J.A. Witz

A parametric study of the post-buckling behaviour of steel plates

This paper investigates the sensitivity of the buckling and post-buckling behaviour of imperfect steel plates used in ship and related marine structures when subject to variations in their initial conditions. The different aspects analysed comprise of variation in aspect ratio, boundary conditions, initial out-of-plane imperfection and material elastic and plastic properties such as strain hardening. The analyses were carried out using the non-linear finite element program ABAQUS. The results obtained for the several plate models show that the three most relevant parameters affecting the buckling and post-buckling of uniaxially loaded plates are aspect ratio, unloaded edges boundary conditions and initial out-of-plane imperfection amplitude.

Rotary bending of marine cables and umbilicals

The stress analysis of a marine cable or umbilical in a rotary bending test rig is presented. It is shown that the finite length of the test sample and the terminations have a significant influence on the stresses in the umbilical which is taken into consideration in determining the cyclic stress distribution in the helical components. Consistency between the theory and test results is demonstrated.

Measurements from the Emerald instrumented flexible riser

Measurements of the loads on a bonded flexible riser connected to the Emerald Producer floating production platform in the northern North Sea are described. This required the strain gauging of the neck of an 8 in water injection riser and its support, and transmitting the signals to a dedicated data processing system. In addition, other sensors were installed to measure the vessels motions (in particular, at the riser top connection). These motions are then related to the measured strains, which are converted into riser axial loads and bending moments. Strong correlations between the riser bending moments and the riser connection heave are shown which support the case that the primary source of riser excitation is vessel motion.

Control of marine vehicles with pneumatic compliances

Abstract This paper is concerned with the use of pneumatic compliances to control the heave, roll and pitch motions of marine vehicles when disturbed by operational loads. The pneumatic compliances are in the form of open bottom air tanks attached to the vessel at the water line and extending above and below still water level. Each tank traps a volume of air above its internal water level. Active operation of these tanks involves controlling the amount of air trapped within the tanks. The air exerts varying forces and moments on the vessel which may be used to counteract disturbing forces. This paper describes a time domain method of dynamic analysis that is used to investigate the performance of such a system. Three application studies are presented which involve the suppression of motion due to vessel loading and crane operation on semisubmersible and monohull vessels.

Steel Plate Serviceability in Marine Structures

Plates are structural components that are able to provide support beyond compressive collapse, i.e. in the post-buckling regime. To date the criterion used to define plate failure under compressive loading is the plate maximum load carrying capacity which is identified as the peak of the associated load versus plate end-shortening curve. Recent advances in structural analysis techniques, such as nonlinear finite element analysis, have enabled the reduction of uncertainty that is embedded into the behaviour and strength prediction of new and in-service structures. Consequently, a new criterion for ultimate plate compressive strength is suggested and justified. This criterion is based on the unloaded edge resultant stress reaching yield. It is considered to be a suitable threshold of serviceability based on the relation between drop of strength versus the gain in plate end-shortening. In practical terms this means that a plate will be able to work in an extended range of end-shortenings.Copyright

Structural and installation analysis of a well control umbilical with bird-caged sections

Abstract The methodology behind the structural analysis of a well control umbilical with bird-caged sections is presented. It is demonstrated that defects in the form of bird-caged sections are detrimental to the mechanical properties of an umbilical. The paper also describes the analysis and real-time monitoring of the umbilical installation in the southern North Sea. The considerable benefits of real-time monitoring in ensuring the structural integrity of an umbilical during installation are demonstrated and discussed.