Offshore Steel Research Articles

Underwater strengthening and repairing of tubular offshore structural members using Carbon Fibre Reinforced Polymers with different consolidation methods

Offshore structural steel members will need strengthening or repairing due to various reasons. Strengthening is often required due to work over demands or increased environmental loads. Repairing becomes essential as the steel members are usually severely corroded due to the marine conditions. Conventional hot work strengthening and repairing solutions become very expensive and retrofitting using Fibre Reinforced Polymers (FRPs) is an economical alternative. This paper investigates strengthening and repairing of offshore steel tubular members using Carbon Fibre Reinforced Polymers (CFRPs) with the help of experimental testing and an analytical study. Two different specimen sizes which were representative of members in offshore structures namely leg and brace were examined in the study. Partial length strengthening of steel tubular members subject to combined loading of axial compression and bending was carried out using different curing environments (in air and underwater curing). Strengthening and repairing of leg and brace specimens with multiple layers of CFRP retrofitted in underwater conditions were also investigated. Two different consolidation methods (underwater vacuum consolidation and underwater stricture consolidation) were compared with the help of full length strengthening of leg specimens. Although partial length strengthening proved to be less effective, partial length repairing of corroded tubular members were successful in restoring their intact capacity. Full length strengthening for underwater leg specimens were experimentally tested and it was found to improve the structural behaviour in many aspects like ultimate strength, load displacement behaviour, elastic slope, energy absorption, and ductility. A novel technique of underwater vacuum consolidation of retrofit laminates was demonstrated through a set of full length strengthened leg specimens. Underwater vacuum consolidation proved to be advantageous compared to the normal underwater stricture consolidation. An analytical check to identify whether a retrofitted tubular member would reach its target section properties was also introduced. Overall, the paper demonstrates multiple successful applications of FRP composites for underwater strengthening and repairing of offshore steel tubular members. Studies like this expanding the current knowledge on how the FRP retrofitted steel tubular behave within a real offshore structure are essential to increase the uptake of this technology in the oil and gas industry.

Estimation of Overall Fatigue Life of Jack-up Leg Structure

Jack-up platforms are designed to work in three conditions: Transit, Preloading and Operating. The fatigue life of the jack-up platforms in operating condition will be determined to be similar to that of offshore fixed steel structures. In preloading conditions, fatigue damage is usually ignored. Up to now, the fatigue damage of the jack-up leg structure in transit conditions has been calculated at approximately 20% of the total fatigue damage of the jack-up leg structure in two conditions (transit and operating). The approximate calculation method is usually accepted by consultants and register agencies. If the approximation is used, the fatigue life of the jack-up leg structure will be calculated only as for the jacket structure of the fixed offshore structure, with 80% of the allowable fatigue life based on standards. The approximation proved to have many disadvantages: the different travel times of each jack-up rig cannot be mentioned; hot spots that need to be maintained during the transit condition have not been pointed out; it is difficult to guarantee the safety of the jack-up leg structures in the transit condition. In order to overcome the limitations of the approximation method, this paper will propose a method to predict the overall fatigue life of the jack-up leg structure in three main problems. Firstly, we use the analysis method of fatigue of fixed steel offshore structures for jack-up leg structures in operating conditions. Secondly, we suggest a method to analyze the fatigue of the structures in transit conditions. Herein, motion analysis and determination of inertia forces on the leg structure are performed by the Boundary Element Model (BEM) in SACS software. Then the inertia forces are assigned to a Finite Element Model (FEM) in SACS to decide the internal forces of the structures. Hotspot stresses are determined by combining nominal stress from FE analysis results with a concentrated stress factor from the analysis of joint local models in the ANSYS program. Then, fatigue damage and fatigue life of hotspots of the structure are determined in the transit condition. Finally, a formula is suggested to determine total fatigue damage in operating conditions and transit conditions with different cases in relation to different fraction factors. These results are used to predict fatigue life corresponding to the most dangerous cases of structural joints. These new suggestions are applied to fatigue analysis for jack-up Tam Dao 05. Currently, the Tam Dao 05 platform has been operating in the Vietnam East Sea. Doi: 10.28991/CEJ-2022-08-03-06 Full Text: PDF

Characterization of Hydrogen Diffusion in Offshore Steel S420G2+M Multi-layer Submerged Arc Welded Joint

As onshore installation capacity is limited, the increase in the number of offshore wind turbines (OWT) is a major goal. In that connection, the OWTs continuously increase in size and weight and demand adequate foundations concepts like monopiles or tripods. These components are typically manufactured from welded mild steel plates with thickness up to 200 mm. The predominant welding technique is submerged arc welding (SAW). In accordance with the standards, the occurrence of hydrogen-assisted cracking is anticipated by either a minimum waiting time (MWT, before non-destructive testing of the welded joint is allowed) at ambient or a hydrogen removal heat treatment (HRHT) at elevated temperatures. The effectiveness of both can be estimated by calculation of the diffusion time, i.e., diffusion coefficients. In this study, these coefficients are obtained for the first time for a thick-walled S420G2+M offshore steel grade and its multi-layer SAW joint. The electrochemical permeation technique at ambient temperature is used for the determination of diffusion coefficients for both the base material and the weld metal. The coefficients are within a range of 10−5 to 10−4 mm2/s (whereas the weld metal had the lowest) and are used for an analytical and numerical calculation of the hydrogen diffusion and the related MWT. The results showed that long MWT can occur, which would be necessary to significantly decrease the hydrogen concentration. Weld metal diffusion coefficients at elevated temperatures were calculated from hydrogen desorption experiments by carrier gas hot extraction. They are within a range of 10−3 mm2/s and used for the characterization of a HRHT dwell-time. The analytical calculation shows the same tendency of long necessary times also at elevated temperatures. That means the necessary time is strongly influenced by the considered plate thickness and the estimation of any MWT/HRHT via diffusion coefficients should be critically discussed.

Mechanism of BN-Promoting Acicular Ferrite Nucleation to Improve Heat-Affected Zone Toughness of V-N-Ti Microalloyed Offshore Steel.

This study examined the effect of boron (B) on the microstructure and toughness of the simulated coarse-grained heat-affected zone (CGHAZ) of normalized vanadium microalloyed offshore steel by using the welding thermal simulation method under different heat inputs for welding. The results showed that when t8/5 (the cooling time from 800 to 500 °C) increased from 14 s to 24 s, In the range of t8/5 of 24–44 s, the impact energy of the CGHAZ rose initially and then remained constant at around 125 J at −40 °C, and dropped to 79 J when t8/5 increased to 64 s. The particles (Ti,V)(C,N)-BN and BN contributed in the generation of acicular ferrite, which minimized the loss of CGHAZ toughness due to the presence of carbon. Furthermore, the microstructural parameters controlling CGHAZ toughness were the contents of the high misorientation grain boundaries and effective grain size at a tolerance angle of 15° at varied heat inputs.

Corrosion in marine and offshore steel structures: Classification and overview

Marine services and industry have emerged as one of the most significant pillars of global economic growth. Corrosion of materials, on the other hand, is still the most important issue for marine structures and equipment. Corrosion is a major contributor to the degradation of marine and offshore structures. It has an effect on the life of process equipment, and might lead to structural failure, leakage, product degradation, contamination, and even death. It is important to investigate the mechanisms of material corrosion in the marine environment, as well as corrosion prevention methods, in order to make effective use of marine resources. Various mitigation techniques, like the utilization of coatings, cathodic protection, and corrosion allowance are utilized to protect offshore structures against corrosion. In general, the marine environment has a significant impact on the protection of offshore structures. Therefore, the objective of this paper is to give an overview about corrosion of marine structures, how it happens and some of the current prevention techniques.

Experimental validation on multi-pass weld distortion behavior of structural offshore steel HSLA S460 using FE-based inherent strain and thermo-mechanical method

Experimental validation on multi-pass weld distortion behavior of structural offshore steel HSLA S460 using FE-based inherent strain and thermo-mechanical method

Fault-tolerant control for nonlinear offshore steel jacket platforms based on reinforcement learning

In this article, reinforcement learning based fault-tolerant control for the nonlinear offshore platform is studied. First, the dynamic model of the nonlinear steel-jacket offshore platform with active mass damper and actuator fault is investigated then an affine nonlinear representation for the offshore platform is obtained. By estimating actuator fault and irregular wave force and inserting their estimation into the cost function, the disturbance control and fault-tolerant control problem (FTC) are converted into optimal control problem. Online policy iteration is used to minimize the performance index. Finally, simulation results show the effectiveness of our method.

Toughness and fracture mechanism at low temperature of offshore engineering steel at different welding heat inputs

The microstructure and low temperature toughness of offshore engineering steel which has a yield strength of 420 MPa were investigated at different welding heat inputs. As the heat input is increased from 50 kJ/cm to 200 kJ/cm, the impact energy in coarse-grained heat-affected zone (CGHAZ) is decreased from 196 to 147 J at –40 ℃. The fracture is changed from ductile to quasi-cleavage and cleavage. The prior austenite grain is increased. The size and area fraction of martensite-austenite (M-A) constituents is increased, while the number density is decreased. The capability on the crack resistance is in decreasing order for fined lath bainite (LB), interlocked LB, coarsen LB and granular bainite (GB), and it is increased by the larger misorientation angle and HAGBs. As the heat input is larger than 150 kJ/cm, the crack is induced by the M-A constituent and propagates along the weak link caused by the M-A constituents. The secondary crack is mostly deflected by the larger misorientation angle larger than 45°, and the cleavage plane prefers {001} plane. Larger welding heat input decreases the CGHAZ strength, thus leads to the increase in crack tip plastic zone (CTPZ). The existence of M-A constituent and coarsening microstructure give rise to the cleavage with the larger CTPZ.

In Situ Observation of Phase Transformations in the Partially Melted Zone of Hsla Offshore Steel During Simulated Welding Process

In Situ Observation of Phase Transformations in the Partially Melted Zone of Hsla Offshore Steel During Simulated Welding Process

Deformation Monitoring of Offshore Steel Pipe Piles Based on Weak Reflection Bragg Grating Technology

Deformation Monitoring of Offshore Steel Pipe Piles Based on Weak Reflection Bragg Grating Technology

Kriging Model for Reliability Analysis of the Offshore Steel Trestle Subjected to Wave and Current Loads

Offshore steel trestles (OSTs) are exposed to severe marine environments with stochastic wave and current loads, making structural safety assessment challenging and difficult. Reliability analysis is a suitable way to consider both wave and current loading intensity uncertainties, but the implicit and complex limit state functions of the reliability analysis usually imply huge computational costs. This paper proposes an efficient reliability analysis framework for OST using the kriging model of optimal linear unbiased estimation. The surrogate model is built with stochastic waves, current parameters, and the corresponding load factors. The framework is then used to evaluate the reliability of an example OST subjected to wave and current loads at three limit states of OST, including first yield (FY), full plastic (FP), and collapse initiation (CI). Three different distributions are used for comparison of the results of failure probability and reliability index. The results and the computational cost by the proposed framework are compared with that from the Monte Carlo sampling (MCS) and Latin hypercube sampling (LHS) method. The influences of sample number on the prediction accuracy and reliability index are investigated. The influence of marine growth on the reliability analysis of the OST is discussed using MCS and the kriging model. The results show that the reliability analysis based on the kriging model can obtain the reliability index for the OST efficiently with less calculation time but similar results compared with MCS and LHS. With the increase of the number of samples, the prediction accuracy of the kriging model increases, and the corresponding failure probability fluctuates greatly at first and then tends to be stable. The reliability of the example OST is reduced with the increase of marine growth, regardless of the limit state.

An Experimental Study of Damage Detection on Typical Joints of Jackets Platform Based on Electro-Mechanical Impedance Technique.

Many methods have been used in the past two decades to detect crack damage in steel joints of the offshore structures, but the electromechanical impedance (EMI) method is a comparatively recent non-destructive method that can be used for quality monitoring of the weld in structural steel joints. The EMI method ensures the direct assessment, analysis and particularly the recognition of structural dynamics by acquiring its EM admittance signatures. This research paper first briefly introduces the theoretical background of the EMI method, followed by carrying out the experimental work in which damage in the form of a crack is simulated by using an impedance analyser at different distances. The EMI technique is used to identify the existence of damage in the welded steel joints of offshore steel jacket structures, and Q345B steel was chosen as the material for test in the present study. Sub-millimetre cracks were found in four typical welded steel joints on the jacket platform under circulating loads, and root average variance was used to assess the extent of the crack damage.

Simultaneous Improvement in Mechanical Properties and Fatigue Crack Propagation Resistance of Low Carbon Offshore Structural Steel EH36 by Cu–Cr Microalloying

Improving the mechanical performance of low-carbon offshore steel is of great significance in shipbuilding applications. In this paper, a new Cu-Cr microalloyed offshore structural steel (FH36) was developed based on EH36. The microstructure, mechanical properties, and fatigue crack propagation properties of rolled plates of FH36, EH36, and normalizing rolled EH36 plates (EH36N) manufactured by a thermo-mechanical control process (TMCP) were analyzed and compared (to simplify, the two rolled specimens are signified by FH36T and EH36T, respectively). FH36T showed an obvious advantage in elongation with the value of 29%, 52.2% higher than the EH36T plates. The normalizing process led to a relatively lower yield stress (338 MPa), but substantially increased the elongation (33%) and lessened the yield ratio from 0.77 to 0.67. Electron back-scattered diffraction (EBSD) analysis showed that SFs of the deformation texture of FH36T and EH36N along the transverse direction (TD) and normal direction (ND) were much higher than those of the EH36T plate, which enhanced the lateral movement ability in the width and thickness direction, enhancing the ductility. Moreover, FH36 plates showed a better fatigue crack propagation resistance than rolled EH36 plates. The formation of the jagged shape grain boundaries is believed to induce a decrease of effective stress intensity factor during the fatigue crack propagation process.

A Case Study of Floating Offshore Super-Long Steel Pipeline Combing with Field Monitoring

How to control deformation and avoid resonance is the key to ensuring the safety of the super-long pipeline when it is floating in the sea. Based on the deformation warning value of pipeline prototype composite material obtained from laboratory tests, the raw water pipeline project in Tong’an Xiamen adopts wireless communication equipment to transmit data, supplemented by aerial photography technology to monitor and feedback the strain and vibration during the dynamic construction of long-distance pipeline floating transportation. Combined with dynamic construction, this monitoring method avoids excessive deformation and resonance of the steel pipeline during floating transportation, and prevents the destruction of the anticorrosive coating. The airtightness test after completion shows that the whole pipeline meets the acceptance requirements. The monitoring results show that the strain at the bent position of the pipeline is large in the process of floating transportation, and the jacking speed and position of the tugboats have an important influence on the deformation of the pipeline. The same type of project should focus on these aspects and timely feedback monitoring data. At the same time, the study also provides detailed strain, modal analysis and effective monitoring technology for the safety of offshore steel pipeline floating transportation.

Analytical Model and Numerical Analysis of Composite Wrap System Applied to Steel Pipeline.

Composite overwraps are a cost-effective repair technology, appropriate for corrosion defects, dents, and gouges for both onshore and offshore steel pipelines. The main benefit of polymer-based sleeves is safe installation without taking the pipeline out of service. This paper presents a new calculation procedure proposed in the form of an algorithm for the sizing of composite repairs of corroded pipelines when the sleeve is applied at zero internal pressure. The main objective of the presented methodology is determination of the effective thickness of the composite repair without its overestimation or underestimation. The authors used a non-linear finite element method with constitutive models allowing analysis of the steel, putty, and composite structures. The validation of the results of numerical computations compared to the experimental ones showed an appropriate agreement. The numerical calculations were applied to compare the analytical results in relation to those obtained by the standards ASME PCC-2 or ISO/TS 24817. The comparison showed that the proposed solution confirmed its effectiveness in reducing the thickness of the sleeve significantly, thus, showing that the current industrial standards provide a considerably excessive composite wrap around the steel pipe corroded area, which leads to an unnecessary increase in the repair costs.

Maximum bending moment capacity of circular perforated offshore steel tubular members

Offshore steel tubular structures may experience perforations during their lifespan due to effects of the corrosive marine environment, which leads to deterioration of their strength capacity and lifetime shortening. There is a lack of procedures to quantify the structural capacity loss in view of the damage dimensions and geometrical characteristics of the tubular members. In this work, the maximum bending moment capacity of circular perforated steel tubular structures is evaluated experimentally and numerically. The effects of the magnitude of the perforation and the diameter-to-thickness ratio are discussed. In a first phase, reduced scale experimental tests and numerical simulations are developed considering tubular structures under pure bending moment load through a four-point configuration. Subsequently, a parametric analysis is conducted employing a simplified numerical model, then a design expression is adjusted and compared with numerical predictions and experimental measures to determine its reliability. The numerical model and the proposed expression showed absolute errors below 3% in relation to the experimental measures. Thus, the proposed expression can be adopted as an alternative criterion to assist in decision-making for supervision, repair or replacement of perforated tubular structures subjected to bending moment.

Fatigue Evaluation of Offshore Steel Structures Considering Stress Concentration Factor

The installation of offshore structures and facilities in the marine environment, usually for the production and transmission of oil, gas exploration, electricity, and other natural resources is referred to as offshore construction. Since offshore structures are subjected to changing threats to the environment year-round. Fatigue behavior prediction noticed on these structures should be considered during the design stage. Fatigue is one of the failure mechanisms of offshore steel structures, and it must be investigated properly during system design. The fatigue analysis of offshore structures under drag wave force, total wave force, total moment about the sea bed, and other variables are reviewed thoroughly. The structure's dynamic response becomes a critical aspect in the whole design process. The fatigue analysis was carried out using MATLAB software, material properties of the offshore structure, and wave spectrum characteristics in this study. This study shows the JONSWAP spectrum and stress concentration analysis prediction. The offshore support structure that is predicted during the design phase will help to keep the stress concentration factor below the fatigue threshold and anticipate safe life design, according to the results of the fatigue study. The fatigue performances of tripod and jacket steel support structures in intermediate waters depth are compared in this project (20-50 m). The North Atlantic Ocean is used as a reference site, with a sea depth of 45 meters. The tripod and jacket support structures will be designed by using current industry standards. Keywords: [Fatigue evaluation, North Atlantic Ocean and Failure].

Structural response of steel lined pipes under cyclic bending

Motivated by reeling installation of mechanically-lined offshore steel pipes, the present paper investigates the structural performance of a bi-material lined pipe under cyclic bending, using advanced finite element tools. A three-dimensional finite element model is developed, simulating the manufacturing process of the lined pipe in the first stage of the analysis, followed by the cyclic bending response of the lined pipe in the second stage. Five bending cycles are considered, representing the two installation cycles and three additional cycles of a failure-repair scenario. The loading cycles impose a bending curvature range corresponding to the strains developed during a typical reeling installation process. Different loading patterns are considered and their effect on liner performance are investigated. The results show that the application of reverse (negative) curvature during the loading cycles, representing the straightener, has a significant influence on the wrinkle size of the liner developed at the two critical generators, and its rate of increase, compared with cyclic bending patterns with non-negative curvature. Numerical results on imperfection sensitivity are obtained, considering two types of imperfection of the liner pipe. In addition, the structural performance of liners with different thickness is examined, and the results show that there exists a minimum value of wall thickness, above which the liner does not exhibit local buckling at the end of the cyclic loading history. The beneficial effect of internal pressure on liner cyclic response is also verified, especially for thin-walled liners, preventing the development of wrinkles. Finally, the effect of manufacturing process is examined, showing the superior structural performance of partially-heated lined pipes, with respect to fully-heated lined pipes, and to lined pipes manufactured by purely-mechanical process.

Hydrogen-induced cracking and corrosion behavior of friction stir welded plates of API 5L X70 pipeline steel

The use of friction stir welding (FSW) has proven to be an excellent alternative to join engineering components. Although FSW has had a significant development in recent years, challenges for new applications have been raised, such as offshore steel parts suffering hydrogen embrittlement in the gas and oil industry. Therefore, in this work, the microstructure, corrosion, and hydrogen-induced cracking were investigated in a two-pass FSW welded joint of API 5L X70 pipeline steel. The electrochemical results indicate an inhibitory effect on corrosion reaction because of a carbonate product generation in the steel surface. The polygonal ferritic and degenerated pearlite bands microstructure in the base metal fixed carbonate deposits in the steel surface. In the welded regions, the bainitic microstructure and the carbide particle distribution are less efficient in setting the weld surface carbonate deposit. HIC tests showed cracks initiation and propagation to be more prone in hard phases.

Formation and Properties of Superhydrophobic Al Coatings on Steel.

Thermal sprayed aluminum coatings are widely scalable to corrosion protection of the offshore steel structure. However, the corrosion rate of the Al coating increases considerably due to the severe marine environment. It has remained a challenge to improve the corrosion resistance and protective ability of Al coatings. The superhydrophobic surface provides a potential way to improve the corrosion resistance of metal materials. Hence, the development of superhydrophobic Al coatings with superior corrosion resistance is of great interest. In this work, the feasibility of the preparation of superhydrophobic Al coatings on a steel substrate was explored. First, Al coatings were prepared onto the steel substrate by the arc-spraying process, followed by ultrasonic etching with 0.1 M NaOH solution, and afterward passivated using 1% fluorosilanes. The effects of the etching time on morphology, contact angle, and corrosion resistance of the Al coatings were evaluated. The schematic model of the fluorosilane passivation process on the Al coating surface was provided. The micro/nanoscale surface structure of the low-surface-energy fluorosilanes promotes the wetting angle of 153.4° and a rolling angle to 6.6°, denoting the superhydrophobic properties. The superhydrophobic Al coating surface displays excellent self-cleaning performance due to its weak adhesion to water droplets. The corrosion current density of the superhydrophobic Al coating (1.36 × 10–8 A cm–2) is 2 orders of magnitude lower than that of the as-sprayed Al coating (1.18 × 10–6 A cm–2). Similarly, the charge-transfer resistance is found to be 12 times larger for the superhydrophobic Al coating and the corresponding corrosion inhibition efficiency reaches 98.9%. The superhydrophobic Al coating displays superior corrosion resistance and promising applications in a marine corrosion environment.