Carr: Tons Per Inch Immersion, or What Was I Thinking?
By Michael Carr – A 70-ton, 75-foot tug hung in the air 10 feet from the bridge windows. The ships master was attempting to maneuver his vessel sideways, out from under the suspended tug, using his vessels twin screws, and bow thruster. But his plan was not working.
A flood current was pushing him forward, and an increasing and unsteady sea breeze was holding him against the dock. The suspended tug, hanging by a single pendant was starting to sway and swing uncontrollably.
“Put if back on the deck, now.” He tried to say calmly into his hand held radio. He was talking directly to the crane operator, who was sitting, out of sight, in his cab atop the crane. Down the tug went, back to the ships deck, oh so slowly. As soon as the tug made contact with the ship’s deck the master sighed in momentary relief, and took a big gulp of coffee.
Time was limited. His crew had to get this tug, and a 2ndtug, off their deck this morning. These tugs were needed for a mission, and the crane was only available today. They had planned this offload maneuver yesterday, discussing with the crane and ship’s crew how the process would go. Maneuver their ship alongside the crane. Hold the ship in place using her twin engines and bow thruster, hook up the tug and lift it, hold it in mid-air, and he would slide his ship sideways out from under the suspended tug. Then the crane would lower the tug into the water. Simple plan.
But it was not working. And now he knew why. When the 70-ton tug was raised off the ships deck, the ship’s displacement changed. He knew that was going to happen, but had not calculated how much. The ship was coming up several inches. It’s called Tons Per Inch Immersion (TPI). For every ton of weight added or subtracted from a ship it’s displacement, and draft changes. We measure this in inches.
When the tug was lifted, the ship came up several inches and that delicate balance achieved using twin propellers and bow thruster, which kept the ship alongside the crane barge, changed. That slight alteration in displacement and draft had wreaked havoc with the delicate balance of forces. Friction, exposed surface, and subtle hydrodynamics all showed their faces.
So how was he going to balance his ship against the barge, and then have the tug raised off the deck without loosing control? There was no room for error, the suspended tug cleared the ships bridge by only a few feet.
He gulped more coffee, looked out the bridge windows, and to himself said “Fuck, fuck, fuck.” He looked around taking in the situation. He walked to the bridge wings and looked at the wind, the tide, the dozen of men standing around looking at him. He knew what they were thinking “What’s the skipper going to do?” and he also knew they were thinking, “This will be one massive fuckup and shit storm if that small tug does not clear the ship’s bridge and superstructure.” “Yup, that’s for sure” he thought.
“Ok” the skipper finally said to himself, I know what to do. He topped his coffee cup from the bridge’s military Bundt coffee pot, and pushed the transmit bar on his hand held radio.
“Bridge to deck”, this is the skipper. Here is what we are going to do. Rig a single breast line from us to the crane barge once we are in place. Rig it for slip. When we start to raise the tug, pull it in tight. Hold it tight until I tell you to let it go, and then pull it in as fast as you can. If you cannot pull it in, or it gets tangled then cut it. Get a fire axe and let me know when you are ready.”
“Here is our new plan”, he told this bridge crew, I will push us up against the barge, we will snug up the breast line and raise the tug. This single breast line will give me something to maneuver against as I hold us in place. Once the tug is high enough to clear our gunwales we will release the breast line and move sideways out from under the tug. That’s it. Lets do this.
The skipper took his place inside the pilothouse on the portside. He could see the tug, his deck, his crew, and the crane barge, but he could not see the crane operator, who was 50 feet above him in the crane’s cab. “Ok, here we go”, he said on his hand held radio.
His words were now forceful and direct.
“Rudders inboard 35 degrees.”
“Thrust left slow.”
“Starboard back dead slow.”
“Port ahead slow.”
“Thrust left half.”
To the crane, “Raise the tug”
As the crane spooled in its cable the tug slowly came off the ship’s deck. As the tug’s last inch of contact with the deck was gone, the tug began swaying in the wind, and the ship began moving about. Both the ship and tug were free of each other.
“Thrust left full, thrust left half, hold that”.
“Deck, pull in the breast line now!”
Slowly the 70-ton tug came off the deck and the ship began to squirm and move. Without that 70-ton deck load the physics had once again changed the balance of forces.
With the added breast line now tight, the skipper had something to leverage the bow thruster and twin engines, but the breast line was now tight as a rail and water was being squeezed out from between its strands. It was helping but not enough. There was too much motion and the suspended tug was swinging too much for the skipper to maneuver his ship out from under the suspended load.
“Put her back on deck,” he said into his radio to the crane operator.
“Just put her down”.
Once the tug was on deck, the skipper stood quietly by the bridge window gathering his thoughts. The bridge crew was quite. They knew the skipper was in a struggle.
But the skipper’s struggle was not just figuring out how to get these tugs off his dam ship, but how to do this in his next move. He could not just keep trying this, or trying that, hoping for success. His crew was becoming as tired as he was, and they would soon begin to doubt his ability.
“I have to get my head in the game and my shit together,” he said to himself.
“Figure this out. There is a solution, don’t just keep doing the same thing.”
About this time the Chief Engineer showed up on the bridge, bearing his coffee cup and looking for coffee. Chief Engineers have a habit of just appearing. You won’t see them for days, and then suddenly they are standing next to you.
“How’s it going skipper?” the Chief Engineer asked.
“Not so good,” said the skipper, and told the Chief Engineer of the two failed lifting attempts. The Chief Engineer filled his coffee cup and stood with the skipper looking down at the cargo deck. Time was running out. After a few minutes, the Chief Engineer said, with no real preamble,
“Would you like me to press up all the voids and empty tanks, I think we could increase our draft by about 6 inches, that would get the bow thruster lower in the water and give us more weight and stability.”
Immediately the skipper knew this could help and asked how long it would take.
“About an hour,” said the Chief Engineer.
“Yes, yes, yes, let’s do that,” said the skipper. Press up all our tanks. Give me all the draft and weight you can.
With an hour to press up the tanks, the skipper had some time to develop a new plan that would work. No more attempts. He had to get the next lift right and get those tugs off his cargo deck. With more draft and displacement his ship should move less, but he could not just hope this would work. He needed more.
“All hands to the mess deck in 10 minutes for a crew meeting” the skipper announced over the ship’s public address system. He was going to brief everyone on his new plan, and give everyone an hour to re-group.
He knew that briefing the crew would also get his plan clear in his mind, and he trusted his crew to give him feedback if the new plan sounded “jacked up”. He could depend on his crew to tell him the truth. Sailors are not politicians or consensus seekers. They just tell it as it is since you cannot BS Mother Nature.
“OK everyone, here is the problem we were having, and here is the solution,” the skipper said to the assembled crew. He then explained what had gone wrong on the first two lifts, and how he and Chief Engineer were correcting the issue.
“We are ballasting down, and we are going to use two snubber or breast lines on the next lift. The added weight will increase our draft forward, giving the bow thruster more bite, and keep us from moving around so much.” He told the crew to take an hour break, and once the ballasting was completed he would let everyone know.
Then the skipper went to wardroom to sit and think through this plan step by step. He rehearsed in his head each command, and how he would handle every possible action. He knew he had to make this work. It was now early afternoon. The sea breeze was steadily increasing and now he heard there were thunderstorm warnings.
“Motherfucker,” he thought to himself, just keep piling on the stress.
“That’s why being skipper is so rewarding!” he thought. You get to make all these fun decisions and you get to drive the boat!
Soon the Chief Engineer appeared and said all the tanks were topped off and overflowing their vents.
“Anytime you want skipper,” he said. “Its all yours, good luck”. And then he added,
“If you don’t fuck this up, I’ll buy you a beer tonight”.
“All hands to cargo loading stations” the skipper piped over the PA system. He was laser focused now. No BS, no more screwing around. These fucking tugs were coming off the deck this time.
He filled his coffee cup, a ritual which carried huge significance. He walked to the bridge window, and then looked around to ensure everyone was in place.
He keyed his handheld radio and called the crane operator “ Bridge to Crane, are you ready to lift?”
“Roger that skipper, ready to go.”
“Ok, no more rehearsals, this time is the real deal. Once you get this first tug above our gunwhale I am going to thrust right and get us out from under her.”
“Roger that Chief.”
There was now a pause. No one was moving, no action was occurring. It was as if time had been stopped. Nothing was going to happen until the skipper keyed his radio and gave commands. He took a sip of coffee and focused.
“Just fucking do it,” the skipper said to himself.
“Rudders inboard 30 degrees. Thrust left slow, port ahead slow, starboard back dead slow.” The ship began to suck herself into the crane barge, the forces from the bow thruster and engines balancing each other and producing a force to port.
Once the ship was firmly pushing against the crane barge, the skipper keyed his radio.
“Snug up both breast lines, and hold them”
“On the crane, raise the tug and keep her going up”
Slowly the crane’s steel cable took up the load. The 70-ton tug came free of the cargo deck again, for the 3rd time. Up she came inches became feet.
The ship began moving about again, squirming as she re-adjusted herself to change in deck load. But this time the skipper was prepared and able to adjust power to keep the situation under control.
“Thrust left half, thrust left full.”
“Port ahead half.”
He was pressing the ship against the crane barge, holding her in place as the tug slowly rose higher and higher. Now it seemed they were at a point of no return, the tug was 30 feet in the air, suspended directly in front of the pilothouse windows, and only 15 feet away. A 70-ton wrecking ball hanging in the air waiting to swing and wreak havoc.
“She is above your gunwale now” came over the radio from the crane operator to the skipper.
“Roger that, hold her there, I am going to come to all stop now and slide sideways out from under her.” Replied the skipper.
Now he had to execute a precise shift from thrusting left to thrusting right and move his ship sideways, out from under the suspended tug. He could not move forward and he could not move aft. He had to slide his ship sideways to starboard using the single bow thruster and twin propellers.
“All Stop, Stop thrust, stop engines.”
“Thrust right slow.”
“Starboard ahead dead slow, port back slow.”
Slowly the ship began to slide sideways to starboard, but with a flood tide from astern and 15 knots of erratic wind, the skipper had to keep adjusting power.
“Thrust right half. Hold that.”
Slowly, oh so slowly the ship moved sideways to starboard and the suspended tug, hanging over the ship began moving to port.
“Dear God, thank you,” said the skipper to himself.
Now the tug was past his ships gunwale, but not yet truly away from potentially wrecking his ship.
His nerves began to release tension as the distance between the ship and tug increased. Once he had enough distance so he could turn the ship, and head into open water, giving himself time to decompress.
“All stop. Right full rudder. Thrust right full, port ahead half, starboard back full”. His ship responded and spun around, now clear of the suspended tug. As the skipper looked astern to ensure he was clear, he could see the tug being lowered into the water.
He took a big gulp of coffee and smiled.
“Fucking A, I love this shit,” he said to himself, turning to his bridge crew and smiled. “Great job on the throttles and rudders!” he said. One tug off and one to go.
He had his ship do a few slow and easy circles out in the harbor, joked with his bridge crew, and thanked the Chief Engineer for ballasting down.
“I think you saved my ass,” the skipper said to the Chief Engineer who had come back to the bridge to check on things and refill his coffee cup.
“No problem” replied the Chief Engineer, “Engineers are always saving you deckies from disaster”.
After a few more minutes the skipper took a deep breath and said to his bridge crew.
“Ok, let’s go back under the crane and get this 2ndtug off our deck. “Rudder amidships, all ahead dead slow.”
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