Machinbird, as Whinging pointed out, those are fuel tank access panels - rather large since a human has to be able to fit through. On the British Airtours 737 fire 30 years ago, an uncontained piece of the engine (part of a burner can) impacted one of those fuel tank access panel. The panel shattered (cast aluminum), resulting an a massive fuel leak onto an engine that had a hole in the burner (whoops).
One of the changes due to that accident is that now all fuel access panels that are within the engine 'burst zone' need to be impact resistant. On the 737 this was done by going to forged aluminum (instead of cast). I'm not sure what they use on the 777 but if I had to guess I'd say they are also forged aluminum.