If you are preparing a specification document and must reference BS 4994 for a legacy tank, always pair it with a current inspection standard (e.g., BS EN 13121-4:2005 for the repair methodology). This hybrid approach is widely accepted in industry practice.
After BS 4994 was withdrawn in the early 2000s, there was a gap. For several years, there was no British Standard for GRP vessels. A transitional document, PD CEN/TR 15729 (Report on the determination of design strength for GRP), was used, but it was insufficient for full vessel design. bs 4994 latest edition pdf
