Wave-current blockage: Reduced forces for the re-assessment of ageing space-frame offshore structures

This paper summarises extensive research work on the accurate calculation of extreme loads from waves and current on space-frame offshore structures. Although relevant to new builds, improved prediction of extreme loads is also key to the re-assessment of old and ageing offshore platforms. Current blockage is a field effect. Due to the presence of the rest of the structure, the flow velocity on each structural member is reduced on average leading to smaller overall loads. The first model to account for this 'current blockage' was first proposed by Taylor [1], and incorporated into standard industry practice (API, DNV and ISO). This is a simple improvement to the original Morison equation (Morison et al. [2]), which predicts forces using the undisturbed open ocean flow properties. New work shows that unsteady large waves on top of a steady current introduces additional blockage, interpreted as wave-current blockage. Large-scale laboratory experiments have been used to validate numerical force calculations. This paper describes a numerical Computational Fluid Dynamics (CFD) model of a porous block with embedded Morison drag and inertia stresses distributed over the enclosed volume of the space-frame as a global representation. At a local member scale, the standard Morison equation is used, but on the local flow. This local flow speed is reduced because of overall interaction between the structural members interpreted as resulting from a distributed array of obstacle. Since the Morison equation is semiempirical, drag and inertia coefficients are still required, consistent with present industry practice. This new method should be useful for assessing the overall structural load resistance and integrity in extreme wave and current conditions when survivability is in question. Results are presented from recent large-scale experiments on a scaled (1:80) jacket model in the Kelvin Hydrodynamics Laboratory in Glasgow. These tests cover force measurements on both a jacket (stiff, statically-responding) and the same model restrained on springs to mimic structural dynamics (the first mode of a deep-water jacket, the second mode of a compliant tower or the first mode of a jack-up). For a jacket structure under all range of wave and current conditions, only a single pair of values of Morison drag and inertia coefficients is required to reproduce the complete total force-time histories on the jacket model. This is in contrast to the present industry practice whereby different Morison drag coefficients are required in order to fit the measured peak forces over the wide range of cases considered. For the dynamic tests, we find that the relative velocity formulation of the Morison equation for space-frame structures is valid for dynamically sensitive structures. All of these effects can be captured using our numerical porous block model.