29-05-2012, 04:26 PM
Evaluation of Ultimate Strength of Jacket Type Structures under Marine Loading
Evaluation of Ultimate Strength of Jacket Type Structures under Marine Loading.pdf (Size: 553.62 KB / Downloads: 109)
Introduction
When designers want to ensure the existing structure can withstand new condition after
changes in jacket structure, for example, increasing design load or inspection reveals some
damages in structure and etc; assessment process is performed [1].
The strength of existing structure under lateral loading can be controlled by appropriate
methods. One can be design level analysis where after linear analysis, design criteria should
be checked. Another would be ultimate strength analysis. Here, through nonlinear analysis,
ultimate strength of structure is estimated and compared to strength of structure in design
level [1, 2 and 3]. Static push-over or nonlinear time-domain analysis can be selected as a
nonlinear analysis [1]. The main result of ultimate strength analysis method is reserve
strength factor introduced by Lloyd and Clawson [4]. Reserve strength ratio (RSR) with same
definition is described as follows by Bea et al [5]:
Modeling
2.1- Software: ABAQUS is a suite of powerful engineering simulation programs and was
used in the investigation for modeling and analysis.
ABAQUS/Aqua is a set of optional capabilities that can be added to ABAQUS/Standard.
It is intended for the simulation of offshore structures, such as oil platforms. Some of the
optional capabilities include the effects of wave and wind loading and buoyancy [7].
2.2- Structural Members Model: Frame element was selected for modeling of structural
pipe members. Main reasons were nonlinearity characteristic, buckling behavior modeling in
nonlinear analyses, and capability of use in AQUA analysis and possibility of use in both
dynamic and static analysis. This type of elements can be used to solve a wide variety of civil
engineering design applications, such as truss structures, bridges, internal frame structures of
buildings, off-shore platforms, and jackets, etc [7].
Nonlinear time-domain analysis results:
three stages performed to indicate maximum tolerable lateral load of
jacket structure. Response of this jacket in design level (load factor=1) and three stages
mentioned above are illustrated in figure 4-4. Locations of partial failure are showed in figure
4-5. In table 4-3 sequences and types of failure at load factor 1.64 is described:
Summery & Conclusions
By doing static push-over and nonlinear time-domain analysis for evaluation of two
typical piled jackets in Persian Gulf region and comparison of results, followings are
concluded, in order:
• Reserve strength ratio for jackets A and B in nonlinear time-domain analysis shows an
increase of 4% and 11%, respectively, compared to those factors derived from static
push-over analysis.
• Maximum lateral deflection of structures increased under dynamical loading, as well.
• In nonlinear time-domain analysis more partial failures can be borne by structure. In
other words structure seems to be stable dynamically, while it behaves unstable
statically.