8/22/2019 Construction of Marine and Offshore Structures, 3rd Edition - Page 276.pdf

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Un-reinforced concrete plugs, both of lightweight and normal-weight concrete, havebeen employed on the concrete offshore platforms in the North Sea to seal the conductorsleeves. The plug is usually 1.52 m thick, with welded shear lugs on the sleeve to transfershear. These plugs are then drilled out prior to driving the conductors.

Increasingly, skirt piles for deep-water offshore platforms are arranged in clustersaround the corner jacket legs and their loading transferred to the jacket by means ofsleeves bracketed out from the sides. The final top of these piles will then be underwatera distance equal to the water depth less the sleeve length. This latter is usually 2030 m.The pile connection is made by grout.

To drive the pile so far below water requires the use of either an underwater hammer ora follower. Several types of hydraulic underwater hammers are now made, two of whichcan fit inside the pile guides which are bracketed out from the jacket at higher levels. Theseslim hydraulic hammers are now employed for piling in deep water since they deliveressentially full energy to the pile, without the losses inherent in the use of followers.However, for shallow water and inland marine structures, followers are usually

employed. The follower is a thick-walled pile section with a machined driving head onits tip which fits snugly over the head of the pile, transmitting axial compression, whilepreventing local buckling.

Occasionally, due to misalignment or minor variances in the pile head, the pile becomesjammed into the driving head and the follower cannot be removed. Then the pile must becut off, either by divers or else by a drill rig using expanding casing-cutter tools. To preventexcessive delays under such a circumstance, the corrective tools should be on board.

Experience shows that with a properly fitting driving head, a square cut on the pile, anda pile wall thickness that is not too small, that is, not less than 25 mm, there is very littleloss in efficiency by use of the follower.

Where excessively hard driving is expectedas, for example, when driving though

limestone or caprock strataa driving shoe should be provided at the pile tip. APIRP2A suggests that this be at least one diameter in length and have a wall thickness 1.5times the minimum thickness of the parent pile section. Experience in driving throughweak limestone containing embedded basalt cobbles has indicated that such a shoe should

be two diameters in length to prevent buckling like an accordion. Steel quality should be ashigh yield as can be properly welded; since the weld is made in the shop, it can be properlypre-heated and post-heated as required.

Cast steel shoes are available for the small- to moderate-diameter piles, also for steel Hpiles. These are more easily affixed and welded (see Figure 8.17). The shoe shouldnormally have the same internal diameter as that of the pile in case it becomes necessary

to drill through the pile as a casing; otherwise, the drill may catch. The slightly largerdiameter tip will relieve some of the skin frictional resistance on the main pile body. Wherethis is judged unacceptable by the geotechnical engineer, an internal pile shoe can be used,with its consequent restrictions on any drilling.

When grouting piles to the jacket sleeves or when grouting between an insert pile and aprimary pile, it is essential that the spaces be completely filled. Experience has shown thatgrout can trap water and bypass it unless great care is taken. As noted earlier, the steelsurfaces should be free of mill scale or varnish. Mud must be excluded from the annulus;this may require the use of wipers when working in very soft muds. The steel surfacesmust also be clean of marine growth (which may form in relatively short periods ofshallow submergence) and free from oil or other contamination. Bentonite drilling mud

should be flushed out with water where this can be done without endangering a drilledhole. If it cannot be safely flushed out, a polymer-based mud should be used.

Both neat cements and expanding cements are used; the latter can give improved bondand shear transfer. API RP2A requires that an expansive, nonshrinking grout be used.

Construction of Marine and Offshore Structures276

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