Offshore Steel Structures Research Articles

The effect of corrosion on the structural reliability of steel offshore structures

This paper considers essential theoretical concepts and data requirements for engineering structural reliability assessment suitable for the estimation of the safety and reliability of corroding ships, offshore structures and pipelines. Such infrastructure operates in a harsh environment. Allowance must be made for structural deterioration since protective measures such as paint coatings, galvanizing or cathodic protection may be ineffective. Reliability analysis requires accurate engineering models for the description and prediction of material corrosion loss and for the maximum depth of pitting. New probability-based models for both these forms of corrosion have been proposed recently and calibrated against a wide range of data. The effects of water velocity and of water pollution are reviewed and compared with recently reported field data for a corrosion at an offshore oil platform. The data interpreted according to the model show good correlation when allowance is made for the season of first immersion and the adverse effects of seawater velocity and of water pollution. An example is given to illustrate the application of reliability analysis to a pipeline subject to pitting corrosion. An important outcome is that good quality estimation of the longer-term probability of loss of structural integrity requires good modelling of the longer-term corrosion behaviour. This is usually associated with anaerobic corrosion. As a result, it cannot be extrapolated from data for short-term corrosion as this is associated with aerobic corrosion conditions.

A new statistical local criterion for cleavage fracture in steel. Part II: application to an offshore structural steel

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.

A new statistical local criterion for cleavage fracture in steel. Part II: application to an offshore structural steel

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.

Environmental and socio-economic impact assessment of Ekoreef, a multiple platform rigs-to-reefs development

The North Sea Ekofisk oil field is approaching the end of its production, and various options are being considered regarding future use or demolition of the platforms. One option is the rigs-to-reefs initiative. OSPAR and Norwegian national legislation contain provisions that allow for the creation of artificial reefs providing they do not hinder other users and do not cause significant pollution. More generally, the creation of sizeable artificial reefs using redundant offshore steel structures is only sensible if positive impacts outweigh negative impacts. At an early stage in the cessation process, we carried out an impact assessment to identify factors that would present special environmental or socio-economic risks or benefits. Thirty-nine impacts, positive and negative, were identified and grouped under the following headings: exploitable fish stocks (8); local biota (7); sediments (4); water column (2); energy and emissions (2); socio-economics (10); other (6). Techniques by which to estimate their likely severity in general and the potential for their reduction (negative impacts) or maximization (positive impacts) are summarized. Semi-quantitative severity scores for all impacts identified were assigned to each of 11 reef-creation options. These were totalled and the options were then ranked accordingly. From an environmental and ecological (including commercial fish stocks) perspective, the results indicate that reef use for protection would be a better option than use for commercial fishing, although differences between the two were not large. The rank order did not discriminate among four of the suggested reef locations. However, toppling in situ was clearly identified as the worst scenario, whereas reef creation at a relatively clean site offered the most promising perspective. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Recycling of paint-contaminated grit.

The impact on the environment of using paint-contaminated grit (PCG) as a partial or full replacement for sand in Portland cement mortar and asphalt concrete mixtures was investigated. The grit waste material originated from abrasive blasting of offshore steel structures. There is a major environmental concern regarding the safe disposal of the spent blasting abrasives that contain paint chips or paint particles and other debris removed from the surface of the steel structures. This work investigated the potential reuse of PCG in Portland cement concrete (PCC) and hot mix asphalt concrete. Several studies were conducted to establish the integrity of the materials containing the recycled grit. These included the chemical and physical characterization of natural sand and PCG, the assay of leaches associated with the grit material for hazardous metal contaminants, such as Cr, Cd and Pb, and the assessment of the mechanical properties of the PCG-substituted mortars by applying special tests (such as Marshall stability and determination of the flow properties) to the PCG-substituted asphalt concrete mixtures. The overall results demonstrated that the potential reuse of PCG in PCC and asphalt concrete mixtures would not pose any environmental threat and could produce several benefits, such as reduced disposal costs, protection of water sources from improper disposal practices and reduced costs in the production of natural aggregates and asphalt cement.

Developments in structural system reliability assessments of fixed steel offshore platforms

In recent years there have been significant developments in the area of system reliability assessments which are becoming increasingly important given the framework of the ‘goal setting’ regime and other changes that are taking place within the offshore industry. The objective of this paper is to review and critically examine recent developments in system reliability methods for fixed steel offshore platforms specifically and identify areas that need to be examined further to maximise the benefits from the use of these techniques. The paper examines the range of proposed methods for system reliability assessment of fixed steel offshore structures under extreme environmental loading. The associated characteristics of the various methods are examined and the paper concentrates in particular on the treatment of the resistance. The various system effects including both deterministic and probabilistic effects and their relative contribution to the overall system reliability are addressed. Key issues such as the modelling uncertainties and sensitivities, validation and benchmarking of the proposed methods are also examined. The study also highlights a number of technical and philosophical issues which need to be addressed to increase the benefits from system reliability applications in design and re-assessment of fixed platforms.

Study on steel corrosion in different seabed sediments

A series of simulation experiments on carbon steel (A3 steel) and low alloy steel (16 Mn steel) in marine atmosphere (MA), seawater (SW) and seabed sediment (SBS) including rough sea sand, fine sea sand and seabed mud were carried out indoors for a year or so by means of individually hanging plates (IHP) and electrically connected hanging plates (ECHP). The corrosion of steels in SBS was mainly due to the macrogalvanic cell effect. The steel plates at the bottom of SBS, as the anode of a macrogalvanic cell, showed the heaviest corrosion with a corrosion rate of up to 0·12 mm/a, approximately equal to that of steel plates in marine atmosphere.The test results showed that the corrosion rates of A3 and 16 Mn steel in marine environment were in the order: MA>SW>SBS by the IHP method; and MA>SBS>SW by the ECHP method. The corrosion rates of steels in the water/sediment interface were directly proportional to the grain size of the SBS by the ECHP method, but those of steels in the water/sediment interface did not vary with the grain size of SBS by the IHP method. The corrosion rate of low-alloy steel was a little higher than that of carbon steel. The results of this study have important applications for design of offshore steel structures such as oil platform, pier, and port.

Calibration Issues in Development of ISO Standards for Fixed Steel Offshore Structures

Abstract A study supported by the American Petroleum Institute established that load and resistance factor design offers improvement in design and is supported by credible data, can yield improved designs of greater reliability and/or economy and is in reasonable accordance with current design practices. This paper discusses the criteria for calibrating the ISO-LRFD based on the API studies. Other topics include actual versus notional reliabilities, regional load factors, member versus system reliability, and selecting a design return period. The paper also describes safety indices for the combined loading interaction formulas. [S0892-7219(00)01301-7]

On alternative approaches to linearization and Morison's equation for wave forces

Restricted accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Wolfram Julian 1999On alternative approaches to linearization and Morison's equation for wave forcesProc. R. Soc. Lond. A.4552957–2974http://doi.org/10.1098/rspa.1999.0434SectionRestricted accessOn alternative approaches to linearization and Morison's equation for wave forces Julian Wolfram Julian Wolfram Department of Civil and shore Engineering, Heriot–Watt University, Edinburgh EH9 1UD, UK Google Scholar Find this author on PubMed Search for more papers by this author Julian Wolfram Julian Wolfram Department of Civil and shore Engineering, Heriot–Watt University, Edinburgh EH9 1UD, UK Google Scholar Find this author on PubMed Search for more papers by this author Published:08 August 1999https://doi.org/10.1098/rspa.1999.0434AbstractThe limitations of the ‘equivalent linearization’ approach are discussed and an alternative approach is advocated based on the idea of ‘corresponding linear forms’ which give unbiased estimates for the output quantity required from an analysis. The application of the alternative approach is illustrated by developing corresponding linear forms for Morison's equation applicable to the estimation of extreme wave loading and the estimation of wave–induced fatigue loading. These corresponding linear forms differ only in a linearization factor and expressions for this are obtained for both cases. It is shown that the linearization factor varies not just with the nature of the output but also with the relative magnitudes of the drag and inertia terms in Morison's equation. The application of the alternative approach to the oscillatory motion of bodies is discussed briefly. Previous ArticleNext Article VIEW FULL TEXT DOWNLOAD PDF FiguresRelatedReferencesDetailsCited by Faedo N, Peña-Sanchez Y and Ringwood J (2020) Parametric representation of arrays of wave energy converters for motion simulation and unknown input estimation: A moment-based approach, Applied Ocean Research, 10.1016/j.apor.2020.102055, 98, (102055), Online publication date: 1-May-2020. da Silva L, Cazzolato B, Sergiienko N, Ding B, Morishita H and Pesce C (2020) Statistical linearization of the Morison’s equation applied to wave energy converters, Journal of Ocean Engineering and Marine Energy, 10.1007/s40722-020-00165-9, 6:2, (157-169), Online publication date: 1-May-2020. Faedo N, Garcia-Violini D, Scarciotti G, Astolfi A and Ringwood J (2019) Robust Moment-Based Energy-Maximising Optimal Control of Wave Energy Converters 2019 IEEE 58th Conference on Decision and Control (CDC), 10.1109/CDC40024.2019.9029578, 978-1-7281-1398-2, (4286-4291) Levchuk K (2018) Engineering Tools and Technologies of Freeing of the Stuck Metal Drilling String, METALLOFIZIKA I NOVEISHIE TEKHNOLOGII, 10.15407/mfint.40.01.0045, 40:1, (45-137), Online publication date: 5-May-2018. Kim H, Lee K and Jang B (2018) A linearization coefficient for Morison force considering the intermittent effect due to free surface fluctuation, Ocean Engineering, 10.1016/j.oceaneng.2018.04.025, 159, (139-149), Online publication date: 1-Jul-2018. Li L, Jiang Z, Vangdal Høiland A and Chen Ong M (2018) Numerical Analysis of a Vessel-Shaped Offshore Fish Farm, Journal of Offshore Mechanics and Arctic Engineering, 10.1115/1.4039131, 140:4, Online publication date: 1-Aug-2018. Gam M, Jang B and Park J (2017) A study on the fatigue analysis for a vertical caisson on FPSO subjected to the nonlinear wave loading, Ocean Engineering, 10.1016/j.oceaneng.2017.03.057, 137, (151-165), Online publication date: 1-Jun-2017. Pryor S and Barthelmie R (2013) Renewable Energy Resources – Ocean Energy Climate Vulnerability, 10.1016/B978-0-12-384703-4.00311-7, (65-81), . Ballard Z and Mann B (2013) Two-Dimensional Nonlinear Analysis of an Untethered Spherical Buoy Due to Wave Loading, Journal of Computational and Nonlinear Dynamics, 10.1115/1.4024887, 8:4, Online publication date: 1-Oct-2013. Karadeniz H, Saka M and Togan V (2013) Water Wave Theories and Wave Loads Stochastic Analysis of Offshore Steel Structures, 10.1007/978-1-84996-190-5_3, (177-252), . Garrè L and Der Kiureghian A (2010) Tail-Equivalent Linearization Method in frequency domain and application to marine structures, Marine Structures, 10.1016/j.marstruc.2010.07.006, 23:3, (322-338), Online publication date: 1-Jul-2010. Arena F and Nava V (2008) On linearization of Morison force given by high three-dimensional sea wave groups, Probabilistic Engineering Mechanics, 10.1016/j.probengmech.2007.12.010, 23:2-3, (104-113), Online publication date: 1-Apr-2008. Socha L Applications of Linearization Methods in Vibration Analysis of Stochastic Mechanical Structures Linearization Methods for Stochastic Dynamic Systems, 10.1007/978-3-540-72997-6_9, (281-340) Socha L (2005) Linearization in Analysis of Nonlinear Stochastic Systems, Recent Results—Part II: Applications, Applied Mechanics Reviews, 10.1115/1.1995715, 58:5, (303-315), Online publication date: 1-Sep-2005. Lin W, Su C and Tang Y (2020) Explicit Time-Domain Approach for Random Vibration Analysis of Jacket Platforms Subjected to Wave Loads, Journal of Marine Science and Engineering, 10.3390/jmse8121001, 8:12, (1001) Uthamanthil N and Drake K (2021) An investigation into the influence of analysis methods upon fatigue damage predictions for jack-up rigs, Ships and Offshore Structures, 10.1080/17445302.2021.1912458, (1-11) Zheng Z, Chen J, Liang H, Zhao Y and Shao Y (2020) Hydrodynamic Responses of a 6 MW Spar-Type Floating Offshore Wind Turbine in Regular Waves and Uniform Current, Fluids, 10.3390/fluids5040187, 5:4, (187) Lemmer F, Yu W and Cheng P (2018) Iterative Frequency-Domain Response of Floating Offshore Wind Turbines with Parametric Drag, Journal of Marine Science and Engineering, 10.3390/jmse6040118, 6:4, (118) This Issue08 August 1999Volume 455Issue 1988 Article InformationDOI:https://doi.org/10.1098/rspa.1999.0434Published by:Royal SocietyPrint ISSN:1364-5021Online ISSN:1471-2946History: Published online08/08/1999Published in print08/08/1999 License: Citations and impact Keywordsfatiguewave forcelinearizationextreme loadingoffshore structuresMorison's equation

Reliability-based methods in the inspection planning of fixed offshore steel structures

For the continued safe operation of a fixed offshore steel structure, it is necessary for any damage resulting from fatigue, collision or impact to be prevented from leading to either progressive failure or sudden collapse. The detection of fatigue damage at an early stage in the life of the structure is, however, difficult and costly, especially sub-sea, but the financial and human consequences of structural failure resulting from inadequate inspection are likely to be much greater, and need to be prevented. The paper describes research that has been undertaken to enable sub-sea inspection strategies to be improved. The findings relate the inspection methods used, the locations inspected and the frequency of inspection to the probability of local fatigue damage leading to unacceptable structural consequences. This provides a practical way of targeting offshore inspection in order to reduce overall life-cycle costs whilst maintaining adequate levels of system reliability for the complete structure.

A normalization of corrosion fatigue behaviour: an example using an offshore structural steel in chloride environments

A normalized presentation of the metal fatigue process is proposed and applied to the corrosion fatigue behaviour of an offshore structural steel under different loading conditions. Corrosion fatigue development in this steel under different stress levels and frequencies is presented in both ‘normal’ and ‘normalized’ terms. Experimental results on pitting and corrosion fatigue‐crack growth are used to introduce the new normalized presentation approach, which may be employed to illustrate the corrosion fatigue behaviour of any given metal/environment system. This allows a description of the initiation and growth of defects to be made, involving both time‐dependent and cycle‐dependent stages. It is suggested that a set of dependencies between fatigue‐characterizing parameters may be presented in graphic form and used as a ‘fatigue certificate’ for any given metal/environment system.

Reliability-Based Criteria for Fixed Steel Offshore Platforms

The key issue addressed in this paper is the accuracy of structural reliability models for the case of fixed steel offshore structures under extreme storm loading. The emphasis is on engineering accuracy for the purpose of use in decision-making, and more specifically to achieve sufficient accuracy to enable the use of reliability models in deriving design criteria for fixed offshore platforms. These reliability models are used to derive partial load factors for use in conjunction with API LRFD to achieve a target reliability level appropriate for permanently manned installations. These load factors are location-dependent. Further load factors are proposed for the design of new, not normally manned installations.

Orientation dependence of microfracture behavior in a dual-phase high-strength low-alloy steel

In selecting the processing conditions and evaluating the reliability of structural materials, microscopic observations and identification of the fracture mechanisms in local cracking behavior are required. An important instance in the failure of the local brittle zone (LBZ) in the welding zone. The LBZ, which is very brittle, is the coarse-grained heat-affected zone near the fusion line, a zone known to be critical to the fracture toughness of welded parts. Thus, maintaining stable fracture resistance by predicting the microfracture behavior is important when using high-strength low-alloy (HSLA) steels in offshore structural steel welds. Depending on the thermal cycles involved during welding, the ferrite/martensite structure can have various morphologies of martensite particles, for example, fibrous and blocky martensite. In summary, in situ SEM fracture tests reveal that in the L-oriented IQ DCB specimen, a microcrack tends to propagate relatively uniformly throughout the ferrite and well-distributed fine fibrous martensite, yielding good elongation with high strength level. Also, the IQ structure in the T orientation shows similar microfracture behavior. On the other hand, in the SQ structure, where blocky-type martensite is mixed with ferrite, strain is localized into shear bands mostly in the ferrite region, and a local microcrack propagates along the strain-localizedmore » band formed in the ferrite, resulting in the SQ structure in the T orientation, where the ferrite-martensite bands are parallel to the notch direction, the martensite cannot act as an efficient barrier to microcrack advance, and thus the tensile ductility is decreased.« less

Experimental investigation on low-temperature fatigue crack propagation in off-shore structural steel A131 under random sea ice loading: Liu, C.- T. and Duan, M.-L. Engng Fract. Mech (1996) 53, 231–237

The objective of this study is to evaluate the reliability of through-aluminum-nitride-via (TAV) substrate by comparing those experimental results with the finite element simulation associated with measurements of aluminum nitride (AlN) strength and the thermal deformation of Cu/AlN bi-material plate. Two reliability tests for high-power LED (Light emitting diode) applications are used in this study: one is a thermal shock test from − 40 °C to 125 °C, the other is a pressure cook test. Also, the strength of AlN material is measured by using three-point bending test and point load test. The reliability results show that TAV substrates with thicker Cu films have delamination and cracks after the thermal shock test, but there are no failure being found after the pressure cook test. The determined strengths of AlN material are 350 MPa and 650 MPa from three-point bending test and point load test, respectively. The measurement of thermal deformation shows that the bi-material plate has residual-stress change after the solder reflow process, also indicating that a linear finite element model with the stress-free temperature at 80 °C can reasonably represent the stress state of the thermal shock test from − 40 °C to 125 °C without considering Cu nonlinear effect. The further results of the finite element simulation associated with strength data of AlN material have successfully described those of the reliability test.

THE EFFECT OF CATHODIC PROTECTION POTENTIAL ON CORROSION FATIGUE CRACK GROWTH RATE OF AN OFFSHORE STRUCTURAL STEEL

Abstract— Duplicate tests have been performed to determine the effect of cathodic protection potential on corrosion fatigue crack growth rate of a modern offshore structural steel, produced by thermo‐mechanically controlled processes. The experiments were carried out using compact tension specimens exposed to artificial seawater at 10°C and subjected to constant amplitude loading at 0.35 Hz. Reproducible results showed that the merits of cathodic protection potentials are strongly dependent on stress intensity ratio R and stress intensity range ΔK. It appears that a specific value of cathodic potential may not give comprehensive protection against corrosion fatigue within the spectrum of variable amplitude loading experienced in service. Fractography showed the initiation of secondary cracks on the fracture surface to be associated with the dissolution of calcium sulphide inclusions, regardless of imposed cathodic potential.

The application of microstructural fracture mechanics to various metal surface states

The application of microstructural fracture mechanics to various metal surface states

The Effect of Nickel on the Mechanical Properties of High-Oxygen Underwater Wet Welds

The use of oxidizing electrodes for wet welding of offshore structural steels, in spite of their low susceptibility to hydrogen HAZ cracking, is limited, in part, by the poor mechanical properties of their weld deposits. Low levels of carbon, manganese, and other deoxidizers, together with high oxygen contents seems to be one of the reasons for this low performance. This work investigated the influence of nickel additions on the tensile strength and impact resistance of wet welds deposited at 1.1 m of water depth. It was found that welds with nickel contents between 2 and 3 weight percent exhibited maximum toughness and tensile strength. Nickel additions also had a strong effect in reducing the grain size of equiaxed ferrite in the reheated region of underwater wet welds, thereby improving their mechanical properties. The drop of mechanical properties for nickel contents higher than 3 weight percent was associated with weld metal solidification cracking.

An assessment of weld mis-matching

A large data bank of information related to welded offshore structural steels has been used to assess the influence of weld metal strength mis-matching on the fracture resistance of these weldments, based on Charpy impact test results, crack tip opening displacement CTOD values and wide plate results, along with supporting welding details, metallography and mechanical properties. The advantage of this approach is that there are sufficient tests for the values to be assessed against a normal statistical distribution, enabling trends to be identified despite appreciable scatter inherent in a smaller number of tests. The practical results are contrasted with the guidelines given in the British Standards Institution publication PD6493:1991 for initially assessing flaws in fusion welded joints in the as welded and post weld heat treated conditions. The important difference between specifying a required toughness K mat and a required CTOD value δ mat has been demonstrated by considering the Level 1 assessment procedure. The improvement in toughness on post weld heat treating these joints has been quantified and distinguished from the practical benefits from weld metal strength overmatching.

Experimental investigation on low-temperature fatigue crack propagation in offshore structural steel A131 under random sea ice loading

An apparatus of low-temperature controlling for fatigue experiments and its crack measuring system were developed and used for offshore structural steel A131 under conditions of both low temperature and random sea ice. The experimental procedures and data processing were described, and a universal random data processing software for FCP under spectrum loading was written. Many specific features of random ice-induced FCP which differed with constant amplitude FCP behaviours were proposed and temperature effect on ice-induced FCP was pointed out with an easily neglected aspect in designing for platforms in sea ice emphasized. In the end, differences of FCP behaviours between sea ice and ocean wave were presented.

Geometry effects on corrosion-fatigue in offshore structural steels

Abstract Fatigue crack growth rates were measured for two offshore steels with yield stresses of roughly 380 and 750 MPa in air and in synthetic seawater under both freely corroding and cathodically polarized conditions. Particular attention was given to the influence of testing frequency and specimen thickness. Small surface cracks were also evaluated. The higher-strength steel tends to exhibit higher fatigue crack growth rates as thickness is increased, this effect becoming more pronounced in an aggressive environment. Furthermore, the frequency dependence under cathodic polarization is more pronounced in the higher-strength steel than in the lower-strength steel. No ‘chemically short crack’ effect (i.e. that small cracks only grow more rapidly than long through-cracks in a corrosive environment) is observed when the ‘reference’ da/dN−ΔK curve for long cracks utilizes specimens with comparable dimensions.