Architecture :: Using Internal Static DAL?

I have designed an application and am wondering if this is the best architecture to use and whether there is any danger.

In namespace Membership I have a public class, MembershipManager, which inherits from webservice

public class MembershipManager() : System.Web.Services.Webservice

The single public web method of this class often needs to read and write to a database.

[WebMethod()] public string DoMembershipWork(...various parameters...)

The database work is done by implementing, in the same namespace,an internal class

internal class MembershipDataAccess

which has a series of internal static methods for data access e.g.

internal static bool UpdateMembership(MembershipManager m)

The MembershipManager class accesses the data access class by calling the appropriate method with itself as a parameter e.g.

MembershipDataAccess.UpdateMembership(this);

Is this a good idea. This application will be processing many transactions simultaneously. Is there a danger of not being thread safe. Note also that sometimes the MembershipDataAccess class will return datasets to the MembershipManager class.