Process Safety · Lesson 56 · 9 min read

A Hydrate Plug Can Move the Hazard When It Clears

A hydrate plug can look like a closed valve, but it is not a designed isolation. It may hold pressure on one or both sides, restrict an instrument connection and then move or disappear when temperature, pressure or composition changes. The moment the plug clears can therefore relocate the hazard: trapped pressure can drive the plug like a projectile, send a liquid slug through downstream equipment or release hydrocarbons through an unexpected path.

The hazard mechanism: a temporary solid can hold permanent pressure

Gas hydrates are ice-like crystalline solids that can form when water and suitable gas components meet at high pressure and low temperature. Offshore systems are vulnerable where gas cools across a choke or valve, during restart after shutdown, in low-flow dead legs, and in pipelines or gas-lift systems exposed to cold conditions. Prevention depends on controlling water, temperature, pressure and inhibitor delivery for the actual operating envelope.

Once a plug forms, a gauge may show pressure on only one side, or its connection may also be blocked. Depressurising one side can increase differential pressure across the plug. Heating, inhibitor injection or natural warming can weaken the solid until the differential suddenly accelerates it. The plug, displaced liquid and released gas can then load downstream equipment.

UK HSE safe-isolation guidance explicitly warns that hydrates can block valves and pipework, make pressure tests misleading and later release trapped pressure when they melt. It also says a pressure gauge indicates that pressure exists but is not, by itself, proof of complete depressurisation. A proven vent or other approved means is needed to confirm the condition before intrusive work.

Official offshore evidence

BSEE Safety Alert 484 describes an offshore incident involving suspected hydrate formation in 3-inch gas-lift piping. Operators isolated the line and relieved pressure downstream of the suspected plug while pressure remained trapped upstream. The alert reports approximately 900 psi remained in the piping. A ball-valve assembly separated from a threadolet and struck an operator. BSEE identified the unequal depressurisation and absence of a job safety analysis among the failures.

BSEE recommends isolating all energy sources, bleeding both sides simultaneously while maintaining the same pressure so the plug does not move or surge, using management of change where applicable, improving dehydration and maintaining accurate P&IDs. Each installation still needs an equipment-specific plan, safe discharge destination and defined operating limits.

Operational analysis: treat the plug as an uncertain boundary

A suspected hydrate changes the job to abnormal-condition management. The team must establish pressure on both sides, other sources, the destination of released inventory and which instruments remain trustworthy. Include connected vessels, common headers and the possibility of more than one plug.

Do not improvise heat, impact a pipe, loosen fittings or open a small-bore connection to “find” the plug. Silence at a bleed point does not prove zero pressure. Conflicting readings require a stop and competent technical support. Select any pressure balancing, heating or inhibitor method from verified conditions and an approved procedure.

Hypothetical FPSO example: gas-lift pressure falls

Hypothetical example—not a documented incident: An FPSO control room sees gas-lift flow fall while upstream pressure rises and a downstream pressure indication becomes unstable. The weather has cooled, and recent dehydration performance has been uncertain. The team suspects a hydrate but does not treat the restriction as an isolation.

Operations reduce risk using the installation's approved abnormal-operations process. They isolate credible pressure sources, establish reliable pressure information on both sides from safe locations, confirm the safe destination for any gas and liquid, and keep people out of the potential line of fire. Process engineering reviews the pressure-temperature history and selects an approved removal method that avoids creating a dangerous differential across the plug. The team records conditions and holds the area until stable flow, pressure and temperature evidence show the blockage is cleared. The example does not prescribe universal pressures, temperatures, inhibitor quantities or valve movements.

Critical controls

Hydrate prevention: keep dehydration, temperature management and inhibitor systems available for the defined operating envelope. Include startup, shutdown, turndown and restart conditions, not only steady production.

Complete pressure picture: use current P&IDs, trace all sources and destinations, and obtain reliable pressure evidence on both sides of the suspected plug. Treat blocked or inconsistent indications as uncertainty, not permission to proceed.

Engineered removal plan: define isolation, pressure management, discharge routing, monitoring, exclusion zones, hold points and stop criteria. Use formal change control when the recovery departs from established operating procedures.

Controlled reintroduction: after the plug clears, verify the receiving system can safely accept the gas and liquid inventory. Watch for slugging, abnormal pressure, low temperature and contamination of common headers.

Leadership actions

Leaders should make hydrate management a defined barrier strategy. Procedures must address prevention, suspected formation, removal and restart; competent support must be available; and decision instruments must be maintained. Exercises should test conflicting pressure information and approval authority.

Recurring hydrate formation requires investigation of dehydration, inhibitor reliability, cold spots, transient operating envelopes and previous workarounds. Production urgency must not turn an uncertain plug into an informal task.

Three practical actions

• Map both sides: identify every pressure source, instrument, bleed route and receiving system before changing pressure around a suspected plug.
• Challenge the evidence: ask which indication could be blocked and what independent observation would confirm the actual pressure condition.
• Protect the recovery: require a competent, approved removal plan with safe routing, pressure balance, exclusion zones and clear stop criteria.

Official video: learning from DuPont La Porte

The U.S. Chemical Safety Board's animation of the 2014 DuPont La Porte release is relevant to hydrate troubleshooting. It is a chemical-plant incident involving toxic methyl mercaptan, not an offshore event. A hydrate formed after water entered the process; the warming plan lacked required safety review and written tracking; and workers did not recognise that high pressure elsewhere was connected to the clearing blockage. Four workers died.

Watch the animation on the official CSB website.

Takeaway and team discussion

Key takeaway: a hydrate plug is an uncertain temporary barrier; control pressure on both sides and the destination of released inventory before attempting to clear it.

Discussion question: if a hydrate were suspected in our gas-lift or production system today, what reliable evidence would show the pressure on both sides and who would approve the removal plan?

Continue in the Process Safety hub, and connect this lesson with verified isolation, zero-pressure proof and flow-path verification.

Verified primary references

1. U.S. BSEE Safety Alert 484: Improperly Depressurizing Piping to Remove a Hydrate Causes Injury.
2. UK HSE: Inspection of Loss of Containment—hydrate formation and management.
3. UK HSE: The safe isolation of plant and equipment (HSG253).
4. U.S. CSB: DuPont La Porte Facility Toxic Chemical Release investigation.

Published and sources checked 7 November 2026. Educational discussion only: apply the installation's safety case, hydrate-management procedure, isolation standard, operating limits and equipment-specific instructions.

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