Experimental Investigation of Subsea Structures during Installation and the Related Wave Loads Added Mass and Damping

This paper presents experiments focussing on the wave loads, added mass and damping of a subsea structure in the splash zone. Two series of tests were carried out: decay tests to determine added mass and damping and captive tests to determine the wave loads in regular waves. Different positions of the subsea structure were tested (in the splash zone, completely submerged and close to the seabed). It was found that for a subsea structure in the splashzone the concept of added mass cannot be applied simply: the added mass is varying strongly with the oscillations of the structure due to the water flow over and inside the structure. Further it was concluded that due to complex effects such as varying buoyancy, first order wave loads, drag dominated effects, water in- and outflow, internal shallow water waves and impulse loads at the bottom, the resulting wave loads are strongly non-linear. Consequently it is not possible to simulate the installation of subsea structure through the splash zone with time domain simulation models with estimated (constant) added mass and damping and simple wave load models. Computation Fluid Dynamics (CFD) methods to determine the complex flow around the subsea structure in waves are presently under development, but need further development and validation. These need to be coupled to the time domain simulation of the motions of the installation vessel to determine the combined response. Until these complete models are available, model tests remain necessary to determine reliably the behaviour of subsea structures in the splash zone.

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