Before HDD (horizontal directional drilling) engineering, it must be make a minimum controlled plan that should meet the method statement and execution procedures of the project:
- Site accessibility and the needs for site preparation.
- Vegetation removal.
- Leveling and soil compaction.
- Availability of workspace necessary to set up drilling rig, ancillary equipment, material (drilling mud), storage tanks, etc.
- Workspace for fabrication and pull back activities.
- HDD (horizontal directional drilling) equipment and ancillary equipment requirements.
- Pullback support equipment requirements.
- Sources of water for the drilling mud.
- Environmental issues including mud recycling and waste disposal.
HDD (horizontal directional drilling) Site Survey
The service provider shall perform site survey to define the topographic conditions (type of terrain, surface grade, soil resistance) and the geotechnical conditions (soil type, stratification, classification, lab tests, and geological anomalies) in order to verify the proper condition to permit the HDD (horizontal directional drilling) activities. Like this, the installer shall perform offshore pre-operation survey to verify the pipeline area condition before start HDD operations. It shall also perform, at least, a bathymetric survey and side scan survey.
Radius of Curvature
The minimum bend radius allowed for the HDD (horizontal directional drilling) section is to be defined according to previously discussion and approved by the owner. All the standards applicable shall be used and applied to minimize the residual stress and extend the project life of this pipeline section in the borehole.
The stresses within the pipeline during all HDD (horizontal directional drilling) related operations cannot exceed the allowable limits provided by pipeline design standard. As a minimum, the following design verification shall be performed:
- Assessment of the installation HDD (horizontal directional drilling) phase, regarding the pulling force and the bending stress resulting from 90% of the design radius of curvature. A proper friction factor shall be considered for calculating the pulling loads.
- Assessment of the external pressure from static head in the drilled hole and overburden pressures. The static head resulting from the maximum envisaged drilling fluid density should be used with a factor of safety of 1.5 to provide conservative estimations.
- Assessment of the post-installation phase, regarding the hoop, longitudinal and combined stress resulting from final hydro tests. The bending stress resulting from 90% of the design radius of curvature shall be incorporated to the longitudinal stress.
Entry and Exit Angles
Entry angles on the beach (onshore side) should be held between 8-deg and 16-deg with the horizontal plane. Exit angles on seabed (pipe side) shall be evaluated by Contractor to allow safe and easy break over support avoiding free-spans at exit point. So, the exit angle shall stay within ≤ 2º (two degrees or lower) and, at maximum acceptable value ≤ 5 (five degrees) as the upper limit angle.
HDD (horizontal directional drilling) Design Manual
The Drilling Company shall organize a Design Manual for the HDD (horizontal directional drilling) design activity, being the Design Manual submitted to owner for approval prior to the beginning of the services. The following items must be included in the Design Manual:
- HDD (horizontal directional drilling) alignment sheets, indicating, as a minimum, the entry and exit angles of the drill, the bend radius, the water depths, design location of the pipeline and its profile using 1:500 and 1:100 horizontal and vertical scale respectively, entire length of the drill, entry and exit co-ordinates, and the installation tolerances at the extremities and along the drilling hole.
- Detail description of the steps to be used in the drilling operation, including the hole diameters for the various steps.
- The anticipated loads during installation.
- The stress analyses for all HDD (horizontal directional drilling) operations and steps.
- Specification of the equipment and support vessels required for pulling operations.
- Stability analysis of the borehole against collapse during construction.
- The allowable limits for the drilling fluid properties and additives, such as type, weight, density, etc.
- The required characteristic for the water to be employed for the mud preparation. The limits for the water contents shall be limited.
- Means of monitoring and controlling the drilling directionality.
- The allowable limits for the drilling directionality.
- The outline of the procedure, indicating number of passes established for achieving the final diameter of the drill.
- Method for disposing the excess drilling fluid and definition of its storage area.
- System for collecting or providing potable and industrial water.
Normally, the owner performs a geotechnical investigation along the axis of the pipeline route, which is compiled in an integrated report and this documentation should be supplied to the Drilling Company. The owner could make available to the Drilling Company the geotechnical information as soil indicative reference. Nevertheless, the Drilling Company is responsible to evaluate the data provided by the owner and assess whether it is sufficient for providing the design and construction work with safety and without potential for environment destruction.
The Drilling Company shall propose and perform additional borings in case consider the information provided not sufficient. The complete soil analyses considering these additional borings shall be supplied to the owner. Any additional borings made by the Drilling Company for further geotechnical assessment should be conducted at least 8-10m off the proposed pipeline centerline. Upon completion, the boreholes should be grouted to reduce risk of frac-out or loss of drilling fluid during the crossing installation. The Drilling Company shall ensure that the drill pass avoids passage through pockets containing cobbles, gravel and boulders. Any adjustments to the proposed crossing profile resulted from geotechnical investigation shall be approved by the owner.
HDD (horizontal directional drilling) Equipment
Horizontal Directional Drilling equipment, for shore approach crossing, furnished by Drilling Company shall include the following components:
- Horizontal Drilling Rig Unit with tensile and torque capacity adequate to crossing length (minimum 200ton), geotechnical conditions and pipe size.
- Control Cab Power Unit.
- Water Pump.
- Slurry Pump.
- Slurry Mixing Tank.
- Mud Recycling System (capacity in pressure and volume).
- Drill Pipe.
- Bentonite Storage.
- Water Storage.
- Clean Mud Storage.
- Dirt Mud Storage.
- Power Generators.
- Downhole survey system.
- Downhole drilling tools.
- Instrumentation sufficient to accurately locate the pilot hole, measure drilling fluid discharge rate and pressure, and measure drill string axial and torsional loads.
- Tanks, cranes and all ancillary support systems.
HDD (horizontal directional drilling) Operational Procedures
Prior to start of work the Drilling Company shall prepare a written installation procedure for the owner review and approval. The crossing installation procedure shall consider possible interference with the prelaid pipelines installed by others. The Drilling Company shall install proper buoys to define the real pre-laid pipeline and structures position. The crossing installation procedures shall include the following:
- Site plan including location, general layout (mud recycling system and other equipment, material, support services), in scale and with dimensions.
- Description of HDD (horizontal directional drilling) rig and ancillary equipment including model, date of manufacture, and specification with capacities and ratings.
- Proposed drill path showing the different soil layers with radius of curvature along the entire path.
- Coordinates and angles of the entry and the exit points.
- Type and amount of drilling mud and additives with the mud pressures anticipated during the installation.
- Drilling process including the pilot hole, surveying, reaming, swabbing, pullback, borehole maintenance, downhole tools, drilling and reaming rates, torque, etc.
- Contingency procedures in case of failure/mistake of one operation.
- Procedure for calibrating the surveys equipment in accordance to the owner references.
- Pre-survey of the subsea area required for pulling operations to assess potential interference of the pipeline to all potential subsea hazards.
- Description of the vessel and equipment required to pull back operations.
- Layout of the vessel-mooring anchors showing the actions taken to avoid interference with the existing seabed structures and pipelines.
- Buoyancy control during pull back operation.
- Detail procedure for the connection pipeline/drill pipe operation.
- Pulling forces and pulling rate during pullback.
- Downhole survey system to determine the horizontal and vertical coordinates along the drilling hole.
- Environmental protection plan, including soil erosion and sediment control devices, water and drilling fluid treatment equipment, waste disposal, and work site restoration.
- Safety and Quality Assurance plan.
Site Inspection and Preparation
A typical drill site (entry side) for HDD spread in shore-approaches crossing shall have minimal dimensions of 50m by 75m of cleared of timber and brush, hard standing and leveled grade. It shall be clear of overhead obstructions, and close to the water source, if possible. The Drilling Company shall perform an initial site inspection to locate and
identify all underground structures, such as pipelines, cables and utilities, notify the owner before working over, under or near these facilities, and set stakes along the site boundaries and at the entry point of the crossing. The Drilling Company shall prepare the site prior to mobilization of HDD. The site preparation, as a minimum, comprises of providing access for heavy trucks and trailers, preparing the soil to be leveled and hard standing, installing soil erosion and sediment control devices, and installation of culverts as necessary to maintain the existing drainage systems. The Drilling Company shall also remove the topsoil from the work areas and store it at a suitable site nearby for replacement after completion of the work. The topsoil shall be placed so it does not affect the natural drainage of the area. The Drilling Company shall prepare adequate foundation for the drilling equipment (at least 1.5 times the maximum capacity of the rig) and after conclusion of the operations this foundation shall be removed in a way not to affect or impact the area.
Protection of Underground Facilities
The Drilling Company shall undertake the following steps prior to commencing drilling operations:
- Contact the utility location/notification service, if available, for the construction area.
- Positively locate and stake all existing lines, cables, or other underground facilities including exposing any facilities that are located within 10m of the designed drilled path. For safety reasons, this may require to hand-dig existing utilities.
- Modify drilling practices and downhole assemblies to prevent damage to existing facilities.
Drilling Fluids and Mud Cuttings
The primary components of drilling fluid used in HDD installation are fresh water and bentonite. The composition of all drilling fluids to be used by Drilling Company including Material Safety Data Sheet (MSDS) shall be submitted to the owner for approval prior to utilization. The materials shall be in accordance with API 13A (American Petroleum Institute, 2004). Two tanks shall be provided for drilling mud, one for clean mud and another for recycled mud.
The Horizontal Directional Drilling operation is to be executed in a manner to avoid the discharge of water, drilling mud and cuttings to the seawater involved during the construction process. The Drilling Company shall provide equipment and procedures to maximize the recirculation or re-use of drilling mud to minimize waste disposal.
A fluorescent non-toxic dye shall be added to the drilling fluid during drilling of the pilot holes so that any frac-outs can be easily detected. Drilling fluids must be free of any additives that will adversely affect the environment. The composition of all drilling fluids to be used by The Drilling Company shall be submitted to the owner for approval prior to utilization. No fluids will be approved or utilized that does not comply with permit requirements and environmental regulations.
Disposal of drilling fluids shall be the responsibility of the Drilling Company and shall be conducted in compliance with all relative environmental regulations, right-of-way and workspace agreements and permit requirements. All cost related to disposal shall be borne by the Drilling Company. Water supply is the Drilling Company’s responsibility, whether purchased locally, hauled in, or pumped from a nearby waterbody. Composition of the water shall be determined by water analyses and supplied to the owner. Always and when possible mud hoses and hoses in general shall be avoided and be replaced by steel pipes with chiksan connections. Mud cuttings shall be analyzed continuously during drilling operations and information supplied to the owner.
HDD (horizontal directional drilling) Environmental and Saftey Requirements
Drilling Waste Disposal
Disposal of drilling fluids and cuttings shall be the Drilling Company ‘s responsibility and shall be conducted in compliance with all relative environmental regulations, right-of-way and workspace agreements and permit requirements. The Drilling Company shall locate suitable sites, obtain necessary approval from applicable local administrative and governmental authorities and the owner. In any case neither drilling fluids nor cuttings will be allowed to enter water bodies.
Inadvertent Drilling Fluid Returns
The mostly frequent kind of environmental injure associated with HDD is frac-out, an involuntary return of drilling fluids to the surface. The occurrences take place when the mud pressure in the borehole exceeds the confined pressure of the overburden. On the other hand, drilling fluids from the borehole could migrate through existing fissures in the formation. Frac-out is also probable when drilling the pilot hole or in back-reaming operations that are performed at an excessive speed and high rate, not lets satisfactory time to the cuttings to be washed and trip
out of the borehole.
Drill Site Maintenance, Clean up and Restoration
Mud pits shall be suitably lined and bermed by the Drilling Company to prevent leakage to the surrounding area. Mud recycling system, tanks, connections, valves and lines shall be maintained in good condition so that leaks do not occur. Any spillage shall be cleaned up immediately and the cause of the leak remedied.
To guarantee the preservation of the Project site, the Drilling Company shall perform the following activities:
- Before commencing of any activity, prepare a photographic record of the entire area, focusing on the vegetation, the topography as well as the general conditions of the environment preservation of the site.
- Prepare an inventory of the vegetation collection, identifying the native and exotic species.
- Prepare a recovery plan of the degraded area to restore the vegetal mass to the previous study and the return the area to its original contour.
- Restore the entire area to its original state upon completion of all services.
Upon completion of crossing installation the Drilling Company shall demobilize HDD spread and clean up the drill site (entry and/or exit sides) including removal of all equipment, material, supplies and temporary structures/foundation, if any, utilized in the course of crossing installation, reinstate the site to the original state.