A lifting arrangement is not just a diagram. It is an engineering package, and every element in it, from the padeye geometry to the shackle selection to the rigging angle, has a direct consequence on whether the lift proceeds or the Marine Warranty Surveyor sends your documentation back. This article walks through the key checks that matter most and explains why each one can make or break your approval.
Offshore and subsea lifts are governed primarily by DNV-ST-N001 (Marine Operations and Marine Warranty) and in the US market frequently also by API RP 2D (Operation and Maintenance of Offshore Cranes). Both standards share the same fundamental logic: the lifting arrangement must be designed to carry not just the static weight, but a set of amplified loads that reflect the reality of crane operations in a dynamic marine environment. Getting this right is the entire job.
As lifting calculations are not straightforward, Grispen has created some applications to guide you through the various steps and check that your lifting arrangement is valid. These tools walk you through load build-up, sling angle checks, padeye assessment, and rigging certification requirements — reducing errors and saving review time.
Visit our Tools page to explore the applications and ask for a demo to see how they can support your project.
Request a DemoThe Weight and Its Multipliers
Every lifting calculation begins with the dry weight in air, but that number is never the design load. Before you write down a single structural check, you need to build up the total applied load using the correct factors. Under DNV-ST-N001, the primary load multipliers are:
- Dynamic Amplification Factor (DAF): accounts for dynamic effects from crane hook motion, vessel heave, wave action. Under DNV-ST-N001, DAF depends on crane type (offshore pedestal crane vs. shear-leg vs. crane vessel), lift point height above water, and the significant wave height for the operating window. Values typically range from 1.10 for sheltered water operations to 1.30 or higher for open-sea lifts. DAF is not a number you choose; it is derived from the standard and the operational metocean criteria, and reviewers will check your derivation.
- Weight Contingency Factor (WCF): added to cover uncertainty in the actual mass of the object. For fabricated items with good documentation, WCF is typically 1.03–1.05. For estimates or early-FEED objects, higher. The WCF must be explicitly justified: "assumed 5%" is not enough; state what the weight certificate or calculation basis is.
- Consequence Class and Method Class: these DNV parameters set the overall safety factor. CC2/MC1 is the most common combination for subsea equipment, giving a design factor of 1.3 applied to the characteristic load. Higher consequence classes (personnel risk, environmental risk, high replacement cost) increase this factor further.
- Skew Load Factor (SKL): applies when more than two sling legs are used and the load distribution between them cannot be precisely controlled. For four-leg arrangements, a skew factor of 1.25 is standard, meaning each leg must be sized to carry 125% of its nominal share. Reviewers will reject a four-leg calculation that ignores SKL.
The interaction between these factors is multiplicative. A lift with a 20t dry weight, WCF 1.05, DAF 1.20, and CC2/MC1 design factor 1.30, with a two-leg bridle, does not produce a design sling load of 20t. It produces a design load significantly above that, so every member, fitting, and termination in the lifting arrangement must be rated to that design load, not the nameplate weight.
"Reviewers reject packages where DAF was chosen without reference to the wave period and crane stiffness, or where the skew load factor was simply omitted from a four-leg arrangement."
Grispen Australia, Offshore Structural Engineering TeamPadeye Design: The Most Scrutinised Component
The padeye is the structural interface between the lifted object and the rigging. It is the component most often under-designed in early packages and most frequently redlined by reviewers. A complete padeye check covers five distinct failure modes.
- Plate bearing stress: the contact pressure between the shackle pin and the padeye hole. The pin diameter must be closely matched to the hole diameter (typically within 3mm), and the bearing area must be sufficient to keep contact stresses within allowable limits. Over-large holes relative to the pin are a common source of rejection.
- Shear-out and tear-out (double shear): the shear planes above and to the sides of the hole must carry the vertical and horizontal components of the sling load without shear failure. This check is sensitive to the geometry of the main plate and the out-of-plane angle of the applied load.
- Tension in the cheek plates: where cheek plates are welded to the main plate to increase local thickness, their weld must be designed for the out-of-plane bending that arises from sling angle. Reviewers check that cheek plates are not merely "tacked on" to improve appearance on a drawing.
- Bending in the main plate: the padeye main plate acts as a short cantilever under combined in-plane and out-of-plane loading. The out-of-plane component comes from the sling angle relative to the plate plane; this is particularly important in four-leg arrangements where the slings do not all pull in the same direction.
- Weld to parent structure: the padeye-to-structure connection must carry the full padeye load into the parent steel without yielding the weld group. This weld check must account for all six load components (Fx, Fy, Fz, Mx, My, Mz) arising from the three-dimensional sling geometry.
Beyond the structural checks, padeye material specification matters. Offshore padeyes are routinely specified in S355J2 (structural) or S355G10+M (offshore grade with improved through-thickness properties) for European projects, or A572 Grade 50 / A36 for US market work. The material selection must match the Charpy impact requirements of the standard and the minimum operating temperature; reviewers will flag a padeye designed for ambient temperature on a project with sub-zero operational exposure.
The Out-of-Plane Angle Problem
Most padeye calculations in standard packages assume the sling pulls in-plane, i.e. that the sling axis lies in the plane of the padeye main plate. In reality, rigging geometry almost always produces some out-of-plane angle, particularly in multi-leg arrangements where slings converge to a single hook or spreader. DNV-ST-N001 requires an explicit check of the out-of-plane bending moment on the padeye, and the design must either limit this angle (typically to a maximum of 5° without penalty, rising to 10° with reduced capacity factors) or the check must explicitly account for the full combined loading. Packages that assume 0° out-of-plane loading but show a plan view drawing where the sling clearly runs at an angle will be rejected.
Rigging Selection and Certification
The rigging, including slings, shackles, swivels, master links and spreader bars, must be selected and documented to the same standard of rigour as the structural steel. This section covers the most common sources of rejection.
- Working Load Limit (WLL) vs. Minimum Breaking Load (MBL): the distinction matters. The WLL is the certified safe working load with an implied factor of safety already embedded. The MBL is the actual failure load. Under DNV-ST-N001, rigging must be selected such that the WLL of each component is not exceeded under the design sling load (DAF × WCF × CC-factor applied). Do not use MBL with a manual factor applied; use catalogue WLL and verify that the design load does not exceed it.
- Sling construction: round slings, wire rope slings, and chain slings have different ratings, stretch characteristics, and sensitivity to heat, chemicals, and abrasion. For subsea lifts, wire rope slings with proper end terminations (spelter sockets or swaged ferrule terminations, not hand-spliced) are standard. Round synthetic slings should not be used for lifts where the sling passes through subsea splash zones unless explicitly rated for that service.
- Shackle pin orientation: this is a detail that experienced reviewers check and inexperienced engineers miss. Shackle pins must be correctly oriented so that the applied load bears on the bow, not the pin. Reversed pin orientation introduces bending into the pin that is not accounted for in the WLL and can cause premature failure. The drawing must show shackle orientation explicitly and the calculation must confirm it.
- Third-party certification: all rigging components must be accompanied by current certificates of conformity to the applicable rigging standards (DNV, Bureau Veritas, Lloyds) traceable to heat number and batch. Certificates must be dated within the required validity period. A calculation that shows the right load numbers but references rigging without traceable documentation will not receive MWS approval.
- Traceability and colour coding: for offshore crane lifts, rigging items must be colour-coded or tagged per the operating installation's rigging management system, with inspection dates current at the time of the lift. This is an operational requirement beyond the design package, but it must be addressed in the method statement if the lifting package includes one.
Sling Angles and Centre of Gravity
The geometry of the rigging arrangement, specifically the sling angles relative to vertical, has a direct and significant effect on sling tensions. This is one of the most frequently mis-stated items in lifting packages reviewed by Marine Warranty Surveyors.
| Sling Angle from Vertical | Load Factor per Sling Leg | Implication |
|---|---|---|
| 0° (vertical) | 1.00 × (W/n) | Ideal; no horizontal component |
| 15° | 1.04 × (W/n) | Negligible penalty |
| 30° | 1.15 × (W/n) | 15% amplification, must be designed for |
| 45° | 1.41 × (W/n) | 41% amplification, significant effect |
| 60° | 2.00 × (W/n) | Double load per leg, high-stress arrangement |
DNV-ST-N001 sets a maximum permissible sling angle from vertical (typically 45°, sometimes 60° with specific justification). Packages that show sling angles approaching or exceeding these limits without explicit calculation of the resulting tension and check of all components against those tensions are rejected. The sling angle must be calculated from the actual hook-to-padeye geometry shown in the drawing, not assumed as a conservative value and then left unchecked.
Centre of Gravity and Trim
The vertical position of the centre of gravity (CoG) must be established and documented. For objects with a CoG above the lifting points, the arrangement is unstable and requires a spreader frame or multiple independent lift points at different elevations. For objects with a low CoG, the main risk is excessive sling angle if the lift points are widely spaced. The CoG calculation must include all appurtenances, installed equipment, trapped fluid in cavities, and any asymmetric mass distribution. This is another area where early-FEED calculations routinely understate the actual CoG position and produce lifting arrangements that do not work in the field.
Horizontal CoG offset from the centroid of the lift points produces unequal sling loads. If the offset is significant, the calculation must distribute the loads between sling legs accordingly and verify that no individual leg is overloaded. A maximum allowable CoG offset is sometimes specified in the project design basis; check this before producing sling tension tables.
Spreader Bars and Lifting Frames
When the geometry of the object does not permit a direct bridle arrangement, or when sling angles would be excessive, a spreader bar or lifting frame is introduced. These components require their own structural design package and are governed by the same DNV-ST-N001 framework.
- Spreader bar compression check: a spreader bar in a four-leg arrangement carries a compressive load from the inward components of the sling tensions. The bar must be checked for column buckling under this compressive load, with the appropriate effective length derived from the end conditions (pinned-pinned in most arrangements). This is frequently missing from early packages.
- Spreader frame bending: for framed spreaders, the bending moment at each node from the combination of vertical sling loads and horizontal thrust from inward-angled slings must be resolved as a full 3D frame analysis. Point-load checks on individual members are not sufficient.
- Pad eye locations on the spreader: the spreader's own padeyes must go through the same five-mode check as the object padeyes. Packages that detail the object padeyes carefully but skip the spreader padeyes are rejected.
- Weight of the spreader in the load build-up: the spreader's self-weight must be included in the hook load. If the spreader is installed first and the object attached below, this affects the order-of-operations in the method statement as well as the load calculation.
- Spreader bar length and sling clearance: the drawing must demonstrate that the slings clear the object at all points in the lift and that the geometry is physically realisable with the available crane hook-to-spreader clearance.
Local Structure Under the Padeye
A padeye that passes all five of its own checks can still fail the review if the structure it is welded to is inadequate to carry the load into the primary frame. This is the "local structure" check, and it is the area where Finite Element Analysis (FEA) adds the most value relative to hand calculations.
The load path from padeye to primary structure must be traced through every intermediate element, including gussets, doubler plates, stiffeners and diaphragms, and each must be verified against yield and stability under the design load. For complex geometry, FEA using STAAD.Pro or ANSYS Mechanical allows the full three-dimensional stress state to be evaluated, including stress concentrations at weld toes, cutouts, and abrupt section changes that hand methods cannot resolve accurately.
Key checks in the local structure assessment:
What the Calculation Package Must Contain
A lifting arrangement deliverable submitted to a Marine Warranty Surveyor for Certificate of Approval must contain a specific set of documents. Missing any one of them will produce a hold that delays the lift. The standard package comprises:
- Lifting study calculation report: covering all load build-up, padeye checks, rigging selection, sling tension tables for all relevant geometries, spreader analysis if applicable, and local structure checks. Produced in Mathcad Prime or equivalent traceable calculation tool, with all inputs, intermediate results, and outputs clearly identified and referenced to the governing standard clause.
- Rigging arrangement drawing: plan view and elevation showing all lift points, padeye details with dimensions and material, sling routing, shackle locations and orientation, hook approach, and any spreader or lifting beam. Angles explicitly dimensioned. CoG position marked. Scale or dimensions sufficient for the MWS to verify geometry from the drawing alone.
- Rigging list or bill of materials: tabulated list of every rigging component with item number, description, WLL, certificate number, certification body, and current validity date.
- Metocean criteria and operational limits: the environmental basis for the DAF and the limiting significant wave height, wind speed, and current for the operation. Derived from the project metocean report or from the standard's operational weather criteria tables.
- Risk assessment or HAZID reference: for lifts above a threshold weight (typically 5t or as specified by the installation's lifting management procedure), a formal risk assessment is required. This is distinct from the engineering package but is submitted alongside it.
Why Packages Get Sent Back
Based on review of lifting packages across projects for Chevron, TotalEnergies, SAIPEM and other major operators, the most frequent reasons for rejection at MWS review are:
| Rejection Reason | Root Cause | Prevention |
|---|---|---|
| DAF not justified against wave period and crane data | DAF picked from a table without derivation | Use the DAF calculation method in DNV-ST-N001 Sec. 4, not just the minimum tabular value |
| SKL omitted from four-leg arrangement | Engineer unaware of requirement | All legs in 4-leg arrangements must be designed to 1.25× nominal share |
| Padeye out-of-plane angle assumed zero | Geometry not modelled in 3D | Calculate actual out-of-plane angle from 3D rigging geometry drawing |
| Shackle pin orientation not shown | Treated as a detail, not an engineering item | Show shackle orientation explicitly on the rigging drawing with a note |
| Rigging certificates expired or missing | Procurement/logistics gap | Check certificate validity dates before submitting the package |
| Spreader bar compression check missing | Spreader treated as a "simple" component | All spreader members must be checked for combined axial and bending |
| Local structure not assessed | Package stops at padeye without tracing load path | Extend analysis to primary structure; use FEA for complex geometry |
| CoG position not verified against fabrication documentation | Weight estimate used instead of certified weight | Use the weight and CoG from the completed weight control report |
When to Use FEA
Not every lifting arrangement requires Finite Element Analysis. A straightforward subsea frame with standard padeyes, two-leg bridle, and simple load path can be fully assessed by hand calculation in a fraction of the time. But FEA adds genuine value, and is sometimes required, in the following scenarios:
- Complex padeye geometry: non-standard plate thicknesses, unusual hole sizes, padeyes welded to curved surfaces (pipes, pressure vessels), or padeyes in areas of geometric discontinuity where hand methods cannot resolve the stress state.
- Thin-walled parent structures: lifting attachments on thin-walled pressure boundary components, manifold risers, or structural frames with plate panels below 12mm where punch-through and plate bending govern over yield of the weld.
- Multi-point lifts with statically indeterminate load distribution: where more than three lift points are used and the load distribution depends on the stiffness of both the object and the rigging, FEA of the complete system is the only way to obtain reliable sling tensions.
- High-value or critical lifts: where the consequence of failure is severe (personnel risk, environmental release, non-replaceable equipment), operators and MWS agents commonly require FEA confirmation of hand calculations even where hand methods are technically sufficient.
- Post-incident review: when a lift has experienced an incident and the root cause needs to be established, FEA is the standard tool for back-calculating the loads and identifying where and why the failure occurred.
Grispen Australia and Grispen Technologies produce FEA using STAAD.Pro and ANSYS Mechanical. Our models capture the full three-dimensional geometry of the padeye and local structure, applied loads from the complete rigging arrangement, and weld stress linearisation in accordance with DNV-RP-C203 hot-spot stress methodology. Results are presented in a format that meets MWS review requirements, with contour plots, stress summaries, and direct mapping to the code acceptance criteria.
Run a First-Pass Screening Before You Start the Full Study
Grispen Australia has developed a browser-based lifting screening application as part of its GATE suite of engineering tools. The tool runs a rapid first-pass assessment of your lift configuration against DNV-ST-N001 and API RP 2D criteria: enter your lift weight, rigging geometry, environmental class, and padeye dimensions, and the tool returns an instant compliance overview covering sling tensions, DAF, dynamic loads, and structural utilisation checks for the padeye and rigging arrangement.
The tool is not a replacement for a certified lifting study submitted to an MWS, but it is designed to flag early whether a configuration has obvious problems before engineering time is spent on a full calculation package. It is useful for:
- Pre-FEED feasibility checks on lift weight and rigging geometry before padeye locations are fixed on the structure.
- Quick comparison of two-leg bridle vs. four-leg arrangements to assess the SKL and sling angle implications of each.
- Internal design reviews where the project team wants a fast sanity check before formal submission.
- Owner's Engineer reviews of contractor-proposed rigging arrangements.
Rapid DNV-ST-N001 lifting compliance check in your browser. Request a demo to see the tool applied to your lift configuration.
Grispen Australia delivers certified offshore lifting and rigging design packages to DNV-ST-N001 and API RP 2D. Our lifting studies have been accepted by Marine Warranty Surveyors from DNV, Bureau Veritas, and Lloyd's Register on more than 50 projects for clients including Chevron, TotalEnergies, BP, SAIPEM and Technip FMC. We produce calculations in Mathcad Prime and ANSYS Mechanical and work directly with your project team or MWS from kick-off through certificate issue.
Based in Perth, Western Australia. Rapid mobilisation for pre-mobilisation lifting reviews, MWS comment resolution, and value-engineering of over-designed rigging arrangements.
Summary
A lifting arrangement that passes MWS review the first time is not the result of following a template. It is the result of understanding what each check is trying to achieve, and making sure nothing in the package contradicts physics or the governing standard. The weight multipliers must reflect the actual operating conditions. The padeye must be checked through all five failure modes. The rigging must be selected against WLL, not MBL. The sling angles must be calculated from actual geometry. The local structure under the padeye must be traced to primary steel. And the documentation must be complete before submission.
If your project has a lift coming up and your package is not in that state, Grispen Australia can review, complete, or produce it from scratch in the timeframe your offshore window requires.
