Vacuum insulated tubing (VIT) has been used successfully to mitigate the potentially harmful effects of annular pressure buildup (APB). In a recent deepwater installation, the subject well had lost alternate APB mitigation capability through a series of events. VIT was then chosen as the only viable technology.
A common design companion of VIT is gelled brine, chosen to decrease annular natural convection driven by heat loss around the VIT connections. There are, however, several drawbacks to the indiscriminate use of gelled brine:
Tight clearances around the tubing hanger running tool, creating the potential for debris plugging and/or tool recovery issues from the subsea wellhead,
Limitations imposed by VIT collapse and hydrostatic packer setting pressures,
Unknown temperature/viscosity response and associated quality control requirements for displacement through a subsea wellhead, and
A need to reduce operational time and cost.
These issues led to the consideration of alternative means of controlling natural convection, and an effort was made to understand, improve, and deploy external coupling insulators.
An in-ground vertical experiment with two connected joints of VIT was conducted with and without coupling insulators. Temperatures were monitored at multiple locations. Test results confirm the effectiveness of external coupling insulators. Several theoretical models using different numerical techniques (finite difference, lumped mass/resistance, finite element, and computational fluid dynamics) were found to be consistent with experimental results. These were compared with critical APB temperatures calculated with a commercial wellbore simulator. The net result of these studies was to adopt the external insulators.
This paper reviews the experimental data and presents several models of a vertically aligned VIT in a deepwater completion. A comparison of thermally effective APB solutions, together with a critical assessment of modeling accuracy, is presented.
Thermal insulation technology is an evolving science covering a wide range of industries and applications, suitable for energy conservation, transport and storage of fluids, and the isolation of critical components, to name a few.
The first oilfield insulated tubing applications, based on silica powder and polyurethane media, were designed to prevent flowline subsidence in the Alaskan tundra. From these simple beginnings, vacuum tubing developed into a highly efficient insulation system capable of carrying large axial loads while fitting within a compact form factor.
The reduction of catastrophic risk associated with annular pressure buildup in a subsea well completion requires specialized technology1. In the late 90's, following one notable2,3,4,5 and several suspected well failures in the Gulf of Mexico, interest in APB solutions increased. The BP Marlin failure was attributed to several potential root causes, one of which was APB. Several companies have recognized VIT as an effective remedy to prevent thermal migration to outer annuli.
In 2002, BP's King West completion was challenged by several unanticipated technical problems. Standard mitigation methods were considered, however, because of wellbore conditions, VIT was chosen as the only viable technology.
VIT consists of an inner and outer tube welded together at both ends. Piping sizes range from 2 in. line pipe to 7 in. pipe in Range 2 or 3. Materials are generally L-80 or 13Cr. The annular space, typically 0.15 in. - 0.5 in. wide, is evacuated and plug welded. This space contains aluminum foil wraps for radiation reduction, and a getter placed to scavenge hydrogen and other deleterious gases in order to preserve vacuum over a typical design life of 10 years.