Underway Replenishment

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 some T-AO UNREP equipment, and show how none of this is possible without systems integration.

A Systems Integration Study

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:

1. Ships maneuver near each other, matching course and speed.  They maintain a fixed separation distance.
2. 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).
3. 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.
4. At the same time, the resupply ship sends pallets of cargo across the highline, using a winch system.
5. 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.
6. Ships are now free to go their separate ways.

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: 

1. Wire size
2. Wire length
3. Load on the winch
4. Winch speed
5. Location and foundations for winches
6. 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. 

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.

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