Please refer to these docs.
How Can Trace and Debug Be Turned On For A Concurrent Request? [ID 759389.1]
Purge Debug Log And System Alerts Performance Issues [ID 332103.1]
How to Purge/Troubleshoot the FND_LOG_MESSAGES table [ID 735138.1]
How to Obtain Debug Log in R12 [ID 787727.1]
What Are Concurrent Reports That Should Be Scheduled Periodically [ID 1066117.1]