Drydock Critical Period & Docking Stability Tool

Critical Period • Block Reaction • Virtual KG Rise • Effective GM • MCTC / Trim • TPC • Free Surface • Block Bearing

Drydock Critical Period & Docking Stability Tool

Screen the loss of transverse stability as buoyant support transfers to the docking blocks. Simple mode starts from an entered block reaction. Advanced can derive reaction from trim/MCTC or buoyancy loss/TPC, applies free-surface moments, uses a reduced-displacement KM, estimates residual GM and heel under an external moment, and checks critical/final block-bearing pressure against the docking-plan basis.

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Use the actual approved docking calculation for the operation.Simple mode is a critical-period screening calculation using the conventional virtual-loss-of-GM approximation.

ReactionBlock support vs displacement

StabilityVirtual GM loss & residual GM

BlocksAverage contact pressure

Critical-Period Condition

Ship displacementtonnes.

Corrected initial GM before block reactionm, including free-surface correction.

KM at docking conditionm above keel.

Block reaction Ptonnes of support transferred to blocks.

User minimum residual GM criterionm. Enter dockmaster/approved planning criterion.

Active block contact aream² supporting the entered reaction.

Permissible block bearing pressurekPa, user-entered docking-plan/block material limit.

TPC at docking draftt/cm, for equivalent buoyancy-loss draft change.

Ship Hydrostatic / Stability Basis

Entering displacement Wt.

KG before docking reactionm above keel.

KM at entering displacementm.

KM at reduced buoyant displacementm from hydrostatics near W−P.

User minimum residual GM criterionm.

External transverse heeling momentkNm from wind/tugs/other transverse effect, if being screened.

Block Reaction Basis

Reaction calculation basis

Direct block reactiont.

Trim change to blocks overallcm.

MCT 1 cmt·m/cm.

LCF-to-first-contact leverm.

Mean draft / water-level losscm.

TPC at docking draftt/cm.

Free-Surface Moment Register

Enter free-surface moments from the ship’s approved stability data for slack tanks in the docking condition. The model applies ΣFSM to the effective KG under reduced buoyant support.

Use	Tank / group	Free-surface moment t·m

Docking Block Bearing

Active bearing area at critical stagem².

Total bearing area when landedm².

Permissible average bearing pressurekPa, docking-plan / block material basis.

Drydocking Checks

Approved docking / blocking plan checkedKeel, bilge and side blocks match the vessel docking drawing and load-bearing structure. Actual docking condition verifiedDisplacement, drafts, trim, KG and tank condition match the pre-docking calculation. Hydrostatics at reduced displacement verifiedKM, MCTC, TPC and LCF/contact geometry are taken from approved data. Free surfaces minimized / accounted forSlack-tank effects are included and tanks are managed per the docking plan. Trim / dock declivity matchedPlanned first contact and full landing are consistent with the block line. Block pressure / material limits verifiedAllowable pressure and bearing areas match actual block construction and contact. Weather / transverse moment controlledWind, tugs and mooring forces will not compromise the critical-period stability margin. Dockmaster pumping / landing sequence agreedDewatering rate, centerline control and verification of full block contact are coordinated.

Critical boundary: this is a screening tool, not an approved docking analysis. Actual drydocking requires the vessel’s approved docking plan, current hydrostatics/stability data, block arrangement and dockmaster calculation. Block reactions and local hull/block stresses can be highly non-uniform; average bearing pressure is only a gross screen.

Engineering basis: the critical period is the interval after first contact with the blocks while buoyant support is transferring to the dock. A conventional screening approximation gives virtual GM loss ≈ P×KM/W. A more explicit weight-transfer method treats the block reaction as removal of support at the keel, giving virtual KG ≈ (W×KG + ΣFSM)/(W−P), which is compared with KM at the reduced buoyant displacement. When reaction is inferred from trimming, P ≈ MCT1cm × trim change / LCF-to-contact lever. Docking/undocking stability must also account for free surface, actual block contact and the approved docking plan.

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