Intelligent flexible pipe can improve tieback design

The oil and gas industry has used flexible pipelines since the 1970s.

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Integrated service umbilicals, production bundles are proven applications

Fern Teo
Henri Morand
Didier Hanonge
Technip
Jason Peacock
AWE Ltd.

The oil and gas industry has used flexible pipelines since the 1970s. Several thousand kilometers have been manufactured, installed, and put in operation. Many things can be done to make flexible pipe "intelligent." This is demonstrated in a technical feasibility study for expanding the producing Tui oil field.

Though intelligent pipelines can take many forms, two significant and proven applications are integrated service umbilical (ISU) and integrated production bundle (IPB). The ISU combines the function of both an umbilical and flexible pipeline. The IPB is essentially the same as an ISU, with an added active heating component. Both are made of a core and an assembly.

The core of either is a standard flexible structure. Flexible structures are made of several different layers. Each layer performs a different function. The inner most layer, known as an interlocking carcass, acts to withstand any hydrostatic collapse. Next is a leak-proof plastic sheath, known as the pressure sheath. This keeps the bore fluid contained. The pressure vault acts to withstand the internal pressure of the bore fluid. Then there are two sets of armor wires cross-wound for torsional stability. These wires take any tensile loading of the flexible pipe. The final layer of a standard flexible pipe is a plastic sheath to prevent water ingress into the annulus between the two plastic sheaths.

The assembly can comprise a bundle of hoses, cables, steel tubes, optical fibers, and insulation wrapped around the core. Assembly components typically are used for gas lift, chemical injection, hydraulic lines, power communication cables, heat tracing wires, and more. The assembly is held together by high-strength tape and a plastic outer sheath.

Feasibility study

Intelligent pipeline benefits have been demonstrated in a technical feasibility study on a Tui oil field expansion. As part of this study, tieback solutions of a production flowline, umbilical, gas lift flowline, and gas export flowline are considered.

Tui is 50 km (31 mi) offshore New Zealand in the Taranaki basin. In operation since 2007, Tui was New Zealand's first standalone subsea development. As part of its subsea field layout, it comprises four wells linked to an FPSO.

Design data

The main criteria for design of the flexible structures in this case are a production flexible flowline, a gas lift flexible flowline, and gas export flexible flowline are as follows (courtesy AWE):

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The umbilical components are part of a previous umbilical design supplied for the project, so each component has been qualified for its function. The feasibility study called for the following:

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Solution

The base case would be to supply three separate flexible pipeline structures and one umbilical. The alternative is to combine the gas lift, production, and umbilical into one pipeline known as the IPB, resulting in the manufacture, supply, and operation of only two pipelines.

In this case, the production line forms the core: the gas lift and umbilical form the assembly. The advantages of an IPB in this situation are reduced installation time, reduced field complexity, and optimized thermal performance.

Reduced installation time and reduced complexity arise because there is one structure instead of three. Reduced installation time is especially beneficial in this case because the Taranaki basin is known for harsh weather. The reduced complexity of the field means fewer pipes on the seabed and a cleaner subsea layout. This is advantageous for fields with existing infrastructure. Optimized thermal performance comes because the gas lift tubes are integrated into the structure. So, where the production line would have the highest temperature, the gas lift would have the lowest. Optimized thermal performance might keep the production line above a critical value to mitigate hydrate formation.

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The Tui oil development is offshore New Zealand.

The IPB design is made up of a standard rough bore (with interlocking carcass) structure which forms the core and an assembly. The assembly consists of six thermoplastic hoses, two cables, and four steel tubes. The thermoplastic hoses can be used either for chemical injection or hydraulic controls. The cables are used for power and communication to subsea equipment, and the 3-in. gas lift line is split into four 1-in. steel tubes. All these components are evenly distributed around the flexible pipe for torsional stability, and are separated by fillers. Fillers keep these components in place as well as transfer any mechanical loading to the core. Both fillers and components are wound in an S-Z manner around the core of the flexible pipe resulting in torsional stability of each of these components.

In detailed design, the following options might bring advantages in cost savings and/or enhanced performance. These include the following:

  • Similar outer diameter components
  • Adding passive insulation
  • Active heating
  • Temperature monitoring.

The use of similar outer diameter components allows the use of one filler type. This reduces the manufacturing complexity of the flexible pipe structure. Passive insulation takes the form of strips of synthetic foam which can be added as part of the assembly.

Active heating

There are three ways to provide active heating to a flexible pipeline: hot water circulation, the use of heat tracing cables within the armor layer, and/or a dedicated active heating section above the core of the flexible pipe. Active heating is useful where hydrate formation is an issue and the bore fluid has to be kept above a critical temperature. It is especially useful for shutdown and restart operations.

Hot water circulation is beneficial because the hot water is warmest where it is injected, which is where the production line is coldest. This can mean an increase in diameter, which is not optimal. This makes the use of heat tracing cables advantageous.

Heat tracing cables replace every few tensile armor wires. The number and location of these cables depend on the heating requirements, and those are governed by factors such as water depth, length of pipeline section, bore fluid temperature, and critical temperature of the bore fluid. If the design of the flexible is governed by tension, a dedicated heating layer above the core of the flexible pipe can be created. The design of the heat tracing cables is unique in the sense that it is a three-phase star connection circuit, which means that the sum of the current phases is nil. Therefore, no return cable is necessary, ensuring a more compact solution.

Temperature monitoring

Technip's temperature monitoring is the distributed temperature sensors (DTS) system. This provides continuous temperature measurement along the length of flowline using optical fibers.

Small bore stainless steel tubes are incorporated in the tensile armor layer during manufacture. Every fourth tensile armor wire is replaced by a steel tube, sometimes with plastic fillers on either side to ensure structural integrity of the steel tubes. At one end fitting termination, the steel tubes join to provide a continuous loop. Post manufacture, the optical fiber is inserted into these steel tubes using a blow down technique.

This involves the use of fluid drag to run the fiber through the control line. A pump pressurizes the system, the tubes provide the drag, and the fittings allow fluid to flow through the system, directing the fiber in the line. The loop in the termination allows the optical fiber to be inserted at one end and retrieved at the other end. This means there is access to both ends of the optical fiber. Double-ended measurements can then be made with no fiber splicing, thereby increasing the accuracy of the measurements.

DTS sends pulses of light down the optical fiber. The ratio of intensities of the two wavelength separated components of the back scattered light yields the temperature at the point of scattering. The time it takes from when the pulse is sent and to when the back scattered light returns gives the location of the temperature. As a result, a temperature versus distance graph for the whole length of the optical fiber can be constructed. The principle is known as Raman OTDR (optical time domain reflectometry).

Graphical user interface

A dedicated system can be created according to project requirements to facilitate the user interface of the system. This can consist of obtaining raw data along the length of the riser system. This can then be split into critical locations along the length of the riser such as touchdown point, gas-lift injection point, and topside. Other functions can be implemented into the system such as alarms in case of detection of cold or hot spots to prevent against hydrate formation and temperature fluctuations in the flexible pipe.

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3D representation of an IPB (left). Cross section of an IPB (right).

Gas export line

The nominal option of the 4-in. gas export line is a standard flexible pipe structure. However, with the use of intelligent options such as active heating, the need for dehydration of gas prior to export onshore could be revisited. This would be useful where there is no offshore processing facility. Active heating could take the form of electrical heat tracing cables integrated into design of the flexible pipe.

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3D representation of an IPB (left). Close up of the DTS system (right).

Qualification and track record

Integrated service umbilicals (ISU) have been used for a number of years. The current record stands at 18. A number of tests have been performed to determine the validity of these intelligent pipeline solutions. These take into account the mechanical behavior of the pipe when subjected to installation loads and hydrostatic loads, the thermal behavior of the pipe due to the integration of active and passive heating, and fatigue behavior of the pipe.

The first test began in 1998-1999. A test sample was fabricated incorporating active heating by hot water circulation. The sample consisted of an 8-in. ID flexible pipe with 11 hoses distributed around the core and passive insulation in the form of 30 mm (≈1.2 in.) of syntactic foam above. This was subjected to several heating and cooling phases. This qualification program resulted in the development of calibrated software in which the global heat exchange coefficient (U-value) of an IPB can be determined accurately as well as software capable of modeling the thermal and hydraulic coupling of an IPB, verifying its performance with regards to flow assurance.

In 2000, a joint industry research and development program (JIP) was formed between Technip and participants to qualify active heating for a flexible pipe. As part of this JIP, two electric heat tracing technologies were tested. The sample incorporated two designs - heat tracing cables as part of the assembly (dedicated heating layer) and heat tracing as part of the armor wires. The sample was then submerged in water and subjected to more than 10 different heating and cooling simulations. Test results were used to validate the design of the IPB as well as software used to design the IPBs.

The JIP also led to the creation of the DTS, which was devised to monitor the temperature along the length of the flexible pipeline during the test. The DTS system was integrated into the test sample as part of the tensile armor wires as well as part of the bundle layer.

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Previously manufactured IPBs.

To complement thermal testing, a full-scale test was done to study the behavior of electrical cables laid in an SZ manner over a core structure. It was found that the dynamic fatigue cycling as well as the heating and cool down phases had no effect on the integrity of the electrical systems. A layer by layer dissection found no significant damage to any of the IPB components.

The JIP paved the way for its first application offshore West Africa. This project is in block 17, 135 km (84 mi) offshore in water depths of between 1,200 m (3,936 ft) and 1,500 m (4,920 ft). For this project, eight 10.75-in. IPBs were supplied. The IPB design consisted of six heat tracing cables, thermal insulation, DTS system, and 24 gas lift tubes evenly distributed around the core of the IPB. A test sample was manufactured prior to final supply which was subjected to a full-scale testing in which the crushing, fatigue and thermal behavior was validated. A full-scale test was performed in a vertical configuration as this is more representative of real-life conditions.

Conclusion

With discoveries in more challenging fields, the implementation of intelligent pipelines is a qualified solution for both new and existing fields. These intelligent pipeline solutions take the form of an integrated service umbilical (ISU) and integrated production bundle (IPB) which can incorporate umbilical functions, active heating components, and the DTS system.

Key advantages of intelligent options can include improved thermal performance, reduced complexity of existing fields/new fields, and minimized installation time. Improved thermal performance can be achieved several ways where the temperature of the bore fluid needs to be kept above a certain critical value. The reduction in complexity of a subsea field layout and minimized installation time arises from incorporating three different flexible structures (gas lift, production, and umbilical) into one pipeline solution.

The ISU design has been in service for many years. IPB is a more recent technology. The IPB is qualified by numerous test programs performed by Technip, which validates the performance of its active heating elements and DTS system. The positive results of these test programs have paved the way for its use on offshore projects; the IPB risers have been successfully implemented on two West Africa field developments and are due to be installed on a project in Brazil.

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