Page 149 - Marine Structural Design

Chapter 6 qffshore Structural Analysis 125

The purpose of the global analysis model is to enable the assessment of the responses resulting
from the global actions. An example of such a model is given in Figure 6.6. For large, thin-
walled structures, 3-dimensional finite element models created in shell (or membrane) finite
elements are normally required. For space frame structures consisting of slender members, a
3-dimensional space frame representation of the structure may be adequate.
The stiffness of major structural connections (e.g. pontoon to column or column to deck)
should be modeled in detail in order to represent the stiffness of the connection The
hydrodynamic loading model may be mapped directly onto the structural model.
Typically, a simplified space-frame model of the structure may be created to obtain the
maximum range of stresses in the tank for a range of tank loading conditions. These load
conditions will include both full and empty pontoons representing the maximum and minimum
sagging and hogging conditions.
The simultaneity of the responses resulting from the global and local analysis models may
normally be accounted for by linear superposition with appropriate load factors applied.
In buckling and ultimate strength checks, relevant lateral pressure applied together with in-
plane forces. The criteria for plated members, stiffeners and stiffened shells are available from
classification rules, industry standards such as NORSOK N-004 (NTS, 1998), API 2U and
API 2V, see Chapters 10 and 1 1.
The ultimate strength criteria for TLP tethers under combined external pressure, tension and
bending may govern its design. These strength criteria may be modified using the formulation
developed in the 1990s for strength design of deepwater pipelines and risers.
The fatigue assessment of TLP, Spar and semi-submersibles is similar to that described for
FPSO, see PART III.

6.6 References
1. ABS (2001), “Guidance Notes on “SafeHull - Dynamic Loading Approach” for Floating
Production, Storage and Offloading (FPSO) Systems”, American Bureau of Shipping.
2. API (2001), “API RP 2A WSD : Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms - Working Stress Design,” Latest Edition.
3. API (1993), “API RP 2A LRFD - Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms - Load and Resistance Factored Design”, First
Edition. 1993.
4. API (1 997), “Supplement 1 to the API RP 2A LRFD, First Edition.
5. AF’I (1997), “API RP 2T - Recommended Practice for Planning, Designing and
Constructing Tension Leg Platforms”, Second Edition.
6. API (2001), “API RP 2A WSD, Recommended Practice for Planning, Designing and
Constructing Fixed Offshore Platforms - Working Stress Design”, American Petroleum
Institute, Latest Edition.
7. API (2001), “API RP 2FPS, Recommended Practice for Planning, Designing and
Constructing Floating Production Systems”, First Edition.
8. Bai, Y., Ayney, C., Huang, E., Maher, J., Parker, G., Song, R. and Wang, M. (2001),
“Design and Construction of Floating Production Systems”, Course Notes for an