DeepStar case study: flowline burial for insulation*

Abstract

Canyon Offshore, Multiphase Solutions and ASGM Engineering recently completed a study for DeepStar to assess flowline burial for insulation for oil and gas deepwater subsea tiebacks with high-pressure/high-temperature (HPHT) reservoir characteristics. The primary goal of the study was to document the scope of application, added value and schemes for successful implementation of burial for insulation of deepwater HPHT flowlines.

The prize in oil production is the extended cool-down time (three plus days), which can potentially reduce the number of occasions where flowline cold shut-in preparations are necessary. The principal prize in gas production is ability to deliver high water-rate gas production, by removing the need for continuous hydrate inhibitor and extending production beyond the point where inhibitor delivery is inadequate.

Lateral and upheaval buckling analyses assessed the expected interaction between flowline, soil and movements associated with expansion due to temperature. Controlled installation sequencing to allow for lateral buckling during the initial startup phase was evaluated to mitigate the risk of excessive bending strains and upheaval buckling in the flowline from thermal expansion. The results indicate installation methodologies can achieve reduced bending strains and avoid upheaval buckling.

Net present value (NPV) calculations were tabulated for the various tieback scenarios to assess the economic merits of flowline burial versus non-burial. The results of the study indicate that burial provides improved thermal insulation characteristics and yielded positive NPV, compared to unburied flowlines. The risk of thermal-induced lateral and upheaval buckling can be mitigated using identified techniques during the installation phase of the work to avoid undesired high bending strains.