Analysis and Design of Columns in Offshore Structures subjected to Supply Vessel Beam Collisions

Abstract

The recommended force-deformation curves for supply vessels collision presented in NORSOK N-004 are based on simplified methods and tests developed more than 30 years ago. The purpose of this work is to contribute to the development of new design guidance by investigating the force-deformation relationships for a supply vessel of 2250 tons displacement colliding with an inclined jacket leg. Reviews of simple calculation models regarding ship side collisions and deformation of platform legs subjected to impact loads have been conducted. Finite element models of unstiffened columns and a side structure in the mid ship area of a supply vessel have been developed using the software Patran. Emphasis has been placed on choosing boundary conditions that reflect the behavior of the connected structures in a realistic way. The jacket leg has been constrained to small axial translation, were the translation is governed by an axial stiffness. The ship side model has been given a constant velocity of 2 m/s and the sides have been constrained from moving in any other degree of freedom than translation in the y-direction. Nonlinear finite element analyses of the ship side impacting a battered unstiffened jacket leg have been performed using the explicit solver LS-DYNA. 1.0, 1.5 and 2.0 m diameter columns with varying wall thicknesses have been used. Force- deformation and energy-deformation curves have been plotted and evaluated for all cases. Most of the analyses were found to be ductile or shared energy design, while the 2.0 m diameter column with a shell thickness of 60 mm was found to be very close to strength design. Impact analyses with rigid columns of 1.0, 1.5 and 3.0 m diameters were performed. The ship force-deformation relationship was found to not be affected by the increase in column diameter.The resistance to local denting of the jacket leg was evaluated and compared to the simplified calculation model in NORSOK N-004. The NORSOK curves for maximum collision contact zone width fit well for the 1.5 m diameter case, but were found to be non-conservative for the 1.0 m diameter columns and too conservative for the 2.0 m columns. The NORSOK curve corresponding to a point load fit well with the columns with small wall thicknesses, especially for small indentations. Analyses using a material model including initiation of fracture were conducted and evaluated. Fracture was in no case found in the jacket leg, but was initiated in the ship side. The overall effect of including fracture on the force- deformation curves proved to be rather small. Local buckling of the jacket leg?s cross section was briefly investigated. The effect on the load bearing capacity of the jacket leg was found to be small, but this was only considered for the analysis with 1.5 m diameter and 30 mm wall thickness and a general conclusion could not be drawn. The recommended NORSOK curves for force-deformation were evaluated and found to underestimate the impact force of a supply vessel. The proposed curve for a 5000 tons displacement ship was found to be suitable for a supply vessel of 2250 tons. The results from rigid column impact implied that the NORSOK standard curves can be applied for 1.0 and 2.0 m diameter columns in addition to the standard 1.5 m diameter.