P1 – Rules for pipes

APPLICATION FIELD

IACS UR P1, “Rules for pipes,” sets the baseline wall-thickness and general strength requirements for pipes covered by classification.

It applies broadly, across essentially every piping system on a classed vessel, unless a more specific UR or unified interpretation displaces it for a particular system.

Three groups of piping fall outside its scope: chemical cargo piping on ships subject to the IBC Code together with shipboard hydrocarbon/chemical process piping, gas cargo/fuel and process piping on ships subject to the IGC Code along with gas fuel piping under the IGF Code, and piping for other low-flashpoint fuels as defined under SOLAS II-1/2.29.

Even for these excluded systems, though, the minimum wall thickness requirement carried in P1.2.4 still applies. The exclusion narrows which parts of P1 govern, not whether any of it does.

PIPE CLASS / SYSTEM TYPE

This is where we most often see confusion in practice, so it’s worth being direct about it: P1 does not classify pipes. The Class I / II / III system, which drives testing regime, joint type, heat treatment and welding procedure, is defined entirely in UR P2.2, based on the pressure and temperature of the service in question. P1’s wall-thickness and strength criteria apply uniformly to piping covered by classification, regardless of which Class the pipe eventually falls into.

In other words, P1 answers “how thick does this pipe need to be,” while P2 answers “what testing and fabrication regime does this pipe need.” Engineers sizing a pipe under P1 should not expect the Class I/II/III designation to change which formula or table they use. It doesn’t.

RELATED RULES

• Direct sibling: UR P2 (“Rules for piping design, construction and testing”) picks up where P1 leaves off, P2.2 defines the Class I/II/III system, and P2’s remaining sections cover materials, welding, non-destructive testing and connection types that P1 does not address.
• Cross-reference within scope: UR F15 governs the minimum wall thickness for ballast lines running through oil cargo tanks, and Table 3 of P1 explicitly defers to F15’s stricter figure in that specific case rather than allowing the general table value to apply.
• Statutory backdrop: The 1988 Protocol to the International Convention on Load Lines, 1966 underlies the minimum wall thickness requirement of P1.2.4, interpreted through Unified Interpretation LL36 on Regulations 19, 20 and 22 of that Protocol.
• Carve-out reference: The three scope exclusions trace back to the IBC Code, the IGC Code, the IGF Code, and SOLAS II-1/2.29 (the definition of low-flashpoint fuel), which together mark out the piping systems governed by dedicated gas- and chemical-carrier rules rather than by P1’s general strength criteria.

RESTRICTION

Beyond the scope exclusions already noted, P1 leaves a handful of situations to the Classification Society’s own judgment rather than settling them with a fixed number.

Design pressure and design temperature for “special cases” are both explicitly left open for special consideration, which in practice means the engineer needs to build a technical assessment. Pipe sizes above 450 mm nominal fall outside the standard minimum-thickness table and must instead follow a recognized national or international standard. Bilge and ballast lines run through deep tanks are likewise called out for special consideration; And same technical assessment are required wherever the calculated wall thickness assumes zero manufacturing tolerance. 

DESCRIPTION

P1 exists to answer a single, deceptively simple question: how thick does a pipe need to be to survive the pressure, temperature and corrosive environment it will see in service, with enough margin that a class surveyor can sign off on it years into the ship’s life.

The rule builds that answer with more logical and scientific “steps”. A calculated wall thickness comes first, driven by design pressure, outside diameter and a permissible stress value that itself depends on the pipe material and its behavior at temperature, including, for steels operating at elevated temperature, long-term creep and rupture strength rather than just room-temperature yield.

On top of that calculated figure, P1 layers a bending allowance where the pipe is curved, and a corrosion allowance drawn from tables that differ by piping service and material, recognizing that a fuel oil line and a seawater line corrode at very different rates even at identical pressure.

Manufacturing tolerance gets added afterward as a separate correction, because the calculated thickness assumes a perfect pipe and real tooling doesn’t deliver one.

Running alongside this calculated route, P1 also sets a floor: a minimum wall thickness by nominal size and service category, published directly in tabular form, which exists precisely so that thin-walled, low-pressure piping doesn’t get sized down to something that fails from ordinary handling, vibration or incidental mechanical damage long before it would ever fail from internal pressure.

IMPORTANT NOTE

• The scope exclusions for IBC/IGC/IGF and low-flashpoint fuel piping do not exempt those systems from the P1.2.4 minimum wall thickness table.

• Ballast piping through oil cargo tanks must satisfy UR F15’s minimum wall thickness where it is more stringent than P1’s own Table 3 figure; sizing purely off P1 in that specific routing is a common and avoidable oversight in tanker documentation.

• The calculated wall thickness from P1.2.2 needs the separate negative-manufacturing-tolerance correction applied before it’s final , submitting the uncorrected calculated value as the design thickness is a documentation mistake class technician remarks regularly.