By Jim-Rex Lawson MOSES

Offshore Drilling

A major difference between onshore and offshore drilling is the type of the drilling platform. Also, in offshore drilling the drill pipe must pass through the water column before entering the seafloor or lake. These type (offshore) of wells have been drilled in waters as deep as 10,000 ft (305 m). Below is an overview of drilling in offshore environments.

Drilling Template

Offshore oil drilling is an oil extraction technique which allows oil companies to access deposits of oil buried under the ocean floor. Most typically, offshore oil drilling sites are situated over the continental shelf, although advancements in drilling technology have made platforms even further out to sea economically and physically feasible.

Offshore drilling requires the construction of an artificial drilling platform, the form of which depends on the characteristics of the well to be drilled. This drilling also involves the use of a drilling template that helps to connect the underwater drilling site to the drilling platform located at the water’s surface.

This template typically consists of an open steel box with multiple holes, depending on the number of wells to be drilled. The template is installed in the floor of the water body by first excavating a shallow hole and then cementing the template into the hole. The template provides a stable guide for accurate drilling while allowing for movement in the overhead platform due to wave and wind action.

Drilling Platforms

There are two types of basic offshore drilling platforms, the movable drilling rig and the permanent drilling rig. The former is typically used for exploration purposes, while the latter is used for the extraction and production of oil and/or gas.

A variety of movable rigs are used for offshore drilling. Drilling barges are used in

shallow (<20 ft [<6 m] water depth), quiet waters such as lakes, wetlands, and large rivers. As implied by the name, drilling barges consist of a floating barge that must be towed from location to location, with the working platform floating on the water surface. In very shallow waters, these may be sunk to rest on the bottom.

They are not suitable for locations with strong currents or winds and strong wave action. Like barges, jack-up rigs are also towed, but once on location three or four legs are extended to the lake bottom while the working platform is raised above the water surface; thus, they are much less affected by wind and water current than drilling barges.

Submersible rigs are also employed in shallow waters and, like jack-up rigs, are in contact with the lake bottom. These rigs include platforms with two hulls positioned above one another, with the lower hull acting like a submarine. When being towed to a new location, the lower hull is filled with air and serves to float the entire platform. Once on location, the lower hull is filled with water, and the rig sinks until the legs make contact with the lake bottom. As with the previous movable rigs, use of this type is limited to shallow water areas.

The most common movable offshore drilling rig is the semi-submersible rig. It functions in a similar manner to the submersible rig, with a lower hull that can be filled or emptied of water. However, this type of rig does not contact the lake floor but floats partially submerged and is held in place through a number of anchors.

This type of rig provides a stable and safe working platform in deeper and more turbulent offshore environments, and when high reservoir pressures are expected. The final type of movable drilling rig is the drillship. These are ships designed to carry drilling platforms great distances offshore and in very deep waters. A drilling platform and derrick are located in the middle of a large, open area of the ship, and the drill is extended through the ship to the drilling template.

When exploratory drilling locates commercially viable oil or gas deposits, a more permanent drilling platform is required to support well completion and oil and/or gas extraction.

A variety of such production platforms are used for offshore drilling. Fixed platforms are typically used in areas with water depths less than 1,500 ft (457 m).These platforms contact the bottom using concrete or steel legs and are either directly attached to, or simply rest on, the bottom. A variety of other production platforms are available for deeper water conditions.