I wrote this clip for the Bangor University School of Ocean Sciences Alumni Newsletter, “The Bridge” (Summer 2023).
Sea level – from nearly under to floating on top
There comes a point in your life when you wonder why you’re doing what you’re doing, and whether you want to carry on doing it.
This point came to me when I sank knee-high in mud installing an ADCP instrument in freezing seawater with a rapidly incoming tide on the mud flats of the Wadden Sea (north of the Netherlands).
I was managing a data collection project which supplied wave data in winter storm conditions to validate a numerical model. The local authorities would use this model to inform their decisions on what to do with an existing dike to protect the area against future sea level rise.
As I tried freeing my legs from the grips of an unnerving combination of cohesive sediment forces and suction, I took stock of the sticky situation I faced. I was on the wrong side of the dike and most of me was about to be on the wrong side of the sea surface. It would be nice to keep at least my head above sea level – and a cup of warm tea back in the office would be nice too.
My first step towards a new career was on a bearing of 135 degrees back to the coastline. Or at least, a part of it where my ankles would once again feel the sting of the cold winter air after being submerged for the better part of two hours (“better part”?). The journey back to dry and solid land started with my team mates pulling me out of the mud, and then a 15 minute walk over the very sticky, very squelchy mudflat. Very prone to losing a shoe. And some dignity.
I lost one of the first and all of the second.
Now I work at MARIN – a Dutch company specialising in maritime research – where the office is warm, dry and just 5 steps away from a coffee machine. MARIN has introduced me to a new world of seakeeping data analysis – collecting and analysing data not from the ocean, but from what floats (and hopefully remains floating…) on top of it.
“Seakeeping” is concerned with understanding how a boat reacts to different sea states. Knowing how vessels move in the sea is valuable information in improving onboard conditions such as crew safety and passenger comfort, as well as reducing the likelihood of damage to cargo and equipment, and informing route optimisation decisions.
My new role at MARIN is calculating seakeeping parameters by analysing measured data from scale-model tests. This is where boats hundreds of metres long are scaled down accurately into models of up to 11 m in length that sail under their own power in a basin nearly 200 m long and 40 m wide. The basin is fitted with some 330 wave paddles distributed along 2 edges so that different sea conditions can be generated. The model boat is surrounded by a carriage which follows and tracks the model as it sails across the basin, measuring its movements and accelerations thanks to a plethora of precise onboard instrumentation.
A whole feast of data is sent from the carriage to the “skybox” – a room overlooking the test basin – where I get to devour it and make sense of it.
We carry out several types of tests, though their nomenclature is somewhat illogical because we refer interchangeably between the vessel’s movement, the water condition, sailing speed or even the test type itself!
For example. “Transit tests” are tests where the boat sails across the test basin, but only in waves and not in calm water. “Calm water tests” are tests where the boat transits across calm water, but doesn’t apply to tests in calm water when its speed is zero. “Zero speed tests” are tests when the boat is in soft mooring (held steady with a system of springs) usually with waves, but not if it’s a “decay test” when the boat is made to wobble in either calm water conditions and at zero speed – or in transit at speed.
Testing a vessel’s response to a carefully determined sequence of wave heights, periods and directions and sailing speeds means test campaigns can run for several days. Often the vessels are set up in various configurations such as different draughts or stabiliser fin and anti-roll tank settings, so the testing time can easily double up.
It’s a busy, hectic time in the basin, and it’s followed by several weeks’ data analysis when the data are worked up into seakeeping parameters. These help address questions such as “What are the effects for the crew, the passengers and the cargo when the boat is hurled about by waves?”
There are parameters for everything, ranging from the motion illness rating (how sick someone feels) to the “response amplitude operator” which helps determine the effect that a tested sea state will have on any of the vessel’s 6 degrees of motion.
Everything is carried out with safety, operability and comfort at heart. For example, translating acceleration forces from the position of the accelerometer instrument on the model to a more useful location, like the lounge deck or cargo hold, helps to inform whether yacht passengers can sip their wine without spilling it, or how securely cargo needs to be fastened. Distribution functions of wave troughs and crests by the keel and deck provide an indication of how often a ship’s propeller might find itself out of water, or how often crew and passengers will be splashed by a wave.
My days as an ocean scientist using satellites in orbit higher than 1000 km to determine sea level rise to millimetre accuracy were fascinating! And ground-truthing the ocean itself with direct measurements was eye-opening (and cold!). The effects of sea level rise are well known; the effects of the sea states on vessels…I’m working on it! In both cases the end goal is the same – to keep our heads above sea level!
The picture of a bridge I’ve attached is the bridge of a ship from MARIN’s virtual reality simulation centre. This provides a realistic simulation of the behaviour of vessels and their interaction between maritime structures, the environment and humans, and is where, for example, captains practice their manoeuvres and directly experience the consequence of their decisions.
Paul Sterlini, B.Sc. Ocean Science (School of Ocean Sciences, Bangor University, 1995)
