Design of a Multibody Dynamics Based Model Scale Test for Towing Operations in Managed Sea Ice

Abstract

AbstractWith a series of physical model tests performed during February to August 2016, Arctic towing operation was investigated while towing a Gravity Base Structure (GBS) in managed sea ice with varying parameters: ice concentration, floe size, towing speed and towing configuration. The CONDRILL™ Arctic driller (Gravity Base Concrete structure) is a promising structural concept for extended exploration drilling operations in limited open water season and in harsh ice conditions. In the studies, the model-scale Arctic driller concept were constructed and tested in different paraffin-made model ice conditions. The tests, designed to shed light on both the physics and the practicalities of moving the GBS in varying ice conditions where towing force and structural stability, influenced by ice resistance are central for a successful platform design. The current model test is the first in a series where the results will be used as design input as well as subsequent marine operations employed for moving the platform in managed sea ice. This paper discusses initial assessments of towing force under varying ice conditions through physically modelling the most significant ice load contributor (i.e., the ice accumulation and clearing process). Based on the model tests, an optimum towing configuration, which involves no permanent ice jamming in all the tested ice conditions, was identified. In addition, it was found that for ice concentrations lower than 60%, the towing speed (or hydrodynamics) governs the towing resistance and the influence from ice floes are minor. However, while at high ice concentration (e.g., ≥70 - 75%), we are shifting from a hydrodynamics governed scenario into multibody dynamic interaction governed scenarios, in which, ice accumulation/clearing and internal friction resistance between the ice floes dominate the tow resistance. The study highlights the importance of an efficient ice clearing mechanism to release the pressure built-up in front of the structure and transport broken ice to the wake region of the structure, which results in lower resistance during towing. The studies reported in this paper contribute to the following items: 1) The multibody dynamic based physical modelling test would be the first of its kind to isolate this important physical process and to study it thoroughly without the influence from other physical processes such as sea ice fractures. 2) The test results are useful to validate currently available numerical simulation tools based on Discrete Element Method (DEM).