【正文】 re drilled from a pad, with the rig being pulled from well to well on metal skids. In order to move equipment from pad to pad, special wheels are mounted directly to the rig substructure and lowered using hydraulic jacks, lifting the rig off the ground. Rigs and related equipment are also transported to remote arctic locations, using Hercules cargo planes, helicopters, or special widetire vehicles called “rolligons.” Often these vehicles move over “ice roads” constructed from fresh water in order to protect the tundra from heavy loads and repeated travel (Gulick 1983).Difficult onshore drilling environments are not always in remote locations. For example, camouflaged and noisedampened drillsite。 are sometimes required for urban drillsites, as happens in the Los Angeles, California, area.Traveling to offshore drilling locations, on the other hand, is not always as difficult as remaining there! The weather and water depth are the two greatest challenges to offshore drilling operations. Conditions at the various areas where offshore drilling is currently conducted can vary significantly. For example, in the Gulf of Mexico water depths can vary from less than 60 ft (18m), where many wells have been drilled, to more than 1000 ft (300 m), where Shell39。s Cognac platform currently holds the depth record for a production platform at 1025 ft (313 m). Although the weather is usually relatively calm in the Gulf of Mexico, occasional hurricanes can bring winds of over 100 mph (45 m/s) and 50 ft (15 m) waves.In the North Sea, rough seas are much more mon and 100 ft (3 3 m) waves must be considered in the design of platforms and drilling equipment (Graff 1981). In Alaska39。s Prudhoe Bay and Beaufort Sea, the annual cycles of ice formation and breakup can plicate drilling activities. Beaufort Sea drilling has been carried out from natural barrier islands and artificial gravel islands that are serviced by a variety of vehicles, depending on ice conditions. Rigs and equipment can be moved by barge during the icefree period of late autumn. After the sea ice has formed between the islands and mainland, thick ice roads can be constructed on top of the sea ice. Prior to road construction, helicopter and STOL (shorttakeoffandlanding) aircraft are utilized. Transportation during the time period in which the ice is breaking up requires the use of a “hoverbarge,” an aircushioned vehicle with a flat barge deck, or an AST (Archimedian Screw Tractor), an amphibious vehicle capable of towing cargo through water and broken ice on two screwshaped pontoons.Offshore drilling in deeper waters must be conducted from icedefended drillships or from stationary platforms built to withstand the tremendous forces generated by moving ice (Gulick 1983). Icebergs can also threaten offshore operations in northern areas, particularly offshore Newfoundland. If they are sighted in time, these icebergs (if small enough) can be towed away to prevent a collision with platforms or drilling vessels.Unfortunately, the sea is not the only thing that moves at some offshore locations. Earthquakeproof platforms must be constructed for areas like the Pacific Coast of North America. In the Mississippi River delta regions, where large volumes of sediment are currently being deposited, platforms are designed such that wells are drilled through the legs of the structure to protect the wells and stabilize the structure against mud flows. Of course, not every drilling environment is hostile, but there are, quite a variety of conditions that the design must take into account. Subsurface EnvironmentsBelow the surfaee, conditions area little less variable but far from pleasant. In general, drilling difficulty increases with increased depth. This is logical because we are required to maintain control over an operation that is taking place at increasingly greater distances via a relatively small linkage. (A string of 5 in ( m) drillpipe drilling a 15,000 ft (4572 m) well is roughly analogous to a string of everyday drinking straws dangling from the edge of a 75 story building). Increased depth also brings increased pressure, and controlling pressure is much of what drilling is all about. Subsurface pressures will be discussed next.Most of the sediment in the sedimentary basins in which drilling is conducted was deposited along with, or later invaded by, water. Consequently, the porous rocks of the petroleum reservoir and the formations above it are full of water. In many cases, the oil and gas accumulations make up little of the total volume of fluids saturating the reservoirs and their associated aquifers. It is the fluid column resulting from all this water, reaching up toward the surface, that causes the pressure encountered in the fluidfilled pore space of the reservoir rock. For example, if you measure the pressure at different depths in a body of water, it will increase according to the density of the water (Fig. ). The rate of pressure increase, or gradient, will increase with increasing salinity. Fresh water exerts a pressure of .433 psi/ft ( kPa/m) and water with a salinity of 55,400 mg/liter total dissolved solids exerts a pressure of .45 psi/ft ( kPa/m). If the body of water bees a basin of watersaturated sand (fig. ), the sand grains can be thought to pack together, supportingone another throughout the column. The fluid pressure in the pore space between the sand grains has not changed, it still varies with depth according to the density of the fluid.Figure Fluid pressure gradient in a body of waterFigure Overburden pressure gradient is the sum of the fluid and rock pressure gradients.The overburden pressure, or lithostatic pressure, on the other hand, is the sum of the pressure caused by the column of fluid and that caused by the column of sediment. Since the column of sediment is porous, it exerts a force per unit area that is only a little greater than that of the water. The variation of this sum with depth is expressed as an