SPECIFYING A LARS OPERATING ENVELOPE

Atlas
Jan 11
10 min read

Updated: Jan 12

Sea State, Dynamic Loads, and Thermal Duty Cycles

As of January 1, 2026, SOLAS Regulation II-1/3-13 makes the definition of a Safe Operating Envelope for offshore lifting appliances a statutory obligation, not a contractual agreement. Masters who authorize operations outside documented envelopes now face Port State Control detention and potential criminal liability.

CTEK Offshore orange Launch and Recovery System (LARS) and subsea deployment basket on a vessel deck.

Launch and Recovery Systems (LARS) rarely fail because they lack static strength. They fail because their operating envelope was ambiguously defined, leading to operation outside the system's dynamic capabilities. Offshore, the difference between a successful campaign and a reportable incident often comes down to how well Significant Wave Height (Hs), Dynamic Amplification Factors (DAF), and Thermal Duty Cycles were specified before the steel was cut.

This article explains how to define a defensible, audit-ready operating envelope that satisfies both the new IMO requirements and the physical realities of the splash zone.

This operating envelope specification builds on the broader systems framework for offshore LARS, DDC, ROV, and diving equipment specification, where reliability is determined during design, not discovered during operations.

Understanding LARS: Manned vs. Unmanned Systems

A Launch and Recovery System (LARS) is equipment designed and built in compliance with standard industry practice and regulatory/Classification Society guidelines, primarily used in offshore operations for manned or unmanned recovery operations as a means to safely transfer personnel or equipment to and from the water surface.

Manned LARS

Manned LARS are used for transfer of personnel such as divers (during diving operations) or vessel crews (during emergency escapes). In the offshore industry, the presence of a human immediately upgrades the entire system's compliance requirements to the highest possible level. The system must be designed, tested, and certified to Man-Riding standards, which are significantly more stringent than those for ROV-only systems.

Unmanned LARS

Unmanned LARS are used for transfer of specialized offshore equipment such as ROVs (Remotely Operated Vehicles), survey equipment, and subsea tooling packages. While these systems must still meet rigorous offshore standards, they are not subject to the additional safety factors and redundancy requirements mandated for personnel transfer.

Industry Guidance and Classification Society Standards

There are multiple industry guidelines and compliance requirements applicable to offshore LARS. Understanding this regulatory framework is essential for proper specification.

Industry Guidance (IMCA)

The International Marine Contractors Association (IMCA) provides the de facto global standards for offshore operations:

IMCA D 018: Code of Practice on the Initial and Periodic Examination, Testing and Certification of Diving Plant and Equipment. This is the baseline for any manned system.
IMCA D 023: Design for Surface Oriented (Air) Diving Systems. Provides the "Diving Equipment Systems Inspection Guidance Note."
IMCA R 018: Guidelines for the Selection of ROV Systems. Applicable for the ROV-specific side of LARS operations.

Classification Society Standards

Below are widely applicable IACS (International Association of Classification Societies) rules and standards for LARS:

DNV-ST-0378: Standard for offshore and platform lifting appliances (formerly DNV 2.22). This includes specific chapters on "Lifting of Personnel" and remains the engineering reference for dynamic load calculations.
ABS Guide for Certification of Lifting Appliances: Includes specific requirements for subsea lifting and man-riding operations.
Lloyd's Register Code for Lifting Appliances in a Marine Environment: Addresses structural design, testing, and certification of offshore lifting systems.

These frameworks establish the foundation upon which operating envelopes must be built. Specifications that ignore this dual structure—operational guidance and statutory compliance—introduce avoidable risk and certification delays.

Why LARS Operating Envelopes Are Misunderstood Offshore

An operating envelope is not a single number. It is a multi-dimensional map of constraints. Many LARS specifications define only a Safe Working Load (SWL, the maximum load a system is certified to handle under specified conditions) at a static depth. This is dangerous.

Offshore, the envelope is the intersection of:

Vessel Motion Characteristics (RAOs - Response Amplitude Operators)
Environmental Limits (Wind, Waves, Current)
System Dynamics (Winch speed, AHC response time)

A system rated for 10 tonnes in calm water may be legally restricted to 4 tonnes when Significant Wave Height (Hs) reaches 2.5 meters. If this Derating Curve is not calculated during the design phase using DNV-ST-0378 methodologies, the vessel Master may be forced to suspend operations unexpectedly to remain compliant with the ship's Cargo Gear Book.

When derating curves are not calculated during design using DNV-ST-0378 methodologies, vessel Masters face an impossible choice: suspend operations to remain compliant with the Cargo Gear Book, or continue work and risk Port State Control detention. Both outcomes are specification failures, not operational failures.

Sea State: Significant Wave Height (Hs) vs. Period (Tz)

The Trap of "Hs" Alone

Specifying a LARS for "Hs = 3.0m" is meaningless without defining the Zero Up-Crossing Period (Tz, the average time between successive wave crests). A 3-meter wave with a 6-second period induces vastly different accelerations on the vessel (and the LARS tip) than a 3-meter wave with a 12-second period.

Short-period seas (Tz < 7s) generate high-frequency vessel motion that Active Heave Compensation (AHC, automated winch control that adjusts payout to compensate for vessel motion) systems struggle to track, while long-period swells (Tz > 10s) introduce large amplitude displacements that can exceed AHC stroke limits. Both conditions degrade load control but require different engineering responses.

Resonance and Vessel RAOs

Specifications must require the LARS manufacturer to analyze the Vessel Response Amplitude Operators (RAOs). The LARS design must account for the worst-case vertical acceleration at the specific deck location (e.g., stern vs. side) where the system will be installed.

Engineering Insight: We recommend specifying a Dynamic Factor (DF) that varies with sea state, rather than a fixed multiplier. This allows for a Sea State Derating Chart that maximizes window of opportunity in calm weather while protecting the asset in rough seas.

Related to Hs is the Peak Period (Tp, the period of the most energetic waves in the spectrum), which further refines understanding of the sea state's energy distribution. Specifications that address Hs, Tz, and Tp provide the complete picture needed for dynamic analysis.

The Physics of Splash-Zone Transit: Avoiding "Snap Loads"

The transition through the splash zone is the most critical phase of subsea deployment. It introduces the risk of Snap Loading—a violent phenomenon where the payload's downward velocity, combined with a wave trough, causes the wire to go slack (Slack Wire Regime).

When the wave passes and the vessel heaves up, the wire re-tensions instantaneously. This impact load can generate forces 3x to 5x the static weight, exceeding the safety factors of even the highest-grade high-performance wire ropes such as compacted strand or rotation-resistant constructions.

Wire ropes typically have safety factors of 5:1 under static load. Snap loads of 4x or 5x static weight reduce effective safety margins to 1.25:1 or 1.0:1—well below acceptance criteria. Repeated snap loading causes fatigue failure within hundreds of cycles rather than the thousands expected under controlled conditions.

The Specification Fix

To prevent Snap Loading, specifications must mandate:

Minimum Winch Speed: The winch must be capable of retracting wire faster than the wave heave velocity (typically >60m/min for Hs=2.0m)
High-Speed Constant Tension (CT): A specialized CT mode designed specifically for splash-zone transit that reacts in milliseconds to prevent slack wire
DNV-ST-0378 Dynamic Analysis: A requirement for a dynamic study simulating the payload's specific drag coefficient and added mass during water entry

Side Loading: Defining Off-Lead and Side-Lead Angles

LARS A-frames and gantries are strongest in the vertical plane. However, currents and vessel drift introduce Side-Lead (lateral) and Off-Lead (fore/aft) angles to the wire.

Standard specifications often assume a vertical lift (0°). In reality, currents can drag a diving bell or ROV to angles of 10° or more. If the sheave head, fleeting mechanism, and A-Frame pivot points are not designed for these lateral load vectors, the structure will suffer fatigue cracking or bearing failure.

Requirement: Specify Design Side-Lead and Off-Lead Angles (e.g., ±15°). This forces the manufacturer to reinforce the A-frame legs and upgrade the sheave bearings to handle the resultant lateral forces.

These lateral loads must be traced through the entire load path from sheave head to vessel structure, a process addressed in detail in our guidance on deck integration, foundations, and load path engineering.

DNV-ST-0378 Section 4.3.2 requires structural analysis for off-vertical loading where operational conditions introduce lateral forces. Classification societies typically require verification that structural stresses remain within allowable limits for side-lead angles up to 15°, though operational experience suggests 10° should be the conservative design target for man-riding applications.

Duty Cycle: AHC, Regenerative Power, and Heat Rejection

The "Continuous" Trap

A winch rated for 20 tonnes is not necessarily rated to hold 20 tonnes continuously while compensating for waves. Modern Active Heave Compensation (AHC) winches constantly reverse direction. When paying out to compensate for a vessel rising, the hydraulic motor acts as a pump, forcing flow back through the system and generating heat that must be dissipated through the cooling system.

Thermal Equilibrium in Hot Climates

In regions like the Arabian Gulf (UAE) or Western Australia, where ambient temperatures exceed 45°C, this regenerative energy creates a massive thermal spike.

Failure Mode:The hydraulic oil overheats (>65°C), viscosity drops, lubrication fails, and the system trips on "High Temp."
Specification Fix:Require a Heat Rejection Calculation based on a "Continuous AHC" duty cycle at 50°C ambient. This ensures the cooling system (seawater or air-blast) is sized to maintain Thermal Equilibrium, not just peak load.

Specifications should require manufacturers to provide heat dissipation calculations showing that hydraulic oil temperature remains below 60°C during continuous AHC operation at 50°C ambient for 4 hours. This duration represents a realistic operational window including deployment, work at depth, and recovery. Verifying these thermal performance claims during AHC system commissioning and sea trials ensures that design calculations reflect actual offshore behavior.

Payload Aerodynamics and Hydrodynamics

Two payloads with the same mass behave differently.

High Drag (e.g., ROV Garage):
Creates massive "added mass" resistance during heave, requiring higher winch torque
Low Drag (e.g., Clump Weight):
Falls fast, increasing the risk of slack wire if the winch braking is too aggressive

The operating envelope must define the Hydrodynamic Coefficients (Cd and Ca) of the intended payloads. "Generic" envelopes force offshore teams to guess, leading to operational restrictions.

DNV-ST-0378 requires that payload drag coefficients (Cd) and added mass coefficients (Ca) be determined through model testing, CFD analysis, or conservative empirical correlations. Specifications that omit these parameters force manufacturers to assume worst-case values that add unnecessary weight and cost, or optimistic values that fail to represent real operational behavior.

How Standards Inform Operating Envelope Definition

International frameworks exist because specific failure modes have already occurred.

DNV-ST-0378 (Standard for Offshore and Platform Lifting Appliances):This is the engineering bible. It mandates the calculation of Dynamic Amplification Factors (DAF) based on Significant Wave Height (Hs) and Hoisting Velocity. It replaces the old "safety factor" approach with a physics-based model.

IMCA D060 & LR011:These guidance notes provide the operational context for defining safe limits, particularly regarding umbilical management and excursion limits.

SOLAS Regulation II-1/3-13 (Effective Jan 2026):

This new regulation makes the maintenance of a Certificate of Fitness and a compliant Cargo Gear Book a statutory requirement. Operating outside the defined envelope is now a violation of the vessel's safety certificate.

IACS UR E27:Unified Requirement for cyber resilience in control systems. This requirement prevents unauthorized modification of operating envelope parameters stored in the control system, ensuring that operational limits remain aligned with certified design capabilities throughout the vessel lifecycle.

Design and Specification Considerations

To ensure your LARS is compliant and capable in 2026, specifications must explicitly require:

Material Selection

Toughness: Materials must have documented Charpy V-notch impact testing values, typically at -20°C or the Design Service Temperature (DST, the lowest temperature at which the material is expected to operate). This ensures structural integrity under low-temperature offshore conditions.

Traceability: All primary load-bearing members (the "Load Path") must have 3.1 or 3.2 Material Certificates, providing full traceability from mill test reports through fabrication.

Dynamic Amplification Factor (DAF)

For offshore lifting, the static load must be multiplied by a DAF to account for the vertical motion of the vessel as stated in DNV-ST-0378. This factor varies with sea state and hoisting velocity and must be calculated, not assumed.

Winch Requirements

The winch is the most critical mechanical component. For Manned LARS, all winches must be fitted with man-riding winches meeting Personnel Lifting criteria for braking systems:

Two independent power-off brakes are mandatory. Each brake must be capable of holding 125% of the Safe Working Load (SWL) independently.
Typically, one is a multi-disc hydraulic brake (integrated into the motor/gearbox) and the second is a band or disc brake acting directly on the winch drum.

These redundancy and braking requirements are part of a broader man-riding engineering philosophy that addresses control interlocks, failure response, and emergency recovery modes where personnel safety depends on system behavior during degraded conditions.

Wire Rope Safety Factors

For Unmanned LARS winch wire ropes, a safety factor of 3 to 5 is common. However, for Manned LARS winch wires, DNV-ST-0378 typically requires a minimum Safety Factor of 8 to 10. This substantial increase reflects the unacceptable consequences of wire rope failure during personnel transfer.

Dynamic Load Charts

Requirement: Dynamic Load Charts showing Safe Working Load (SWL) vs. Significant Wave Height (Hs)Why: Static ratings alone do not support operational decision-making or Port State Control compliance

Snap Load Analysis

Requirement: Snap Load Analysis verifying winch speed prevents slack wire in specified sea statesWhy: Unanalyzed splash-zone behavior leads to wire rope failures and payload loss

Thermal Duty Cycle Definition

Requirement: Thermal Duty Cycle Definition with explicit "Continuous AHC" cooling capacity at Max Ambient TemperatureWhy: Undersized cooling forces operational shutdowns in warm climates

Side-Load Ratings

Requirement: Side-Load Ratings with structural approval for off-lead/side-lead angles (typically ±15°) to account for current dragWhy: Unverified lateral loading causes fatigue cracking in A-frame structures

Cyber Resilience

Requirement: Cyber Resilience compliance with IACS UR E27 for the LARS control system (PLC), protecting the envelope parameters from unauthorized modificationWhy: Parameter drift without documentation undermines Cargo Gear Book validity

Payload Hydrodynamic Coefficients

Requirement: Payload Hydrodynamic Coefficients defining drag (Cd) and added mass (Ca) for all intended payloadsWhy: Generic assumptions force conservative operational restrictions or create unanalyzed risk

Practical Takeaways for Engineers and Project Teams

Define the "Derating" Curve:
Don't ask for a "20 Tonne LARS." Ask for a "LARS capable of lifting 20 Tonnes in Hs=2.0m with Tz=8s"

Specify Material Requirements:
Require Charpy V-notch testing at Design Service Temperature and 3.1/3.2 Material Certificates for all load-bearing members

Distinguish Manned vs. Unmanned:
Recognize that man-riding applications require wire rope safety factors of 8-10, not 3-5, and dual independent braking systems

Test for Heat:
During Factory Acceptance Testing (FAT), run the AHC system for 2 hours at maximum ambient temperature to prove the cooling system works. Static lifts do not generate heat—thermal validation requires sustained operational testing

Respect the Splash Zone:
Ensure the "Constant Tension" modes are tuned specifically for the high-speed requirements of water entry/exit with verified winch speeds exceeding wave heave velocity

Audit the Envelope:
Ensure the Cargo Gear Book onboard matches the actual equipment capabilities. Discrepancies here are a primary target for Port State Control inspectors under the new SOLAS regime

Specify Side-Load Angles:
Require structural verification for off-vertical loading to ±10° minimum, ±15° for general handling applications

Demand Hydrodynamic Data:
Require manufacturers to define payload Cd and Ca coefficients or provide the basis for their conservative assumptions

Verify DAF Calculations:
Ensure Dynamic Amplification Factors are calculated per DNV-ST-0378, not assumed from generic tables

Operating Envelopes Are Where Reliability Is Won or Lost

Under SOLAS II-1/3-13, the operating envelope documented in the vessel's Cargo Gear Book is now the legal boundary of authorized operations. Specifications that define envelopes ambiguously or optimistically create compliance gaps that appear during Port State Control inspections, not during engineering reviews. The new regulatory reality makes envelope definition a statutory engineering function, not a contractual optimization exercise.

A well-defined operating envelope gives offshore teams clarity. It defines when operations can proceed, when they must pause, and why. More importantly, it aligns design intent with real operating conditions before equipment arrives on deck.

When operating envelopes are engineered with the same care as structural strength or control logic, LARS performance becomes predictable. That predictability is the foundation of offshore reliability.

This specification approach applies equally to other deck-mounted systems. Related guidance on man-riding redundancy and braking philosophy addresses similar envelope constraints where personnel safety depends on system behavior during degraded conditions.