Offshore drilling refers to the act of extracting crude oil which lie underwater near the shoreline in the oilfield . Offshore drilling is used to describe oil extraction off the coasts of continents.
The first offshore drilling was done at the end of the 19th century in Baku at the Bibi-Eibat field.[citation needed] In 1896 oil companies began drilling from wharves built out over the Pacific Ocean at Summerland field, California.
This is how offshore drilling done in detail preparation:
The crew makes the rig ready and starts the drilling perations. From the first hole, they drill a surface hole down to a pre-set depth, which is above where the oil trap is placed. There are five sequences to drilling the well:
1. Put the drill bit, collar and drill pipe in the hole.
2. Mount the kelly and turntable and begin drilling.
3. As drilling runs, circulate mud through the pipe and out of the bit to float the rock cuttings out of the hole.
4. Add new sections (joints) of drill pipes as the hole gets deeper.
5. Remove (trip out) the drill pipe, collar and bit when the pre-set depth (anywhere from a few hundred to a couple-thousand feet) is reached.
when they reach the depth they are expected, they operate and cement the casing -- put casing-pipe sections into the hole to avoid it from collapsing in on itself. The casing pipe has spacers around the outside to make the pipe keep staying in the center of the hole.
The crew installes the casing pipe in the hole. The cement crew pumps cement down the casing pipe using a bottom plug, a cement slurry, a top plug and drill mud. The pressure from the drill mud causes the cement slurry to flow through the casing and fill the space between the outside of the casing and the hole.
You might interested to read about the types to do offshore drilling and where is it done??
Wednesday, March 25, 2009
Offshore Drilling In Detailed
Monday, March 23, 2009
How Difficult Is Offshore Drilling?
Offshore drilling in the oilfield can be done with 3 types of "vehicles" or Oil Drilling Rig, depending on the depth of water
1. Offshore drilling For depth of 7 - 15 ft (shallow sea) is often used type of oil rig "Swamp Barge." How to mobilize the oil rig to the location of the wells? after the rig was drowned by filling the tanks Ballast with water. After the rig sitting on the base and the Spud Can be fixed into its sea floor, we can start the offshore drilling. To prevent the oil rig from the sea flow , The oil rig is stabilized first by attaching it to the poles. If they are not stable and the position of rig is moving, there will be a serious problem, especially the well.
2. Offshore drilling for depth 15 - 250 ft, is usually used jack-up rig (typically 3 or 4 feet, and the independent legs with spud legs on its leg. There is also a non-independent leg with type "mat foundation" . The oil riglegs with a mat foundation is usually used in the sea that have soft seabed .
Jack-up oil rig can be used to drill exploration wells in the oilfield . The most critical time is when moving the oil rig aproaching to the platform, because the rig should be close to a certain distance from the platform. It should be done carefully, The oil rig might hit the platform and can cause significant damage. The distance between the oil rig and platform has been determined according to the design,so oil rig floor and derrick located on the cantilever on the deck that can moved so those all can reach the well slots on the platform . One platform can contain 4, 6, 9, 12 or more slots depending on the size of the well platform. Platform for approaching oil rig is guided by 2 or 3 towing boats, and supported by 2 or 4 anchors in the rig. After the rig in the final locked position, the rig legs are given the initial burden with water. The Oil Rig hull is increased slightly above sea surface until the rig legs is not going down at 100% preload. Usually after the oil rig is stable for three hours the "initial burden" is removed and the rig can be jacked up to the height of a certain mode position for Drilling on the platform. That's how the offshore drilling done for water depth 15 - 250 ft.
3. Offshore drilling for depth (> 250 ft), used drillships (floater) or semi-submersible.
This Floater Drilling Rig is usually used for drilling well since this type can be attached to developments wells.This type of oil rig is usually equipped with 8 anchors, which are spread around the rig. After the rig is in the position of the wells, all anchors are deployed and given tension (pretension) to 300000lbs for each anchor. Similar to 'preloading' in type jack-up oil rig, 'pretension' during mooring operations is very important to be done to floaters oil rig ,so it can be stable at the Drilling mode. In addition, the rig is also equipped with "motion compensator" system to overcome the problem heave, pitch and roll, so the relative position of the hole is stable all the times. After all these steps are done,we are ready to do offshore drilling. That's how the offshore drilling done for water depth more than 250 ft