Marine Cargo Securing and Lashing Systems

The forces on cargo during a loaded ocean passage are not trivial. A 30-tonne container stowed on deck amidships can experience a transverse inertia load of 15 tonnes or more when a 100-metre vessel rolls in confused seas. That load has to go somewhere: into the lashing rods, the twistlocks, the cell guides, the deck fittings, and ultimately the ship’s structure. When any link in that chain is incorrectly sized, improperly installed, or not checked during the voyage, cargo shifts. Cargo shifting has sunk ships, killed crew, and caused hundreds of millions of dollars in property damage across a series of incidents from the loss of the P&OSL Provence through to overside container losses on post-Panamax containerships in North Pacific winter weather.

The international framework that governs cargo securing sits across three primary instruments: SOLAS Chapter VI (Carriage of Cargoes), the IMO Code of Safe Practice for Cargo Stowage and Securing (CSS Code), and the IMO/ILO/UNECE Code of Practice for Packing of Cargo Transport Units (CTU Code, MSC.1/Circ.1497). The Cargo Securing Manual required by SOLAS VI/5 is the vessel-level document where those instruments converge into ship-specific approved arrangements. This article covers the regulatory hierarchy, the engineering method for lashing force design, securing equipment certification, the container and RoRo securing disciplines, and the operational inspection duties that keep the system working during the passage. The container lashing force calculator implements the CSS Code Annex 13 balance-of-forces method described below.

SOLAS Chapter VI: the statutory foundation

SOLAS Chapter VI has carried the carriage-of-cargoes obligations since the 1974 convention, but its regulatory weight has grown substantially through amendments. Regulation 5 of SOLAS VI, headed “Stowage and Securing,” is the direct statutory hook for cargo securing. It requires that all cargoes, cargo units, and cargo transport units be loaded, stowed, and secured throughout the voyage in accordance with the ship’s Cargo Securing Manual approved by the Administration. The operative phrase is “approved by the Administration”: flag state endorsement of the CSM is mandatory, not optional, and a CSM not submitted for approval is a SOLAS deficiency.

The 2016 VGM amendment stands alongside Regulation 5 as the most operationally consequential change to Chapter VI in recent decades. SOLAS VI Regulation 2, as amended by Resolution MSC.380(94) accepted on 1 January 2016 and entering force on 1 July 2016, made verification of a packed container’s gross mass a precondition for loading. Two methods are permitted. Method 1 is weighing the packed container on calibrated and certified equipment. Method 2 is weighing all packages and cargo items individually, including dunnage, pallets, and other packing material, and adding the container tare mass to the sum to produce a total gross mass. The associated guidelines in MSC.1/Circ.1475 require the shipper to provide the VGM to the master and to the terminal representative in advance of loading, with enough lead time for use in stowage planning. An inaccurate container mass distorts stack load calculations, lashing adequacy assessments, and ship stability: a container declared 14 tonnes but actually 24 tonnes can place a lashing rod into its failure zone before the ship has left the anchorage.

SOLAS Chapter VII (Carriage of Dangerous Goods) feeds into the cargo securing obligation for dangerous goods consignments. The IMDG Code and CTU Code both specify securing requirements for dangerous cargo transport units, and those requirements are incorporated by reference into the vessel’s CSM chapter on dangerous goods.

Chapter VI Regulation 6 covers the safe loading of bulk carriers, a distinct obligation from general cargo securing, but the same Administration-approval requirement for associated documentation applies.

The Cargo Securing Manual: structure and approval

The Cargo Securing Manual is the operational core of the cargo securing framework. MSC.1/Circ.1353/Rev.2, issued on 7 December 2020 and superseding Rev.1, sets out the revised guidelines for CSM preparation. The document is vessel-specific: a CSM prepared for one ship cannot be transplanted to a sister vessel without adapting the ship-specific data on deck fitting locations, stowage positions, and approved stack weights.

Under MSC.1/Circ.1353/Rev.2, a CSM covers five functional areas. Chapter 1 is the preamble: definitions, the regulatory basis, and scope. Chapter 2 covers the securing devices and arrangements, with a full catalogue of all permanent and portable securing equipment on board, their certified MSL or SWL, their condition requirements, and the inspection procedures. Chapter 3 addresses the principles of securing, including the acceleration data applicable to the ship, the force calculation method (referencing CSS Code Annex 13), and approved arrangement tables for each cargo type the vessel carries. Chapter 4 provides the instructions for the specific cargo types carried, including worked examples and pre-calculated tables where available. Chapter 5, a requirement added in Rev.2 for containerships and certain other vessel types, is the Cargo Safe Access Plan (CSAP): a set of plans and procedures defining how crew safely access container stacks for lashing, unlashing, and inspection without creating fall-from-height or entrapment hazards.

The CSM must be approved by the flag Administration before the vessel enters service carrying cargo. In practice, most Administrations delegate the review and approval function to a Recognized Organization (class society). Lloyd’s Register, DNV, Bureau Veritas, ABS, ClassNK, and the other major societies all operate CSM approval services, and the society’s stamp on the CSM is accepted as Administration approval under SOLAS. Approval covers both the document and any subsequent revisions that change approved arrangements, add new cargo types, or modify equipment lists. A minor editorial correction to the CSM does not require fresh approval; a change to a maximum permitted cargo weight at a specific location does.

The master is responsible for ensuring that cargo is actually secured in accordance with the approved CSM during each loading operation. Port state control officers routinely check CSM availability, its approval endorsement, and whether the crew can demonstrate understanding of its contents. A CSM that is on board but that no officer has read is not a meaningful safety instrument.

The CSS Code and its Annexes

The Code of Safe Practice for Cargo Stowage and Securing has been the engineering reference behind cargo securing since its original adoption by IMO as MSC/Circ.745. The current consolidated edition incorporates amendments through MSC.1/Circ.1623 of 7 December 2020. The CSS Code’s main body addresses general principles: stowage quality, dunnaging, and the categories of cargo for which additional specific guidance applies. The Annexes carry the technical substance.

Annex 1 covers guidelines for preparing cargo securing manuals, now displaced for most practical purposes by MSC.1/Circ.1353/Rev.2. Annex 5 addresses the safe carriage of vehicles on ro-ro ships. Annex 7 covers semi-processed wood products. Annex 8 covers containers and flats. Annex 12 provides guidance on safe handling and securing of heavy cargo items. Annex 13, “Methods to Assess the Efficiency of Securing Arrangements for Non-Standardised Cargo,” is the mathematical heart of the system, and it is the annex that ship officers, marine surveyors, and cargo securing specialists actually use when they need to verify that an unusual cargo is properly lashed. The 2020 revision via MSC.1/Circ.1623 updated the applicability criteria and the acceleration tables to align with the IGC Code (Resolution MSC.5(48)) probability level of 25 days.

The CSS Code is formally non-mandatory for SOLAS purposes: it is a code of practice, not a mandatory convention. Its engineering methods become mandatory in practice because the Cargo Securing Manual prepared to MSC.1/Circ.1353/Rev.2 is required to incorporate them, and the Administration-approved CSM is mandatory under SOLAS VI/5. A CSM that diverges from the CSS Code methods without equivalent alternative analysis will not receive approval.

CSS Code Annex 13: lashing force design

Annex 13 defines the balance-of-forces and balance-of-moments method for determining whether a proposed securing arrangement is adequate for the cargo weight, position, and voyage conditions. It applies to non-standardized cargoes: items that are not containers handled through a pre-approved container stowage arrangement, not vehicles lashed per a pre-approved RoRo arrangement, and not bulk solid cargo. Heavy-lift items, project cargo, steel coils, abnormal unit loads, and deck cargo on general cargo ships are typical Annex 13 applications.

Acceleration basis

The acceleration figures in Annex 13 Table 2 (as updated by MSC.1/Circ.1623) are calibrated to ships with lengths 50 m to 200 m, speeds 9 knots to 24 knots, and a B/GM ratio of 3 or greater. They represent a 25-day probability level: the accelerations that the cargo will statistically experience over a representative voyage. Three orthogonal acceleration components are defined:

• Transverse acceleration $c_y$ (combining roll and lateral): highest at the ship sides, at forward and aft positions; lower amidships
• Longitudinal acceleration $c_x$ (combining pitch and surge): highest at bow and stern; lower amidships
• Vertical acceleration $c_z$ (combining heave, pitch, and roll): highest forward; lower aft

For a vessel of 100 m length at 15 knots, typical midship transverse design accelerations are around 0.5g to 0.6g; longitudinal values are lower, around 0.3g to 0.4g. These values increase for shorter, faster vessels, and for cargo stowed in forward or after positions. For ships with B/GM below 13, Annex 13 Table 4 supplies a correction factor that reduces the transverse acceleration to account for a stiff ship’s reduced roll amplitude. Use of an uncorrected table value on a very stiff ship would overstate the lashing requirement; use on a tender ship with B/GM below the table range would understate it, requiring a direct dynamic analysis.

Wind force contributes to the balance. The simplified Annex 13 approach treats wind loading as 1 kN per square metre of exposed projected area, applied separately in transverse ($F_{wy}$) and longitudinal ($F_{wx}$) directions based on the cargo profile area normal to each axis.

Maximum Securing Load and Calculated Strength

MSL (Maximum Securing Load) is the rated capacity of a securing device: the load it can sustain without failure under field conditions. Table 1 of the CSS Code defines MSL as a fraction of the device’s catalogued breaking strength:

The Safe Working Load (SWL) marked on certified equipment may substitute for MSL provided it equals or exceeds the MSL calculated from the table.

Annex 13 introduces a further reduction to obtain the Calculated Strength (CS) of each securing device:

The factor of 1.5 accounts for uneven distribution of load among multiple securing devices in the same arrangement, and for strength reduction from imperfect assembly (non-ideal lead angles, improperly tightened turnbuckles, residual curvature in wire rope). A lashing rod with an MSL of 120 kN contributes CS = 80 kN to the securing balance. Stacking multiple devices multiplies the available CS but does not eliminate the individual device ceiling.

Balance-of-forces equations

Three equilibrium checks are required for each cargo unit: transverse sliding, longitudinal sliding, and transverse tipping. The checks evaluate whether the combined restoring effect of friction and securing devices exceeds the combined destabilizing effect of inertia and wind.

Transverse sliding. The condition that must be satisfied is:

where $\mu$ is the friction coefficient between cargo and stowage surface, $m$ is cargo mass in tonnes, $g$ is 9.81 m/s², $c_y$ is the dimensionless transverse acceleration coefficient, $F_z$ is the vertical component of lashing forces lifting the cargo, and $f_{y,i}$ is the transverse efficiency factor of device $i$ as a function of its securing angles $\alpha$ (vertical) and $\beta$ (horizontal from the transverse axis). A lashing device at $\alpha = 45°$ and $\beta = 0°$ (pure transverse) has $f_y = \cos(45°) = 0.707$.

Longitudinal sliding. The check is:

where $f_{x,i}$ is the longitudinal efficiency factor, equal to $\cos(\alpha) \cdot \sin(\beta)$ for a device running at horizontal angle $\beta$ from transverse.

Transverse tipping. The moment equilibrium check is:

where $a$ is the height of the cargo centre of gravity above the tipping point, $b$ is the horizontal distance from the cargo centre of gravity to the tipping axis, $c_i$ is the moment arm of securing device $i$ about the tipping axis, and $e$ is the height of the wind force application point. Tipping is typically the governing check for tall, narrow cargo units with a high centre of gravity relative to their base width. A slab-sided unit 4 metres tall and 1.2 metres wide with its CG at 2 metres height will tip before it slides in most standard ship motion scenarios.

A longitudinal tipping check follows the same structure in the fore-aft plane where the cargo geometry warrants it.

Friction coefficients

Friction between cargo and stowage surface reduces the net securing load demand. Annex 13 tabulates representative coefficients:

Steel on steel wet is assigned zero because a greased or wave-wetted steel deck surface provides essentially no friction resistance. Anti-slip mats (rubber-backed, rated by adhesion tests) increase the effective $\mu$ but must be included in the CSM with their certified values. Using an assumed $\mu$ of 0.3 for steel-on-steel deck cargo without a physical anti-slip layer is a common and dangerous error.

The container lashing force calculator applies the Annex 13 balance-of-forces method with user-selectable ship length, speed, cargo position, cargo dimensions, mass, and lashing geometry, returning the CS demand versus available CS and the governing failure mode (sliding or tipping).

The Cargo Securing Manual in practice

A CSM is only as useful as the people using it. During cargo planning, the chief officer consults the CSM to confirm that the intended stowage position, cargo type, and securing arrangement match an approved combination. For standard cargoes and approved positions, the CSM typically contains pre-calculated tables: “X tonnes of bagged cargo in a hold position Y requires Z lashing rods at arrangement W.” For unusual items, the Annex 13 calculation must be carried out explicitly, either by hand or using approved software.

Several lashing calculation software packages are in use commercially: MacLash (marketed by Pactor GmbH), various class-society proprietary tools, and the DNV StowLash3D system for container ships. Where such software is used to generate securing arrangements that are not pre-calculated in the CSM, the software output must generally be approved or the software itself must be approved by a class society as equivalent to Annex 13. DNV StowLash3D uses a finite element model that captures torsional loads on lashing bridges and hatch cover deformation effects, which the standard balance-of-forces method does not address.

The CSM must be revised whenever: the ship’s securing equipment changes; new cargo types are approved; approved stack weights are altered; or the ship undergoes structural modifications that affect lashing point locations or capacities. A revision requires resubmission to the flag Administration or its delegated RO. Operating a vessel with an outdated CSM is a SOLAS deficiency that port state control surveyors will act on.

Container securing systems

Container securing on cellular container ships is a distinct sub-discipline within cargo securing, governed by a combination of SOLAS VI, the CSS Code, individual classification society container lashing rules, and the VGM requirement of SOLAS VI/2. The system integrates passive structural securing elements (cell guides, hatch coaming interaction) with active securing elements (twistlocks, lashing rods, turnbuckles).

Cell guides and below-deck stowage

In the container holds of cellular container ships, containers are guided by vertical steel cell guide rails fitted to the hold structure. Each container bay has four guide rails running the full hold height, one for each corner of the container stack. A container lowered into the hold slots between the guides and is retained against transverse and longitudinal movement by the guide clearance, typically 25 mm to 50 mm per side. Below-deck containers in cells are not individually lashed: the cell guide structure provides securing against ship motion loads, and the twistlocks between tiers provide vertical restraint. The stack weight and the resulting compression in the lower-tier twistlocks must remain within the rated capacity of the corner casting: ISO 1496-1 specifies a minimum static compression capacity of 848 kN (approximately 86 tonne-force) per corner casting for a standard 20-foot or 40-foot ISO container.

The cell guide rail loads are taken by the hold transverse web frames and bulkheads. On a heavily-rolling ship carrying heavy containers, the guide reaction loads can approach 100 kN per guide rail per bay, and the class rules for the ship’s structure incorporate those loads.

On-deck stowage: twistlocks and lashing rods

On-deck container stowage does not have the benefit of cell guide lateral restraint. The securing system relies on twistlocks between container tiers for vertical and limited lateral restraint, lashing rods from deck lashing eyes to container corner castings for primary lateral restraint, and on some ships, lashing bridges providing elevated lashing attachment points for the second or third tier.

Twistlocks are the container-to-container and container-to-deck connection at each corner casting. Three operational patterns are in use:

• Manual twistlocks: The fitting is placed on the lower container’s upper corner casting; the upper container is lowered over it; a worker rotates the cone element by hand to engage the lock. Unlocking requires the reverse hand operation. Manual twistlocks are reliable but labour-intensive.
• Semi-automatic twistlocks: The fitting locks automatically under the weight of the stacking container. A retaining pawl holds the cone engaged until manually released from the side. Unlocking still requires manual access.
• Fully automatic twistlocks: Both locking and unlocking are gravity-actuated. Engagement occurs as the container is landed; release occurs when the container is lifted during discharge. No worker needs to climb the stack for twistlock operation. Fully automatic twistlocks have become standard on high-throughput terminals.

A typical twistlock assembly has a breaking strength in tension of 500 kN and shear of 420 kN, with an MSL of 250 kN (50% of breaking strength per CSS Code Table 1). The class-society approval for a specific twistlock design involves load testing per the manufacturer’s stated breaking strength, verification of materials and dimensional compliance with ISO 3874, and a review of the manufacturing quality plan.

Lashing rods (also called lashing bars) connect from the ship’s deck lashing eyes to the lower corner castings of the second tier of containers. They resist the lateral force that twistlocks alone cannot carry at height. A standard lashing rod for deck container securing runs 15 mm to 22 mm diameter high-tensile steel, with an MSL typically in the range 120 kN to 200 kN depending on manufacturer and grade. Grade confirmation through manufacturer certification is mandatory: a steel rod of unmarked grade used in a lashing arrangement is a SOLAS deficiency.

The lashing bridge is a structural gantry welded or bolted to the hatch covers or deck structure at a height of approximately 1.5 m to 2.0 m above the container top, providing lashing attachment points at a geometrically advantageous height for the second or third tier. By raising the lashing attachment point, the effective moment arm in the tipping balance increases, and the vertical component of the lashing rod load (which reduces the net downward load and thus friction) decreases. Class societies approve the lashing bridge as part of the container stowage arrangement, and the bridge’s structural adequacy under design lashing loads is verified by calculation during ship design.

Stack weight and the VGM

Container stack weight limits govern how many tonnes can be piled in a given tier before the corner casting load in the lowest container exceeds ISO limits or the lashing system reaches its design ceiling. On modern deep-sea container ships operating 7 to 10 tiers on deck, the maximum permitted weight typically drops with tier height: a position that allows 30 tonnes on tier 1 may permit 25 tonnes on tier 3 and 20 tonnes on tier 5. Those figures are in the approved stowage plan and are enforced through the ship’s cargo planning computer, which integrates VGM data received from terminals.

The VGM requirement under SOLAS VI/2 feeds directly into this system. Before 1 July 2016, declared container weights were frequently inaccurate, sometimes substantially so. A 2012 study cited in IMO MSC 94 documents found that a proportion of containers on major trade routes had declared weights differing from actual weights by more than 3 tonnes. An overweight container in a high tier slot compromises the structural integrity of the lower containers, the lashing rod adequacy, and ship stability simultaneously. The VGM requirement addressed all three by mandating that the actual mass be determined before loading.

The container stack weight calculator evaluates whether proposed per-tier weights comply with ISO 1496-1 corner casting limits and the ship’s approved stack arrangement.

RoRo cargo securing

Roll-on/roll-off vessel cargo securing addresses a different loading geometry from container ships. Vehicles, trailers, heavy construction equipment, and project cargo on flatbeds arrive under their own power or by tractor and are parked on multi-deck cargo decks with lashing points built into the deck structure. Cargo does not stay in a fixed vertical stack: it moves horizontally onto the deck and is then secured in place. The ship’s motion in service generates the same transverse and longitudinal inertia loads as on any other vessel type, and the securing arrangement must resist those loads throughout the passage.

RoRo lashing equipment

Standard RoRo lashing uses alloy steel chain, Grade 80 (specification designation T) or Grade 100 (designation V). Grade 80 chain has a minimum breaking force of approximately 212 kN for 13 mm diameter, giving an MSL of 106 kN per the 50% CSS Code rule. Grade 100 chain of the same diameter breaks at around 265 kN, MSL 133 kN. The choice between grades determines how many lashings are required for a given cargo weight and voyage; Grade 100 chain reduces lashing count and shortens lashing time, which matters on vessels with tight port turnarounds.

Chain tensioners (load binders) apply pre-tension to the lashing chain, preventing slack that would allow cargo to move before the lashing engages. A chain lashing with no pre-tension is essentially not securing the cargo against the initial surge of a wave impact: it only engages after a finite movement, which can be enough to break a securing eye or damage adjacent cargo. Standard practice for road trailers on RoRo ferries is a minimum of four lashings, one at each corner of the trailer frame, with wheel chocks as supplementary restraint.

Wire rope lashings provide flexibility for awkward securing geometries where chain’s link-by-link adjustment is insufficient. Wire rope lashings are tensioned with turnbuckles. Single-use wire rope lashings have an MSL of 80% of breaking strength, but reusable wire rope lashings are rated at only 30% because cyclic loading and residual curvature from storage substantially reduce their effective strength.

RoRo-specific CSS Code provisions

CSS Code Annex 5 addresses the safe stowage and securing of vehicles on ro-ro ships. It specifies that: vehicles must be loaded, stowed, and secured in accordance with the ship’s Cargo Securing Manual; the master must satisfy themselves that all vehicles are secured before departure; and the securing inspection must be repeated at regular intervals throughout the voyage, particularly after significant changes in weather.

The RoRo lashing trailer calculator applies the CSS Code Annex 5 methodology to verify lashing adequacy for trailers, vehicles, and rolling equipment under ship-motion loads.

The European Gateway casualty in 1982, where a RoRo ferry capsized with the loss of six lives partly because a bow door was open, was not a lashing failure per se. But the general pattern of RoRo incidents including the Herald of Free Enterprise (1987, 193 lives), where cargo access doors were left open, illustrates how the whole RoRo safety system, from structural weathertightness through to cargo securing, must be treated as an integrated operational discipline. Lashing failures on RoRo vessels tend to cause cascading damage rather than single-cargo losses: an unsecured trailer can impact and release other trailers, and a cascade of moving trailers on a car deck can cause stability loss.

Breakbulk and general cargo securing

Breakbulk and general cargo ships carry the widest range of commodity types, and their CSMs must address each category. The key cargo types and their specific securing considerations follow.

Steel products

Steel coils, slabs, and plates are among the heaviest and most force-concentrated cargo units that general cargo ships carry. A single slab of ship plate may weigh 30 tonnes and rest on a contact area of less than 4 square metres, producing point loads that require timber dunnage to spread. CSS Code Chapter 9 and associated guidance address steel product stowage.

Steel coils are round in cross-section and will roll under ship motion if not crated or chocked. The securing arrangement uses purpose-built cradle blocks that constrain the coil against rolling, combined with chain or wire rope lashings over the coil tied to deck fittings fore and aft. The CSS Code requires a minimum of four securing devices per coil, and the individual device MSL must be verified against the calculated securing load from the Annex 13 balance-of-forces check.

Project cargo and heavy lifts

Project cargo, encompassing large-diameter pipe, pressure vessels, wind turbine components, industrial machinery, and similar units, often pushes the limits of CSS Code Annex 13 because the items are large enough for ship motion accelerations to vary measurably across the cargo footprint, and because the centre of gravity and rotational inertia are not simple to determine. For items above approximately 100 tonnes, a dedicated marine engineering calculation is typically required, performed by a marine warranty surveyor.

The marine warranty surveyor (MWS) approval process for project cargo involves reviewing the grillage design (the platform of steel beams on which the cargo rests and to which the lashings attach), the lashing arrangement, the voyage risk assessment, and the weather routing. The MWS issues a warranty certificate that satisfies the cargo insurer and, in many cases, satisfies the charterer’s requirement under the contract of carriage. The MWS calculation uses Annex 13 as its base method, often supplemented by direct dynamic analysis where the cargo geometry or the voyage route falls outside the Annex 13 applicability range.

Timber deck cargo

The 2011 TDC Code (Resolution A.1048(27), adopted 30 November 2011) provides the specific guidance for ships carrying timber deck cargoes. It applies to all ships of 24 m or more in length engaged in the carriage of timber as deck cargo. The Code is non-mandatory as an IMO instrument, but most flag states have incorporated it into their national requirements, and its provisions feed into the CSM.

Timber deck cargo lashing must resist the high exposed-surface wind forces and the wave wash that deck stowage entails. The 2011 TDC Code specifies lashing wire sizes and spacing relative to the timber stack width and height, requires that the lashings pass over the full width of the timber package, and mandates intermediate lashings where package length exceeds 3.5 m. A minimum timber freeboard (from the uppermost cargo to the ship’s rail) is also specified to limit the risk of deck inundation wave loads on the cargo. These provisions connect to the vessel’s weathertight integrity requirements, which are discussed in the companion article on marine hatch covers and weathertight closures.

CTU Code and the container-packing obligation

The IMO/ILO/UNECE Code of Practice for Packing of Cargo Transport Units (CTU Code, issued as MSC.1/Circ.1497 on 16 December 2014) addresses what goes on inside a container or other cargo transport unit before it is sealed and shipped. The CTU Code is a tripartite instrument: it was developed jointly by the IMO, the ILO, and the UN Economic Commission for Europe, reflecting that the CTU’s journey starts before it reaches the port gate and continues after discharge. It is non-mandatory under international law but is referenced as best practice in both the IMDG Code (for dangerous goods) and the CSS Code.

The CTU Code’s four core obligations for the packer are: the CTU must be structurally adequate for the load; cargo must be packed to prevent movement within the CTU; the total mass must not exceed the CTU’s rated capacity; and, for dangerous goods, the packing must comply with the IMDG Code segregation and marking requirements. The securing-within-the-CTU aspect is important because a ship’s Cargo Securing Manual addresses the forces on the CTU as a unit: it does not reach inside the container. If cargo packed inside a container shifts during the voyage and crushes against the container end walls, the CSM has been complied with (the container moved within its approved stack) but the CTU Code was not.

The practical consequence of poor CTU packing becomes apparent when dockers open a 40-foot box to discover a toppled pallet of machinery 3 metres from where it was loaded. In the worst cases, opening a container with shifted heavy cargo has caused fatal accidents. The CTU Code’s guidance on packing includes specific techniques for blocking and bracing cargo within the unit, friction-increasing measures (dunnage bags, anti-slip mats), and weight distribution requirements that prevent the container’s corner casting loads from exceeding ISO 1496-1 limits.

Securing equipment certification and marking

Every piece of portable cargo securing equipment on board must carry certification identifying its type, rated MSL or SWL, manufacturer, and the approval basis. Equipment without traceable certification cannot be assigned a confirmed CS value in a lashing calculation and therefore cannot be counted in a CSM-compliant securing arrangement.

Chain must be marked with its grade designation: the grade 80 mark is “8” stamped on every link or on a tag attached to the chain; grade 100 is marked “10.” An unmarked chain found in a cargo securing locker must be treated as unknown grade and assigned the lowest applicable MSL. The CSS Code Table 1 does not distinguish grades because the MSL is defined as 50% of the actual breaking strength, which must be established by certificate, not inferred from marks.

Wire rope certifications identify the construction (6x36, 8x36, or other), the core type (IWRC or fibre core), the nominal diameter, and the minimum breaking force per ISO 2232 or equivalent. A wire rope certificate that pre-dates the current period by more than 5 years, or that does not accompany the coil or reel it certifies, is not adequate for cargo securing use.

Lashing rods are certified by the manufacturer to a specific breaking load and are marked with their MSL or SWL and the manufacturer identification. After any deformation, permanent kink, or load event that exceeded the MSL, a lashing rod must be removed from service regardless of its apparent visual condition. A rod that has been stretched to its yield point has a reduced cross-section at the necked zone and cannot be relied on to sustain its rated load.

Annual class survey covers the portable securing equipment inventory. Surveyors check that the certification records match the actual equipment on board, that equipment condition is consistent with continued service, and that quantities match the CSM. A containership with 600 certified lashing rods in the CSM inventory but only 540 available for inspection is deficient regardless of the condition of the 540 on hand.

The IACS Hatch Load Calculator addresses the related structural aspect of cargo securing at the hatch-cover level.

Voyage inspection and heavy-weather response

Cargo securing does not end when the hatch is battened or the last lashing rod is tightened. SOLAS VI Regulation 5 requires that securing be maintained throughout the voyage. MSC.1/Circ.1353/Rev.2 Chapter 3 instructs that securing inspections be carried out at regular intervals and after any event (heavy weather, course alteration, speed change) that may have affected securing integrity.

Daily inspection of accessible lashings checks for: rod tension (slack rods are retightened with turnbuckles); rod integrity (cracks at the threaded section, corrosion, deformation); twistlock engagement (unlocked twistlocks on deck containers have been found post-voyage on multiple audit reports); and chain/shackle condition for RoRo and general cargo. The inspection must be documented in the cargo log.

Heavy weather management is not a substitute for adequate initial securing. Reducing ship speed cuts the wave-excited roll angle and the resulting transverse acceleration: at half speed, the effective $c_y$ on a vessel governed by rolling may drop by 30% or more depending on hull form and loading. Course alteration to reduce beam seas to a quartering approach similarly reduces roll. These measures buy time for additional securing or for crew to check existing lashing integrity safely. They do not convert an inadequate lashing arrangement into an adequate one; they reduce the demand on an already-adequate arrangement. Heavy weather management for cargo security is covered in detail in the companion article on heavy weather operations.

The threshold for additional securing during a voyage is the point at which forecast or encountered weather is expected to produce accelerations materially above the Annex 13 design level used when planning the securing arrangement. That decision is the master’s, drawing on voyage weather data, ship motion monitoring (where fitted), and judgement from cargo securing experience.

Containers lost overboard represent both an environmental hazard and a navigation danger. A standard 20-foot box floating 90% submerged is essentially invisible on radar until a vessel is very close. Carriage of misdeclared or overweight containers, combined with securing arrangements calculated from the wrong (lower) declared mass, has been identified as a contributing factor in several overboard container events. The VGM requirement addresses the mass accuracy element; ensuring lashing calculations use the VGM figure and not a pre-VGM declared weight is the operator’s responsibility.

Comparison: securing system characteristics by cargo type

Limitations

The CSS Code Annex 13 balance-of-forces method has documented range limitations that practitioners must apply correctly. The acceleration tables apply to ships between 50 m and 200 m in length, speeds from 9 to 24 knots, and B/GM ratios of 3 or above. A 250-metre container ship, a 6-knot heavy-lift barge, or a vessel with an atypically low GM falls outside the parametric range. For those cases, direct dynamic analysis using a validated ship motion model is the correct approach, not extrapolating from Annex 13 tables.

The Annex 13 method treats each cargo unit as a rigid body. It does not model the interaction between the cargo unit’s flexibility and the securing loads. For certain long, flexible items (pipe bundles, structural beams), dynamic amplification at the cargo’s natural frequency can produce peak lashing loads that exceed the quasi-static Annex 13 estimate.

The friction coefficient values in Annex 13 are representative, not measured values for specific surface conditions. Anti-slip mats, ice, oil contamination, or paint on deck surfaces all change the actual friction coefficient from the tabulated standard values. Using the standard 0.3 timber-on-steel value for an oily steel deck under timber dunnage is non-conservative. Where surface condition differs from the Annex 13 assumptions, the conservative approach is to use $\mu = 0$ for any surface that is wet, contaminated, or uncertain.

Lashing geometry assumptions in the balance-of-forces method treat each securing device as acting at its stated angles. Rigging a lashing rod at a different angle from the one used in the calculation invalidates the CS contribution assumed for that device. This error is common when cargo dimensions differ from the planning dimensions used at the calculation stage.

The CSS Code does not address the effect of resonance in the ship’s roll cycle on securing loads. A vessel whose roll natural period is excited by wave encounter frequency in following or quartering seas can experience roll amplitudes substantially larger than the Annex 13 design level. This scenario, encountered in following sea conditions, can produce transverse accelerations that temporarily exceed the design $c_y$, with no margin if the CSM arrangement was designed to the exact Annex 13 tabulated value.

The CTU Code, while covering internal cargo securing within containers, is non-mandatory under international law. Compliance depends on national transposition and on shippers’ awareness and practices. The gap between CTU Code intent and actual container packing quality remains a recognized problem across the supply chain, as documented by the TT Club’s cargo integrity campaigns.

Port state control inspection verifies CSM availability and crew familiarity, but does not routinely verify that the lashing calculation in the CSM is correct or that the calculation method is properly applied to the specific cargoes being carried on the voyage being inspected. The system depends on competent CSM preparation and honest application of the Annex 13 method.

See also

• Cargo Securing Manual: vessel-level document requirements, approval process, and revision obligations
• Marine Hatch Covers and Weathertight Closures: structural weathertightness requirements that interact with cargo securing for deck and tween-deck stowage
• Heavy Weather Operations: master’s obligations and operational decisions during severe weather affecting cargo
• SOLAS Convention: the treaty framework from which Chapter VI Regulations 2 and 5 derive
• Container Ship: vessel type overview, including the structural integration of the cell guide and lashing system
• Ro-Ro Vessel: vessel type overview for roll-on/roll-off cargo operations
• General Cargo Ship: vessel type overview for breakbulk and general cargo operations
• Marine Cargo Handling Cranes and Derricks: lifting equipment used during cargo loading and discharge

Calculators:

• Container Lashing Force Calculator: CSS Code Annex 13 balance-of-forces for deck containers
• Container Stack Weight Calculator: ISO 1496-1 corner casting load limits per tier
• Container Open Top Lashing Calculator: lashing assessment for open-top container units
• RoRo Lashing Trailer Calculator: CSS Code Annex 5 lashing adequacy for trailers and vehicles
• IACS Hatch Load Calculator: hatch cover load capacity under cargo stack