Mobile Offshore Rigs

UNIT 4

The Role of Environment in Rig Design

Text 1. Rotary Rig Types

Many kinds of rotary drilling rigs are available, particularly offshore where the marine environment plays an important role in rig design. Two broad categories of rigs are those that work on land and those that work offshore. Some experts like to create a third category: rigs that work in inland waters. Inland rigs usually drill in lakes, marshes, and estuaries, places that are neither land nor offshore. For our purposes, dividing rotary rigs into land and offshore types is acceptable, because inland rigs also drill in water, even if it is shallow.

Land Rigs

A major difference is their size, and size determines how deep the rig can drill. Well depths range from a few hundred or thousand metres to tens of thousands of metres. The depth of the formation, that contains, or is believed to contain, oil and gas, controls well depth. Classified by size, land rigs are light duty, medium duty, heavy duty, and very heavy duty.

Keep in mind, that a rig can drill holes shallower than its maximum rated depth. For example, a medium-duty rig can drill a 750-metre hole, although a light-duty rig can also drill it. On the other hand, a rig cannot drill too much beyond its rated maximum depth, because it cannot handle the heavier weight of the drilling equipment required for deeper holes.

Another feature of land rigs share is portability. A rig can drill a hole at one site, be disassembled if required, moved to another site, and be reassembled to drill another hole. Indeed, land rigs are so mobile that one definition terms them “portable hole factories”. The definition sounds odd, but it is accurate.

Mobile Offshore Rigs

A widely used offshore drilling rig is a mobile offshore drilling unit, or MODU, for short. Another is a platform. Although drilling occurs from platforms, companies employ them on the producing side of the oil and gas business. This book concentrates on drilling, so it does not cover platforms. MODUs are portable; they drill a well at one offshore site and then move to drill another. MODUs are either floaters or bottom-supported. When drilling, floaters work on top of, or slightly below, the water’s surface.

Floaters include semi-submersibles and drill ships. They are capable of drilling in waters thousands of metres deep. MODUs that contact the ocean bottom and are supported by it are bottom-supported.

Bottom-supported units include submersibles and jackups. Submersibles are further divided into posted barges, bottle types, inland barges, and arctic. Generally, bottom-supported rigs drill in waters shallower than floaters.

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