Herøya petrochemical industrial complex is located just south of town. See the map (photo) to get a feel for the way there. You can stay in Porsgrunn's hotels and have easuy access by local bus or taxi or own vehichle to Herøya, just 10 minutes south. There are sightseeing tours of the area per appointment. Check with tourist authorities in Porsgrunn for details. I incidentally followed directions to the tourist info and came to the "House of Pain Tatoo".
Porsgrunn is close enough to E18 (Oslo-Kristiansand (Stavanger) highway to be able to comfortably drop by for a stop. The E18 runs closer to the coast, so about 10 km upriver is the city area. It's also a possibility to run up to the mountains of Telemark from E18 through Porsgrunn-Skien.
Check the map.
There are several trains and bus-train connections with Porsgrunn on the Vestfold railway line. Trains connect Porsgrunn to Skien and Oslo-Lillehammer. For international transfers from Porsgrunn, get off at Larvik for the Hirtshals and Fredrikshavn Ferries (Denmark), at Sandefjord for connecting to the Torp Airport (better take the direct bus from Porsgrunn, though), Oslo for trains to Denmark, Sweden and ferries to Germany, Denmark, and Gardermoen airport for main international air connections. Travel time Porsgrunn-Oslo: about 2 1/2 hrs, Gardermoen 3 hrs.
The station and the bus terminal are in walking distance a bit to the north of the main street/downtown area.
Tickets must be bought at the Narvesen kiosk near the platforms. You can also buy no-frills-tickets on board.