Page 60 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 45

Torsional failure of a wire rope mooring line during

installation in deep water

C.R. Chaplin

Department of Engineering, University of Reading, Reading RG6 6A Y, W.K.

Received 8 October 1998; accepted 12 October 1998

Abstract

During the deployment of mooring components for an FPSO in deep water excessive torsional distortion
was induced which led to failure of all six spiral strand mooring ropes. A mechanism has been investigated
which accounts for the failures. The foilowing issues are fundamental to the incident being considered:
0 the different torque/tension characteristics of chain and wire rope
0 the sensitivity of different components to twist
0 the interactions between chain and a work wire.
(0 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Fatigue assessment; Fatigue design; Load history; Offshore failures; Rope failures

1. Introduction

Beneath the South Atlantic waters off the coast of Brazil there are several large reservoirs of oil.
Much of this oil is located in water depths of between 500 and 2000 m. This makes the Brazilian
offshore oilfields the deepest in the world, in terms of water depth, and it is inevitable therefore
that floating production systems installed by Petrobras are continually breaking new ground. This
is especially true of the mooring and anchoring systems employed, and the methods used to install
them. This paper analyses a particular problem encountered during the installation of compound
chain and spiral strand mooring lines on the FPSO (floating production, storage and offloading
system) P34 in the Barracuda field of the Campos Basin [ 11.

2. Background

Wire ropes are used in combination with chain, anchors and now fibre ropes, not only as
component parts of mooring systems, but also as the principal tension element for raising or
lowering mooring components: the work wire. Whether installed as components in a mooring line,

Reprinted from Engineering Failure Analysis 6 (2), 67-82 (I 999)