GATE Petroleum Engineering Offshore Drilling and Production Practices Syllabus

What is a marine drilling riser?

A large-diameter pipe connecting the subsea BOP (on the wellhead) to the floating drilling vessel. It guides the drill string, returns drilling mud to the surface, and houses the choke, kill and other auxiliary lines.

What is the purpose of a telescopic (slip) joint in a marine drilling riser?

It is placed at the top of the riser to compensate for the relative vertical motion (heave) between the floating vessel and the fixed riser/BOP, allowing the vessel to move up and down without imposing axial load on the riser.

Why are buoyancy modules and a riser tensioner system used on a marine riser?

Buoyancy modules reduce the riser's submerged weight, and the riser tensioner applies constant top tension to keep the long riser in tension (preventing buckling/collapse) despite vessel motion and weight.

Differentiate a drilling riser from a production riser.

A drilling riser is a temporary, large-bore conduit used only during drilling to carry the drill string and return mud. A production riser is a permanent conduit that carries produced hydrocarbons (and injection fluids/control lines) from the seabed/subsea wells to the production facility throughout field life.

What is the difference between a dry (surface) tree and a wet (subsea) tree completion offshore?

A dry-tree completion has the wellhead/Christmas tree located on the platform above the waterline (accessible for intervention, used with TLPs/spars). A wet-tree (subsea) completion has the tree installed on the seabed, controlled remotely, allowing wells distant from the platform and use of floating production vessels.

What is a subsea Christmas tree?

An assembly of valves, chokes and fittings installed on the subsea wellhead that controls and regulates the flow of produced fluids (or injected fluids) from the well, located on the seabed.

What is a subsea manifold in offshore well completion?

A seabed arrangement of piping and valves that gathers and combines production from several subsea wells (or distributes injection fluid) into common flowlines, reducing the number of risers/pipelines to the host facility.

Define 'tieback' in the context of subsea completions.

Connecting a subsea well or template, via flowlines/risers, back to an existing host production facility (platform or FPSO), allowing satellite or marginal fields to be developed economically without a dedicated platform.

What functions are performed on an oil processing (production) platform?

Separation of the well stream into oil, gas and water; oil treating/stabilization; gas compression and dehydration; produced-water treatment; and metering/pumping of stabilized crude for export.

What is the primary purpose of a three-phase separator on a production platform?

To separate the incoming well stream into its three phases—oil, gas and water—using gravity and retention time, so each can be further processed, exported or disposed of.

Why is gas-oil separation often done in stages (multistage separation) on a platform?

Reducing pressure in successive stages maximizes the amount of stabilized liquid (stock-tank oil) recovered and minimizes light-component losses, improving liquid recovery and producing a more stable crude with lower vapour pressure.

What is the function of a water injection platform?

To treat seawater (or produced water) and pump it at high pressure down injection wells into the reservoir for pressure maintenance and secondary recovery (waterflooding), sweeping oil toward producing wells.

List the main seawater treatment steps before water injection.

Coarse/fine filtration (removal of solids), deaeration/de-oxygenation (to limit corrosion), biocide and oxygen-scavenger/chemical dosing, and scale-inhibitor injection—then boosting and high-pressure injection pumping.

Why must dissolved oxygen be removed from injection seawater?

Dissolved oxygen causes severe corrosion of pipework and wells and promotes bacterial growth (e.g. souring); deaeration towers and oxygen scavengers reduce $\ce{O2}$ to a few parts per billion.

What is reservoir voidage replacement and how does it relate to water injection?

It is replacing the underground volume of produced fluids with injected water (or gas) to maintain reservoir pressure. The Voidage Replacement Ratio is $$\text{VRR}=\frac{\text{reservoir volume injected}}{\text{reservoir volume produced}}$$ with a target near 1 to keep pressure stable.

What is an FPSO?

A Floating Production, Storage and Offloading vessel: a ship-shaped (or converted tanker) floating unit that receives, processes and stores produced hydrocarbons and periodically offloads crude to shuttle tankers. It has no export pipeline requirement and is ideal for remote/deep fields.

What does the acronym FPSO stand for, and how does an FSO differ?

FPSO = Floating Production, Storage and Offloading. An FSO (Floating Storage and Offloading) only stores and offloads crude (no processing facilities); it receives already-processed oil from a nearby platform.

What is a Single Point Mooring (SPM)?

An offshore loading/mooring buoy anchored to the seabed that allows a tanker to moor at a single point and weathervane (rotate freely) around it with wind/current, while transferring crude oil through a swivel and hoses/risers.

What does SBM stand for and what is its relation to SPM?

SBM = Single Buoy Mooring, a common type/term for a Single Point Mooring (SPM). Both denote a single anchored buoy that lets a moored tanker weathervane while loading/offloading oil; 'SBM' often refers to the buoy hardware itself.

What is the function of the rotating/swivel assembly in an SPM/SBM buoy?

The fluid swivel allows the moored tanker (and the buoy's rotating upper part) to weathervane 360° around the fixed lower buoy while maintaining continuous, leak-free transfer of crude oil from the fixed seabed risers to the rotating export/transfer hoses.

Why is the 'weathervaning' ability of an SPM important?

It lets the moored tanker freely align itself with the prevailing wind, wave and current direction, minimizing environmental loads on the mooring and allowing safe loading in open, exposed offshore locations.

Name two common types of single point mooring systems.

Catenary Anchor Leg Mooring (CALM) buoy—a buoy held by several catenary anchor chains with the tanker moored by hawser; and Single Anchor Leg Mooring (SALM)—a buoy held by a single articulated/tensioned anchor leg to the seabed.

How does a CALM buoy differ from a SALM system?

A CALM buoy is held by multiple catenary anchor chains spread around it, and the tanker connects via a hawser. A SALM uses a single vertical (often articulated, buoyant) anchor leg providing the restoring force; it has a smaller footprint and different motion characteristics.

What advantage does offshore crude storage in a GBS, FPSO or tanker provide?

It buffers continuous production against intermittent tanker offloading, eliminating the need for an export pipeline to shore and allowing economic development of remote fields where the crude is shuttled away by tankers.

What 'utilities' are typically provided on an offshore production platform?

Power generation, instrument/utility air, fuel gas, fresh/potable water, firewater and fire-fighting systems, chemical injection, drains and waste handling, HVAC, inert gas/nitrogen, and crane/material handling—supporting the main process systems.

What is flaring and why is a flare system a key utility offshore?

Flaring is the controlled burning of excess or relief gas. The flare/vent system safely disposes of gas from pressure relief, blowdown and process upsets, protecting equipment and personnel; it is a mandatory safety utility on production platforms.

Why are offshore platforms designed with a buoyancy/wave loading consideration using the Morison equation?

Because hydrodynamic wave and current forces on slender members must be estimated for design. Morison's equation gives the force per unit length as a sum of drag and inertia: $$dF = \tfrac{1}{2}\rho C_D D\,|u|u\,dz + \rho C_M \tfrac{\pi D^{2}}{4}\dot{u}\,dz$$

How does water depth generally dictate the choice of offshore structure (shallow to ultra-deep)?

Shallow water: jack-ups and fixed jacket/GBS platforms. Intermediate: compliant towers. Deep water: TLPs, Spars, semisubmersibles. Ultra-deep: Spars, drillships, and subsea tiebacks to FPSOs—fixed structures become uneconomic as steel weight rises sharply with depth.

What is the key station-keeping difference between a TLP and a Spar?

A TLP is held by vertical high-tension tendons that use excess buoyancy to eliminate heave, whereas a Spar relies on its deep-draft, low-centre-of-gravity hull (with catenary/taut mooring) to minimize heave; the Spar floats with conventional mooring while the TLP is tethered taut.

What is a subsea template and why is it used in offshore completions?

A pre-fabricated steel framework set on the seabed that locates and guides the drilling of multiple wells in a fixed pattern and supports wellheads/manifold connections, organizing cluster/subsea developments and easing tieback to the host facility.