Fatigue of welded structures at sub-zero temperature [AVIF-No. A 301]

Research partner: Technische Universität Hamburg-Harburg, Ship Structural Design and Analysis (Prof. DSc. (Tech.) Sören Ehlers), Center of Maritime Technologies e.V. (Dr. Roland)

Time period: 01.01.2016 to 30.09.2019

 


Motivation/Background

Within the last years, it has been a continuously growing interest in exploration of oil and gas in the arctic region. Up to 25% of the undiscovered oil and gas resources are expected to be found in sea areas with at least seasonal ice coverage and harsh and cold environments (Gautier et al., 2009). As a result of the latter, structures exposed to cold climate are required to function reliably in their design conditions including exposure to low temperatures. Current regulations however do not cover the expected temperature range and thus the reliability of structures used in such conditions may be reduced. DNV GL provides some guidance down to -30°C (DNV, 2013), while NORSOK is limited to -14°C (Norsok, 2013). Consequently, there is a lack of guidance in structural behavior at low temperatures.

While the strength of steel increases under cold temperatures, the inherent risk of unexpected brittle fracture increases as well. All ferritic structural steels suffer from reduced fracture toughness at low temperatures due to the ductile-to-brittle transition behavior (DBT), which is characteristic for steels with body-centered cubic (bcc) crystal structure. At lower temperatures, the mechanism of stable crack growth behavior changes from plastic blunting and tearing to cleavage controlled brittle fracture. To this day, the transition behavior of welded structures is just barely understood. For this reason large parts, in particular fatigue critical once, are made of lower strength steel. By carefully assessing service conditions, fabrication processes and improving current international standards for ship and offshore structures, the sales of higher strength steel could significantly being increased.

 

Summary of planned objectives

This research project seeks to improve the applicability of higher strength steel in fatigue critical regions of ship and offshore structures. For this purpose the material behavior of base materials and welded structures made of higher strength steel will be analysed for temperatures up to -50°C by means of tensile, fracture toughness and SN tests. Based on the results state-of-the-art fatigue assessment methods will be checked and if need updated.

The research is focused on the following objectives:

1. To test the static and dynamic material properties for different structural details and material strengths at changing temperatures from room temperature to -50°C

2. Numerically simulation of the material behavior based on the test results

3. Verification of state-of-the-art fatigue assessment methods for welded structures at low temperatures like nominal stress, structural stress, notch stress and strain energy density methods

 

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