Sand Cell Structures

A clean and fast offshore pad construction in environmentally protected areas.

SEEDA is an association of experienced Canadian engineers and associated professionals offering a wide variety of services to clients in North America and overseas. SEEDA understands that some structures could be built in 10 to 25-meter water depths to explore for petroleum resources. There is extensive Canadian experience in the development of unique temporary structures for this purpose. In particular, a Canadian engineering company developed and patented sand structures system that was designed in the mid-1970s, and a prototype island was constructed off the south coast of England in 15 meters of water.

The technology is well-known in the geotechnical community – when non-saturated sand is compressed by external pressure, the shear strength increases to that of lean concrete. This allows islands to be constructed with near-vertical side slopes reducing the quantity of sand and construction time required to achieve a given working area above the surrounding water level. This is made possible using a differential pressure between the exterior of an enclosing membrane and the interior where a standard dewatering system reduces the water pressure. Larger structures can be constructed using a series of abutting sand cells to form a perimeter and then the center filled with more sand. Such system vastly reduces the quantity of sand required to construct an island with a given surface area above sea level.

Between 1974 and 1976 a series of model tests were carried out in the laboratories of Kings College in Central London. The first test was on a 40 cm high model and then a special concrete testing tank was constructed to allow a 2-meter high model to be built and tested. With successful test results, the technology was patented, and a consortium of Canadian, British and Dutch companies was established to construct a prototype in 15-meter water depth in a real marine environment of the South Coast of England. The prototype was constructed in 1976 using the following plan. An octagonal floating steel deck weighing about 40 tons was built in a shipyard near Southampton in Southern England. It was 10 meters in diameter, and the neoprene “bag-shaped” membrane was carefully folded membrane and attached, and a tug towed the deck to the selected location in Christchurch Bay where a vessel was anchored that contained the sand and delivery system. There are strong tides in Christchurch Bay, so construction did not start until the current reduced to zero when the tide was turning. A hopper full of gravel was quickly dropped to anchor the bag to the seabed, the drainage pipes were lowered to the bottom of the neoprene bag, and a conveyor belt delivered 5000 tons of sand and the de-watering system activated. Twenty-four hours later a stable sand island “sandcastle-shaped” structure named “SANDISLE ANN” was complete, and two days later the construction crew left the structure which had been equipped with an automatic pumping system in case there were leaks where the membrane was connected to the steel deck. SANDISLE ANN survived for almost a month unattended by design or construction personnel. A violent autumn storm likely eroded the sandy seabed around the base of the island causing it to fall over sideways.

Individual sand islands can be constructed side by side to form the outside “wall” of a larger island surface above sea level. At the end of drilling the de-watering pumps can be turned off and in time the membrane will rupture and the island collapse back into the seabed.