2019-09-13 18:26:00,598 8282 INFO ? odoo: Odoo version 11.0-20190112
2019-09-13 18:26:00,599 8282 INFO ? odoo: Using configuration file at /etc/odoo/odoo.conf
2019-09-13 18:26:00,599 8282 INFO ? odoo: addons paths: ['/var/lib/odoo/addons/11.0', '/usr/lib/python3.7/site-packages/odoo/addons']
2019-09-13 18:26:00,605 8282 INFO ? odoo: database: odoo@default:default
2019-09-13 18:26:00,709 8282 INFO ? odoo.service.server: HTTP service (werkzeug) running on localhost:8069
2019-09-13 18:26:01,681 8282 INFO ? odoo.addons.base.ir.ir_actions_report: You need Wkhtmltopdf to print a pdf version of the reports.
2019-09-13 18:48:00,790 8282 INFO ? odoo.service.server: Initiating shutdown
It's not visible from the logs what's triggering the shutdown of the service, but it seems to happen often.
Do you know what happened during that time, and what you did within that ~20 minute timespan ?
But later in the log it happens more often. You'd have to ask upstream about that.