Pipe-in-pipe solutions

Considerations for pipe-in-pipe designs for high-pressure, high-temperature fields were set out by Subsea 7 engineers at this year’s OTC. Jerry Lee takes a look.

Subsea 7’s flowline solutions (from left: bundle, direct electrical heating pipe, wet insulated pipe, electrically heat traced flowline, pipe-in-pipe, lined pipe and single pipe). Image from Subsea 7.

When producing from high-pressure, high-temperature (HPHT) fields, oil and gas production retains much of the heat it had in the reservoir. But, when it gets to the pipeline, this heat tends to dissipate to the environment, as it is exposed to low seafloor temperatures. Here the possibility of hydrate, wax, and asphaltene formation, resulting in pipeline blockages, increases, particularly during periods of prolonged shutdowns. One option to limit the cooling effect is to use pipe-in-pipe (PIP) systems.

In order for a single pipe system to be used in a deepwater field, the pipe would need to be covered with thick, multi-layered insulation. However, this would result in a low pipeline specific gravity and cause stability issues, according to a paper presented by authors from Subsea 7 at this year’s Offshore Technology Conference (OTC) in Houston. PIP systems, on the other hand, offer a low overall heat transfer coefficient (OHTC) value (0.5-1 watts per square meter per Kelvin [W/sq m-K]), or U-value, as well as a longer “no touch time” before intervention is needed, says Thurairajah Sriskandarajah, Pasupathy Ragupathy, and Venu Rao, of Subsea 7, in their paper “Design Aspects of Pipe-in-Pipe Systems for HPHT Applications” [OTC-27046-MS].

Some PIP systems are made of two concentric pipes (an inner pipe or flowline, and an outer pipe or carrier pipe) with centralizers, waterstops, and intermediate and end bulkheads. The centralizers are placed between the carrier pipe and flowline to keep the flowline centered, minimizing contact with the environment, which means the flowline is not directly exposed to the cold seawater, reducing the amount of insulation needed. The annulus between the two pipes can be filled with air or a passive thermal insulation, such as aerogels, to reduce heat loss. To improve thermal performance even more (U-value < 0.5 W/sq m-K), the annulus can also be made into a partial vacuum or the annulus gas can be changed, according to Subsea 7.

PIP systems also shield the flowline from hydrostatic over-pressure and potential damage from trawling, anchors, etc. Furthermore, they can enable the use of fiber-optic systems to log production data along the length of the pipeline, leveraging the availability of a dry annulus, according to Subsea 7’s paper, as well as use fiber-optic sensors to detect and monitor pipeline deterioration, according to Subsea 7’s Technology Manager, Gordon Drummond, in the company’s publication, Deep 7.

Other PIP systems can be installed in bundles with or without centralizers. With bundled pipelines, the carrier pipe can host multiple inner pipes or umbilicals, which are allowed to slide along the length of the bundle, except at the ends where they are restrained by the bulkheads. These systems are useful when the subsea field design needs flexibility, such as areas with challenging seabed conditions (e.g. boulders) or areas that are highly congested, says Subsea 7. If greater thermal efficiency is required, a partial vacuum or reduced pressure in the annulus can be combined with an appropriate annulus material.

Recent developments have expanded the depth limits to 1100m for a carrier bundle and 1400+m for open carrierless bundle, according to Martin Goodlad, Subsea 7’s strategic technology manager, Bundles, in Deep 7.

Installation

Bundled systems are towed out in sections, this has the benefit of increased installation weather windows and does not affect the PIP system during installation activities. For other PIP systems, installation offshore can be done using the reel-lay, S-lay, or J-lay methods. Though this may seem straightforward, the installation process can actually affect the PIP system post-lay, says Subsea 7’s OTC paper.

During installation, the bending moment capacity and residual out-of-straightness (OOS) of the pipe will change depending on the load transfer between the inner and outer pipe associated with each method. Post-lay, the carrier pipe is fully straightened. However, the flowline is more sinusoidal when using reel-lay, rather than S- or J-Lay methods. Also, during installation, with the vessel’s tensioners carrying the carrier pipe load and the vessel’s equipment carrying the flowline load, a residual axial compressive load is induced on the inner pipe that has been laid on the seabed. When this compressive installation stress is combined with the hoop stress induced by high internal pressures and the operating conditions, a high equivalent stress close to the pipe’s yield stress can develop, according to the paper. Factor in the differential axial load and bending moment capacities of a section – a result of the variations in yield strength and wall thickness due to manufacturing process – and a localized axial deformation, wrinkling of the inner pipe wall, and eventual failure of the inner pipe can occur, particularly at weak sections.

Installation causes stress on the carrier pipe, as well. During installation, tensioners induce tension on the carrier pipe, and when the pipe is laid on the seabed, the tension is gradually relieved and the carrier pipe goes into compression. At the same time, hoop compression resulting from the hydrostatic pressure causes small axial tension, due to Poisson’s ratio effect that will act together with the installation induced stresses, outlines the Subsea 7 OTC paper.

Corrosive tendencies

If the fluids produced from the HPHT field are corrosive, the PIP system may require corrosive resistant alloy (CRA) lined pipe for the inner pipe. Under these conditions, however, the PIP system can be installed using reel-lay with fresh water in the pipe, so that the internal pressure will prevent wrinkles in the CRA pipe from forming, despite the compressive forces induced during installation. The solution has been successfully applied during the installation of single lined pipe for the Guara-Lula project, and according to the OTC paper authors, the same solution can be adopted for PIP systems with CRA lined inner pipe. During normal operations, the high temperatures can also cause wrinkles, however, the effects of high temperature are overcome by the high pressure, during operation. Though, wrinkles can form when the system is shut-in and compression is induced due to the pressure dropping quicker than the temperature. But, if the residual curvature of the pipe has a nominal strain of 0.4% or less, the issue can be avoided; otherwise, a minimum internal pressure of 7-10 bar may be necessary to mitigate wrinkling issues during shutdown, says the paper authors.

High temperatures in the system can also affect the centralizers and thermal performance. Exposed to high temperatures, the centralizers can be susceptible to thermal creep and progressive deformation. This deformation can then lead to the insulation deforming, which would affect its performance.

How PIP systems are installed also needs consideration. Generally, a PIP system can be trenched and buried, completely rock dumped, or just laid on the seabed. But, as water depth increases, trenching and burying and rock dumping become prohibitively expensive. Along with how the pipe will be laid, engineers must also mitigate the pipe’s tendency for upheaval or lateral buckling and walking-gradual axial displacement of the whole pipeline towards one end or buckle sites creeping from their originally formed positions, the paper says, resulting from pipe-soil interaction, seabed slope, axial ratcheting, and residual bottom tension.

To mitigate buckling, the pipeline can be “snake laid” to form control sites for buckling, laid over sleepers, spot rock dumped, or buoyancy units can be strapped to the pipe to supplement the formation of seabed induced buckling, say the paper authors. The curved section of the snake lay are triggers for buckle mitigations, as well as the sleepers and buoyancy units. To prevent excessive loading on riser connections, spools, and jumpers, walking tendencies can be mitigated by tying the PIP system to suction piles or anchoring the system with clump weights and tether clamps.

The pipeline bundle system expands the limits of HPHT flowline design, the high axial compressive forces generated by the high termperature are balanced by tension in the sleeve and outer carrier pipe. The balanced forces with the added weight of the bundle system mitigates the need for global buckling mitigation measures and therefore offers a cost efficient alternative solution.

HPHT fields can be demanding on the equipment used to produce from them. Engineers not only need to be concerned with the impact of fluid properties on host facilities, but also the properties of the pipeline used to transport them, and the condition in which the pipe is installed.

* Sriskandarajah, T., Ragupathy, P., & Rao, V. (2016, May 2). Design Aspects of Pipe-in-Pipe Systems for HP-HT Applications. Offshore Technology Conference. doi:10.4043/27046-MS