Rozprzestrzenianie się solanki w wodach Zatoki Puckiej

Abstract

Spreading of brine in the Puck BayA b s t r a c t. Polskie Górnictwo Naftowe i Gazowe (PGNiG) is planning to construct underground gas storages at Kosakowo near Gdynia. The storages, encompassing 10 caverns (total volume 25×107 m3) and situated at a depth of 800–1600 m, will be located about 4 km onshore from the Puck Bay. The gas storages will be created by drilling of boreholes and leaching of rock salt (approx. 5.6×106 t). The concentration of brine will reach up to 250 kg/m3 (salinity 250 PSU), and its discharge will attain a rate of 300 m3/h (0.083 m3/s). Due to the unique biological significance of Puck Bay (the Natura 2000 region), the location of brine discharge and the system of its release required detailed analysis. One of the most important criteria of the project was to ensure that brine spreading occurs to such an extent that salinity increase in the near-field does not exceed 0.5 PSU. The discharge system (diffuser) was designed by Przedsiębiorstwo Projektowo-Wdrożeniowe Emporium, while investigations of nozzles were carried out by the Szewalski Institute of Fluid-Flow Machinery PAS. Analysis of brine dilution in the Puck Bay was conducted by IBW PAN in co-operation with Deltares (the Netherlands) and was carried out in the following steps: 1) determination of kinematic parameters of brine stream coming from a single nozzle; 2) determination of mixing conditions of a single stream in the surrounding water; 3) spreading of brine in the near-field of diffuse and 4) spreading of brine in Puck Bay. Based on the analysis of the near-field dilution of brine, the following solution was selected: 48 nozzles (0.008 m each) located in 16 heads (3 nozzles per head, distributed every 120°). The stream of brine will be directed 45° towards the free surface, while the nozzles will be located 3 m above the bottom. The brine stream will move about 2 m towards the free surface, and later downwards. When reaching the sea bed the stream will be about 7.5 m from the head axis; the surface covered by diluted brine will reach a circle of 2 m diameter. After initial dilution in the near-field of the diffuser, and upon reaching the sea-bed, this mixture does not behave as a stream; further mixing takes place, in accordance with advection-diffusion equations. For modeling of brine spreading in Puck Bay (far field) the 3D hydrodynamic model, set-up based on Delft3D-FLOW, was applied. Calculations were carried out for 4 sites (accepted by Maritime Office in Gdynia) where depth increases from 8 m to 34.5 m with increasing distance from the shore. In all the considered cases, similar near-field discharge of brine in the Puck Bay was assumed. Based on the results of the calculations, the following conclusions were reached: 1) applicability of the proposed technical solution will lead to limited salinity increase in the near-field of diffusers (not exceeding 0.5 PSU); 2) in any of the sites considered, salinity increase will not exceed 0.5 PSU, regardless of hydro-meteorological conditions. Even under the most unfavorable conditions, i.e. those of long-term of wind and in the site located at 8 m depth, salinity will increase by a maximum of 0.5 PSU.