Structure and Function of Ship Towing Chains
In ship towing operations, the towing chain is the core component connecting the tugboat and the towed vessel, undertaking the crucial functions of transmitting towing force and maintaining course stability. This article will systematically introduce the technical characteristics of ship towing chains from aspects such as structural composition, core function, and technical parameters.
I. Definition of Ship Towing Chains
A ship towing chain, also known as a towing bridle chain, is a large steel chain connecting the towed barge or buoy, and is a primary attachment in towing operations. When steel cables are used instead, it is called a bridle cable.
A typical bridle chain consists of two chains forming a set. One end of each chain is connected to the towing derrick with shackles, and the other end is connected to the towing eyeplates on the port and starboard sides of the towed vessel with shackles. This Y-shaped forked structure is the core feature that distinguishes towing chains from ordinary anchor chains.
II. Structural Components of a Towing Chain
(I) Link Types
A ship's towing chain is composed of various links, each with its own function:
Chain Link Types and Functions
| Link Type | Code | Main Function |
|---|---|---|
| Common Link | C | Forms the main body of the chain, transmits towing force |
| Enlarged Link | EL | Size transition, connects components of different specifications |
| End Link | E | Connects to shackles or towing padeyes |
| Swivel | SW | Prevents chain from twisting and kinking |
| End Shackle | ES | Connects to anchor or towing point |
The dimensions of a standard link are: diameter d, width 3.6d, length 6d. Enlarged links are 10% larger than standard links, and the diameter of the end link is 1.2d.
(II) Stowed Chain Structure
Towing chains must use stowed chains. Stowed chains feature a central brace (crossbar) on each link, offering several advantages:
1. Increased strength: Stowed chains are approximately 20% stronger than stowless chains of the same size and material.
2. Reduced deformation: The brace supports both sides of the link, reducing the rate of deformation under stress.
3. Prevention of entanglement: Stowed chains are less prone to tangling when stacked, facilitating handling.
4. Enhanced tensile strength: The link's major and minor diameters are designed in a 1.6-1.7 ratio, allowing it to withstand greater tensile forces.
(III) Surface Treatment
To withstand the corrosive marine environment, towed chains typically use hot-dip galvanized coatings or rust-proof paint coatings. Hot-dip galvanized chains exhibit excellent corrosion resistance in seawater environments and have a longer service life.
III. Position of the Towing Chain in the Towing System
In a complete towing rigging system, the towing cable consists of three parts:
Tugboat → Main Towing Cable → Short Cable (Bridge Cable) → Triangle Plate → Torn Chain (Two Strands) → Towed Vessel
- Main Towing Cable: The main towing wire rope on the tugboat, bearing the main towing force.
- Short Cable (Bridge Cable): Connects the main towing cable and the towing chains, generally not less than 30m in length.
- Triangle Plate: An isosceles triangle connector, with its three corners connecting the main towing cable and the two towing chains respectively.
- Torn Chain: The core component of this article, a Y-shaped fork connecting the two sides of the towed vessel.
The length of the towing chain is usually taken as the distance between the two towing points of the towed vessel, and the included angle between the two towing chains should not exceed 90°.
IV. Core Functions of the Towing Chain
(I) Maintaining Course Stability
The most important function of the towing chain is to maintain the course stability of the towed object. Without a chain, a single towline connecting the towed vessel to the center will cause it to yaw during navigation, making it difficult to control its direction. The chain branches off to connect the two sides of the towed vessel, forming a stable triangular structure that ensures the towed vessel always faces the tugboat.
(II) Uniform Distribution of Towing Force:
The chain evenly distributes the tugboat's pulling force to the towed vessel's two sides, preventing excessive stress on any single point and protecting the vessel's structural safety.
(III) Buffering Impact Loads:
Compared to steel cables, chains have better wear resistance and a certain degree of flexibility, providing a buffer against wave impacts and reducing the impact load on the towline.
(IV) Meeting Regulatory Requirements:
According to the "Statutory Inspection Rules for Marine Towing" and classification society regulations, large FPSOs and other non-self-propelled floating bodies must be equipped with a main rigging system featuring a chain and triangular eyeplates.
V. Material Grades and Technical Parameters
(I) Material Grades
Marine anchor chain steel is classified into several grades:
Anchor Chain Steel Grades
| Grade | Code | Characteristics |
|---|---|---|
| M1 | AM1 | Killed steel, general strength |
| M2 | AM2 | Fully killed fine-grained steel, medium strength |
| M3 | AM3 | Fully killed fine-grained steel, high strength |
According to requirements, anchor chain steel of grade 2 or 3 (i.e., M2 or M3 grade) should be used. M3 grade steel has higher tensile strength and fatigue performance, suitable for large towing operations.
According to requirements, anchor chain steel of grade 2 or 3 (i.e., M2 or M3 grade) should be used. M3 grade steel has higher tensile strength and fatigue performance, suitable for large towing operations.
(II) Safe Load Requirements
The safe load (SWL) of the anchor chain must match the towing force of the tugboat bollard, and must have a factory certificate and a safe load certificate. The breaking strength of each anchor chain should not be less than the breaking strength of the main towing cable.
VI. Technical Requirements for Connectors
The connecting components of the towing chain are equally crucial:
- Triangular plates: Made of forged steel (e.g., quenched and tempered steel No. 35), with a safe load capacity of up to 714kN.
- Connecting shackles: Must be bolt-type, equipped with nuts and cotter pins.
- Strength requirements: The ultimate load of all connecting components should not be less than 1.5 times the maximum breaking load of the towing cable.
- Verification test: Connectors should be verified under a calculation working load of 2 times, and there should be no permanent deformation.
VII. Summary
The ship towing chain (dragon whisker chain) is an indispensable safety component in towing operations. Its Y-shaped forked structure connects the towing points on both sides of the towed vessel, achieving uniform transmission of towing force and stable course control. Using a chain with a stop structure, M2/M3 grade high-strength steel, hot-dip galvanized anti-corrosion treatment, and standardized triangular plates and shackles, it constitutes a complete and reliable towing connection system.
Whether it's port towing, maritime rescue, or long-distance towing of large FPSOs, the correct selection and configuration of the towing chain is the foundation for ensuring operational safety.
This article is for technical popularization purposes. For specific selection, please contact us. SMEOCEAN provides you with professional advice and can provide all classification society certifications.