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Seafastening Design and Knowing the Vessel Motions to Consider

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.

Construction Accident Involving Faulty Winch Leaves Worker with Serious Injuries

This incident was reported by a worker that was aboard the ship at the time of the failure. On the day of the accident, the boat was on-shore unreeling pipe from an on-board reel. A winch was used to help with the removal of the pipe from the reel during the final phase. The winch was under tension just before the event occurred. It was at this time that the worker mentioned to their supervisor that they believed that the base plate under the winch was flexing. When the winch finally came to a complete stop the welds that held the winch to the deck suddenly came apart. This resulted in the winch breaking from its sea-fastenings and hurtling along the deck in such a way that the winch and base plate crushed his left leg.

There was an investigation by the members of the IMCA (International Marine Contractors Association). Their investigation discovered a few major pieces of information as follows:

•    About two months prior to the sea-fastening welds breaking, the base plate and attachments began to buckle underneath a load. The attempted fix was to add more weld to the base to hold it where it was.

•    There was initially a 32Te constant tension winch which was malfunctioning on multiple occasions. It was replaced with a 10Te winch.

•    The new 10Te winch that was installed was not constant tension, but also was not even fitted with a tension measurement.

•    If the Management of Change process was completed, it did not correctly identify that the new winch was inadequately fastened to the deck.

•    When the initial event occurred two months prior to the incident in question, it was not investigated and fixed in an acceptable way. This could have significantly contributed to the break on this particular day.

Here are some of the investigation teams ideas’ on how this situation could have been avoided.

•    One of the main priorities should have been to identify all problems with the Management of Change requirements, and fix these problems immediately.

•    When the winch was changed, a structural engineer should have been asked to check the position and sea-fastenings to ensure the load on the winch wire could be adequately transferred to the vessel deck structure.

•   In situations where welds are in tension, often the case for winches, all welds including under deck welds should be non-destructively tested (NDT).

The result is a very serious lost time injury which is going to require time, money, and medical attention to rectify. Had the event occurred just a split second earlier this accident could have easily resulted in death? Additionally, if the vessel was offshore at the time, it may have been impossible to get the required medical attention in a timely manner. An injury like this can remain with the worker for the rest of their life. A safety incident like this will remain with the company for a very long time.

This is just part of the investigation that was done by the IMCA the entire report can be read in the IMCA Safety Flash 12/14 Newsletter.
The incident detailed above could have been avoided by having Seafasten Offshore review the challenges at hand.  Seafasten Offshore is the world leader in engineering design of vessel seafastening solutions and sea transport. If you are unsure about the condition, or adequacy in which cargo or equipment has been secured to a vessel’s deck it is best to ask an expert. Contact Us @ seafasten.com