Technology assists in assembly of insulated pipe-in-pipe joints

As oil and gas field production moves into greater water depths, sea water temperatures at the wellhead drop nearly to the freezing point, creating a variety of problems, such as hydrate, paraffin, and wax formation in piping. British Steel believes that pipe-in-pipe insulation technology may solve many of the temperature problems, and could prove to be more cost effective than other proposed solutions.

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Steelmaker planning US Gulf pipe facility

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The sleeve in this system is able to slide over the flowline pipe. This allows the sleeve pipe to be closed by means of a single butt weld. Once the flowline has been welded, the sleeve pipe is allowed to slide until it abuts the adjacent sleeve, at which point the butt weld can be performed.


As oil and gas field production moves into greater water depths, sea water temperatures at the wellhead drop nearly to the freezing point, creating a variety of problems, such as hydrate, paraffin, and wax formation in piping. British Steel believes that pipe-in-pipe insulation technology may solve many of the temperature problems, and could prove to be more cost effective than other proposed solutions.

The steel producer is considering plans to build a complete insulated pipe-in-pipe facility in the Gulf of Mexico area within a year to provide its newly developed Hydrotherm product to deepwater developments in the region.

Pipe-in-pipe systems are difficult to engineer. British Steel claims that if done right, the systems generate real reductions in the full life cost of a development, and solve the problem of insulating deepwater pipelines. Comments made recently by some pipeline engineers indicate that in the end there may be no other way to keep oil and gas flowing in extreme depths. The counter-argument has long been that pipe-in-pipe systems are too complex and too expensive to manage.

British Steel engineers say that pipe-in-pipe systems are field specific. One insulation technology is not the answer to every challenge. Insulation materials and field jointing methods must be selected to meet the thermal and structural requirements of the pipeline throughout its entire life cycle.

Hydrotherm system

The Hydrotherm system is highly versatile, according to British Steel.

  • The system can be laid by conventional pipelaying, reeling, or towing. In addition, the sections can be designed to contain thermal strains.
  • For S-lay operations, the field joints can be assembled using a sliding collar or half shell arrangement to give homogeneity of insulation and the maintenance of compliant characteristics across the joint.
For a typical deepwater project a major advantage is that the pipe-in-pipe field joints can be assembled in an efficient way during the J-laying operation.

The systems proposed have been designed such that the outer pipe (the sleeve) is able to slide over the flowline pipe. This allows the sleeve pipe to be closed by means of a single butt weld. Once the flowline has been welded, the sleeve pipe is allowed to slide until it abuts the adjacent sleeve, at which point the butt weld can be performed.

This field jointing design is made possible by the use of specially developed EPDM friction grip bulkheads (a British Steel patented invention). These provide enough friction to grip the flowline and sleeve during assembly, preventing sliding until the selected point in the field jointing process.

This jointing method can also be used in the assembly of stalks for pipelines that are to be installed by reeling. Insulation options for sub-sea applications include PU foam, microporous silica, ceramic microspheres, and polymer cements.

Insulation materials

Specifications and characteristics of the three insulation materials are as follows:

  • PU foams have been widely used in the Gulf of Mexico for the purposes of pipe-in-pipe insulation. An open cellular structure of density 0.03 pounds force/cu ft has a coefficient of thermal conductivity of better than 0.014 Btu ft/sq ft/hr °F (0.025 Watts/meter K). The product can be supplied in half shell segments, sprayed directly onto the pipe or injected into the annulus of the pipe-in-pipe sections.
  • Microporous silica is an open cellular structure comprising bonded ceramic powders with reinforcing ceramic fibers, in which the ultimate cell size is less than the mean free path of an air molecule. This results in the lowest theoretically possible conductivity, typically 0.021 Watts/meter K. The product is supplied in quilted panel form or solid segments, encased within a skin of glass cloth for attachment to the flow line.
  • Alumina silicate microspheres are free-flowing hollow silicate spheres which are watertight, do not creep, and are resistant to thermal and oxidative aging. They exhibit a melting point of 1,400°C with an average collapse depth of 4,500 meters of water. Thermal conductivity values are typically 0.11 Watts/meter K. The specific heat capacity is ~0.78 kilojoules/kilograms K at 80°C. The typical particle size is 30-300 microns, and the density is 425 kg/m.

Thermal performance

Typical heat transfer coefficients are quoted for the pipe body and the overall system. The latter value reflects the contribution of the bulkheads and spacers to the thermal performance. The values are based on the reference plane of the product pipe inside diameter and are calculated relative to the seawater temperature.

British Steel has used computer design programs to calculate the required "U" values based on operational temperature, product physical properties, and flow conditions, and can advise on the required system components and dimensions. The flexibility of the analytical software allows rapid assessment of alternate design options to ensure that the most cost effective and structurally acceptable solution is achieved.

Full-scale project-related thermal conductivity verification trials by an independent third party have previously been commissioned to assess the accuracy of the theoretical British Steel computations. The tests were completed under controlled internal flow conditions.

The theoretical "U" value was shown to be 18% conservative, compared to that determined from the full-scale test.

The use of composite arrangement allows the level of insulation to be tailored to suit project specific thermal and structural criteria, system geometry, and economics. Mathematical modeling can also be employed to predict cool-down characteristics where a minimum temperature is required to be maintained at a specified time period following production shutdown.

Simulated shutdown trials, including field joints, undertaken on the Hydrotherm system for previous qualification trials have concluded that the thermal performance of the individual field joint assemblies was similar or better than predicted mathematically proving the integrity of the British Steel design approach.

Compliant behavior

Based on the principle of compliance, where the outer sleeve is maintained concentrically around the flowline by means of spacers and longitudinally by means of EPDM bulkheads, it can be shown that the carrier pipe (outer sleeve) contributes to the overall section stiffness. For analytical purposes, therefore, an equivalent pipe section can be calculated. This offers benefits in areas such as calculation of allowable spans and mitigation of thermal buckling.

Internal pressure and temperature generates large compressive axial forces in the flowline and at either end of the flowline. Expansion movement relieves these forces. For the pipe-in-pipe system these forces are distributed between the flowline and the sleeve pipe at the ends by means of steel bulkheads designed for the purpose.

At the end of the flowline, the expansion induces a tension in the sleeve pipe which gradually reduces to zero at the point of fixity. The compressive force in the flowline increases in proportion. The end expansion is therefore reduced due to the induced tension in the sleeve pipe.

Buckle resistance

The axial stress and the force to mobilize thermal expansion is induced in the flowline only, the Hydrotherm sleeve serving to lessen the expansion of the flow line due to the ability of the compliant insulation material to translate shear and hence longitudinal force from the inner flow line to the outer sleeve.

The incorporation of the increase in axial stiffness into the conventional theory needs to be accounted for in two forms, one being related to axial strain when deflected, and the other due to axial compliance under restrained thermal expansion.

The first of these is accounted for by assuming that the axial stiffness used in calculating the axial strain in the deflected shape is the product of Young's Modulus and the sum of the cross-sectional area of both pipes. The behavior of the system under unrestrained thermal expansion will result in less extension for a given temperature rise than would in a single pipeline.

The magnitude of the buckle is governed to a large extent by the length of pipeline which feeds into the buckle. To restrict these forces below threshold limits the pipeline can be "snaked" to ensure that it buckles in a predictable controlled manner. The spacing of the route bend apexes, typically a few miles, effectively limits the maximum "feed-in" lengths. This method of lay has already been successfully used in a pipe-in-pipe project in the North Sea.

Full-scale verification trials to determine the minimum allowable post-buckle bend radius on a 16-in. diameter pipe have demonstrated the ability of a fillet welded (leg length 1/2-in.) sliding collar field joint to be bent to a radius of 30 ft in the end of life condition (having previously undergone simulated installation and in-service static and dynamic loading). An extreme curvature representing localized deflection at the apex of a predicted upheaval buckle geometry assuming a 330°F thermal driving force.

Collapse performance

In proposing a pipe-in-pipe solution for a given project, British Steel identifies an outer sleeve pipe to resist collapse at the specified water depth. Wall thickness calculations are performed against normal design codes to show that buckle propagation would not occur.

The choice of insulation material is relevant, as some support the outer pipe allowing a reduction in thickness and hence a saving in overall system weight.

The compliant nature of the microsphere insulant has been shown to offset the onset of ovalization extending the predicted linear and non-linear behavior of the sleeve pipe and thereby significantly increasing the measured bending and collapse response of the system.

Hydrostatic collapse tests undertaken to date have confirmed the improved pressure resistance and the potential for wall thickness reductions associated with a microsphere packed annulus.

For deepwater applications, British Steel has been developing and testing composite insulation incorporating a thermal cement and microporous silica insulation, designed to provide good thermal and structural characteristics. Test results have shown the potential for reducing the overall submerged weight by up to 40% at depths in excess of 5,000 ft, increasing the lay window of conventional barges.

Buckle arrestors can also be provided. Development work on the collapse performance of the systems continues and it may be possible to establish that the EPDM bulkheads in the system act as buckle arrestors. This would eliminate the need for welded steel buckle arrestors saving weight and cost.

Assembly, layability

The fastest lay rates for S-lay are achieved using the sliding collar field joint. A lay rate of some 1.25 miles per day was achieved on the most recent project whilst installing a 2-in. by 0.433-in. outer sleeve, 16-in. by 0.811-in. flow line.

The field joint comprised a sliding collar abutting a swage at one end of the adjacent pipe spool. The joint was completed by means of a partial penetration butt weld at the swaged end and a circumference fillet weld between the collar and sleeve pipe outside diameter at the other.

Extensive project specific 3D finite element modeling of the field joint has been undertaken for several field joint geometries to simulate installation bending and tension loads to determine stress and strain concentrations and post yield behavior as the load is shed from the apex of the curved system. Supportive strain monitored full-scale trials to validate the theoretical predictions were also carried out.

For J-lay deepwater applications, British Steel has identified the means by which multiple spools, such as 80-ft "doubles", 160-ft "quads," or 240-ft "hexes," can be assembled from 40-ft, single pipe-in-pipe spools. This is achieved using the sliding sleeve technology, and can be done onshore or on the lay vessel.

The advantage of building the multiple joints on the vessel is that the logistical difficulties of transporting the relatively slender spool lengths offshore are avoided.

Bulkheads as waterstops

British Steel's EPDM bulkheads can be designed to act as a waterstop to prevent water penetration through the system in the unlikely event of a perforation of the sleeve pipe. Designs have already been successfully produced and tested for more shallow water depths, up to 2,000 ft., and development work is in hand to prove a seal in ultra-deepwater.

Where the bulkhead is used as a seal, finite element analysis is used to predict stress relaxation and consequently the level of induced bolt load required at installation to maintain sufficient sealing pressure over the full design life of the pipeline.

Pipe-in-pipe systems combine a range of technologies to achieve the thermal and structural performance required for a particular application. The requirements of a deepwater project are very different to one at high temperature in shallow water. British Steel has developed a range of insulation and field jointing options.

Acknowledgment

Stephen Triggs of Mentor Project Engineering in Houston assisted in providing test data and material for this article.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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