Production Semi-submersibles (Semi) and Tension Leg Platforms (TLP) are typically constructed in Southeast Asia and dry transported to near installation site by Heavy Lifting Vessel (HLV). Platform columns and/or pontoons are partially or entirely extended outside of HLV deck and exposed to sea. Dry transportation takes one to two months from the fabrication yard to the offloading site and the environmental conditions during voyage are normally very rough. The overhanging portions of the Cargo (Semi or TLP) will experience severe wave slamming loads and this could result in possible shifting, lifting and overturning on the HLV. To ensure the safe transport of the Cargo during the voyage, tie downs are nominally considered, designed and installed.
Wave slamming has been characterized as a difficult subject with a complicated structural-hydrodynamic interaction. Analytical prediction is generally less reliable and model testing has become an important and widely accepted design tool to derive reliable sea fastening loads for tie down designs. However, tie down stiffness in prototype-scale is very difficult to model and scale properly in model tests. The measured results cannot be directly applied without considering correction of stiffness differences between prototype and model scales. Therefore, it is necessary and crucial to develop a methodology to account for the stiffness differences and maximize the value of model testing.
The objectives of this paper are in three folds, the first is to establish a systematic and consistent methodology to model tri-axial stiffness in model testing and explore load cell stiffness effects on high-frequency system responses of the measured results; the second is to account for stiffness differences between prototype and model scale and establish a correlation method; the third is to employ correlated results to provide guidance for optimizing final tie down design.