The use of fracture mechanics in failure analysis in the offshore diamond mining industry

doi:10.1016/j.engfailanal.2004.12.014

Engineering Failure Analysis

Volume 12, Issue 6, December 2005, Pages 893-905

https://doi.org/10.1016/j.engfailanal.2004.12.014 Get rights and content

Abstract

This paper considers two major failures of offshore diamond mining equipment, which should not have occurred had both fracture mechanics aspects and materials behaviour been more fully understood. The two case studies include (a) failure of a wire rope swivel, and (b) failure of a load cell (used for monitoring rope load). In case (a) the swivel shank had failed in reverse bending fatigue, although it was thought initially to experience only tensile stresses. The linkage system to the haul rope, through a form of clevis plates, led to high bending fatigue stresses, and the failure was exacerbated by inappropriate remedial changes, including poor choice of material, as well as stress concentrating effects induced by a shrink fit stainless steel collar. In case (b), complete collapse of a gantry, sheave wheel system and partial A frame support resulted from the prior brittle failure of the load cell, and this could be attributed to incorrect heat treatment of the load cell material. In both cases quantitative use is made of fracture mechanics in the failure assessment.

Introduction

The offshore marine diamond mining industry provides numerous technical and engineering challenges, which need to be met before the diamonds themselves can be liberated. These challenges include aggressive environments, relative inaccessibility and extreme weather, frequently leading to unclear or even unknown loading conditions, with consequent wear, corrosion and fatigue.

Under such circumstances it is hardly surprising that failures are not infrequent, despite stringent design efforts to overcome the engineering challenges. In certain circumstances nowadays, greater use is being made of fracture mechanics in trying to prevent failure, although the application of this powerful technology is not yet as widespread as it might be. The present paper serves to highlight the use of fracture mechanics in the failure analysis field, as applied to the offshore diamond mining industry, by considering two case studies of diamond recovery equipment. The case studies include (1) failure of a large swivel system, and (2) failure of a load cell shaft, used for monitoring rope load.

Section snippets

Introduction and background

One of the more developed (third generation) techniques of offshore diamond recovery, off the west-coast of southern Africa, is by means of a remote operating vehicle (ROV) mining tool, with associated airlift of the diamond bearing gravel to the ship. This system typically operates in water depths between 30 and 150 m and the ROV facility (rather like a bulldozer with the scraper blade replaced with an articulated digging device) being hoisted to the mother ship by a 150-mm diameter steel rope

References (3)

American Society of Metals (ASM). Handbook on failure analysis;...

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Citation Excerpt :
Note, however, that the conclusions should not be based exclusively on fracture mechanics but substantiated by additional information from other sources unless the fracture mechanics analysis including its input parameters is beyond any doubt. Two failure cases on mining equipment were provided in [8]. A wire rope swivel (Fig. 6) was “improved” by a new design because of previous failures.
Although fatigue crack propagation and fracture cause a large part of failure events in industrial practice, fracture mechanics in failure analysis seems to be still a side issue. Starting from an introduction into important basic questions of failure analysis and fracture mechanics, the authors specify what kind of questions in failure analysis can be effectively solved by fracture mechanics (and which can't). They illustrate their discussion with a number of 13 case studies from the literature. Much more pronounced than in the design stage the benefit of fracture mechanics in failure analysis depends on its accuracy. This is limited by both, intrinsic factors of the method and the availability and quality of the input information. The authors discuss the various aspects and provide the reader with some background information which, as they believe, will be helpful for better understanding the prospects and limitations of fracture mechanics in failure analysis and the conditions of its application.
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