Pipeline temperature control in high pressure high temperature environments

The exploitation of hydrocarbon reserves within offshore oil and gas is strongly linked to several thermodynamic processes that determine the state and nature of the product at the point of arrival at the processing plant. Changes in temperature and pressure are the principle drivers as these also steer changes in phase, composition and viscosity.

Considerations of these complex interactions should be at the heart of flow assurance, impacting the design, development and ultimately, the reliability of installations. Temperature is the easiest change to manipulate, by controlling thermal loss with specialist insulation, or by the provision of energy to the system through electrical or hot water heating. Integrating these two approaches can provide a particularly robust and flexible level of control, expanding operating windows.

Subsea thermal insulation, based on polymeric materials, is a proven solution for ensuring predictable heat loss in flowlines, risers, manifolds, Christmas trees and peripherals.  Materials such as epoxies, urethanes, silicones, polyolefins and rubbers, have all been used successfully within their respective operational capability.

Over time much of the "easily available" offshore oil and gas reservoirs have been exploited with Greenfield sites typically being found in deeper water. This can cause issues due to the more demanding conditions in these environments.

Closed cell foams such as blown polypropylene, polymer syntactic polyurethanes and light urethane foams, widely used in the 0 to 300-meter interval, are unsuitable for deep water environments. They are no longer capable of operating without excessive compaction and the corresponding deterioration in performance.

Extended water depth capability for foamed materials can be achieved using mechanical foams, or "syntactic" materials with small hollow glass microspheres dispersed in the polymer matrix. This offers an alternative solution to the above for water depths of around 2,500 meters.

For ultra-deep-water environments greater than this, only incompressible, solid materials can be reliably used. These are generally associated with higher thermal conductivity and thereby poorer performance. Deeper wells with significantly higher operating temperatures also pose a challenge for both the insulation system in terms of chemical degradation or structural collapse and the corrosion protection system. Current requirements of an operating temperature of +150 oC to +180 oC, limit the number of viable material types suitable to less than a handful.

Recent developments within rubber technology have produced an innovative hand-applied product for application to Christmas trees, manifolds and jumpers. This high temperature rubber compound, capable of continuous hot-wet exposure at up to +180 oC, is an incompressible compact material containing no glass microspheres and has a low thermal conductivity of 0.165 W/m/K.

Hand-applicable or "pack in place" systems have been discussed for HPHT environments over many years and have some distinct advantages over castable systems during the application phase.  As there is no requirement for casting molds or tooling, there is a significant reduction in front-end engineering, design and manufacturing time.  Hand-applicable systems are quick and easy to mobilize on site. Using automated mixing equipment, with a low user threshold, enables a high level of local manpower, allowing for increased project flexibility.

Vulcanization temperature and time, along with a requirement for using autoclaves in the curing process have previously been concerns for project operations, limiting the applicability of rubbers for the insulation of complex structures. The development of low temperature vulcanization chemistries has obviated these concerns. Today, high operating temperature rubber systems can be vulcanized under control at temperatures as low as +40 oC without the need for autoclaves.

In conclusion, with new developments in technology, rubber systems are seeing a resurgence for high temperature, ultra-deep-water fields, having advantageous thermophysical properties, convenient application practices and a high level of application flexibility.