Page 142 - Marine Structural Design

118 Part I Structural Design PrincipreS

Long-term service at a fixed location
Seas approaching predominantly from a narrow sector ahead
Zero ship speed
Range of operating loading conditions
Tank inspection requirements
Different return period requirements compared with normal trading tankers
For ocean-going vessels, classification Rules specify corrosion control, coating requirements,
corrosion prevention equipment/operation and wall-thickness allowance based on 20 years of
operating life. For FPSOs, additional wall-thickness allowance may be required considering
factors such as,
An FPSO may have longer operating life,
An FPSO requires no dry docking inspection and
The cost for coating repair and reduced production is high.

Analysis Models
Five typical levels of modeling may be developed for the finite element analysis of the hull
structure including:
Global Structural Model (Model level I),
Cargo Tank Model & Turret Model for FPSO (Model level 2),
Frame and Girder Model (Model level 3),
Local Structural Analysis (Model level 4),
Stress Concentration Models (Model level 5).
The 3D FEM models are developed for the following:
Cargo tank area at midship,
Fore end area including the structure supporting the flare boom,
Module supports and the supporting structure,
Main crane pedestals supporting structure,
Porches for productiotdinjection risers and export risers including the supporting
structures, pull-in supports,
Spread mooring attachments.
The structural design shall give due consideration to the loads imposed by the topsides, risers
and mooring connection and this should be reflected in the FEM model. The main stress
contributions include:
Primary stress due to global hull girder bending,
Secondary stress due to panel bending between bulkheads,
Tertiary stress due to local plate bending between web hes.