When submarines move beneath regional sea lanes - which are among the most congested in the world - the crew better know what they are doing. There could be tragic consequences if a submarine that is less than shipshape, or has an ill-trained crew, collides with a surface vessel or undersea object.

The Republic of Singapore Navy (RSN) invests heavily in submarine training. Every submariner can, for example, find his way about the vessel and to essential equipment while blindfolded. The RSN has also introduced its own submarine rescue vessel, the Swift Rescue, which is on permanent standby to support 171 Squadron, the RSN's submarine unit. But the bigger unknown is whether other maritime users will know what to do to avoid colliding with a submarine.

With this in mind, the RSN has proposed a code of conduct that aims to promote safer underwater operations for naval forces with submarines and for ships at sea. This code would fill an essential need - now there is no code of conduct for incidents at sea governing the underwater domain.

The RSN's Code for Unplanned Encounters at Sea (Cues) governing the underwater domain seeks to give submariners a set of rules to avoid collisions with other undersea vessels (such as other submarines or unmanned underwater vehicles) or surface ships. It was put forward at the recently concluded May 29-June 2 Submarine Operational Safety Conference in South Korea.

Cues builds on 20 years of RSN submarine operations in regional waters as well as the Singapore navy's experience operating submarines in the Baltic. The RSN has a submarine training detachment in Sweden.

If adopted by the more than 10 regional navies which operate over 200 submarines, submariners would benefit from a common code of conduct that provides useful cues on how to safely steer when submerged vessels encounter one another. More importantly, the code would provide surface ships - especially merchant ships and civilian vessels - with important cues on what to do when they spot red smoke flares fired from a submarine about to conduct an emergency surfacing. Such a procedure would see a submarine of several hundred tonnes shoot to the surface in seconds.

This aspect will strengthen maritime safety as civilian vessels do not have sonar and are, therefore, unaware of what lurks beneath them.

The sea may seem vast. But the risk of collision is not theoretical.

In February 2001, the United States Navy's (USN) nuclear- powered submarine, the USS Greeneville, collided with a Japanese fisheries training ship, the Ehime Maru. An emergency ballast blow executed by the Greeneville brought it to the surface suddenly and the submarine struck the Ehime Maru. Nine Japanese aboard the Ehime Maru were killed when it sank after the collision.

The underwater Cues recommends that submarines about to conduct an emergency surfacing release a red pyrotechnic such as a smoke flare that would float as a warning to ships in the vicinity and give them time to move away.

And in January 2005, the USN sub, USS San Francisco, collided with an underwater sea mountain while travelling at full speed. The sub was nearly lost with all hands but managed to limp to the surface. This incident underscores another aspect of the RSN's outreach to regional sub operators: the sharing of information, best practices and agreement on common standards for how subs are made and operated safely.

While information on sub movements is sensitive, the RSN holds the view that navies can still collaborate by sharing non-sensitive information that affects the safety of submerged navigation. This includes seismic activity (that could interfere with sonar), fishing activity and real-time movements of deep-water oil rigs and deep draft vessels such as Very Large and Ultra Large Crude Carriers whose hulls project tens of metres below the waves.

To promote information sharing, the RSN has developed a Submarine Safety Information Portal at the Information Fusion Centre at Changi Naval Base to facilitate the sharing of "live" updates of ships at sea. This big picture is useful, as it can be used to coordinate submarine rescue assets, especially vessels identified beforehand with the equipment to assist with the rescue of submarines involved in accidents at sea.

The sea lanes in the Malacca Strait and South China Sea are not only congested, but also traverse relatively shallow water, with the southern reaches of the South China Sea typically around 60m to 70m in depth.

What challenges do submariners face in shallow water?

Think of Changi Airport's iconic control tower, which stands 78m tall. The height from the bottom of the hull to the waterline - a measurement known as the draft - of a full laden Very Large Crude Carrier is about 20m. So an underwater submarine in the South China Sea has a distance of about two-thirds the height of the Changi control tower to avoid colliding with the hull of deep draft vessels like tankers, container ships, ocean liners and even oil rigs. It is not a lot of room to manoeuvre.

In Singapore waters, the number of tanker arrivals has charted a steady climb over the past five to 10 years, from about 21,000 tankers of all classes (oil, chemical, liquefied petroleum gas and liquefied natural gas) to 22,000 tankers in 2015. Not only are more tankers calling, such vessels are bigger in size and tonnage. Fully laden oil tankers are a hazard to submarines because their massive size and cargo make them hard to spot on sonar.

Add to this the rise in deep draft vessels such as ocean liners (which can embark thousands of passengers) and oil rigs (whose legs can reach the seabed), as well as expectations that the regional submarine fleet will jump by 100 hulls to around 300 diesel-electric subs by 2020 - and one can appreciate the urgency of efforts to promote safer underwater navigation.

In years to come, one can expect unmanned underwater vessels to also ply beneath the waves, adding a new challenge to submarine operations.

Congested sea lanes and shallow seas have not deterred regional navies from adding even more submarines to regional waters. The underwater space will get even busier as more subs patrol regional sea lanes.

These challenges underline the importance of a code of conduct for the underwater domain to enhance maritime safety for all sea users.

•The writer, a former defence correspondent of The Straits Times, is a member of the Advisory Council on Community Relations in Defence (Accord).