Thats a very good point from Mr Veit. In a project where you are involved with GRP piping, make sure you involve your prospective GRP Vendors from a very earlier stage as the product properties can be differ from vendor to vendor.
Most of the execution approches are to include the Stress Analysis of GRP portion in the GRP vendor scope or a specialist agency.

Another excellent Vendor site is WWW.SARPLAST.COM.

You can also refer WWW.ADPF.AE, AMIANTIT (http://www.afilpipe.com/)and others.