What Is a Tie-In List? Interface Schedule

A tie-in list (also called a tie-in schedule or tie-in register) is an engineering document that identifies every point where new piping or equipment must be connected to an existing (live or operating) system. Each tie-in point is assigned a unique number and tracked with its technical details, execution method, required shutdown duration, and planned execution date. The tie-in list is a critical planning tool for brownfield projects, plant expansions, and pipeline modifications where new construction interfaces with operating facilities.

Tie-ins are among the highest-risk activities in construction because they involve working on or near live systems, often under time pressure during planned shutdowns.

Tie-In List Field	Description	Example
Tie-in number	Unique identifier	TI-001, TI-002
P&ID reference	Drawing showing the connection	PID-200-015
Line number (existing)	Existing pipe or equipment tag	8”-HC-20001-B2A
Line number (new)	New pipe connecting to existing	8”-HC-30005-B2A
Tie-in type	Hot, cold, or shutdown tie-in	Shutdown (SD)
Connection method	Weld, flange, clamp, hot tap	Butt weld
Size	Connection diameter	8” (DN 200)
Pipe class	Material specification	B2A
Isolation method	How existing line is isolated	Double block and bleed
Preparation scope	Pre-work before shutdown	Prefab spool, install weldolet
Execution scope	Work during shutdown window	Cut existing pipe, fit-up, weld, NDT
Shutdown duration	Required downtime	12 hours
Test requirement	Post-connection testing	Hydrotest to 22.5 barg
Reinstatement	Post-test activities	Remove blinds, open isolation valves
Planned date	Scheduled execution date	During Turnaround 2027

Types of Tie-Ins

Tie-In Type	Definition	Risk Level	Planning
Cold tie-in	Connection to a system that is out of service, depressurized, and drained	Low	Can be scheduled flexibly
Shutdown tie-in	Connection during a planned shutdown; system is isolated, depressurized, and purged	Medium	Requires shutdown window, detailed permit-to-work
Hot tie-in (hot tap)	Connection to a live, pressurized system without shutdown	High	Requires hot tapping machine, specialized crew, detailed procedures

Cold tie-ins are the simplest and safest. Shutdown tie-ins require careful coordination with operations to minimize downtime. Hot tie-ins (hot taps) are performed when the system cannot be shut down and involve cutting and welding on a pressurized pipe or vessel—a high-risk operation requiring specialized equipment and procedures.

Tie-In Execution Planning

Successful tie-in execution depends on maximizing the amount of pre-work completed before the shutdown window:

1. Pre-fabrication: All spool pieces, fittings, and supports fabricated and tested in advance
2. Pre-installation: Weldolets, branch connections, and supports welded to the existing pipe while the system is still operating (where safe to do so)
3. Material staging: All materials, gaskets, bolts, welding consumables staged at the tie-in location
4. Crew briefing: Detailed method statement, safety review, and permit-to-work completed
5. Execution: During the shutdown window, the existing pipe is cut, the new spool is fitted and welded, NDT is completed, and testing performed
6. Reinstatement: Isolation blinds removed, valves opened, system returned to operations

Common Mistake

Underestimating the time required for NDT and testing after the tie-in weld is a common cause of shutdown overruns. A single failed radiographic test (RT) on a tie-in weld can add 4-8 hours for repair, re-welding, and re-inspection. Always build contingency time into the shutdown schedule (typically 25-50% buffer) and have backup welders and NDT technicians on standby. Pre-qualify all welders on the specific material and joint configuration before the shutdown day.

The tie-in list is managed alongside the broader project management schedule and is a key input to shutdown/turnaround planning.

Read the full guide to piping engineering