Page 147 - Marine Structural Design

Chapter 6 Offshore Structural Analysis 123

The fatigue hot-spot stresses are obtained by combining stress components fiom the global
hull girder bending, secondary and tertiary bending and locally imposed loading. The stress
concentration factors may be obtained using parametric equations or fine mesh finite element
analysis of critical regions. Principal stresses are used in the evaluation of fatigue damage. The
selection of S-N curves and methodologies for fatigue damage assessment are discussed in
detail in PART ID.

6.5.4 TLP, Spar and Semi-submersible
A column-stabilized structure (semi-submersible or TLP) is defined as a floating installation
consisting of a deck structure with a number of widely spaced, large diameter, supporting
columns that are attached to submerged pontoons.
Some special components of column-stabilized structures include:
Ring (continuous) pontoons,
Twin pontoons,
Multi-footing arrangements,
Tension legs (TLPs).
Such structures may be kept on station by using a passive mooring system (e.g. anchor lines),
an active mooring system (e.g. thrusters), or a combination of both.
In recent years Spar structures became a type of popular floating installations for use in the
Gulf of Mexico when the water depth is deeper than 1000 m. Production risers are supported
by air-filled buoyancy cans in the central moonpool of the hull. For truss spars, the bottom half
of the spars consists of tubular truss and heave plate structures.
In the conceptual design phase, the design and analysis for TLP, Spar and semi-submersible
include:
Establish design basis,
Select facilities and conduct system design,
Determine layout,
Size hulls and estimate global performance,
Design topside and hull structures,
Design risers and foundations such as piles for mooringltethering,
Estimate weight, schedule and costs for fabrication and installation,
Review HSE compliance and quality assurance.
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Successful deepwater development depends on an experienced team using a systems approach
to select a concept such for floating installations. Dorgant et a1 (2001) presented primary
drivers for system selection for three major field development projects, and discussed
technical/commerciaYfeasibility/regulatory issues for alternative facility systems (TLP, Spar,
FPSO and semi-submersible).
Demirbilek (1989) edited a couple of interesting articles on the various design topics for TLP
design and analysis such as environmental criteria, hydrodynamic loads, structural analysis
and criteria, foundation design and analysis, riser analysis, tendon analysis, fatigue design and
fracture mechanics analysis, material selection, model tests and measurement.