Driven tubular steel piles

How does Driven tubular steel piles work, step by step?

1. 1

   Step 1: Confirm the ground model and the driveability assessment

   Marine ground investigation is expensive and always thinner than anyone would like, so the first move is to reconcile the boreholes, the geophysics and the seabed survey into a single ground model the whole team works to. The designer and the specialist contractor then run the driveability assessment: whether the tube as designed can reach the founding level with the plant proposed, where refusal is likely, and what the contingency is if the pile stops high. Obstructions, boulder horizons, buried debris and old structures are all mapped as far as the data allows. The wall thickness, the toe detail and the driving plant all come out of that assessment rather than out of a rate.

2. 2

   Step 2: Fabricate, coat and mark the piles ashore

   Tubes arrive as cans and are welded into pile sections in a fabrication yard where the welding can be done properly and tested. Circumferential welds are inspected, the coating is applied to the specified extent in controlled conditions, and lifting points, driving shoes, internal stiffeners and anode brackets are fitted while the pile is still on land. Each pile is marked with a graduated scale so that penetration can be read off during driving, and the marks are checked against a tape before it goes to the water. Everything that can be done ashore is done ashore, because the same operation over water costs several times as much and is harder to prove.

3. 3

   Step 3: Set up the marine plant and the position control

   The marine contractor either moors a piling barge on an anchor pattern or jacks a platform down onto the seabed. A jack-up is steady and precise once its legs are proven, but slow to move; a floating spread moves quickly and works within tighter weather limits. Positioning runs on satellite positioning checked against shore control, with pile positions translated into the working frame of the plant. The daily plan carries the tide windows, the wave and wind limits for each operation, and the point at which work stops. On most projects the seabed at each pile position is re-confirmed by survey before the pile is pitched.

4. 4

   Step 4: Pitch the pile and hold it to line and rake

   The pile is lifted by crane and pitched into a template frame, into the leaders, or into a gate arrangement fixed to the plant. The template is the single most important piece of temporary works on the project, because it holds position and rake while the tube takes its own weight and starts to run under self-penetration. Verticality and rake are checked by inclinometer and by survey before driving starts, and are re-checked as the pile goes down. Raked piles are harder in every respect - the lift is awkward, the hammer has to be supported in line with the pile, and small errors at the top become large errors at the toe.

5. 5

   Step 5: Run the soft start and the noise mitigation

   Before full driving energy is applied, the marine environmental controls are put into effect. On most projects that means a monitored exclusion zone around the pile, a period of observation before work starts, a soft start in which energy is introduced gradually so that mobile species can move away, and a noise mitigation system deployed around the pile where the marine licence calls for one. Observers have the authority to stop the operation. These controls are set by the marine licence and the project's environmental team, and the piling crew works to them without discretion.

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   Step 6: Drive the pile and record the drive

   Driving usually begins with a vibratory hammer while the pile is running freely, then changes to an impact hammer for the final penetration. The blow count is logged against penetration for the whole drive, and dynamic testing is carried out on a proportion of the piles so that capacity and driving stresses can be assessed. Where the pile meets material it will not penetrate, the sequence agreed in advance is followed - commonly drilling out the plug, advancing and re-driving. Refusal criteria, maximum driving stresses and the acceptable range of blow counts are set by the designer, and any pile that behaves unexpectedly is stopped and reported rather than driven harder.

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   Step 7: Splice, clean out and complete the pile

   Where the pile is longer than a single section, the next can is lifted, aligned and welded on with the drive paused, and the weld is tested before driving resumes. Once the pile is at founding level the top is cut to the design level, usually by burning from a floating access platform, and every cut edge and site weld in the splash zone is made good to the coating specification. Where the design requires it the plug is cleaned out by airlift or grab, reinforcement is lowered in and the pile is concreted, which turns the tube into a composite section and closes the top against the sea.

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   Step 8: Fit the corrosion protection and hand over the records

   Anodes are fitted and bonded, or the impressed current system is installed and commissioned, and the continuity of the protection is tested. Coating damage from handling and driving is repaired to the specification, with particular attention to the tidal and splash zones where loss is fastest. The pile record - as-built position, rake, founding level, driving log, weld tests, coating inspection and protection commissioning - is handed to the designer and to the port operator as the baseline against which every future inspection will be read.