Ursa TLP, Tendon & Foundation Design, Fabrication, Transportation and TLP

Abstract

Ursa is the largest TLP in the Gulf of Mexico having 98.000 tons displacement and is the deepest TLP in the world located in 3,800 feet-of-water, MC Block 809. The URSA tendon and foundation design, fabrication and installation were built on the wealth of experience of previously installed TLPs1,2,3,11. In this paper, we focus on the new challenges that had to be met for this record setting project. Unique features of the Ursa Tendons, Foundation and Installation include:More tendons, 16 (4/corner) versus 12 (3/corner);Larger tendons, 32"OD × 1.5" wall (compared to the previous 26" × 1.3" and 28" × 1.2" wall)Longer Tendon Main Body Sections than the previous TLP's, resulting in 283.5' individual sections. This reduced the number of connections required and offshore handling time.New tendon top connector devdoped for Ursa resulting in simpler operations and significant cost savings.The foundation system consists of 16 individual piles. They are 417' in length and 96" OD constructed from 60-ksi material.The potential for significant fatigue-damage accumulation due to pile driving required special design and fabrication solutions.Design, Construction, and Installation strategy developed early on helped reduce cycle time, cost, and accommodated changes during construction.On site temporary mooring of the TLP was accomplished using a Flex-Yoke system.Improved procedures for tendon handling, stabbing and tensioning were developed. Introduction The Ursa TLP tendon, foundation and installation design and fabrication extended over a period of 3 years. In order to meet the fast-tract project schedule, material orders had to be placed early, before design completion. Design decisions were focused on reducing both cost and schedule, taking advantage of learnings from the previous TLP's, Early attention was given to the TLP installation design to allow modifications to be incorporated into the planned schedule, tendon design and pile driving requirements. The modification brought out by the site and water depth change6,7,9 were significant. New soil information had to be obtained, designs verified, installation procedures adjusted and schedules maintained. Tendon Design, Fabrication and Transportation TLP and Tendon Sizing After concept selection in the summer of 1995, the TLPSIZE program was used to size and optimize the URSA TLP. For the initial site, the water depth was determined to be 3,950 feet. Later, due to the site relocation, the water depth was reduced to 3,S00 feet7. Cost optimization was focused on sizing the hull and tendons such that the minimum tendon bottom tension was slightly above zero and the tendon met stress, collapse, and fatigue design criteria with minimal steel cross section area and maximized buoyancy. The program was calibrated based on technical and cost data from the MARS TLP.

Centrifuge Model Tests on Anchor Piles for Tension Leg Platforms

“…The URSA TLP in Mississippi Canyon Block 809 (Digre et al, 1999;Gatlin, 1999) and the properties from one of the geotechnical investigation sites conducted as part of the design process served as the prototype for the centrifuge tests. The centrifuge tests simulated the lateral response of a 100-inch diameter pile embedded to 200 ft in a soil that modeled the geotechnical strength properties from one of the Ursa site investigations.…”