Page 352 - Drilling Technology in Nontechnical Language

Chapter 14 – SAFETY AND ENVIRONMENTAL ISSUES 343

Offshore rigs

Solid waste from rigs includes drilled cuttings. Where water-based
muds are used, these cuttings can often be safely discharged overboard
where they simply pile up on the seabed. In sensitive areas where even
clean cuttings (that is, without any hydrocarbons present on them) might
damage the environment, cuttings can be disposed of in other ways, as
can cuttings with oil on them. First, they can be collected in containers

and shipped to land for disposal in a landfill. Second, they can be ground
up into a slurry and injected into one of the casing annuli, if suitable
formations are exposed to that annulus that allow this injection. Food
waste is generally ground up so that it will pass through a coarse mesh
and dumped overboard, but again, in sensitive areas, this can be shipped
to land.
Sewage can be treated so that it can be safely discharged overboard
without pollution.
Liquid discharges can be somewhat trickier due to the volumes involved.
Water-based muds and cements can usually be discharged overboard, as
no lasting harm will result. Oil-based muds based on mineral oils are
not discharged but are stored and often recycled, resulting in savings to
the operator.
Scrap steel, rope, plastic containers, medical waste, and other rubbish
are not discharged but are returned to the shore base for responsible
disposal.

Onshore rigs

As part of location preparation, land rig locations have waste pits
dug. One of these waste pits—the largest—is positioned next to the mud
tanks. Drilled cuttings and mud are dumped into this pit. It is a good idea
to dig a large U-shaped pit and to dump mud and cuttings at one end.
At the other end of the U, the liquid that drains off can be sucked out.
Sometimes it can be recycled into the mud system (as it is full of expensive
chemicals), and this is pretty useful in arid regions where water supply is
expensive. Alternatively, it can be sucked into a tanker and disposed of or
processed elsewhere.

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