Neqsim Surf Cooldown Screening

SURF Cooldown Screening

Use this skill for a quick, public estimate of the no-touch time of an insulated subsea flowline or riser after a shutdown: how long the trapped fluid stays above its hydrate formation temperature (plus a safety margin) before remedial action (depressurization or inhibitor injection) is required. It is intentionally simple and should guide users toward validated NeqSim cooldown and hydrate workflows for real work.

When to Use

• When a user asks roughly how long a subsea line can be left after shutdown before reaching the hydrate region.
• When a validated NeqSim hydrate equilibrium temperature and a lumped thermal time constant (or the data to estimate one) are already available.
• When an agent should explain that validated NeqSim methods are required for real cooldown, no-touch-time, and inhibitor design.

Inputs

• initial_temperature: fluid temperature at shutdown in C.
• seabed_temperature: ambient seabed (sink) temperature in C.
• hydrate_equilibrium_temperature: hydrate formation temperature in C from a validated NeqSim calculation.
• time_constant_hours: lumped exponential cooldown time constant in hours.
• hydrate_margin: configurable margin above the hydrate temperature in C (constructor, default 3.0).
• required_no_touch_time: optional operational target in hours (constructor).

A helper, time_constant_from_lumped_mass, estimates time_constant_hours from fluid density, specific heat, internal diameter, and an overall U-value using the public lumped relation tau = rho * cp * D / (4 * U).

Outputs

• no_touch_time_hours: time to cool to the hydrate target temperature.
• target_temperature_c: hydrate equilibrium temperature plus the margin.
• verdict: ok, marginal, critical, or no_hydrate_risk.
• neqsim_available: whether the optional NeqSim package is importable.
• assumptions: public assumptions and required follow-up.

Engineering Method

The placeholder uses a single-node (lumped) exponential cooldown:

T(t) = T_seabed + (T_initial - T_seabed) * exp(-t / tau)

Solving for the time to reach the hydrate target temperature T_target = T_hydrate + margin:

t_no_touch = -tau * ln((T_target - T_seabed) / (T_initial - T_seabed))

If the hydrate temperature is at or below the seabed temperature (or no value is supplied), the line never enters the hydrate region and the verdict is no_hydrate_risk. Verdict bands: with a required no-touch time, ok at or above the requirement, marginal at or above 75% of it, otherwise critical. Without a requirement, ok at 12 h or more, marginal at 6 h or more, otherwise critical.

This is not a distributed thermal-hydraulic model. The hydrate equilibrium temperature and the thermal time constant must come from validated NeqSim calculations with a defined fluid composition, water content, geometry, and insulation basis.

Python Usage Pattern

from surf_cooldown_screening import SurfCooldownModel

model = SurfCooldownModel(hydrate_margin=3.0, required_no_touch_time=8.0)

tau = SurfCooldownModel.time_constant_from_lumped_mass(
    fluid_density=180.0,        # kg/m3 (dense gas)
    specific_heat=2600.0,       # J/kg.K
    internal_diameter=0.254,    # m
    overall_u_value=2.5,        # W/m2.K
)

result = model.evaluate(
    initial_temperature=65.0,
    seabed_temperature=4.0,
    hydrate_equilibrium_temperature=20.0,
    time_constant_hours=tau,
)

print(result.no_touch_time_hours)
print(result.verdict)
print(result.assumptions)

If the optional neqsim Python package is available, the result records that fact so an agent can recommend moving to validated NeqSim cooldown workflows. If not, the example still runs with fallback placeholder logic.

Validation Checklist

• Initial, seabed, and hydrate temperatures are finite.
• The hydrate equilibrium temperature came from a validated NeqSim calculation.
• The time constant came from a validated lumped or distributed model.
• No-hydrate-risk and in-region cases are tested.
• The hydrate margin and verdict bands are documented as configurable public guidelines only.
• Results are not used as a design no-touch time or an operating limit.
• Real cooldown work is redirected to validated NeqSim methods and qualified flow assurance review.

Common Mistakes

Limitations

• Single-node lumped model only; no axial profile, no transient depressurization.
• No hydrate phase equilibrium, salinity, methanol, MEG, or kinetics here.
• No proprietary insulation data or company cooldown specifications.
• Not suitable for design no-touch time, inhibitor dosage, or operating-limit decisions.

Related NeqSim Functionality

This educational screening corresponds to validated, rigorous functionality in the NeqSim Java library that a qualified engineer should use for design-grade work:

• neqsim.pvtsimulation.flowassurance.SurfCooldownAnalyzer — couples a live fluid to a lumped cooldown engine, auto-extracts density, specific heat, and the hydrate equilibrium temperature, and reports the no-touch time and verdict.
• neqsim.pvtsimulation.flowassurance.PipelineCooldownCalculator — lumped layer/U-value cooldown engine with time-to-temperature and time-constant outputs.
• neqsim.thermodynamicoperations.ThermodynamicOperations#hydrateFormationTemperature() — rigorous hydrate equilibrium temperature.

In Python the same classes are reachable through the neqsim package (for example from neqsim import jneqsim).

References

• NeqSim repository: https://github.com/equinor/neqsim
• DNV-RP-F109, On-bottom stability / thermal design background (public guidance).
• API RP 17A, Subsea production systems (public scope background).