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Compliant Tower

Compliant towers are similar to fixed platforms in that they have a steel tubular jacket that is used to support the surface facilities. Unlike fixed platforms, compliant towers yield to the water and wind movements in a manner similar to floating structures. Like fixed platforms, they are secured to the seafloor with piles. The jacket of a compliant tower has smaller dimensions than those of a fixed platform and may consist of two or more sections. It can also have buoyant sections in the upper jacket with mooring lines from jacket to seafloor (guyed-tower designs) or a combination of the two. The water depth at the intended location dictates platform height. Once the lower jacket is secured to the seafloor, it acts as a base (compliant tower) for the upper jacket and surface facilities. Large barge-mounted cranes position and secure the jacket and install the surface facility modules. These differences allow the use of compliant towers in water depths ranging up to 3,000 ft. This range is generally considered to be beyond the economic limit for fixed jacket-type platforms.

The portion of the tower that contains the drilling, production, and crew quarter modules is the surface facility. Individually, size is dictated by the dimensions needed to handle production, drilling operations, and crew accommodations. The surface facilities are smaller by design on compliant towers than on fixed platforms because of the decreased jacket dimensions that support them.

The supporting structure, for a compliant tower; it may consist of a lower and upper section. Typically, the tower's jacket is composed of four leg tubulars that can range from 3 to 7 ft in diameter and are welded together with pipe braces to form a space-frame-like structure. The lower jacket is secured to the seafloor by weight and with 2- to 6-ft piles that penetrate hundreds of feet beneath the mudline. Both the lower and upper jacket dimensions can range up to 300 feet on a side. The water depth the structure will reside in dictates the height of the jacket.

A series of buoyant tanks (up to 12) located in the upper part of the jacket places the members in tension, reducing the foundation loads of the structure. The tanks can range up to 20 ft in diameter and up to 120 ft in length. The amount of buoyancy is computer controlled, keeping the appropriate tension in the structure members during wind and wave movements. This buoyant system can also be incorporated into some member designs, minimizing the size and placement of the tanks.

For compliant towers in general, mooring is only used in the guyed-tower design. For guyed-towers, several mooring lines (up to 20 lines measuring 5 -inch dia.) are attached to the jacket close to the waterline and are spread out evenly around it (up to 4,000 ft of line). Clump weights (120 ft x 8 ft, up to 200 tons) may be attached to each mooring line and move as the tower moves with the wind and wave forces. To control the tower motions better, the lines are kept in tension during the swaying motions. The portion of the lines past the clump weights are anchored into the seafloor with piles (as many as 20, each 72-inch dia., 115-ft long, penetrating 130 ft, and weighing up to 60 tons).

A system of connected lengths of pipe that transports hydrocarbons. A pipe-lay barge usually lays or buries them on the seafloor. Pipe diameters generally range up to 36 inches. They may be coated in concrete or use some type of cathodic protection for long-term integrity. Distances between the production facility and its onshore destination dictate pipeline length.

Support Services that make everyday operations possible include supplies, materials, and workers, which can be transported by workboats, crewboats, supply boats, and helicopters.

During the onshore fabrication of the jacket, the mooring system for the guyed-tower is installed. A specially designed, dynamically positioned crane barge that consists of a 100-foot crane, guyline winch module, anchor pile module, and clump weight module can be used to install the compliant tower. The installation procedure starts with the anchor piles, then the chains, then the clump weights, and finally the remaining mooring lines that attach to the jacket as needed. Until the jacket is installed, anchor buoys hold the remaining lines in position.

After onshore fabrication, the jacket is towed in one or two pieces out to the site on a specially designed barge. The process is similar to how it is done for fixed platforms, such as Shell's Cognac or Bullwinkle project. For normal compliant towers, the upper and lower jacket can be joined at sea or vertically joined at the site. For the guyed-tower design, the jacket is in one piece and does not need joining. The jacket can be launched from the rear or side of the barge. For compliant towers, up to 10,000 hp per jacket section (upper and lower) may be needed. A series of tugs with a combined horsepower of up to 25,000 hp can be used to tow guyed-towers. The design of the compliant tower and whether the jacket is in one or two pieces dictate the number of tugs needed. One or two dynamically positioned crane barges (up to 2,000 tons) are used to install the jacket as a whole (guyed-towers) or join them vertically onsite or at sea.

After the jacket is installed, a deck barge brings the surface facilities out from shore to the site and installs them either module by module or as a complete unit. The crane barge can be moored to the jacket or to the seafloor with up to 12 lines. The twelve-point layout is most commonly utilized.

A dynamically positioned pipe-laying ship installs the pipeline or pipeline bundle. Since the dynamic-positioned system eliminates the need for anchors, the ship can operate in and around compliant towers having mooring systems without interfering with the guylines or mooring of the crane barges. The complete installation process can take up to eight months before any additional drilling or production can start. Maintenance of compliant towers is completed in a method similar to that of a fixed platform. Considerations in the guyed-tower design for the mooring system must be addressed when any maintenance vessels are moved. The installed component and the weather conditions dictate the extent and duration of the maintenance needed. A remotely operated vehicle (ROV) would do any anticipated maintenance of the jacket and would inspect anything within the boundary of the jacket (buoyancy tanks, risers etc.) and the mooring system (in guyedtower design). A crane barge would complete any retrieval or replacement.

The crew can maintain any surface facility component such as the drilling, production, and crew quarters modules and repair them with parts brought in by workboats. If major repairs or replacements are needed, a crane barge would transfer the needed large materials or complete modules.

Monitoring of the pipelines is accomplished by ROV inspection and watching for signs of pressure drops or increased fluid volumes. If leaks are detected, 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 are normally a few feet long and whose cross-section equals the inner diameter of the pipeline.)

In compliant towers, the seafloor footprint is the base-structure dimension mooring system of the tower (if guyed-tower design) and the mooring systems of crane barges and workboats. Base dimensions can range up to 300 feet on a side. The mooring systems of the crane barges and workboats may vary, but they commonly use the jacket structure and the seafloor for anchoring.

Operations are similar to those of fixed platforms but differ mainly in size and quantity. During normal operations of the surface facility, air emissions from the separation, compression, and cogeneration components apply. The prime movers for the drilling operations and the operational components for the living quarters also add to the air emissions. Stored chemicals 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 residue from the rock cuttings, and cleaning agents.

From the drilling operations, the cuttings account for most of the depositing. The distribution of the cuttings and the concentration of the drilling mud still left on the cuttings are dictated by the water depth and current. In deepwater, cuttings have longer distances to travel to reach the seafloor and create larger arrays of disturbance. These cuttings are not focused in a small area as in shallow water, where piles of cuttings may accumulate over a long time. Other impacts may include any dropped objects such as tools, spare parts, and trash.




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