API 4F Drilling Rig Substructure Steel Framework For Rig Floor Support

The substructure is the large steel framework assembled directly over the drilling site that supports the entire rig floor, mast, rotary table, drawworks, and all associated drilling equipment. It is the foundation upon which the entire drilling operation rests. Unlike the mast which rises vertically to support the travelling block and hook load, the substructure provides the elevated working platform for the driller and crew, houses well control equipment such as the blowout preventer (BOP) stack, and transmits all operational loads—hook load, rotary load, setback load, and mast loads—safely to the ground.

Substructures are essential components for truck-mounted drilling rigs, trailer-mounted drilling rigs, and workover rigs. Their primary functions include:

Supporting the mast and derick – Providing a rigid base for mast erection and load transmission.

Elevating the rig floor – Creating adequate clearance beneath the rotary table for BOP installation, wellhead access, and operational space.

Supporting rotary table loads – Bearing the full torque and weight transmitted through the drill string during rotation.

Providing setback capacity – Holding the weight of the drill pipe stand when the string is tripped out of the hole.

Housing wellhead equipment – Creating space below the rig floor for BOP stacks, choke manifolds, and well control systems.

The substructure height is generally determined by two key factors: rig type (truck-mounted, trailer-mounted, or modular skid) and the required blowout preventer (BOP) stack clearance. A sufficient clear height under the rotary beams—typically ranging from 2 m to over 7.5 m—is essential to accommodate BOP equipment and provide safe access for well control operations.

All our substructures are designed and manufactured in compliance with API Spec 4F (Specification for Drilling and Well Servicing Structures), the industry standard covering steel derricks, masts, substructures, and crown block assemblies for petroleum drilling and well servicing operations. We also incorporate design verification through Finite Element Analysis (FEA) using ANSYS Workbench or equivalent software to optimise structure, stability, and load bearing performance.

Key Features

Substructure Classification by Design Type

Depending on the structure, our substructures are divided into the following main types:

1. Integral Substructure (Box on Box / Block Combination Type)

The simplest and most economical design. The substructure consists of two or more fabricated steel boxes that are stacked and bolted together on location. The mast, drill floor, and all equipment are installed at ground level and raised together using the drawworks power or hydraulic cylinders.

Advantages: Simple construction, high rigidity, excellent load distribution, easy to manufacture and maintain.
Typical Application: Truck-mounted rigs, workover rigs, shallow to medium depth drilling where frequent relocation is not required.

2. Scoping (Telescoping) Substructure

The substructure features an inner and outer frame arrangement that allows the drill floor height to be adjusted by telescoping the upper section upward from the lower base. The mast is typically of two section or three section telescoping design, and can be moved with the rig main module or separately.

Advantages: Adjustable height for different BOP stack requirements, reduced transport height (telescoped down), good stability when fully extended.
Typical Application: Medium-depth workover rigs, trailer-mounted drilling rigs, wells requiring variable BOP clearance.

3. Foldable Substructure (Parallelogram Structure)

The substructure uses a parallelogram (scissors) linkage mechanism that folds flat for transport and then lifts into position hydraulically or mechanically on location. The drill floor and mast can be installed at low position, then raised together with the mast. The drill floor is of folded type or parallelogram structure, complete with V-door, escape chute, walkways, handrails, stairs, mouse hole, and rat hole.

Advantages: Extremely low transport height (as low as 2.7 m / 9 ft), fast rig-up (often within 30 minutes), no crane required for erection on many models, ideal for multi-well pad drilling.
Typical Application: Fast-moving land rigs, pad drilling operations, deep well workover rigs, ZJ series drilling rigs.

4. Split Foldable Substructure (Two-Part Modular)

A more advanced version of the foldable design where the substructure is split into two separate modules (left and right) that are transported independently and then connected on location. Each module contains its own hydraulic raising system.

Advantages: Reduced individual transport weight, excellent for road-legal transport in regions with strict axle load limits, suitable for larger rigs (ZJ40 and above).
Typical Application: Large drilling rigs, remote locations with challenging transport conditions, deep well drilling programs.

5. Trailer-Mounted Substructure (Semi-Trailer Type)

The substructure is integrated into a semi-trailer chassis. The front level of the substructure may be telescoping while the rear module is semi-trailer mounted. The mast is typically two or three sections telescoping and can be moved with the rig main module or separately.

Advantages: Extremely fast move (rig-up in hours rather than days), eliminates need for separate transport trailers, self-contained design, ideal for well-servicing and workover operations in mature fields.
Typical Application: Workover rigs (XJ series), shallow drilling rigs, well-servicing fleets requiring rapid rig-move capability.

Serial Substructures – Technical Specifications

Our substructure product range covers the following models and corresponding technical parameters:

Technical Notes for Specification Table

Max. Static Load (Hook Load): The maximum vertical load that the substructure can support through the mast and travelling system during hoisting operations. This includes the weight of the drill string, casing string, or completion equipment being run in or pulled out of the hole. For perspective, a 2,500 kN rating (approx. 250 tonnes) is typical for medium-depth land rigs; 3,000 kN (approx. 300 tonnes) is required for deeper wells.

Max. Rotary Beam Load: The maximum load that can be supported by the rotary table beams while the drill string is rotating. This load is transmitted through the rotary table to the substructure foundation. Rotary beam loads are typically lower than static hook loads because rotation induces dynamic forces and vibration. Note that these loads may be applied simultaneously with hook load and setback load, as defined in API Spec 4F Section 7 Design Loading.

Setback Capacity: The maximum height of drill pipe stands that can be racked in the derrick when the drill string is tripped out of the hole. Setback capacity is expressed as the length of a stand of drill pipe (typically two or three singles joined together) that can be set back in the racking board. A higher setback capacity indicates the ability to handle longer stands, which reduces tripping time. For example, 6,000 ft setback capacity means the substructure can support the full weight of a 6,000 ft column of drill pipe stood in the derrick.

Clear Height Under Rotary Beams: This is the vertical clearance between the rotary table beams and the ground (or substructure base). For models with higher drill floor heights (e.g., SZT3000/6.7-FZ with 6.7 m / 22 ft overall height), the clear height under the rotary beams is typically 1–2 metres less than the total drill floor height, depending on the beam depth. Clear height is critical for BOP stack installation. API specification typically requires minimum 21 ft (6.4 m) clear height for standard drilling operations, as shown in industry tenders.

Nominal Static Hook Loads: The static load rating is measured with a specified number of drilling line parts (usually 8, 10, or 12 lines) under defined wind load conditions (typically 36 m/s / 80 mph). API Spec 4F requires the rating to be displayed on the substructure nameplate, along with the date of manufacture, serial number, and PSL level.

FEA Design Verification: All load ratings are verified through 3D finite element analysis using ANSYS Workbench, SolidWorks Simulation, or equivalent software. The FEA models evaluate the substructure under multiple load case combinations as defined in API Spec 4F Section 8, including operating loads, wind loads (with and without setback), dynamic loads, erection loads, transportation loads, and where specified, earthquake loads and ice loads.

Substructure Components – Detailed Breakdown

A complete substructure assembly consists of the following key components:

Boat basement – The lowest section that directly contacts the ground or foundation pads. Main function is to support the entire rig weight and distribute loads evenly to the ground.

Mast substructure – The dedicated support structure that carries the mast feet and transmits mast loads (including hook load and dead line anchor loads) into the boat basement and substructure body.

Substructure body – The main vertical and horizontal framework that supports the rotary table load and provides standing storage (setback area). This is the primary load bearing element of the entire assembly.

Floating platform – An elevated work area (sometimes called the "monkey board" or auxiliary platform) that stores wellhead tools and provides working space for the derrick-man and wellhead crew during tripping operations.

Rotary beams – Heavy fabricated steel beams directly beneath the rotary table. These transmit rotary torque and vertical loads from the rotary table into the substructure body. For larger rigs, the rotary beam load rating can exceed 2,500 kN (approx. 560,000 lb).

Drill floor – The working platform consisting of steel checker plate (typically ¼" thickness for standard areas, ⅜" thickness around the rotary area for added durability) with anti-slip matting around the rotary table for crew safety. Includes V-door, rat hole, mouse hole, escape chute, walkways, handrails, and stairs.

BOP installation device – A trolley or skid system for installing, removing, and servicing the blowout preventer stack. Provides guided movement under the substructure for safe and efficient BOP handling.

Slide way – Used to lift or lower the drill pipe during tripping operations. Aligns pipe with the V-door and catwalk.

Ladders and access platforms – Typically two ladders are provided (one from substructure floor to ground on the driller's side, one from substructure floor to carrier floor on truck-mounted rigs). Handrails are 1 m (3 ft 6 in) high with toe plates for perimeter fall protection.

Why Partner with Sunrise for Drilling Substructure

As a specialized trading company with strong API-certified manufacturing partnerships, Sunrise offers:

API Spec 4F certified substructures – Full compliance with 5th Edition (2023) requirements, with PSL-1 or PSL-2 available.

Complete design type range – Integral, telescoping, foldable, split foldable, and trailer-mounted configurations to suit any rig type or operational requirement.

Verified FEA design – Every substructure is analyzed using ANSYS Workbench or equivalent software, with design reports available for customer review.

Full NDT documentation – MPI, UT, and DPT inspection reports included with every shipment.

Low temperature capability – Special materials and welding procedures available for -45°C operation for arctic and cold climate drilling programs.

Global logistics support – Export documentation, sea/air freight, and delivery to remote drilling locations worldwide.

Contact us to order the right substructure for your drilling or workover rig. We provide the correct design type, drill floor height, load ratings, and API compliance documentation for your specific well depth and operational requirements — with full FEA verification, NDT inspection, and on-time delivery.