I am agree with the @HotLicks this code must work. Are you sure between the sort code and log there is no code. If there is than please add it.
Only problem i see is that You have added your array name instead of NSArray in [self.orderArray sortUsingDescriptors:[self.orderArray arrayWithObject:sortDescriptor]];
this line.
Do it like this :
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"orderid" ascending:FALSE];
[self.orderArray sortUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]]; //Change in this line
[sortDescriptor release];
NSLog(@"self.orderArray : %@",self.orderArray);