Page 140 - Marine Structural Design

116 Part I Structural Design Principres

stochastic analysis) is recommended. In order to linearize the actual non-linear soil response,
the stiffness matrices for structure-soil interaction should be developed based on a wave
height, which contributes most significantly to the fatigue damage.
Assessment of Existing Platforms
An existing platform should be re-assessed if the design conditions change such as:
If the original operating load is significantly exceeded due to the addition of facilities,
If the structure is altered, degraded due to fatigue and corrosion damages.
If the structure has an inadequate air gap, is operated under different environmental and
operating conditions.
When the life-safety level becomes more restrictive.
API RP 2A (1997) give a comprehensive recommendation on survey, metocean, seismic and
ice load criteria, structural analysis methodologies and evaluation criteria as well as mitigation
alternatives.

Fire, Blast and Accidental Loading
API Rp 2A (1997) proposed risk-based structural assessment for fire, blast and accidental
loading. The assessment includes the following tasks:
For the selected platform, assign a platform exposure category based on consequence of
failure in terms of human life and cost,
For a given event, assign risk levels Low, Medium and High to the probability (fiquency)
of the event occurring,
Determine the appropriate risk level for the selected platform and event based on a risk
matrix,
Conduct further study or analysis to better understand frequency, consequence, and cost of
mitigation and set acceptance criteria based on (As Low As Reasonably Practicable)
ALARP principle.
Reassign a platform exposure category andor mitigate the risk or the consequence of the
event.
For those platforms considered as high risk for a defined event, a detailed structural integrity
assessment is to be conducted for fire, blast or accidental loading based on nonlinear finite
element analyses or experimental tests.
For a comprehensive reference list and guidance on the design against fire and blast loads,
reference is made to ISSC (1997). The panel V.2 report for ISSC (1997) summarized design
and assessment philosophy, preventive and protective measures for fires, analysis methods for
fire loads and load effects, analysis methods for blast loads and load effects, probabilistic
analysis and design recommendations.
6.5.3 Floating Production and Offloading Systems (FPSO)
Structural Design General
The design of FPSO should comply with the classification requirements and industry
standards, e.g. NORSOK (NTS, 1998) and API RP 2FPS. In cases where the FPSO is