Page 575 - Marine Structural Design

Chapter 30 Risk Assessment Applied to Wshore Siruciures 551

instance, Gilbert et a1 (2001) presented a study to compare the risks of FPSO, which have
never been used in the Gulf of Mexico, with the risks for existing deepwater floating
production systems (TLP and Spar) in the Gulf of Mexico, and a shallow-water jacket serving
as a hub and host to deepwater platforms. The whole production systems were considered,
from the wells through the transport of product to the shore. Three risk measures were
assessed and analyzed for each system in a 20-year production life: the total number of the
human fatality risk, the total volume of oil spilled as a measure of the chronic environmental
risk and the maximum volume spilled in a single incident. It was concluded that there are no
significant difference in the fatality risks, environmental risks among of the four types of
systems studied. The study has been very useful for the regulatory agency and offshore
industry to accept the use of FPSO in the Gulf of Mexico.
30.6.6 Risk Based Inspection
Three fundamental questions are to be answered in the planning of risk-based inspections (Xu,
2001),
what should be inspected ?
how much effort should be made on individual components or details ?
when the inspection should be conducted ?
The key step in inspection planning is the ranking of the components for inspection. A rating
system should be created including analysis of frequency and consequence and the detection
of defects through inspection. The frequency analysis may be based on databases for failure
frequency and analytical methods or a combination of the databases and analysis.
The consequence of failure to be considered in inspection of the FPSO structures include the
following,
Structures including vessel hull and topside structures
catastrophic - loss of stability and structural integrity or leading to downtime of more thn
one year;
critical - loss of structural integrity that requires excessive dry dock repairs or down time
of between 6 months and one year;
severe - moderate structural damage that requires minor dry dock repirs or downtime of
between 1 months and 6 months;
minor - minor damage that requires a quick onboard repair or a down time of less than one
month.
Mooring systems and the thruster system that assists the station keeping system
catastrophic - resulting in a big loss of asset or downtime of more than one year
critical - resulting in major collision and grounding with downtime of between 6 month
and one year;
severe - leading to minor collision and downtime between 1 month and 6 months.
minor - leading to repair or replacement of one line at site and two or more lines damaged.