DNV-RP-N103: Modelling and Analysis of Marine Operations
DNV-RP-N103 is the recommended practice document underpinning the design calculations required by DNV-ST-N001. It defines how engineers model the hydrodynamic forces on lifted payloads: drag, added mass, slamming, splash-zone forces and snap-load assessment. It is, in practical terms, the reference document every offshore lifting calculation cites — including the calculations that size heave compensators.
Practical application: For practical application of this topic, see engineering studies and analysis and product / system design.
What RP-N103 covers
RP-N103 provides:
- Hydrodynamic coefficients for typical payload geometries (drag CD, added-mass CA)
- Methods for calculating hydrodynamic forces in the splash zone and below
- Snap-load assessment for lifting operations where wire slack is possible
- Modelling guidance for time-domain and frequency-domain analyses
- Recommended approaches for irregular sea states
It is technical reference material — design engineers, marine warranty surveyors and equipment vendors all use it.
Drag forces on offshore lifts
Drag is the velocity-squared force a payload experiences when moving through water:
FD = ½ ρw CD A⊥ v²
where ρw is water density (~1025 kg/m³), CD is the drag coefficient, A⊥ is the area perpendicular to motion, and v is velocity. RP-N103 provides typical drag coefficients:
- Sphere: CD ≈ 0.5
- Horizontal cylinder (cross-flow): CD ≈ 1.2
- Vertical cylinder (axial flow): CD ≈ 0.9–1.1 depending on aspect ratio
- Cube (face normal to flow): CD ≈ 1.05
- Rectangular plate (normal to flow): CD ≈ 1.10 + 0.02(L/W + W/L)
For irregular shapes (typical of subsea structures) bounding boxes give conservative results. CFD or model testing is the alternative.
Worked drag examples are in Subsea Lifts.
Added mass — accelerating water
When a payload accelerates underwater it must also accelerate the surrounding water mass — the “added mass” or “virtual mass”. This adds an inertial force:
FA = mA × ä
where mA = ρw CA VR is the added mass and ä is acceleration. RP-N103 provides added-mass coefficients:
- Sphere: CA = 0.5
- Cylinder (axial motion): CA = 0.62–1.00 depending on aspect ratio
- Rectangular plate: CA = 0.58–1.00 depending on aspect ratio
- Disc: CA = 2/π ≈ 0.64
For subsea lifts close to the seabed, added mass increases due to confinement — an important effect in landing operations.
Slamming and splash zone
When a payload first enters the water, slamming forces dominate. RP-N103 provides:
- Slamming coefficient Cs for typical geometries
- Time-history of slamming during partial submergence
- Methodology for combining slamming with quasi-static buoyancy
The splash zone is the worst phase of any subsea lift. See Splash Zone Crossing for the operational implications and how a compensator manages it.
Snap loads
A snap load occurs when a partially-submerged payload momentarily becomes weightless (relative motion exceeds free fall) and the wire goes slack — followed by sudden re-tensioning. The peak snap load can far exceed the static weight of the payload.
RP-N103 provides:
- Criteria for snap-load risk assessment (when can it happen?)
- Methods for calculating peak snap load when it does
- Mitigation guidance — primarily heave compensation and shock absorption
A well-designed PHC eliminates snap-load risk by maintaining tension throughout the wave cycle — its primary safety function in addition to motion reduction.
Combined hydrodynamic analysis
RP-N103 specifies combined analysis approaches:
- Quasi-static — used for slow operations and rough sizing
- Time-domain — full wave-by-wave simulation, used for critical lifts and snap-load assessment
- Frequency-domain — RAO-based analysis, useful for narrow-band sea states
- Spectral — combining vessel response and payload response in irregular seas
Tools commonly used: OrcaFlex (time-domain), in-house spectral codes, and dedicated lift-analysis software.
Norwegian Dynamics maintains tools that combine RP-N103 hydrodynamic analysis with our compensator dynamic models — useful for case-specific sizing.
How RP-N103 feeds compensator sizing
For a given lift case, the RP-N103 calculation produces:
- Maximum payload force (gravity + drag + added mass + slamming)
- Force time-history during the critical phases (splash zone, free-hanging, landing)
- Snap-load risk assessment
Compensator sizing then uses these inputs to:
- Determine required stroke (must accommodate the calculated relative motion)
- Determine spring stiffness (must match the natural-period requirement vs sea state)
- Determine damping (must dissipate calculated energy)
- Verify load capacity at peak combined load
This is the engineering loop that turns a customer’s lift case into a recommended compensator. Norwegian Dynamics uses RP-N103 inputs in our standard sizing workflow. For the math behind passive compensation efficiency, see Passive Heave Compensation Basics.
Related on Norwegian Dynamics
- DNV Standards for Offshore Lifting — The pillar overview that frames RP-N103 alongside ST-0378 and ST-N001.
- Subsea Lifts — Worked examples using RP-N103 drag and added-mass coefficient tables.
- Passive Heave Compensation Basics — PHC efficiency math built on RP-N103 hydrodynamic inputs.
- Dynamic Amplification Factor (DAF) — How RP-N103 inputs combine into the DAF a compensator must reduce.
Working on a lift that needs this?
We use RP-N103 inputs in our standard sizing workflow — drag, added mass, snap-load assessment, time-domain combined analysis. Send your lift case and we’ll come back with a recommended compensator and the supporting calculation summary.