Offshore Platform Extension

[engrAlan]

One of our projects is to install electrical equipment on an existing 15m x 10m offshore platform. The platform was built more than 20 years ago. Since there is no enough space to install the new equipment on the existing deck, we were forced to provide deck extension to accommodate our new equipment approximately 2 Metric ton total weight on 2m x 4m skid.

My concern is on performing the structural analysis of the existing 20 year old structure on the sea, is there any requirement on applying reduction factor to account structure aging? One of the solution that we are trying to avoid is to conduct NDT, this activity maybe more expensive than our project cost. Any idea?

 

[Ussuri]

I think you will have no choice but to survey the structure, how else would you know what condition it is in. API RP 2A states:

Sufficient information should be collected to allow an engineering assessment of a platform’s overall structural integrity. It is essential to have a current inventory of the platform’s structural condition and facilities.

A structure should be evaluated based on its current condition, accounting for any damage, repair, scour, or other factors affecting its performance or integrity.

Typically you would need a topside survey and an underwater survey. Things to look out for include damage, deterioration due to age, major modifications, lack of accuracy of platform drawings.

 

[engrAlan]

Thanks Ussuri. We have conducted survey and we don't find any visual defects on the structural members on the top side. No modifications were done but most probably dents will be present on the underwater structures.

On the design basis of these structural platforms, Deck Beams are designed for a live load of 4.80KPa = 0.50 Mton/sq.m distributed load or 44KN = 4 Mton concentrated load. If we consider the total area of the platform to carry 4.80kPa it would take 15m x 10m x 4.80kPa = 720KN. The additional load that we are about to install is 2 Mton = 20 kN which is about 20/720 = 2.75% of the total capacity.

Is there any standard that would say complete structural integrity check (underwater survey) will not be required since my additional load is very small say less than 5% of the total structure designed capacity?

Thank you.

 

[Ussuri]

I don't know of anything i'm afraid. You could also look at ISO 19902 and Norsok N006.