Page 145 - Marine Structural Design

Chapter 6 offshore Structural Analysis 121

Modeling for Compartmentalization and Stability
The relevant detrimental effects in the compartmentalization and stability assessment of an
FPSO are:
Environmental actions
Relevant damage scenarios
Rigid body motions
The effects of free-surface
Boundary interactions (e.g. mooring and riser systems)
In order to determine the vessel’s mass and position of the center of gravity, an inclining test is
conducted when the construction is near completion. In the Operational Manual, the vessel’s
center of gravity is recorded.
The number of openings in watertight structural elements should be kept to a minimum.
Arrangements for access, piping, venting, cables, etc., arrangements should be made to ensure
that the watertight integrity of the structure is maintained.
The stability of an FPSO should satisfy the requirements stated in relevant Codes. The
requirements for stability are given in IMO regulations (resolution A167, A206 and A502,
superseded by A749(18)), IMO MODU Code (issued in 1989), and classification Rules.
Adequate stability should be established for all relevant in-service and temporary phase
conditions. The assessment of stability should consider both the intact and the damaged
conditions.

Modeling for Fatigue Analysis
Fatigue sensitive details and the materials selected should be documented to have sufficient
fatigue strength for transportation and in-place conditions. Three levels of fatigue analysis may
be conducted:
fatigue check based on simple stress formulae for scantling (primarily aimed at
connections between longitudinal stiffeners and transverse web frames in the hull
structure), see Section 19.6.
simplified fatigue assessment to check the allowable stress range assuming the long-term
stress range follows the Weibull distribution, see Section 19.3,
spectral fatigue assessment based on the first principles, see Chapter 20.
The spectral fatigue assessment, makes use of the wave scatter diagrams for the installation
sites for in-place conditions and route-specific wave conditions for the transportation phase,
see Chapter 3. The wave scatter diagrams define the occurrence probability for various sea-
states defined by significant wave height and period. The analysis also takes into account the
direction of the sea and swell conditions relative to the vessel heading.
Particular attention should be given to connection details of the following:
Integration of the mooring system with hull structure,
Main hull bottom, sides, and decks,
Main hull longitudinal stiffener connections to transverse frames and bulkheads,
Main hull attachments, seats, supports, etc,