Lena Guyed Tower Decommissioning Engineering

The Mississippi Canyon 280-A platform, also known as Lena Guyed Tower located in 1,000 ft water depth in the Gulf of Mexico (GoM), was successfully decommissioned and reefed in-place in 2020. To meet the regulatory requirements on offshore platform decommissioning, i.e. BSEE decommissioning requirements and the LDWF artificial reef program, an environmental study and an engineering study were performed to ensure that the platform decommissioning satisfies these critical requirements on environmental protection and operational safety. The environmental study revealed the abundance of marine lives residing on the tower and rare coral species only observed in deepwater region. This justified the environmental benefit of turning the tower into an on-site artificial reef and functioning as a vibrant marine life habitat at the end of its production service life. The engineering study on Lena decommissioning, as addressed in this paper, involved three (3) stages of engineering work: tower decommissioning concepts and feasibility study; selected concept definition; and detailed tower reef-in-place laydown (toppling) plan and design. High-level tower reef-in-place criteria were set up in the early engineering stage as guidelines to the detailed laydown design, including specific project requirements on minimal environmental impact and safe offshore field operations. This engineering study relied heavily on multiple expert brainstorm sessions on various decommissioning concepts and options, and advanced structural modeling and computer simulations for assessment and design. A large number of tower structural models were developed to verify the plan and check various factors and risk scenarios that may affect the tower performance in different stages of the decommissioning operation. Advanced analytical techniques were developed and applied, which feature the driving mechanisms of the problem such as soil-structure-interaction, guyline tension, structural member contact and sliding, large displacement simulation, structural collapse, and hydrodynamics. Stringent quality assurance and testing procedures were followed to ensure the credibility of developed analytical techniques and engineering technologies. The most challenging part that differentiates the decommissioning engineering from typical structural design is how to properly estimate the platform in-situ "as-is" condition and carry out the assessment as such to make a regulatory compliant, feasible, reliable, and efficient plan. Therefore, engineering judgment had to go beyond the scope covered by existing industry codes and standards, and rely on the most recent advances in industry research and technology. This engineering study resulted in a clearly defined tower decommissioning plan and procedure, backed up with contingency plans for various risk scenarios and potential deviations in field operation due to limitations and/or uncertainties. The decommissioning plan minimized the offshore field construction scope and risk, and kept the required tower laydown pull loads within the capacity range of typical GoM offshore tugboats. As a result, the Lena tower was successfully laid down on seafloor in an anticipated reefing position, with no major deviations from the plan. Lena Guyed Tower is the first deepwater compliant structure decommissioned. This decommissioning effort may provide the most valuable information and lessons learned to the industry for similar decommissioning and abandonment operations in the future.