Underway Replenishment

1.0 Introduction

I have an idea. Let’s
run a zip line between two ships, while underway, and pump hazardous fuel along
it; should be easy! Well, T-AO vessels
do exactly that. Some time ago, I had
the opportunity to do some work aboard these engineering marvels, which
performs underway replenishment (UNREP) of naval vessels. While studying the forethought that went into
the ship’s design, I realized this was an excellent example of systems
integration.

Naval architects work daily as systems integrators, but it’s
difficult to point to any one system and show where this adds value. So today I want to review the details of T-AO
UNREP equipment, and how none of this is possible without systems integration.

2.0 UNREP Sequence

To oversimplify, naval ships need fuel and food to keep
going. UNREP operations allow naval
vessels to resupply without entering port.
The basic UNREP sequence allows transfer of fuel and palletized cargo in
large quantities:

Ships maneuver near each other, matching course
and speed. They maintain a fixed
separation distance.
Messenger lines are sent across, which then pull
over heavier lines, eventually leading to large steel cables. [1] These wires act like the industrial version
of zip lines. One wire to carry the fuel
hoses (span wire) and one to carry the cargo (highline).
The fuel hoses are lowered across on the
spanwire. These fuel hoses have a
special fitting that allows quick to connect or disconnect. Once connected, the resupply ship starts
pumping across fuel.
At the same time, the resupply ship sends
pallets of cargo across the highline, using a winch system.
After all cargo and fuel are transferred, we
reverse the connection sequence. Pull
back the fuel hoses and cargo winch cables.
Disconnect the spanwire, highline, and any distance markers.
Ships are now free to go their separate ways.

Figure 2‑1: Example of UNREP Sequence [2]

This entire operation is a testament to excellent seamanship
of sailors on both ships. But we are
talking about unforgiving industrial equipment.
And most skilled sailor doesn’t stand a chance without the efforts of
engineers. So I want to focus on the
systems integration necessary to bring the industrial world back into the realm
of human capabilities.

3.0 Avoiding a Collision

Before those ships ever approach each other, the naval
architect goes to work. The first
concern is avoiding a collision.
Operating in such close proximity takes immensely skilled
seamanship. But the best mariner in the
world doesn’t stand a chance if their ship gets overpowered by the sea.

The naval architect performs seakeeping studies to
investigate the relative motion between these two vessels under different storm
conditions. How strong will the waves
push against the hull? Do the engines
and rudder have sufficient power to correct against the waves? The naval architect takes these questions and defines
safe operating limits for the ships.
Maximum sea state, minimum separation distance, minimum speed, etc. This is the type of work that DMS does in
system integration. Start with a general
question, add in environmental information, and generate safe operating limits.

Safety also requires us to check for unusual
conditions. As the two ships approach
one another, the two hulls generate a suction force that tries to pull them
together. Something that never occurs
with the ship in open water. This is a
prime example of the dangers of industrial scale ships. Without proper engineering, this suction can
easily overpower the rudder. The naval
architect anticipates the danger and checks that the vessel has sufficient
power to hold against that suction. Due
to these safeguards, the crew know about the danger and skillfully hold against
it.

4.0 Fuel Hoses

The fuel hoses demonstrate another great example of system
integration and safety. This system
starts as a simple idea: zip line with a
rubber hose. (Figure
4‑1) But when you amplify ideas to industrial
scales, the risks also increase.

These fuel lines are not small. How much do they weigh, and what damage can
they do? That fuel probe is made of
solid steel. If we let it run down the
spanwire, out of control, it could smash straight through someone’s skull. (Figure 4‑2) For the safety of the crew and the ship, we
need to control the motion of the hoses at every point of deployment. All those saddle winches and messenger lines
are safety features, meant to hold the hoses in check. But what are the requirements for those
winches? The naval architect steps in
again, listing a host of specific details:

Wire size
Wire length
Load on the winch
Winch speed
Location and foundations for winches
Define different operational scenarios

The goal is to quantify the different environmental and
operational demands, which is not easy. Picture
these two ships traveling alongside, refueling.
When a naval architect pictures this, they don’t see a pleasant sunny
day; they see a storm with whitecaps on the waves. Their challenge is to imagine the worst
scenario possible, check that the scenario remains reasonable, and then design
to that.

Figure 4‑3: Emergency Breakaway – Example of Design Contingencies [3]

5.0 Cargo Transfer

Hoses aren’t the only innovation that goes into the UNREP. The highline system also showed some
impressive system integration. First, it
addressed the issue of managing cable tension.
In Figure 5‑1
the two ships politely stay a fixed distance apart. But in reality, they move back and forth a
few meters. Small movements on a ship
scale are huge for the steel stable.
This could easily stretch the cable past its breaking limit.

This demonstrates the job of a naval architect to understand
the limits of the machinery and pair that against the demands of the
environment. We need to quickly adjust
the length of the highline, compensating for ship movements. A normal winch can’t react fast enough. So we add a ram tensioner into the mix,
designed to rapidly adjust the cable length and maintain a safe tension on the
highline. (The same system applies to
the span wire on the fuel lines.)

System integration for the highline also focused on
practical matters. The cargo normally
comes over on pallets, which means it starts sitting on the deck. But to transfer onto the highline, we need to
lift the cargo off the deck and suspend it from the trolley. Do you plan to lift it with your bare hands? This cargo weighs several tonnes. We need a crane to lift it from the deck and
transfer it to the highline system. And
this is where systems integration looks for opportunities to simplify. Rather than installing a separate crane, turn
the highline into a crane.

Figure 5‑2: Highline Lifting Cargo [4]

The T-AO can adjust the height of the transfer head, lifting
the trolley and its cargo above the deck.
Once suspended from the trolley, the highline sends the cargo
across. This simple system design
improves cargo transfer speeds. Systems integration
is also about improving performance.

6.0 Integrating Disciplines

Larger projects (like designing an entire T-AO vessel and
UNREP system) draw on the expertise of several different engineering
disciplines. Dozens of different
engineers, each specialized in their own fields, without a common technical
language or basis of knowledge. For
example, a pure mechanical engineer may not know about the details of USCG fire
protection requirements and sprinklers, but they understand how to design a
piping system. With these larger
projects, the naval architect acts as a generalist, translating requirements
and sharing information across disciplines.
Don’t underestimate the value of an adaptable engineer who knows a
little something about everything. That
is also part of systems integration.

7.0 Conclusion

Systems integration can be difficult to recognize. As we examine a ship, we cannot identify any
single machine labeled “designed by system integrator.” But the systems integrator provides key
support, defining operational limits.
They provide environmental requirements for individual machines, and
they determine the worst-case scenarios involved for safe operation of those
machines. If we widen our perspective to
include the entire vessel, the systems integrator makes the difference between
a harmonized ship versus a pile of components.

8.0 References

[1]	Robert Sherman, “Underway Replenishment (UNREP),” FAS Military Network Analysis, 06 Mar 1999. . Available: https://fas.org/man/dod-101/sys/ship/unrep.htm. .
[2]	Military Archive, “How it Works: Refueling Ships at Sea,” YouTube, 29 Apr 2019. . Available: https://youtu.be/NZOFY1z2biI. .
[3]	Alex Torres, “Emergency Breakaway,” YouTube, 20 May 2014. . Available: https://www.youtube.com/watch?v=LOdoLkF1Y2Y. .
[4]	D&K Engineering, “UNREP: Transfer to Carrier,” YouTube, 13 May 2016. . Available: https://www.youtube.com/watch?v=u-zuKrhfbOA. .
[5]	YouTube Creator, “Virtual Reality Ships Bridge Training Simulator Demo by Konsberg Maritime,” YouTube, 17 Dec 2012. . Available: https://www.youtube.com/watch?v=6cLvkTCryBY. .
[6]	Wikipedia Authors, “Pecos T-AO-197 replenishes Blue Ridge LCC-19,” Wikimedia Commons, 10 Oct 2011. . Available: https://commons.wikimedia.org/wiki/File:Pecos_T-AO-197_replenishes_Blue_Ridge_LCC-19.jpg. .
[7]	Website Author, “2012 version of ShipConstructor CAD/CAM program released,” 3D CAD World, 19 Aug 2011. . Available: https://www.3dcadworld.com/2012-version-of-shipconstructor-cadcam-program-released/. .

Underway Replenishment

1.0 Introduction

2.0 UNREP Sequence

3.0 Avoiding a Collision

4.0 Fuel Hoses

5.0 Cargo Transfer

6.0 Integrating Disciplines

7.0 Conclusion

8.0 References

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