Low cycle fatigue of lamellar TiAl with 8.5 at.% Nb was studied with a total strain amplitude of 0.28% at three temperatures: room temperature, 750 and 900 °C. At room temperature, the material exhibited cyclic hardening and the fracture mode was mainly interlamellar. At 750 and 900 °C, the material showed cyclic softening and the fracture mode was translamellar. The lattice strain in γ phase was almost tensile and larger tensile lattice strain in γ phase seems detrimental. Besides, opposite direction of {201}γ and {100}α2 lead to crack propagation along α2/γ interfaces. B2/βo phase always suffered compressive lattice strain in the tests. The destruction of lamellar microstructure was the reason for colony refinement at 750 °C and 900 °C.
Corrosion damage of steel in concrete structures is a major concern. NDE techniques based on variations in induced magnetic fields due to loss of steel have been shown to be an effective tool. This paper includes results from laboratory investigation, field tests, and numerical analysis based on this concept. Test specimens included reinforcing bars and prestressing cables with simulated flaws as well as flaws from real corrosion. Variations in the magnetic field were recorded as electrical signals that were characterized to aid in the detection of corrosion and evaluation of the condition of steel in concrete. It was found that the amplitude of the signals could be related to the extent of the corrosion. Loss of cross sectional area in bars and cables of approximately 3 percent could be detected by the technique. The results of a finite element analysis yielded a good agreement with those from the experiment. The technique offers significant capabilities for field assessment of the condition of steel in concrete structures.
One key decision operators make when planning field developments is whether the production system will use only subsea equipment or will also include dry trees. Although dry tree floating platforms have primarily utilized SPAR and TLP hull forms, semisubmersible options are being developed and qualified for the Gulf of Mexico. A key component of the dry tree semisubmersible is the riser tensioning system, which must be carefully matched to hull motion characteristics and arrangements. The SPAR platform utilizes self-supporting buoyancy cans or long stroke riser tensioners in a protected location within the SPAR centerwell and in combination with a keel guide. The TLP utilizes short stroke riser tensioners with or without a keel guide and in a location open to interaction with waves and currents. A dry tree semisubmersible is likely to use long stroke riser tensioners in an open wellbay configuration either with or without a keel guide. This paper uses computer analysis and motions calibrated to model test data to evaluate the strength and operational performance of a riser tensioning system for a specific dry tree semisubmersible configuration. The system uses existing riser tensioning equipment in an arrangement that is open to wave and current interaction. The paper compares arrangements with and without a keel guide and presents differences in strength, stroke range and cost. The semisubmersible hull form includes columns arranged in pairs at each corner of the platform and the paper investigates the influence of the paired-column arrangement on wave loading on the risers and supporting structures. The results of the paper indicate what modifications are necessary, if any, for riser tensioning equipment integration. The information and results presented in this paper are applicable to operators evaluating dry tree interfaces on proposed new floater concepts in addition to equipment suppliers planning to provide or qualify riser tensioning equipment for semisubmersibles. The results will also contribute to further development of a dry tree semisubmersible option for Gulf of Mexico projects, which will provide cost and execution plan improvements compared to existing options for deepwater.
In previous Spar designs where pull tubes were used to board the risers (either export or flowline risers), the pull-tube extended a considerable distance beyond the keel and used a tapered design to form a bend restrictor that supported the riser throughout the riser/hull interface. In a current Spar design, the pull-tube is terminated at the hull keel and the bending loads are carried by a double sided stress-joint in the riser that pivots on a centralizer located near the bottom of the pull-tube. Essentially, this is an adaptation of the double-sided stress joint used for top tensioned risers exiting the bottom of their buoyancy can stems to the similar condition of an SCR exiting a pull tube terminating at the Spar’s keel. This new pull-tube and SCR configuration can be applied for both Truss and Classic Spars. SCRs boarding Spars through pull tubes have several advantages over stress joints or flex-joints anchored in porches, notably, eliminating both the need for divers to make large piping connections at 500′ to 600′ water depths and the possibility of those connections leaking over time. Moving the bend restrictor function from the pull tube to the riser provides the additional advantage of adding flexibility for the Spar to accommodate future risers whose size and weight are not known at the time the pull tubes are designed and the platform is installed. With the stress joint as part of the riser, the bend restrictor can be custom designed for each riser since the pull tube works the same for all risers. The SCR and stress joint, pull-in and in-place analyses have been performed by using the finite element program ABAQUS. The nonlinear capabilities of ABAQUS including the hybrid, gap and contact element formulations are utilized in the analysis of the pull-in process. The nonlinear contact elements with finite sliding capability are modeled with an exponential over-closure relationship.
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