A fixed platform consists of a welded tubular steel jacket, deck, and surface facility. The jacket and deck make up the foundation for the surface facilities. Piles driven into the seafloor secure the jacket. The water depth at the intended location dictates the height of the platform. Once the jacket is secured and the deck is installed, additional modules are added for drilling, production, and crew operations. Large, barge-mounted cranes position and secure the jacket prior to the installation of the topsides modules. Economic considerations limit development of fixed (rigid) platforms to water depths no greater than 1,500 ft.
Surface facilities (also known as topsides) are the part of the platform that contains the drilling, production, and crew quarter modules. The size of each module is dictated by the volume of fluid to be handled, the number of personnel needed to operate the facility and operations, and the potential expansion needed to accommodate future production from other fields. Combined, the topsides dimensions could be 200 feet by 200 feet per deck level, with four decks, resulting in an overall height of 100 feet.
A jacket is a tubular supporting structure for an offshore platform consisting of four, six, or eight 7- to 14-ft diameter tubulars welded together with pipe braces to form a stoollike structure. The jacket is secured to the seafloor by weight and 7-ft diameter piles that penetrate several hundreds of feet beneath the mudline. Typical base dimensions are 400 feet by 500 feet. Skirts are also added to aid the jacket in fixing it to the seafloor. At the water line, dimensions can range up to 150 feet on a side. The water depth that the topsides will reside in normally dictates jacket height.
A pipeline is a system of connected lengths of pipe that transports hydrocarbons; the pipe is usually laid or buried on the seafloor by a pipe-lay barge. Pipe diameters generally range from 4 to 36 inches. The pipes may be coated in concrete for weight and use some type of cathodic protection for long-term integrity. Distances between the production facility and its onshore destination dictate length.
Support services that make everyday operation possible include supplies, materials, and workers that can be transported by workboats, crewboats, supply boats, and helicopters.
After the onshore fabrication of the jacket is completed, it is loaded onto a very large barge (dimensions up to 850 ft by 200 ft by 50 ft) that will transport the jacket to its location. The towing of the jacket may involve the use of several tugs (up to 52,000 hp combined) over hundreds of miles, the distance determined by where the jacket is fabricated and where the intended site is located. In some designs, there is a jacket base section that may be in place before the actual jacket is installed. The placement of the jacket base section prior to the jacket could provide better support during installation. Once the jacket arrives on location, it is launched, up-ended, and lowered into position with two or more tugs. With a beacon system or with a remotely operated vehicle (ROV) assist, the jacket is placed in position on the seafloor. The beacon system consists of homing devices laid on the seafloor around the area of the jacket's intended site; the beacons allow computer-aided control and monitoring of the installation process. Then a pile and hammer-handling barge is brought in to drive the piles into the seafloor, through guides in the legs. A second method of pile driving is the use of an underwater hammer with ROV alignment. Once this work is completed, the jacket is secure on location and the surface facilities can be installed.
The surface facilities are fabricated onshore and towed out on one or more crane barges. Once on location, the crane barge(s) is moored in whatever fashion needed and installation begins. Mooring can be done by different methods such as lines to the seafloor only or a combination of lines to the jacket and the seafloor. A crane on the barge(s) transfers the modules as a whole or separately from the barge(s) to the deck where workers complete the final connections.
A pipeline is connected to the jacket via a jumper at its base. A pipe-laying barge or ship installs the pipeline over the distance needed to connect the platform to shore or another facility. For deepwater applications, these vessels may be dynamically positioned and do not require any mooring system.
The nature of a component as well as the weather dictates the extent and duration of the maintenance performed on a platform. Either divers in shallow water or an ROV in deeper water would inspect the jacket or anything inside its boundary to determine the extent of maintenance required. A crane barge would attempt any retrieval or replacement. The crew can maintain any surface facility component, such as the drilling, production and crew quarters module, and repair it with parts brought in by workboats. If major repairs or replacements are needed, a crane barge transfers large materials or complete modules.
The pipelines are monitored for pressure changes in the lines and through ROV inspection by video. If leaks are detected, repairs are begun. Clamps can be used to minimize lost fluids until a new line is laid and put on line. Pigs are pumped through sections of the pipe to clean out the inner walls, clearing them of any paraffin or hydrate coating. (Pigs are wipers that can be several feet long and whose cross-section equals the inner diameter of the pipeline.)
With platforms, seafloor footprints are limited to the dimensions of the base of the jacket and the mooring systems of crane barges and workboats. These dimensions are stated earlier in this chapter. The mooring systems of the crane barges and workboats may vary, but they commonly use the jacket structure and the seafloor for anchoring.
During normal operations of the surface facility, air emissions occur from the separation, compression, and cogeneration components, and from other sources. Emissions can occur during the installation and maintenance of any of the components. The prime movers for the drilling operations and the operational components for the living quarters also add to the air emissions. Stored chemical may spill or ignite, adding to the emissions.
Water discharges from the surface facility can occur from many sources during normal operations. Discharges may also occur during the installation and maintenance of any component. Any of the many liquid chemicals used in everyday operations have the potential of being spilled. These include but are not limited to any glycol or methanol used in chemical injection, dispersant agents used in oil-spill response, produced waters, mud residues from the rock cuttings, and cleaning agents.
From drilling operations, the cuttings account for most of the discharges. The water depth dictates the distribution of the cuttings and the concentration of the drilling mud still left on the cuttings. In deepwater, cuttings have longer distances to travel to reach the seafloor and are distributed over a larger area. The size and type of particle, as well as the ocean currents, affect the distribution. This disturbance is not focused in a small area as in shallow water, where piles may accumulate after a long time. Other impacts may include any dropped objects such as tools, spare parts, and trash.
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