22 Ground investigations over water
COMMENTARY ON 22.1
Ground investigations conducted over water are more expensive and time consuming than comparable investigations conducted on land, and there can be a temptation to reduce the scope of the investigation; economies in this direction can turn out to be false.
The scope and extent of ground investigations over water should be realistically assessed so as to be commensurate with the proposed construction.
NOTE 1 Further guidance on undertaking ground investigations over water can be found in BS EN ISO 19901-8.
Geophysical surveys should be used at the planning stage where needed to provide geological information for the ground beneath the construction site and the positioning of the investigation boreholes.
Owing to the difficulties in sinking boreholes below water (particularly with regard to rotary core drilling, rotary open hole drilling, cone penetrometer testing (CPT) and certain in-situ testing and sampling techniques), working platforms should be fixed/elevated or floating with heave compensated drilling equipment fitted. A conductor pipe should be suspended from the platform to protect the flexible investigation string from the forces of currents. Percussion boring and rotary drilling techniques, both conventional and wireline, can be employed. Geophysical logging techniques should be employed where needed to augment the information obtained from the borehole programme.
NOTE 2 Increasing use is being made of a variety of penetration testing and sampling techniques, originally developed for work further offshore, where specially designed boring, drilling or penetration testing equipment is lowered to the seabed to be operated by remote control or by a diver. Penetration depths vary from less than 5 m for some devices, to 20 m or more for others providing the ground conditions allow.
Over water work is always subject to a risk of delay because of unsuitable environmental factors (weather conditions, sea state and currents); the need to find a balance between a higher daily operating cost and an extended programme with long environmental delays should be taken into account at the planning stage. The scope of the work, including the methods of boring, sampling and in-situ testing, should be planned with the particular difficulties of the site being taken into account.
NOTE 3 For any given weather condition, the amount of delay depends on the type and size of the installation or working platform. In general, the larger the working platform, the smaller the risk of delays due to weather but, on the other hand, the greater the operating costs.
When working over water, health and safety requirements, navigational warnings, and the regulations of governmental departments and other authorities should all be taken into account. All marine craft utilized, whether floating or elevated, should have the correct marine certification, documentation and should ideally be in class.
NOTE 4 Class certification is not needed for all near shore/inland waters.
22.2 Stages and work platforms
COMMENTARY ON 22.2
Where stable working platforms are available or can be provided, such as jack-up drilling platforms, jetties or purpose-built scaffold stages and drilling towers, it is normally possible to use conventional dry-land ground investigation boring equipment and conventional methods of sampling and in-situ testing.
If borehole locations allow, it might be possible to work from existing structures; if so, it might be necessary to construct a cantilever platform over the water on which to mount the drilling rig.
When drilling close to the shore in very shallow water, it might be convenient to construct a scaffold or other tower at the borehole location. In this case, some means of transporting the boring equipment to the work stage is needed as well as all precautions for working over water including a safety support vessel.
An environmental loading analysis is also needed prior to mobilizing the work stage to ensure it is robust enough to withstand the local conditions. Some towers are so constructed that they can be moved from one borehole location to another without having to be dismantled.
Jack-up platforms can be floated into position and then jacked out of the water to stand on their legs. These fulfil the requirement for a fixed working platform and provide stable highly manoeuvrable safe craft from which to operate. Special craft fitted with spud legs can be floated into position but do not elevate out of the water like a jack-up platform. They conform more to floating craft and offer manoeuvrable work platforms but are more environmentally sensitive than jack-up platforms.
The design of all staging, towers and platforms should be deemed to be "temporary works" and designed accordingly. The design should take into account the capability of the bed to withstand the foundation loads including those associated with the drilling operations. The design should also take into account the effects of the fluctuating water levels and currents due to tides, waves and floating debris.
The choice of jack-up platform or spud leg barge should take into account the bed conditions with regard to leg penetration and in particular rapid leg penetration (punch through). Any jack-up platform used should have a pre-load procedure to ensure that rapid leg penetration is mitigated. Small, near shore, three-legged jack-up platforms should not be used as they do not have a secure pre-load mechanism.
22.3 Floating craft
When selecting a floating craft suitable as a boring vessel, the following should be taken into account:
- a) the anchor holding properties of the bed;
- b) the likely weather conditions;
- c) the depth of water;
- d) the strength of currents;
- e) whether the water is sheltered or open; and
- f) whether accommodation is required on board for personnel.
In inland sheltered waters or rivers, a small anchored barge might suffice, but in less sheltered waters a barge should be of substantial size, and anchors should be correspondingly heavy.
NOTE 1 In offshore conditions, rather than using a floating barge, a jack-up platform could be the chosen work platform; however, in deeper waters a ship can be employed.
If a barge is used for the work, an auxiliary vessel should be employed to handle the moorings, but in certain cases a ship might be able to lay and pick up its own moorings. Four or six point moorings should be used and anchors should have the best holding capacity feasible for the expected bed conditions. In water depths greater than about 80 m conventional moorings are difficult, and vessels that can be maintained in position by computer-controlled thrusting devices (i.e. dynamically positioned) should be used.
In order to achieve high quality sampling and coring, constant vertical pressure should be maintained between the drill bit and the bottom of the hole.
NOTE 2 For shallow water investigations, coring is usually carried out using an hydraulic power swivel suspended in the drilling mast or from a fixed feed frame. These methods are only suitable if the drilling machine is mounted on a jack-up or fixed platform, as any significant movement from an unfixed or floating work platform can have a detrimental effect on core quality and risk the loss of broken drill strings.
NOTE 3 Dynamically positioned vessels can typically be used in water depths greater 20 m and offer the benefit of not interacting with the seabed apart from at the borehole location meaning that a smaller area of the seabed needs to be assessed to be free of obstructions.
NOTE 4 For deeper waters, particularly with swells and where jack-up platforms cannot operate, for high quality coring operations, special techniques of heave compensation such as piggyback coring can be used.
22.4 Inter-tidal working
Sinking boreholes between high and low tide levels should be carried out using scaffold stagings or platforms (see 22.2), by using flat-bottomed pontoons or shallow draft jack-up rigs or by moving boring rigs to the location during periods permitted by the tides.
NOTE Shallow draft jack-up rigs allow for a higher level of operability and safety to all of the above methods as they remain level and stable at all states of the tide.
22.5 Setting-out and locating borehole positions
COMMENTARY ON 22.5
Modern positioning systems, such as the Global Positioning System (GPS) and the Differential Global Positioning System (DGPS) are now commonly used, because they are simple to operate and provide accurate surveying data.
Guidance notes for surveying, including GPS, and for different purposes and equipment are available from the Survey Association's website.10) More generic guidance is produced by the Ordnance Survey.11)
The location of investigation points should be established in relation to an appropriate coordinate system. Electronic methods of position fixing should be used wherever possible. Close inshore, boreholes may be set out by using sextant observations to features onshore or by lining in from previously placed shore markers. Where necessary, theodolite observations can be taken from land-based stations. Further offshore, electronic methods of position fixing should be used; electronic methods are also advantageous in poor visibility conditions.
22.6 Determination of reduced level of bed
Reduced levels of the bed should be determined. This should be done:
- a) by determining the elevation of a fixed point on the platform or vessel using DGPS at the same time as measuring the vertical distance to the bed level and the water surface; or
- b) by transferring reduced levels of the water surface to a boring vessel from a tide gauge set up close inshore, which is read at frequent intervals throughout the tidal cycles, to readings of water depth taken at the same time on the boring vessel.
Corrections should be carried out, where necessary, to allow for tidal variations when the readings of the tide gauge and the vessel vary significantly.
NOTE The depth of water can be difficult to determine where the bed is very soft and reduced levels of strata boundaries would then become less accurate.