Conductor service life improved by detailed soil analysis

Initial and historical costs at stake

Brian McKenzie
Fugro

• This figure shows the relationship which can be derived linking maximum stable open hole depth (the maximum wellbore depth before stability becomes a concern) with the mud weight used to support the open hole. The different contours represent soils of different sensitivity. Through being able to predict the onset of sloughing or hole instability, the driller can anticipate problems and a suitable mud program can be prepared [19,285 bytes].

• Soil strength gain follows a logarithmic relationship with time. In this example, the plot on the left shows two conductor resistance curves - axial resistance versus penetration. The lower, red curve is the installation resistance, which then increases - as the soil strength recovers - to a much higher value.
  In this case, the conductor is jetted in against a resistance of 1.5 MN. The top hole section is then drilled, but now the driller wants to hang off some casing and land the wellhead (total load 2.57 MN). The curve, however, indicates that the conductor can't actually support this weight until at least a day after jetting is completed. Anticipating this process can avoid the costly problems associated with well subsidence which have been experienced outside the Gulf of Mexico [23,272 bytes].

Geotechnics is traditionally associated with foundations, for instance for piled steel jackets. However, a well conductor is actually a foundation too, since it is a seabed-supported structure which must sustain loading of some nature. In fact, the loading regime acting on a well conductor is complex, and varies throughout the conductor's life.

One of the key considerations when planning and installing conductors is the need to avoid geohazards - or to design around them. Typically, that means over-pressured shallow formations, boulders, excessively hard or soft soils or sensitive soils, faults and slopes. Geotechnics can help identify, avoid, or design around most of these hazards.

For example, when driving conductors in hard, boulder-strewn ground as often encountered in the North Sea, or in areas farther afield where gypsum strata or cold-water coral reefs are found, geotechnical models can allow a minimum conductor shoe thickness to be specified in order to resist tip buckling. The conductor can then be drilled out for the first inner casing without the risk of the drill string getting stuck in a deformed conductor.

Geotechnics may even lead to the elimination of some design concepts and installation methods at an early stage of the project. This detection is imperative in order to avoid wasting money pursuing what turn out to be "no go" options.

Determining size

As an example, during the FEED stage of a proposed deepwater Norwegian Sea development, comprising a template-less array of conductors, it was possible to eliminate the initial proposed conductor size as inadequate because of the excessive soil movements predicted. However, the geotechnical exercise was performed sufficiently early for there to be flexibility in the design. It became possible therefore to remedy the design and bring it back within the bounds of feasibility. In that way the cost-saving, template-less array concept could be pursued.

It is vital to understand the soils at a new well site. Geotechnical practitioners like to avoid transferring blindly what may be "best practice" or "rule-of-thumb" from a mature area to a new area where the soil conditions render such practice inappropriate. The message is, if you don't understand the soils, you may not end up with the same integrity that you were hoping for.

Jetting conductors

Jetting conductor installation provides a good example of this principle. While jetting is well established and considered the norm in the Gulf of Mexico, it is still a novel method throughout most of the world's other deepwater prospects, with problems reported off West Africa and Norway.

Initial attempts at jetting offshore West Norway have run into difficulties, partly due to the lack of site-specific soils data, but also because of the operator's reluctance to believe the geotechnical prediction that the "off-the-shelf" conductor design would not have sufficient soil support, and that the wellhead would subside when the first inner casing was hung-off. The costs of the geotechnical exercise which can predict such unwelcome events are probably in the region of 1% of the cost of the wellhead recovery operations.

Setting depth

Setting depth assessment is another important aspect of conductor planning. Application of geotechnical data may lead to a cost saving through reduced setting depth requirements. The benefits of a shorter conductor go way beyond the minimal saving incurred on conductor steel. A shorter conductor is easier and quicker to install, so the time savings can be of much greater value.

Another aspect that can benefit from geotechnical input is the avoidance of damage to other wells or foundations during the conductor installation and top-hole drilling operations.

Once a workable conductor design has been identified, the next step is to assess the cheapest way of getting it in the ground, while still fulfilling technical requirements. Installation methods can range from the conventional to the novel, but the feasibility of all concepts depends heavily upon the soil conditions.

Conductor installation

For instance, conventional driving techniques can often lead to excessive ground damage and unacceptable conductor deviation whenever refusal is reached and drilling-ahead has to be performed. However, an appreciation of the soil conditions enables a simple exercise to be performed to devise a driller-friendly installation program that avoids drilling ahead in strata susceptible to wash-out.

Conductors cemented in a pre-drilled hole - drilled and grouted conductors-are another well-established practice. But what about the problems peculiar to deep water and the very soft, often highly sensitive soils associated with these areas?

With jetting, the conductor is washed into place using fluid injected through a jet-head at the end of the drill string. The drill string also serves as the lowering tool for the whole conductor. The jet fluid erodes the soil inside and at the tip of the conductor, reducing the soil resistance, allowing the assembly to penetrate under its own weight. Use of drill collars or heavy donut weights around the string assists the process.

The big incentive here is the ability to disengage the drill string, then immediately start drilling the top hole section, without waiting on cement. The potential time and costs savings are immense, especially given today's rig rates. However, the big unknown is the degree of soil disturbance caused by the jetting process.

An advancing zone of disturbed, wetted soil can be created, which the conductor then relies on for support. To make matters worse, it is often necessary to reciprocate the conductor to get it in, and also the jet-head is often badly positioned. In these cases, the disturbance is simply too great for the conductor to ever be of any use.

Conductor loading

What happens once the conductor is successfully installed? It then has to undergo and sustain a complex long-term loading history, which may last tens of years if the well becomes a producer.

For axial loading, the immediate requirement is to support the first inner casing string - and probably a BOP stack too. This may all have to be supported by the conductor before the first casing is cemented up. If the soil isn't strong enough, there can be subsidence problems. Upon hooking up of the drilling riser, the axial loading may then switch to tension, with significant cyclic variations from drilling vessel movements and current loads on the riser.

Returning to the requirement to support the first inner casing, this is a particular issue with jetted conductors because of the excessive soil disturbance mentioned earlier. It takes time for the soil strength to recover, and it might not become strong enough until quite a while after jetting is completed.

Good soils modeling is also essential in predicting wellhead deflections and conductor stresses. Particularly in soft soils, the maximum conductor curvature under snag loading can occur quite far below the seabed, so the maximum stresses may even coincide with a connector. There is no escaping snag loading even in deep water - trawling activities can affect wells in 900-1,000 meters of water.

From that follows the problem of fatigue. Only a few cycles at extreme storm or snag loads can eat up a huge proportion of the fatigue life. If fatigue is critical, it becomes important to incorporate a good soil model in the riser analysis.

Copyright 1999 Oil & Gas Journal. All Rights Reserved.