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BURIAL RITES

INNOVATIVE GEOTECHNICS

Specially developed offshore investigation techniques are proving invaluable for assessing buried submarine cable routes.

Andy Barwise reports.

Installation of multi-billion dollar submarine telecommunication cables reached a peak during the late 1990s. Although it was slowed by the dotcom crash of 2000, there is a continuing need for systems that enable rapid transfer of data between traditional business centres, such as London and New York, and developing markets like India and China.

Site investigation company Lankelma is in the middle of a contract for a major cable route between Europe and the Far East. The firm is about to start the final leg of the contract, which involves collecting about 1,400km of continuous resistivity data and more than 600 CPT tests from the seabed.

Both natural and man-made hazards need to be considered in installing submarine cables.

They may have to traverse steep slopes, often in areas of seismic activity. Fishing and ship anchoring can also cause problems.

This means detailed and accurate survey and investigation of the seabed is critical to ensure cable systems are engineered and installed safely and cost-effectively.

As with large onshore projects, the first step is a detailed desk study to identify an outline route and potential hazards. Where possible, route engineering avoids clearly defined hazards such as steep slopes or licensed dredging areas. Additional protection is usually achieved by burying the cable.

Submarine cable installation is a complex and time-consuming task requiring a range of disciplines and skills. For trans-ocean networks, cables need to be placed in water depths of up to 9km. Particular consideration needs to be given to tides and currents, the direction and magnitude of which can vary significantly within the water column.

Sophisticated computer modelling packages have been developed to allow highly accurate installation in deep water: a cable installed in the Pacific off the coast of California in water depths of 2km was placed with an accuracy of 5m.

Installation is even more difficult when the cable needs to be buried. Ploughing is the preferred method and although based on the same principle as agricultural ploughs, submarine cable ploughs are complex electro-hydraulically powered machines. They are computer controlled from the cable ship via a fibre optic link and in deep water may be several kilometres behind the vessel.

Before cable burial became routine, standard offshore surveys for choosing cable routes consisted of bathymetric, side scan sonar and seismic reflection surveys covering a wide corridor.

Seabed sampling was limited to widely spaced (5km to 10km) surface grabs or cores.

These investigations are clearly not detailed enough for ploughing projects. One option initially adopted by the cable industry was a plough assessment survey where an instrumented grapnel or mini plough was pulled along the route.

Data recorded, including penetration and tow force, was reviewed against the route survey and burial performance assessed. But this approach had a number of shortcomings, as results were sometimes difficult to interpret and costs high.

Cone penetration tests (CPTs) were introduced to burial assessment surveys in 1997.

Specially designed CPT and resistivity systems are now available to investigate the seabed in water up to 1km deep.

The CPT has the advantage of providing primary geotechnical parameters, allowing mathematical models to be developed for plough performance using classic soil mechanics theory.

CPTs are typically carried out every 5km, with additional tests at targeted locations. But plough performance predictions can only be made with confidence for the area at or near the test location.

Additional investigation techniques are needed between the test sites. One is continuous resistivity profiling using a towed array.

Resistivity surveying is commonly used for land geophysical investigations but is relatively new to deep water investigations.

The technique maps lateral and vertical changes of electrical resistivity of the seabed continuously and quickly. By normalising the data against the electrical resistivity of the seawater, it is possible to determine a 'formation factor'which can be related to porosity.

Formation factors are derived using processing software. The formation factor is a constant value within a certain range of water resistivities. The electrical resistivity and formation factor is low for loose sand and soft clay. Higher values are found with consolidated sediments.

Rocks with very low porosity have the highest formation factors.

The burial assessment survey may be carried out using a wide range of vessels. The key criteria are the ability to hold station and accommodate a launch and recovery system able to deploy the CPT and resistivity systems.

A survey may take several months and is carried out 24 hours a day by a team of 10 to 12, including two positioning surveyors, two geotechnical engineers, two electro-technicians and two CPT and resistivity operators.

The ship's crew may consist of another 20 people. Typical day rates are up to £15,000, excluding the cost of the vessel.

In June, Lankelma mobilised a Neptune CPT, a DSR resisitivity system and a dedicated launch and recovery system on a cable laying vessel.

Lankelma's DSR3000 seabed resistivity system is mounted on a bottom-towed sled trailing a streamer configured with an inverted Schlumberger array.

A pair of electrodes at the centre of the streamer injects a current of up to 25A at 48V, giving potential penetration of about 5m. Seven pairs of potential electrodes are spaced along the streamer, each pair looking at a deeper 'slice' of seabed. The innermost pair will only return data for the superficial sediments and the outermost pair will return an average of all units down to a maximum depth.

The sled carries a CTD (conductivity, temperature, depth) probe and constantly monitors any changes in salinity that will affect resistivity data.

In some areas of the globe, seawater conductivity is very stable (for example in the Mediterranean), but it can vary over relatively short distances. It is therefore necessary to measure seawater conductivity to allow derivation of the formation factor.

The Lankelma Neptune CPT was specifically developed for the cable industry. The 1.5t unit uses a 2.5cm2 piezocone measuring tip resistance, local friction on the sleeve and pore pressure behind the cone shoulder. It is possible to identify changes in seabed type and to target CPT locations using resistivity survey data.

Both the resistivity and CPT systems are operated via a control umbilical and monitored in real time. Acoustic transponders are used to provide accurate positioning of the systems on the seabed. Initial data processing is carried out aboard ship before being completed onshore.

CPTs are interpreted using published empirical relationships between cone data and soil type. Estimates of relative density for cohesionless soil and undrained shear strength for cohesive soils are made.

While it is possible to use these values to develop mathematical relationships of plough performance, the preferred approach is to use empirical correlation factors derived from back analysis of burial records.

Using the results of the burial assessment survey, it is possible to divide the cable route into sections of similar burial conditions.

Predictions of burial depth, speed and tow tensions are made.

Results are normally presented on a series of alignment charts.These present the route alignment with a number of panels summarising the geophysical route survey, the CPT and resistivity survey and burial predictions.

These are used to plan the cable laying and burial operations and to identify potential hazards to both the cable and plough.Where insufficient burial is likely to be achieved, additional protection in the form of cable armour and/or rock cover is provided.

On the current job, Lankelma has been collecting continuous resistivity data from the 10m contour to 1km of water depth contour. Test results are reviewed in relation to the geophysical data available on board to enable the burial assessment to be made.

This work has been managed offshore by Lankelma's Andy Barker, who was on board for the first eight weeks of the survey and will shortly start another eight weeks for the final leg. The final review and interpretation of all the data into a detailed burial assessment is performed by Lankelma's sister company and offshore geotechnical consultant Setech.

Andy Barwise is technical director at Lankelma CPT.

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