1.1.1 Definition of Offshore Structures

An offshore structure has no fixed access to dry land and may be required to stay in position in all weather conditions. Offshore structures may be fixed to the seabed or may be floating. Floating structures may be moored to the seabed, dynamically positioned by thrusters or may be allowed to drift freely. The engineering of structures that are mainly used for the transportation of goods and people, or for construction, such as marine and commercial ships, multi-service vessels (MSVs) and heavy-lift crane vessels (HLCVs) used to support field development operations as well as barges and tugs are not discussed in detail in this book. While the majority of offshore structures support the exploration and production of oil and gas, other major structures, e.g. for harnessing power from the sea, offshore bases, offshore airports are also coming into existence. The design of these structures uses the same principles as covered in this book, however they are not explicitly included herein.

We focus primarily on the structures used for the production, storage and offloading of hydrocarbons and to a lesser extent on those used for exploration.

1.1.2 Historical Development

The offshore exploration of oil and gas dates back to the nineteenth century. The first offshore oil wells were drilled from extended piers into the waters of Pacific Ocean, offshore Summerlands, California in the 1890s (and offshore Baku, Azerbaijan in the Caspian Sea). However, the birth of the offshore industry is commonly considered as in 1947 when Kerr-McGee completed the first successful offshore well in the Gulf of Mexico in 15 ft (4.6 m) of water off Louisiana [Burleson, 1999]. The drilling derrick and draw works were supported on a 38 ft by 71 ft (11.6 m by 21.6 m) wooden decked platform built on 16 24-in. (61-cm) pilings driven to a depth of 104 ft (31.7 m). Since the installation of this first platform in the Gulf of Mexico over 50 years ago, the offshore industry has seen many innovative structures, fixed and floating, placed in progressively deeper waters and in more challenging and hostile environments. By 1975, the water depth extended to 475 ft (144 m). Within the next three years the water depth dramatically leapt twofold with the installation of COGNAC platform that was made up of three separate structures, one set on top of another, in 1025 ft (312 m). COGNAC held the world record for water depth for a fixed structure from 1978 until 1991. Five fixed structures were built in water depths greater than 1000 ft (328 m) in the 1990s. The deepest one of these is the Shell Bullwinkle platform in 1353 ft (412 m) installed in 1991. The progression of fixed structures into deeper waters upto 1988 is shown in fig. 1.1.

Since 1947, more than 10,000 offshore platforms of various types and sizes have been constructed and installed worldwide. As of 1995, 30% of the world’s production of crude came from offshore. Recently, new discoveries have been made in increasingly deeper waters. In 2003, 3% of the world’s oil and gas supply came from deepwater (>1000 ft or 305 m) offshore production [Westwood, 2003]. This is projected to grow to 10% in the next fifteen years [Ibid.] The bulk of the new oil will come from deep and ultra-deepwater production from three offshore areas, known as the “Golden Triangle”: the Gulf of Mexico, West Africa and Brazil. Figure 1.2 illustrates the recent growth in ultra-deepwater drilling in the Gulf of Mexico. Drilling activity is indicative of future production.

The importance of deepwater production to the US is illustrated in fig. 1.3. US oil production is on the decline, dropping from about 7.5 MM BPD in 1989 to 5.9 MM BPD in 2001. The current US oil consumption is about 20 MM BPD. Experts do not believe there are significant new resources onshore in the US. Deepwater production has grown from 9.5% of US production in 1989 to 26.4% in 2001 (from 750,000 to 1,500,000 BPD). The drilling activity shown in fig. 1.2 suggests that this percentage will continue to grow.

Fixed structures became increasingly expensive and difficult to install as the water depths increased. An innovative and cheaper alternative to the fixed structure, namely, the Lena guyed tower was introduced in 1983. The platform was built in such a way that the upper truss structure could deflect with the wave and wind forces. Piles extending above the sea floor could bend, and horizontal mooring lines attached midway up the platform could resist the largest hurricane loads. The Lena platform was installed in 1000 ft (305 m) of water. Two more “compliant” towers were installed in the Gulf of Mexico in 1998: Amerada Hess Baldpate in 1648 ft (502 m) and ChevronTexaco Petronius in 1754 ft (535 m). Petronius is the world’s tallest free-standing structure.

Although nearly all of these platforms are of steel construction, around two dozen large concrete structures have been installed in the very hostile waters of the North Sea in the 1980s and early 1990s and several others offshore Brazil, Canada and the Philippines. Among these, the Troll A (fig. 1.4) gas platform is the tallest concrete structure in existence. It was installed offshore Norway in 1996. Its total height is 1210 ft (369 m), and it contains 245,000 m3 of concrete, equivalent to 215,000 home foundations.

Gravity structures differ from other fixed structures in that they are held in place strictly by the weight contained in their base structures. The Troll platform, for example, penetrates 118 ft (36 m) into the seabed under its own weight.

The first floating production system, a converted semi-submersible, was installed on the Argyle field by Hamilton in the UK North Sea in 1975. The first ship-shaped floating production and storage system was installed in 1977 by Shell International for the Castellon field, offshore Spain. There were 40 semi-submersible floating production systems (FPSs) and 91 ship-shaped floating production and storage systems (FPSOs) in operation or under construction for deepwaters as of 2002 [Offshore, 2002]. Petrobras has been a pioneer in pushing floating production to increasingly deeper waters in their Campos Basin fields, offshore Brazil. Table 1.1 lists the progression of field development offshore Brazil in ever-increasing water depths. Some of the unique features of innovation and records are included in the last column.

1.1.3 Selection of Deepwater Production Concepts

The types of production concepts available for...