I suspect you could probably get away with 'fixed swell'.
It probably infers 'fixed swell direction'.

"la variation des hydrostatiques en présence d’une houle figée" (http://www.beamer-france.org/BanqueDocument/pdf_408.pdf)

"Persistent swell conditions, particularly the Southern Ocean long-period swells, are largely responsible for resonant roll motions resulting in significant downtime in both new-build and converted FPSOs in this region. " (http://www.kbr.com/Newsroom/Publications/Technical-Papers/FP...

"The CFPSO concept has been targeted toward the West African gas fields where metocean conditions may be described as benign but with persistent swell. Extreme design environmental criteria have been assumed to be..." (http://e-book.lib.sjtu.edu.cn/otc-03/pdffiles/papers/otc1529...

"My son Kevin and I had no trouble detecting a regular swell running in the same direction as the wind, roughly southeast, but it took us an hour or more of close observation to realize that among the breaking waves a smaller but persistent swell was running almost at right angles to it — presumably the remains of waves generated in the Roaring Forties, hundreds of miles to the south." (http://johnvigor.blogspot.fr/2013_03_01_archive.html)

"Both methods reduce the heave RAOs when the wave period is greater than 12 s, and they may be applicable to situations where the installation window may contain prolonged periods of persistent swell. " (http://ittc.info/downloads/Proceedings/27th Conference/1-7 O...

I haven't found an example where it's possible to "prove" the equivalence between the terms (the closest I came was here https://3w2.ensta.fr/~dfr/Cours/index.php?usebdd=ensta&sigle... , unfortunately they haven't bothered translating the timetable into English...), but it seems reasonable to me. Perhaps people with more maritime experience could confirm or discount this possibility..?