The pressure rating of a blowout preventer represents a working pressure differential from the inside to the outside of the body. This paper documents two important points about the subsea use of blowout preventers. They are that working pressure of preventers must be reduced and in general operating pressures increased as water depth and mud weight increase. Working pressure adjustments are required because when a preventer is installed subsea and the drilling fluid (in the riser) is heavier than sea water, there is an initial static pressure difference across the preventer. The amount of this static pressure differential must be subtracted from the published working pressure to obtain the subsea working pressure. Operating pressures must increase because as a machine, the blowout preventer has a closing system that applies sufficient force to a rubber sealing element to contain well pressure. Subsea, the magnitudes and relative importance of the various forces on the rubber element may change from shop measured values. To obtain the same preventer sealing characteristics, the closing pressure may have to be increased. For all the preventers studied, the closing pressure increase required is well within the capability of a conventional rig closing unit. Equations and graphs give the needed pressure adjustment for preventers commonly used subsea.
Numerous tests have been performed on blowout preventers commonly used in subsea operations. These tests by manufacturers and others seek to determine the equipment reliability and frequency of maintenance under various operating conditions. Most testing, however, is done at atmospheric conditions. For subsea operations, the reported forces on the preventer element must be examined and corrected. The analysis presented here shows what pressure adjustments must be made to subject the seal element to nearly the same forces subsea as it would encounter on the surface.
Blowout preventers are designed for a specific maximum working pressure. This pressure is the maximum differential pressure allowed across either the sealing element or the preventer walls. Preventers are initially tested with atmospheric pressure on the outside and the full working pressure on the inside. When used subsea the external ambient pressure is the hydrostatic pressure of the sea water at the depth of the preventer. The internal pressure is the hydrostatic pressure of the drilling fluid column in the riser down to the preventer. Therefore, the maximum effective working pressure (pwork) as measured through a circulating line containing the same fluid as the riser is:
Equation 1
where: (pwork)s is the surface working pressure
Î"p is the difference between the hydrostatic pressure inside the preventer and the ambient hydrostatic pressure outside the preventer.
The factor Î"p is plotted in Fig. 1 as a function of water depth and mud weight. This reduction in pressure, or derating of the preventer, is identical for all preventers regardless of type or manufacturer. For most cases of interest, the reduction is less than 10% of the working pressure commonly used (68,940 KPa).
