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Published By Lankelma

Lankelma is the foremost contractor for onshore in-situ soil testing in the UK. An acknowledged specialist in CPT, Lankelma also offers a worldwide consultancy and training service.

A.P. van den Berg develops, designs and manufactures geotechnical and environmental soil investigation equipment for onshore and offshore applications. Specialists in CPT systems and equipment.


Gardline Geosciences offers worldwide marine geotechnics, in-house consutancy and services with marine investigations ranging from nearshore to full ocean depth (down to 3000m).

About the Author

Hans Brouwer studied civil engineering at Delft University in The Netherlands. He has worked as a part-time lecturer at Amsterdam Polytechnic and was senior partner in a structural engineering consultancy. He has written a standard textbook in Dutch about the design of building foundations. He now lives in England where he writes technical textbooks in English, hopefully to reach a bigger readership.

Chapter 9

Near-shore marine testing

Jack-up platforms

Figure 80
A jack-up platform (Figure 80) generally consists of a base platform (either fixed or modular pontoons) with either three or, more commonly, four legs. The legs run through ‘bearing’ and ‘collar’ mechanisms which enable the platform to jack against the legs by hydraulic driven pin-andladder, inflatable bladder or rack-and-pinion action, elevating the platform out of the water.
Most jack-up platforms are modular and container transportable. This means that the platform components can be easily transported by lowloader and ship. Moreover, the jack-up platform can be constructed in a range of sizes. 
Jack-up platforms are particularly suited to operations in remote coastal sites, open water and high tidal ranges, particularly where deep exploratory investigations, long duration (10–72 hours) or a stable station is required.
Depending upon size and leg length, most geotechnical jack-ups can operate in water depths of 2–35 m. The main factors include water depth, leg penetration into the seabed, and prevailing wind and seastate conditions. 

It is important that significant leg penetration into superficial soils (ie >50% of leg length or leg section) is avoided as torsional forces resulting from waves and currents exert significant stresses on legs which may lead to leg or bearing failure. Similarly, leg toes and spuds must be stable on the seabed to prevent loss of bearing strength (eg through scour).
This is normally achieved by pre-loading the seabed to twice the point load of each leg, and then jacking on the legs – in the case of four legs, first one diagonal pair, then the other pair – once they are in position; obviously this jack-up operation is performed only with 4-leg platforms. Generally, 3-leg platforms should not be used in particularly exposed sites or where scour may be expected; however, some large 3-leg jack-up platforms are designed specifically to operate in extreme marine environments.
Cone penetration tests: operation
The CPTs are performed using either of the following methods.

  • As an integrated CPT system as part of a drill derrick
  • As a stand-alone fixed system installed over a moon-pool (a
    hole  in a platform deck)
  • As a cantilever system mounted over the side of the platform.

Once the jack-up rig is on location, the main riser casing is installed from
deck to seabed (or mud line). Usually this will be a drill casing of 300 mm
diameter. Further casings of 150 mm diameter can also be installed to
add rigidity. A final casing (heavy duty HWY drill collar or 55 mm diameter
CPT casing) is then lowered within the drill casing to the seabed; the
final casing has been specially bored out at the joints to enable a 15 cm2
penetrometer, or a 10 cm2 penetrometer and friction reducer, to pass
through it. This provides the required lateral support for applying the
maximum thrust on the CPT rods while reducing significantly the risk of
buckling. It is important, where possible, that the casings remain proud
of the seabed so that the seabed is not disturbed, allowing a continuous
CPT to be performed. This is not always possible, so often the casings
are bedded into the seabed by up to 0.5 m to ensure lateral stability. It
is critical, though, that the final CPT casing sits above and is not pushed
into the seabed since a soil plug could prevent the penetrometer being
pushed out of the casing at the start of testing.
Where the CPT beam is incorporated into a drill derrick and a drag bit is
attached to the leading section of the HWY casing, drill outs and retests
of refused CPTs is possible. This enables deep CPTs to be performed
rapidly and with confidence.
After the casings are lowered, the CPT system, placed on the derrick
beam, cantilever or deck clamps, is set in place to enable the CPT test
to be performed.
Typically, top-push systems are used to perform CPTs from jack-up
platforms (Figure 81). The hydraulic rams have a 20 tonne push
capability. The rams are powered directly from the main powerpack of
the platform. An auxiliary powerpack and generator is mobilised as
back-up to ensure 100% reliability of the equipment.
A calibrated electric cone is pushed into the soil using 1 m long
threaded rods inside the CPT casing. The deck-to-mud-line distance
and all relevant casing depths are recorded to enable the processor to
calculate the actual bed level and depth of seabed disturbed by the


casing. The cone penetration test will be performed in accordance with BS1377-9[12] andamended to incorporate the international reference test procedure for the CPT[1].
Unless otherwise dictated by seabed conditions,
it is wise to perform tests using a piezocone
penetrometer (CPTU) as this records the best
parameters for characterising marine superficial
soils, particularly soft alluvial and laminated soils.
The information below summarises the key points highlighted by these standards.

 Cone calibration
  • The cones, umbilical cable and recording
    equipment will have been calibrated for use in a wide range of soil types and consistencies. Copies of all calibration certificates will be available for inspection at the vehicle. The zero
    values are recorded prior to and post each CPT test.
  • Before each test the seals between different elements of a tip will be cleaned and inspected to check their integrity.

CPT testing

  • The test should be carried out under continuous penetration at the
    rate specified in BS1377-9[12]. The penetration rate is maintained at
    20 mm/sec ± 5 mm/sec throughout each stroke of the thrust
    machine. The readings are recorded at intervals of not more than
    20 mm depth
  • Each test is terminated at ‘refusal’ or as instructed by the supervising
    engineer. Refusal is defined as follows:
    ♦ total thrust equals nominal reaction provided
    ♦ cone end-load is 90% of the cone capacity
    ♦ sleeve friction is > 15% of the cone axial capacity
    ♦ with a string of pushrods, deviation from the vertical is > 15°
      between the tip of the cone and the top of the hole
    ♦ rod deviation is greater than 3° over a length of penetration of 1 m
      or less
    ♦ the operator feels that further penetration would damage the
  • The engineer has the ability to measure dissipation at specific
    locations and depths. In order to do this the test is temporarily
    stopped and the decrease in porewater pressure is monitored with
  • Copies of data can be printed out after each test, or on request, as
    preliminary data. After processing and quality assurance (QA)
    checking of the data, the final data can be printed out.

The jack-up rig should be equipped with an email facility to enable each
test to be emailed on completion for processing and interpretation.

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