14
STRUCTURES IN THE OFFSHORE ENVIRONMENT
a smaller diameter polyuréthane pipeline, a seafloor wire rope to shore, and
concrète blocks on the sloping sea floor. This was the world s first and so far
the only OTEC System at sea to generate net useful power. In the year 2001,
further development work on OTEC Systems was underway in India.
1.5
CHALLENGES
To meet the need for new sources of energy and minerais, marine engineers must
work at the frontiers of known technology. Their main challenges are to design,
deploy, and operate facilities and equipment in environments where none hâve
before existed—in deeper and deeper water, on the slopes of the continental
shelves, and in the hostile Arctic seas. To meet the challenges, marine engineers
continue their research and development efforts in the following four broad and
necessarily overlapping topics of concern.
Envîronmental Forces
Every offshore facility is subjected to several types of environmental loads
during its lifetime. Thus, site-dependent databases are being developed to characterize the time-varying fluid-induced loads of winds, currents, and waves.
Such loads occur both on a short time scale of seconds and minutes, induced
by periodic vortex shedding, wind gusts, and wave slamming; and over longer
periods of hours, days, and even years, where the loads are induced by steady
waves, tides, and hurricanes. At some sites, data are also needed on subsea
earthquake intensity, on the scouring of sea floor foundations by currents, and
on the total and differential settlements of the sea floor due to the withdrawal
of hydrocarbons during the lifetime of the structure. At présent, there is the
particular challenge of minimizing differential settlements for gravity platforms.
For the Ekoflsk structural complex in the North Sea, however, this challenge
appears to hâve been met by employing water injection. In Arctic régions, data
are needed on the rates of ice accretion and on the velocity, yield strength,
and mass of floating ice. Reliable deterministic and statistical methods are being developed to measure and interpret time-dependent field data suitable for
predicting structural loadings.
Structural Materials
An offshore structure should hâve a high ratio of strength to self-weight.
For instance, for each added unit of deck weight for a tension leg platform,
an additional 1.3 units of hull weight are required for buoyancy support, and
an additional 0.65 unit of mooring pretension force is needed. In this case,
high-strength steels with high fracture toughness are being investigated for the
purpose of reducing hull weight. Hollow cylindrical Steel link chains or synthetic
mooring line materials, such as Kevlar with an abrasion-résistant polyethylene
coxer, are being developed to increase mooring capacity. To détermine the
suitability of new, high-strength steels and composite materials in the offshore
environment, test data are being generated. These data involve measures of corrosion fatigue, fracture toughness. stress corrosion cracking, and weldability for
STRUCTURES IN THE OFFSHORE ENVIRONMENT
a smaller diameter polyuréthane pipeline, a seafloor wire rope to shore, and
concrète blocks on the sloping sea floor. This was the world s first and so far
the only OTEC System at sea to generate net useful power. In the year 2001,
further development work on OTEC Systems was underway in India.
1.5
CHALLENGES
To meet the need for new sources of energy and minerais, marine engineers must
work at the frontiers of known technology. Their main challenges are to design,
deploy, and operate facilities and equipment in environments where none hâve
before existed—in deeper and deeper water, on the slopes of the continental
shelves, and in the hostile Arctic seas. To meet the challenges, marine engineers
continue their research and development efforts in the following four broad and
necessarily overlapping topics of concern.
Envîronmental Forces
Every offshore facility is subjected to several types of environmental loads
during its lifetime. Thus, site-dependent databases are being developed to characterize the time-varying fluid-induced loads of winds, currents, and waves.
Such loads occur both on a short time scale of seconds and minutes, induced
by periodic vortex shedding, wind gusts, and wave slamming; and over longer
periods of hours, days, and even years, where the loads are induced by steady
waves, tides, and hurricanes. At some sites, data are also needed on subsea
earthquake intensity, on the scouring of sea floor foundations by currents, and
on the total and differential settlements of the sea floor due to the withdrawal
of hydrocarbons during the lifetime of the structure. At présent, there is the
particular challenge of minimizing differential settlements for gravity platforms.
For the Ekoflsk structural complex in the North Sea, however, this challenge
appears to hâve been met by employing water injection. In Arctic régions, data
are needed on the rates of ice accretion and on the velocity, yield strength,
and mass of floating ice. Reliable deterministic and statistical methods are being developed to measure and interpret time-dependent field data suitable for
predicting structural loadings.
Structural Materials
An offshore structure should hâve a high ratio of strength to self-weight.
For instance, for each added unit of deck weight for a tension leg platform,
an additional 1.3 units of hull weight are required for buoyancy support, and
an additional 0.65 unit of mooring pretension force is needed. In this case,
high-strength steels with high fracture toughness are being investigated for the
purpose of reducing hull weight. Hollow cylindrical Steel link chains or synthetic
mooring line materials, such as Kevlar with an abrasion-résistant polyethylene
coxer, are being developed to increase mooring capacity. To détermine the
suitability of new, high-strength steels and composite materials in the offshore
environment, test data are being generated. These data involve measures of corrosion fatigue, fracture toughness. stress corrosion cracking, and weldability for