What Is Mooring Analysis?

Mooring analysis is a key process in offshore engineering and naval architecture, used to evaluate and design systems that secure vessels or offshore structures in fixed positions. These mooring systems consist of elements such as anchors, chains, and cables that stabilize ships, floating platforms, or other maritime installations. Mooring analysis aims to ensure these systems can endure environmental factors like wind, waves, and currents, providing safe and steady positioning.

Mooring
Image for representation purposes only.

Key Elements of Mooring Analysis

  1. Environmental Forces: Analyse the impact of wind, wave, and current forces on the structure. These forces are influenced by location, water depth, and environmental conditions.
  2. Vessel or Structure Features: Assessing the size, shape, and draft of the vessel or structure helps predict how it will react to environmental forces.
  3. Mooring System Components: Evaluate the performance of components such as anchors, chains, and connectors. Their material properties, arrangement, and strength are critical for system reliability.
  4. Dynamic Behaviour: Analyse the system’s behaviour under various conditions, including vessel motions, changes in mooring line tension, and potential resonance. This ensures effectiveness in diverse sea states.
  5. Holding Capacity: Calculate the system’s capacity to maintain the structure’s position, especially during extreme weather. This includes assessing the ability of anchors and the overall arrangement to resist movement.
  6. Safety Margins and Compliance: Apply safety factors and adhere to industry standards, ensuring compliance with regulatory requirements and guidelines from classification bodies.
  7. Simulation and Modelling: Use advanced software tools to simulate the behaviour of the mooring system in various conditions. These simulations help engineers refine designs and detect potential issues early.

Necessity of Mooring Analysis

Mooring analysis plays a vital role in the development and operation of offshore platforms, floating production systems, vessels, and similar structures. By assessing and optimizing mooring systems, engineers ensure their safety and stability across different environmental scenarios, mitigating risks and enhancing reliability.

Types of Mooring Systems and Their Working Principles

Mooring systems are essential for stabilizing vessels and offshore structures. Different types of mooring systems are used depending on operational requirements, environmental conditions, and the nature of the structure. Below are detailed explanations of three commonly used mooring systems and their working principles.

1. Single Point Mooring (SPM) System


A single-point mooring system secures a vessel to a single mooring buoy connected to anchors or subsea pipelines. It is commonly used for loading and unloading hydrocarbons in deep-water locations, where fixed infrastructure is impractical.

Working Principle:

  • The SPM system allows a vessel to weathervane freely around the mooring buoy, adjusting its orientation to wind, waves, and currents.
  • The buoy is connected to the seabed by multiple anchor lines or chains arranged radially.
  • Flexible hoses connect the buoy to subsea pipelines, enabling the transfer of fluids while maintaining vessel stability.

    Single Point Mooring
    Image for representation purposes only.

Advantages:

  • Facilitates fluid transfer in remote offshore locations.
  • Allows vessels to align naturally with environmental forces, reducing strain on the system.

Applications:

  • Tanker operations for oil and gas loading or offloading.
  • Offshore production systems require a mobile and adaptable setup.

2. Multi-Point Mooring (MPM) System


A multi-point mooring system involves anchoring a vessel or structure using several mooring lines connected to fixed points on the seabed. This system provides enhanced stability and is widely used for offshore operations requiring precise station-keeping.

Working Principle:

  • The vessel or floating structure is tethered using multiple mooring lines, typically arranged in a spread configuration.
  • Each line connects to an anchor or pile, ensuring forces are distributed evenly across the system.
  • The symmetrical arrangement provides resistance to multidirectional environmental forces, such as waves and currents.

Advantages:

  • Superior stability in harsh conditions.
  • Allows for longer-term station-keeping compared to single-point systems.

    Multi-point mooring system
    Image for representation purposes only.

Applications:

  • Offshore construction and installation vessels.
  • Floating platforms for drilling or production.

3. Catenary Anchored Mooring (CAM) System


The catenary mooring system uses heavy chains or cables arranged in a curved (catenary) shape to secure a vessel or platform. It is commonly employed in moderate to deep water depths.

Working Principle:

  • The curved anchor lines provide flexibility and act as shock absorbers, dissipating forces from environmental loads.
  • When external forces, such as wind or waves, push the vessel, the catenary shape of the lines allows them to lift slightly off the seabed, increasing tension and providing restoring force.
  • Stability is restored as the forces balance, ensuring the vessel remains in position.

Key Characteristics:

  • Anchor lines are typically long and made of heavy materials to ensure the catenary effect.
  • The system’s design ensures that anchors stay embedded in the seabed under varying loads.

    Catenary Anchored Mooring
    Image for representation purposes only.

Advantages:

  • High stability due to the damping effect of the catenary lines.
  • Effective in managing dynamic forces from waves and wind.

Applications:

  • Offshore drilling units and production platforms.
  • Barges and other floating installations in moderate to deep waters.

Comparison of Mooring Systems

Each mooring system has its unique features and applications. Selecting the right system depends on the operational requirements, environmental conditions, and the type of vessel or structure to be moored.

Understanding Offshore Mooring Systems

In the offshore environment, professionals often find themselves stationed on vessels to conduct various complex operations, particularly related to subsea pipelines and installations. This summary sheds light on the intricacies of mooring systems essential for this type of work.

Operations on Offshore Structures

Personnel working on the sea must install or repair offshore structures to facilitate subsea activities. Effective vessel management ensures safety and operational efficiency. Key elements include:

  • Accommodation and Work Vessels: These serve as platforms for personnel to live and work.
  • Mooring Lines: Play a crucial role in securing vessels to the seabed.

Vessel Types:

Environmental Challenges

The ocean environment presents numerous challenges, including wind, waves, and currents. These phenomena result in:

  • Oscillatory Motions: Movements caused by environmental forces.
  • Drift Motion: A slow, unintended movement of the vessel that poses risks.

Risks of a Drifting Vessel:

  • Danger to personnel on board.
  • Potential collisions with surrounding installations.

Importance of Mooring Systems

To mitigate risks, a robust station-keeping arrangement is required. The best solution involves mooring lines, which work as a defence against drifting through the following elements:

Typically, eight mooring lines are utilised, designed as a spread mooring system that spreads force in all directions.

Mooring Analysis and Safety

Assessing the performance of a mooring system occurs through mooring analysis, which ensures both operational safety and environmental integrity.

mooring lines
Image for representation purposes only.

Key aspects of mooring analysis include:

  1. Site Modelling: Involves creating an accurate seabed profile, assessing depth, and understanding nearby structures. High accuracy is critical for seabed modelling.
  2. Vessel Modelling: This includes:
    • Geometry and positioning of the vessel.
    • Dynamics—how the vessel interacts with environmental forces.
  3. Mooring System Modelling: This encompasses:
    • Detailed simulation of mooring lines and their connections.
    • Structural properties critical for realistic performance simulation.
  4. Environment Modelling: It’s essential to define:
    • Wind, wave, and current conditions applied to the vessel.
    • Various combinations of these factors for comprehensive case analysis.

Ensuring Safety

The primary goal of mooring analysis is safety, which entails:

  • Security of the vessel and personnel on board.
  • Protection of surrounding installations.
  • The integrity of the mooring system itself.

Working Principle of Catenary Anchored Mooring Systems

In this informative video, we delve into the mechanics of a straightforward catenary anchoring arrangement. The discussion encapsulates the fundamental principles of how the system operates, specifically focusing on a barge moored with two catenary anchor lines. Each line plays a crucial role in maintaining the stability and position of the barge in the water.

Overview of the Catenary System

The system consists of a barge moored via two catenary anchor lines:

  • AFT Anchor Line
  • Power Anchor Line

Here’s a breakdown of the arrangement:

  • Both lines are positioned at equal distances from the barge’s bow (BGE) and stern.
  • The lengths of the mooring ropes are equal, and the mooring lines have a symmetrical shape.

Forces Acting on the Barge

Key Forces:

  1. Weight of the Barge (W): This force acts vertically downward at the centre of gravity.
  2. Buoyant Force (B): This force counteracts the weight, acting vertically upward.
  3. Tension Forces: 
    • Tension at Aft (TA): Acts tangentially at the aft fairlead.
    • Tension at Power (TF): Acts tangentially at the forward fairlead.

System Assumptions

To simplify understanding, several assumptions are made:

  • The catenary is flat with a constant depth.
  • Both anchor lines experience equal tension due to symmetry.

These assumptions lead to the conclusion that the vertical components of both tension forces are equal while acting in opposite directions.

Analysing External Forces

With a steady wind applying pressure on the barge:

  • The vessel surges forward due to the wind force (Fwind).
  • The movement disturbs the symmetry of the catenary lines, leading to an increase in tension in the aft line as more of it is lifted from the seabed.
  • Resulting tension changes create unequal forces acting on the vessel: 
  • As the aft line tension (TA) increases, the forward line tension (TF) decreases.

Net Force and Stability

As a result of these tensions, a net horizontal force acts backwards against the wind-induced motion of the barge. When the forces equal each other (TA ≈ TF), the barge reaches a stable equilibrium. In the absence of the mooring lines, the barge would drift continuously due to wind without counteracting forces.

Restoring Mechanism

When wind ceases:

  • The catenary arrangement works to restore the barge to its original position due to the balance of tensions across the system.

If, however, all catenary lines lift off the seabed entirely, conditions depend on the anchor’s capacity to withstand the pull—if it cannot, the barge may drift unfettered.

Overview of Mooring Line System Modelling in Bentley MOSES

Now we will see a detailed explanation of the Mooring line system, specifically the modelling and simulation aspects associated with it. The discussion revolves around the coupling of different lines and the implications of environmental influences on line tension and positioning.

Introduction

The Mooring system comprises interconnected lines where specific environmental processes can induce positional shifts affecting the overall line tension. This outlines the methods to model such systems, the computational tools used, and the post-processing of results produced from simulations. Key aspects include defining segments in the modelling phase and visualizing outputs for better understanding and analysis.

Key Components

  1. Mooring Line Modelling:
    • A single property line can be modelled into segments.
    • Each segment’s connection point is critical, defined by a unique identifier.
    • Information such as diameter and weight per length are essential for accurate representation.
  2. Visualization and Graphical Feedback:
    • Users can visualize the modelling process and ensure synchronization between various elements.
    • A defined coordinate system aids in placing segments accurately within the graphical interface.
  3. Handling Forces on Lines:
    • Mid-line modelling can be executed by creating intersections in the segments.
    • Weight and additional forces can dynamically affect line behaviour during simulation.
  4. Simulation of Extreme Cases:
    • The presentation demonstrates how sudden tensions can lead to conditions where lines break under maximum stress.
    • Sensor integration allows for monitoring specific threshold limits, facilitating automatic line disconnection when safety limits are reached.
  5. Post-Processing of Results:
    • A key feature in the system involves dealing with extensive result sets after simulations are complete.
    • Important outputs can be filtered and visualized using built-in tools which generate relevant graphical interpretations.

Conclusion

The article concludes with a review of various Mooring types, showcasing their unique characteristics and application scenarios. In addition, emphasis is placed on the sophisticated functionalities of the modelling software, illustrating its versatility in simulating real-world scenarios. Understanding these systems is crucial for efficient marine operation management, and hence, learning how to effectively utilize such tools becomes imperative for professionals in the field.

Through proper modelling techniques, dynamic simulations, and a robust post-processing framework, participants are equipped with the knowledge necessary to handle Mooring Line Systems effectively. 

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About Author

Hrituraj Singh is a final-year undergraduate student specializing in
Naval Architecture and Ocean Engineering at the Indian Maritime
University. He is having a strong hold on Naval Arch subjects and
keen interest in writing. Founder of BuildNEO (a student initiated
society active in the university).

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