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SEABED SECRETS

SEABED NVESTIGATION

Scientific ocean drilling has entered a new era. Dan Evans and Ali Skinner of the British Geological Survey chart the latest developments.

The history of international scientifi c ocean drilling extends back to the 1960s, but a new phase was initiated in 2003 with the start of the Integrated Ocean Drilling Program (IODP). IODP is a multinational research initiative for scientific drilling operations using specifically designed drillships or platforms.

Its purpose is to improve understanding of earth system processes ranging from changes in the earth's climate to the rifting and drifting of continents. This is achieved through marine drilling and logging, and undertaking research on the cores and data obtained.

IODP drilling operations focus on a capability provided by three scientifi c ocean drilling platforms.

Each partner fiances their own platform through public funds, and also contributes to a central pool of funds that is distributed by the central management organisation.

The non-riser Joides Resolution (or its replacement from 2007) from the US continued from its work on the Ocean Drilling Program (ODP) where it operated as the single platform. It has recently addressed problems including overpressure and fiuid fi ow in the Gulf of Mexico and the development of ocean crust at the Atlantic mid-ocean ridge.

This is now joined by the Japanese drilling vessel Chikyu with riser capability, and mission specifi c platforms (MSPs) provided by Europe, adding a new facet of international scientific ocean drilling .

The appropriate platform is chosen to meet specific objectives on the basis of science prioritisation undertaken by the IODP Science Advisory Structure. As the name implies, MSPs will be selected on a case by case basis to best address each scientific problem.

Launched in 2002, the Chikyu was delivered at the end of July 2005 and will now embark on many months of training off Japan before starting work for IODP in autumn 2007.

With the derrick top 121m above sea level, it will be able to handle a drill string up to 10,000m long, and the capability to drill to 7km below seabed in 2.5km of water.

It is intended that her targets will be in the Nankai Trough, to drill into the seismogenic zone associated with two tectonic plates that are colliding along a shallow-dipping fault plane. Several observatories will then be installed into the boreholes to monitor seismicity and fi uid movement in an attempt to understand the genesis of major earthquakes.

MSPs are the European contribution to IODP under the auspices of ECORD, the 17 country strong European Consortium for Ocean Research Drilling, that also includes Canada.

The ECORD Science Operator (ESO) consortium of European scientifi c institutions has been formed specifi cally to undertake MSP operations, comprises the University of Bremen, the European Petrophysics Consortium (Universities of Montpellier, Aachen and Amsterdam led by Leicester University), and the British Geological Survey (BGS) which acts as co-ordinator.

The first expedition for MSPs in IODP was to the Arctic Ocean.

For three weeks in August to September 2004, the Vidar Viking was engaged in drilling the Lomonosov Ridge to a depth of 440m below

seabed, an accomplishment not previously achieved in the Arctic sea ice. The objective was to reach an unconformity at 405m depth and recover the overlying sequence of essentially normally consolidated sediments that records the climatic history of the Arctic Ocean over the last 65M years (Shipboard Scientific Party, 2005). Data was not previously available, but the evidence is that the Arctic enjoyed a Mediterranean-type climate at the start of that period. The total cost of the expedition was $12.5M (£7M), and a lot of work is now being carried out on the cores in laboratories around the world.

The Vidar Viking normally works as a Baltic icebreaker in winter and an anchor-handling vessel during the summer. It was chosen for the

task as it was necessary for the drilling vessel to have icebreaker class, even though it was accompanied by two Arctic-class icebreakers, the nuclear-powered Sovetskiy Soyuz and the diesel-powered Oden (Evans and Skinner, 2005).

Considerable modification was required, including a 2m diameter moonpool, and a complete drilling system. This was installed by the

drilling contractor (GE May 2004), Seacore which designed, built and installed a new-concept ram rig. BGS supplied wireline coring equipment, and Schlumberger was the logging contractor. Both the ice management and fleet management teams based on the Oden were crucial to the success of the operation. The ice management team gathered and analysed data to predict the speed and direction of ice movement, and based on this data, the . eet-management team then co-ordinated the actions of the three ships so that the icebreakers were able to break down the ice floes and the Vidar Viking could hold position and keep drilling for long periods.

Another hazard that faces millions of people around the world is rising sea level, particularly in response to climatic warming and melting ice sheets. The MSP expedition in 2005 is addressing these and other issues by undertaking drilling of coral reefs around Tahiti. Coral reefs are excellent sea level

indicators as their growth keeps pace with sea level rise. Taking cores from successive reef terraces in this tectonically inactive reef setting located remotely from glaciated regions allows details of the 120m rise of sea level during the last deglaciation to be charted. Determining the rate at which

sea level rose at that time is important as it provides an indication of the rate at which sea level could rise with global warming.

The Tahiti drilling starts this month and goes on until November using the DP Hunter as the drilling vessel. This dynamically positioned, 104m long vessel has a large moonpool and has generally been used for diving and ROV work. The drilling contractor is again Seacore. The water depths range from 25m to 310m, and the project calls for the drilling of three transects, amounting to a total of 19 holes with penetrations of between 45m and 105m. Seacore

will once again be using its R100 rig, but for very high recovery rates required in porous coral, a miningtype wireline coring system will be piggy-backed on top of the conventional API rig which will deploy the conductor pipe, so that all coring is carried out in compensated mode.This type has a good track record in other coral reef situations worldwide. Tahiti is a volcanic island, and this drilling system will also be suitable for recovery of the basalt that

underlies the coral reef and marks the termination of drilling at each location. The conductor pipe will be a heavyweight drill pipe or casing with an ID of 100mm and a casing shoe to set it 1m into the seabed. A seabed baseplate of around 1.5m diameter and weighing 10t to 12t at the end of the conductor

will make re-entry possible. A wellproven wireline overshot core-barrel retrieval system allows continuous coring operations without havingto pull the drillstring to recover each core run. A typical core run is expected to be 3m. Living reefs are highly sensitive ecosystems and great care will be

taken to minimise the impact of drilling. Seawater will be used as the drilling lubricant, and the dynamic positioning of the vessel restricts seabed footprint to that of the hole and the baseplate. On location, the drillstring will be positioned by colour video camera to avoid live corals, and experience elsewhere suggests that few if any cuttings will come to the sea bed due to the porous nature of the reef. Sites will be occupied for only two to three days, so influence on the ecosystem will therefore be very brief, and should not have any significant impact on the living reef.

IODP is a 10-year programme, and much has already been achieved and planned. With the addition ofthe Chikyu, the programme will have greatly enhanced capablity to address exciting and relevant scientific questions through marine drilling.

References

Evans, D and Skinner, A C. 2005. Seabed coring in the ice-covered waters of the Arctic Ocean for IODP. Underwater Technology, Vol 26, 73-75.

Shipboard Scientific Party, 2005. Arctic Coring Expedition (ACEX): paleoceanographic and tectonic evolution of the central Arctic Ocean. IODP Preliminary Report, 302. www.ecord.org/exp/acex/302PR.pdf. More information on the IODP and the vessels and organisations mentioned can

be obtained from: www.iodp.org, www. jamstec.go.jp/chikyu or www.ecord.org

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