Deep Ocean Mooring Design Technology - Present Status and Future Development
Authors: McNatt Tobin R. 
Published: October 2008

Offshore developments in energy related industries have led to requirements for high capacity mooring systems in deeper waters and more hostile environments. As these requirements have emerged, the design technology available for such systems has had to develop as well. Today's design technology for mooring floating offshore facilities in deep ocean locations (2,0006,000 feet) is presented in this paper. Aspects of the design process are described including ocean mooring design criteria, hardware components and configurations, environmental data requirements, analysis techniques, deployment, operation, and inspection, maintenance and repair considerations. Also included are recommendations for component testing and small and large scale system tests. Areas requiring further development are identified and recommendations are made which will enhance the designer's ability to complete deep ocean mooring designs.

Introduction

Mooring systems have traditionally been designed using well established rules of thumb and extensive background experience with the mooring problem at hand. These systems typically involved water depths of one and hold to several times the draft of the ships which had to be moored. These "relatively" shallow water depths have been addressed for centuries by sea-going operators, and any mooring which considered depths of up to 1,000 feet was, and usually still is, considered a deep water mooring. Over the past 10-15 years developments in the offshore industry have placed greater demands on the mooring system designer. As exploration and production operations have moved into deeper waters and more hostile environments, mooring requirements have increased significantly. The platforms which must be moored not only represent large capital investments, but often their efficiency is related directly to their ability to maintain station above a specific site on the ocean floor. To meet these more severe mooring requirements new technology has been developed including improved hardware, such as lightweight high efficiency anchors and high strength chain, computer based analytical tools, and operations equipment such as centralized tension monitoring and control stations for multi-leg moorings.

As exploration has recently moved into even deeper waters using techniques such as dynamic positioning for accurate station keeping, a new set of mooring requirements has developed, namely, the need to place development or production facilities into waters of 2,000 - 6,000 feet for significant periods of time. As the traditional bottom mounted concepts become impractical at these water depths, floating or compliant bottom mounted concepts have emerged which require mooring systems for long term station keeping. A viable mooring system design for this type of application must address a comprehensive set of requirements. This paper will describe today's design technology for deep ocean moorings in the following areas:design criteriaconfigurations and hardwareenvironmental conditionsanalytical toolsperformance analysisdeployment and operationinspection, maintenance and repairtesting and demonstration. In addition, areas requiring further development are identified and recommendations made which will enhance the feasibility of deep ocean mooring designs.

Design Criteria

Specific design criteria must be established at an early stage to form a basis for the design along with necessary operational criteria. Key categories of design criteria are described below.