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A talent for making the model match the real thing makes Dr Wim van der Molen’s research into the numerical and physical modelling of moored ships a highly sought-after skill in the context of layout and maintenance of South Africa’s port infrastructure. Van der Molen is part of a team of specialised researchers in the field of coastal engineering and port infrastructure (as a subset of infrastructure engineering), which operates from the CSIR’s offices in Stellenbosch.
A civil engineer by training, Van der Molen specialised in hydraulic and coastal engineering for his postgraduate degrees. He completed both his Master’s and PhD degrees at the Delft University of Technology in The Netherlands and has been working for the CSIR since 2007
Natural variables: How to roll with the punches
The productivity of a harbour is measured by throughput of containers and volume of goods such as coal and iron ore. The total throughput in South African ports for 2008 was 185 million tons of bulk cargo and 3,9 million containers. Operations should ideally run continuously for 365 days per year for optimal maritime trade and return-on-investment in the infrastructure. It costs approximately R10 billion to build a new harbour which includes, amongst others, the channel, the breakwater, the buildings and inland connections.
However, natural variables common to coastal areas such as storms, wind and waves cause ‘downtime’. This is defined as days lost due to conditions that do not allow safe mooring or safe loading or offloading of containers.
Waves are the biggest variable in this mix, particularly in South Africa. South Africa has eight major, well-equipped harbours and associated infrastructure located on its coastline, which spans 2 700 km from Namibia to Mozambique.
In harbours with terminals, it is therefore important to understand the ‘what’ and the ‘how’ of waves and particularly, their effect on moored ships.
One of the model ships used in the hydraulics laboratory at CSIR's Stellenbosch site
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Keeping ships tied down tight
Waves vary greatly in size. The heavy winter storms in the south Atlantic Ocean have been known to whip up waves of 17 m over the Cape peninsula. Although wave forces can be dissipated through breakwaters and other infrastructure, moored vessels need to be secure within a harbour environment.
A large tanker of 200 000 tons and over 300 m in length is moored to the quay in a harbour by means of 16 mooring ropes which secure it in the front and at the back. Each mooring rope has a breaking strength of 100 tons; by comparison, a rope used to tow a vehicle has a breaking strength of 2 tons.
However, tying down a ship may not always be enough. Documented cases of mooring ropes breaking due to wave forces come from Australia and New Zealand, with loss of life and major recovery cost implications. Mooring lines have had to be replaced in the Port of Saldanha, in a strictly controlled process.
Understanding how waves penetrate harbours and how they affect moored ships is therefore essential to realise the benefits of maritime transport. Tight criteria are in place on wave heights. To this end, Van der Molen’s specialities – moored ship modelling and manoeuvring modelling (a new field) – are invaluable in enabling decision makers to plan and implement harbour infrastructure and operations effectively.
Keofloats allow researchers to determine small wave size accurately in a specific harbour layout
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The smaller the waves, the better ...!
Van der Molen and his colleagues have a number of research techniques at their disposal. “Numerical modelling is a useful starting point,” he explains. “Using specialised software it is possible to make runs for different environmental conditions, factor in uncertainties and to play around with different layouts.”
The next step is to build a physical model to check the findings from the numerical modelling in a ‘real’ environment, in this case, the hydraulics laboratory at Stellenbosch .
A port layout is constructed precisely to scale for testing, while wave generating equipment allows researchers to generate waves of a predetermined height.
The innovative keofloat system is one such physical model that has earned Van der Molen accolades from his peers. The novel wave measurement system is used to verify attenuation or weakening of waves in a port. The system relies on the use of small cylindrical polystyrene blocks called 'keofloats' (4 cm in diameter) to measure small waves in a harbour basin constructed in the hydraulics laboratory at a scale of 1:100. The keofloats are agitated by waves produced by a wave generator and monitored by a video camera.
Data captured by video image processing specialists are analysed using a computer algorithm. It is possible to determine the vertical position of the keofloat at sub-pixel accuracy. The estimated accuracy is 0,2 mm or better for waves smaller than 1 cm, whereas conventional probes guarantee 0,5 mm to 1 mm accuracy.
He explains, “We are able to accurately determine the size of small waves in a specific harbour layout taking into account the physical wave phenomena of diffraction around breakwaters, refraction (curving of wave propagation) around shipping channels and in shallow areas, and wave reflection against revetments and quay walls.”
Other modelling techniques include Keoship, the building of model ships to scale (carriers, tankers or container ships), which includes mooring lines and fenders, with measuring devices. Specific methodology is used to monitor and capture the effect of waves on these ships.
Taking expertise abroad
The CSIR has used keofloats in a project for a marina in the Seychelles and a port in the Persian Gulf. Van der Molen keeps up his contacts with international clients, notably consultancies based in The Netherlands, the UK, the US and Australia with whom he worked on projects in Nigeria, Australia and Southeast Asia.
As a relative newcomer to South Africa who was ‘recruited’ by longtime staff member, Hans Moes, he enjoys the good South African weather to pursue his passion for sport – cycling, running and golf. He is keen to travel more and discover other parts of the country
in the foreseeable future, even places with no ports!
CSIR Communication:
Biffy Van Rooyen, email: BvRooyen@csir.co.za
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