Design Approximations for Offshore Tubulars Against Collisions

[摘要] The aim of this study is to derive simple design formulae for estimating the probable extent of damage to offshore tubular members due to lateral impacts, and for evaluating the residual strength of damaged tubular members subjected to combined axial compression and hydrostatic pressure. Existing models and methods are reviewed for predicting the probability of offshore collisions and consequential probable extents of damage, and for evaluating the residual strength of damaged members. Lateral impact tests are reported conducted on small-scale tubes having simply supported roller end conditions. The aim of the tests was to provide more realistic experimental information for local denting deformation of the tube wall at the point of impact and overall bending deformation of the tubular member as a beam under lateral impact. A simple numerical model is developed for simulating the dynamic response of a tubular member having simply supported roller end conditions. In the analysis, the tubular member is reduced to a spring-mass system with two degrees-of-freedom, one for local denting and the other for overall bending. Strain-rate sensitivity of the material and other dynamic effects upon the response of the tubular member have been considered by multiplying an empirically derived modification factor to the spring coefficient for overall bending. Combined axial compression and hydrostatic pressure loading tests are also conducted on damaged tubes whose form of damage are realistic. An analytical method is also developed to evaluate the residual strength of damaged tubular members under combined axial compression and hydrostatic pressure. The method involves two separate phases of calculation: derivation of bending moment - external axial compression - hydrostatic pressure - curvature relationships for dented tubular cross-sections using the tangent stiffness method; and determination of the residual strength of a damaged tubular member using the bending moment - curvature relationship based on the Newmark integration method. Rigorous parametric studies are performed using the theoretical models which have been validated with the experimental results obtained from the tests conducted as part of this study and other test data available in the literature. Finally, simple design formulae are derived using the parametric study results. A direct fit is attempted for design equations to predict the probable extent of damage to unstiffened tubular members subjected to lateral impacts, while the Perry formula is adopted as the basis of a formulation to estimate the residual strength of damaged tubular members under combined axial compression and hydrostatic pressure. Conclusions regarding the experimental and theoretical studies and the proposed design formulae are included, and an extension to this study is proposed in order for the design formulae to directly be applicable to the design of offshore structures against collisions. An approximate equation is presented in Appendix 1 for bending moment-external axial compression - hydrostatic pressure - curvature relationships of damaged tubular cross-sections. In Appendix 2 an approximate expression is derived for von Mises elastic buckling pressure of circular cylinder under pure radial pressure Appendix 3 describes the derivation procedure of a strength formulation for ring-stiffened cylindrical shells under combined axial loading and radial pressure, where the quadratic Merchant - Rankine formula in generalised form is adopted as the basis of the formulation. Volume II of this thesis is ref. 82 and contains the full experimental report and test data for the lateral impact tests.

[发布机构] University:University of Glasgow

[关键词] Ocean engineering, Mechanical engineering, Hydraulic engineering