Anchor Leg System Integrity - From Design Through Service Life
Authors: Duggal Arun S. 
Published: July 2010

The paper focuses on the authors' experience with the long-term performance of anchor leg components for floating production facilities. The paper shows that good long-term performance starts with design and is also influenced by the installation of the system, and in-service monitoring and maintenance programs. The paper also presents data on chain corrosion from anchor leg systems collected over the past twenty years. Guidelines based on the authors' experience for design, installation and monitoring and maintenance are also provided.

Introduction

Anchor leg components are not like fine wine; they do not improve with age! Thus it is important to recognize that anchor leg components degrade with use and exposure to the environment and that this should be considered in the design, installation, inspection and maintenance of the system.

The long-term, in-service performance of mooring system components is becoming increasingly important given the exponential increase in the number and complexity of floating production facilities worldwide. The trend of floating production systems being developed in deeper waters and harsher environments, coupled with longer service life requirements make incorporating the knowledge and understanding of long-term anchor leg component performance in the design and maintenance of future systems even more important. This paper provides some insight into the design and specification of anchor leg components from the fairlead to the anchor based on the authors' experience with the long-term performance of these systems.

The industry has a lot of experience with the long-term performance of mooring systems due to the use of these components for permanent mooring systems for over 30 years. However, until recently data on anchor leg component performance has not been readily available to the industry as a whole. The Mooring Integrity Joint Industry Project (JIP) has allowed the collection of some data from various sources to be presented in the public domain (Brown et al., 2005, HSE Report 444, 2006). The data collected demonstrated the degradation of anchor leg components over time and provided insight into where improved design methodology and details can improve the long-term performance. The data also shows that anchor leg failures tend to occur in regions of highly dynamic behavior, i.e., at or near the fairlead, at the interfaces with other components (mooring support buoys, subsea connectors), and the touch down area. The data also shows that connector design, especially devices to prevent rotation or disassembly, is not robust enough in many cases to provide the desired long-term performance. It is important to note that the majority of permanent mooring systems have performed well and have good long-term performance of the anchor leg components other than expected wear and corrosion. The paper's intent is to learn from the existing systems and provide some guidance on improving long-term performance of permanent mooring systems in the future.

The paper focuses on permanent mooring systems which typically have a service life from about 10 years to over 30. For permanent mooring systems, the anchor leg systems are designed by code to ensure sufficient ultimate strength, fatigue life and corrosion and wear allowance for the service life. However, the paper shows that other factors in engineering design, system dynamics, installation and maintenance also play a role on the integrity of the system over time.