I'm sorry that I can't offer any help on the specific Stress aspects of this problem.
However i would like to point out something to everyone who might read this post. You wrote "the client, in fast start of the project verbally agreed to the proposal by our stress engineer."
When will people learn, "If it is not in writing then it does not exist"
At the front end of a project, Fast Track or not, there should be a process called "Scope of Work" development. A "Scope of Work" for any process plant project should be prepared by every Engineering and Design discipline and include two parts.
Part one shall be that discipline's understanding of what plant includes. This is called the Scope of Facilities.
Part two shall be that disciplines understanding of what they will do, how you will do it and what deliverables will be produced. This is called the "Scope of Services".
After the Company Project Manager and the Client agree on it and approve it, it then becomes a part of the Contract.
"Piping Engineering Leadership for Process Plant Projects"
By: James O. Pennock - Published by: Gulf Publishing.
You can find it here:
http://www.amazon.com/gp/product/088415 ... s&v=glance