SBM’s article in edition 30 of Port Technology International touched upon the benefits of separating dry cargo traffic from oil cargoes in ports and in coastal waterways, thereby reducing the opportunity of ship collision or groundings and so contributing towards safe navigation and the protection of the environment.
In the 50 years that has elapsed since the first introduction of offshore loading for oil tankers, the marine transportation industry has seen many significant changes, notable among which is an enormous increase in the size of oil tankers. In the 1950s and 60s the average size of oil tanker was less than 100,000 tonnes deadweight with a draft of about 12 metres.
Today, crude oil carriers of 350,000 tonnes dwt are the norm with vessels of 500,000 tonnes dwt (and greater) in service, having drafts in excess of 25 metres. With the introduction of these much deeper hulls and the reduced manoeuvrability that comes with very large crude carriers it became an imperative to develop suitably safe and efficient deep water berthing facilities. Natural deep water harbours were in short supply and the building of conventional jetty-type facilities for large carriers was, and is today, enormously expensive, often requiring breakwaters plus significant amounts of dredging, sometimes with maintenance dredging needed afterwards. And, of course, jetties utilise extensive stretches of expensive coastal real estate.
Further, the support of two or sometimes three tugs is needed to safely position a large oil carrier alongside a jetty and, then again, to un-berth it and clear port after cargo transfer is complete. By moving the terminal ‘offshore’ into deep enough waters, not only were terminal costs reduced but the possibility of a tanker grounding was effectively eliminated and, being in open sea, the limited manoeuvrability of the larger ships became less of an issue.
The technical solution that made offshore loading universally attractive, however, was Single Buoy Mooring or ‘SBM’. The possibility to berth a ship by bow lines only, without the aid of tugs and for the ship to be able to swing freely, or ‘weathervane’ around the mooring point in response to any change in the direction of wind, wave and current, always taking up a position of least resistance to the environment, meant that the mooring structure could be optimally designed and therefore extremely cost-effective to manufacture and install. The ease and speed of berthing and unberthing at an SBM together with rapid cargo transfer ensured the fastest possible ship turnaround times and that a greater number of cargoes could be carried by any one ship. An SBM system allows visiting tankers to approach from any direction, to berth without the aid of tugs and in much higher sea states than is possible at a fixed-direction mooring structure.
Once moored, the tanker can remain on the buoy in even higher sea states, without concern about changes in direction of the weather. All of this added up to improved terminal availability and the elimination of demurrage charges and SBMs became an instant success with tanker owners and terminal operators alike.
The position today
More than 500 offshore loading and offloading terminals have been installed worldwide, of which SBM Inc. has supplied more than 80 per cent. Many are located in major oil producing regions such as the Middle East and West Africa where they are dedicated to the export of crude oil. Others are at the receiving end, delivering feed stock to oil refineries that produce fuels such as petrol, diesel oil, aviation spirits etc. Some are used to directly handle refined petroleum products, either importing or exporting, or both, or to deliver liquid feedstock to petrochemical complexes, or handle gas in liquid form.
The style of SBM buoy selected by the vast majority of terminal operators is the ‘turntable’ type. In this configuration the buoy hull fixed to the sea floor by large chains and a rotating ‘turntable’ is mounted at the deck (see Figure 3). The configuration has been incredibly successful over the years in always providing free rotation, even when the terminal is unoccupied, and in offering ease of inspection and maintenance. There are turntable buoys still in active service more than 40 years after first installation.
The principle benefits of offshore loading by SBM system may be summarised as follows:
• Lower capital cost than equivalent fixed jetty-type facilities
• Lower terminal operating costs with no tugs required (yet retaining the service elements normally provided by port authorities, such as bunkering etc)
• Much faster project implementation from inception to commissioning
• Faster ship turnaround times with elimination of demurrage
• Minimum use of valuable coastal real estate
• Separation of oil traffic from busy dry cargo ports and coastal waterways