COMBINED LOADING CRITERIA FOR TITANIUM RISERS

ABSTRACT DNV has, in cooperation with partners from the industry, carried out a joint industry project with the aim to develop recommended practice with respect to the design of titanium risers. As a part of this work, calibrated design formulae for combined loading have been established. The considered load situation is a combination of internal overpressure, bending moment and axial force. The data basis for the calibration study encompasses results from 12 finite element simulations with varying diameter to thickness ratio and internal pressure exposed to bending moment and axial force. With the design equation for steel risers, taken from the DNV Offshore Standard (OS-F201) Dynamic Risers, as a basis, the titanium data basis was investigated using state-of-the art methodology with an uncertainty modeling for load effects in compliance with recent research and development projects for risers and pipeline design. The outcome of this work is a design equation with reliability based calibration of safety factors that comply with the overall safety objective in the above offshore standard. INTRODUCTION Successful design of titanium risers has been carried out for a limited number of cases over the last decade. In lack of relevant codes, the principles of standards such as DNV-OS-F101 Submarine Pipelines and the later DNV-OS-F201 Dynamic Risers have been used. Significant modifications are, however, necessary in order to use these standards for titanium risers. The fact that no established design codes for titanium risers are available, introduces extensive demands to the design process. The general impression is that the lack of a good and reliable titanium design code is one of the obstacles to utilizing titanium risers in a reliable and economical way

ACHIEVEMENTS IN APPLICATIONS OF MARINE HYDRODYNAMICS

ABSTRACT The paper gives a review of the achievements in the applications of marine hydrodynamics to problems in offshore and maritime engineering. In particular focus will be on numerical methods for analysis of ships and floating production systems and how the work of Professor Newman has influenced the development of computational tools for use by the industry. The paper also presents some recent applications of such numerical tools for analysis of fixed and floating production systems, seakeeping of ships with forward speed including problems related to sloshing and moonpool dynamics. The challenges for each of these applications will be discussed from a physical viewpoint and how the numerical tools are applied to solve the problems. Validation of computer tools in light of their intrinsic limitations will be discussed

Addressing failure rate uncertainties of marine energy converters

publication-status: Publishedtypes: ArticleThe interest in marine renewable energy is strong, but has not led to significant commercial-scale investment and deployment, yet. To attract investors and promote the development of a marine renewable industry a clear concept of project risk is paramount, in particular issues relating to device reliability are critical. In the public domain, reliability information is often scarce or inappropriate at this early stage of development, as little operational experience has been gained. Thus, reliability estimates are fraught with large uncertainties. This paper explores sources and magnitudes of failure rate uncertainty and demonstrates the effect on reliability estimates for a notional marine energy converter. If generic failure rate data forms the basis of a reliability assessment, reliability estimates are not robust and may significantly over- or underestimate system reliability. The Bayesian statistical framework provides a method to overcome this issue. Generic data can be updated with more specific information that could not be statistically incorporated otherwise. It is proposed that adopting such an approach at an early stage in an iterative process will lead to an improved rate of certainty

STRESS MEASUREMENTS AT HOT SPOTS USING STRESSPROBE

ABSTRACT The estimation stress concentration factor (SCF) of weld details of floating structures is critical parameter for fatigue life predictions. The common practice for predicting the SCF value is the use of code-specified empirical equations and through detailed finite element analysis (FEA) for critical joints. Under certain conditions, it becomes necessary to reassess the SCF value while the structure is in service. The most accurate approach is to measure the SCF value on the real joint since variations in FEA results always exist due to variation on modeling techniques by different analysts. While the use of strain gauges is the standard approach, applying strain gauges in service can be extremely complicated. Therefore, an alternative approach for direct measurement that does not require removal of coating and bonding to the surface is attractive. The StressProbe that takes advantage of the change in the magnetic permeability of steel due to the presence of a mechanical stress offers the required alternative as a non-contacting strain measurement method. The paper presents the results of a study to assess that capability of the StressProbe in measuring SCF value by measuring variations in strain in a high strain gradient region of a hopper corner detail that is typical in floating structures and compare the results with those obtained using both strain gauges and finite element analysis. The results show that the StressProbe can be used for the determination of the SCF at a weld under various scenarios and it can also be used to monitor cyclic stresses during periods when there is wave loading. While there is some variability in the StressProbe results but given the variability found with strain gauges it is considered that the StressProbe could be used to replace strain gauges

Disease activity level, remission and response in established rheumatoid arthritis: Performance of various criteria sets in an observational cohort, treated with anti-TNF agents

<p>Abstract</p> <p>Background</p> <p>Most composite indices of disease activity and response criteria in RA have been validated and compared in clinical trials rather than routine care. We therefore wanted to compare the performance of the DAS28, SDAI and CDAI activity indices, their activity states, their response criteria, and also compare with the ACR response criteria in an observational clinical setting.</p> <p>Methods</p> <p>Agreement between the criteria sets was investigated using κ statistics in a non-randomized cohort of 1789 RA patients from southern Sweden, starting their first course of anti-TNF-treatment. Mean disease duration was 12 years. Completer analysis was used.</p> <p>Results</p> <p>Agreement between high, moderate and low activity states was moderate or substantial, with κ = 0.5 or better for all criteria. Agreement between SDAI and CDAI disease states was > 90% in these categories with κ > 0.8. DAS28 original and modified cut point remission had good agreement (κ = 0.91). Agreement between responses was substantial at the overall/ACR20 level (about 95%, κ = 0.7 or better) for all criteria. By contrast, agreement was poor between moderate and high level responses.</p> <p>Conclusion</p> <p>Disease activity states according to the various indices perform similarly and show substantial agreement at all levels except remission. Agreement between SDAI and CDAI states is excellent. Response criteria, applied at the individual patient level, are hard to interpret and show poor agreement, except at the lowest level of response. Thus, they should not be applied uncritically in clinical practice.</p

Mooring line fatigue damage evaluation for floating marine energy converters: Field measurements and prediction

publication-status: Publishedtypes: ArticleThe vision of large-scale commercial arrays of floating marine energy converters (MECs) necessitates the robust, yet cost-effective engineering of devices. Given the continuous environmental loading, fatigue has been iden- tified as one of the key engineering challenges. In particular the mooring sys- tem which warrants the station-keeping of such devices is subject to highly cyclic, non-linear load conditions, mainly induced by the incident waves. To ensure the integrity of the mooring system the lifecycle fatigue spec- trum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly as- sessed through numerical modelling of representative load cases. However, for new applications such as floating marine energy converters numerical models are often scantily validated. This paper describes an approach where load measurements from large- scale field trials at the South West Mooring Testing Facility (SWMTF) are used to calculate and predict the fatigue damage. The described procedure employs a Rainflow cycle analysis in conjunction with the Palmgren-Miner rule to estimate the accumulated damage for the deployment periods and individual sea states. This approach allows an accurate fatigue assessment and prediction of mooring lines at a design stage, where field trial load measurements and wave climate information of potential installation sites are available. The mooring design can thus be optimised regarding its fatigue life and costly safety factors can be reduced. The proposed method also assists in monitoring and assessing the fatigue life during deployment periods

Unsteady hydrodynamics of a full-scale tidal turbine operating in large wave conditions

Tidal turbines operate in a highly unsteady environment, which causes large-amplitude load fluctuations to the rotor. This can result in dynamic and fatigue failures. Hence, it is critical that the unsteady loads are accurately predicted. A rotor's blade can experience stall delay, load hysteresis and dynamic stall. Yet, the significance of these effects for a full-scale axial-flow turbine are unclear. To investigate, we develop a simple model for the unsteady hydrodynamics of the rotor and consider field measurements of the onset flow. We find that when the rotor operates in large, yet realistic wave conditions, that the load cycle is governed by the waves, and the power and blade bending moments oscillate by half of their mean values. While the flow remains attached near the blade tip, dynamic stall occurs near the blade root, resulting in a twofold overshoot of the local lift coefficient compared to the static value. At the optimal tip-speed ratio, the difference between the unsteady loads computed with our model and a simple quasi-steady approximation is small. However, below the optimal tip-speed ratio, dynamic stall may occur over most of the blade, and the maximum peak loads can be twice those predicted with a quasi-steady approximation

Genic SNP markers and legume synteny reveal candidate genes underlying QTL for Macrophomina phaseolina resistance and maturity in cowpea [Vigna unguiculata (L) Walp.]

<p>Abstract</p> <p>Background</p> <p><it>Macrophomina phaseolina </it>is an emerging and devastating fungal pathogen that causes significant losses in crop production under high temperatures and drought stress. An increasing number of disease incidence reports highlight the wide prevalence of the pathogen around the world and its contribution toward crop yield suppression. In cowpea [<it>Vigna unguiculata </it>(L) Walp.], limited sources of low-level host resistance have been identified, the genetic basis of which is unknown. In this study we report on the identification of strong sources of host resistance to <it>M. phaseolina </it>and the genetic mapping of putative resistance loci on a cowpea genetic map comprised of gene-derived single nucleotide polymorphisms (SNPs) and amplified fragment length polymorphisms (AFLPs).</p> <p>Results</p> <p>Nine quantitative trait loci (QTLs), accounting for between 6.1 and 40.0% of the phenotypic variance (R²), were identified using plant mortality data taken over three years in field experiments and disease severity scores taken from two greenhouse experiments. Based on annotated genic SNPs as well as synteny with soybean (<it>Glycine max</it>) and <it>Medicago truncatula</it>, candidate resistance genes were found within mapped QTL intervals. QTL <it>Mac-2 </it>explained the largest percent R²and was identified in three field and one greenhouse experiments where the QTL peak co-located with a SNP marker derived from a pectin esterase inhibitor encoding gene. Maturity effects on the expression of resistance were indicated by the co-location of <it>Mac-6 </it>and <it>Mac-7 </it>QTLs with maturity-related senescence QTLs <it>Mat-2 </it>and <it>Mat-1</it>, respectively. Homologs of the <it>ELF4 </it>and <it>FLK </it>flowering genes were found in corresponding syntenic soybean regions. Only three <it>Macrophomina </it>resistance QTLs co-located with delayed drought-induced premature senescence QTLs previously mapped in the same population, suggesting that largely different genetic mechanisms mediate cowpea response to drought stress and <it>Macrophomina </it>infection.</p> <p>Conclusion</p> <p>Effective sources of host resistance were identified in this study. QTL mapping and synteny analysis identified genomic loci harboring resistance factors and revealed candidate genes with potential for further functional genomics analysis.</p