PLATFORMS
7
Compilant Platforms
An alternative class of offshore structures meant for depths from 300 to 800
m is the compilant tower such as that shown in Figure l.ld. Such a tower
may or may not hâve mooring lines. It is a pile-supported Steel truss structure
designed to comply or flex with the waves and has considerably less structural
material per unit height when compared with a common jacket template tower.
The first compilant tower was the Lena, which was installed in the early
1980s in the Gulf of Mexico. Including its three-level drilling and production
deck and its drilling rigs, this tower reaches a total height of 400 m. Each
of the 20 stabilizing cables, attached 25 m below the water line and arranged
symmetrically about the structure, extends a horizontal distance of about 1000
m to a line of clumped weights that rest on the sea floor, to an anchor cable
and an anchor pile. Under normal weather or small storm conditions, the cables
act as hard springs, but with severe storms or hurricanes, the cable restraints
become softer or compilant. That is, the amplitude of tower rotation increases
at a rate greater than that of the loading, since the clumped weights lift off the
sea floor to accommodate the increased storm loads on the tower. When storms
or hurricane conditions are anticipated, operations on compilant towers cease
and the crew is evacuated.
Installation of the Lena cables was more difficult and costly than anticipated.
Subsequently, compilant towers without cables hâve been designed by Exxon,
and two such designs were installed in 1999 in the Gulf of Mexico. Unlike
the jacket-template structures, the compliant towers hâve natural frequencies
in bending or sway near 0.03 Hz, or well below the 0.05 Hz frequency of the
highest energy sea waves in the Gulf of Mexico during storm conditions. Thus,
an important feature of such structures is that they are designed to hâve natural
sway frequencies well removed from the frequency range of the highest energy
waves for normal seas (0.1 to 0.15 Hz) and for storm seas (0.05 to 0.1 Hz). This
frequency spread is necessary to avoid platform résonance, which can lead to
failure. The sway frequencies of two platforms in comparison to the frequency
range for the spectrum of the highest energy storm waves in the Gulf of Mexico
are depicted graphically in Figure 1.4. The measurement and meaning of this
wave height spectra, which is highly site-dependent, will be discussed in detail
in subséquent chapters.
Buoyant Platforms
The tension leg platform (TLP) can be economically compétitive with compliant towers for water depths between 300 m and 1200 m. The schematic design
of the TLP is depicted in Figure 1.1e. In such designs, the total buoyant force
of the submerged pontoons exceeds the structure’s total gravity or deadweight
loading. Taut, vertical tethers extending from the columns and moored to the
foundation templates on the océan floor keep the structure in position during
ail weather conditions. The heave, pitch, and roll motion are well restrained by
the tethers; but the motions in the horizontal plane, or surge, sway, and yaw,
are quite compliant with the motion of the waves. The first production TLP
7
Compilant Platforms
An alternative class of offshore structures meant for depths from 300 to 800
m is the compilant tower such as that shown in Figure l.ld. Such a tower
may or may not hâve mooring lines. It is a pile-supported Steel truss structure
designed to comply or flex with the waves and has considerably less structural
material per unit height when compared with a common jacket template tower.
The first compilant tower was the Lena, which was installed in the early
1980s in the Gulf of Mexico. Including its three-level drilling and production
deck and its drilling rigs, this tower reaches a total height of 400 m. Each
of the 20 stabilizing cables, attached 25 m below the water line and arranged
symmetrically about the structure, extends a horizontal distance of about 1000
m to a line of clumped weights that rest on the sea floor, to an anchor cable
and an anchor pile. Under normal weather or small storm conditions, the cables
act as hard springs, but with severe storms or hurricanes, the cable restraints
become softer or compilant. That is, the amplitude of tower rotation increases
at a rate greater than that of the loading, since the clumped weights lift off the
sea floor to accommodate the increased storm loads on the tower. When storms
or hurricane conditions are anticipated, operations on compilant towers cease
and the crew is evacuated.
Installation of the Lena cables was more difficult and costly than anticipated.
Subsequently, compilant towers without cables hâve been designed by Exxon,
and two such designs were installed in 1999 in the Gulf of Mexico. Unlike
the jacket-template structures, the compliant towers hâve natural frequencies
in bending or sway near 0.03 Hz, or well below the 0.05 Hz frequency of the
highest energy sea waves in the Gulf of Mexico during storm conditions. Thus,
an important feature of such structures is that they are designed to hâve natural
sway frequencies well removed from the frequency range of the highest energy
waves for normal seas (0.1 to 0.15 Hz) and for storm seas (0.05 to 0.1 Hz). This
frequency spread is necessary to avoid platform résonance, which can lead to
failure. The sway frequencies of two platforms in comparison to the frequency
range for the spectrum of the highest energy storm waves in the Gulf of Mexico
are depicted graphically in Figure 1.4. The measurement and meaning of this
wave height spectra, which is highly site-dependent, will be discussed in detail
in subséquent chapters.
Buoyant Platforms
The tension leg platform (TLP) can be economically compétitive with compliant towers for water depths between 300 m and 1200 m. The schematic design
of the TLP is depicted in Figure 1.1e. In such designs, the total buoyant force
of the submerged pontoons exceeds the structure’s total gravity or deadweight
loading. Taut, vertical tethers extending from the columns and moored to the
foundation templates on the océan floor keep the structure in position during
ail weather conditions. The heave, pitch, and roll motion are well restrained by
the tethers; but the motions in the horizontal plane, or surge, sway, and yaw,
are quite compliant with the motion of the waves. The first production TLP