Dry docking
Relevant documents and M-notices: SOLAS Chapter I, MGN 546 (In Water Surveys), MSN 1751 (HSSC), MSN 1845
Dry docking questions are common at Mates and Masters level oral exams. It is entirely feasible that you might go for your Mates exam, or even your Masters, without having experienced a dry dock; perhaps your company didn’t send 2nd officers to dry dock to save money, or you left ships before they went to dry dock and joined ships after they’d come out. At some point in your career though it is very likely that you will take a ship into and out of dry dock, so you do need to understand the requirement, the process and the practicalities.
When and why do ships go to dry dock?
Ships go to dry dock for two reasons; to allow an inspection of the ships bottom, and to correct any defects found as well as removing biofouling, allowing painting etc.
Passenger ships and cargo ships have different requirements for inspections of the ships bottom.
Passenger ships: Once every 12 months (SOLAS Chapter I Regulation 7)
Cargo ships: Twice in a 5 year period, with not more than 36 months between inspections (SOLAS Chapter I Regulation 10)
These inspections are required for the issue and maintenance of the Passenger Ship Safety Certificate and the Cargo Ship Safety Construction Certificate.
So, does an inspection of the ships bottom always mean going to dry dock? No; under the Harmonised System of Survey and Certification some of these inspections may be done as In Water Surveys (IWS)
‘For passenger ships the inspection is required every year. On the years in which the out of water inspection does not take place, an in water inspection of the ship’s bottom shall be carried out. As a minimum, two of these surveys in any five year period, at intervals not exceeding 36 months, shall be conducted with the ship out of the water.
For cargo ships there shall be a minimum of two inspections during any five year period at intervals not exceeding 36 months. Inspections should normally be carried out with the ship out of the water, however consideration may be given to alternate inspections being carried out with the ship afloat.’
We’ll cover In Water Surveys in more detail in another article. For now though, understand that they have to be ‘nearly as effective’ as a dry dock, and that permission to carry out In Water Surveys is granted by the vessels Certifying Authority, normally her Classification Society.
It is worth mentioning also the Extended Dry Docking scheme. This is a scheme whereby the dry docking period is extended from five years to 7.5 years, by replacing some dry docks with in water surveys.
There is another occasion when a vessel might find herself in dry dock; that is following an incident, for an emergency dry dock. If this is following a collision, grounding or similar incident there will be additional stability and safety considerations and you would expect to receive expert advice from your classification society.
Preparation for dry dock.
Liaise with the dry dock master. Emergency dry docks excepted, you will know well in advance when you’re scheduled for a dry dock. It’s important to engage with the dry dock master long before arrival in order to give them the information they need to do their job. Their role is to prepare the facility and make it safe for us to use. This will mean sending them copies of our:
General arrangement plan
Docking plan
Fire plan
Ship Security Plan (basics)
Shell expansion plan
Detailed plan of the keel
Underwater fittings plan
Capacity plan
Arrival stability condition
Inform the flag state, classification society and P&I club.
Why does the dry dock need our shell expansion plan, plan of keel, underwater fittings plan etc?
The ship will sit on blocks in the dock; these could damage certain parts of the keel, or get in the way of work that needs to be done i.e. draining tanks. The underwater fittings plan/keel plan show the rudders, propellors, sea suctions, echo sounder and doppler log transducers, tank plugs, thrusters, probes etc and ensure that none of these are damaged by the having the weight of the ship resting on them.
Information that we’d expect to receive from the dry dock master includes:
Access arrangements for the vessel for the duration
Provision of fresh water
Provision of shoreside power
Provision of pressure for fire mains
Security arrangements for the dock
Waste disposal arrangements
Bathroom arrangements (the ships sewage system will be inoperative for at least some of the dry docking period)
Pitch of dry dock
Arrangements for air testing prior to tank entry
Permit to work system for the dock and vessel
Arriving at the dry dock
Note that, when asked in your exam about preparing for dry dock, the main aspect that the examiner wants to hear about is likely stability, so pay particular attention to this section.
When waiting to enter the dry dock, the vessel should be upright with as little ballast as possible (minimal deadweight). She must have a precisely calculated stern trim (slightly more than the pitch of the dry dock). This is to reduce both P-force (explained below) and the duration of the critical period.
It is good practice for the Chief Officer to sound all the vessels tanks to ensure that she is ballasted exactly as required. Free surface should be reduced as far as possible and GM increased. The ships architect should have provided a dry dock stability condition. Having as little deadweight as possible also helps to reduce the stress on the vessels structure, and resultant ‘bellying out’ once she is on the blocks.
Entering the dock
When coming into the dry dock it is not possible to use the vessels thrusters and prop; that would very likely move the blocks. Instead she will be taken in by tugs, by lines from the shore or by a combination of these measures. Each dry dock will be different and the docking master and dry dock master will talk you through the process. There may be a pilot on board.
Once the ship is in position divers may go down and check that everything is OK (i.e. that the vessel is centred and the blocks are in the correct place); the Doppler log and echo sounder should be switched off before the divers enter the water.
It is good practice to secure the anchors prior to entering dry dock. Once the dock bottom is dry they should be lowered to the dock bottom at the first opportunity.
P-force
Once the ship is in position, the dock is sealed by closing the caisson or mitre gates and the dock pumps are started. As the dock is drained, the following happens:
The stern post of the ship comes to rest on the blocks. This is the start of the critical period. This acts as if a weight had been discharged from low down, and as the centre of gravity (G) moves away from a discharged weight, the ships centre of gravity rises. This effective upthrust is known as P-force.
As the ships G continues to rise, this reduces the vertical distance between G and M, the metacentric height. This is a reduction in stability and increases up to the critical instant, the moment just before the vessels bow takes the blocks. At this moment GM will be at a minimum.
Once the bow touches, p-force is spread through the vessel and its affect on stability is reduced. At this point G and M are in the same place and stability is provided by the blocks and by shoring.
There are so many things that need to happen in dry dock that to list them all would make this article unwieldy. Here are a few to think about.
Superchlorination of the fresh water system from machinery space to furthest outlet
Anchors lowered to dock bottom
Measuring and marking of anchor chain
Cleaning of biofouling
Painting of the hull
Painting of draught marks and load lines
Inspection of sewage system
Inspection of fresh water tanks
Inspection of all ballast water tanks
Inspection of rudder (including removal of rudder plug)
Removal (and replacement!) of all drain plugs. Normally witnessed by the CO.
Vacuum testing drain plugs of empty tanks
Replacement of sacrificial anodes
MCAQs
What are the reasons for dry docking?
How often would you expect to go to dry dock working on a cruise ship?
How often would you expect to go to dry dock working on a container vessel?
What are the safety considerations for dry dock, with regards to stability?
What is P-force?
What is the critical period?
What is the critical moment?
What services would you expect to be provided to the vessel by the dry dock?
When would you stop the ships generators? (think about the water intake for their cooling system)