Depletion occurs as reservoir fluids are produced. As fluids are extracted, the reservoir pressure gradually declines.


Dual gradient drilling is applicable in the deepwater industry. The drilling riser is filled with two different fluids mirroring the environments it runs through – between surface and mudline the riser is filled with seawater, and under the mudline the riser is filled with mud. Reducing the total pressure pushing down on the wellbore and the impact of water depth on well design, the pressure at the bottom of the wellbore nevertheless remains the same as seen in Single Gradient Drilling.

Distribution Manifold

Distribution manifolds, also known as buffer manifolds, are used to divert a single gas or liquid feed line to multiple locations or devices. They are also used to evacuate unwanted fluid or condensation from the vent valve before it gets to the operating machinery and causes corrosion.


An annular preventer used to divert a kick away from the rig.

DP Drillship

Dynamically positioning drillship. Maritime vessels with a drilling rig, drillships are typically used for deepwater exploratory drilling operations. As they are required to stay in the same location for periods of time, various means of positioning equipment is used. Dynamic positioning refers to computer-controlled propellers and thrusters which automatically maintain the drillship’s fixed position despite any movements from wind, wave or currents.

Drill Pipe

Steel pipe supplied in sections, which are joined together by threaded couplings to increase the total length to connect the rig surface to the BHA. The drill pipe rotates the bit and circulates drilling mud.

Drill String

The total combination of the BHA, transitional drill pipe, and the drill pipe in use within the well.

Drilling Hazard Mitigation

Common drilling hazards are lost circulation, gas zones, fluid loss, kicks and differential sticking.

Drilling window

The drilling window is the range of fluid gradients between the pore and fracture pressure gradients. If pressure in the wellbore drops below the pore pressure gradient, an influx is likely to occur. If the pressure in the wellbore exceeds the fracture pressure gradient, well fluids may be lost to the formation and can cause the rock to crack.


A Drill String Isolation Tool closes the annulus and diverts flow using a hydraulically activated packing element.


Diving support vessel. A dedicated vessel for subsea saturation diving and installation work.