DNO’s USD 110 million Peshkabir-Tawke gas project has captured 12 billion cubic feet of otherwise flared gas equivalent to 766,000 tonnes of CO2 equivalent, since start up in mid-2020 through the first quarter of 2022. Phase 2 is a USD 25 million expansion underway at the Tawke field to capture breakthrough gas starting in the fourth quarter of 2022. The Company is also debottlenecking the Peshkabir gas plant originally designed for 50,000 bopd to handle larger volumes of associated gas from higher field production now averaging 65,000 bopd. DNO holds a 75 percent operated interest in the Tawke and Peshkabir fields with partner Genel Energy plc (25 percent).
Pechkabir Oil & Gas Field GAS Plant Facility Construction Project site is located in Zakho, Iraq. The project consists of Gas Plant Facility Expansion in Peshkabir Oil & Gas Field and Inlet Gas and Injection Compressor Unit in Tawke Oil & Gas Field.
GTL were appointed during detailed design and EPC as Geotechnical Consultants and detailed design for piled
foundations and structures.
Initially GTL were requested to provide a technical/commercial proposal for helical (screw) piles, however, it was GTL’s professional opinion that this technology would not best suited to the ground conditions and the associated load conditions.
Helical piles would not be able to penetrate the lower soils and that pre-drilling would be required to reduce the installation torque. This would increase the operational period and reduce the pile’s load capacities especially lateral, dynamic, and seismic loads mainly due to reduction in α values. Pre-drilling of a helical pile should only be
undertaken with a qualified Engineer being present as it is fraught with hazards.
Preliminary testing would need to be undertaken as the helical piles are required to be pre-drilled,
therefore, testing should be conducted under the exact same installation conditions.
The additional installation torque would also require a larger circular hollow section (CHS) to form the pile stem, this increase would provide the higher torsional capacities required without detrimental damage to the pile’s structure.
Due to the soil conditions to accommodate the dynamic loads the helical would need to be raked.
However, this increased system stiffness also results in greater bending moments in the pile cap, which would be concern, especially for highly seismic regions if not designed correctly.
A hollow stem Micropiles has been proposed due to several factors including subsurface soil conditions. It is also important to note that, in this situation, the proposed micropile will offer greater load capacities.
The micropile has the benefit of being able to penetrate any obstruction that can sometimes cause premature refusal while installing using helical piles.
The pile itself consists of a hollow threaded rebar system which is also known as a self-drilling hollow bar system. In this system, the hollow bar will function as a sacrificial drill bit which be converted to a central reinforcement and will be used to flush out debris. This system can eliminate predrilling, frequent removal of pipes and drills, etc and can handle any condition relating to the installation of micropiles.
On completing the final stage of drilling, the cement mortar is injected into the hollow core of the drill bit by an injection adaptor mounted with the drilling unit. The grout flushing simultaneously serves in stabilising the borehole and filling the area.
Scope of Works
GTL were retained by GPS to design micropiles for various equipment skids, pipe supports, and electrical and instrumentation supports. The project is located at the Tawke Gas Injection plant, Iraq.
GTL successfully completed piling and grillage design for the various facilities based on provided design criteria and loads per support.
GTL were then requested to provide comments on how client could undertake value engineering scope of work to investigate opportunities to reduce pile count considering a newly proposed piling layout for the flare line goal posts (with pile group per support), air cooler structures, and LER/Substation structure.
Based on the geotechnical site investigation program illustrated in the Geotechnical report six (6) boreholes having a total depth of 53.0 m were drilled and six (6) test pits were excavated.
The referenced geotechnical report includes the idealised soil profile for each borehole, in addition to the strength and stiffness properties of each soil layer based on the conducted field and laboratory tests. These idealised soil profiles and soil properties were utilised in the conducted analyses for the design of the micropiles.
The main objective of the geotechnical capacity calculation is to evaluate the length of the micropile (bond zone) required to resist the applied compression and uplift loads with a prescribed factor of safety (2.5 and 3, respectively).
The analysis of lateral performance of the micropiles under the applied axial, shear, and moment for each foundation was conducted using LPile v2019.
The analysis was based on the solution of a differential equation describing the behaviour of a
beam-column with nonlinear supports, which has been in use in the practice of civil engineering
since the 1950’s.
The solution ensured that the computed deformations and stresses in the foundation and supporting soil are compatible and consistent.
Over 450 piles were designed at various depths and diameters to accommodate a modular type construction.
The design of the considered micropiles includes checking their axial geotechnical capacities.
Dynamin Compression Design
Performed dynamic analysis on one (1) of two identical steel skids supporting a reciprocating compressor package. The minimum required pile layout and the external loads imposed by the compressor skid on the piles are provided to GTL Where practically possible, we were required to ;
- Ensure resonance avoidance within the frequency range off 80% to 120% of the equipment operating frequency (primary and secondary).
- Ensure vibration amplitude at any point on the steel skid is limited to 3.8 mm/s
- Ensure vibration amplitude on the compressor frame is limited to 10 mm/s
- Ensure vibration amplitude on the compressor cylinders is limited to 20 mm/s
- Provide recommendations for additional piles as required.
The minimum number of piles (30 piles) for the compressor skid and the initial pile layout were
provided by the compressor package vendor along with pile loads considering fixed and pinned
The piles were then sized (i.e., shaft diameter and pile length) by GTL considering the provided loads and deflection criteria. Thirty (30) micro-piles with 273 mm DIA steel pipe casing and 6 m long were found to be adequate.
The objective of this study is to verify whether the designed piles considering the initial layout and pile count satisfy the dynamic performance criteria and to provide piling recommendations to ensure satisfactory performance otherwise.
Upon review of the initial pile layout provided by the vendor, it was found that many piles were situated off-centre from the main skid beams, which could result in undesirable performance.
GTL requested that those piles be relocated in such a way to be aligned with the main skid beams of the compressor package.
The dynamic analysis was then performed considering the adjusted pile layout.
The analysis considering the initial pile count and pile layout (adjusted layout) shows that the vibration response of the skid and the motor do not meet the vibration limits. In addition, it was found that there are some major natural modes that are within the operating frequency range of 700 to 930 RPM (i.e.,resonance condition).
A revised pile layout which contains 11 additional piles was investigated (i.e., total of 41 piles). The revised pile layout was found to yield acceptable performance. The results of the compressor package supported by the revised pile layout show a satisfactory vibration response for the skid, compressor frame, compressor cylinders, and motor. The results also show that there are no major resonances within the operating range.
GTL were also requested to consider updated loads and design criteria (into the value engineering scope.
Three typical micropile sizes were selected as the foundation for various equipment skids, pipe supports, electrical and instrumenting supports, and other miscellaneous supports.
The lateral performance under the applied external loads imposed by the superstructures, which are provided by others, and the structural capacity of the selected piles have been investigated with the aid of LPile v2019.Moreover, the ultimate axial geotechnical capacities of the selected piles have been estimated under compressive and uplift forces.
The results demonstrated that the designed micropiles yield acceptable lateral performance with maximum lateral deflection at the pile head lower than the allowable lateral deflection.
The geotechnical capacity of each of the considered micropiles is sufficient to carry the external loads applied on the micropiles from the superstructure with an acceptable margin of safety (factor of safety . 2.5 for compressive loads and factor of safety . 3.0 for uplift loads). The structural capacity of each of the considered micropiles was also checked against the factored loads.
The micropiling system shall consider the health and safety implications of the construction phase,
in particular the statutory requirement to guard rotating parts.
The rig operator and supervisor shall be competent and experienced in the selected
construction type and technique.
Normal construction tolerances for micropiles shall be in accordance with the standard
installation tolerances (SPERW section B1.8.)
Installation of self-drilling hollow bar
GTL propose to undertake the piling using a rotary installation head attached to an excavator which will use either an air or water system for flushing. The air jet or water Drilling flush shall be maintained throughout the installation process (to maintain the annulus around the hollow bar). Refer to annex B of BS EN 14199. The contractor shall confirm details of the proposed drilling and flushing medium after final design stage.
Once the final set has been reached the remaining pile shaft extension will be cut to length at the appropriate level specified by the Contractor and the pile cap plate attached ready for the skid.
The piling operation for each installation team will be supervised by a GTL engineer to ensure the highest standards of technical assurance, health and safety and QA/QC. The engineer will work with a team who will be trained to undertake the operations and assist in the field.