2
STRUCTURES IN THE OFFSHORE ENVIRONMENT
In perspective, offshore structures include a great deal more than the towers
and platforms. They include moored or mobile ships whose positions may be
precisely controlled. They include the guy lines for compliant towers, the cables
for buoys and for tension-leg platforms, and the associated pipelines without
which the platforms and submerged oil production Systems would be useless.
Detailed descriptions of such installations may be found in the references at the
end of this chapter. Of particular note is the review article on compliant offshore
structures by Adrezin et al.(1996), with its 130 citations to the world literature
on the subject up to the mid-1990s. For descriptions of current practice in ail
types of offshore installations, the reader is referred to the yearly conférence
proceedings such as found in the References at the end of this chapter.
This chapter begins with a short history of offshore structures, describes typical state-of-the-art installations, and concludes with a discussion of engineering
challenges for future designs. Subséquent chapters address in some detail both
the mathematical modeling and the environmental loading of offshore structures, together with ways to predict their dynamic responses and structural
integrity, from both the deterministic and the statistical viewpoints.
1.1 HISTORICAL PERSPECTIVE
The earliest offshore structure for oil drilling was built about 1887 off the coast
of Southern California near Santa Barbara. This was simply a wooden wharf
outfitted with a rig for drilling vertical wells into the sea floor. More elaborate
platforms supported by timber piers were then built for oil drilling, including
installations for the mile-deep well in Caddo Lake, Louisiana (1911) and the platform in Lake Maracaibo, Venezuela (1927). Soon after these early pier Systems
were built, it became apparent that the lifetime of timber structures erected
in lakes or océans is severely limited because of attacks by marine organisms.
For this reason. reinforced concrète replaced timber as the supporting structure
for many offshore platforms up to the late 1940s. Over the next 50 years about
12,000 platform structures were built offshore, usually of Steel but more recently
of precast concrète. The chief features of these structures, together with their
support ing components such as mooring Systems and pipelines, are discussed in
this chapter. See also Gerwick (1999) and Will (1982).
Offshore mooring Systems hâve a variety of configurations. Ail hâve anchors
or groups of piles in the seabed with flexible lines (cables, ropes, chains) leading
from them to buoys, ships, or platform structures. The function of a mooring
System is to keep the buoy, ship, or platform structure at a relatively fixed
location during engineering operations. Engineering efforts in mooring Systems
ha\e focused in recent years on the development of new anchor configurations
with higher pullout loads, larger capacity and lower cost of installation for deeper
water applications.
W hen pipelines were first laid offshore, no extraordinary analyses or deployment techniques were needed since they were in shallow water and were of small
diameter. somewhat flexible, and made of relatively ductile Steel. As platforms
were built in deeper and deeper water with multiple well slots, larger diameter
STRUCTURES IN THE OFFSHORE ENVIRONMENT
In perspective, offshore structures include a great deal more than the towers
and platforms. They include moored or mobile ships whose positions may be
precisely controlled. They include the guy lines for compliant towers, the cables
for buoys and for tension-leg platforms, and the associated pipelines without
which the platforms and submerged oil production Systems would be useless.
Detailed descriptions of such installations may be found in the references at the
end of this chapter. Of particular note is the review article on compliant offshore
structures by Adrezin et al.(1996), with its 130 citations to the world literature
on the subject up to the mid-1990s. For descriptions of current practice in ail
types of offshore installations, the reader is referred to the yearly conférence
proceedings such as found in the References at the end of this chapter.
This chapter begins with a short history of offshore structures, describes typical state-of-the-art installations, and concludes with a discussion of engineering
challenges for future designs. Subséquent chapters address in some detail both
the mathematical modeling and the environmental loading of offshore structures, together with ways to predict their dynamic responses and structural
integrity, from both the deterministic and the statistical viewpoints.
1.1 HISTORICAL PERSPECTIVE
The earliest offshore structure for oil drilling was built about 1887 off the coast
of Southern California near Santa Barbara. This was simply a wooden wharf
outfitted with a rig for drilling vertical wells into the sea floor. More elaborate
platforms supported by timber piers were then built for oil drilling, including
installations for the mile-deep well in Caddo Lake, Louisiana (1911) and the platform in Lake Maracaibo, Venezuela (1927). Soon after these early pier Systems
were built, it became apparent that the lifetime of timber structures erected
in lakes or océans is severely limited because of attacks by marine organisms.
For this reason. reinforced concrète replaced timber as the supporting structure
for many offshore platforms up to the late 1940s. Over the next 50 years about
12,000 platform structures were built offshore, usually of Steel but more recently
of precast concrète. The chief features of these structures, together with their
support ing components such as mooring Systems and pipelines, are discussed in
this chapter. See also Gerwick (1999) and Will (1982).
Offshore mooring Systems hâve a variety of configurations. Ail hâve anchors
or groups of piles in the seabed with flexible lines (cables, ropes, chains) leading
from them to buoys, ships, or platform structures. The function of a mooring
System is to keep the buoy, ship, or platform structure at a relatively fixed
location during engineering operations. Engineering efforts in mooring Systems
ha\e focused in recent years on the development of new anchor configurations
with higher pullout loads, larger capacity and lower cost of installation for deeper
water applications.
W hen pipelines were first laid offshore, no extraordinary analyses or deployment techniques were needed since they were in shallow water and were of small
diameter. somewhat flexible, and made of relatively ductile Steel. As platforms
were built in deeper and deeper water with multiple well slots, larger diameter