If you work for an engineering consultancy, check if your organization has a corporate subscription to IHS Markit, ASTM, or BSI. The standard may already be sitting in your company's digital library. Conclusion
EN 13121 directly inherits most of the design equations from BS 4994. If you simply need to design an FRP tank, EN 13121 is legally required in the UK and Europe, and it is available for purchase (but not free).
: Detailed guidelines for wet lay-up and filament winding processes. Where to Find BS 4994 PDF
K=K1×K2×K3×K4×K5cap K equals cap K sub 1 cross cap K sub 2 cross cap K sub 3 cross cap K sub 4 cross cap K sub 5 Breakdown of Sub-Factors Description Typical Values K1cap K sub 1
If you need a PDF for professional use that you can download, print, and keep, you will need to purchase it from an official standards reseller.