You're going to need not only a topic but a research question about which it will be possible for you to collect relevant data. For a PhD, you'll need either a question that no-one has yet addressed or a new approach to a question that has been researched but remains unsettled.
Before you even start brainstorming, you need to do some preliminary research. This will stand you in good stead, as your reading at this stage will form the basis of the literature review section in your proposal and the literature review chapter in your dissertation.
Read the literature review sections of several of the most recently published studies in the area of risk management. If you find a topic that intrigues you, go deeper on that by reading as many studies as you can. What ought to happen is that, as you read, research questions will begin to form in your mind. In order to facilitate that, make up a little worksheet on which you list the topic and research question for every study that you read (or read about).
Once you've got a few possible topics/questions in mind, we can help you frame and assess the viability of various research questions so that you choose one that will be feasible for you to research and write up.
Simone, EssayForum.com