Over the years legislation has changed. The following is relevant in this context:
CE - European Conformity
Especially with regards to radiofrequency emission and susceptibility many older systems fail resulting in unnecessary shutdowns or operational inconvenience when using radios and such. Also outside Europe it makes sense to ensure equipment is CE marked.
This refers to the explosion hazard always present in the oil and gas industry, and the approved (!) technologies to overcome this.
This refers to high pressure systems, for example transport gas, fuel gas and steam.
Safety Integrity Level refers to the level in which a system acts safe in case of malfunctioning in order to prevent injury or death.
CO/NOx, CO2 and CH4 emission reduction measures through taxes and limits.
Legislation was changed to improve transparency of the market, and to overcome flaws in previous standards having resulted in accidents. It is allowed to continue using an installation if it was truly installed according to the legislation in place at the time. But it is not always wise to do so, as it makes it hard to carry out improvements on the installation without breaking current legislation. With respect to the environment it is often better to demonstrate that a company is already taking it's measures rather than waiting for the authorities to step in with law.