Heavy lift vessels are diverse, ranging from relatively low-capacity deep-sea vessels to massive capacity vessels capable of lifting over 1,000 tonnes. These vessels are equipped with cranes mounted on a floating body, presenting a unique challenge compared to land-based cranes. As the cranes slew the cargo inboard or outboard, the ship’s movement and the changing load distribution impact the ship’s stability. To counteract these effects and ensure safe lifting operations, stability pontoons are utilized as additional measures. In this article, we will explore the role of stability pontoons in enhancing heavy lift vessel operations and their impact on stability, buoyancy, and metacentric height.
1. The Challenge of Crane Stability on Geared Ships
When cranes are mounted on a floating body like a ship, their stability is influenced by the ship’s movement and load adjustments. Unlike land-based cranes with a stable ground, the ship’s “ground” is constantly shifting. The act of slewing cargo over the ship’s rail introduces a heeling moment, causing the ship to rotate around its longitudinal axis and altering the ship’s list. As the crane operates, the ship’s angle may change, necessitating the addition or removal of water ballast in the ship’s tanks to counterbalance these effects.
2. Introduction to Stability Pontoons
Stability pontoons are deployed on some heavy lift vessels to enhance lifting operations. These pontoons serve two primary purposes: increasing the outreach capacity of the vessel and accelerating cargo loading. By reducing the amount of ballasting required, stability pontoons allow vessel owners to maintain high crane capacity without sacrificing seakeeping, fuel economy, and speed characteristics.
A stability pontoon is typically a large watertight steel cuboid that is stored onboard the heavy lift vessel when not in use. Vessels may have multiple stability pontoons onboard, which can be deployed in different orientations to support specific lifting operations. The figure below provides examples of stability pontoons, highlighting their scale and deployment options.
3. Deploying Stability Pontoons for Enhanced Crane Capacity
Stability pontoons are typically fitted onto outriggers that protrude from the vessel hull. These outriggers have special tracks with locking pins or hydraulic clamps, allowing the pontoon to be fixed at a specific height depending on the operation’s requirements. In some cases, the stability pontoons can also be ballastable, enabling additional weight to be added.
Stability pontoons offer several benefits that enable heavy lift ships to utilize their full crane capacity effectively. Let’s explore two significant advantages provided by stability pontoons: roll damping and reducing the angle of heel.
3.1 Roll Damping
The presence of a stability pontoon introduces an additional damping coefficient for dynamic motions, specifically roll motion. As cargo is lifted and moved, or as waves and swell encounter the vessel, the roll motion caused by these dynamics can be reduced by the stability pontoon. This roll damping effect is also recognized within industry regulations, such as DNV’s Rules for Classification for Vessels for Special Operations.
3.2 Reducing the Angle of Heel
During lifting operations, the main objective is to minimize the angle of heel. This can be achieved through two approaches: balancing the weight or improving stability.
One method involves capitalizing on buoyancy with a stability pontoon deployed on the same side of the vessel as the cargo being lifted. With the stability pontoon deployed, an additional buoyancy force is generated. As the vessel lifts the cargo, it tilts toward the side of the lift, causing the pontoon to submerge deeper and increase the buoyancy force. This additional buoyancy acts to maintain the vessel at a level heel, thereby reducing the overall heel angle during the lift operation.
3.2.2 Shifting the Center of Gravity
Stability pontoons can also counteract the reduction in stability caused by the crane’s weight by shifting the center of gravity. When the stability pontoon is positioned adjacent to the main hull on the opposite side from the lift, it pulls the center of gravity back to the vessel’s centerline, reducing the vessel’s heel. Ballasting the pontoon further enhances this effect by pulling the center of gravity closer to the centerline and slightly lowering it with the additional weight.
3.3 Waterplane Area and Transverse Moment of Inertia
Another significant benefit offered by stability pontoons is their influence on the waterplane area and the transverse moment of inertia of the waterplane. The waterplane refers to the plane created by the intersection of the hull and its appendages with the water’s surface. The waterplane area represents the area of this shape, while the transverse moment of inertia measures the shape’s second moment across the vessel’s beam.
Increasing the waterplane area and transverse moment of inertia directly impacts the transverse metacentric radius (BM), a crucial parameter in stability. By increasing the BM, stability pontoons contribute to an improved measure of stability (GM). The relationship between waterplane area, transverse moment of inertia, and metacentric height is influenced by various factors, including vessel dimensions, stability pontoon dimensions, and the distance of the pontoon from the centerline.
4. Practical Application and Benefits of Stability Pontoons
In practical heavy lift vessel designs, making significant changes to the vessel’s length or beam is often impractical due to their impact on hydrodynamics and seakeeping. Stability pontoons offer a reasonable solution by providing substantial benefits without major modifications to the overall vessel design.
While stability pontoons contribute to enhanced stability, it’s important to note that several other factors also influence the stability of lifting operations. Therefore, vessel operators and project partners carefully consider stability pontoons and other measures to ensure safe and stable lifting operations.
Stability pontoons play a vital role in enhancing heavy lift vessel operations. By counteracting the effects of ship movement and load adjustments, stability pontoons improve the stability, buoyancy, and metacentric height of geared ships during lifting operations. These pontoons offer benefits such as roll damping, reducing the angle of heel, and influencing waterplane area and transverse moment of inertia. Through the strategic deployment of stability pontoons, heavy lift vessels can achieve maximum crane capacity without compromising other essential performance characteristics.
1. How do stability pontoons enhance heavy lift vessel operations?
Stability pontoons enhance heavy lift vessel operations by providing additional stability, reducing roll motion, and minimizing the angle of heel during lifting operations. They contribute to improved buoyancy, metacentric height, and waterplane area, enabling vessels to utilize their full crane capacity effectively.
2. Are stability pontoons a standard feature on all heavy lift vessels?
Stability pontoons are not a standard feature on all heavy lift vessels. Their inclusion depends on the specific requirements of the vessel’s intended operations. Vessel owners and project partners carefully consider factors such as lifting capacity, seakeeping, fuel economy, and speed characteristics when deciding whether to equip a vessel with stability pontoons.
3. How are stability pontoons deployed and adjusted on heavy lift vessels?
Stability pontoons are typically stored onboard heavy lift vessels when not in use. They can be deployed by attaching them to outriggers that protrude from the vessel hull. These outriggers have special tracks with locking pins or hydraulic clamps, allowing the pontoon’s height to be adjusted as required for different lifting operations.
4. Do stability pontoons impact the overall design and performance of heavy lift vessels?
Stability pontoons offer benefits to heavy lift vessels without significantly impacting the vessel’s overall design and performance. They allow vessel owners to maintain high crane capacity while preserving essential characteristics such as hydrodynamics, seakeeping, and fuel efficiency. Stability pontoons provide a practical solution for improving stability during lifting operations.
5. How do stability pontoons contribute to reducing the angle of heel?
Stability pontoons reduce the angle of heel by influencing the vessel’s buoyancy, shifting the center of gravity, and increasing the transverse moment of inertia of the waterplane. The additional buoyancy force generated by the stability pontoon counteracts the vessel’s inclination during lifting, while the center of gravity is adjusted to restore balance. These factors collectively contribute to reducing the angle of heel during heavy lift operations.