PSI - Issue 18

Fokion Oikonomidis et al. / Procedia Structural Integrity 18 (2019) 142–162 Dr Fokion Oikonomidis / Structural Integrity Procedia 00 (2019) 000 – 000

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Keywords: mooring chain, fracture toughness, cathodic protection, seawater

1. Introduction

High strength C-Mn mooring chains are widely used to keep floating offshore oil and gas platforms in position and as structural components in subsea production systems. These chain links, and associated mooring ropes, may be used to connect the floating platforms to the anchors at the sea bed, resulting in chain lengths of several kilometres for each offshore platform. An additional use of these components has been adopted in the last two decades for subsea production systems, usually associated with buoyance systems employed in the risers system. Especially in the last application, the material is submerged in seawater constantly, which means it is subject to failure due to corrosion. In this case, Cathodic protection (CP) can effectively prevent general corrosion of the chains from the surrounding seawater. The main disadvantage of this corrosion protection method is that hydrogen atoms in the water can be liberated at the surface of the links because of the cathodic reaction and diffuse into the steel. The presence of hydrogen atoms in the steel causes a drop in the fracture toughness due to hydrogen embrittlement. This paper presents and discusses results from full-scale fracture toughness testing of a studless mooring chain link grade R5 in NaCl solution under CP. This was part of a larger testing programme carried out by TWI Ltd (not presented in detail in this paper), in which the tensile properties and fracture toughness properties from small scale specimens were tested in air and in the environment. A studless mooring chain link grade R5 was used for testing (see Fig. 1 for dimensions). The chain link was delivered to TWI after being subjected to the standard proof load of 17811kN (DNV, 2013). The average yield stress of 952MPa and average ultimate stress (UTS) of 1032 MPa, for the chain tested in air, at room temperature were provided by the manufacturer. Tensile testing has previously been carried out at TWI, in air and in 3.5% NaCl solution under CP at room temperature (R.T.) on R5 grade studless chain material (extensive details of the testing programme are not provided in this paper). Those tests used round tensile specimens extracted from the leg region of similar chain links and the results were considered representative of the current material. The minimum yield stress (0.2% offset) in air at R.T. was 878MPa, and the minimum ultimate tensile stress (UTS) was 951MPa. The minimum yield stress (0.2% offset) in 3.5% NaCl solution under CP was 870MPa and the minimum UTS was 945MPa. Small scale fracture toughness testing in 3.5% NaCl solution under CP using single edge notch bend (SENB) specimens extracted from an identical chain link to the one examined here, coming from the same production batch was previously carried out at TWI. Two test results of K 0.2 (determined from J 0.2 ) were available from that testing representing the estimated driving force at initiation of crack extension in the leg region: 2344N/mm 1.5 and 2399.6N/mm 1.5 . These properties, measured from previous tests on a similar material, are considered to be representative of the chain link examined here. 2. Material

3. Notching and Pre-cracking

3.1. General

The chain link was notched on the intrados leg region, opposite to the weld (shown in Fig. 1), using Electro Discharge Machining (EDM). The EDM notch shape and dimensions are illustrated in Fig. 2. The EDM wire diameter was 0.2mm. A fatigue pre-crack was extended from the EDM notch by subjecting the chain link to cyclic bending (see Fig. 2). The aim was to generate a pre-crack with total maximum depth of 18mm in the centre of the leg region of the chain link, in order to be consistent with the pre-crack location of the corresponding SENB specimens used in previous tests of similar chain links. The cyclic loading details (i.e. number of cycles, and cyclic load) were estimated by using Finite Element Analysis (FEA) to achieve a target pre-crack extension. Because of the particular geometry of the chain link and the residual stress distribution, numerical modelling was used to assist the fatigue assessment.