Replacement scrum-half Rhys Webb scored twice as Ospreys completed a European Champions Cup double over struggling Northampton with a 32-15 bonus-point victory at a wet Liberty Stadium.

Ospreys, who backed up their 43-32 success at Franklin's Gardens, have given themselves a fighting chance in the Pool 2 but will need to secure wins over Saracens and Clermont in the new year.

They were indebted to tries from prop Dmitri Arhip, Webb and the returning Justin Tipuric, with 12 points coming from the boot of Sam Davies.

After a scoreless first half, Northampton -- without the England pair of Dylan Hartley and Courtney Lawes -- came back briefly with a try for Harry Mallinder and a penalty from Piers Francis.

They also grabbed a consolation try for Rhys Marshall in stoppage time, converted by Mallinder.

Ospreys made an imposing start, scoring the game's opening try after two minutes.

It was scored in the right corner by prop Arhip, but conceived close to halfway with a break by Ben John, a weaving run from Tom Habberfield and good link work from Tipuric.

The touchline conversion attempt was, however, missed by Davies.

Northampton looked sure to score a try after moving the ball from a line-out but Mallinder knocked on in the tackle with Ahsee Tuala free outside him.

And Saints had two goes at a drive from a line-out and got pushed back off the ball by Ospreys before the home side earned a scrum penalty and were able to clear their lines.

In the 24th minute Ospreys extended their lead to 8-0 with a 45-metre penalty for Davies after Saints had been caught with hands in a ruck.

That became 11-0 five minutes later as Davies found the target with a second penalty after Saints were penalised at another scrum.

Saints tried to respond in the latter stages of the half but the Ospreys' defence was too good.

The pendulum swung back Northampton's way after the break to cut the deficit to only three points.

Webb came on at half-time but when he was caught in possession in his 22 that led to Northampton's try, with Mallinder going over after a kick from Francis, who failed with the conversion attempt.

And four minutes later Francis kicked a penalty goal from in front of the posts after the home side infringed at a ruck.

Some patient build-up play put Ospreys back in charge with a try from Webb in the 56th minute. Davies' conversion gave the hosts a 10-point lead.

And they put the result beyond doubt with two tries in the space of three minutes.

In the 66th minute a break by Dan Evans saw Owen Watkin put Tipuric over, with Davies' conversion extending the home lead to 17 points before Webb dived over following another concerted attack.

Davies converted as Ospreys led 32-8 with 10 minutes remaining.

In stoppage time Marshall squeezed over for a second Saints try, converted by Mallinder.