2013: Advent Oil & Gas: Pipes and tankers

by on December 11, 2013

So we have all this useful stuff… but how do we get it to people who will pay for it?  The answer is, simply enough, “in bulk”.  Oil and gas are valuable commodities but it still makes sense to save as much as possible by transporting the largest quantity possible at once.  Fortunately fluids are relatively easy to transport, and the compressible nature of gas also helps.

A well-known and reliable way of moving fluids is in pipelines.  These can be very long and have very high capacity – virtually all of Eastern Europe’s gas consumption and much of its oil is supplied by pipeline from Russia, much of which ultimately comes from Siberia.  Pipelines are also much cheaper and more efficient than road or rail tankers.  They can work all hours and require relatively little power or labour for operation (though they need pumps or compressors to keep oil or gas, respectively, flowing).

Pipelines can be made from a variety of materials.  In mediaeval China natural gas was transported short distances using bamboo pipes.  The nineteenth-century gas industry used cast iron, which resists degradation but is brittle; ductile iron was briefly used, but is even less robust.  In modern times, steel is used for high pressure, high-grade polyethylene (PE) for medium to low pressure, and copper used for some low-pressure fittings.  These materials are cheap (apart from copper) and tough; PE has the additional advantage that it can be stored in rolls and bent to fit just about any route.

Pipelines are not, of course, free from problems.  They are a major piece of infrastructure in their own right, and are expensive to build; provision needs to be made for crossing roads, railways, and rivers, as well as for mitigating ecological damage.  In addition the machinery of pumps and valves and their associated instruments requires a robust control system (referred to as SCADA, for supervisory control and data acquisition).  They are also subject to concerns over sabotage, terrorism, or accidents, especially if they are above ground or in an area prone to earthquakes (or in a James Bond film).

While it is possible to lay pipelines on the seabed, this is very expensive, and can only be done over limited distances and depths (the length record is held by Nord Stream in the Baltic, at 1222km).  For transport over long stretches of sea, the solution is a tanker ship.  Oil and liquefied natural gas (LNG) tankers are rather different in design, although both are large ships with holds adapted to carry liquid fuel.

Oil tankers are relatively simple: they are broadly similar to bulk grain or ore carriers, except that holds are enclosed, oil-tight, and have pumps for moving oil.  Originally some of the oil tanks also served as ballast tanks, which led to oil spillage being inevitable whenever the ballast was discharged; later this was changed so that ballast tanks were always separated, meaning that if all went well, oil spillage was zero.  This still failed to prevent disasters such as the Exxon Valdez spill: the tanker in question ran aground on rocks and the hull was punctured, spilling a huge volume of oil from the tanks.  Since that time, legislation has changed to force new tankers to be built with a double hull, so that there is space between the outside of the hull and the oil tank (rather than the hull being completely filled with oil).

Natural gas is transported in liquid form as LNG even though methane cannot become a liquid at room tepmerature – it is said to be supercritical.  Instead it has to be cooled to a chilly -162°C.  As a result, LNG requires a lot of insulation, and tanks are normally made spherical in order to keep the surface-area-to-volume ratio as low as possible, thereby minimising insulation cost for a given transport volume.  LNG carriers therefore tend to have a number of huge spherical tanks along the hull.  These have the advantages of being self-supporting and resistant to sloshing; however, they are an inefficient use of hull space, and give the ship a high windage (area above the water), so prismatic membrane tanks are being introduced for some ships.

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