1) dual casing riser;

2) single casing riser; and

3) tubing riser.

Each riser system was analyzed at two water depths (4,000'and 6,000'), with two platform types (SPAR and TLP), and with both 6 and 12 wells tied into the platform. Phase I of this study (MMS did not participate) estimated costs for capital expenditure and operating expenditure for different dry tree riser configurations.

The study used fault tree analysis to determine the safety of the three riser designs. All of the scenarios mentioned above were evaluated. The study also looked at both normal producing operations and the safety of well interventions (i.e. workovers). Statistical failure records were used where available, but "engineering judgment " was often used to assign reliability values to components.

Results of the analysis can be summarized as follows: environment. This system benefits from the fact that dual independent barriers exist even without a mud column during well interventions. environment. ( This seemingly counter-intuitive result is explained by the fact that the tubing riser is removed and a separate workover riser is installed for well interventions). environment and the highest calculated total cost.