Mooring Chock “Jacket” – Which Material Will Win?

Foreword 

Our group recently undertook a project in collaboration with a large shipyard to develop mooring chock casting. The following will detail the application scenarios of this project. 

The Road to Innovation for Marine Mooring Chocks

As a core component in ship mooring operations, marine mooring chocks bear the crucial mission of guiding cables, distributing stress, and protecting the hull structure. However, due to frequent use, environmental corrosion, and other factors, this seemingly simple device is highly susceptible to becoming a consumable item. Damage to these locks not only severely impacts ship operational efficiency but can also lead to major safety accidents. This article will provide an in-depth analysis from the perspectives of damage causes, maintenance difficulties, solutions, and future prospects. 

I. Current Status of mooring chocks: Why Do They Keep Breaking Down? 

Causes of Damage:

1.Design Defects: Some mooring chocks suffer from unreasonable structural design issues, such as insufficient hole wall thickness, excessively small fillet radius, and lack of reinforcement at the connection points with the hull. These problems lead to stress concentration, making them prone to cracking or even breakage after prolonged stress. Statistics show that mooring chocks with such design defects have an average service life of only about 18 months. 

2.Manufacturing Defects:Internal defects such as slag inclusions and porosity during carbon steel casting, or improper heat treatment leading to coarse grains and reduced toughness, will significantly reduce the mechanical properties of the mooring chock, becoming a potential source of crack initiation. 

3.Improper Installation: Uneven installation surfaces, improper welding parameters, and uneven bolt preload will subject the mooring chock to additional stress, leading to cracks after prolonged use. 

4.Overuse: Exceeding design loads during mooring, excessive cable angle deviation causing localized stress concentration, frequent impact loads (such as ship swaying in wind and waves), and inadequate corrosion protection leading to accelerated rust and material deterioration will all accelerate mooring chock damage. 

Material, Manufacturing Process:

The primary manufacturing process for mooring chocks is casting; the choice of material directly affects its durability and safety. 

Quantity: 

The number of mooring chocks on a vessel depends on the vessel type, size, and mooring requirements. Large vessels typically require dozens of mooring chocks to meet mooring needs in different positions and orientations. Is it a design flaw? 

The damage to the mooring chock is not caused by a single factor, but rather by the combined effects of design, manufacturing, installation, and use. Although some early designs of mooring chocks had structural flaws and insufficient strength, their durability and safety can be significantly improved through design improvements. 

II. The Maintenance Dilemma—"Removal" is More Expensive than "Replacement"

When the mooring chock is severely damaged, the repair process is extremely complex and costly. Because the mooring chock is usually fixed in a specific location on the hull and closely connected to the hull structure, repairs require the vessel to be sailed to a repair shop, where professionals remove the damaged mooring chock and replace it with a new one. This process requires not only specialized tools and equipment but also a significant amount of manpower and resources, resulting in extremely high repair costs. Furthermore, the vessel cannot operate normally during repairs, further exacerbating economic losses. 

III. Conventional Approach: Modifying the Design Scheme

Modifying the design and recertification:

Comprehensively optimizing the mooring chock design, improving its durability and safety in terms of structure and materials. However, this approach requires recertification by the classification society, a time-consuming process. Furthermore, many ships still use the existing mooring chocks. A complete replacement would not only involve a huge investment of manpower and resources but also disrupt normal ship operations, making implementation difficult. 

Strengthening Manufacturing Process Control:

By strictly controlling various parameters in the casting process and optimizing heat treatment processes, the manufacturing quality of the guide holes can be improved, internal defects reduced, and mechanical properties enhanced. However, this method can only improve the quality of the mooring chock to a certain extent and cannot fundamentally solve the damage problems caused by design and usage factors. 

IV. Innovative Approach: Giving the "Hole" a Replaceable "Cover"

Equipping the mooring chock with a protective cover is a more feasible and innovative solution. The protective cover material can be stainless steel or cast nylon. 

Cast Nylon Protective Cover: Parameters and Advantages: Low density, flexural strength 90–110 MPa, notched impact strength >15 kJ/m², operating temperature range −40 ℃～+110 ℃. The design features an integral conical horn shape with a quick-release slot structure, reducing wear on polymer cables by over 30%; it is also lighter in weight. Theoretically, its service life can be extended by 12 years. 

Stainless Steel Protective Sleeve

Parameter Comparison: Stainless steel protective sleeves are characterized by high strength and corrosion resistance, with a bending strength of 200–300 MPa, a notched impact strength of approximately >50 kJ/m², and an operating temperature range of -40℃ to +800℃. They are heavier and require welding for installation. Theoretically, their service life can be extended by 15-20 years. 

Cost and Advantages/Disadvantages Analysis of Cable Guide Hole Protective Sleeves

Cost-wise: The cost of stainless steel protective sleeves is comparable to that of synthetic material protective sleeves. 

Advantages/Disadvantages: Stainless steel protective sleeves are durable and have a long service life, providing long-term reliable protection for mooring chocks; however, they are heavy and complex to install. Cast nylon protective sleeves are lightweight and easy to install, quickly providing protection for cable guide holes; however, their durability is relatively weak, and long-term use may lead to wear and aging, requiring periodic replacement.

In Mechanical Testing Experiments 

Currently in the testing and trial phase, damage to marine mooring chocks involves multiple stages, including design, manufacturing, installation, and use. To effectively solve this problem, factors such as cost, durability, and ease of installation must be comprehensively considered. Among existing solutions, adding a protective sleeve to the mooring chock is a feasible and innovative method. Cast nylon protective sleeves, in particular, excel in cost, weight, and durability and are currently attracting considerable attention; while stainless steel has an undeniable advantage in terms of material properties—it can withstand rigorous testing, and safety and durability are paramount.

It is believed that after rigorous testing by shipyards, including various mechanical tests, fatigue tests, and simulated scenario tests, the optimal solution for protecting mooring chocks will be promoted and widely applied, letting the data speak for itself.

The author would like to say: There are many foundries in China, but a foundry with a soul is truly one in a million.

More Links:

What is Green Casting, And How Can It Be Achieved?

What is the Casting Process ?

The Application of Metal Castings