Offshore Drilling and Production Practices: Ocean Environment, Buoyancy and Stability, Platforms and Mobile Units, Station Keeping, Risers, Subsea Systems and Offshore Production

Offshore work follows the onshore sequence — exploration, appraisal, development drilling, production and abandonment — but every operation must survive the sea. The ocean environment loads a structure through waves (the dominant load for most structures), currents (tidal, wind-driven and ocean circulation), wind on the topsides, tides and storm surge that change the water level, and in some regions ice, earthquakes, seabed instability and marine growth that adds weight and drag. Design uses metocean criteria: extreme conditions a structure must survive, and operating conditions within which drilling or offloading can continue. In deep water (depth greater than half a wavelength) the wavelength of a regular wave of period T is L = gT²/(2π), about 156 m for T = 10 s, and water-particle motion decays exponentially with depth.

Wave force on a slender cylindrical member of diameter D is given by Morison’s equation, the sum of a drag term and an inertia term per unit length: f = ½ρC_D D u|u| + ρC_M (πD²/4)(du/dt), where u is the water-particle velocity, ρ the sea-water density (about 1025 kg/m³) and C_D and C_M the drag and inertia coefficients. Drag dominates for slender members in large waves; inertia dominates for large-diameter members, and for very large bodies (compared with the wavelength) diffraction theory replaces Morison.

A floating unit obeys Archimedes’ principle: it sinks until the weight of water displaced equals its own weight, so its displacement is Δ = ρV. Its stability depends on three points on the centreline: the keel K, the centre of gravity G, and the centre of buoyancy B (the centroid of the displaced volume). When the unit heels through a small angle B moves sideways, and the vertical through the new B cuts the centreline at the metacentre M. The metacentric height GM = KB + BM − KG, with BM = I/V, I being the second moment of the waterplane area about the heel axis. If M is above G (GM > 0) the buoyancy and weight form a righting couple and the unit is stable; if G is above M it capsizes. For a box-shaped barge of beam B and draft T, BM = B²/(12T) and KB = T/2. Raising deck loads raises G and reduces GM; slack tanks reduce it further through the free-surface effect. A semi-submersible gets its stability from widely spaced columns, whose waterplane has a large I.

A floating unit must be kept over the well within a small fraction of the water depth, or the riser and wellhead are overstressed. Mooring does it passively: a spread mooring of catenary lines (chain and wire whose hanging weight provides the restoring force) or, in deep water, taut lines of polyester rope anchored by suction piles or drag anchors; a ship-shaped unit may instead use a turret mooring that lets it weathervane into the prevailing weather. Dynamic positioning (DP) does it actively: position references (satellite navigation, hydroacoustic transponders, taut wire), wind and motion sensors and a computer that drives azimuthing thrusters to cancel the environmental forces. DP needs no anchors and suits very deep water and frequent moves, at the cost of fuel and dependence on power and control systems; DP equipment classes set the level of redundancy against single failures.

From a fixed platform or a jack-up the well is drilled much as on land, with a surface BOP on top of a conductor that runs from the deck to below the seabed; the platform rig drills many deviated wells from one jacket through a well-slot template. From a floating unit the BOP sits on the seabed wellhead (subsea BOP), and a marine drilling riser — a large-diameter pipe with choke, kill and booster lines — joins it to the rig and carries mud returns back to surface. Because the rig heaves with the waves while the riser and string must not, the riser is held by riser tensioners at the top and passes through a telescopic (slip) joint, and a heave (drill-string) compensator keeps the weight on bit steady; a flex joint at the bottom accommodates the rig’s offset. The conductor is the first string, jetted or driven into the seabed, that carries the wellhead and the first loads.

Production risers carry well fluids to the host: top-tensioned risers for dry trees on a TLP or spar, steel catenary risers hanging freely in a curve from a floater, and flexible risers of layered steel and polymer in lazy-wave or other configurations for large motions. Umbilicals bundle hydraulic control lines, electrical power and signal cables and chemical-injection tubes to operate and monitor subsea equipment. Offshore completion is either dry-tree — the Christmas tree on the platform deck, with direct access for wireline and workover — or wet-tree — a subsea tree on the seabed wellhead, controlled through the umbilical, which needs a vessel for any intervention but allows wells far from any platform.

Subsea production systems place the wells and much of the equipment on the seabed: subsea trees, templates and manifolds that gather several wells, jumpers and flowlines, control modules fed by umbilicals, and increasingly subsea boosting (multiphase pumps), separation and compression. A subsea tie-back connects such a cluster to an existing host platform or to shore, which is how small or distant fields become economic. Offshore production facilities do what an onshore gathering station does in a small space: oil-processing platforms separate, treat and pump oil and compress gas; water-injection platforms treat sea water (filtration, deaeration, often sulphate removal) and inject it for pressure maintenance; utilities provide power generation, fresh water, instrument air, flare and relief systems, firewater and accommodation.

Where there is no pipeline, oil is stored offshore — in an FPSO (floating production, storage and offloading vessel), an FSO (storage only), or the cells of a gravity-base structure — and offloaded to shuttle tankers. Offloading and loading use single-point moorings (SPM), about which a tanker can rotate freely with wind and current. The commonest is the CALM (catenary anchor leg mooring) buoy, a single buoy mooring (SBM) anchored by several catenary chains, with a turntable to which the tanker is moored by a hawser and floating hoses; the SALM (single anchor leg mooring) uses one vertical tensioned leg; turret systems build the single point into an FPSO. Transportation is by subsea pipeline to shore or a hub, or by tanker.