ABSTRACT

In recent years several incidents related to failures of high strength mooring chains used in mobile offshore units mooring systems have been reported.

Investigations of some of these incidents have been carried out by, or on behalf of, the chain owner, the rig owner or the operator involved, resulting in a set of explanations for why they occurred. As a result, a set of possible measures to counteract them have been launched.

In this paper a systematic review of the available data is made and discussed, and some conclusions on the possible root causes and failure mechanism for the failures are presented. A gap analysis on the specific material knowledge was performed and the resulting suggested research and test program is presented.

INTRODUCTION

Higher strength material mooring chains have become widespread for offshore mobile units mooring systems on the Norwegian Continental Shelf (NCS) and are expected to be essential for the viability of Floating Offshore Wind Turbine (FOWT) installations. The high failure rate observed in some productions of R5 grade material has resulted in dedicated investigations and actions across the industry (Barros et al., 2022).

The failures considered in this paper occurred in chain which were fully compliant with the existing standards. It is observed however, that an apparent heightened sensitivity to hydrogen cracking which associated with surface damages caused by handling of the chain onshore and offshore, in combination with high load events and CP exposure, has resulted in the reported failures. Mitigation actions have been implemented mainly focusing on reducing damages from handling and high load events, but a more refined and clear criteria to properly certify high strength material mooring chains needs to be established A research and testing program is proposed and described with the aim of confirming the identified root causes and failure mechanism and to provide quantitative background for the mitigation actions implemented.

MATERIALS AND METHODS

The information presented in this paper has been obtained from the available failure investigation reports, bibliographic review, presentations at seminars, workshops and interviews with different industry players. Chain manufacturers, chain rental companies, anchor handling operators, rig owners and license owners have been consulted.

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