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Engineering on the sea floor

by  Keith Lewis

Swept by strong ocean currents and waves of sand or gravel, and shaken by earthquakes, the ocean floor around New Zealand is a hostile place. Underwater cables, pipelines and huge oil-drilling platforms have to be strong enough to withstand the full force of the elements.


A hostile seabed

Much of New Zealand lies in or near the stormy latitudes known as the roaring forties, between 40° and 50° south. The country’s elongated shape and north–south orientation means that it is buffeted by prevailing westerly storms, ocean swells and currents. In addition, it lies astride one of the world’s major plate boundaries, where the seabed is ruptured by faults.

New Zealand’s dynamic sea floor can be both a challenge and a nightmare for offshore engineers. Marine geologists can determine the risks posed by natural processes to structures built on the seabed.

Cook Strait

Some of New Zealand’s largest offshore engineering projects have been around Cook Strait, where storms and strong currents are funnelled between the North and South islands. They scour the seabed, or stir it into abrasive sand or gravel waves.

Submarine avalanches

In deeper water off the continental slope the effects of storms, waves and tides may be less important, but underwater avalanches and landslides are hazardous to the submarine communication cables that form vital communication links between continents.


Submarine cables

The first Cook Strait cable

In 1866, only a few decades after the arrival of the first European colonists, a simple copper telegraph cable was laid across the stormy and tide-scoured seabed of Cook Strait. It was not reliable. Further attempts were made in the late 19th century, but it was not until the 1920s that a reliable telephone cable link was established between the North and South islands.

Early cables were laid with little understanding of the extreme conditions of Cook Strait’s seabed. In the late 1950s plans were underway for a submarine cable linking South Island hydroelectric power generators with North Island towns, and only then were serious attempts made to understand how such cables were affected by the underwater environment.

Testing the Cook Strait seabed

In 1959 Henry Pantin was appointed New Zealand’s first marine geologist. His initial job was to determine whether Cook Strait’s well-known tidal rips would affect the seabed along the proposed route of the power cable. Scientists in other parts of the world had traced sand movement, but the seabed that Pantin found was made of pebbles.

He devised a unique way to measure pebble movement. Magnetic ironsand from the North Island’s west coast beaches was made into concrete blocks, then crushed into pebbles. These were dumped in heaps on the seabed at marked locations. Samples of the sea floor were taken at regular intervals in a grid around the dump sites, and tipped into a chute, where magnets sorted out the ironsand pebbles. The pebbles had moved far from where they had been dumped, indicating that the sea floor was swept by strong currents. Pantin’s study showed that the proposed cable would have to withstand constant battering by shifting pebbles. Re-routing the cable was considered too expensive at the time.

Despite being heavily armoured, the power cables laid in 1964 had a troubled history. They were replaced on the same route, along with new telecommunications cables, in 1991.

To avoid hazards in the strait’s narrows, in 2000 a new fibre-optics telecommunications cable was laid along the comparatively benign (but longer) route between Levin and Nelson.

International cables

Since 1876, when the first telegraph cable from Sydney came ashore at Cable Bay, near Nelson, New Zealand has been linked to the world by a web of fragile cables. These are laid across oceans that are typically 5 kilometres deep and studded with rocky seamounts.

Since the 1980s fibre-optic cables capable of carrying huge amounts of data have been laid along the ocean floor. They stretch from near Auckland to Sydney, and onto South-East Asia and North America, with links to the Pacific Islands. Underwater volcanic vents, rocky ridges and abyssal trenches, as well as avalanche-prone areas and deep-ocean channel systems, are all hazards that must be avoided by trans-oceanic cables.


Drilling the Māui gas field

In 1969, one of the largest gas fields yet discovered was found on New Zealand’s doorstep. The Māui gas field, as it came to be called, lay about 30–50 kilometres offshore from Taranaki in open ocean. At the time, however, it was one of the most adverse environments ever to be considered for gas extraction. The engineering problems were immense – new technology was required, under tight deadlines.

Environmental challenges

The Māui field lies kilometres beneath thick sedimentary rocks that are 110 metres below the water’s surface. The area is exposed to Southern Ocean swells and Tasman Sea storms. Measurements obtained during exploration suggested that the production platform which sat above the sea must be able to endure waves up to 22 metres high and wind gusts of 260 kilometres per hour. The field is near active faults, so the tower, rising 275 metres from the sea floor, has to be capable of withstanding earthquakes up to magnitude 8.

The pipelines that carry oil and gas to the shore are 610- and 254-millimetres in diameter and covered in concrete. They cross underwater beds of boulders the size of small cars, dumped by catastrophic fluid avalanches (lahars) from the volcanic cone of Mt Taranaki. Supports and protective rock embankments had to be built to buttress the pipes between mounds of boulders.

Māui A platform

After many trials in appalling weather, the Māui A platform was completed and functioning in 1979. Since then it has produced about 80% of New Zealand’s gas and oil.

In stormy seas

The Māui A oil and gas tower was built in a Japanese dry dock. Two tugs then towed it 8,400 kilometres to New Zealand. The voyage was easy compared with the task of upending and pinning the tower to the sea floor in rough weather. In 1976 New Zealand experienced its worst summer weather for 25 years. Experienced oil workers based at the tower began to rate the Tasman Sea as more inhospitable than the treacherous northern North Sea.

Māui B platform

As the flow of gas and condensed light oil began to run out at Māui A, another part of the field, richer in light oil, was brought into production. 1993 saw the construction of the un-manned Māui B platform. It is 15 kilometres further offshore than Māui A, and has pipelines linking the platforms and running to shore. However, this platform’s days are also numbered. Other fields in the same area have been explored, and some of these off the north and south Taranaki coasts are to be exploited in the late 2000s.

Drilling other fields

The technology to develop shallow offshore gas fields is now well established, but drilling deeper fields is a challenge. Economic gas and oil reserves have not yet been found elsewhere around New Zealand (outside of Taranaki). Although gas in the form of ice-like hydrate seems to be widespread under the continental slope to the east of the North Island, no one has yet devised a way to exploit these deposits.


Coastal works

Breakwaters

For at least 150 years New Zealand has been pushing its harbours, towns and effluent out beyond the shifting shoreline. Wharves and harbour breakwaters reach seaward, often causing damage to the coastal seabed. When structures are built on the water’s edge, the sand that naturally moves along a coastline can become trapped. This can lead to coastal erosion in some areas and a build-up of silt or sand in others.

Reclaimed land

In many places along New Zealand’s shores, shallow seabeds have been reclaimed to supplement scarce flat land. As its name suggests, Wellington’s downtown street, Lambton Quay, used to be the waterfront. The engineering technology involved in reclamation is relatively simple. Rocks and rubble are poured into the shallow water until they rise above it. This fill is compacted and allowed to settle.

A bit of Bristol

The beach at Oriental Bay, Wellington, has only a small natural supply of sand. A sandy beach had been engineered there long before it was re-nourished with South Island sand in the early 2000s. On 9 October 1944 the Evening Post reported that 10,000 tons of clean, Bristol Channel sand, originally used as ships’ ballast in vessels from England, found a new use and was tipped over the sea wall.

Pipelines

As standards of hygiene improved around 1900, reticulated sewerage systems discharged waste, often through short seabed pipelines, into the ocean. Except in a few places where ocean currents carried the effluent away, this replaced one health hazard with another. Swimming and collecting shellfish had to be banned in areas close to many of these outfalls. More recent offshore pipelines have been kilometres long, and carry only treated effluent to areas where coastal currents will disperse it away from populated coastlines.

Pipelines also carry other products, mainly oil and gas. One off Waikato carries ironsands from the beach out to Japanese ships moored 3 kilometres offshore.

Environmental issues

There has been some belated respect in recent times for the fragility of the seabed and shoreline. This has been largely due to the Resource Management Act 1991, which placed tight restrictions on coastal engineering works and structures built along the shore and on the sea floor.


External links and sources

More suggestions and sources


How to cite this page: Keith Lewis, 'Engineering on the sea floor', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/engineering-on-the-sea-floor/print (accessed 22 October 2019)

Story by Keith Lewis, published 12 Jun 2006