Numerical modelling complements physical testing in staged design of ocean wave-driven pump

Abstract

National Research Council Canada (NRC) is collaborating with College of the North Atlantic (CNA) to develop an ocean wave-driven pump to supply the CNA shore-based aquaculture centre in Lord's Cove, Newfoundland. NRC provides a combination of physical model testing under controlled laboratory conditions and computer simulation using numerical modelling tools developed at NRC and commercial software. The CNA plans to demonstrate the sustainability of a shore-based aquaculture centre in the context of rural Newfoundland. The efficiency of the centre depends in part on its energy consumption, a major part of which is supplying seawater to a facility some distance from the ocean and above sea level. The development of a seawater supply pump driven by the power of ocean waves would reduce the electrical costs for the centre. The NRC is supporting a staged design program through numerical and physical model testing to select a robust design optimized for the local sea conditions at the Lord's Cove mooring site. The wave pump is inspired by designs for wave energy converters. But since wave energy converters have not converged on a single design, there are major risks and costs in designing a wave pump, from construction and testing of physical models for competing designs to mooring failure and potential loss of the device in the hurricane-force storms typical of southern Newfoundland. Using lessons learned in the energy sector, risks can be mitigated by systematically testing the behaviour and response of proposed concepts through a staged design program, starting from characterizing the hydrodynamic characteristics of a simplified model through progressively larger, more complex physical and numerical models used in functional tests. This approach systematically directs the design process toward a single prototype optimized for performance in specified wave conditions using scaled model tests in advance of the most costly stage of sea trials. The goals of the staged design program for the CNA wave-driven pump are to (1) evaluate various design concepts (2) select a design suited to the CNA mooring site (3) optimize the design for a typical sea state and (4) specify a mooring robust enough to keep the platform on station during the sometimes extreme sea states at Lord's Cove. The program began with characterization of the chosen mooring site outside Lord's Cove. Processed wave buoy data from this site informed the estimation of mooring line loads using numerical tools developed by NRC and typical sea states on which to focus design optimization. In the second stage, a numerical tool was developed to estimate pump output and validated against test results for a physical pump. In the third stage, various wave pump platform design concepts were evaluated based on estimation of hydrodynamic characteristics. The designs were characterized using a combination of physical model tests in the NRC 200-metre wave tank in St. John's and computer simulations using Ansys Aqwa and Matlab. A sample pump platform design is shown as a scale model and a numerical mesh in Fig. 1. In the fourth stage, which will precede a full-scale prototype deployment at sea, a design has been selected and an operational scale model wave pump platform has been tested in the NRC wave facilities. Numerical modelling is used to choose a mooring for the full-scale deployment. In this staged design program, the use of physical model tests and computer simulation are complementary methods used alternatively to solve potentially costly and time-consuming problems. A selection of the problems encountered in the design of the wave pump platform is presented along with a critique of the solutions applied by NRC to build confidence in a wave-driven pump design tailored for the CNA site.