Spill prevention and Response

Offshore Well

offshore Oil and natural gas deposits are pressurized by the weight of the layers of soil and rock above them. When an exploratory drill taps into one of these deposits, the seal is broken so that the pressurized oil and/or gas flows up and out of the well. Modern wells control the pressure of the flow to keep oil and gas from escaping into the environment. For example, most wellheads are equipped with two types of blowout preventers (BOPs) and additional shut-off valves are installed on the lines carrying oil and waste products to field processing centers. All of these safety devices are linked to a master switch that can shut down the entire system in the event of severe weather or other potentially hazardous circumstances.

Prevention Tactics and Technologies

Blowout Preventers (BOPs)

Introduction - Modern drilling practices control well pressure to keep oil and gas from escaping into the environment. A well's mud and casing program are the primary means of controlling subsurface well pressures. Additionally, a series of large valves and other devices - known individually as a blowout preventer (BOP) and collectively as a BOP stack - are installed on top of the well, allowing personnel to manage pressure increases or close the well if necessary.

BOP Function

If, during drilling, indications are that fluids may be entering the wellbore from subsurface formations - a condition known as a kick - the first response is to manually activate the BOP, which isolates the wellbore from the surface and closes the well.

BOPs can also activate automatically in case of pressure decrease in the well or increase in fluid flow up the wellbore.

Annular BOPs

are used to seal off blowouts in the space between the wall of a well and the well pipe or drill string. The sealing element resembles a large rubber doughnut that is mechanically squeezed inward to close tightly around the pipe.

BOP Operation and Types

BOPs come in a variety of styles, sizes and pressure ratings. Depending on the level of urgency and need for response, the specific combination of BOPs in a BOP stack provide the drilling crew with series of response options in the event of a significant pressure change.

By closing the appropriate valve or series of valves, the drilling crew can establish control of the formation and then initiate procedures to increase drilling mud density until the BOP can be opened and formation pressure controlled.

An annular BOP is essentially a large rubber donut-shaped ring that, when activated, squeezes against any object in the hole, thereby closing the well.

There are also a series of BOPs known as rams. Ram BOPs quickly seal the top of a well in the event of kick. They consist of two halves of a well cover, split down the middle. Large-diameter hydraulic cylinders, which are normally retracted, force the two halves together to seal the wellbore. The covers are constructed of steel for strength and fitted with elastomeric components on the sealing surfaces.

A pipe ram uses two round half plates with a gap in the middle that, when closed, allows room to accommodate the drill pipe in the well.

Blind rams use two round half plates with no gap in the middle and are designed to close over an open wellbore, or one that has no drill pipe or other object in the well.

A shear ram is fitted with two hardened-steel shearing surfaces that can actually cut through drill pipe and close to seal the well.

Most BOP stacks contain at least one annular BOP at the top of the stack and one or more ram-type BOPs below.

Summary

Because BOPs are critical to the safety of the crew, the rig and the wellbore itself, they are inspected, tested and refurbished at regular intervals.

Drilling mud and casing are the primary means of controlling subsurface pressures to prevent oil and natural gas from reaching the surface. The BOP stack is the secondary line of defense to prevent formation fluids from reaching the surface.