offshore term

7/31/2019 Offshore Term.

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OFFSHORE TERMINALS (OT)


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Definition: OT means a port facility for loading and /or unloading cargo at which the berthing facility is notin direct contact with shore but is connected viavessels, trestle, causeway, tramway, tunnel pipelineor other means.

Offshore terminals are classified generally as offshorepiers (sea islands), multibuoy moorings, and single-

buoy mooring facilities. Each of these systems hasthree principal parts:

A means for holding the tanker in position.

A means for transferring the cargo from the tankersmanifold to manifold on the loading platform or onthe sea bottom.

An underwater pipeline between the manifold andshore.


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Sea Islands (offshore Piers):

A fixed berth consisting of a platform with unloading facilities connected to; shore by

submarine pipelines, and breasting and mooring dolphins comprises a sea island.

The breasting dolphins take the impact load during berthing and the loadsimposed while moored.

The mooring dolphins contains bollards or quick-release hooks to which the ships

wire are connected. These wires hold the ship in a fairly fixed envelope and

permit the use of loading arms.

Multiple-Buoy Mooring:Three to seven moored buoys are installed in a general semicircular patterns around

the stern of a tanker. The ships anchors are normally used for the mooring

points forward. The buoys are oriented in the direction of prevailing wind and

waves. The tanker ties up with the help of her own anchors and mooring launch.

This system is used in relatively calm waters and locations where wind and waves

are consistently from one direction.

In this system, the ship is held in position rigidly. The connection to the ships

manifold is made by the use of submerged hoses that are lifted from the sea

bottom once the ship is moored. Submarine pipeline connect the pipeline and

manifold (PLEM) to shore.


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Fixed Berth: The simplest version of this facility is thefinger pier, built out into deep water. In a protected area

this is the most preferred system from an operational pointof view. Its use is restricted to comparatively sheltered areas

where deep water lies close to shore.

Single-Buoy Mooring are very common for; mooringVLCCs. The types in use are:

Catenary anchor-leg mooring (CALM)

Single anchor-leg mooring (SALM)

Single point tower with rotating trussed arm

Single point tower with floating hose.

Two most common types are the CALM and SALM systems.


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CALM System:This facility is a moored buoy to which a tanker is connected

by a mooring line. Cargo transfer takes place from thebuoy to the tanker through a floating hose that isconnected to the ships manifold and to a fluid swivel onthe buoy. The connection PLEM is made by an under buoyhose. The buoy is fixed but a turntable on top of the buoy,to which the mooring line is connected allows the tankerto weathervaneabout the buoy in response to changes inwave, current and wind.

Other characteristics of the CALM system are:

Uses four to eight anchored catenary chain legs

The buoy always stays on the surface of the water The buoy stays fixed during operations - only the turntable

mounted on the buoy turns


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SALM System:A typical SALM consists of a mooring buoy at the sea surface held to a

mooring base on the sea floor by a single anchor leg. The buoy is drawn

down against its buoyancy by tension in the anchor leg. Tankers aremoored by mooring lines to the buoy, and a mooring swivel in theanchor leg allows the tanker to swing around the mooring point.

At the SALM, the tanker is free to weathervane, that is to align itself withthe predominant wind, waves and currents.

To transfer cargo through the mooring, a fluid swivel is mounted

concentrically about the anchor leg, either on the top of the base or onthe top of a riser pivoted at the base and forming part or the anchor leg.A submarine pipeline, carrying cargo to or from the mooring, is jointedto the base or to bottom or the riser by hoses. Cargo hoses connected toan arm on the fluid swivel rise to the surface where they float andextend to the tanker manifold.

Other SALM characteristics are:

It has a base which is piled to hold against mooring line loads.

Its hose connection is made below the active wave zone.

The buoy turns with the weathervaning ship.

The buoy is designed to submerge with increasing mooring line loads.


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Design Considerations Offshore TerminalsOffshore terminals are designed for the same loads normally expected at

any port facility plus other additional loads that exist because of theexposed location of the terminal.

One major consideration is the rate of corrosion of metal elements and theerosion of other materials. However, in general, following loads andforces are considered in the design of offshore terminals / structures.

Dead load of structure and equipment

Line loads

Impact loads associated with live loads Wind forces

Wave forces

Ship berthing forces

Ship mooring forces

Hydrostatic pressure

Buoyant forces

Lateral earth forces

Earthquake forces

Vibratory loads


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ONSHORE TERMINALSAn onshore terminal may be describes as docking facilities for the shore loading and unloading

of petroleum. However, a full description would include the following terms and

respective definitions in as much as they form the general scenario in which onshore oil

terminals are installed and operated.

Channel: The buoyed, dredged and policed fairway through which ships proceed from the sea to their or

from one berth to another within a harbor.

Berth: The water area, at the waterfront edge or a wharf, reserved for a ship.

Harbor: An area or water affording a natural or artificial haven for ships. In a more limited sense, an

area separated by natural or artificial indentations of shore line from the main body of water, as the area

within two headlands or points between which run the main ship channels leading to an open sea. Harbor facilities: Those aids or conveniences provided for ships as opposed to those offered by ports for

cargo. These facilities include : channels, anchorages, anchorage basins, mooring posts, mooring basins,

dry docks, ship repair plants, tug boats, etc.

Port: A harbor plus terminal facilities. Also, a harbor provided with terminal and transfer facilities that

enable it to be used in commerce. It involves some amount of development for purposes of commerce

and physically, if there are marked indentions of shore lines, ports may exist without harbors.

Port facilities: Waterfront terminals, including structures, equipment and aids for loading/unloadingcargo. Other facilities included are wharves, pier, sheds, warehouses, rail roads, water or street

connections, etc.

Terminal: The end of a movement in transportation. Also, the buildings, structures and equipment at

the end of a transportation movement for the transfer, delivery and storage of cargo.


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Design Considerations Onshore TerminalsDocking facilities for the loading and unloading of oil,

typically consists of wharves, piers, bulkheads, or

mooring berths with equipment for handling cargohoses that connect the ships manifold with pipelineson the dock.

Some of the preliminary factors are :

Size of the tankers that will use the terminal.

The quantity and characteristics of the crude oils tobe handled.

The rate of loading and unloading the cargo.

Ballast and bunkering facilities that are to beprovided i.e. utility requirements.

Storage requirements.


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Planning the Fixed-Structure Berthing Facility

Depth of Berth: As for offshore terminals, thedepth needed is the draft of the maximum shipplus an allowance of about 10 % for under keelclearance. This allowance is variable and is afunction of wave height, length and direction.

In general the dock should not be broadside to theprevailing wave front. It is also better to havethe ship anchored parallel to the direction of the

prevailing winds or, if this is not possible,aligned so that the wind is holding the ship offthe deck.


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Site Selection

Preliminary site investigations are generallyrestricted to a local check of charts,

geological maps and meteorological

information together with the taking of smallnumbers of borings if dredging is needed.

Final investigations need a full-site survey;

with current observations on spring and tides

as well as borings over the likely sites of the

main components of berthing structure.

offshore term

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