Static non-linear three-dimensional analysis of a riser bundle by a substructuring and incremental finite element algorithm

The problem of static, non-linear, large three-dimensional deformation of riser bundles used in offshore oil and gas production is studied within the limits of small strain theory. The mathematical model consists of the models of component-risers and connectors which hold risers together. Each riser is modelled as a thin walled, slender, extensible or inextensible tubular beam-column. It is subject to non-linear three-dimensional deformation dependent hydrodynamic loads, torsion and distributed moments, varying axial tension, and internal and external fluid forces. The problem is solved numerically by developing an algorithm which features substructuring, condensation and non-linear incremental finite elements. Substructuring is used to decompose the riser bundle problem into those of individual component-risers and equilibria of connectors. Condensation is used along with the connector equilibrium equations to produce connector forces and moments. Strong non-linearities present in the model are handled by an incremental finite element approach. Accuracy of the computer code is verified by solving simple three-dimensional cases. Two three-dimensional applications are solved for a bundle with seven component-risers and up to a total of 1267 degrees of freedom. Finally, a comparison is made with numerical results of a two-dimensional analysis code. The influence of problem size on total CPU time is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50097/1/1620281104_ftp.pd

Effect of fluid static pressure on the immediate postbuckling behavior of heavy tubular columns

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26654/1/0000198.pd

Nonlinear incremental inverse perturbation method for structural redesign

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76715/1/AIAA-1983-892-392.pd

Three-dimensional nonlinear statics of pipelaying using condensation in an incremental finite element algorithm

The problem of static, nonlinear three-dimensional deformation of pipelaying used in construction and installation of underwater pipelines is studied within the limits of small strain theory. The mathematical model consists of the pipeline model and geometric constraints imposed by the seabed and the lay vessel in stinger or J-type pipelaying. The pipeline is modeled as a thin-walled, slender, extensible or inextensible tubular beam-column. It is subject to gravity, lateral friction from the seabed, nonlinear three-dimensional deformation dependent hydrodynamic loads, torsion and distributed moments, varying axial tension, and internal and external static fluid forces. The problem is solved numerically by developing a nonlinear incremental finite element algorithm which features condensation and principles of contact mechanics. Condensation is used along with the geometric constraints to formulate a condensed problem which produces reaction forces. Strong nonlinearities present in the model are handled by an incremental finite element approach. The developed computer code is used to study stinger pipelaying for various stinger configurations, investigate the effect of water depth, and compare stinger to J-type pipelaying in deep water.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28834/1/0000669.pd

Redesign of marine structures by perturbation

Finite element (FE) methods are extensively used for analysis of static and dynamic behavior of marine structures. Often predicted response is unacceptable from the point of view of design or operation. Improvement of response then, becomes a design goal which can be achieved by redesign or reduction of operational threshold. Traditional trial and error techniques using FE methods make redesign expensive and are often inconclusive. In this paper a perturbation-based method is developed to solve redesign problems with both static and modal dynamic objectives using data only from the FE analysis of the baseline structure. Code RESTRUCT implements this method and functions as postprocessor to general or special purpose FE codes. Several simple numerical applications are used to illustrate the efficiency of this redesign method and how it can be used to resolve conflicts caused by incompatible redesign requirements. A 192-degree-of-freedom tower with repeated eigenvalues is redesigned subject to frequency and displacement constraints. Finally the impact of perturbation-based redesign on marine structural design is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27488/1/0000532.pd

Structural model correlation using large admissible perturbations incognate space

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76957/1/AIAA-10863-901.pd

Admissible large perturbations in structural redesign

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76363/1/AIAA-10551-828.pd

Invariant and consistent redundancy by large admissible perturbations

Structural perturbation theory has been developed over the past 16 years to relate two structural states modeled by the same Finite Element (FE) model but described by different values of the design variables. Relating an intact/damaged (initial) structure to a limit state structure produces the reserve/residual redundancy. Invariant and consistent redundancy, redundancy functions, and injective mappings are defined and related to the design variables. General perturbation equations are derived to relate the two states and produce failure surface equations. Individual and joint failure points are identified and redundancy is computed without linearization of failure surfaces, enumeration of failure paths, trial and error, or repeated FE Analyses (FEAs). This is achieved by large admissible perturbations using a prediction-correction algorithm and postprocessing FEA results of the initial structure only. The latter may differ from the limit state structure in stiffness, mass, geometry, or response by as much as 100-300% depending on the size of the FE model. Structural perturbation theory treats discrete and continuous structures as the FE method does; modeling of the structure as a simplified system of components is not needed. To introduce this new approach to redundancy, modal dynamic and static deflection failure criteria are used in the elastic range. Numerical applications on a beam, a small, and a large offshore tower are used to test the method. Future developments and impact to design are discussed as the new methodology introduces an alternative to systems reliability and stochastic FE.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30259/1/0000657.pd

Large deformation three-dimensional static analysis of deep water marine risers

The problem of static three-dimensional, nonlinear, large deformation response of a marine riser is formulated within small strain theory and solved numerically. This type of analysis is necessary, for the new generation of drilling and production risers. The mathematical model takes properly into account the effects of internal and external pressure and the complete nonlinear boundary conditions, without linearizing the follower forces. The extensibility or inextensibility condition is used as the constitutive relation in the tangential direction. Torsion and bending are coupled. The external load and the boundary conditions are deformation dependent. A solution method is developed based on an incremental finite element algorithm, which involves a prediction-correction scheme. In the correction phase deformation dependent quantities are updated. The extensibility or inextensibility condition is used to reduce the degrees of freedom of the system. The numerical results of the developed computer code compare very well with available semi-analytical and numerical solutions. Three numerical applications are used to demonstrate the importance of large deformation, nonlinear and three-dimensional analyses.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25547/1/0000089.pd

Finite element structural redesign by large admissible perturbations

In structural redesign, two structural states are involved: the baseline (known) state S1 with unacceptable performance, and the objective (unknown) state S2 with given performance specifications. The difference between the two states in design variables and performance may be as high as 100% or more depending on the scale of the structure and the design problem considered. A perturbation approach to redesign (PAR) is presented to relate any two structural states S1 and S2 that are modeled by the same finite element model but represented by different values of the design variables. General perturbation equations are derived expressing implicitly the natural frequencies, dynamic modes, static deflections, static stresses, Euler buckling loads and buckling modes of the objective state S2 in terms of its performance specifications, and only state S1 data and FEA results. Large admissible perturbations (LEAP) algorithms are implemented in code RESTRUCT to define the objective state S2 incrementally without trial and error by postprocessing FEA results of state S1 with no additional FEAs. Systematic numerical applications in redesign of a 10-element 48-d.o.f. beam, a 104-element 192-d.o.f. offshore tower, a 64-element, 216-d.o.f. plate, and a 144-element 896-d.o.f. cylindrical shell show the accuracy, efficiency, and potential of PAR to find an objective state that may differ 100% or more from the baseline design.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30329/1/0000731.pd