Browsing by Author "Braithwaite, Jarryd"

Now showing 1 - 6 of 6
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    Analysis of tightening sequence effects on preload behaviour of offshore wind turbine M72 bolted connections
    (MDPI, 2019-11-20) Braithwaite, Jarryd; Mehmanparast, Ali
    Offshore wind turbines in shallow waters are predominantly installed using a monopile foundation, onto which a transition piece and wind turbine are attached. Previously, the monopile to transition piece (MP-TP) connection was made using a grouted connection, however, cases of grout failure causing turbine slippage, among other issues, were reported. One solution is to use bolted ring flange connections, which involve using a large number of M72 bolts to provide a firm fixing between the MP-TP. It is in the interest of offshore wind operators to reduce the number of maintenance visits to these wind turbines by maintaining a preload (Fp) level above the minimum requirement for bolted MP-TP connections. The present study focuses on the effect of the tightening sequence on the Fp behaviour of M72 bolted connections. A detailed finite element (FE) model of a seven-bolt, representative segment of a monopile flange was developed with material properties obtained from the available literature. Three analyses were made to examine the effect on Fp after tightening, including the initial Fp level applied to the bolts, the tightening sequence and the effect of an additional tightening pass.
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    Fatigue life assessment of offshore wind support structures in the presence of corrosion pits
    (Elsevier, 2023-08-14) Shamir, Muhammad; Braithwaite, Jarryd; Mehmanparast, Ali
    Offshore wind turbine support structures, which connect the wind turbine transition piece and/or tower to the seabed, are located below the sea level and are in direct contact with seawater during the entire lifespan; therefore, they are highly susceptible to corrosion damage and cracking. In particular, the pitting corrosion is very crucial in these support structures, as it leads to local stress concentrations and thus affects the fatigue life. Although corrosion protection mechanisms are commonly implemented in offshore wind turbines, they have a finite life and therefore corrosion damage cannot be completely avoided during the entire life cycle, and this can lead to pitting corrosion on the steel surface. This paper aims to investigate the impact of pitting corrosion on fatigue durability of steel structures by performing tests on lab-scale coupons, made of S355 structural steel which is widely employed in fabrication of offshore wind support structures. For this purpose, cross-weld uniaxial samples were initially exposed to seawater for different time durations and then tested under cyclic loading condition. Furthermore, the durability analysis of corroded samples was carried out using a modified NASGRO equation. The results show that the pitting corrosion significantly reduces the fatigue life, and its level of life reduction is strongly dependent on the seawater exposure time. Moreover, the results show that employment of a linear trend for lower stress ranges would result in significant underprediction of the fatigue life, hence over-conservatism in the design life, at longer seawater exposure times.
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    The influence of laser shock peening on corrosion-fatigue behaviour of wire arc additively manufactured components
    (Elsevier, 2023-01-26) Ermakova, Anna; Braithwaite, Jarryd; Razavi, Javad; Ganguly, Supriyo; Berto, Filippo; Mehmanparast, Ali
    The need for increased manufacturing efficiency of large engineering structures has led to development of wire arc additive manufacturing (WAAM), which is also known as direct energy deposition (DED) method. One of the main barriers for rapid adoption of the WAAM technology in wider range of industrial applications is the lack of sufficient performance data on the WAAM components for various materials and operational conditions. The present study addresses this essential need by exploring the effects of laser shock peening surface treatment on corrosion-fatigue crack growth (CFCG) life enhancement of WAAM components made of ER70S-6 and ER100S-1 steel wires. The experimental results obtained from this study were compared with the CFCG trends from nominally identical specimens without surface treatment and prove the efficiency of the examined surface treatment method for corrosion-fatigue life enhancement and crack growth retardation of WAAM built steel components, regardless of the material type and specimen orientation. Furthermore, the residual stresses in the WAAM built specimens with and without surface treatment were measured to validate the influence of beneficial residual stresses, arising from surface treatment, on subsequent CFCG behaviour of the material. The residual stress profiles show the beneficial compressive stress fields in the surface treated areas which result in CFCG life enhancement. The results from this study make significant contribution to knowledge by evaluating the suitability of WAAM built steel components for application in offshore environments.
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    Material pre-straining effects on fatigue behaviour of S355 structural steel
    (Elsevier, 2021-04-25) Anandavijayan, Satya; Mehmanparast, Ali; Braithwaite, Jarryd; Brennan, Feargal; Chahardehi, Amir
    A commonly used material in offshore structures is S355 structural steel. For example, during the monopile fabrication process, the material is pre-strained to different levels at different depths through the thickness. Therefore, the influence of pre-straining on fatigue life and crack growth behaviour of the material needs to be examined and considered for design and life assessment procedures. In the present study, uniaxial fatigue and fatigue crack growth tests have been conducted on materials with different pre-strain levels and the results are compared with the un-strained material state. From the test data, it has been seen that the S-N fatigue life will reduce with increasing pre-straining level, while the fatigue crack propagation rate remains largely unchanged in pre-strained material. The results from this study are compared with the recommended S-N fatigue and fatigue crack growth trends available in standards and are discussed in terms of the applicability and level of conservatism in the recommended curves to account for the material pre-straining effects on the fatigue life assessment of offshore structures.
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    New shape function solutions for fracture mechanics analysis of offshore wind turbine monopile foundations
    (Elsevier, 2018-05-03) Bocher, Mathieu; Mehmanparast, Ali; Braithwaite, Jarryd; Shafiee, Mahmood
    Offshore wind turbines are considered one of the most promising solutions to provide sustainable energy. The dominant majority of all installed offshore wind turbines are fixed to the seabed using monopile foundations. To predict the lifetime of these structures, reliable values for shape function and stress intensity factor are needed. In this study, a new equation is developed through finite element simulations which have been performed for a wide range of monopile geometries with different dimensions, crack lengths and depths, to evaluate shape function and stress intensity factor solutions for monopiles. The new solutions have been verified through comparison with the existing solutions provided by Newman & Raju for small hollow cylinders. The empirical shape function solutions developed in this study are employed in a case study and the results have been compared with the existing shape function solutions. It is found that the old solutions provide inaccurate estimations of fatigue crack growth in monopiles and they underestimate or overestimate the fatigue life depending on the shape function solution employed in the structural integrity assessment. The use of the new solution will result in more accurate monopile designs as well as life predictions of existing monopile structures.
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    Sensitivity analysis of friction and creep deformation effects on preload relaxation in offshore wind turbine bolted connections
    (Elsevier, 2020-06-08) Braithwaite, Jarryd; Gómez Goenaga, Iñigo; Tafazzolimoghaddam, Behrooz; Mehmanparast, Ali
    A considerable portion of the preload loss in offshore wind turbine foundation to transition piece flanged connections is caused by short-term relaxation of the bolts. This relaxation can be influenced by several factors including the friction coefficient between the contact pairs in the flange bolted connections as well as room temperature creep. The present study focuses on the effect of these two factors on the performance of M72 bolts in offshore wind turbines. A detailed finite element model of a one-bolt-segment of the flanged connection is developed with material properties obtained from the literature. The first analysis examines the response of the bolt to a change in friction coefficient between contact surfaces. In the second analysis, the effect of room temperature creep is investigated over the first 48 hours of operation. Although creep is usually neglected in such low temperatures, for high load levels close to the material's yield stress, significant creep strain rates can occur. Finally, a sensitivity analysis on both friction and creep properties is carried out to investigate the influence of these two factors on the preload relaxation of M72 bolted connections.