An FEA Based Methodology for Assessing the Residual Strength of Degraded Mooring Chains

Crapps, Justin M. (ExxonMobil Upstream Research Company) | He, Haiping (ExxonMobil Upstream Research Company) | Baker, David A. (ExxonMobil Upstream Research Company)

OnePetro 

ABSTRACT

The safe operations of floating production assets rely on the integrity of the mooring system. Mooring systems are exposed to vessel motions and a seawater environment, both of which influence chain degradation. As such, mooring system designs account for a projected amount of degradation through selection of chain size, but all degradation modes may not have been foreseen and/or the actual degradation rates may be higher than anticipated in the design. In these situations, an immediate concern is understanding the integrity of the mooring system and then determining the time frame needed to safely manage the mooring system life cycle.

In this paper, the authors present a finite element analysis (FEA) based methodology for estimating the residual strength of degraded mooring chains. The paper presents work investigating FEA modeling parameter sensitivities and selection of appropriate parameters for FEA modeling. In addition, validation against break testing of full-scale chains is presented.

INTRODUCTION

A typical offshore (e.g. Floating Production Storage and Offloading (FPSO)) mooring system may contain multiple components: chain stopper, fairlead or bending shoe, chain segments, wire rope, polyester rope, sockets, thimbles, anchors, and connectors or jewelry of various types. Due to its robust nature, chain is usually used at locations along the mooring line that are prone to the highest damage - at the top under high tension and in the touchdown or ‘thrash zone.’ Two types of chain may be considered for mooring systems: studded and studless. Studded chains have a stud, or brace placed between the bars in the midsection of the chain to prevent flexure and aid in fatigue endurance. Studless chains do not contain a bracing stud.

During the design process, an allowance is added to the diameter of chain segments to accommodate for potential material loss due to inservice corrosion and wear. Typically, a design code e.g. API 2SK (API, 2005), or a company specific practice, provides a recommendation or requirement relating an allowed annual corrosion/wear rate. This rate is multiplied by the target design life of the mooring system to reach the total material loss allowance for the mooring chains.