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Various concepts of pipe-in-pipe design and fabrication.

Pipe-in-pipe (PIP) is a pipeline design concept that minimizes heat loss from the fluids being transported. Engineers resort to this method when alternative methods of insulating a pipeline can't meet the design's heat loss criteria, particularly in deep water developments.

Maintaining heat is necessary for flow assurance and other operational considerations. PIP is a special type of fabricated pipeline with two concentric pipes with insulating material contained within the annuli.

The inner pipe, which carries the fluids is known as "carrier pipe," while the outer pipe is called the "jacket." The design has evolved in recent years and consists of several design and fabrication methodologies. PIP design, as well as fabrication, and installation requires special considerations depending on the type considered.

Design, Fabrication Methodologies

Engineering design of PIP is no different than an ordinary subsea pipeline. The same principles of mechanical design still apply. Generally, the jacket is designed for lay stresses, even though the fixed type segments require composite section geometric properties for proper analysis. The hydrodynamic stability calculations are almost entirely based on the geometric properties of the composite section.

Stress distribution at the joints of the jacket pipe and the carrier pipe in the case of a fixed type PIP segment is complex and generally a finite element method (FEA) model is required to analyze stresses.

There are a number of options available for fabricating a PIP pipeline segment. The fabrication method used for a PIP segment differentiates the pipeline more so than the design methodology. There are essentially three main types of PIP segments based on fabrication method used: fixed, sliding and restrained. They refer to the movement of the carrier pipe with respect to the jacket pipe.

In this type of segment, there is no movement between the carrier pipe and the jacket pipe, which are connected at the ends either by welding donut rings/bulkheads or forged mbps. This aligns the two pipes axially and fixes them in all three directions for rotation as well as for translation. Since the two pipes are connected at the ends, each pipeline segment is like a sealed cylinder. Therefore, can allow a vacuum in the annulus for insulation.

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Custom-designed components in this method PIP design consist of a pipeline segment fabricated with two concentric pipes of two different diameters, which are separated by two donut rings at the ends, creating an annulus. The annulus is filled with an insulating material, its size determined by the insulation requirements as established by flow assurance requirements for the project.

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The segments are joined on the lay vessel by welding the inner pipe (carrier) ends. Portion of carrier pipe between the two welded segments does not have jacket protection until after the welding is completed for connecting two carrier pipes of the two adjoining segments. This is essentially a custom design for every scope and each project. The sizes of the carrier and jacket pipe are sized to fit the criteria of each project.

The joint between the two segments, known as a "field joint," can be handled in various ways, the simplest being with wet insulation applied and then covered with insulated wrap. Since the two pipes are connected at the ends using metallic centralizing donut rings/bulkheads, each pipeline segment is like a sealed cylinder. Therefore, it can permit a vacuum in the annulus for insulation, if required. There will be some heat loss from the carrier pipe (flowline) to jacket pipe through the metallic centralizing donut rings/bulkheads.

The behavior of this type of PIP at the joints is complex. Therefore, generally a finite element analysis is performed to ascertain stresses in the bulkhead and the pipes during the pipe lay as well as for thermal loads under operational conditions. This PIP system can be installed by S-lay and J-lay methods. Reel lay should be avoided, because of the complicated stress distribution at the joints and the strong possibility of creating torsional and fatigue stresses in the carrier pipe.

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Fabrication of this type of PIP joints is accomplished with swaged connectors or forged tulips and is much easier than using the donut rings. In the case of swaged and tulip type ends, both are initially welded to the jacket pipe, then welded to the outer surface of the carrier pipe. This is done at both ends of the joint.

In a restrained type of fabrication of PIP segments, polymer bulkheads are used to hold insulation material in place and to the two pipes coaxially aligned. These bulkheads transfer the load during installation and provide concentric alignment, but are not attached to the jacket pipe. The flowline is concentrically located inside the jacket pipe by spacers in the main body of the joint and by non-metallic bulkheads at both ends.

The bulkheads provide axial and lateral concentricity between the flowline and carrier pipe during installation. However, it does allow some axial movement of the flowline with respect to the jacket pipe during lay operation. The insulation material is either pre-attached to the flowline prior to insertion into the jacket pipe or the annulus is filled once the flow line has been inserted inside the jacket pipe.

The field joint is usually done by welding steel half shells to close the gap between consecutive joints, and using foam or other insulating material to fill the space. Installation of restrained PIP systems can be accomplished essentially using Reel lay; S-lay or J-lay. For Reel-lay, it is advisable to keep the outer and inner surface of the bulkheads rounded and smooth, so as to keep the resistance to movements of the flowline with respect to the jacket pipe during the lay operation to a minimum.

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In this type of fabrication, the flow line is free to move inside the jacket pipe. The jacket pipe is joined by a butt weld to attach each segment to the next segment and thus allowing the jacket pipe to slide freely over the flowline. Alignment spacers are generally used to keep the outer jacket pipe and the flowline pipe aligned.

A temporary device may have to be used to keep the flowline and the jacket pipe aligned during transportation, handling and laying operation. This device is removed prior to welding. Thermal insulation may be the outer most layer if flow-through-annulus active heating is used. Since this can be installed using Jay-Lay, S-Lay of Reel-Lay, any vessels available and consistent with the pipeline diameter and water depth can be used.

Field Joint Options

In an S-lay a PIP system is typically welded on the lay vessel with half shells used at the end of each joint to cover the carrier pipe insulation, which could be wet type, foam or equivalent.

An alternative PIP insulation system for S-lay installation is offered by fabricator ITP. The insulation is provided by Microtherm[R] (Izoflex[R]). This material is strapped onto the flowline prior to insertion into the jacket pipe. The jacket pipe is then swaged down onto the flowline and welded at the end of every pipe joint. This effectively creates a bonded system, and the field joint is completed by sliding a steel sleeve over the joint and bonding in place.

For a reeled system, the pipeline is fabricated onshore, and the requirement for field joints is reduced as typically the pipe in pipe system allows axial sliding of the flowline within the sleeve. Such a sliding system allows the flowline and sleeve to be welded separately, before the insulated flowline is inserted into the sleeve pipe.

The pipe in pipe flowline is then stored at the spool-base in long stalks prior to final joining of the stalks as the pipeline is reeled onto the installation vessel. It may require field joints with half shells between stalks, or it may be possible to weld the flowline and then slide the sleeve along and weld the sleeve without requiring half shells.

Insulation Of Segments

Insulation is an important part of the PIP segment. Since, effective and efficient functioning of the insulation is of paramount importance in order for the system to work and achieve the objective of mitigating heat loss from the flowline/carrier pipe.

Selection process of pipe-in-pipe insulation systems is most times designer and fabricator dependent and can be complex. And therefore, no recommendations are given here for any particular system. Instead it is recommended to examine the properties of various insulating materials available, and to consider the generic system designs suitable the type of PIP desired.

Polyurethane Foam (PUF) systems are assembled in standard joint lengths by first centralizing the flow-line pipe inside the carrier pipe, sealing the annular gap, then injecting the reactants plus blowing agent into the annulus. The PUF expands and cures, bonding to the inner and outer pipes in the process. PUFs can be blown at low densities (110kg/m3) and provide extremely low conductivity (0.03 Wm-2K-1).

Dry-wet insulation, or Vikotherm, which is manufactured by Trelleborg Technologies of Norway comes in two types of high-performance insulation and may eliminate the need for PIP requirements in some cases:

* Vikotherm-R2 is a three-layer coating system capable of operating at -45[degrees]C to +155[degrees]C and at depths of 3000-plus meter.

* Vikotherm-S1 is based on non-syntactic silicone technology and has an operational range of -40[degrees]C to +135[degrees]C and depths up to 3000 meters.

Granular materials, or microspheres, can be poured into the annular space between the flowline and jacket pipe. The granules are usually alumina-silicate microspheres, or fly ash, a waste product from coal-fired power plants. These microspheres range from 10 to 150 mm in diameter. This material is inert and can be used without any reservation in the annuli of the PIP segments.

Thermal conductivity of microspheres is between 0.09 and 0.11 W/m2K. The pipe assembly can consist of single or double joints. During the application of this insulation, the segments are normally inclined at an angle to receive the material and then vibrated to ensure optimum compaction.

Microporous silica materials are ultralow density solids formed from a gel in which the liquid components are displaced by a gas to produce an aerogel. Aerogels are, in effect, a silica foam with pore spaces smaller than the mean free path between air molecules. The fact that the pores are so small gives the material exceptionally low density and thermal conductivity. Aspen Aerogels, Cabot and Microtherm are some of the manufacturers of these materials. These insulating materials have been used by installation contractors, including Subsea 7 and Technip.

Mineral wool is a form of spun silica manufactured from molten rock spun at temperatures as high as 1600[degrees]C into interlocking fibers. The main supplier of mineral wool products for subsea pipeline insulation applications in the North Sea is Rockwool.

Rockwool Aquaduct CL product is designed for pipe-in-pipe applications. The material is manufactured with radially orientated fibers when installed, which provide compression resistance at low thicknesses (down to 10mm). It is supplied in pre-formed sheets (with a "C" shaped cross section) with a bonded aluminum foil outer facing to reduce radiation heat transfer in the annulus between the insulation and the carrier pipe.

A perfect vacuum is the best insulation possible. However, achieving this is difficult, since some gases under certain conditions are prone to diffusion through steel and can create partial pressures. This can cause conditions for corrosion of pipeline, as well have derogatory effects on the heat transfer coefficient. Vacuum can be combined with other type of insulation to achieve desired heat transfer coefficient.

The insulation system choice will depend on the installation/PIP contractor.

Installation Of PIP

PIP pipelines can be installed using three lay methodologies: reel-lay, S-Lay or J-lay:

* J-lay is the second slowest of the three pipe lay methods, but the speed of the installation depends on the number of welding stations, pipe diameter, water depth and pipe joint shape. It is similar to the S-lay, however, it is the least stressful pipe-lay method of the three. This is especially important when laying a PIP type joints. Minimum depth of water required for J-lay depends on the tower angle and size of pipe. In general, J-lay cannot be used for water depths less than 600 feet.

* The S-lay is the slowest of the three pipe lay methods. It subjects the line pipe to high stresses, as well as stress reversal within a short span of time. This is especially important when laying a PIP type joints as it can lead to fatigue stresses. An S-lay system is typically a bonded PUF system with no relative axial movement of flowline and sleeve. The insulated PIP joints are welded on the S-lay vessel, typically using half shells to complete the field joint of the sleeve.

* Reel-lay is the fastest of the three methods, but only useful for up to 16-inch outer diameter and Normal reel-lay can install as much as 24 km per day, depending on the pipe size, water depth, etc. The pipe ovality and flattening effects due to reeling are important issues, and should be given due consideration in the detailed engineering phase before finalizing the pipe lay method. The only problem is that while it may be safe to reel single pipe on to a reel without much impact on the pipe integrity, the effect of reeling on the fixed PIP sections cannot be easily ascertained.

In general, a reeled sliding PIP system will allow axial movement of the insulated flow-line relative to the sleeve and the flow-line is typically located within the sleeve by centralizers/spacers.

Repair, Heat Loss Strategies

Pressurizing inlet valves are recommended when there is possibility of gelling or waxing, in case of an insulation failure due to a damaged segment. Where it is possible, use pressure to dislodge gelled material and open the pipeline for operation and or repair.

Inlet valves are installed on the pipeline, for example, at 1 km intervals are close shut when the pipeline is installed. It is important to make sure these valves do not get damaged during pipeline installation and are upright during installation.

The pipeline system should be designed assuming that if one segment remains uninsulated, the allowable heat loss from the pipeline fluids stays well within the design criteria. This is only possible for fixed-type PIP segments, since each segment and its insulation are totally independent of the rest of the pipeline system.

Use of double pipeline loop with diverter mechanism is an expensive, but reliable solution to guard against unwanted shut ins. This essentially requires two pipe lines, each designed to carry all the produced fluids. The loop is designed with a mechanism to divert flow in either pipeline at will if one pipeline is damaged. This design is prone to slugging under normal operational conditions, since each pipeline carries essentially half the quantity of the fluid it is designed to carry.

Prefabricated clamps can only be used in fixed PIP segments, and only if the damage is limited to the outer pipe/jacket and the insulation has not been damaged. The clamps have been in use in industry for a long time to control leakage in single pipe pipelines.

Replacing damaged segments completely is difficult and can only be done with fixed PIP segments. This will require pipeline system shut in, cleaning, drying and re-commissioning with revenue loss and added expense as the consequence. However, all this cost may seem small when compared with the cost of replacing an existing PIP pipeline system, especially if the field production has not peaked and the pipeline is long. The proposed operation and sequence is as follows:

* Shut in the pipeline system.

* Remove the field joints on both ends of the damaged segment, exposing the carrier pipe/flowline.

* Cut off and remove the damaged segment, replacing it with a new segment.

* Leave field joints exposed after the new PIP segment is in place, provided the amount of heat loss is acceptable.

* Re-commission the pipeline system as needed.

* Develop conclusions and recommendations.

Conclusions

PIP systems allow a range of advanced and highly efficient insulation materials to be used in achieving overall heat transfer coefficients of less than 1 W/m2K. These systems are important components of subsea developments where untreated well fluids may have to be transported large distances and wax and hydrate problems must be managed.

However, as a result of this insulation, thermal expansion challenges are increased and techniques, such as probabilistic analysis, upheaval buckling, snake lay and cooling spools are employed to mitigate high-expansion loads.

There is bending of the PIP joints in the S-lay, as well J-Lay. However, both sagbend and overbend stresses can be controlled by manipulating the tension, forward pushing of the vessel and use pf stinger type/geometry. However, there is no such freedom in reel-lay to control stresses while loading the pipe on the reels. The pipe must be bent to conform to the reel radius, and stay that way for many hours or perhaps days. This is unlike S-lay and J-lay stresses (both sagbend and overbend), which exist for a short time compared with the reel-lay.

By JC Suman, Owner, Energy Transport & Infrastructure, LLC, Richmond, TX

JC Suman is an engineer and owner of Energy Transport & Infrastructure, LLC, a Richmond, TX-based consulting firm, specializing in construction and project management of pipelines, terminals and structures. Suman is an author of several technical papers that have appeared in ASCE, ASME, OMAE, Oil & Gas Journal and the Pipeline and Gas Journal.
Table 1: Insulation and PIP segment type compatibility matrix.

PIP/INSULATION   Granular Material   Micro-porous   Phase Change
                                       Material       Material

Fixed                    Y                Y              Y
Restrained               Y                Y              N
Sliding                  N                Y              N

PIP/INSULATION   PUF (injected)   PUF   Vacuum

Fixed                  Y           Y      Y
Restrained             Y           Y      N
Sliding                N           Y      N

Table 2: Compatibility of installation method and type of PIP segment

Installation method/   Fixed   Restrained   Sliding
joint type

S-lay                    Y        Y/N          Y
J-lay                    Y         Y           Y
Reel-lay                 N         Y           Y
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Title Annotation:2015 International Offshore & Gulf of Mexico Report
Author:Suman, J.C.
Publication:Pipeline & Gas Journal
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2015
Words:2994
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