The influence of pull-out load on the efficiency of jetting during spudcan extraction

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Abstract

Mobile jack-up drilling rigs are deployed at many locations during their service life. This necessitates retrieval of the platform’s legs and spudcan footings before the rig move. In soft soils, where the spudcans embed deeply, the extraction process can be difficult, time consuming and therefore costly. Water jetting systems, devised to ease spudcan extraction, are a common feature on modern jack-up units. However, their effectiveness in reducing the pull-out load required is questioned by the offshore industry. To investigate their efficiency, centrifuge experiments of a reduced scale spudcan model with jets have been performed at the University of Western Australia. The footing was extracted from penetrations of up to 1.5 diameters in normally consolidated clay. Similar to spudcan extraction in the field, these were carried out under load control, applying a constant extraction force. Both influences of pull-out load magnitude and jetting flow rate were investigated. The study demonstrates that jetting is efficient in facilitating spudcan extraction, as it reduces the required uplift load. Practical guidance is provided in applying the results to field conditions.

Introduction

The majority of the world’s offshore drilling in water depths up to 120 m is performed from self-elevated mobile “jack-up” units (Fig. 1). Jack-ups are temporary structures designed to spend a short period at one site before being relocated. Prior to each move, the jack-up must extract its inverted conical “spudcan” footings from the seabed by pulling its independent truss work legs through the hull. For deeply embedded spudcans, this process can be difficult and time-consuming. Extraction from penetration depths of one or two spudcan diameters are reported to regularly take one or two weeks, and in some cases up to ten weeks have been required to free the legs.

The jack-up’s buoyant hull is used to develop the “pull-out force”. In order to apply tensile load to the legs, the hull is jacked down into the water beyond floating draft. The allowable over-draft, typically about 0.6 m, is set out in the operations manual of the rig and therefore determines the maximum pull-out force the jack-up can apply. This is estimated to be between 8 and 25 MN, depending on the rig, and corresponds to approximately 20%–50% of the original vertical compressive force the spudcan was subjected to when installed.

Depending on the depth of embedment and the nature of the soil, this pull-out force is not always sufficient to extract the spudcan. Most modern mobile drilling rigs are therefore equipped with a water jetting system integrated into the spudcan bottom and/or top face to assist in the leg extraction. The water is supplied from pumps located on the hull through hoses down the jack-up legs. In clay material, where significant negative pore pressures (referred to here as suction) are developed at the spudcan invert, jetting applied at the spudcan base aims to reduce the extraction resistance by counteracting the suction. Top jetting, which is not discussed further in this paper but has been investigated by Lin [1] in sand, is intended to loosen the soil above the spudcan.

The research presented in this paper aims to provide guidance to the offshore industry on the efficiency of water jetting. Centrifuge experiments that mimic in-situ jetting procedures by providing a constant pull-out load are discussed. It complements a previous study of Gaudin et al. [2], [3] that was conducted under displacement-control and that identified the mechanisms governing jetted spudcan extraction.

Section snippets

Parameters influencing spudcan extraction

In the early stages of extraction of a deeply embedded spudcan, the mechanism identified comprises reverse end bearing in combination with uplift of the soil on top of the spudcan [4], [2]. The mechanism is shown schematically in Fig. 2(a). This transforms gradually to a flow mechanism around the spudcan edges while the soil above the spudcan continues to be lifted upwards (Fig. 2(b)). The change in mechanism occurs at maximum extraction resistance, and is accompanied by the generation of

Facility and test set-up

The experiments were performed in the beam geotechnical centrifuge at the University of Western Australia [8] at an acceleration of 200 times the Earth’s gravity (known as 200 g). The initial spudcan penetration, holding of a constant “operational” load and finally the extraction, were all controlled using a two-dimensional actuator mounted on top of the strongbox. Attached to the actuator via a cylindrical leg and a load cell was the model spudcan (Fig. 5(a)). It represents a 17.11 m diameter

Experimental results

All results are presented in prototype dimensions, unless otherwise stated.

Discussion

In the extraction phases of the three load-controlled centrifuge tests reported here where V exceeded 30 and the peak excess pore pressure was recorded, the prototype direct applied tensile load Qdirect was 15.6 MN, 24.0 MN and 17.7 MN for tests LCJ1, LCJ2 and LCJ3 respectively (Table 3). This results in Qdirect/Qult ratios of 0.15, 0.23 and 0.31 respectively (accounting for the difference of shear strength between samples). The filling ratios associated with these loads were 0.86 for test

Estimation of required water jetting

Based on the framework introduced in this paper, this section provides a step-by-step guide to estimating the water jetting required.

As the available extraction force provided by the jack-up’s buoyancy is known, comparison to the estimated non-jetted undrained extraction resistance, Qult, can be used to determine the required jetting flow rate using the following procedure:

  • 1.

    Determine the available extraction load from the jack-up’s buoyancy, Qdirect.

  • 2.

    Estimate the non-jetted undrained extraction

Conclusions

This paper investigates the efficiency of water jetting to ease spudcan extraction of a reduced scale model tested in a geotechnical centrifuge, mimicking the in-situ procedure by applying a constant pull-out force during the process. The results support a conceptual framework developed from displacement-controlled centrifuge experiments, which relates the applied tensile load and the water jetting flow rate (via the pore pressure generated at the spudcan invert) required to achieve successful

Acknowledgments

This project has been partially funded by Keppel Offshore Technology Development. Their contribution and permission to report these results are gratefully acknowledged, along with the fruitful discussions with Dr Okky A. Purwana. The authors acknowledge the contribution of Don Herley, beam centrifuge operator, who assisted with the centrifuge experiments, and Tuarn Brown, who assisted in the development of the experimental device. The authors would like to thank Prof. Mark Randolph for his

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