
ID	0115
Class	KNOWN-LIMITATION
Severity	2
Status	ready

Models: —

Found: 2026-10-18, feature seabed-contact (plan D1 phase 2, S1; owner ruling 2026-10-18: accepted for now)

Evidence

include/fhsim_environment/seabed/SeabedLaw.h:158-173 (UpliftResistance, UpliftSpeedScale): the uplift of a fully buried body is U u_up(v) with ‘v_up = max(v_reg, U / (c’ ℓ)),c'=upliftDampingLimit= 100 N s/m². It is a viscous resistance to a rising speed, not a hold: at rest it is zero. -include/fhsim_environment/seabed/SeabedLaw.h:135(dampingTimefloor) andSoilForceinSeabedContact.h: the normal force of a body deeper than its height is the capacity R (Kondner saturates), which pushes the body out (up). Nothing in the law is a state: no suction, no consolidation, no stick. -tests/SeabedContact_Test.cppSeabedContact_Sphere.FullyBuriedItIsHeldBackFromPullingOutByTheUpliftResistance,SeabedContact_Cylinder.AFullyBuriedCylinderResistsBeingPulledOut,SeabedContact_ExplicitStepping.*(a body started 2, 10 or 40 diameters deep rises to rest on the bed).
Measured inFEATURE.md‘ (S1): a deeply buried sphere that was held at 6 m depth before is expelled; a bridle element 23 m deep rises at about R/(c’ ℓ), 8 m/s for a 7.5 m, 20 mm warp element (less with water drag).

Effect

A body that is placed or dropped below its own height is pushed out of the soil until it rests with s < D; it does not stay where it was put. Fixtures that start with the cable inside the sediment (TrawlCable_BedRegression) show the expulsion; an anchor that has penetrated deep does not stay embedded. The real soil closes over a buried body and holds it.

Possible fix

A state per contact (Dahl or LuGre friction, or a consolidation/set-up time of the remoulded soil) that holds a buried body; or remove the rebound R at cover H > 0. Either needs state in the law, which the law family does not have (pure functions of pose and velocity) and a model SimObject would have to carry.

Test that would prove it

A sphere placed at 5 D and left at rest in a soil with s_u(0) = 5 kPa stays within a centimetre of where it was put for 60 s, and one pulled out at 1 m/s meets the uplift resistance.

Risk

Adds a state to a stateless interface (SeafloorForcesInterface evaluates by pose and velocity); the numerical constants dampingTime and upliftDampingLimit were chosen for explicit steps at 1 ms (MARE-0397) and would have to be checked again.