Platform supply vessel

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Platform Supply Vessel

A PSV

A platform supply vessel, or PSV, is a cargo vessel designed to support offshore oil platforms. As cargo vessels, they do not perform any towing or anchor handling, with winches and other associated deck machinery sacrificed in order to maximize deck cargo area (see AHTS).

Contents

Use

PSV at shore base

PSV's transit drill supplies from shore bases to oil rigs. Efficiency and dependability are key, particularly as many rigs are situated in harsh and deep water environments. Weather windows can be short, so supplies must be able to unloaded to the rig quickly. PSV's are sometimes fitted for crew transfer operations, as well as rescue zones for recovery operations. PSV's may provide additional support operations, such as limited fire-fighting or oil recovery. On return trip, PSV's carry wastes and rig by-products for processing.

Size and Arrangement

PSV at sea

PSV's tend to have capacities ranging from 1500-5000 tonnes deadweight, lengths of roughly 30-100 metres, and beams from 15 to 18 metres. Superstructure and accomodations are generally forward, with a transom stern; this allows for maximum deck area.

Design

As per most cargo vessels, stowage of cargo takes precidence in the design of PSV's. Underdeck, specialized tanks carry drilling supplies: drilling fluids such as brine and mud, and dry bulk such as cement. These tanks are specially designed to allow these often thick or non-fluid materials to be pumped (or blown by compressed air) up to the rig while the vessel lies under. Often, these tanks will be fitted with wash-down systems or 'clean' bulkheads to allow for quick cleaning and interchange of tank contents.

As PSVs work in close proximety to rigs, the bridge will have windows situated on the visor for view of the platform. Lifting devices are generally limited, as the platform and shore base are likely to have ample capacity. Deck cargo is carried pre-rigged and containerized to minimize unloading times. There will be a bridge control station with clear view of the working deck, which often means a second, aft, console. Modern PSV's will have some level of dynamic positioning, DP-2 or DP-3.

Stability

PSV's typically have large forecastles and relatively tall superstructures, which move downflooding points high above the waterline. The hullforms are full, and of rugged construction, making for a seaworthy vessel. Care must be taken to account for damaged stability; a loaded vessel will have limited freeboard aft; and as this deck is long and flat, if it starts to submerge the waterplane area will quickly decrease (risking capsize).

Cargo Systems

Cargo on a PSV in port

PSV's may have a handling system, consisting of one or two smaller cranes; however, most rely on shore or rig-based facilities for actual lifting. Complex cargo discharge systems will be fitted, consisting of powerful pumps and compressed air systems. Some supplies may be flamible or explosive, requiring extra care in the carriage and handling. Decks are typically sheathed in thick wood, with cargo rails outboard. Lashing points may be integral to the rail system, deck, or both.

Propulsion

Conventional shafting is common, providing a simple, cheap and reliable layout suitable to the primary role of a PSV. Alternative arrangements are considered on a case-by-case basis i.e.,

  • Where fitted for DP, azimuthing propulsors eliminate the need for stern thrusters, stern skegs, and rudders while providing full thrust in any direction - greatly increasing station keeping and maneuverability, though with increased weight, slightly lower efficiency, and increased cost.
  • Diesel-electric drives eliminate shaftlines through cargo areas and allow more flexibility in the disposition and placement of main engines. The operational profile of a PSV usually does not favour electric drives (as much of the operational time is spent at transit speed or on harbour generators/shore power) for overall efficiency, and there is the associated up-front capitial cost to the system, however these may be mitigated by auxillary functions or envelope restrictions.

Differentiation

As vessels in constant demand and use on the oil fields, PSV's are often modified to take on additional roles as required. This may include large cranes, drill support and sub-sea equipment installation/salvage equipment, helidecks, etc.

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