Page 138 - Marine Structural Design

114 Part I Structural Design Principles

6.5 Structural Modeling
6.5.1 General
This section gives a general overview for the design of marine structures using a finite element
modeling technique. Reference is made to recommendations described in NORSOK N-004
(NTS, 1998), which is one of a few codes that provide guidance on the finite element
modeling in marine structural design. This section shall address structural modeling defined by
industry codes for fixed platforms and floating production installations.
6.5.2 Jacket Structures
A Jacket structure is a welded tubular space frame consisting of vertical or battered legs
supported by a lateral bracing system. The function of the jacket is to support the topside
facilities, provide supports for conductors, risers, and other appurtenances and serve as a
template for the foundation system. Graff (1981) and Dawson (1983) give an introduction to
the design and analysis of jacket structures, including basic formulations for environmental
loads, modeling of foundation, finite element analysis, dynamic response and stress
acceptance criteria. In general the design activities include:
Identi@ the project needs
Evaluate environmental conditions and soil conditions
Develop preliminary design proposals focusing on the methods of installation
Evaluate the installation methods in terms of technical and economical feasibility,
. construction and installation challenge, foundation requirements and cost etc.
Dimension the structure to resist the in-place load during operating condition, for each
mode of operation such as drilling, production, work over, or combinations thereof.
Evaluate the design to ensure that it can resist actions due to transportation from the
fabrication yard to the installation site, including load-out, sea transportation, installation,
mating, and hook-up.
Account for abandonment of the structure after decommissioning
Meet quality and HSE requirements.
Analysis Models
The global analysis of platforms starts from defining the geometrical and material properties
of the structural members, the foundation properties and functional, environmental and
accidental loads.
Two types of structural analysis may be conducted:
A linear analysis to check ultimate strength and fatigue criteria based on industry codes
(such as API FW 2A) using internal member forces.
A non-linear finite element analysis of structural response to accidental loads (such as ship
collision, dropped objects, fire, explosion and earthquake) or extreme response to wave
load as part of the re-assessment of exiting platforms.
The basic formulation for linear finite element analysis is given in Section 6.3. The nonlinear
finite element analysis is detailed in Chapter 12.