OFF-SHORE DRILLING
Offshore drilling is a mechanical process where a wellbore is drilled below the
seabed, just like in case of onshore drilling, at first seismic survey is needed for exploration of the resource. It is
typically carried out in order to explore for and subsequently extract petroleum which lies in rock
formations beneath the seabed. Most commonly, the term is used to describe
drilling activities on the continental shelf, though the term can also be applied to
drilling in lakes, inshore waters and inland seas.
There are many
different types of facilities from which offshore drilling operations take
place. These include bottom founded
drilling rigs, combined drilling and production facilities either bottom founded or floating platforms, and deepwater mobile offshore drilling units (MODU) including semi-submersibles and drill ships.
These are capable of operating in water depths up to 3,000 metres
(9,800 ft). In shallower waters the mobile units are anchored to the
seabed, however in deeper water (more than 1,500 metres (4,900 ft) the semisubmersibles or drill ships are maintained at the required drilling location using dynamic positioning.
HOW DO SEISMIC
SURVEYS WORK?
Sound waves help
scientists or geologist map the ocean floor and geology beneath it, these
sounds waves are produced by bubble explosion or blasting under seabed. These
released sound waves into the water gets reflected off subsurface rock layers
and are “heard” by sensors(hydrophone). Scientists or geologist analyze the
collected data and use it to create maps of geologic structures that could
contain energy resources beneath the ocean floor. The sound produced during
seismic surveys is comparable in magnitude to many naturally occurring and
other man-made ocean sound sources, including wind and wave action, rain,
lightning strikes, marine life, and shipping.
SOME TYPES OF OFFSHORE DRILLING RIGS
There are different types of rigs which are
designed to work according to the specific needs of oil and gas companies.
These rigs are designed to perform in hard conditions like shallow water, the
deep depths of the ocean or areas with unstable and difficult weather
conditions.
SUBMERSIBLE
RIGS
A submersible oil rig
can be used in shallow water where the depth of water is about 80 ft or less.
These rigs are towed to the location of the oil reserves and submerged in the
water until the rigs lie on the ocean floor. Anchors are sometimes used to
secure the position of the submersible rigs. The submersible drilling platform
is supported on large pontoon-like structures. These pontoons provide buoyancy
allowing the unit to be towed from location to location. Once on the location,
the pontoon structure is slowly flooded until it rests securely on its anchors,
of which there are usually two per corner.
The
operating deck is elevated 100 feet above the pontoons on large steel columns
to provide clearance above the waves. After the well is drilled, the water is
pumped out of the buoyancy tanks and the vessel is re-floated and towed to the
next location.
SEMI-SUBMERSIBLE
RIGS
The semi-submersible
design was first developed for offshore drilling activities. These rigs are
built to withstand harsh weather conditions. Depending on their design, the
semi-submersible oil rigs may be either self-propelled or towed to the
location. Once the rig is in position, ballast is used to drown the rig into
the depths of the ocean. The movement of a semi-submersible rig can be
controlled with the help of computers in case of modern rig or anchors in the
case of older rigs.
PRODUCTION
PLATFORMS
Production platforms are
constructed on the surface of the spot where oil or gas reserves are found. It
is only constructed when exploratory drilling at the spot reveals oil or gas
reserves which is worthy of a massive expense. These production platforms are
permanent structures that are design to last decades and cannot be moved once
they are built. They are often constructed far from land and in some of the
most worst waters on Earth.
JACK-UP
RIGS
When oil drilling moved
into offshore waters, fixed platform rigs and submersible rigs were built, but
were limited to shallow waters. When demands for drilling equipment was needed
in water depths greater than 100 feet, then the jackup rigs were built.
Jack-up
rigs are kept in position over the surface at the spot where the oil reserves
are located. The jack-up rigs are maneuvered into position with the help of legs
that are jacked down from the surface. These rigs can be used at depths of
about 600 feet below the surface. The rig should rest about fifty feet above
the surface of the water before the drilling work begins.
DRILLSHIP
Drill-ships are useful
in undertaking drilling work of new oil and gas reserve wells even at
significant depths. A drillship is a vessel that has been fitted with a
drilling device and can propel itself to where the oil deposits are present.
The drillship can also be constructed from an existing ship. Older drillships
use anchors to maintain the ship’s position in the ocean. Newer drillships
utilize computer-controlled thrusters to better control the position of the
ship. The drillships that are controlled by computers can be used to navigate
even in very deep water. They have extensive mooring or positioning equipment,
as well as a helipad to receive supplies and transport staff. Typically
employed in deep and ultra-deep waters, drillships work in water depths ranging
from 2,000 to more than 10,000 feet (610 m to 3,048 m).
IMPACTS ON THE ENVIRONMENT DUE TO DRILLING
Offshore
oil production involves environmental risks, most notably oil spills from oil
tankers or pipelines transporting oil from the platform to onshore facilities,
and from leaks and accidents on the platform. Produced water is also generated,
which is water brought to the surface along with the oil and gas; it is usually
highly saline and may include dissolved or un-separated hydrocarbons.
