shut-in valves, high-low pressure pilots, high-low level controls, high-temperature shutdowns, gas detectors, shielded ignitions, fire prevention and detection equipment, and pressure relief systems. Drain and sump systems are also designed to collect any spillage that might occur on the platform. The sequence of drilling operations for production wells is essentially the same as for exploratory wells. As water depths increase, the economic desirability of seafloor completions and, in fact, an entire subsea production system increases. Considerable progress has been made in this direction. One system allows non-divers, oil field mechanics and technicians, to work on the wellhead on the seafloor in a one-atmosphere climate by lowering them in a work chamber which can be sealed to the base of the wellhead and evacuated to atmospheric pressure. Other systems comprising complete producing facilities (pumps, separators, treater, etc.) are in various stages of developments. The safety equipment used in subsea systems is similar in concept to that used in current operations and will provide the same high degree of environmental protection. The systems are designed such that a failure condition will automatically shut-in the affected part of the system. As with platform wells each subsea well will be equipped with a subsurface safety valve that can be actuated and tested on a frequent basis. Subsea completions will result in seafloor obstructions that could foul trawling gear. Only a small portion of the total trawling effort takes place at depths where subsea completions are most likely to be used. Should a trawl snag a subsea completion the possibility that it would damage any of the well head assembly to the extent of causing uncontrolled flow is extremely remote because of the strength and durability of the materials that would be used in the sea floor structure. Wells usually are produced through tubing placed inside the final or production string of casing. During tubing installation, the blowout preventers remain in use to ensure control of the well A system of in-tubing safety valves, plus other casing and tubing valves at the surface or seafloor, is installed to control well Actuation is usually at the producing platform. A wellhead consisting of several redundant control valves, is installed at the platform lower deck level and subsurface safety valves are installed at depths varying from a few hundred to several flow. thousand feet in the tubing string. Of major concern in the operation and control of every production platform are the downhole control devices. Production tubing is fitted with one or more safety valves that are installed and located at least 100 feet below the mud line or seaflooor. In the past, velocity choke valves ("storm chokes") designed to shut off production when the flow rate exceeds predetermined limits have been used. Such valves should close if surface equipment failure results in an excessive flow through the tubing. These chokes are particularly susceptible to failure from internal erosion in areas where sand is produced along with the oil and gas. Certain types of subsurface fail-safe valves do not depend on the velocity of well fluids for actuation, but are held open by hydraulic or other fluid pressure applied from the surface. The valve is designed to close automatically, shutting off the flow of fluid from the well in the event of some undesirable situation of the platform. Essentially all wells drilled since December 1, 1972, are equipped with valves that are actuated from the surface. These valves provide highly reliable protection and may be tested frequently to insure proper operation. Their use will increase costs significantly, but the need for more reliable valves has been shown by recent incidents in the Gulf of Mexico and elsewhere. The increased degree of safety offered by use of the fail-safe valves probably justifies their installation. Blowout preventers as well as downhole control devices have proven to be extremely valuable, in time of accidents and emergencies, in preventing large amounts of oil from escaping into the environment. When hurricanes have passed through offshore oil and gas fields, entire platforms have been swept away with only a minimal spillage of oil (see Section II.B.6.). b. Drilling Programs As with exploratory drilling, the casing program and mud program for each well must be approved by the Geological Survey before a drilling permit is issued. The following information was furnished by the Geological Survey and petroleum industry and describes the mud and casing program and cuttings generated by a representative offshore well. This well is assumed to be: 3) using a standard platform mounted rig a "normal" well, i.e., one in which no special The representative 10,000-foot offshore well generates approximately 1,674 bbl. of cuttings weighing about 695 tons. To drill this well approximately 7,000 bbl. of seawater drilling mud containing 237 tons of mud components are used. The drill cuttings are separated from the mud by screens and discharged overboard and the mud is saved and transported for reuse at another well site. Average drilling time is 10 to 14 days. The casing program for this well consists of four strings: 1) the structural casing, about 30 inches in diameter. 2) It is set to a minimum depth of 100 feet to provide the 16-inch conductor pipe, set at 900 feet 3) the 10 3/4 inch surface casing, set at 3,500 feet 4) the 7-inch production string, set at 10,000 feet. Drill Cuttings Table 30 shows the volume of cuttings generated by the representative 10,000-foot well and the types of drilling muds that are used to drill the well. As the drilling fluid-drill cuttings mixture is circulated to the surface, drill cuttings are separated from the drilling fluid by shale shakers, desilters, and desanders and discharged overboard. |