Abstract Correlations between Charpy energy transition temperatures and other fracture mechanics parameters have been widely used for many years in steel failure analysis and design. The primary value transferred often has been a transition temperature, such as the 27 J transition temperature. This approach does not consider the scatter in the transition temperature or in the overall Charpy data. This work uses a scatter assessment tool developed by Orynyak et al. and applies it to a wider variety of steels and weldments to see whether it can provide a second parameter to be transferred to fracture assessment. Examples include pipeline steel and structural steel, as well as weldments in both the weld metal and heat-affected zone. While Charpy transition scatter is expected to be high in heat-affected zones, examples can be found in base metals with similarly high scatter.

The objective of this work is to present and review the recent developments in the experimental and numerical assessment and simulation techniques on the welding induced distortions and residual stresses. The temperature distribution, welding induced distortion and residual stresses in thin walled welded structures, originating from different experimental tests are reviewed and discussed. Different mathematical models and their numerical applications in representing the heat source are analysed and their advantages and drawbacks are discussed. Thermal stress analyses employing the three-dimensional nonlinear thermo-elasto-plastic approaches and finite element simulations with inherent deformation applicable to large-scale and complex welded structures are also revised and discussed. Discussions on the material properties of the base metal, heat affected zone (HAZ) and weld metal, the effect of the welding sequence, and the pattern of residual stress distribution presented are given a special attention.

In order to further improve welded pipeline performance, a detailed knowledge of the key and interlinking relationships between the chemistry, microstructure and mechanical properties of the weld joint is needed. In this paper, the results of optical emission spectroscopy analysis on the as welded chemical properties of a submerged arc welded API-5L grade X65 linepipe are first presented. The microstructure of the various weld regions is then assessed against the results of the chemical analysis using a series of microscopy techniques. A fine grained ferrite-degenerate pearlite microstructure was observed within the base metal of the linepipe along with large (1.5 μm) cuboidal Ti (C, N) precipitates. Within the heat affected zone (HAZ) close to the molten weld joint, grain growth occurred with small volume fractions of induced upper bainite present within the microstructure. The fusion zone of the submerged arc weld joint consists of predominantly acicular ferrite with a small volume of grain boundary phases and a high number of large (0.8 μm) spherical Ti (C, N) precipitates. The results of Vickers hardness tests carried out at two length scales (macro + micro) show clear relations between the hardening effects of the cementite enriched degenerate pearlite and induced upper bainite phases within the base metal and HAZ respectively. Fractography analysis of Charpy impact test samples across the submerged arc welded joint found that the large Ti (C, N) precipitates within the fusion zone appear to be acting as microvoid initiation sites for the ductile fracture and as such contributing to the relatively low toughness properties within the fusion zone. Finally, the potential benefits of reducing the Ti content in both the welding wires and X65 base metal for further improvement of the mechanical properties of the linepipe weld joint are discussed in regards to reducing the size of the coarse Ti (C, N) precipitates within the base metal, HAZ and fusion zone.

The most important issue associated with liquefied natural gas storage systems, such as LNG carriers, LNG FPSO and FLNG, is the structural safety. One of the most common materials for the LNG storage systems has been 9% nickel steel over the last 50 years as it has excellent mechanical properties under cryogenic temperature. Recently, the consideration for lowering the nickel content becomes necessary due to the increase of the nickel price and the high price of nickel based welding consumables. In this respect, 7% nickel steels are developed for cryogenic applications. Nippon Steel and Sumitomo Metals Corporation have developed 7% nickel steels with improved toughness comparable to that of 9% nickel steels by TMCP and micro-alloying technology. The major objective of this study is to evaluate the fatigue performance of 7% nickel steels with a special attention to Type B LNG carrier applications. Cyclic fatigue and fatigue crack growth rate (FCGR) tests for 7% nickel steels were conducted at room and cryogenic temperature. Fatigue tests were carried out with three types of specimens such as base metal, butt weld and fillet weld to characterize the fatigue properties. Also FCGR tests were carried out using compact tension specimens. The difference of FCGR characteristics among base, weld and HAZ (Heat Affected Zone) are investigated for three types of specimens. The results of 7% nickel steels are evaluated and compared with those of 9% nickel steels.

High pressure tubing and associated tubing couplers are critical components required for the operational control of subsea oil and gas production equipment. Tubing couplers used in subsea oil and gas developments are commonly made from Nitronic 50 HS® (N50HS) due to its high strength, corrosion resistance, and resistance to galling. Nitronic couplers are typically welded to several dissimilar metals including super duplex stainless steel (SDSS) control tubing using SDSS filler metals such as AWS A5.9 ER2594. Recent evaluations have found that sigma (σ) phase forms in N50HS weldments and its effect is not broadly understood by industry. During N50HS solidification, Scheil solidification conditions establish compositional gradients in the unmixed zone located along the fusion line adjacent to the N50HS base metal. This solidification-induced segregation promotes compositions that are susceptible to interdendritic intermetallic compound formation when they are reheated by subsequent weld passes such as in multipass welding or at weld start-stop locations. Decreasing heat input is a common approach to reduce or eliminate the formation of intermetallic compounds in SDSS. Although decreasing heat input can reduce the amount of energy available to drive the solid state transformation from ferrite to σ, it does not change the solidification mode (AF) or solidification conditions from Scheil to para-equilibrium within the range of cooling rates possible with arc welding processes. As such, the compositional gradients that promote intermetallic compound formation along the N50HS fusion line can only be minimized through heat input control and cannot be eliminated in arc welds regardless of the heat input used. The effects of σ on toughness and corrosion resistance of Nitronic weldments were evaluated. N50HS solidified samples with up to 2 volume percent σ were found to have CVN of >40J at −40°C, and no evidence of pitting at 25°C in the ASTM G48 test.

API X80 grade UOE double submerged arc-welded pipe has been applied to steam injection oil sand recovery systems to increase the volume of steam to be injected and decrease the installation cost. The pipes for the systems are subjected to high temperature for a long period, such as 350 °C for 20 years. Therefore, it is important to ensure the reliability of the pipes during and after long-term operation. In this study, based on the recent development of high-frequency electric-resistance-welded (HFW) linepipe with a high-quality weld seam, the durability of newly developed API X80 grade HFW linepipe for long-term high-temperature operation was investigated. The change in the microstructure of the pipe body and weld seam was small after exposure to 400 °C and lower temperatures. The tensile strength of the base metal and weld seam after heat treatment with temperatures as high as 400 °C can be determined using the Larson-Miller parameter, which depends on the temperature and holding time of the heat treatment. The newly developed API X80 grade HFW linepipe was considered to have sufficient tensile strength during and after long-term operation at 350 °C for 20 years, similar to API X80 grade UOE pipe. No significant change in the Charpy absorbed energy during long-term heating was observed. Creep tests indicated that the time to rupture at 400 °C or lower exceeded 10 6 hours, and the creep effect was considered almost negligible at temperatures less than 400 °C. The rupture stress at approximately 350 °C was estimated to be far higher than the typical hoop stress of approximately 200 MPa on the steam distribution system. High-temperature fatigue properties were also measured to ensure reliability under varying stress conditions.

Recent extensive use of Corrosion Resistance Alloy (CRA) as internal protection layer of standard carbon-steel pipes (clad and lined pipe) in the oil and gas industry requires an intensive use of bimetallic welds. Since some degree of defects in welds is inevitable, and in codes and standards (such as BS7910) the case of bi-metallic joint is usually not considered, some R&D’s activities are ongoing to define specific design guidance for an Engineering Critical Assessment (ECA) aimed at determining flaw acceptance criteria for fabrication of bimetallic joints. Based on the limited guidance in the literature, proposed procedures for ECA on CRA welds seem not cover the root/hot pass weld region, for which the requirement of “zero defect” became mandatory. As direct consequence, it penalizes the weld fabrication rate, particularly if “J-lay” or “S-Lay” methods are adopted. Furthermore, they are investigating on cases where weld material is overmatching the base metal or for a limited partial overmatching, despite for CRA welds, such conditions, seem quite difficult to be fully met, if current consumable materials present in the marked are selected. Aim of present paper is to describe how any standard ECA procedure (ordinarily used to assess carbon-steel welds) may be alternatively adopted to assess CRA welds for clad & lined pipe material, if specific conditions are respected. For this purpose a few number of elastic-plastic Finite Element Analysis (FEA) is required to identify and/or extends the validity limits which have to be met in order to be conservative in the use of selected standard procedure. Outer, inner and under clad flaws, located along the weld fusion line, were investigated. Such approach, certainly leads to a quite conservatism, but gives the advantage to provide a safe flaws acceptance criterion in root/hot pass weld, and it may be also applied for any level of weld partial overmatching condition. Despite proposed simplified approach is suitable until moderate plastic straining, it may be appropriated for any ECA on CRA pipe when “J-lay” or “S-lay” installation method is adopted, and/or for many riser’s configuration, and/or for several flowline routing also if exposed to post-buckling condition. It is demonstrated that the proposed simplified approach, when applied under moderate plastic strain conditions, provides accurate J-integral solutions compared to the complex method as proposed by current R&D.

Saving costs is a major attraction of wet welding application in structural repairs of offshore installations. Nevertheless, improve the quality of wet welding to get it as close as possible to plain structural steel quality and also qualify welding procedures in AWS D3.6 class ‘A’ have been challenges not consistently overcome. This paper describes wet welding trials in shallow waters (5m and 10m) with a rutile and an oxy-rutile type, both commercial electrodes. Two different base metal compositions were employed in the preparation of butt and fillet joints. The main objective is to amend new results of wet weds properties to those already published aiming the application of this welding technique under more reliable conditions and the qualification of welding procedures. The weldments were tested by Vickers hardness, Charpy V notch, tensile, shear strength, bending and fillet weld break tests, chemical and macrographic analysis. Some of these properties and diffusible hydrogen, obtained in laboratory with a mechanized gravity system, will also be presented in order to complement or explain the field testes results obtained. Both electrodes produced class AWS E 70XX weld metals and overall results according class “B” requirements of the AWS D3.6M:2010 code. Some good elongation results obtained encourage future trials to achieve class “A”. Barriers to the class “A” qualification of welding procedures in shallow waters are also discussed.

The growing demand of the transport of gas and oil under severe conditions, such as in deepwater and onshore in cold climates, requires heavy wall thickness line pipes with both high strength and excellent toughness at low temperature in order to reduce the cost of gas transportation and constructions. In particular, when developing and efficiently manufacturing high strength line pipes with heavy wall thickness, achieving excellent heat affected zone (HAZ) toughness of seam weld is one of the key subjects. In general, steel plate for heavy wall thickness line pipes relatively contains large amount of alloy elements to secure required mechanical properties, whereas addition of alloy elements cause deterioration of HAZ toughness of seam weld. This paper deals with the method of improving HAZ toughness in UOE pipe seam weld. Relationship between microstructure and toughness of simulated HAZ was investigated, and volume fraction of martensite-austenite constituent was reduced by optimization of chemical composition of steel plates. Moreover, low heat input double submerged arc welding (DSAW) process with using welding wires with smaller diameter was newly developed to enhance HAZ toughness. With this new DSAW process, decrease of HAZ width and reduction of austenite grain size were also achieved. Based on the above knowledge, the manufacturing condition was optimized and heavy wall thickness X70 UOE pipe with excellent toughness was successfully developed.

9% Ni steel has been used for LNG storage tanks for more than four decades although 5.5% Ni steel (N-TUF CR196) was developed in the 1970’s using a special heat treatment method named L-treatment. The reason why the actual application of 5.5% Ni steel has not been attained to LNG storage tanks is mainly because the requirement of fracture properties is not confirmed for the tanks. Under the circumstances of expanding demand for natural gas and double-integrity in LNG storage tanks, we restarted developing low Ni steel for LNG storage tanks by using both conventional and advanced techniques. For the application of low Ni steel to the present LNG storage tanks, both fracture initiation and propagation properties of base metal plates and welded joints should be concerned. The fracture initiation and propagation properties of base metal were compensated with the intercritical reheating process (L-treatment), and the propagation property was additionally enhanced by combining TMCP with L-treatment. In addition, the chemical composition adjustment and the homogenization treatment of solute elements were conducted for improving the fracture initiation and propagation properties of welded joints. 6% Ni steel plates were manufactured by the process of continuous casting, reheating, hot rolling, direct quenching (TMCP), L-treatment, and tempering, and their chemical composition was 0.05C-0.06Si-1.0Mn-6.3Ni-Cr-Mo. As the results of fracture property evaluation including large-scale fracture tests such as the duplex ESSO test and the wide plate tensile test, it was demonstrated that 6% Ni steel has good characteristics regarding brittle fracture initiation and propagation in base metal plates and welded joints.

Steel welds representing three generations of pipeline steel were studied against hydrogen effect, using the fracture toughness parameter J integral which is a measure of the plastic work. The influence of hydrogen on the growth of a partial wall defect is studied and comparisons with the growth of the same defect in air are presented. The tests conducted were three-point bending fracture toughness tests, in air and in a hydrogen environment simulated by electrolysis. Reduction in J 0 was observed in all pipeline steel grades as the current density increases, which was more pronounced in the base metal rather than in the heat affected zone. The different microstructures of the welded steels are observed and correlated to the reduction in plasticity. It is concluded that microstructure seems to be the decisive parameter for the selection of a pipeline steel for service in a hydrogen environment.

Titanium is increasingly being applied to offshore installations due to its high strength, low modulus, environmental resistant properties and high fatigue capacity. These combined properties make certain titanium alloys ideal in the application as a stress joint (TSJ) where flexibility and fatigue capacity are the main design limitations. Over 80 TSJs are currently in service worldwide and are now being designed for more arduous fatigue service involving high motion floaters and locations with more severe sea states. This paper explores the application of BS7910 level 2 & 3 ECA for an ASTM Grade 29 titanium production riser TSJ to a 100 year storm operational limit, and a 1000 year storm survival event for a high motion floater in the GOM. Two critical locations are addressed: 1) A section at the top of the tapered wall immediately below where the TSJ terminates at the platform in the stress joint and where the base material properties apply, and 2) the girth weld location farther down the tapered section where the wall is thinner. Typical measured J-R curves together with Paris-law constants with mean stress effects incorporated are presented and used in the ECA. Comparisons between the BS7910 level 2 and 3 show the increase in allowable defects due to the more elaborate procedure at level 3. However, since the fatigue loading (and S-N endurance) of this particular case is high, the allowable initial defects turn out to be small for the weld location, and the weld location was more critical than the base metal location in spite of lower fatigue loading. This is due to the weld metal having a higher crack growth rate in the Paris region, a reduced crack growth threshold, and a reduced J-R curve. Other approaches to fatigue performance integrity for cases where allowable defects may be too small to detect are suggested. These include the local thickening of the wall at the weld and the redesign of the taper profile optimized for fatigue rather than extreme load only. It is concluded that: a) Level 3 analysis gives larger allowable initial defects level 2; b) It is important to carry out ECA at an early stage in the design process to identify potential problem areas; c) Non-conventional NDE methods should be investigated for future TSJs which are designed to operate under demanding fatigue loadings.

Fracture mechanics SENT testing and FE simulation to establish hydrogen influenced cohesive parameters for X70 structural steel welded joints have been performed. Base metal and weld simulated coarse grained heat affected zone have been included in the study. The base metal did not fail at net section stresses lower than 1.29 times the yield strength and reveals low sensitivity to hydrogen embrittlement. The weld simulated coarse grained heat affected zone was prone to fracture at stresses above 64% of the yield strength, which indicates hydrogen embrittlement susceptibility. The cohesive parameters best fitting the experiments are δ c = 0.3 mm and σ c = 1700 MPa (3.5·σ y ) for the base metal and δ c = 0.3 mm and σ c = 2100 MPa (2.6·σ y ) for the coarse grained heat affected zone.

Porosity is a common defect observed in underwater wet welding. Several research programs have been developed to understand how pores form in order to mitigate the problem. No superficial pores and a limited number of internal pores (based on size) are important requirements to classify underwater wet welds according to the American Welding Society – AWS D3.6M standard. The main objective of this work is to study the effect of base metal and core rod carbon content on weld metal porosity. A pressure chamber with 20 atmospheres capacity was used to simulate depth with fresh water. To perform the welds, a gravity feeding system able to open an electric arc and deposit the weld automatically was used. Beads-on-plate were made using Direct Current Electrode Negative (DCEN) configuration on two base metals with different carbon contents (C2 – 0.1 wt. pct. and C7 – 0.7 wt. pct.) at 50 meters water depth. Commercial E6013 grade electrodes were used to deposit the welds. These electrodes were produced with core rods with two different carbon content (E2 – 0.002 wt. pct. and E6 – 0.6 wt. pct.) and painted with varnish for waterproofing. Samples were removed from the beginning, middle and end of the BOP welds and prepared following metallographic techniques including macroetching and image analysis for weld porosity. A data acquisition system was used to record current, voltage and welding time at 1.0 kHz rate. The porosity measurements indicated an increase of about 85% and 70% when E6 electrodes were used instead of E2 electrode on C2 and C7 steel plates, respectively. Simultaneously, the increase in porosity was followed by an increase in short circuiting events, an increase in weld bead penetration and a decrease in welding voltage. These observations seem to confirm, a direct effect of carbon content of the core rod on weld metal porosity and that porosity is associated with the CO reaction that can occur during metal transfer in that molten droplets carry gas bubbles to the welding pool. On the other hand, the increase of carbon content in the base metal was seen to decrease the porosity in the weld metal. This result can be related with the decrease in penetration observed when changing C2 to C7 plates. The smaller participation of carbon from the base metal in the weld pool reactions should then reduce the CO formation and, consequently, the amount of pores in the weld.

This paper presents the method and procedure of CTOD test which are used for the toughness evaluation for welded joints. Two types of high strength steel (E38 and E43) are chosen as the object of fracture toughness evaluation. The contents of CTOD test include three-point crack tip open displacement (CTOD) bending tests for base metal (BM) specimens, weld position (WP) specimens and heat-affected zone (HAZ) specimens of the high strength steel, considering different thickness of steel plate and different test temperature (−20°C and 20°C). The CTOD test can achieve the P-V curves of samples, and the CTOD values are calculated. On the basis of the above work, the results of toughness evaluation are obtained. Meanwhile, some factors which affect the toughness of high strength steel are discussed in this paper, such as thickness of steel plate and test temperature, and many valuable conclusions are achieved.

In recent years, the strain based design for pipeline has been widely accepted by the industry, but the definition of a rational flaw acceptance criteria for girth welds subjected to axial strain within the context of the existing codified fracture mechanics based assessment procedures is problematic since these are essentially stress based. To extend the FAD method to the large strain conditions, several challenges i.e. weld strength mismatching, fracture toughness, and welding residual stresses have to be understood. With appropriate modifications as per DNV-RP-F108 [1], the assessments procedure detailed in BS7910 document for stress based situations have been used successfully for several projects to develop acceptance criteria for pipeline installation involving plastic straining. But only weld metal strength over-match comparing with base metal is considered in DNV-RP-F108 [1]. High strength line pipes are required to reduce the transmission cost of natural gas in long distance and internal clad with corrosion resistant alloy (CRA) is used for transportation of sour gas. Steel manufactures have developed such line pipes to develop new oil and gas field. The inconel filler metal was selected as weld consumable for the production girth weld in the lay budge. From the all weld tensile tests, it was found that the yield strength of the weld is under-match comparing the base metal, and the pipeline maybe subjected to a strain level up to 1.0% due to the lateral buckling. In this research the effect of weld strength mismatching on the structural integrity of the pipeline subjected to large strain was studied. The Engineering Critical Assessment (ECA) was performed to derive the critical flaw acceptance criteria for the AUT system. The segment tests and numerical analysis were performed to validate the assessment procedure, and the finite element analyses of the pipeline girth weld with surface crack in the weld centre were carried out to investigate the effect of bi-axial loading on the ECA results.

Grade 23 and/or 29 titanium alloy pipe and forgings are typically butt-welded together in the fabrication of offshore riser components such as tapered stress joints (TSJs) for top-tensioned risers and as hang-offs for dynamic catenary risers. Although Grade 29 titanium base metal in a relevant wrought/forged product form has already been evaluated in regards to sustained-load cracking (SLC) resistance, minimal data is available to ensure that the SLC resistance of typical GTA butt-welded joints in these thicker-wall titanium alloy components will also meet design requirements. As part of a TSJ production weld qualification, conservative fracture-mechanics based SLC tests were conducted at room temperature on 1G-position machine GTA butt-welded Grade 23 titanium pipe utilizing Grade 29 titanium filler metal. Test results revealed no significant SLC susceptibility in the weld and a minor effect in HAZ metal, producing K SLC values similar to K Q values. These values safely meet typical TSJ fracture mechanics requirements, and are consistent with published SLC information on this alloy system.

Macroscopic porosity in underwater wet welds is one of the main defects that deteriorate the mechanical properties of the wet welded joints. It is well established that weld metal porosity is a function of pressure, thus water depth. However, the mechanism of porosity formation is not well understood, therefore the problem is not yet mitigated to acceptable levels, particularly at water depths close to and beyond 100 m. To purposely produce porous welds similar to those obtained in wet welding, bead-on-plate (BOP) welds were deposited in air with gas metal arc welding (GMAW) with no shielding gas, with autogenous gas tungsten arc welding (GTAW) and GTAW with cold wire feed using insufficient shielding gas (8 CFH). During welding with both processes, oxygen from the atmosphere readily reacts with the alloying elements in the molten tip of the wire and in the weld pool. Under these conditions, droplets that detach from the wire electrode will generally contain a gas bubble, which is transported into the weld metal. These two welding processes were selected because there is no slag produced in the process. Slag slows down the cooling while giving enough time for degassing to occur, as in the case of shielded metal arc welding (SMAW) in air. Even with insufficient shielding gas, the autogenous GTAW welds did not exhibit porosity because there was no metal addition in the form of droplets. However, when a wire was fed into the arc, droplets detached from the wire in the oxidizing atmosphere transported gas into the weld pool, manifested as external and internal weld metal porosity. Similarly, the GMAW BOP welds exhibited internal porosity. When quenched in water, the droplets that detached from the electrode in these oxidizing conditions exhibited internal voids. Metal transfer analysis performed on the GMAW BOP welds associated short circuiting mode with large droplets and high porosity contents (10 pct.). Conversely, small droplets are expected to transport less gas and produce less porosity. Proof of concept welds using the pulsed current GMAW (GMAW-P) process resulted in higher droplet detachment frequency, smaller droplets and a low number of short circuiting droplets. Even though a few short circuiting events were still present, the GMAW-P process drastically reduced porosity to only 0.2 pct. Chemical reaction between oxygen and carbon generates CO gas at the bottom surface of the droplets in flat welding position, this gas ascends and is partially trapped inside the droplet. However, when the welding torch and base metal are rotated 90 degrees or in horizontal welding position, the CO gas generated escapes. Consequently there is no CO bubble in the pendant droplet or porosity in the weld metal. Wet welds were made with pulsed current using AWS E6010 electrode at a pressure equivalent to 50 m water depth. Porosity was reduced from 3.9 with constant current to 2.5 pct with pulsed current. Even when porosity was reduced with pulsed current, higher pulse frequency needs to be tested along with different peak and background current values to further reduce porosity. Flux covered electrodes with ferro-manganese, ferro-titanium and boron additions were extruded for wet welding. These electrodes produced wet welds with an average porosity of 1.2 pct., which could be further reduced to 0.85 pct. by better control of the arc at the beginning side of the weld.

This Paper describes the testing of Segment Specimens and Full Scale Pipes carried out at DNV within the Joint Industry Project “Fracture Control for Installation Methods Introducing Cyclic Plastic Strain – Development of Guidelines for Reeling of Pipelines”. The JIP was a cooperation between DNV, TWI and Sintef. A total of 24 full thickness Segment Specimens, with cracks/notches introduced in the Base Metal, Fusion Line Root and Fusion Line Cap, were tested. The specimens were loaded both monotonically and cyclically, simulating reeling installation. Two Full Scale Pipes were subjected to a bending loading program simulating a reeling installation. Each pipe contained three test welds and cracks/notches were introduced both at the 6 o’clock and the 12 o’clock positions, i.e. six cracks/notches in each pipe. In one pipe the cracks/notches were introduced in the Weld Metal and in the other in the Fusion Line from the Cap side. After the loading program all the cracks/notches were broken open and the stable crack growth were measured and compared to predictions based on fracture mechanics principals (essentially following BS 7910-1999). A method for adjusting the analysis procedure, in order to obtain good agreement between the predictions and the experimental observations, is suggested. The JIP also included Materials Testing and FEM Analyses. A Guideline Document was developed which is currently being used by the project participants and it is the intention to issue a DNV Recommended Practice in support of the DNV Offshore Standard OS-F101 “Submarine Pipeline Systems – 2000” based on the experience gained and feed back received from the participants. The Guideline document is described in another paper (OMAE2004-51061) at the conference.

Offshore structure steel with high strength of YS550MPa has been investigated. As for offshore structure steel, high toughness in welded joints is required in addition to that in base metal. TMCP type steel of up to YS420MPa grade is used widely, and up to YS500MPa grade is reported in some papers. However, steel of higher strength grade with good toughness and weldability will be beneficial to structures in strict conditions. To reach the YS550MPa requirement, hardening effect by Cu precipitation was utilized. Steel plates were designed with micro-alloyed low C-Mn-Cu-Ni-Cr-Mo system. The combination of the copper precipitation and TMCP technology can increase strength without deteriorating toughness and weldability. Heat treatment for Cu precipitation was carried out to optimize the balance of strength and toughness of the base metal. The developed steel also shows good HAZ CTOD toughness up to 76.2mm thickness in several welding conditions including after PWHT. The newly developed steel has the possibility to increase the flexibility to design large-sized structures.