SPA-FRANCORCHAMPS, Belgium - Formula 1 drivers sought reassurance from tyre supplier Pirelli on Friday after World champion Sebastian Vettel and Ferrari's Fernando Alonso suffered punctures in second practice for the Belgian GP.Red Bull's Vettel was fastest in the dry session but the sight of him nursing his stricken car back to the pits with strips of right-rear tyre buckling and flailing was more of a talking point.Pirelli had to strengthen it tyres after a spate of blowouts at the British GP in June triggered safety fears and threw the sport into crisis.'PROBLEM NOT SIMILAR'Alonso had a rear-right puncture too, also at around turn 14, at the end of the session and reserved judgement on the causes."I don't think it's a similar problem to what we saw in Silverstone, maybe more of a random set of circumstances, but all the same it needs careful analysis." said the Spaniard who was quickest in the morning.Compatriot Pedro de la Rosa, who heads the Grand Prix Drivers' Association, told Reuters they had asked FIA race director Charlie Whiting for clarification at their regular Friday briefing.Pirelli motorsport head Paul Hembery assured reporters that Friday's incident was completely different to Silverstone and due to "external sources" - such as debris - rather than anything structural."It looks on the Red Bull as though something has been rubbing on the surface and then has just cut through," he said, before talks with Whiting and FIA officials."And on the Ferrari there are two quite clear holes through the top of the tread. So we have to go and look at the track later to see what is between turns 13 and 15."Hembery was still at the track late in the evening and was due to provide an update on Saturday.'CLEARLY SOMETHING...'July's Hungarian GP, the last race before the summer break, was free of tyre-related incidents but the Hungaroring is one of the slowest circuits while Spa is the second fastest track with heavy loads on the tyres.Any failure or track issue, with cars racing at full throttle for long stretches, could be catastrophic."There is clearly something but I cannot tell you what it is," said Hembery. "It is from external sources. It is a worry for the sport because we have to go out and find what it is. We will try our very best to identify it and give us an indication of what is causing it."Vettel had earlier lapped with a best time of 1min49.331 with the sun coming out after a damp start to the day in the Ardennes forests where the weather is notoriously capricious.The championship leader's time looked ominous, 0.059 quicker than second-placed team mate Mark Webber and comfortably clear of Frenchman Romain Grosjean in the Lotus with a time of 1min50.149.Alonso set the largely unrepresentative pace in the opening session with a lap of 1min55.198 as drivers shrugged off the long August break and got back up to speed.CATERHAM CRASHEDThe Spaniard, third in the championship after 10 of 19 races, was ahead of the Force Indias of Paul di Resta and Adrian Sutil with the top three setting their best times in the final half hour of an incident-free session.The afternoon was more lively, with Dutchman Giedo van der Garde losing control and crashing his Caterham into the barrier at Stavelot. Hembery said the tyres were not to blame.The McLarens of Briton Jenson Button, the 2012 race winner, and Mexican Sergio Perez showed signs of progress, despite the team backing away from earlier assertions that they could challenge for a victory.Perez was fourth fastest in the morning and eighth in the afternoon, when the team tried out some developments for 2014.Vettel, who has a 38-point lead over Lotus's Kimi Raikkonen, had been sixth-fastest behind fellow German Nico Rosberg in the Mercedes in the morning."This morning was tricky with a few raindrops and the track was slippery; it's a lot colder here than Hungary, but that's Spa and part of this circuit," said Vettel. "This afternoon we had stable conditions and both of us seemed happy with what we had."BIGGEST RIVALRaikkonen, who cried off sick from media engagements on Thursday after weeks of speculation about his future and talk of moves to Red Bull or Ferrari, was sixth in the afternoon.Mercedes's Lewis Hamilton, winner in Hungary and increasingly looking like Vettel's biggest rival after three pole positions in succession, had a quiet day and was 15th and 12th respectively for Mercedes."The car feels very different here with the low downforce set-up, compared to the last race, and we need to refine a few areas to get the balance sorted," said the Briton.Second practice session times1 Sebastian Vettel (Ger) Red Bull 1min49.331 2 Mark Webber (Aus) Red Bull 1min49.3903 Romain Grosjean (Fra) Lotus 1min50.1494 Felipe Massa (Bra) Ferrari 1min50.1645 Jean-Eric Vergne (Fra) Toro Rosso 1min50.2536 Kimi Raikkonen (Fin) Lotus 1min50.3187 Fernando Alonso (Spa) Ferrari 1min50.5108 Sergio Perez (Mex) McLaren 1min50.5369 Nico Rosberg (Ger) Mercedes 1min50.601, 10 Paul di Resta (Gbr) Force India 1min50.61111 Adrian Sutil (Ger) Force India 1min50.62912 Lewis Hamilton (Gbr) Mercedes 1min50.75113 Nico Hulkenberg (Ger) Sauber 1min50.97214 Pastor Maldonado (Ven) Williams 1min50.99115 Jenson Button (Gbr) McLaren 1min51.19516 Daniel Ricciardo (Aus) Toro Rosso 1:51.44717 Valtteri Bottas (Fin) Williams 1min51.56818 Esteban Gutiérrez (Mex) Sauber 1min51.64419 Giedo van der Garde (Ned) Caterham 1min53.15720 Charles Pic (Fra) Caterham 1min53.25121 Jules Bianchi (Fra) Marussia 1min53.48222 Max Chilton (Gbr) Marussia 1min54.418Stay with Wheels24 for the 2013 Belgian GP weekend.