Consideration on the Scatter of COD and Fatigue Crack Propagation Characteristics of Heavy Section C-Mn-V Forged Steel for Offshore Structure
Authors: Fukuda Takashi , Iwadate Tadao , Shimazaki Masahide 
Published: October 2008

A 197 mm thick C-Mn-V forged steel plate including welds was subjected to mechanical and corrosion fatigue tests and several engineering data useful for the design of offshore structures were obtained. The chemical and mechanical properties of base metal and welds satisfy typical current requirements in the North Sea projects and the heavy wall forged tubular is proved to be applicable to the offshore structures.

The scatter of critical COD (?c) in the transistor temperature range was studied and a new statistical method was presented for estimating the lowest ?c value. Corrosion-fatigue tests in sea water were carried out on the base metal and heat affected zone (H.A.Z.) of SAW (Submerged Arc Welding) at a cyclic frequency corresponding to sea wave frequency. No difference in fatigue crack propagation rate was observed between in the base metal and H.A.Z.

Based on these data obtained, manufacture of seamless forged tubules for offshore platforms has started and at present, several tubules have been successfully produce in spite of complicated configuration and extremely heavy wall thickness of the products. The forged tubular have fulfilled the requirements for mechanical properties of offshore structure of the North Sea.

Introduction

For leg structures of oil and gas offshore production platforms, heavy wall Togged seamless tubular is most attractive from the welding cost and reliability view points. Namely, it precludes longitudinal butt weld seam and reduce number of internal stiffeners, thus providing easier fabrication and maintenance including in-service inspection. However, heavy wall tubular such as over 120 mm in thickness have not yet been applied for offshore structures, although some new materials b Q have been presented, mainly because of lack of engineering data, especially elastic-plastic fracture toughness and corrosion fatigue data. To cope with this problem, a 197 mm thick C-Mn-V forged steel plate including welds was subjected to mechanical tests including COD, J and corrosion-fatigue tests.

Typical current requirements 3,4) for offshore structures in the North Sea are as follows;Ceq.= C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15O:47%Yield strength&gt;310 N/mm2, Tensile strength&gt;490 N/mm2Reduction of area at through thickness tensile test&gt;30%V-Charpy impact energy at -400°C&gt;35JCritical COD at -10°C, using full thickness test specimen&gt;0.35 mm

Material Selection

The chemical composition of the forged steel plate used for test is complying to Bs4360-50D, as is shown in Table 1. The low C-Mn-V steel was chosen because of its well balanced tensile properties, toughness and weld ability. Vanadium content was controlled so low as not to cause stress relief embitterment in grain coarsened H.A.Z. of welds. Grain refining was accomplished by aluminum addition. Supine shape control was done by additions of small quantities of calcium in order to minimize the directionality in mechanical properties and enhance the resistance to lamellar tearing.

In this study, a 197 mm thick forged plate was used instead of forged tubular, to maximize the efficiency of specimen production.