Effect of Mo content on nano-scaled particles,prior austenite grains and impact toughness of CGHAZ in offshore engineering steels

The effect of Mo on nano-scaled particles,prior austenite grains and impact toughness of coarse-grained heat-affected zone(CGHAZ)in offshore engineering steels with Ca deoxidation was studied.The heat-affected zone(HAZ)toughness of Mo16 steel is obviously higher than that of Mo8 steel at all the heat inputs of 50,100,150 and 200 kJ/cm,with HAZ toughness of both steels decreased with increasing the welding heat input.When the Mo content is increased from 0.08 to 0.16%,the size of nano-scaled particles in HAZ is decreased from 18 to 15 nm,and their number density is increased from 0.7 to 0.9μm^(−2).Thus,the Zener pinning force is increased,and the prior austenite grain size(PAGS)is decreased,leading to the improved HAZ toughness.Microstructural characterizations show that the nano-scaled particles in both steels are Ti(C,N)with the solute elements of Nb and Mo.The calculated critical particle size of TiN is 10.2 and 8.4 nm in Mo8 and Mo16 steels at 1350℃,and the particles larger than the critical size are stable during the welding process.From the Zener pinning force calculation,Ti(C,N)particles play the more important role in the pinning effect on the prior austenite grain growth.Based on the regression analysis by the MATLAB results,the predicted values of PAGS at different heat inputs are well fitted with the experimental data.

Application of the monitoring and early warning system for internal solitary waves:Take the second natural gas hydrates production test in the South China Sea as an example

Internal solitary waves(ISWs) contain great energy and have the characteristics of emergency and concealment. To avoid their damage to offshore engineering, a new generation of monitoring and early warning system for ISWs was developed using technologies of double buoys monitoring, intelligent realtime data transmission, and automatic software identification. The system was applied to the second natural gas hydrates(NGHs) production test in the Shenhu Area, South China Sea(SCS) and successfully provided the early warning of ISWs for 173 days(from October 2019 to April 2020). The abrupt changes in the thrust force of the drilling platform under the attack of ISWs were consistent with the early warning information, proving the reliability of this system. A total of 93 ISWs were detected around the drilling platform. Most of them occurred during the spring tides in October–December 2019 and April 2020, while few of them occurred in winter. As suggested by the theoretical model, the full-depth structure of ISWs was a typical current profile of mode-1, and the velocities of wave-induced currents can reach 80 cm/s and30 cm/s, respectively, in the upper ocean and near the seabed. The ISWs may be primarily generated from the interactions between the topography and semidiurnal tides in the Luzon Strait, and then propagate westward to the drilling platform. This study could serve as an important reference for the early warning of ISWs for offshore engineering construction in the future.

Elaboration of a conceptual solution for the development of the Arctic shelf from seasonally flooded coastal areas

Currently,construction of facilities in the Arctic is carried out on sand islands,but there are limitations for remote coastal conditions.These restrictions are associated with the lack of quarries with construction sand in the immediate vicinity of the work site,as well as the risk of seasonal flooding of the territories.This article includes conceptual solutions for construction of modular piling in Arctic nearshore conditions,upon which drilling rigs and auxiliary equipment can be placed for the purpose of subsequent wildcatting,development and exploratory extended reach drilling wells(with measured depth about 15000 m).During the construction of the modular piling foundations,a mathematical simulation of the thermodynamic condition of the frozen ground was conducted,and the bearing capacity of the foundations was assessed.The proposed solutions will solve these problems by constructing modular pile foundations and placing the necessary infrastructure on them,taking into account the distribution of loads during operation in seasonally flooded areas.

A review on flow-induced vibration of offshore circular cylinders

As a fundamental fluid-structure interaction(FSI)phenomenon,vortex-induced vibrations(VIVs)of circular cylinders have been the center of the FSI research in the past several decades.Apart from its scientific significance in rich physics,VIVs are paid great attentions by offshore engineers,as they are encountered in many ocean engineering applications.Recently,with the development of research and application,wake-induced vibration(WIV)for multiple cylinders and galloping for VIV suppression attachments are attracting a growing research interest.All these phenomena are connected with the flow-induced vibration(FIV).In this paper,we review and give some discussions on the FIV of offshore circular cylinders,including the research progress on the basic VIV mechanism of an isolated rigid or flexible cylinder,interference of multiple cylinders concerning WIV of multiple cylinders,practical VIV suppression and unwanted galloping for cylinder of attachments.Finally,we draw concluding remarks,give some comments and propose future research prospects,especially on the major challenges as well as potentials in the offline/online modelling and prediction of real-scale offshore structures with high-fidelity CFD methods,new experimental facilities and applications of artificial intelligence tools.