Abstract

The ability to maintain stationkeeping throughout severe environmental loading is critical to the safe operations of permanently moored floating facilities. Failures of forged mooring components have been observed on these facilities and noted by the issuance of multiple safety notices by United States regulatory bodies (U.S. Department of the Interior Safety Alerts No. 259 on 16 January 2008 and No. 296 on 12 May 2011). These alerts highlighted the need for Operators to remain vigilant in ensuring that the mooring components present are of sufficient quality and toughness to prevent the observed failures from occurring in the future. This paper describes a study which includes the following three main components:

  1. a survey of existing facilities to determine the extent of forged low-toughness mooring connectors in service,

  2. develop a high-level assessment procedure to assess such connectors, and

  3. identify potential mitigation activities.

The main findings of this paper confirm the existence of low fracture toughness connections. Operators, manufacturers and the classification societies have actively pursued improved manufacturing and certification processes which significantly improve the toughness values, which will be demonstrated in this paper. Assessment and mitigation processes are also summarized in this paper which can be employed by operators to reduce the risk exposure from the low toughness mooring components.

1. Introduction

Several incidents involving failure of forged mooring line anchoring system connectors have been observed on permanently moored facilities. Post-failure analyses indicated that manufacturing processes were defective for each case, namely the heat treatment applied at the manufacturer resulted in the material being unable to meet specified Charpy impact energy. Failure to meet this specification indicates a lower material toughness that may adversely affect fitness-for-service. These events led the U.S. Department of the Interior Minerals Management Service Gulf of Mexico OCS Region (MMS) and the U.S. Coast Guard, 8th District (USCG) to issue Safety Alert No. 259 on 16 January, 2008. This alert identified several recommendations including:

  • Operators review mooring component specifications to ensure manufacturing and testing activities provide a product that meets usage demands

  • Operator specifications will include sufficient Charpy testing to ensure adequate material toughness for components

  • Operators review required destructive and non-destructive testing of critical mooring component elements

  • Operators will review equipment inspection and handling requirements to ensure no damage is induced during transportation or installation

While the safety alert does not impose explicit mitigation for existing mooring systems, it does indicate that a proper understanding of existing mooring system state should be developed and procedures in place to prevent below specification connectors from entering service in the future.

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