Mooring Design and Installation Considerations for the Mirage and Telemark Field Development
Authors: Veselis Todd , Frazer Ross , Huntley Mark 
Published: July 2010

The design and installation of the mooring system for the MinDOC deep draft floating platform for the Mirage and Telemark field development presented several unique challenges. This system was the first permanent mooring to be designed with post Hurricane Katrina and Rita metocean conditions. Another factor adding complexity and stringent performance requirements was the use of dry trees on the development. The combination of these requirements required an efficient, robust mooring design based on a polyester taut-leg system consisting of twelve moorings and driven piles.

During the design stage, it was not known which vessel would install the mooring or perform the hookup to the hull. Therefore careful consideration was given to the type, size, and component weight, as well as the length of the polyester segments. This was done in an effort to maximize flexibility and allow the mooring to be installed by either an Anchor Handling Tug Supply (AHTS) vessel or a construction vessel.

The mooring was designed to be installed in three (3) phases to increase flexibility during the offshore campaign. The advantages of this approach were that the project could install the mooring off the critical path, utilize smaller less expensive vessels for key phases, and reduce the time required for hookup.

Over the last several years, the practice of prelaying polyester moorings on the seabed has been widely accepted and relatively common for MODUs in the Gulf of Mexico and around the world. However, approval for this practice was still unprecedented for a permanent mooring system. The approval process included a combination of field and laboratory testing to demonstrate that exposure to the seabed would not have a detrimental effect on the strength and fatigue characteristics of the polyester mooring ropes over the service life of the mooring.

The ability to prelay the moorings on the seabed offered cost savings, risk reduction, and scheduling advantages to the project and allowed the project to maintain installation flexibility.

Introduction

The Mirage and Telemark fields are located in Mississippi Canyon Block 941 (MC-941) and Atwater Valley Block 63 (AT-63), respectively. These fields are being developed from a single Deep Draft Floating Platform (DDFP) which is located in MC-941 in approximately 4,000 ft of water. Development of these fields presented several unique and interesting challenges related to the mooring system design and installation. One of the major goals of the project was to maintain flexibility throughout the installation and commissioning of the platform. In order to achieve this, a multitude of options and careful logistics planning were required during the design phase. Planning was especially critical because there were a number of subcontractors working on the project with each having their respective areas of responsibility.

Several aspects of the design basis figured more prominently in the mooring design process. One of these was the increased site specific metocean design conditions in the Gulf of Mexico due to recent hurricanes such as Katrina and Rita. This elevated criteria significantly increased the design forces on the moorings. In addition to metocean conditions, the proposed location had bathymetry and geological features which impacted the design. The design also had to satisfy the offset requirements for the use of dry trees and their associated tensioners.