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Offshore production operations
Offshore production operations can be either very similar to or radically different from land-based installations. Discussions on artic operations are covered in the artic operations page
Except for a few innovative installations, wellheads and Christmas trees on platforms are basically the same as for land wells (see Fig 1). Control valves, safety valves, and piping outlets are configured the same and use the same or similar components. Some of the valves probably will have pneumatic or hydraulic actuators to facilitate remote and rapid closure in an emergency. Also, some Christmas trees may have composite block valves instead of individual valves flanged together.
The major difference, however, between land and platform well completions is the economics incentive on platforms to reduce equipment weight wherever possible and to minimize space requirements. Simply put, lighter, smaller equipment and more compact installations result in less expensive platforms. A good example is the use of composite block valves to reduce Christmas-tree size and weight. Another example is the spacing of wellheads, as close together as drilling operations will permit, with just enough room for safe and efficient operations of the tree valves, control valves, and well-workover equipment. Typically, this means centerline distances of 6 to 10 ft between wellheads.
Where only one drilling rig is on a platform, all the wellheads usually are located in one bay. Larger platforms that are designed to accommodate two drilling rigs may have two well bays (one for each rig) with two or more rows of wells in each bay.
The primary function of process equipment, whether on a platform or on land, is to stabilize produced fluids and prepare them for shipping or disposal. Well production is separated into components of oil, gas, and water (and sometimes condensate). The separated fluids are measured and then either shipped, injected back into the reservoir, or flared.
Differences between the process equipment (oil and gas separators, free-water knockouts, gas scrubbers, pumps, compressors, etc.) installed on a platform and those installed on land are minor (see Fig. 2). Where possible, consideration is given to using vessels and machinery that are compact and lightweight (e.g., electric motors are commonly used instead of gas engines for driving pumps and compressors). Vertical clearance between decks may impose height limitations and dictate, for example, the use of horizontal instead of vertical separators.
There is a major difference, however, in the way equipment is packaged. If it is to be installed offshore after placement of the platform jacket and decks, process equipment usually is built in modules at a land site. The module assemblies then are barged offshore, lifted onto the platform, and hooked up. This significantly reduces expensive offshore installation and hookup time. In any event, the equipment and its piping, wiring, and controls are installed as compactly as possible. The extra engineering and fabrication cost needed to reduce deck area to an absolute minimum are more than offset by savings in platform structure cost.
Well servicing and well workover
On relatively small platforms with no more than 5 to 10 wells, it is common practice in some areas to drill all the wells before any of them are placed on production. The drilling rig is removed after the last well is drilled, and future well workover is performed with a portable workover rig well pulling unit or hydraulic workover unit (HWO). Downhole work that does not require pulling tubing (e.g., replacing safety valves, gas lift valves, or standing valves) normally is accomplished with a wire-line unit.
On larger platforms with more wells, drilling and production operations generally are carried on concurrently. In this case, the drilling rig performs well workover if it is still on the platform.
In the great majority of cases, crude-oil production is transported from platforms by subsea pipelines. Because most offshore producing areas involve multiple platforms and more than one operating company, the pipelines are generally common carriers.
An offshore pipeline can be the single most expensive element of an offshore installation, sometimes far exceeding the cost of one or more platforms, depending on:
- Pipe diameter
- Need for burial
- Need for coatings and cathodic protection
- Water depth
- Various construction considerations
In the great majority of cases, however, piping is still the safest, most economical way to transport crude-oil production to a land site.
Occasionally, an offshore oil field is too remote, production rates are too low, or the field is too short-lived to justify a pipeline economically. The alternative is to transport the oil using tankers. This usually requires some type of loading system installed 1 to 2 miles from the platform, such as a moored buoy or articulated loading tower. A seafloor pipeline connects the loading facility during the transfer of oil.
The two most important drawbacks of tanker-loading operations are sensitivity to weather and the need for separate oil storage. Tanker loading is best suited to mild weather areas to minimize downtime from storms. Oil storage requirements will depend on total field producing rates and reservoir characteristics (i.e., whether the wells can be shut in for short periods without lost productivity) as well as tanker downtime. This has led to the development of permanently moored storage tankers.
Disposal of gas from an offshore production site will depend on a combination of reservoir and economic factors. If well production is primarily oil, the gas may be handled as a byproduct and be disposed of in the most economical way. Piping the gas to a land site for sale and use as a fuel is generally preferred if it can be done economically. Injection back into the producing formation is a common alternative. This helps maintain reservoir pressure and conserves the gas for possible future sale. In some areas, gas flaring is still acceptable, but many countries now forbid it except for the short test periods and for the disposal of small amounts of residual waste gas.
Produced water is normally cleaned so that it may be either discharged offshore in accordance with governmental regulations or re-injected into the reservoir. In either case, a combination of mechanical and chemical methods may be used to condition the produced water before disposal. Tankage and filtration are used to remove oil and other contaminants from the water. Chemical treatment is common to control bacteria and corrosion in injection wells.
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