Subsea Soil Investigation
Subsea soil investigations are performed by geotechnical engineers or engineering geologists to obtain information on the physical properties of soil and rock around the subsea field development for use in the design of subsea foundations for the proposed subsea structures. A soil investigation normally includes surface exploration and subsurface exploration of the field development. Sometimes, geophysical methods are used to obtain data about the field development. Subsurface exploration usually involves soil sampling and laboratory tests of the soil samples retrieved. Surface exploration can include geological mapping, geophysical methods, and photogrammetry, or it can be as simple as a professional diver diving around to observe the physical conditions at the site. To obtain information about the soil conditions below the surface, some form of subsurface exploration is required. Methods of observing the soils below the surface, obtaining samples, and determining physical properties of the soils and rocks include test pits, trenching (particularly for locating faults and slide planes), boring, and in situ tests.
- 1 Offshore Soil Investigation Equipment Requirements
- 2 Seabed Corer Equipment
- 3 Piezocone Penetration Test
- 4 Drilling Rig
- 5 Downhole Equipment
- 6 Laboratory Equipment
- 7 Subsea Survey Equipment Interfaces
- 8 References
Offshore Soil Investigation Equipment Requirements
The general requirements for soil investigations are as follows:
- Drill, sample, and downhole test to a minimum of 120 m below seabed.
- Carry out relevant seabed in situ testing, for example, a cone penetration test (CPT) to a maximum of 10 m depending on soil conditions.
- The actual sampling and subsequent handling are carried out with minimum disturbance to the sediments. The choice of sampler and
sampling tubes reflects the actual sediment conditions and the requirements for the use of the sediment data. Therefore, different types of equipment are required.
- All equipment capable of electronic transmissions is designed to sustain the water pressure expected in the field.
- Records of experience with the use of the equipment and routines and procedures for interpretation of measurements for assessment of sediment
parameters are documented and made available. A detailed description of the sampling is provided as is testing equipment, which includes the following:
- Geometry and weight in air and water of all sampling and testing equipment;
- Handling of the seabed equipment over the side, over the stern, or through the moon-pool as applicable;
- Required crane and/or A-frame lifting force and arm length;
- Any limitations as to crane and A-frame capacity, water depth, sediment type, penetration depth, etc.;
- Zeroing of the PCPT before deployment;
- During testing, recording of the zero readings of all sensors before and after each test. Calibration certificates for all cones are presented on commencement of operations. Sufficient spare calibrated cone tips should be provided to ensure work can be completed.
Seabed Corer Equipment
The coring equipment used should be of well-proven types and have a documented history of satisfactory operation for similar types of work. The seabed corers have a nonreturn valve at the top of the tube to avoid water ingress and sample washing out when pulling the sampler back to the surface. Both penetration and recovery are measured and recorded. The main operational requirements for the corers are as follows:
- The corer is capable of operating at seabed.
- The corer is monitored continuously in the water column using a transponder.
Piezocone Penetration Test
The main operational requirements for the PCPT are as follows:
- PCP equipment is capable of operating at seabed.
- All cones are of the electric type, and cone end point resistance, sleeve friction, and pore water pressure are continuously recorded with depth
- The PCP rig is monitored continuously in the water column using a transponder.
- Typical penetration below the seabed is up to 5mpending soil conditions.
- During PCPT operations, prior to the start of the penetration of the push rods into the soil, the following data are recorded: water head, the
resistance at the penetrating probe, the lateral friction, and the pore pressure starting from an elevation of 1 m above the seabed.
- The penetrometer is positioned in such a way as to provide the perfect verticality of push roads.
The drilling rig should be provided with all drill string components: the drill pipe, drill bits, insert bits, subs, crossovers, etc. The capability for the drill string on the drilling rig to be heave compensated such that the drill bit has a minimum of movement while drilling and performing downhole sampling and testing is very important. Borings are drilled, using rotary techniques with a prepared drilling mud, from the seabed to the target depth. The objective of the borings is to obtain high-quality samples and perform in situ testing.
Equipment for performing sampling and testing in downhole operation mode through a drill string is relevant to the investigation:
- Push sampling;
- Piston sampling;
- Hammer sampling.
An ample number of cones and sample tubes should be available. Push sampling is performed with thin-wall or thick-wall sample tubes, depending on the soil conditions. The main operational requirement for downhole equipment is that the equipment be used in the maximum relevant water and drilling depths.
The vessel is provided with either a room or a container to act as an offshore soil testing laboratory with sufficient equipment and personnel for 24 hour per day operation. All necessary supplies and equipment for cutting liners and sealing and waxing samples, including transportation boxes for shipping of samples to the onshore laboratory, have to be carefully provided for. The offshore laboratory varies depending on the nature of the project. Equipment is required for performing the following types of standard laboratory tests:
- Extrusion of samples;
- Description of samples;
- Bulk density;
- Specific gravity;
- Water content;
- Shear strength of cohesive sediment.
Subsea Survey Equipment Interfaces
Sound Velocity Measurement
Velocity profiles are recorded whenever necessary to ensure that the correct speed of sound in seawater is utilized for the calibrations of the geophysical and bathymetric instruments. The velocity of sound in seawater can be calculated with a recognized formula. All equipment is operated in accordance with manufacturers’ published instructions and conforms to manufacturers’ specifications. The velocity probe and winch system are capable of operating efficiently in survey water depths. Data are to be recorded on the descent to the seabed and recovery to the surface. The instrumentation is calibrated to the standards set by the National Bureau of Standards within the 12 months prior to the mobilization date. Calibration certificates should be included with the survey procedures.
Sediment Handling and Storage Requirements
Sediment samples are carefully marked, handled, and transported. Samples from the corer are cut in 1-m sections. The core samples are then stored in a cool place, but not frozen where shaking and shock are limited to a minimum. The sealed cylinders or waxed samples are clearly labeled with:
- Top (nearest seabed);
- A “Top Up” indicator (arrow pointing upward);
- Core location, attempt number, date, and company project number;
- Section number and depths at top/bottom;
- Length of core in meters. An identification label is placed inside the top cap. The sealed and marked sample cylinders and waxed samples are placed in boxes suitable for transportation. If possible, the core barrels should be stored vertically. Rooms adjacent to heavy engines or generators, which generate excessive vibrations, are avoided. The boxes with sealed sediment material are transported to the onshore laboratory with caution and handled with care. Special precautions are made to prevent shock and impact loads to the sediment material during handling of the boxes.
The sediment must not be exposed to temperatures. Whether samples are air freighted or trucked to the onshore laboratory must be decided in each case. Each cylinder and waxed sample is registered and stored for convenient retrieval. On completion of the fieldwork, a sample log for each sample is prepared. The sample log includes the following information:
- Project number;
- Site area;
- Borehole or core number;
- Sample number;
- Water depth;
- Date of sampling;
- Type of sampler;
- Diameter of sampling tube;
- Length of core material;
- Length of sediment penetration;
- Core catcher material;
Whether core material is extruded on board or sealed in a tube or liner; A short description of sediment type should be prepared based on contents in the core catcher and in each end of the liners.
Onboard Laboratory Test
The cores are cut into sections no more than 1 m in length. Disturbance of the cores is avoided during cutting and at other times. The following tests are conducted at each end of the 1-m samples:
- Pocket penetrometer;
- Motorized miniature vane.
Sediment samples obtained by the Ponar/Van Veen grab sampler are described, bagged, and sealed for transportation with the cores. A motorized miniature vane measurement is conducted within the box core sample near the center of the core where the soils are undisturbed.
Prior to sealing, a visual classification of the sediment types is performed. Pocket penetrometer and shear vane tests are undertaken at the top and bottom of each core section. All cores are then labeled and sample tubes are cut to minimize air space, sealed to prevent moisture loss, and then stored vertically. Minimum labeling includes this information:
- Project name;
- Core location reference number;
- Water depth;
- Clear indications of the top and bottom of the core (e.g., use different color caps or mark the cores “Top” and “Bottom”);
- An “UP” mark indicate proper storage orientation.
Onshore Laboratory Tests
The following tests, as applicable depending on soil types and locations, are carried out in a geotechnical laboratory on core samples sealed and undisturbed in the field as soon as possible after recovering the samples:
- Sample description;
- Sieve analysis;
- UU (Un-consolidated, undrained) and triaxial (cohesive soil);
- Miniature vane (cohesive soils);
- Classification tests (Atterberg limits, water content, submerged unit weight);
- Carbonate content;
- Ferrous content;
- Thermal properties;
- Organic matter content;
The onshore laboratory program is approved prior to commencement of testing.
Near-Shore Geotechnical Investigations
To carry out geotechnical investigations in near-shore areas, a self-elevating jack-up is fully utilized or, as an alternative, an anchored barge for drilling operations in up to 20 m of water depth (WD) and as shallow as 2 m of WD. The general requirements for certification, integrity, and safe/efficient working described in preceding sections are applied. In addition, the acceptable sanitary conditions and messing conditions are guaranteed which can reduce the impaction of environment in near-shore areas. For support of the geotechnical drilling unit, any small boat operations should comply with the following guidelines:
- Small boats will be equipped with spare fuel, basic tool kit, essential engine spares, radar reflector, portable radio, mobile telephone, potable water, first aid kit, and distress signals/flares (secure in a water proof container).
- Small boats will only be driven by members of the crew or other personnel who have undergone a specialized small boat handling course.
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