In iceberg prone regions, subsea substructures placed on the seabed are atrisk of impacts from free-floating and scouring iceberg keels. Here themethodology for assessing iceberg loads and two mitigation strategies aredescribed. The iceberg load model was an extension of previous work forestimating iceberg impact loads on offshore surface-piercing structures. Components of the algorithms were modified such that global design loads fromkeel contacts account for the change in contact location (i.e., longer leverarm in the vertical direction resulting in greater rotation effects). Theiceberg eccentricity model and the relationship between contact area andpenetration distance were also modified to account for iceberg keel contactswith a generic low profile structure on the seabed. One concept considered wasa single wellhead structure fitted with a special weak shear link incorporatedinto the design at the expected scour level. The shear link, or failure joint, would act as a mechanical fuse designed to fail in a combination of shear, tension and buckling during keel loading. The failure joint minimizes downholestructural response during iceberg keel loading on the production tree. Thedesigned failure mechanism would allow the well to be re-entered by protectingthe well casing from damage. Another concept considered was a steel truncatedcone structure installed over the well installation and fixed to the seabed byone of several identified foundation concepts. The protection structure absorbsenergy through crushing of the ice keel and encourages the iceberg to deflectaround and over the structure. The steel structure would be designed accordingto ultimate limit states accounting for energy absorption through elastic andplastic deformation of the structure. Design loads would correspond to anAbnormal Level Ice Event (ALIE) with an annual exceedance probability of 10–4. The size of the frame is governed by the size of the wellhead and tree system, Remotely Operated Vehicle (ROV) access requirements, and slope to encourageiceberg keel deflection. Piles may be the best option for securing a protectionstructure to the seabed, especially if a local vessel can be sourced to performthe installation. As an alternative to piles, using a drill rig to install wellcasings may be an option; however, market conditions for drilling rigs maydictate economic feasibility.
Impact Loads and Protection of Subsea Structures on the Seabed from Floating Iceberg Keels
Ralph, Freeman, Soper, Steve, Stuckey, Paul, McNeil, Andrew, and John Barrett. "Impact Loads and Protection of Subsea Structures on the Seabed from Floating Iceberg Keels." Paper presented at the OTC Arctic Technology Conference, Houston, Texas, USA, December 2012. doi: https://doi.org/10.4043/23788-MS
Download citation file: