The void between the drill string and the casing of the well borehole.


American National Standards Institute. The ANSI coordinates development of safe systems, processes and products through standards development and conformity assessment programs. Standards provide dimensions, ratings, terminology and symbols, test methods, and performance and safety requirements for personnel, products, systems and services in hundreds of industries.


American Society of Mechanical Engineers

Auxiliary Line

Service lines running parallel to the riser. These include control system fluid line, buoyancy control line, mud boost line, hydraulic supply.

Back Pressure

The pressure existing at the surface on the casing side of the drill string/annulus flow system.

Ball Joint

A ball and socket assembly that has a central through-passage equal to or greater than the internal diameter of the riser. It may be positioned in the riser string to reduce local bending stresses.


The bottomhole assembly (BHA), is the lower portion of the drill string, from the bit to drill pipe. It includes the bit, drill collars, stabilizers and other drilling components.


An uncontrolled release of underground pressure or fluids into the wellbore, or to the surface. Such events can be catastrophic to the surroundings, causing of loss of life, environmental damage, and damage to property.


A blow out preventer (BOP) is a large valve at the surface of a well which closes off the wellbore. It is activated by hydraulic actuators due to loss of pressure control, or due to well interventions.

Box and Pin Stabs

A conventional tool-joint, a mating box and pin assembly which provides pressure-tight mating of two pipe joints.


A contingency well kill operation. Fluid is forcibly pumped into a formation to block well bore fluids or gas reaching the surface, or to force them back into the reservoir. Bullheading can be performed as a contingency operation with minimal advanced planning, however it can present subsequent safety and environmental risks.


Constant bottom hole pressure. Bottomhole pressure is kept constant during connections to compensate loss of AFP when the mud pumps are off.


A mechanical device which controls the flow of oil or gas in a well or flowline.

Christmas Tree

Assembled at the top of a completed well both surface or subsea, the Christmas Tree is an assembly of valves, fittings, chokes, and gauges used in monitoring and controlling the flow of hydrocarbons and other fluids from production or injection wells. Their primary function is to control the flow out of the well.


Closed loop drilling.

Connection Gas

A small influx of gas in the drilling fluid when a pipe connection is made.


Term referring to a range of mechanical devices used to provide a pressure containing structural connection between adjacent end fittings. May consist of a flange arrangement bolted to the end fitting, or an assembly comprised of clamped hub, gasket and seal ring(s), pulled into place with hydraulics and locked in.

Conventional Drilling

A conventional drilling operation (also referred to as open-loop drilling ) uses the drilling fluid hydrostatic pressure to balance between the pore and fracture pressure gradients without the assistance of an annular pressure control system. Use of a control head at the surface does not directly imply a rig is using MPD.

Crossover Joints

In Riser gas handling, crossover joints (used with riser flanges) provide integration of the RGH system with the rig’s riser below the tension ring and the termination joint.


Low temperature processing, generally subzero. For LNG this can be as low as -162˚C.


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.


Extended factory acceptance test


A detachable hinged grip mechanism that may be placed around the drill pipe to lift or lower it into the wellbore.

Entrained Gas

Formation gas that enters the drilling fluid in the annulus.


Engineering procurement construction. A prominent form of contracting agreement, where the engineering and construction contractor carries out the detailed engineering design of the project, procurement of necessary equipment and materials, and then construct to deliver a functioning facility or asset to their clients. Also called the execution phase, this phase usually follows the FEED.


Engineering Procurement Construction and Installation. A common contracting arrangement. Unlike an EPC contract, the offshore facility or vessel that is constructed has to be transported and installed at the project location.


Factory acceptance test


Finite element analysis


Front end engineering design. The FEED study is conducted after Conceptual Design, or the Feasibility Study. It includes basic engineering, focusing on project specific technical requirements and rough investment cost for the project. The FEED package is used as the basis for bidding the Execution Phase Contracts such as EPC or EPCI, and is used as the project design basis.

Flex Joint

Flex joints are flange assembles which allow for angular misalignment between the riser and the BOP, reducing stresses and reaction forces.

Flow Spool

Routes diverted flow to the rig or MPD choke manifold.


Floating, Production, Storage and Offloading. A floating ship-shaped vessel which contains processing equipment and storage for hydrocarbons in a double hull. Once processes, an FPSO stores oil or gas before offloading processed petroleum to shuttle tankers or through pipelines.

Frac Gradient

The slope and pressure (psi/ft) at which a rock formation fractures or cracks and accepts fluid.

Frac Pressure

The pressure required to cause fractures in rock.


Flowline termination assembly


A seal clamped between two mating surfaces to prevent leakage of fluid or gas. Gaskets are made of deformable materials so they can be forced to mold to the space between the surfaces, compensating for any irregularities which could cause leak paths.


Protects the riser string by acting as a shock absorber. It sits underneath the spider and assists it supporting the weight of the drill string.


Hazards and Operability Study. Team meetings aiming to examine planned or existing process in order to identify and evaluate possible risks to personnel, equipment, or efficient operation.


Pressure based on fluid column


High pressure high temperature. An HPHT environment has a pressure rating greater than 15,000psi, and a temperature rating greater than 350°F.


Hydraulic power units deliver steady hydraulic pressure and flow.


Health Safety and Environment


Health, Safety, Security and Environmental


The flow of fluid from the formation into the wellbore, such as gas, oil, salt water.


International Organization for Standardization. Comprised of 162 member countries, ISO provides specifications for products, services and systems to ensure quality, safety and efficiency.


A section of flexible pipe used in subsea and topside, static, or dynamic applications. With connectors at either end, subsea jumpers connect flowlines and subsea facilities together, such as a flowline to a riser.


The undesirable entry of formation fluid into the wellbore during drilling operations. Kicks occur when formation pressure is greater than the hydrostatic pressure in the wellbore.


Liquefied natural gas. Natural gas (NG) condensed into liquid form once condensates are extracted, for ease of transport or storage. NG in liquid form is 1/600th the volume of NG in its gaseous state. The gas is cooled to approximately −162 °C (−260 °F).


An arrangement of piping or valves designed to collect or distribute a fluid to or from multiple flow paths, also including points to connect flowlines or umbilicals. Subsea manifolds can be used to gather produced fluids and direct selected wells to a well test line, as well as to distribute injected fluids (gas or water) or gas lift gas to individual wells.


A support structure placed on poorly consolidated, soft or unstable seabed as a footing for structures such as manifolds, jack up rigs etc.


Mud gas separator. A device used to safely vent pockets of free gas from the drilling fluid system, encountered while drilling.


Opening in the hull of a drilling platform, drilling platform or vessel allowing equipment to be past into water in controlled and safe conditions.


Managed Pressure Drilling is an adaptive drilling process used to precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly. It is the intention of MPD to avoid continuous influx of formation fluids to the surface. Any influx incidental to the operation will be safely contained using an appropriate process.

MPD wellbore sealing device

A device which is made up above the BOP which creates a pressure-tight obstruction in the annulus which blocks the further passage of fluid and allows the annulus to be pressurized from the surface while drilling. When an MPD wellbore sealing device is in use, return fluids must exit the annulus through a flow spool or return flow line. ACDs and RCDs are both types of MPD wellbore sealing devices.


Liquid drilling fluid. Drilling mud provides hydrostatic pressure on the borehole wall, lubricates and cools the drill bit and suspends cuttings so they can be carried out.

Mud Weight

The density of the drilling mud, measured in pounds per gallon (lbm/gal) (ppg) or pound cubic feet (pcf).


Standards were developed by the Norwegian petroleum industry. The specifications serve as references in the authorities regulations. NORSOK is an acronym for: Norsk Sokkels Konkurranseposisjon.


A commonly used acronym in the oilfield meaning ‘Non-Productive Time’.

Over-pressured zones

Over-pressured zones are formations in which the pore pressure exceeds the hydrostatic pressure gradient. These may occur due to fluid migration from deeper formations and fluids being trapped and heated.


An abbreviation for ‘pipeline inspection gauges’, pigs are used to assess internal pipeline conditions, to clean, and perform maintenance, known as pigging. The name is thought to derive from the squealing sounds early devices made.


Pipeline end manifold.


Pipeline end termination. A termination structure between the main pipeline and a rigid spool or flexible jumper.


Pressurized mud-cap drilling. One of four common drilling techniques in MPD, used to mitigate fluid loss when drilling in depleted zones or formations with fractures or large voids, such as caverns. All drilling fluids and gases produced are returned to the formation during drilling, maintaining pressure in the formation and the annulus fluid column. This mitigates drilling hazards such as fluid loss, kicks and differential sticking. PMCD deduces costs and NPT, and improves safety.

Pressure gradient variations

Pressure gradient variations occur due to the presence of multiple formations, each with its own pore and fracture pressure. As different types of rock are drilled, it becomes necessary to set casing to isolate low pressure zones from higher pressure zones.


The process of bringing formation fluids to surface in order to process hydrocarbons for sale.


Pressure relief value is a required safety device that releases gases and liquids to control or limit pressure within a system or a vessel.


The rotating control device seals around the drillstring while it rotates during operations, to stop hydrocarbons or other fluids releasing into the atmosphere.


Riser Gas Handling. Integral to the drilling riser, AFGlobal’s RGH system provides solutions for retrofit and newbuild applications using specialty riser joints and equipment.


A pipe within which fluids flow upwards, for example, from a wellhead to a platform. In a drilling riser, drilling mud and cuttings flow back up the annulus to the surface, after the mud has passed downwards to the BHA, through the riser’s internal drill pipe.

Riser Adapter

Crossover between riser and flex/ball joint.

Riser Handling Tool

Tooling to enable the safe lifting and deployment of single or multiple risers offshore. It attaches to the upper end of a riser joint, and lifts and lowers it in the derrick.

Rotary Table

Device that joins to the upper end of a riser joint to permit lifting and lowering of the joint and the assembled riser string in the derrick by the elevators.


Remotely operated vehicle (ROV). An unmanned, free-swimming subsea vehicle controlled remotely from a vessel or offshore platform. The ROV is equipped with manipulator arms and tools that enable it to perform tasks such as valve operations or hydraulic functions and other general tasks. ROVs can also carry tooling packages for undertaking specific tasks such as pull-in and connection of flexible flowlines and umbilicals, and component replacement.

Running Tool

Device that joins to the upper end of a riser joint to permit lifting and lowering of the joint and the assembled riser string in the derrick by the elevators.

Seal assembly

A seal assembly is the removable portion of wellbore sealing device.


A floating vessel, platform or rig. Ballast tanks within submerged pontoon columns are filled with water, enabling the structure to achieve stability when anchored on-site.


Metal gripping elements which hold the drill pipe in place.


Society of Petroleum Engineers


A set of slips which support the riser when it is being run or retrieved, or while connections are being made or broken.


A short section of rigid pipe with flanges or connections at each end. Created to the exact length required for make-up, spools are commonly used to connect flowlines and or subsea facilities together.

Stick Slip

A sudden jump in velocity from standstill as a reaction to built-up torsional energy along the length of the drill string. While ‘stuck’, the bit stops drilling but torque continues to be applied from the turning rotatory table on the rig floor. The build-up of torque causes the bit to giveaway and ‘slip’, resulting in a significant increase in rotational speed. Vibrations are sent along the drill string, which can cause damage.

Syntactic Foam

A composite material consisting of a polymer matrix containing hollow particles or microspheres.


Target depth. The depth the producing formation is expected to be encounter, or the depth to which a well is being drilled. Target depth is measured along the length of the well bore, and as such is not to be considered a true vertical or an independent measurement.

Telescopic Joint

Positioned at the top of the riser, the telescopic joint is comprised of an inner and outer barrel which can be stroked to compensable for rig heave or movement placed on the riser.

Tension Ring

Attached to the telescopic joint, it is used to pull tension on the riser string, to increase stability. The tension ring forms a termination point between the riser system and tensioning system on a floating rig.


The action of connecting one pipeline to equipment, or to another pipeline.

Tie-In System

Connection equipment, typically connecting flowlines, umbilicals and jumper spools. Tie-In systems are predominantly vertical or horizontal application methods.


True vertical depth. Measured vertically from a point in the surface to a certain point below. In comparison, TD is the length measured along the path of the well bore, which might not be vertical.


Underbalanced operation. Activity where pressure exerted in the wellbore is intentionally less than the pore pressure in any part of the formations, in order to bring formation fluids to the surface.


A flexible hose used to connect instruments, for the transmission of electrical or hydraulic signals, control fluid, chemical injection, or fiber optic communications to and from a power source to a remote station. They can be single or multiple connections bundled in a single line. Umbilicals connect surface installations to subsea developments, such as a tree or manifold.

Umbilical System

AFGlobal’s Umbilical System has an integrated RGH/RCD umbilical for HMI interface, HPU power, and PLC communications between the rig and the RGH riser assembly.

Under-pressured zone

Under-pressured zones are formation in which the pore pressure is less than the hydrostatic pressure gradient. These may occur due to fluid migration or production depletion .

Weather Window

A period of good weather suitable for scheduling drilling or installation operations offshore, before periods of bad weather. Weather windows can be short term, ie between storms, or long term, ie the summer season.

Well Kill

A term referring to a range of oil well control procedures performed to stop a well from flowing.


Temporary storage of subsea equipment after deployment but before installation.


Work-class remotely operated vehicle. A large ROV typically equipped with standard set of tools, a five-function grabber and seven-function manipulators. They also have controls that enable a range of additional tooling packages to be operated without the need for a dedicated umbilical system.