
ID	0366
Class	KNOWN-LIMITATION
Severity	1
Status	ready

Models: TrawlCable, RundDorg

Found: 2026-10-12, feature wake-receiving P1 review of a237d92 (owner ruling A1 of 2026-10-12: file, do not fix)

Related: owner ruling R110 (MARE-0313); MARE-0314 (added mass, which names the door and the clump weight as not included); MARE-0365 (wet fraction).

Evidence

src/trawl_mooring_interaction/CableSegmentWaterForce.h:177-207 (DoorForce) and :218-236 (ClumpWeightForce) take only the water velocity: weight less buoyancy, drag (and lift for the door), a damping moment. No wave particle acceleration, no Froude-Krylov force, no added mass; the doc comments say so (:170, :211). The door and clump bodies are plain rigid bodies; only the cable elements are AddedMassRigidElements.
The cable elements do it instead (InWaterForce, :99-110): the Morison inertia force (1 + Ca) rho f V a_n normal to the axis from the mean wave acceleration of the ends (body_submergence::MeanWaveAcceleration, cylinder_hydro::MorisonInertiaForce, R104 extending R67, MARE-0314), and the matching added mass Ca rho V f in the element's inertia (AddedMassRigidElement.h:59, Ca = 1).

Effect

In waves, a door or a clump weight near the surface feels the wave orbital velocity through its drag only: no inertia excitation and no added mass in its dynamics, while the cable elements on either side have both. At the depths the TrawlCable and RundDorg pages describe (R110) the wave acceleration is small and the effect with it. The missing added mass also matters without waves: a clump weight or door accelerated by the rig (shooting, hauling, snap loads) is lighter than it should be (a steel clump: rho/rho_steel = 13 %; a door: the added mass of a plate normal to itself is large compared with its mass).

Possible fix

Clump weight: the cylinder law of the elements, cylinder_hydro::InertiaForceFactor with Ca = 1 normal to the axis, and make the body an AddedMassRigidElement.
Door: a plate, added mass normal to the plate (rho pi/4 · c² · span for a flat plate per unit span, DNV-RP-C205 Table A-2) and Froude-Krylov rho V a over its displaced volume. Needs a coefficient choice (owner).

Test that would prove it

TrawlCableWater / CableWaves: a clump weight in a regular wave takes (1 + Ca) rho V a_n; its inertia carries Ca rho V normal to its axis. A door takes the Froude-Krylov force rho V a and the plate added mass normal to Ny.

Risk

Moves results in waves and in transients for every rig with a door or clump weight.