Page 139 - Marine Structural Design

Chapter 6 Offshore Structural Analysis 115

The finite element model for analysis of jackets includes:
(2) Loads: The loads include:
Functional loads such as gravity load
Environmental loads due to wind, waves, currents, earthquakes, and
Accidental loads that may occur during its service life.
The increase in hydrodynamic and gravity actions caused by marine growth should be
accounted for. The hydrodynamic model of the structure should include appurtenances such as
conductors, I-tubes and risers, caisson, ladders and stairs, launch box, boat landing, guides,
and anodes. Depending upon the type and number, appurtenances can significantly increase
the global wave forces. In addition, forces on some appurtenances may be important for local
member design. Appurtenances not welded to the main structure are generally modeled as
non-structural members that only contribute as equivalent wave forces.
(2) Foundation: The foundation system for the jacket temporary on-bottom condition prior to
installation of the permanent foundation system should be documented to have the required
foundation stability for the specified environmental conditions, and for all relevant limit-states.
Throughout the analysis, structure-to-ground connections should be selected in order to
represent the response of the foundations. They may normally be simulated using linear
stiffness matrices. The finite element analysis may model behavior of axial and lateral soil-
foundation systems explicitly.
(3) Structures: The stiffkess of the deck structure shall be modeled in suficient detail to
ensure compatibility between the deck design and the jacket design. In a linear analysis
normally it is sufficient to model one member using only one element. However, in order to
account for member buckling and local dynamic response, one or more beam-column elements
are required to model each member depending on the element formulation, distribution of
actions. Major eccentricities of load carrying members may be modeled as rigid ends in the
model.
Modeling for Ultimate Strength Analysis
The load cases include each mode of operation, such as drilling, production, work-over, or a
combination thereof. According to NTS (1998), it is necessary to perform analyses to
establish:
maximum base shear of wave and current actions for dimensioning jacket bracing's,
maximum overturning moments for dimensioning jacket legs and foundation systems.
maximum local member forces which may occur for wave positions other than that
causing the maximum global force.
Modeling for Fatigue Analysis
Fatigue analysis should include all relevant actions contributing to the fatigue damage both in
non-operational and in operational design conditions. When calculating fatigue damage, local
action effects due to wave slamming and vortex shedding should be included if relevant.
While jackets in shallow water depths are normally insensitive to dynamic effects, non-
linearity's associated with wave theory and free-surface effects may be important. A
deterministic analysis is normally recommended for such jackets. For deepwater jackets where
the dynamic effects are important, a fatigue analysis in the frequency domain (dynamic