Stability Test

An overview of what to expect from engineering services for a stability test.

The purpose of a stability test is to determine the weight and center of gravity for a vessel.  This is very important.  When you look at stability assessments and loading conditions, you will notice that the ship’s lightweight is a very large component of those load conditions.  It is very important to get this number accurate.

You need a stability test at several times during a vessel life.  There should always be at least one stability test when the vessel is first built.  And then at periodic times as the vessel ages.  When the weight changes beyond a certain percentage, a stability test is required.  This is usually any 4% change in weight.  But ask you engineer.  The exact number depends on who your regulatory body is and the scenario.  Basic rule of thumb:  anytime you replace, add, or remove a major piece of equipment, you will need a stability test.

A stability test consists of four major procedures:

1. Stability test plan
   1. Engineer completes this weeks before the big day.
2. Deadweight survey
   1. Determines the lightweight of the vessel
   2. Determines the LCG and TCG of the vessel
3. Incline Experiment
   1. Determines the VCG of the vessel, the most important number for center of gravity.
4. Stability test analysis
   1. Engineers process all the data from the stability test and prepare a test report.
   2. The important number from the test report is the light ship weight and center of gravity.
   3. There is a lot of math to prove that you can trust the numbers on the test report.

Stability tests are big, complicated and expensive.  They easily cause trouble.  Why?  Because the stability test a precise scientific experiment.  It requires a level of precision that ships rarely ever operate on.  Who normally cares if a piece of cargo was one inch to the left?  On a stability test, it matters.  So pay careful attention to the engineer and follow their advice.

The engineer may seem like the villain in a stability test.  They are actually your friend trying to protect your from all the things that can go wrong during a test.  The only thing more frustrating than doing a stability test is this:  when the regulator refuses to accept the results and you need to do a second stability test.  So follow the engineer’s advice and let the protect you from that scenario.  A little extra prevention is worth it.

In preparing for this test, there will be plenty of times where you will be tempted to ask the engineer if you can compromise on something.  My advice, don’t.  The engineer will do it.  They want to help you. And they have the methods to work with your compromises and still get the test passed.  But say you ask for a compromise right in the planning phase.  And then something happens the day of and now you need two compromises.  Will the test still pass?

Here is a good rule of thumb:  plan to allow no compromises during the planning phase of the test.  And allow for only one compromise the day of the test.  The engineer should be able to easily accommodate that.  Discuss your concerns with the engineer.  Go over contingencies.  Ask the engineer which things are more risky than others.

Deliverables

There are two major deliverables from the engineer.  The first is a stability test plan.  This must be submitted to your regulator for review before you do the test.  Ask your engineer about this FAR in advance.  Some regulators need months to review the stability test plan.

The second deliverable is the stability test report.  This is the big one.  It tells you the important number that you want to know.  And it has lots of math and data to prove that it is correct.  All your future stability assessments will be based on this test report.

Budget

You can expect 2 – 4 weeks to prepare the stability test plan.

Expect 3 – 6 days for the actual stability test.  There may also be preparatory meetings beforehand.

And then expect another 1 – 2 weeks to analyse the test data and prepare the test report.

There are additional costs beyond the engineer’s costs.

1. Weight rental:  You will need to rent several tonnes of weights to incline the ship for the test.
   1. These are often ecology blocks / k rails / steel weights
   2. The engineer will often ask for details of what is available at your location
   3. The engineer will tell you exactly what you need for the test
   4. You can not use water test weights.  This is only allowed in extremely rare occasions and casts suspicion on the entire test.
2. Crane rental:  You will need to rent a crane to move the test weights around.
   1. If your vessel has a crane, you may be able to use that for the test.  Ask your engineer.
   2. Many vessel cranes have insufficient capacity for the test weights.
3. Tug rental:  You may need to rent a tug to pull the vessel off the pier during the test.
   1. As an alternative, you can moor the vessel against Yokahama fenders.  Those allow the vessel to heel correctly for the test.
4. Tank cleaning and gas free
   1. Tanks need to be either pressed full or completely dry during the test.  There are some compromises to this rule.  This is the part you will ask the engineer to be flexible on.  And this is the part that can quickly get you into trouble.  Ask your engineer about the risks.
   2. The regulator needs to inspect any empty tanks to ensure they are completely empty.
   3. I mean EMPTY.  Pumped to no suction head.  Stripped dry.  Sucked dry with a hand pump.
   4. Gas free.  The regulator needs to crawl in the tank and ensure all corners are dry.  So any empty tank must be gas free.
   5. This includes fuel tanks.
5. Opportunity cost:  Your vessel will be completely unusable for a few days before and after the test.
   1. The crew need to be off the vessel on the day of the test.  Only the captain, chief engineer, and a few line handlers should be on the ship.
   2. You need to take out as much weight as possible from the ship.  There is a limit to how much deadweight you can have on the vessel during the test.
   3. Engine room stores.  Take them out and store them somewhere secure on the pier.
   4. Bosun room stores.  Take them out and store them somewhere secure on the pier.
   5. Think of this as an opportunity for spring cleaning.

Yes, that all sounds very scary and expensive.  Find an engineer you trust for this.  They will walk you through each step and help you as best as possible.  Plan frequent communication with your engineer.

Schedule

The schedule can be measured in months.  Planning will start months ahead of time.

The actual test will only take a few days, weather permitting.  Typically the engineer arrives on the vessel a few days beforehand.  They perform the deadweight survey on the days leading up to the test.  On the day of the incline experiment, all work must stop.  The crew must be off the vessel during that day.  The galley is not useable.  The heads are shutdown.  And once an incline experiment begins, it can not be interrupted until finished.  The incline experiment typically takes 6 – 8 hours.  But if problems arise, it may take much longer.  Some take up to 18 to 24 hours in the worst case.

The analysis after the test will take 1 to 2 weeks.

Client Input

There needs to be a lot of input from the client on this.  The engineer will work with you.  They will help you as best as possible.  But let them look out for your interests.  Let them protect you from the risk of doing two stability tests.

One of the major things you need to provide is the vessel, available for the engineer.  The vessel will not be useable when the engineers do the stability test.  There can still be work done on the vessel, but that work needs to stop on the day of the incline experiment.  No workers on the ship during that day.  No crew on the ship during that day.

Also, do not expect the vessel to depart for sea the day after your test.  It will need refueling.  All the stores need to be reloaded onto the vessel.  And the day of the stability test may slip a day or two, depending on the weather.