ARTICLES

Critical line selection list

Due to the fact that only 35-50% of all piping systems in the North Sea can be documented to meet the piping codes requirement to pipe stress and flexibility analysis, DNV has chosen to slightly modify and rename a typical traditional critical line selection list taken from the NORSOK standard L-002, Rev. 2, 1997, that has been used in the North Sea for a decade, to pipe stress priority piping. A few items have been added to the original NORSOK standard L-002, Rev. 2, 1997 critical line selection criteria. This implies that piping belonging to one or several of the below listed criteria should always be given priority in the pre-, basic- and detail engineering phases of the project. Piping that is sorted out based on the selection criteria for pipe stress priority piping should be listed in a pipe stress critical line report.

Those lines that do not fall into these criteria should however, in some way, be documented to meet the code requirement to sufficient pipe stress flexibility. Such documentation should always be finished and reviewed by a stress engineer before oil-and gas production or start-up takes place. The pipe stress priority piping selection criteria are listed below.

Critical lines - pipe stress priority piping

The following selection criteria apply:

a) All lines at design temperature above 180°C.

b) 4" NPS and larger at design temperature above 130°C.

c) 16" NPS and larger at design temperature above 105°C.

d) All lines with design temp. below -30°C and where the largest possible ΔT >190°C.

e) Lines 4”and larger with design temp. below -30°C and where the largest possible ΔT >140°C.

f) Lines 16” and larger with design temp. below -30°C and where the largest possible ΔT >115°C.

Note:

The ΔT temperatures in d), e) and f) above are based on a design temperature of 30°C above maximum operating temperature. When the maximum design temperature is defined to equal the maximum operating temperature then the ΔT values above should be reduced by 30°C.

g) Lines 3” NPS and larger with a wall thickness larger than 10% of the outside pipe diameter. (Typical are water injection piping and high pressure API piping used in drilling units).

h) Thin walled piping of 20” NPS and larger with wall thickness less than 1% of the outside pipe diameter (typical is gas turbine power generator exhaust piping).

i) All lines 3" NPS and larger connected to sensitive equipment such as rotating equipment. However, lubrication oil lines, cooling medium lines etc. for such equipment shall not be selected due to this item.

j) All piping expected to be subjected to vibration due to internal and external loads such as pressure transients, slugging, vortex shedding induced oscillations, high gas velocities and herby acoustic vibrations of the pipe wall membrane.

k) All piping connected to pressure relief valves and rupture discs.

l) All blow-down piping 2" NPS and larger excluding drains.

m) All piping along the flare tower.

n) All piping above 3" NPS likely to be affected by movement of connecting equipment or by structural deflection.

o) GRE piping 3" NPS and larger.

p) All piping 3" NPS and larger subject to steam out.

q) Long vertical lines (typical 20 meters and higher).

r) All production and injection manifolds with connecting piping.

s) The ring-main firewater line including the deluge headers and all hydrocarbon lines containing oil and gas if the installation is going to be designed for a safe shut-down after an accidental design blast/explosion.

Any other lines defined in the project DAL specification or similar project document to be intact after an explosion. (Check national regulations and any relevant class rules such as DNV-OS-A101).

t) Lines falling into category III according to PED for installations going to be placed in Europe.

u) Other lines requested by the owner, class society, the project or responsible pipe stress analyst to be critical.