What is the minimum number of heavy-duty tie-down straps required to secure an item of heavy cargo on a flatbed trailer?

1. At least two tie-downs for any cargo piece over 1.5 metres in length, with additional tie-downs proportional to weight and length
2. One thin rope regardless of cargo weight
3. Cargo requires tie-downs only if it weighs more than 20 tonnes
4. No tie-downs are needed if the flatbed has side rails