Dyanamic Loads On Helical Piles

GTL International LLC (GTL) provided consultancy service to provide a suitable foundation design for three Siemens gas turbines (SGT-300N) for a field development project near Abu Al Khasib, Basra, Iraq.??

Rapid construction of a suitable foundation for the gas turbines was project-critical. This, therefore, favors a foundation solution that can take advantage of pre-fabrication as much as possible and GTL proposed a foundation using helical screw piles with a steel beam grillage to form the base onto which the gas turbines were connected.

GTL was requested to provide Consultancy to provide structural analysis services to assess the proposed foundation concept to determine the extent to which its vibration performance might meet the project foundation vibration performance criteria.??

GTL undertook a concept evaluation study involving a review of the available information, modeling, and analysis to estimate the vibration performance and compare it with the project design criteria.

Description of Plant

The foundation will support a centrifugal Gas Turbine (SGT-300) made by Seimens that runs at a speed of 14048 rpm and an Electric Generator (and associated gearbox) made by Seimens that runs at a speed of 1500 rpm.??

The dynamic operating loads of the gas turbine act at its operating speed, 14048 rpm (234.1 Hz or 1471 rad/sec), and the dynamic operating loads of the electric generator act at its operating speed, 1500 rpm (25 Hz or 157 rad/sec).

The operational dynamic forces due to the unbalance of the gas turbine and the electric generator are calculated assuming a balancing quality of G2.5 and the rotors weights provided by the vendor included in the dynamic analysis package delivered to GTL for the purpose of evaluating the dynamic performance of the proposed foundation.? The configuration, layout, and weights of the turbine and generator were also included in the analysis package.

The subject foundation system proposed to support the new equipment is a helical piled foundation with a steel beam grillage connecting the pile heads.?

The steel beams grillage consists of tie beams and cross beams.?

The connections between the tie beams, cross beams, and piles The elevation of the foundation base (pile cut-off elevation) is 0.35 m. The helical pile has a steel shaft of 273 mm in diameter, 12.5 mm wall thickness, and is 16 m long. The steel shaft if fitted will have three helical plates each 450 mm in diameter.? The steel grillage is relatively flexible and the dynamic response should be evaluated using finite element analysis to account for its flexibility.

TOPOGRAPHY

The Siba field itself is approximately 21km long and 6-13 km wide. It is located within the Zagros Foreland Basin. This Basin is bounded to the east by the Zagros Mountains, to the west by the Arabian Plate, and to the south by the offshore areas of the Arabian Gulf. To the northwest lies the Euphrates Valley. The general gradient of the plain is 1 m per 20 km. The main geomorphological units of the accumulation fluvial origin are terraces, alluvial fans, sheet run-off plains, flood plains, shallow depressions, marshes and lakes, and sabkhas. There are a number of individual marshes and lakes that have developed in shallow depressions in different parts of the Mesopotamian Flood Plain and are distributed in three main areas. The oil field is located in an area known as Central Marshes, which lies to the north of Euphrates

DESCRIPTION OF THE SOIL

KEIL commissioned a local Iraqi company by the name Al-Fao Company to conduct a soil investigation program for the site, which involved 12 boreholes drilled to depths 31.5 to 35 m below existing grades.? The borehole logs indicate that the subsoil encountered by the ground investigation can be divided into 3 sections as follows:

Superficial Holocene horizons, encountered predominantly as firm, clayey to very clayey, slightly sandy silt. The thickness of Zone A varies considerably across the investigation site between 3.5 m and 16.5 m. ?The SPT values measured within Zone A varied between 10 and 20.

Zone A is underlain by predominantly stiff silt, normally present as clayey to very clayey, slightly sandy silt. It has been concluded, that Zone B is also of Holocene origin. The bottom of Zone B varies between 15.0 m and 25.0 m. The SPT values measured within Zone B vary between 20 and 30.

Zone C comprises presumably Pleistocene sediments encountered embedded pebbly, medium dense to dense sands, sandy gravels,? silts and stiff to very stiff clayey, sandy silts. This formation extended to the end of the boreholes. The SPT values for Zone C is greater than 30.

In addition, six soundings of seismic down-the-hole tests were performed shear wave velocity and primary wave velocity were measured and the results were used to evaluate the Poisson’s ratio for the different soil layers. The measured shear wave velocity values, their average or best estimate (BE), lower bound (LB), and upper bound (UB) are provided in Table 3.?

Due to the variability of the measured shear velocity values across the site, GTL considered three cases of dynamic soil properties to account for the uncertainty in the value of the shear modulus of the soil underneath the foundation.

Case 1: Shear wave velocity profile based on the average of the measured values as presented in Table 3. This represented what was expected to be the average (best estimate) value of the soil shear wave velocity.

Case 2: Shear wave velocity profile based on the lower bound values. This modeled what was expected to be the lower bound on the values of the soil shear velocity.??

Case 3: Shear wave velocity profile based on the upper bound values. This modeled what was expected to be the upper bound on the soil shear wave velocity values.

Based on the results of the geotechnical investigation and the geophysical study, the representative dynamic soil properties of the soil at the investigation site used in the calculation of the stiffness and damping constants of the foundation system.

DYNAMIC ANALYSES?

GTL performed two different sets of dynamic analyses for the foundation as follows. ?The DYNA6 program was used to calculate the dynamic stiffness and damping constants of the piles system considering the loading frequencies (i.e. frequencies for the gas turbine, 14045 rpm, and electric generator, 1500 rpm). These analyses considered the three soil conditions, i.e., LB, BE, and UB soil shear wave velocity profiles. In addition, an analysis was performed to consider a disturbed soil zone in the vicinity of the pile shaft due to the installation effects of the helical piles. The disturbed zone is annular and its size is equal to the diameter of the helix minus the pile shaft diameter (i.e. 450 mm – 273 mm). ?The calculated piles' stiffness and damping constants.

simulate the piles-soil structure interaction. The dynamic response of the machine, steel under base (skid), and steal beam grillage was calculated for the unbalanced forces from the electric generator (acting at a speed of 1500 rpm) and the gas turbine (acting at 14045 rpm). The equipment and the associated steel skid were considered rigid in the SAP2000 analyses. The machinery masses were assigned to their center of gravity.? The steady-state response of the foundation systems to the unbalanced forces was calculated at the most critical locations for vibration. These points are located at the top corners of the steel skid and the centers of the electric generator and gas turbine.

The original steel beam grillage design resulted in vibration amplitudes higher than the acceptable performance levels of vibration for high-speed turbine generators. Several revisions to the design were attempted including fixing the pile head into steel beams and stiffening the steel beams in both directions.? The response of the revised system. The vibration amplitudes represent acceptable dynamic performance.?

The important natural frequencies of the system (with significant modal participation) were found to be much lower than the rated speed of the equipment.? In addition, the calculated highest vibration velocity is below 1.5 mm/sec, which represents acceptable performance.