The Utilization of Temporary Buoyancy Tanks for Transportation and Installation of Arctic Gravity Base Structures

Transportation distances from potential construction sites in the Pacific Basin to the Beaufort Sea range between 2000 to 3000 nautical miles, thus presenting a unique challenge to the successful completion of the towage and installation of Gravity Base Structures (GBS). Further complexity is added by the rapidly changing atmospheric conditions along the transportation routes (figure 1) as well as the short duration of ice clearance and shallow draft available for sea passage around Point Barrow. This paper presents the results of an analytical investigation concerning the applicability of large size Temporary Buoyancy Tanks (TBT) to GBS for Arctic environments. The current discussion examines primarily the advantages in utilizing TBT to improve the designer\´s ability to control the hydrostatic stability and motion response characteristics of GBS during tow. It is shown that these tanks, if properly designed, may reduce excessive motions by creating damping due to the formation of a "reversible flow" mechanism. Various design considerations and system selection criteria are indicated and it is shown that the application of TBT to GBS is entirely feasible. A combined evaluation of the relative weight, strength, stability, practicability and cost for several TBT systems indicated superior performance of a "hybrid" section made of two concentric steel skins and Lightweight Concrete (LWC) placed in between.