Stig Berge

Constrained nonlinear control allocation with singularity avoidance using sequential quadratic programming

Control allocation problems can be formulated as optimization problems, where the objective is typically to minimize the use of control effort (or power) subject to actuator rate and position constraints, and other operational constraints. Here we consider the additional objective of singularity avoidance, which is essential to avoid loss of controllability in some applications, leading to a nonconvex nonlinear program. We suggest a sequential quadratic programming approach, solving at each sample a convex quadratic program approximating the nonlinear program. The method is illustrated by simulated maneuvers for a marine vessel equipped with azimuth thrusters. The example indicates reduced power consumption and increased maneuverability as a consequence of the singularity-avoidance.

Low Cycle Fatigue of T-Tubular Joints With Out-of-Plane Bending Loading

Tubular T-joints were fatigue tested with out-of-plane bending loading. Six models were tested, three models with R-ratio of 0 and three with R = −1. Hot spot stress was measured for the brace and for the chord using the ECSC linear extrapolation procedure. Fatigue loading was applied in load control, to obtain through thickness cracking at a number of cycles in the range 4 000–200 000 cycles. The data were analysed and compared with published data and with current fatigue design criteria for tubular joints.Copyright

Environmental Effects on Fatigue Strength of Armour Wire for Flexible Risers

Fatigue of armour wires is in many cases a limiting factor for the design life of flexible risers. Until a few years ago, fatigue design was based on SN data obtained by component testing in air, with the implicit assumption that the environment in a pipe annulus is benign with regard to fatigue of armour wires. Service experience has shown that a pipe annulus may contain species that are aggressive with respect to steel, and could affect fatigue strength significantly. In a consistent design methodology these effects should be taken into account. MARINTEK in co-operation with SINTEF Materials and Chemistry is running a Joint Industry Project (JIP) with the aim of developing a basis for fatigue design of armour wire in which the effects due to the chemical environment in a pipe annulus are accounted for. The project started in 2001 and is now into a Phase III to be completed in 2008. A Phase IV is proposed, to be carried out in 2009–2011. Testing is carried out on tensile armour wire in air, and in aqueous environments and with H2 S and/or CO2 at various partial pressures. SN curves have been obtained for more than 50 different combinations of material grade, environmental composition and loading parameters. In this paper the methodology of the testing is presented, with some general results. The following aspects of corrosion fatigue are discussed: - Procedures for fatigue testing of armour wire in corrosive environments, test protocol. - The scope for establishing common fatigue design criteria for armour wire, based on strength classes. - The effect of loading frequency in corrosion fatigue, assessment of fatigue strength criteria for long lives. - The fatigue limit in corrosion fatigue.Copyright

FATIGUE ASSESSMENT OF WELDED JOINTS TAKING INTO ACCOUNT EFFECTS OF RESIDUAL STRESS

Residual stress may have a significant effect on the fatigue strength of welded joints. As a non-fluctuating stress, it has an effect similar to that of the mean stress. Recently the International Association of Ship Classification Societies (IACS) has issued Common Structural Rules (CSR) for respectively tankers (IACS 2006a) and bulk carriers (IACS 2006b). The effect of mean stress in fatigue design is taken into account in both sets of rules. However, the treatment is quite different, in particular with regard to residual stress and shakedown effects. In the present paper a comparative study of fatigue design procedures of the IACS rules is reported, with emphasis on residual stress effects. Testing was carried out with longitudinal attachment welds in the as-welded condition. The initial residual stress was measured by a sectioning method using strain gages. Hot spot stress was determined experimentally by strain gauges and numerically by finite element analysis using different types of elements. Fatigue testing was carried out and SN-curves were plotted according to the relevant stress as specified by the rules. In order to investigate the shake-down effect of residual stress, testing was performed for several pre-load conditions which could be taken to represent maximum load levels in a load history. The aim of the study is to contribute towards better understanding of the effect of residual stress and shakedown on fatigue strength of welded joints.

Corrosion Fatigue Testing of Flexible Riser Armour: Procedures for Testing and Assessment of Design Criteria

Fatigue design of armour wires in flexible risers is reviewed, with particular emphasis on effects of corrosive environments in pipe annulus. Test procedures for corrosion fatigue testing of armour wire and assessment of fatigue design criteria are described.© 2003 ASME

Qualified Assessment for Lifetime Extension of Flexible Risers

There have been many challenges with respect to long term operation of flexible pipes, in many cases with the need to replace or repair them before the end of the initial design life. The number of identified failure mechanisms and observed failure events is also increasing.In the present paper, some typical issues that arise during the process of integrity assessment and qualification for lifetime extension are outlined. More detailed considerations are illustrated in relation to a particular case study. This involves elaboration of the different steps which are required in order to be able to extend the lifetime of a flexible riser for the case of corrosive annulus environments. The particular case of an outer sheath damage is applied for the purpose of illustration.Copyright

Fatigue Assessment of Aluminum Ship Details by Hot-Spot Stress Approach

This paper presents a robust methodology for fatigue assessment of aluminum ship details using a hot-spot stress range approach. A series of fatigue tests of a typical aluminum ship detail was carried out to obtain a design S-N curve. The test detail was analyzed by the finite element method (FEM) using several modeling techniques and element types. Recommendations are given on finite element modeling practice, extrapolation methods and design S-N curves.Copyright

Low Cycle Fatigue Strength of X65 Pipeline Girth Welds

During pipe laying at large water depth using S-lay over a stinger, the pipe may be subjected to reversed plastic strains which could lead to low cycle fatigue failure. X65 pipeline girth welds with wall thickness 22mm were tested in cyclic strain control. Undermatched and overmatched welds were tested. Two types of cyclic strain programs were applied. In the first test series the welds were subjected to a tensile-tensile cyclic strain, simulating pipe going over the stinger, to validate that a pipe going over a stinger would not violate criteria for low cycle fatigue design. In the second test series, the specimens were initially pre-strained to 2% and subsequently fatigue-cycled with a maximum strain of 2% to obtain design criteria for a pipe laying stationary over the stinger for a period of time, subjected to cyclic loads due to waves. The results are presented in terms of Δe–N curves, with proposed design curves for the two types of welds.Copyright

Fatigue of 28-inch Titanium Riser: SN Data and Defect Assessment

Qualification testing was carried out on full scale titanium pipes for a gas export riser from the Asgard B platform. Prototype pipe sections with girth welds produced by the TIG process were delivered from three different suppliers. The wall thickness was 30 mm. The pipes were sectioned into fatigue specimens and tested in constant amplitude loading in air. The loading was transverse to the weld, simulating axial and bending loading of the pipe. Secondary stresses were measured on each specimen. SN data were obtained for the welds and compared with proposed design curves. The fracture surfaces were investigated with scanning electron microscopy, and defects were categorised and sized. Fracture mechanics analysis was carried out and the results were compared with experimental data. Problems relating to fracture mechanics modelling for fatigue life prediction and defect assessment of titanium welds were discussed.© 2002 ASME

Surface Characterisation and Fatigue Strength of Corroded Armour Wire

Tensile armour wire from two flexible risers was fatigue tested for assessment of SN-curves. Both risers had been subjected to sea water flooding of the annulus for several years. The wire was corroded in the form of pits, however, the cross section was not significantly reduced. Reference wire from the same batches of material that had not been subjected to corrosion was also tested. Testing was carried out in air, to investigate the effect of accumulated corrosion damage in terms of a fatigue notch factor. The fatigue strength of the corroded wire was significantly reduced, corresponding to a fatigue notch factor in the range 1.5–1.7. The wire was examined by optical and by electron scanning microscopy and the surface defects were characterised.© 2014 ASME