How to make a professional approach for delay analysis to support a claim against the employer?

There are no universally agreed methods of analyzing/quantify delay, but legal precedence showed that "critical path method" or "time impact analysis" are preferred to support a claim against the Employer.

you must identify the event which caused the delay....then who is responsible for that event.....then you should apply the effect on the time schedule.....the difference between the original schedule and the new one is the time claim .....

There are many techniques delay analysis each of them suitable depending upon the premise. In broad term, they can be categorised into one of the four

1. Impacted As Planned

2. CollapseddAs Built

3. As Planned vs As Built

4. Time Impact Analysis (TIA)

Each one of the above can either be performed either prospectively or retrospectively. The most accepted of all is TIA prospectively, but it depends on many factors such as Contract terms and conditions, legal jurisdiction etc.

Apart from the techniques it is important to check if contract condition such as time barring clause is respected or not, does the contract defines float ownership or consequences of concurrent delay, whether the delay analysis performed is based on cause and effect relationship or it is a global claim. Also, all claim should have minimum:

1. Cause

2. Effect

3. Entitlement

4. Substantiation.

In any case, it does not have one perfect answer which is the best technique to analyse time extension claim.

You can refer AACE RP29 and SCL Delay and Disruption Protocol for detail.