An offshore field is a small industrial site planted in open water, and almost nothing on it makes its own way there. The mud that drills the well, the cement that lines it, the fuel that runs the rig, the food the crew eats, the anchors that hold the rig over the wellhead, and the technician who replaces a failed turbine gearbox all arrive by sea on a purpose-built support vessel. The offshore support vessel (OSV) fleet is the supply line, the tug, the rescue boat, and the floating hotel for the upstream oil and gas industry & the offshore wind industry, and the way each vessel type is built reflects exactly which of those jobs it does. This article is the hub for the offshore support and marine operations cluster: it sets out the OSV families, the marine operations they perform, & the planning rules that decide whether a job goes ahead, then routes down to the deep-dive articles and the eight calculators that carry the arithmetic.
The defining constraint is the weather. A truck can deliver to a factory in any conditions short of a closed road; an OSV working a rig 150 nautical miles offshore can be stopped for days by a sea state its equipment could physically survive but its operation cannot safely tolerate. That gap, between what the steel can take and what the job can take, is why marine operations have their own discipline, their own surveyors, & their own standards. The same gap drives most of the calculators in this cluster, from the AHTS bollard pull calculator that sizes the towing force for a rig move to the escort tug operating-speed-window calculator that bounds where a tethered escort can hold a tanker.
The offshore support vessel families
OSVs are classified by the work they do, & the work dictates the hull, the propulsion, and the deck layout. A platform supply vessel is built around a flat cargo deck and a tank farm; an anchor-handling vessel is built around a stern roller and a winch that pulls hundreds of tonnes; a crew transfer vessel is built around speed and a fendered bow. The families overlap at the edges, but the core distinctions are stable enough that a charterer specifies the type before anything else.
The platform supply vessel (PSV) is the workhorse. Its job is to carry consumables out to a rig or production platform and bring back returns: empty containers, used mud, slop, & scrap. The whole design serves cargo. A long clear after-deck, on a mid-size PSV around 700 to 1,000 square metres, takes containerized stores, drill pipe, casing, & equipment, all lashed down against the vessel’s roll. Below the deck sit the bulk and liquid tanks: drilling mud in water-based and oil-based grades, brine, barite & cement carried as pressurized dry bulk, base oil, rig fuel, potable water, drill water, and methanol. The PSV deck-area and deadweight calculator works the two numbers a charterer actually checks, because a heavily-tanked load can exhaust the deadweight before the deck fills, and a deck of light containers can leave deadweight on the table.
The anchor-handling tug supply (AHTS) vessel does the heavy work. It tows mobile drilling rigs and floating structures to location, sets and recovers their mooring anchors, & can carry supply cargo when not on a tow. The hull is shorter and beamier than a PSV’s, the engines far larger, and the stern carries a roller and a towing & anchor-handling winch rated in hundreds of tonnes. The single number that defines an AHTS is bollard pull, the static pull it develops at zero speed. The AHTS bollard pull calculator estimates that force, and the rest of this article returns to it because the rig-move plan stands or falls on it.
The fast support and crew vessels serve people and light cargo rather than bulk. In oil and gas, fast supply or crew boats run personnel and urgent stores at speed. In offshore wind, two distinct vessel types have grown up: the crew transfer vessel (CTV) and the service operation vessel (SOV), covered in detail below. The emergency response and rescue vessel (ERRV), also called a standby vessel, has one job that hopefully never happens: it stands near a manned installation ready to recover people from the water and provide a refuge if the platform has to be evacuated. UK installations in particular carry a dedicated ERRV requirement, and the vessel’s fast rescue craft, recovery capability, & survivor accommodation are what it’s rated on, not its cargo.
| OSV family | Primary role | Key capability metric | Typical figure |
|---|---|---|---|
| Platform supply vessel (PSV) | Carry deck cargo & bulk/liquid consumables to installations | Clear deck area & deadweight | 700 to 1,000 m2 deck; 3,000 to 5,000 t DWT mid-size |
| Anchor-handling tug supply (AHTS) | Tow rigs, set & recover moorings, supply | Bollard pull | 150 to 400 tonnes-force; winches 400 to 800 t |
| Fast supply / crew boat | Run personnel & urgent light cargo at speed | Service speed | 20 to 30 knots |
| Crew transfer vessel (CTV) | Daily technician transfer to wind turbines | Transfer limit (significant wave height) | up to ~1.5 m Hs; ~60% workability |
| Service operation vessel (SOV) | Offshore hotel + walk-to-work transfer | Gangway transfer limit (Hs) | up to ~2.5 m Hs; ~80% workability |
| Emergency response & rescue vessel (ERRV) | Standby, person-in-water recovery, refuge | Survivor recovery & accommodation | rated rescue capacity, not cargo |
The table compresses a lot of variation. PSVs run from small 50-metre coastal units to 90-metre deepwater vessels with 5,000 tonnes deadweight; AHTS bollard pull spans the small harbour-tug end up to the 400-tonne deepwater anchor handlers built to move semi-submersibles & drillships. The point is the metric, not the exact figure: each family is bought and chartered against the one number that bounds its job.
Two further families round out the fleet at the technical end. The dive support vessel (DSV) carries a saturation diving system, a moonpool, and a hyperbaric lifeboat, and holds station over a worksite while divers work the seabed from a bell; its governing constraint is station-keeping precision, because a metre of drift with divers in the water is a fatality risk, not a delay. The construction support or cable-lay vessel carries a crane or a carousel and installs the hardware the field is built from. Both sit closer to the subsea and offshore installation campaign than to routine supply, but they share the OSV hull form, the dynamic positioning, & the same marine-operations planning discipline, so a charterer scoping a field build counts them in the same fleet.
Dynamic positioning and station-keeping
A supply vessel can’t moor to a fixed installation, so it holds position by power. Dynamic positioning (DP) is the system that keeps a vessel at a fixed point and heading using its own thrusters, taking position from satellite and reference sensors and computing the thrust each unit needs against the wind, current, & wave drift pushing the hull off station. Almost every modern OSV that works close to an installation carries DP, & the redundancy class of that DP system is one of the first things a charterer specifies, because it sets how the vessel behaves when something fails.
The IMO grades DP by the consequence of a single fault. A DP1 vessel has no redundancy, so a single failure can cause a loss of position; it’s acceptable only where a position loss has no serious consequence. A DP2 vessel keeps position after any single failure in an active component (a thruster, a generator, a control element), achieved by splitting the thrusters, power, and control into independent groups. A DP3 vessel keeps position after any single failure including the total loss of one compartment to fire or flood, achieved by physical separation as well as electrical. The dynamic positioning article carries the system detail; the operational point for marine operations is that the work being done sets the class. Cargo runs alongside an unmanned platform may accept DP1 or DP2; diving, with people in the water, and work over a live wellhead, demand DP2 or DP3, because a drive-off or drift-off there kills.
DP shapes the way a vessel works the 500 metre zone. A vessel on DP alongside an installation runs a continuous footprint check, an escape route, & a set of alarm limits, so that the moment position error grows past a yellow limit the operation pauses and past a red limit the vessel drives clear on a pre-planned heading. The whole approach is built so the vessel always has somewhere safe to go, because the alternative, a powered vessel driving onto a platform riser, is the event the field’s safety case fears most.
Bollard pull and the rig move
A rig move is the set-piece marine operation of the oil and gas OSV world, and bollard pull is the number it turns on. Bollard pull is the steady towline force a vessel develops while pulling against a fixed point at zero forward speed. It’s certified by a physical test: the vessel pulls against a shore bollard or a moored anchor with a load cell in the line, holds the pull, and the steady-state tension is recorded, usually as the maximum sustained value over a defined period rather than a momentary peak. The certified figure on the bollard pull certificate is the working number; brochure horsepower is not, because the conversion from installed power to pull depends on propeller design, nozzle type, & how much of the power reaches the water as thrust.
The reason the certificate matters is that the tow analysis is built on it. Towing a mobile offshore drilling unit, whether a jack-up under tow or a semi-submersible, requires enough pull to overcome the structure’s towing resistance: the hydrodynamic drag of the hull through the water plus the wind load on the exposed structure plus a current allowance, all multiplied up by a safety factor for the weather the tow may meet. Where one vessel can’t cover the required pull with margin, the move uses two or more in tandem, and the plan adds their certified pulls with an interaction allowance because vessels towing in company interfere with each other’s wake. The IMO Code for Mobile Offshore Drilling Units (the MODU Code, IMO Resolution A.1023(26)) governs the units being moved, and the towing arrangement itself goes to a marine warranty surveyor for approval before the rig leaves location.
Anchor handling is the other half of the AHTS job & the more dangerous one. To moor a semi-submersible drilling rig over a wellhead, the AHTS runs each of the rig’s anchors out from the rig, lowers it to the seabed at the designed position, & tensions the mooring line; to move the rig it recovers each anchor in reverse. The forces are large and the geometry is unforgiving: a heavy anchor and chain hang off the stern roller, and if the wire leads off to the side as the vessel is pushed by weather, the transverse pull can heel the vessel dangerously. The loss of the AHTS Bourbon Dolphin off Scotland in April 2007, during anchor handling for a semi-submersible in deep water, killed eight crew and led the Norwegian inquiry to recommend tighter limits on anchor-handling stability and clearer roles between rig and vessel; it remains a reference incident for the operation’s hazards. Stability during anchor handling, not bollard pull alone, is what a modern AHTS design and operating manual are scrutinized on.
The mechanics of a deepwater mooring set the demands on the vessel. A semi-submersible may carry eight to twelve mooring lines, each a combination of chain at the rig and seabed ends with a long span of wire or polyester in between, and each anchor can weigh fifteen tonnes or more before the chain and the catenary are counted. The AHTS picks up the anchor on a chaser or a permanent chain system, runs out along the line’s bearing while paying out wire from the anchor-handling winch, then lowers the anchor to a position fixed by the vessel’s own DP, often hundreds of metres from the rig. Working depth matters because the weight of the deployed wire itself loads the winch and the stern roller before the anchor is even on the bottom; a 1,500-metre mooring run can put more wire weight over the roller than the anchor. This is why the winch on a deepwater anchor handler is rated at 400 to 800 tonnes while the bollard pull is 150 to 400: the winch must hold the suspended catenary, not just pull the anchor.
The roles on a rig move are deliberately split & written down before the vessel sails. The rig (the towed or moored unit) is responsible for its own watertight integrity, its draft, & the condition of its mooring equipment; the AHTS is responsible for the towing connection or the anchor-handling work and for its own stability; the marine warranty surveyor approves the whole arrangement against the analysis. The Bourbon Dolphin inquiry’s central finding was that those boundaries had blurred under schedule pressure, with the vessel taking on a heading and a wire lead its stability couldn’t tolerate. A modern operation carries a stability-limiting curve in the bridge, a defined maximum wire angle, & a quick-release on the towing pin and the winch, so the crew can dump the load before a transverse pull capsizes the vessel.
Cargo: deck, bulk, and liquids
What a support vessel carries divides cleanly into deck cargo and tank cargo, & the two compete for the same deadweight. Deck cargo is everything lashed on the open after-deck: offshore containers (themselves built and certified to a higher standard than ISO road boxes because they’re lifted in a seaway), drill pipe and casing in racks, riser joints, subsea equipment, and bagged or palletized stores. The deck has a maximum uniform load in tonnes per square metre and a total area, and cargo is planned so neither the area nor the local strength nor the deadweight is exceeded.
Tank cargo is the liquids and pourable solids carried below deck, and it’s where the chemistry shows. Drilling mud, the dense fluid that lubricates the bit, carries cuttings up the hole, & holds back formation pressure, is carried in dedicated tanks and pumped up to the rig through hoses; water-based and oil-based muds need separate tank groups to avoid contamination. Brine, a dense clear salt solution, is carried for completion work. Barite and cement, dry powders, are carried as pressurized dry bulk in cylindrical tanks and blown ashore or to the rig with compressed air. Base oil, methanol, rig fuel, potable water, & drill water each have their own tanks. The vessel also takes returns: used oil-based mud and slop are hazardous and have to come back to shore for disposal, so a working PSV is rarely empty in either direction.
The competition for deadweight is a real planning constraint, not a formality. A supply vessel has one deadweight figure that every cargo draws on at once: deck cargo, tank liquids, the vessel’s own fuel and stores, & water ballast. Load the tanks with dense oil-based mud and brine and the deck may have to sail half-empty to keep the vessel within its loadline; fill the deck with heavy casing and the tanks go out partly empty. The loading is planned as a stability and strength problem too, because a tank that’s partly full of liquid has a free surface that cuts into the vessel’s stability, & a heavy deck load raises the centre of gravity. So the cargo plan that the PSV deck-area and deadweight calculator supports isn’t just an inventory; it’s a trim, stability, & deadweight calculation the master signs before the vessel sails.
Bunkering at sea is its own operation. Transferring fuel from a supply vessel to a platform, or topping up a vessel offshore, is governed by a transfer procedure that limits the conditions, sets the hose and connection arrangement, & requires monitoring against spills, because a fuel spill inside a field is both a pollution event and a fire risk. The offshore bunker transfer calculator works the transfer rate and time against the tank volumes and pumping arrangement, and the operation sits under the platform’s permit-to-work like every other job inside the field.
Offshore wind: CTV, SOV, and walk-to-work
Offshore wind has reshaped the OSV fleet in fifteen years, because a wind farm’s needs differ from an oil field’s. A wind turbine has no mud or fuel demand; what it needs is people, repeatedly, to install and then maintain it. Two vessel types answer that, & they split by distance from shore.
The crew transfer vessel (CTV) is the near-shore answer. It’s a fast aluminium catamaran, usually 20 to 30 metres, that carries a dozen technicians from a port out to the array each morning and back each evening. To put a technician onto a turbine, the CTV drives its rubber-fendered bow firmly against the turbine’s boat-landing & holds it there with engine thrust; the friction of the fender against the ladder steadies the bow enough for the technician to step across and climb. The method is cheap and quick, but it stops in rough water: transfers are generally not attempted above about 1.5 metres significant wave height, which on an exposed North Sea site limits year-round workability to roughly 60 percent. A CTV that can’t transfer is a CTV that turns around.
The service operation vessel (SOV) is the far-shore answer & a different kind of ship. It’s a larger vessel, often 80 to 90 metres, that goes offshore and stays for two to four weeks, housing 40 to 60 technicians in single cabins with a galley, workshops, & a store of spares. It holds station near the turbines on dynamic positioning, & instead of pushing a bow against a ladder it deploys a motion-compensated walk-to-work gangway: a telescoping bridge whose tip is held steady against the turbine by active motion compensation while the ship moves underneath it. The gangway lifts the transfer limit to around 2.5 metres significant wave height, taking workability toward 80 percent, & it lets technicians walk across with tools and parts rather than climb a ladder one-handed. Some intermediate designs sit between the two, transferring at about 2.0 metres. The economics are simple: close to shore the CTV’s daily run is cheap, but as the array moves further out the transit eats the working day, & at some distance the SOV’s stay-offshore model wins even though the day rate is far higher.
| Wind transfer method | Vessel | Transfer mechanism | Sea-state limit | Workability |
|---|---|---|---|---|
| Bow-push step-across | CTV (20 to 30 m catamaran) | Fendered bow held against boat-landing | ~1.5 m Hs | ~60% |
| Motion-compensated gangway | SOV (80 to 90 m, DP) | Active-compensated walk-to-work bridge | ~2.5 m Hs | ~80% |
| Intermediate gangway vessels | CSOV / day-boat hybrids | Smaller compensated gangway | ~2.0 m Hs | between the two |
The walk-to-work gangway came across from the oil and gas world, where motion-compensated access bridges had been used to move people between vessels and fixed platforms, and offshore wind adopted it because turbines are too numerous and too small to justify a helideck on each. Guidance for these personnel-transfer operations sits with bodies such as the International Marine Contractors Association (IMCA) and, for the wind sector specifically, the Global Offshore Wind Health & Safety Organisation (G+), whose good-practice guidelines cover marine transfer and lifting. The same far-shore logic drives the offshore, cruise and specialised operations overlap, where accommodation and complex marine work meet on a single hull.
The 500 metre safety zone
Around a fixed or floating installation sits a regulated exclusion: the safety zone. Under Article 60 of the United Nations Convention on the Law of the Sea (UNCLOS), a coastal state may establish safety zones around offshore installations in its exclusive economic zone, & those zones may extend up to 500 metres measured from the outer edge of the installation, unless a different distance is authorized by a competent international standard. The 500 metre radius is therefore the worldwide maximum, and most states adopt it directly. The United Kingdom sets it through annual Offshore Installations (Safety Zones) Orders made under the Petroleum Act 1987, naming each installation; the United States establishes installation-specific zones on the Outer Continental Shelf under 33 CFR Part 147, often at the full 500 metres.
Inside the zone, only vessels on the installation’s business may enter, & they do so under control. A PSV approaching to deliver cargo, a standby vessel, or a vessel servicing the installation operates inside the zone under a documented approach procedure & the installation’s permit-to-work; any other vessel that enters without consent is committing an offence in most jurisdictions. The reason is collision risk. A vessel that loses propulsion or suffers a dynamic-positioning failure while working alongside an installation can drift onto it in minutes, and a drive-off or drift-on collision with a platform leg or a riser is one of the highest-consequence events in the field’s safety case. So vessels working inside the zone hold position on redundant systems, monitor an escape route continuously, & abandon the approach the moment a fault appears. The 500 metre line is where that discipline starts.
The standby and rescue function
A manned installation in open water can’t be evacuated by road, so the rescue cover is a vessel. The emergency response and rescue vessel (ERRV), historically called the standby vessel, stays on station near a manned platform with three jobs in escalating severity: recover a person who has fallen in the water, provide a refuge and muster point if the platform is partly evacuated, & take off and shelter the whole complement if the installation is abandoned. The UK regime, shaped after the Piper Alpha disaster of July 1988 and the Cullen Inquiry that followed, requires a standby vessel for manned offshore installations, and the vessel is rated on its rescue capability rather than any cargo it can carry.
The capability that matters is the recovery of people from a cold sea, fast. An ERRV carries fast rescue craft (FRC), small powered boats launched and recovered by davit even in a seaway, & a means to lift survivors who can’t help themselves, such as a scramble net, a rescue basket, or a daughter craft with a recovery cradle. It also carries survivor accommodation, heated space, & first-aid facilities sized to a defined fraction of the installation’s complement, because a North Sea evacuation can put dozens of people in survival suits into 6-degree water where the survival time without recovery is measured in tens of minutes. The offshore platform abandonment calculator works the evacuation side that the ERRV exists to receive: how many people, in what survival craft, in what time.
Weather windows and limiting sea states
The single fact that organizes offshore marine operations is that the sea is only workable some of the time, & nobody controls when. Every operation has a limiting sea state, the significant wave height (and often wind speed and current) above which it can’t safely proceed, and the planning question is whether a long enough stretch below that limit, a weather window, will appear when the spread is on location and ready.
Significant wave height (Hs), the mean height of the highest third of the waves in a record, is the usual governing parameter because it tracks the motion a vessel and its equipment feel. The limit varies by operation: a CTV transfer stops around 1.5 metres, an SOV gangway around 2.5 metres, a heavy offshore lift may stop well below 2 metres because the crane and the rigging can’t tolerate the dynamic load, & an anchor handling job has its own limit set by the vessel’s stability. A persistence statistic, how long conditions stay below a given Hs once they drop below it, decides whether the window is usable: a 12-hour operation needs a window longer than 12 hours plus a margin, not just a single hour of calm.
Forecast uncertainty turns the limit into two numbers. The design limit is what the equipment can physically take; the operational limit is lower, because the forecast might be wrong & the sea worse than predicted when the point of no return passes. Standards reduce the design limit to the operational limit with an alpha factor, a coefficient below 1.0 that’s smaller for longer operations and longer forecast horizons, where the forecast is less reliable. A 6-hour operation forecast a few hours ahead carries a high alpha factor close to 1.0; a 48-hour operation planned against a 3-day forecast carries a lower one, so the crew is told to start only when the forecast Hs sits well under the equipment’s true limit. The escort tug operating-speed-window calculator applies the same kind of bounding logic to a tethered escort’s safe operating envelope.
Wave period matters as much as wave height, & ignoring it is a common planning error. A vessel and its access system respond to the frequency of the waves, not just their size: a long-period swell of 2 metres can move a hull far more than a short-period 2.5-metre wind sea if the swell period sits near the hull’s roll or heave natural period. So the limiting criterion for a sensitive operation, a heavy lift or a gangway transfer, is often given as a pair, a maximum Hs together with a workable peak-period band, and a forecast that clears the height but not the period still stops the job. Swell direction relative to the heading the vessel can hold adds a third axis, because a beam swell rolls a ship that a head swell would only pitch.
The persistence problem also shapes how the spread waits. Offshore weather doesn’t deliver a clean window on demand: a North Sea winter can run weeks where the workable hours come in scattered patches too short to use, & the cost of a spread of vessels and a rig waiting on weather runs to hundreds of thousands of dollars a day. So the planning trades window length against season, sometimes deferring a weather-sensitive installation to a summer campaign rather than paying standby through a winter of broken windows. The offshore rig day-rate calculator puts the number on that wait, because every day the weather costs is a day of the rig and its support fleet billed at full rate.
Marine operations planning and the warranty surveyor
A marine operation is planned as a documented sequence with defined start and abort points, not as a continuous activity. The governing distinction is weather-restricted versus weather-unrestricted, and it decides which design basis applies. A weather-restricted operation is short enough that it can be started and finished inside a reliable forecast; the conventional cut-off in DNV-ST-N001, a recognized standard for marine operations and marine warranty, is a reference period of 72 hours or less, including a contingency. Such an operation is designed against the forecast limit reduced by the alpha factor. A weather-unrestricted operation is one that can’t be completed inside a forecast or has no safe abort, such as a long ocean tow or a structure left exposed for weeks; it has to be designed against extreme statistics instead, a seasonal return-period sea state (often 10-year for the season, 100-year for a permanent condition), because no forecast covers it.
The operation is built around defined hold points and a point of no return. A documented marine operation lists the sequence of steps, the go/no-go criterion at each one, & the last moment at which the operation can still be reversed to a safe condition. Before that point the crew can abort cheaply; after it, the only way out is to finish, so the weather forecast has to cover the whole reference period from the point of no return forward, with contingency. A load-out of a jacket onto a barge, for instance, has a point of no return once the structure’s weight is off its quayside supports and onto the barge; from there the ballasting and the tow have to proceed regardless, so the window is checked against the full duration, not the load-out alone.
The independent check on all of this is the marine warranty surveyor (MWS). The MWS is appointed under the project’s insurance, usually as a warranty in the cargo or construction-all-risks policy, & acts for the insurer. Before a rig move, a heavy tow, a load-out, or an offshore lift can proceed, the MWS reviews the procedures, the vessel and equipment certificates (the bollard pull certificate among them), the towing or lifting analysis, the seafastening design, & the weather forecast, & when satisfied issues a Certificate of Approval for that specific operation. No certificate, no cover, so the MWS sign-off is a hard contractual gate on the operation, not advice. The MWS works to recognized standards including DNV-ST-N001 and the relevant marine guidance from the International Marine Contractors Association (IMCA), whose M-series documents cover marine and diving operations, & for industry-wide upstream practice the guidelines of the International Association of Oil & Gas Producers (IOGP).
The planning also has to integrate with the rest of the field’s operations. A rig move blocks a subsea and offshore installation campaign that needs the same anchor corridors; a helicopter crew change needs its own weather window on the same day the SOV wants to transfer technicians; the marine helicopter operations on the installation deck have limiting wind and visibility criteria that don’t always coincide with the sea-state window for marine work. A marine operations coordinator sequences all of it so that the weather windows, the vessel availability, & the MWS approvals line up, because a window that opens with the spread not ready, or the certificate not issued, is a window wasted.
Where each cluster article goes deeper
This hub stays at the level of families and principles; the cluster’s leaf articles & calculators carry the detail and the arithmetic. The two deep-dive wiki articles cover the production end of the field: FPSO (floating production, storage and offloading) for the unit that turns a moored hull into a producing facility, and marine helicopter operations and helidecks for the air link that moves people in and out and the deck standards that govern it.
The eight calculators each work one of the numbers this article has named. The AHTS bollard pull calculator estimates the towing force for a rig move. The PSV deck-area and deadweight calculator bounds the cargo a supply vessel can take. The offshore bunker transfer calculator works the at-sea fuelling rate and time. The offshore helicopter operations calculator and the helideck clear-of-obstructions calculator cover the air-side limits and the deck obstacle-free geometry. The platform abandonment calculator works the evacuation side the ERRV exists to support. The offshore rig day-rate calculator puts a cost on every day the weather steals, & the escort tug operating-speed-window calculator bounds where a tethered escort can safely work.
Limitations
This article describes the OSV fleet and marine operations at the level a charterer, planner, or surveyor uses to scope a job; it isn’t a substitute for the governing procedure, the class rules for a specific vessel, or a marine warranty surveyor’s approval. The vessel figures are representative ranges, not specifications: bollard pull, deck area, deadweight, & transfer limits vary widely between individual vessels, and the certified figures on a particular vessel’s certificates always govern. The sea-state limits quoted (about 1.5 metres for CTV transfer, about 2.5 metres for SOV gangway transfer) are typical industry figures for North-European conditions; the actual limit for any vessel and access system is set in that system’s operating manual and depends on the wave period and direction as well as the height. The 500 metre figure is a worldwide maximum under UNCLOS Article 60; the zone, its exact extent, & the entry rules for a given installation are set by the coastal state’s own legislation, which must be checked for the field in question. The weather-restricted threshold (a reference period of 72 hours or less) and the alpha-factor approach follow DNV-ST-N001; other recognized standards and a project’s own MWS may set different criteria. None of the calculators in this cluster replaces the formal tow analysis, lift study, stability assessment, or MWS approval that an actual operation requires.
See also
- FPSO: floating production, storage and offloading vessel
- Marine helicopter operations and helidecks
- Dynamic positioning
- Subsea and offshore installation
- Offshore, cruise and specialised operations
- AHTS bollard pull test calculator
- PSV deck area and deadweight calculator
- Offshore bunker transfer calculator
- Offshore helicopter operations calculator
- Helideck clear-of-obstructions calculator
- Offshore platform abandonment calculator
- Offshore rig day-rate calculator
- Escort tug operating-speed-window calculator