Weblogic Smart Update tool (bsu) provides you a feature named Rolling upgrade which helps you to apply patch in cluster environment.
Please refer to the below document in OTN
Rolling Upgrade is the process of upgrading a running WebLogic Server cluster with a patch, maintenance pack, or minor release without shutting down the entire cluster or domain. During the rolling upgrade of a cluster, each server in the cluster is individually upgraded and restarted while the other servers in the cluster continue to host your application.
Before the WebLogic Server 9.2 release, to upgrade a cluster, you had to either shut down the entire domain, or install the upgraded server version on a parallel domain on additional hardware and use a load balancer to transfer load from the old domain to the new domain. The rolling upgrade process minimizes downtime and allows you to install a patch, maintenance pack, or minor release while the domain is still running.
Hope it helps!!