ROSEAU is a slender deepwater comp1iant platform designed to support drilling and production facilities. It is rigidly piled to the bottom like a conventional jacket .Its compliancy and its large first natural period are provided by the flexibility of the steel structure and by a stabilizer which entrains a large mass of water. The basic design described here is for a structure in the Gulf of Mexico (Fig. 1) but other structures in different environments have been studied.
Mathematical models have been used to analyse the behaviour of the structure and to calculate the maximum stresses and fatigue life of the members.
Model tests have been run to correlate the calculations.
For many years, the exploitation of deep water oil fields has stimulated the engineer's imagination. Many concepts have been developed. Even if subsea completion appears feasible in deep waters, the preference in the industry is to try to keep the wellheads above water in order to limit maintenance cost and to obtain a high level of safety.
This trend is illustrated in Figure 2 by a selection of jacket platforms for exploiting oil fields in water depths now up to 400 m (1300 ft).
Amongst various concepts, compliant structures have great advantages for water depths beyond the range of applicability of conventional fixed structures.
Fixed platforms such as jackets become very difficult to design and very heavy in deep water, because they have to sustain the total wave loading. In addition the first natural period of vibration of these structures is close to that of waves, thus creating a dramatic dynamic amplification.
On the contrary, comp1iant structures such as TLP, guyed tower or articulated tower, are designed to move slightly in response to the dynamic environmental forces. The wave loads, for a large part, are balanced by inertia forces. ROSEAU is also a compliant structure. As such, it has a more favorable dynamic behaviour than a conventional jacket whilst offering the same operational advantages:
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small displacement at topside level
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continuous support of the conductor pipes
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structure rigidly piled to the seabed preventing localised bending of the conductors at the mud line.
Since 1981 several ROSEAU structures have been evaluated for various different conditions such as the Gulf of Mexico, the Mediterranean Sea and the North Sea. The design has been improved in terms of simplification and safety. Sophisticated calculations have confirmed the reliability of the design against extreme conditions and the fatigue analysis has verified the design life of a ROSEAU installed in 600 m (2000 ft) in the Gulf of Mexico.
Although the concept of ROSEAU has been derived from that of the compliant structures such as the guyed tower or the articulated tower, it may be distinguished from these two well known concepts by the way its stability is provided.
