Extraction of oil and gas from ocean

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.