What are the functions of an Emergency Shutdown Valve (ESDV)?
Emergency Shutdown Valves (ESDVs) are an essential risk reduction measure for onshore and offshore installations; the primary intent on ESDV closure is the isolation of pipeline inventories from topsides plant, the isolation of plant sections to minimise hydrocarbon inventory and to isolate wells from processing plant facilities during an emergency event.
The closure of ESDVs will, therefore, serve to reduce the available hydrocarbon inventory during a loss of containment event and minimizes the potential for escalation to other sections of plant or equipment. This article discusses ESDVs and the maintenance challenges thereof for safe production operations.
Why are ESDVs Important?
An ESDV must close in a timely fashion and once closed it must seal off the flow of fluids when it is called upon to do so during an emergency event; functional reliability of ESDVs is therefore vital for safe production operations. In order to ensure that ESDVs remain functionally reliable, they must be maintained. However, maintenance on ESDVs must be optimal and must be targeted to identify the specific failure modes that each valve is susceptible too; it must also be conducted at a suitable frequency to ensure that the required levels of reliability are being realized and impact on production is minimized.
What are the Consequences if ESDVs are Left Unmanaged / Uncontrolled?
If ESDVs are in a failed state, or not functioning to their required performance standard, then uncontrolled escalation of hydrocarbon events can result. “It was the lack of an ESDV on the Piper Alpha riser that allowed gas to flow uninterrupted causing the eventual destruction of the installation killing over 160 people on the platform.” [1]
Figure 1: Piper Alpha Incident [2]
It is therefore critical to have ESDV designed into installations and test them to ensure operation as and when required in an emergency situation. The testing frequency in a maintenance regime is also important as ESDV ‘reliability’ is a function of test frequency.
ESDV Maintenance and Testing
ESDV maintenance is vital for the assurance of functional reliability; in the event that ESDV maintenance is suboptimal and/or mismanaged this will serve to increase the likelihood of ESDV functional failures. ESDV functional failures were recently shown to be strongly correlated to ESDV age and to the failure to learn and implement lessons from previous incidents; however, just because an ESDV is ‘old’ does not mean that it is no longer serviceable, but it does mean that the maintenance regime is even more important in order to ensure functional reliability.
ESDV maintenance tasks should be specifically chosen to test each of the specific failure modes that they are susceptible to during normal operations; a successful outcome following each test would then serve to confirm functional operability. There are several ways in which ESDVs may ‘fail’, neglecting SDV controls systems and the potential for controls systems malfunctions, there are three key modes which may lead to SDV failure when called upon to operate in an emergency; given that the primary function of an ESDV is to close within a specific timescale and seal off the flow of fluids, the primary failure modes of interest are:
- Failure to close
- Closes too slow
- Excessive leakage across the ESDV when closed
Figure 2: ESDV Fault Tree – Failure to Isolate Hazardous Fluids
What do we offer?
AIE considers that the potential for ESDV “failure” should be anticipated in order to enable the prevailing maintenance regime to be proactive in form and function; AIE further advocates that a preventative approach will serve to provide assurance that ESDVs will function as required and as originally intended. The time at which ESDV functional failures manifest themselves are generally when they are least expected and the worst possible moment would be during an emergency situation where the need for hydrocarbon inventory isolation is critical.
We work to deliver optimized ESDV maintenance strategies that significantly enhance reliability and safety through the reduction of technical risks. Once developed we then ensure the maintenance strategy is fully up-loaded and rolled out through the clients CMMS whilst ensuring that suitable critical spares are also available and contingency plans are in place to deal with ESDV functional failures if and when these arise.
AIE has delivered a range of major projects for clients in relation to ESDV maintenance, testing, and verification and we have developed strategies which help to significantly reduce OPEX costs whilst meeting minimum safety and reliability standards. Please feel free to contact our technical team for any support requirements on this topic.