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January 2012 www.sname.org/sname/mt including NaKika (2001), Thunder Horse (2005), and Atlantis (2006). These units had massive hulls designed for very large payloads and installed water depths ranging from 6,340 to 7,100 ft. (1,930 to 2,160 m). under Horse was the largest steel semisubmersible ever built, and it included a full drilling system with dual derrick, accommodations for 188 persons, and a 16-point mooring system. Atlantis was some- what smaller without the drilling system, and it had accommodations for 100 and a 12-point mooring system. These units re-established the semisubmersible as a preferred option for very deep water, harsh environment appli- cations. Later developments included Blind Faith, Independence Hub (in 7,920 ft., or 2,414 m water depth) and under Hawk FPUs. ese smaller units demonstrated the possibility of building the semisubmersible hulls internation- ally with the topsides fabricated in Gulf Coast yards, and then completing the platforms on the Gulf Coast at shore before installation at the site. Together, under Horse and Atlantis accounted for some 37% of total oil production in the U.S. Gulf of Mexico for the year 2009.Finally, Cascade and Chinook will deploy the rst FPSO to be used in the Gulf of Mexico, to come online in 2012. This unit will be installed in 8,200 ft. (2,500 m) water depth and will feature a disconnectable turret- mooring system to permit the vessel to move out of the path of approaching hurricanes. Modern deepwater oaters Performance was relatively simple to achieve in early-generation semisubmersible FPUs. It became a much more challenging objec- tive from a naval architecture or marine engineering point in the modern FPU. As the size of the topsides increased, so did the complexity in the motion characteristics, hull systems, structural design, construc- tion, and onboard safety systems. The evolution from the early units has been dramatic in terms of growth in physi- cal dimensions, improvements in safety, and increase in production capacity. Figure 2 shows the relative sizes of three large Gulf of Mexico semisubmersibles. Transporting hydrocarbons to shore is always an issue. One look at a map of the Gulf of Mexico indicates the sheer number of platforms along the Gulf Coast in shallow water (the shelf), the smaller number of plat- forms located in the ultra-deepwater elds, and the varying presence of pipeline infra- structure (the ?spaghetti bowl?). e Gulf of Mexico and the North Sea are mature areas noted for their extensive pipeline systems. Brazil and West Africa, on the other hand, have less developed pipeline networks and must rely on FPSOs with storage capacity and shut- tle tankers to produce and deliver the product. Shelf facilities are interconnected by an extensive network of shallow water oil and gas pipelines. At the extreme margins of the Gulf of Mexico, pipelines are rare, and devel- opment options are limited. Constructing pipelines in extreme water depths is costly and challenging, and is somewhat pred- icated on discoveries with cooperating partners willing to share the significant investment. All of the current FPUs men- tioned here are tied to trunk lines, or to nearby platforms via ineld pipelines. e engineering activities associated with con- necting the platform to the pipeline present significant challenges to development of ultra-deepwater oating facilities. Early FPUs produced anywhere from 10,000 to 30,000 BOPD. The later units deployed in the North Sea were designed to produce from 100,000 to 190,000 BOPD. Increased throughput directly impacts the production equipment weight, crew requirements, and production riser loads, which then require greater payload capac- ity and hull displacement. The current generation of Gulf of Mexico FPU?s has pro- duction rates from 40,000 to approximately 225,000 BOPD or up to 1 billion cubic feet per day of gas production. Constructability Fabrication of large FPUs is truly an interna- tional business, and the Gulf Coast plays a key role in the process. e recent trend has been to produce newbuild oater hulls in the Far East (Korea, Singapore, and Malaysia) and construct the topsides (deck structure, production process modules, quarters, power generation modules) on the Gulf Coast. Hull dimensions and top- sides weight must be tailored to this method so the topsides can be lifted onto the hull inshore at a protected dock. ese oaters generally use the ?truss deck? topsides conguration. Several Gulf Coast yards have the facili- ties and equipment to carry out the inshore deck lift and assembly. Another common FPU configuration is known as the ?deck box? design, whereby the topsides incorpo- rates a watertight upper hull with equipment and accommodation spaces, plus external modules placed on top of the box deck. is conguration requires the hull and box deck to be assembled at the hull fabrication yard. Both methods employ large submersible dry transportation vessels to deliver the hull from the hull yard to the Gulf Coast. Inuences on FPU designs The impact of water depth on hull size may not be obvious, but can be explained as follows. Deeper water requires lon- ger risers, which generate more riser load, and longer mooring lines that translate to more buoyancy required from the hull. Recent discoveries made in U.S. Gulf of Mexico, beyond 5,000 ft. water depth, show greater potential reserves than the shallow water elds, which mean more oil and gas throughput, more risers, and larger topside processing facilities. All these factors con- tribute to the increase in hull size. þÿ 1970s þÿ 1980s þÿ 1990s þÿ 2000s þÿ 2010s 446 ft (136 m) FIGURE 2. RELATIVE SIZES OF GULF OF MEXICO SEMISUBMERSIBLES 32_37_Praughtfeature_SNAME_Jan12_P3.indd 3512/22/11 3:39 PM