Design of Dynamic High Voltage Cables for Floating Substation

This paper presents the design and performance assessment of 220kV dynamic export cables for a floating substation characterized by a ring-shaped floater known as Damping Pool. The main originality of the design presented is that the cables considered have dry conductors. They are shielded from the water by a longitudinally welded corrugated copper sheath. Similar cables have been operating at lower voltage levels and thus with thinner insulation thicknesses. The export cable configuration has been designed considering environmental conditions representative of both the Central North Sea, Pacific Coast of Japan or the US, in 100m water depth. Ultimate and fatigue limit-state design verification of the configuration are made through nonlinear time-domain analysis using coupled models comprising the floating substation hull, the mooring system and dynamic export cables. Fatigue limit-state design verification is based on the fatigue properties of the cable section, combined with appropriate S-N curves of the armour layers and metallic screen-sheath. Design verifications show that the dynamic export cable configuration proposed could satisfactorily meet the performance requirements for a service life over 25 years, considering proven cable equipment such as a bend stiffener remaining within today’s manufacturer molding capacities.

Wave Load Mitigation by Perforation of Monopiles

The design of large diameter monopiles (8–10 m) at intermediate to deep waters is largely driven by the fatigue limit state and mainly due to wave loads. The scope of the present paper is to assess the mitigation of wave loads on a monopile by perforation of the shell. The perforation design consists of elliptical holes in the vicinity of the splash zone. Wave loads are estimated for both regular and irregular waves through physical model tests in a wave flume. The test matrix includes waves with Keulegan–Carpenter ( K C ) numbers in the range 0.25 to 10 and covers both fatigue and ultimate limit states. Load reductions in the order of 6%–20% are found for K C numbers above 1.5. Significantly higher load reductions are found for K C numbers less than 1.5 and thus the potential to reduce fatigue wave loads has been demonstrated.

Evaluation of methods for calculating reinforced concrete structural members for the fatigue limit state

В нормах по проектированию железобетонных конструкций зданий и сооружений (начиная с 1962 г. и по настоящее время) содержится методика расчета на выносливость, которая была составлена с учетом обобщения и анализа данных многочисленных экспериментально-теоретических исследований. Последующее использование данной методики в практике проектирования железобетонных конструкций показало, что при эксплуатации конструкций, рассчитанных с учетом требований по выносливости, разрушений не происходило. Вместе с тем, анализ показал отличие отечественной методики от подходов иностранных норм в части расчета по растянутой арматуре. Дальнейшие исследования показали некоторые неcовершенства нормативной методики расчета по растянутой арматуре (как ненапрягаемой, так и предварительно напряженной). С учетом данных проведенных расчет-но-теоретических исследований, а также в целях гармонизации с основными положениями расчета на усталость, принятого в нормах проектирования ряда ведущих стран, представляется полезным в расчете на усталость по растянутой арматуре наряду с максимальным напряжением в пределах цикла нагрузки учитывать и предельную амплитуду напряжений. В этой связи предполагается проведение актуализации существующей методики расчета на выносливость, которая будет дополнена новыми положениями расчета по растянутой арматуре. При актуализации методики наиболее правильным будет максимально учесть другие положения существующей методики. В частности, будет сохранен прежний подход к определению действующих напряжений в бетоне и арматуре, а также расчет на усталость по сжатому бетону.

A Comparative Study between Three-Legged and Tripod Sub-structures in Design of Offshore Wind Turbines in the Transition Water Depth

This research aimed at investigating tripod and three-legged offshore wind turbine substructures. A comparison between the two substructures based on their weight as well as the installation method of piles, i.e. pre-piling and post-piling, was carried out. The in-place (Ultimate Limit State), Dynamic, natural frequency check and fatigue (Fatigue Limit State) analyses were conducted considering aerodynamic and hydrodynamic loads imposed on substructures in 50m water depth. An optimisation process was carried out in order to reduce the mass of substructures. The results revealed that the three-legged substructure is more cost effective with 25% lesser structure mass. However, the construction of the three-legged structure usually takes more time due to increased number of members and subsequently welding joints. The results, furthermore, showed that the pre-piling method reduces the time and cost of offshore installation, and reduces the weight of piles by 50%.

Assessment and propagation of mechanical property uncertainties in fatigue life prediction of composite laminates

A probabilistic model for estimating the fatigue life of laminated composite materials considering the uncertainty in their mechanical properties is developed. The uncertainty in the material properties is determined from fatigue coupon tests. Based on this uncertainty, probabilistic constant life diagrams are developed which can efficiently estimate probabilistic ɛ-N curves at any load level and stress ratio. The probabilistic ɛ-N curve information is used in a reliability analysis for fatigue limit state proposed for estimating the probability of failure of composite laminates under variable amplitude loading cycles. Fatigue life predictions of unidirectional and multi-directional glass/epoxy laminates are carried out to validate the proposed model against experimental data. The probabilistic fatigue behavior of laminates is analyzed under constant amplitude loading conditions as well as under both repeated block tests and spectral fatigue using the WISPER, WISPERX, and NEW WISPER load sequences for wind turbine blades.

Fatigue strength assessment of ship structures accounting for a coating life and corrosion degradation

Purpose Fatigue strength and reliability assessment of complex double hull oil tanker structures, based on different local structural finite element approaches, is performed accounting for the uncertainties originating from load, nominal stresses, hot spot stress calculations, weld quality estimations and misalignments and fatigue S-N parameters including the correlation between load cases and the coating life and corrosion degradation. Design/methodology/approach Ship hull wave-induced vertical and horizontal bending moments and pressure are considered in the analysis. Stress analyses are performed based on the nominal, local hot spot and notch stress approaches. A linear elastic finite element analysis is used to determine the stress distribution around the welded details and to estimate structural stresses of all critical locations. Fatigue damage is estimated by employing the Palmgren-Miner approach. The importance of the contribution of each random variable to the uncertainty of the fatigue limit state function is also estimated. The probability of fatigue damage of hot spots is evaluated taking into account random coating life and corrosion wastage. Fatigue reliability, during the service life, is modelled as a system of correlated events. Findings The fatigue analysis showed that the fatigue damage at the hotspot, located at the flange of the stiffener close to the cut-out, is always highest in the cases of the structural hot spot stress and effective notch stress approaches, except for the one of the nominal stress approach. The sensitivities of the fatigue limit state function with respect to changes in the random variables were demonstrated showing that the uncertainty in the fatigue stress estimation and fatigue damage are the most important. Fatigue reliability, modelled as a parallel system of structural hot spots and as a serial system of correlated events (load cases) was evaluated based on the Ditlevsen bounds. As a result of the performed analysis, reliability and Beta reliability indexes of lower and upper bounds were estimated, which are very similar to the ones adopted for ultimate strength collapse as reported in literature. Originality/value This paper develops a very complex fatigue strength and reliability assessment model for analysing a double hull oil tanker structure using different local structural finite element approaches accounting for the associated uncertainties and the correlation between load cases and the coating life and corrosion degradation. The developed model is flexible enough to be applied for analysing different structural failure modes.

Reinvestigation of Ice Speed Effect on Ice-Induced Vibration of Conical Structure

Offshore installations designed to withstand extreme ice actions in ice-covered waters have experienced ice-induced vibrations (IIVs). ISO 19906 states that the dynamic ice actions and the corresponding IIVs shall be considered in the design and characterized with respect to the fatigue limit state (FLS). Ice-breaking cones have been effectively adopted in several full-scale structures to reduce the magnitude of the ice loads. Full-scale experience from Bohai Bay has shown that a structure utilizing a conical waterline geometry reduces the magnitude of IIVs relative to the analogous vertical structure. The same data indicated that structures with a narrow cone at the waterline can still experience IIVs and the magnitude of acceleration varies by ice speed for a given ice thickness. Researchers have performed comprehensive model tests on sheet ice approaching flexible conical structures and found the same phenomenon. In this paper, both full-scale field data and model test data were reinvestigated in consideration of ice speed. Two dimensionless variables were constructed for this purpose, one dimensionless variable with respect to the ice velocity and the other with respect to the acceleration of the structure. It is found that these two dimensionless variables show a linear correlation. The proposed methodology can be used to investigate the ice-induced acceleration of the flexible conical structure subject to a moving ice sheet.

Insights on the design of free-spanning pipelines

The design of free-spanning pipelines is performed with the aim of ensuring their integrity against permanent loads generated by seabed roughness, functional loads induced by internal pressure and temperature, and dynamic loads induced by marine currents and direct wave action. In particular, a load and resistance factored design is applied that focuses on extreme environmental loads, and a fatigue limit state approach is applied as a consequence of free-span dynamics due to vortex shedding-induced vibration and direct wave action. The pipeline free-span scenario can be permanent, when generated by seabed roughness, or characterized by short- to long-term evolution, when generated by seabed mobility and scouring in shallow waters. Free-span analysis is generally a task involving a number of disciplines and should be carried out using a multidisciplinary approach. The paper illustrates various themes related to free-span analysis: (i) free-span scenarios, (ii) characterization of the environment from deep to shallow water related to proper seabed properties, (iii) hydrodynamic load regimes, (iv) pipeline free-span design assessment aiming to reduce overstress and fatigue damage, (v) erodible seabed mobility and local scour, and (vi) some experiences of inspection surveys chosen as representative of a free-spanning pipeline in sandy soils.