Introduction

Over the past few years there have been several accidents and incidents that are related to drill rig instability and / or movement while drilling.

1. Several examples where drill rigs have toppled over causing significant equipment damage and injury and, in at least one event, the incident resulted in a fatality. Figure 1, 2 and 3 show examples of drill rigs that toppled over.

2. I am aware of at least three reports of drill rigs moving out of alignment during drilling operations due to subsidence of an outrigger. In both cases the quill rod (kelly rod) broke but in none of the incidents was anyone harmed.

3. Several instances where large washouts / sinkholes have formed either at the collar of the borehole or adjacent to the collar of the borehole. Figure 4 and 5 show examples of large washouts that occurred, one at the collar of the borehole and one adjacent to the borehole.

In almost all these cases the incident was reported to be caused by “poor or uneven ground conditions” and the recommended controls to prevent further occurrences did not, in my opinion, go far enough to address the real causes of the incidents. There is no doubt that poor surface conditions may contribute to rig instability, but there are a number of other possible causes that must be identified and controlled.

In this brief summary, the causes of drill rig instability are identified, and a series of controls are suggested to reduce the likelihood of an incident occurring.

Causes of drill rig instability

Drill rigs may move out of their set-up alignment for several reasons:

1. Accidental or deliberate activation of one of the outrigger controls.

2. Mechanical failure of an outrigger or outrigger mounting.

3. Hydraulic failure in one or more outriggers.

   3.1 Seals at the outrigger piston rod may leak and allow oil to bypass.

   3.2 The outrigger piston seal may leak and allow oil to bypass.

   3.3 Hydraulic oil may leak past the spool of the hydraulic control valve.

4. One or more outriggers may subside or sink into the ground. This can be caused through two mechanisms:

   4.1 Inherently poor ground conditions may be unable to support the weight of the drill rig, i.e. the ground bearing pressure (GBP) may be too high for the given surface conditions.

   4.2 Induced poor ground conditions, caused through:

• drilling fluid or rain saturating the area around the base of the outriggers or,

• sub-surface washout either due to insufficient casing being installed or installed casing being poorly seated.

Causes 1, 2 and 3 are easily understood and controlled. Drill rig subsidence however is less easy to understand and detect and therefore to control.

Drill rig subsidence

An outrigger will subside (sink into the ground) when the pressure exerted by the foot of the outrigger (its ground bearing pressure) exceeds the pressure that the surface is able to carry before deforming. In the context of a drill rig, ground bearing pressure (GBP) can be thought of as the pressure exerted on the Earth’s surface by an outrigger.

It has been suggested that a ground bearing pressure greater than 100kPa would potentially result in subsidence in soft ground conditions and so, if we assume that an upper limit of 100kPa is acceptable then, it is relatively easy to calculate the required dimensions of outrigger footings to ensure that GBP is maintained below the 100kPa threshold.

Determination of ground bearing pressure (GBP)

Most drill rigs utilise 4 outriggers plus a mast dump facility to level and stabilise the drill rig base so that the mast can be aligned to the specified inclination. The total outrigger footing area must therefore include the surface area of the mast dump pad.

The following two examples illustrate how Ground Bearing Pressure can be calculated.

Example 1: Calculate the GBP at each ground contact point for a drill rig of mass 15 MT if the outrigger pads are 0,68 metres diameter and the mast dump pad is 0,4 metres x 0,4 metres.

We know:

Drill rig mass= 15MT,

Outrigger footings are circular with a diameter of 0,68 metres,

The mast footing dimensions are 0,4 x 0,4 metres.

We need to determine the total area of the outrigger and mast footings. 

Since we assumed that the load is equally distributed between the four outriggers and the mast footing, GBP is below the 100kPa limit, and it seems unlikely that rig subsidence will occur.

In reality, it is unlikely that all surface contact points will carry an equal load, the centre of gravity of the drill is unlikely to be at the geometric centre of the outriggers and so, ground bearing pressure will not be equally distributed across all outriggers.

In addition, the mass of the drillstring will effectively increase the total mass of the drill rig and so add to the GBP when pulling or lowering. We must therefore add the drillstring mass to the drill rig mass in the calculation of GBP.

We must also bear in mind that ground bearing pressure at each outrigger footing will vary as different operations are carried out. When pulling the drillstring, for example, the mast and the front outriggers will carry substantially greater load than the two back outriggers. Similarly, when applying weight on bit, the front outriggers will tend to lift and so the back outriggers will carry substantially greater load than the front outriggers. These factors should be considered in the calculation of actual GBP and are illustrated in the following example.

Example 2: Consider the drill rig in Example 1, but assume also that the drillstring mass is 8 MT (i.e. the total mass of the drill rig and drillstring = 23MT) and that the front outriggers carry 70% of total load when pulling, calculate the maximum ground bearing pressure at the front outriggers and the mast dump pad.

In a fully loaded condition therefore, with the front outriggers carrying 70% of the pulling load, it seems likely that GBP would be much greater than the notional 100kPa limit and in order to reduce GBP to 100kPa it will be necessary to increase the dimensions of the front outrigger and mast dump footings accordingly.

It is important to recognise that simply increasing footing areas will not necessarily reduce GBP to the desired level, it is essential also that careful attention is paid to the design and construction of the outrigger pad material.

If this is not done correctly, the footing may deform and so effectively reduce the area of the footing in contact with the ground. This will negate the benefit of the increased surface area. Figure 6 illustrates this very clearly.

Increasing footing dimensions may also introduce other hazards in certain circumstances and so very careful attention must be placed on this change.

Sub-surface washout

In boreholes with deep sand cover, it is sometimes difficult to correctly seat casing and so the opportunity exists for drilling fluid to return past the seat of casing rather than up the inside of the casing. In extreme cases, this flow of drilling fluid can cause a washout of the unconsolidated material behind the casing that will manifest as a sinkhole at surface. These sinkholes can occur at the collar of the borehole or adjacent to the collar of the borehole.

Typically, these washouts do not result in rig movement over a period of time, they occur suddenly and often with dramatic results. In Figure 4 above, the sinkhole occurred a distance from the collar of the borehole and resulted in the front end of a rod stack to falling into the sinkhole. In Figure 5 above, the washout occurred at the collar and eventually resulted in the drill rig sinking into the void that was created. Fortunately, in neither of these events was any harm caused but it is very clear that serious injury or loss of life could have resulted from either incident.

Detection of sub-surface washout

Detection of the formation of a washout is difficult and requires very careful observation of return fluid flow.

If casing is correctly seated, the seat of the casing should provide a fluid seal so that the total pumped volume of drilling fluid returns up the inside of the casing. Any flow of drilling fluid outside of the casing will therefore indicate that the casing is not correctly seated and that the potential for a washout exists.

In a normal collar configuration, it is impossible to detect fluid flowing upwards outside of the casing and so, in high-risk boreholes, it is recommended that a rotating diverter is used. The rotating diverter screws into the casing and diverts return fluid through a flexible hose to the surface sumps or surface tanks. The collar of the borehole is therefore relatively dry, and it is easy to see if the return flow is by-passing the casing. The diverter also serves to keep the area around the front of the drill rig relatively dry.

Monitoring rig alignment

It seems likely that rig subsidence will take place over a period of time and will get progressively more severe as borehole depth increases. It is important therefore to monitor rig stability on a regular basis so that movement can be detected early.

In most exploration projects, rig alignment is set and checked using either a clinorule or sometimes even a spirit level! While these tools may be sufficient for set-up purposes, they cannot monitor small changes in rig alignment and so it is recommended that all exploration drills are fitted with a pair of digital inclinometers mounted on the mast of the drill and on the deck of the drill. These digital inclinometers are very inexpensive and readily available and since they are digital, they are capable of reporting very small changes in rig alignment and so can act as an early warning device if procedures are implemented where they are checked pre-shift and during a shift.

If necessary, the inclinometers can be permanently fitted to provide a continuous readout of drill rig attitude and so provide a visual and/or audible warning if drill rig movement exceeds a pre-set limit.