2013: Advent Oil & Gas: Drilling for riches

by on December 6, 2013

Rigs and platforms are not all created equal.  There are a number of different types, each designed for a specific task and environment, from the most basic land rigs to the very advanced (and expensive) platforms used for deep-water drilling.

Land rigs are the original and simplest form of mechanised drilling.  These are large structures built on land, as the name implies, and have some similarities with cranes.   Like cranes, they may be semi-permanent (and this was commonly the case in the early days of oil drilling), but modern rigs can typically be dismantled over a few weeks and rebuilt on another site.  A land rig can also be built on a barge and used to drill in shallow water: this is known as a swamp barge.

The next logical step from the swamp barge is the jack-up rig.  These are similar in that they move from place to place as barges, but they have three or more legs that can be lowered onto the seabed and used to lift the rig above the water place.  The rig then rest on the seabed in water up to about 300m deep.

A related alternative is the platform proper with non-moveable legs and foundations.  Traditionally these are made of concrete or heavy steel, but this rigid construction is used only for relatively shallow water.  For greater depths – up to 900m – a flexible structure called a compliant tower is used in conjunction with piled foundations.

In deeper water still, the semi-rigid compliant tower is unfeasible, so floating structures are used.  Tension-leg platforms use stiff tethers kept in tension that anchor the rig to the seabed; they are used for depths of up to 1500m.  Spar and semi-submersible rigs instead have a low centre of gravity to keep stable, and sit low in the water so they are affected less by waves, allowing them to drill in depths of 3000m.  Both types can be moored to the seabed with conventional (albeit very strong) chain and rope, but most modern semi-submersibles use dynamic positioning systems (DPS) to keep their position in the water.  DPS consists of an array of computer-controlled pivoting thrusters; due to the disastrous consequences in case of failure, only the most robust DPS technology (IMO Class 3) is permitted on oil rigs.

For the very deepest water (and many scientific or exploratory tests) a drillship can be used.  This is just what it sounds like – a ship with a derrick on it – held in place by DPS and capable of drilling in up to 3500m of water.  That’s more than ten times the height of the Eiffel Tower, not even counting the 7000m or more of the well itself.  Not bad considering it’s drilled from a boat without any sort of anchor or support…

Leave a Reply