulation system is based on a bundled pipeline system where the fluid production fluids are indirectly heated by

looped water line. Figure 5 presents the temperature warm up trend plots of the pipeline at two areas; the platform end and

production inlet with varying hot water circulation rates.


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Figure 5 - Hot Water Circulation Pipeline Warm up at Production Inlet and Platform end

Figure 5 demonstrates the importance of considering the lag time between production pipeline warm up at the platform end

and at the production inlet at the far end of the production pipeline when planning a start-up operation. In this case it is shown

that for all water circulation rates the production fluids will heat up to the target temperature in around 5 hours at the platform

end of the pipeline. However at the production inlet section of the pipeline the warm up times range from 18 - 45 hours. The

trends plots of the pipeline warm up utilizing an Open Loop DEH system and Trace Heated PiP system are given in Figures 6

and 7.

Figure 5 - DEH System Pipeline Warm up

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60 70

Temperature[oC]

Time [h]

Production Inlet - 30 m3/h Platform End - 30 m3/h Production Inlet - 40m3/h

Platform End - 40m3/h Production End - 50m3/h Platform End - 50m3/h

Production End - 60m3/h Platform End - 60m3/h

Target Temperature = 25oC

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250 300 350 400 450 500

Temperature[oC]

Time [h]

50 W/m 75 W/m 100 W/m 125 W/m



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Figure 6 - Trace Heated PiP System Pipeline Warm up

The uniform heating provided by electrical heating systems across the pipeline means no lag time will exist between

temperatures across the pipeline as is the case with Hot Water Circulation systems. On comparison of the two electrical

heating system results indicate that although the Trace Heated PiP system may require comparatively longer warm up times

compared with DEH, power requirements are significantly lower - a factor of five in this case study. The performance of

insulation in each system is demonstrated Figure 8 showing the cooldown trends in both liquid and gas filled sections of the

pipeline.

Figure 7 - Cooldown of gas and liquid filled sections for Active Heating Systems

As expected the use of high performance insulation (in this case in a vacuum) gives the ETH-PiP system the longest

cooldown time. This is considerably higher than the DEH system providing a much longer response time particularly in

unplanned shutdown cases.

0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350 400 450 500

Temperature[oC]

Time [h]

10 W/m 15 W/m

20 W/m 30 W/m


0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

Temperature[oC]

Time [h]

ETH-PiP - Liquid Filled

ETH-PiP - Gas filled

DEH - Liquid FilledDEH - Gas filled

Bundle - Liquid Filled

Bundle - Gas filled


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Impact of Future Industry Trends on Active Heating Systems

From the perspective of flow assurance strategies, active heating methods in the last 15 years have evolved from being

considered novel to more commonly and successfully used but is yet to still be fully accepted as a flow assurance strategy by

operators. Initially hot water circulation systems were only the heating systems considered; overtime the development and

usage of DEH systems in industry has increased. In the last few years after a significant amount of qualification work the

installation and operation of an ETH-PiP was realised. This progression of active heating system development and

deployment is illustrated in Figure 8.

Figure 8 - Past Present and future for Active Heating Systems

As is shown the trend for the next 10 - 20 years in the Oil and Gas industry is the development of marginal fields with tie

back distances of 100 km (particularly in arctic areas) and beyond, as project cost inflation has made these the most economic

option, where flow assurance is critical to making these a reality. The implementation of ETH-PiP in particular will be a

major driver for these developments moving forward.

A summary of the current status each heating system in terms of tie back distance and largest water depth is provided in

Table 2 to understand where further work is needed.

0

5

10

15

20

25

30

35

40

1999 2005 2013 2040

No.in

Op

eration

Year

ETH PiP DEH HWCDeep Water Long

Distance Smart

Fields in Operation

is common place

with substantial

experience in use

of:

Low power high

thermal

performance ETHPiP technology

utilising Fibre optic

Temperature

monitoring systems

to process pipeline

temperature and

predict parameters

such as liquid hold

up in shutdown

scenarios


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Active Heating System Longest current tie back distance Water Depths

Hot Water Circulation

Systems

- 27 km Dual Flowline PiP, 15km for Bundle

Pipeline (In Operation)

- Recent project study shown it to be

technically feasible at distance of 50km

- 1670 m in King PiP system

- Bundles installed at depth of 410m

Direct Electrical- 44 km (In operation)

- 55km (In development for North Sea)

- 1000m - PiP Systems (In operation)

- 1070 m - Open Loop (Indevelopment for West Africa Field)

ETH-PiP

- 6 km (Successful Islay Pilot Scheme in North

Sea)

- 14 km (In development for West Africa field)

- 700 m (In development for West

Africa field)

Table 2 - Summary of status of current operational and developing Active Heating Systems

In general Table 2 demonstrates that there is large scope for continual development of active heating technology for longer

distance deeper water fields. Recent concept selection studies have identified that the use of hot water circulation systems can

be implemented on tie back distances of 50km and much further. In the future this will need to be measured against the

increasing topside pumping and heating requirements which will lead to increased project CAPEX and hence determine the

feasibility of the design.

With DEH one of the main constraints is the maximum heated section length which is currently restricted to a maximum

voltage. Qualification work will be required to further increase the maximum cable voltage level. To improve the efficiency

of the system it has been proposed to move components such as the transformers and capacitors from the topsides

(conventional design) to subsea and design the system into heated sections of up to 50km (Lervik et al 2012) thus reducing

supply cabling heat losses. Although experience exists in the application of subsea electrical equipment including

transformers and no showstoppers were identified, further innovation will be required if the DEH is to be applied to tie backs

in excess of 100km.

The advancement and implementation of ETH PiP technology is seen as being next major step in the evolution of active

heating systems. With its low power, high thermal performance design and the capability to include an online monitoring

system as was done on Islay it can be seen to be the key to smart oil field developments of the future which in itself is an

industry trend. Flow Assurance will be critical to this as we monitor/validate subsea system performance and make sure the

strategies are correct. Further qualification of higher capacity power cables will be crucial in implementing the ETH-PiPtechnology in fields with long tie back distances

Conclusions

On review of the Active Heating Systems currently in operation and those being developed it is clear that for continued

offshore oil and gas development particularly in ultra deepwater fields, their implementation is crucial to the successful life of

field flow assurance strategy as a whole. Particularly during shutdown and restart periods heating systems provide a means of

ensuring the flowline temperature remains above the wax appearance and hydrate formation temperatures before operation is

resumed. The real benefit may only really be realised in later field life when the rates have declined and the operation of the

field becomes more problematic

Hot Water Circulation Systems are a proven industry option using both PiP and Bundle pipeline technology that has been

shown to lower CAPEX particularly in a recent application where produced water is now being used as both a heatingmedium and injection for further oil recovery. Bundle Pipeline Systems have been successfully installed and operated for

over 30 years and represent a cost effective approach for integration of the subsea production system within one flowline

system.

Direct Electrical Heating systems have been implemented successfully on several deepwater fields lowering the reliance on

chemical injection for hydrate and wax inhibition. Furthermore the planned use of DEH in a 55km flowline system which

will make it the worlds longest, demonstrates the on-going improvements being developed for this technology.

Electrically Trace Heated PiP technology is next evolution of active heating systems that combine high performance thermal

insulation with a low power design making it a very attractive alternative to conventional DEH systems. It has successfully

been installed recently is now being considered for a number of projects.


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In the future it is expected that the major industry drivers will be towards the development of smart oil fields of the future

which will require continual development and innovation of active heating systems technology. ETH-PiP systems will be

crucial to unlocking the potential within these fields particularly through the monitoring systems that forms part of this

technology, which offer enhanced control of operation across life of field.

Flow assurance will be a critical discipline in the selection design and operation of active heating systems for the life of field

operation. It is important that the flow assurance engineer has a large involment from concept through to operation in the life

of the cycle of the project with continuous interfacing with all other disciplines.

Acknowledgements

The authors would like to acknowledge Subsea 7 for permission to publish this paper. It is emphasized that the conclusions

put forth reflect the views of the authors alone, and not necessarily those of Subsea 7.

References

Denniel, S., Perrin, J. and Felix-Henry, A. 2004. Review of Flow Assurance Solutions for Deepwater Fields, Offshore

Technology Conference, Paper OTC 16686.

Riviere, L., Decrin, M-K., Sibaud, E, and Anres, S., 2011. Oil Field Developments - Long Tie-Back Concepts UK Limited,

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