Subsea and Surface Well Control Systems and Procedures the "Zane Barnes"

ABSTRACT Cooperative efforts between contractor and operator have resulted in a BOP and surface gas handling system of unique, practical, and field proven design. This system meets the demands for both well and riser gas control in moderate to very deep water. Particular features, e.g. subsea BOP and outlet placement, state of the art arrangements, and the rationale behind them are described. Procedures for proper use are outlined. In addition, a recommended generalized approach is presented for preparing deep water vessels to solve the special problems of handling gas in the marine riser. INTRODUCTION The well control and riser gas handling systems on the "Zane Barnes" were designed and built to enable maximum safety in up to 6000 ft. water depths and for the deepest, highest pressure wells to be encountered offshore. Materials for all components with well bore fluid wetted surfaces were prepared for sour service. The special problems associated with gas entrapment in the stack during a kill and the expansion of this gas in the riser after its release and its migration to surface were successfully addressed. Figure 1-System Schematic illustrates key features of the system. The subsea BOP stack is an 18-3/4" bore unit with four 15,000 psi WP rams and two 10,000 psi WP annulars. Control is by a multiplexed electro-hydraulic system. The 21", 5/8" wall marine riser carries two 3" ID, 15,000 psi WP choke/kill lines, a 2.25" ID hydraulic power fluid conduit, and a 3.375" ID riser circulating line. Surface equipment includes a 3" 15,000 psi WP choke/kill manifold, a main and a "mini" diverter system, two independent mud/gas separators, one of which is dedicated to the riser and one to the choke manifold. In addition to the primary trip tank, a small "mini" trip tank is attached to the choke manifold. Instrumentation includes a state of the art electronic as well as a mechanical monitor for the main trip tank, mud pit total volume, and a heave corrected flow line monitor. No system, and especially not a comprehensive one like that on the Zane Barnes, is workable without carefully thought out procedures and thorough crew training. This has been done, and the reward has been the handling without incident of one complex well control situation and several smaller ones during the first year of operation. There is a high degree of confidence that this can be done whenever required. In the succeeding discussions of individual sub-systems emphasis will be placed on the unique features, the remainder being mutt the same as those used in floating drilling with subsea BOP systems worldwide. SUBSEA BOP STACK As illustrated in Figure 2, the subsea BOP has an outlet between annulars. This is used to minimize trapped gas volume during kills. Procedures call for two-line kills in all "routine" instances, using the upper annular BOP and the uppermost outlets on both the choke and kill sides.