Seafastening Design and Knowing the Vessel Motions to Consider

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For those doing seafastening design for the first time, it is worth knowing the type of loads the secured cargo will be subjected to.

There are various guidelines and standards which depend on the mode and type of transport, however most codes share the same tips when it comes to ensuring that the cargo is transported safely.

1. Heave
Heave is simply the vertical motion of the vessel moving up and down. Translation in this direction causes the cargo to weigh more when the vessel heaves up, and weigh less, when the vessel heaves down.

2. Surge
Surge is the linear longitudinal (front/back) motion. This causes the cargo to want to move laterally to the front of the vessel and and the rear of the vessel.

3. Sway
Sway, much like Heave is the linear lateral (side-to-side) motion. This ignores any rotations of the vessel which we will cover later. This causes the cargo to want to move laterally across the deck in a port / starboard direction.

4. Roll
The roll of a vessel is the rotation about an imaginary axis running horizontally along the ships length.

5. Pitch
The pitch of a vessel is the rotation about the axis running across the width. Pitch motion can be thought of as an up-or-down movement of the bow of the ship.

6. Yaw
Yaw rotation of the vessel on its vertical axis. This is the axis running vertically through the ship and through its centre of gravity. Another way of thinking about yaw motion is side-to side movement of the bow of the ship.

 

Heave Vessel motion

Heave vessel motion

Surge Vessel Motion

Surge Vessel Motion

Sway Vessel Motion

Sway Vessel Motion

Yaw Vessel Motion

Yaw Vessel Motion

Roll Vessel Motion

Roll Vessel Motion

Pitch Vessel Motion

Pitch Vessel Motion

The 3 most important seafastening tips

Here are some of the most important things that you should think about before before performing any sea-fastening design.

1. Check for vessel appurtenances not shown on drawings.

One of the most important checks we should do before starting anything else is ensuring the cargo does not clash with appurtenances. A clash may be with vents, deck protrusions, or other miscellaneous steelwork on the deck. If it is found that the cargo must be relocated due to a clash, it can cause a number of costly issues.

Often the engineering calculations must be redone, and re-approved by marine warranty surveyors. The rushed manner in which this design rework is performed brings into question the quality and safety of the new design. There is also the issue of time. A vessels departure time may be seriously affected by having to relocate cargo to a new location due to a clash.

In order to avoid such issues it is important to seek out as many photos of the vessel and the location on deck. You cannot rely purely on the vessel drawings for showing all the potential clashes. Often vessel drawings will exclude various items. Check your photos and then, check them again.

2. Check the under-deck welds.

Under-deck welds are often overlooked in seafastening design. These are the welds from the deck plate to the under-deck beams. An “out-of-sight, out of mind” mentality can be dangerous regarding under-deck welds.

If the vessel is old, the under-deck welds can often be corroded, or display hairline cracks. Whenever welds are in poor condition they will not possess the full structural capacity which would normally be expected.

In particular, welds that are in tension tend to have less than the expected structural capacity which could lead to a catastrophic failure.

3. No Weld Zones.

“No Weld Zones” are defined as those locations on a vessel deck where welding should not be performed. This is often due to the presence of fuel and oil tanks or other combustible materials. Each vessel will generally have its own “No Weld Zone” drawing showing these locations.

It is extremely important that the person performing the seafastening design has access to this drawing in order to avoid welding in any dangerous locations.

These are some of the most important factors to consider when embarking on seafastening design.