Page 144 - Marine Structural Design

120 Part I Structural Design Principles

The hull girder strength should be evaluated to withstand relevant combinations of still water
and wave induced bending moments and shear forces. The extreme hogging and sagging
conditions are considered in the analysis. The appropriate site-specific environmental data
should be applied in the analysis.
Transverse strength refers to the hull's ability to resist lateral pressure and racking actions in
combination with longitudinal action effects. This resistance is provided by means of
transverse bulkheads, web frames, girders, and stringers. Transverse strength should be
evaluated using a finite element model of a specific portion of the hull, and the effects of deck
equipment actions should be included.
Usually buckling and ultimate strength for plated members and stiffeners are checked based on
NTS (1998) or API 2V or classification Rules. Typical criteria for plated members and
stiffeners are discussed in Chapter 10. The strength checks are camed out for main structures,
secondary structures and structures supporting hull appendages.
In some cases, the FPSO hull is also designed for collision, such that the collision with supply
vessels and shuttle tankers does not cause penetration of the side or inner longitudinal
bulkhead. Impacts from the bow, stem and side of the supply vessels and shuttle tankers are
considered. Fire fighting, explosion protection and heat protection are designed based risk
assessment, see PART V.
ABS (2001) gives guidance on SafeHull-Dynamic Loading Approach (DLA) for Floating
Production Storage and Offloading Systems (FPSO). The DLA approach provides enhanced
structural analysis to assess the capabilities and sufficiency of a structural design. A pre-
condition to use DLA is that the initial scantling for hull structures is based on Rule
requirements. The results of a DLA analysis may not be used to reduce the dimension of the
hull structures. However, if an increase of basic scantling is identified through the DLA
analysis; such an increase is to be accomplished to meet the DLA requirement. The DLA
analysis procedure consists of the following (ABS, 2001):
Create sea-keeping analysis models,
Assemble hull loading scenarios & create still-water load file,
Obtain & veri@ environmental data,
Conduct analysis of ship motions and predict wave-induced loads & extreme value for
each DLP (Dominant Load Parameters, such as vertical hull girder bending moment
amidships),
Derive equivalent wave for each DLP,
Establish wave-induced load effects,
Create structural analysis for the defined load cases,
Conduct global and local structural analysis,
Check structural analysis results against acceptance criteria.
The benefits from conducting a finite element analysis like DLA analysis is the increased
safety (by increasing scantling in the weak areas), reducing possible future renewals and
providing structural models that may be used immediately in the events of emergency
situations.