0118 — Seabed law: a resting sphere sinks 4 to 6 % of its diameter

ID	0118
Class	KNOWN-LIMITATION
Severity	2
Status	blocked

Models: —

Found: 2026-10-19, plan C1 task 5 (seafloor sanity criteria, feature old-vs-new), default SoilModel and default SoilState

Decision needed: Is 4 to 6 % of the diameter acceptable for a sphere on the default soil, or must the stiffness cap or the sphere capacity change? Physics: owner.

Evidence

• include/fhsim_environment/seabed/SeabedLaw.h:119-125 (ElasticForce, Kondner rule) with SoilModel::stiffnessFactor 200 (SeabedLaw.h:42) and the sphere capacity N_c0 = 6 (SeabedLaw.h:47).
• Failing test: tests/SeabedContact_Sanity_Test.cpp, SeabedSanity_VALIDATION.StaticPenetrationOfTypicalLoadsBelowOnePercentOfDiameter. Criterion (C1-D9): static penetration < 1 % of the diameter.
• Static penetration / diameter at force balance (default soil): sphere D 0.1 m, 35 N: 4.19 %; sphere D 1 m, 5 kN: 5.96 %. The cylinders pass: D 0.1 m, 100 N/m: 0.18 %; D 1 m, 2 kN/m: 0.47 %.
• A sphere has a small contact length (stiffness length D, bearing area of a spherical cap), so the same weight per contact width gives a deeper cap than a cylinder's.

Effect

A resting net node, clump weight or other sphere on the default soil sinks to several percent of its diameter, which also moves its lower end (bed contact point) by centimetres. Cylinders (cables) are fine.

Possible fix

Options: raise stiffnessFactor for spheres, or a different stiffness length for a sphere; or accept and loosen the criterion for spheres. Owner decision.

Test that would prove it

The failing test, with the criterion unchanged. The drop-test, energy and element-length criteria pass (see the test file).

Risk

A stiffer contact needs a shorter explicit step (the damping floor dampingTime is tuned for the present stiffness).