RIASSUNTO
ABSTRACT
The recovery and redeployment of Christmas trees is an important factor in the economics of subsea field development because the time expense involved in well maintenance is a major cost item.
This paper examines 18 trees that have been recovered and redeployed by Petrobras off the Campos Basin in Brazil and analyzes the reasons for recovery and the time involved in completion, reinstallation and tree maintenance operations. As a rule, tree maintenance has been very minor and the tree redeployment operation has generally been performed in slightly less time than the initial installation.
The data presented here are useful for estimating field operating costs of future subsea field developments. Additionally, the recommendations given, based on the experience analyzed, will help design completions that are faster and less expensive to install and maintain, particularly for deepwaters.
INTRODUCTION
The economic viability of a Subsea Production System is affected by the costs of tree installation, recovery and redeployment because they are major expenses during the productive life of the field.
our data show' that during the life of Campos Basin fields, wells have had to be reworked mainly due to changes in downhole conditions. This intervention has taken the same or slightly less time than the first installation. The required maintenance is simple and. relatively inexpensive, involving more tree testing and verification than actual repairs.
In this work we evaluated more than 100 completions performed by Petrobras near the Campos Basin and analyzed the 18 trees that have been pulled and rerun through the period end January 1, 1987. These data are used to estimate the costs associated with intervention operations. Additionally, the discussion points out features that have proven troublesome and recommends design practices for the new generation of trees being designed for waters beyond 1000 m deep.
Previous work on this subject was performed by Gunderson (1) in a post-mortem analysis of a tree retrieved after 20 years of operation and found to be in excellent condition except for minor problems with the valves.
The post-mortem analysis of the Cobia tree by Volkert and Shaw (2) showed that leakage of well fluids into the control system due to scssv failure compromised tree control, and that anode consumption was above that expected. Hammet and I1Jke (3) made a statistical analysis of subsea completion failures, grouping them by categories and concluding that downhole equipment, control systems and connectors are the main sources of subsea completion malfunctions.
Barbosa, Moreira and Robson (4) gave a detailed analysis of the cost and time involved in the installation, recovery and maintenance of two trees that were reinstalled three times and converted from diverless to diver-assisted configurations.
Jouti (5) made an early study of the operational difficulties of subsea completions, pointing to problem areas of those early times, some of which are still troublesome. The methodology employed in this present paper is derived from operations identified in that work.