TACKLING JACK-UP RIG NO-GO LOCATIONS. Prakasha Kuppalli ABSTRACT

Transcription

1 TACKLING JACK-UP RIG NO-GO LOCATIONS Prakasha Kuppalli ABSTRACT Jack-up rigs are deployed for drilling and work-over (maintenance) of offshore oil and gas wells for producing hydrocarbons. Rigs drill through the well slots provided in the pre-installed offshore platforms. Deployments of jackup rigs often face problems of punch-through, slippage due to previous pugmarks and leg retrieval. In addition, small jack-up barges have the problem of insufficient leg length. These can be classified into problems related to bearing capacity and those related to contrast in soil strengths. Punch through occurs due to contrast in soil strength in the vertical plane i.e., when a weaker layer underlies a stronger layer. Leg slippage occurs due to contrast in soil strength in the horizontal plane. Though the natural soils generally show very little variation in strength over the diameter of the spudcans, previous deployment of rigs remold the soils over the footprint and create a strength contract between the virgin and the remolded soil in the horizontal plane. This creates a strength contrast across the diameter of the spudcan when a jack-up rig having a different footprint than that deployed earlier is mobilized. Hence these spudcans have a tendency to slide into the weak depressions left by the earlier deployment. Thick layers of soft clays at the seabed often necessitate that the legs penetrate deep in to the sea bed to get enough bearing capacity. This sometimes limits deployments of rigs which have limited leg length. Since these Jack-up rigs are intended to move from location to location, their legs have to be pulled up (retrieved) after the drilling operation. However problems during leg retrieval are encountered when spudcans rest on a soil layer of very high capacity due to suction created. The problems are compounded when a thick soft clay layer overlies a hard clay layer and the rigs with conical spudcans have been deployed earlier. Locations where rig owners refuse to go to certain platforms due to these problems are popularly known as No-Go locations. Though considerable research has been carried out, solution to such problems require a comprehensive understanding of the various rig and geotechnical issues. Solutions to such problems include dumping stones on the seabed, constructing stone columns for soil improvement, mobilizing lighter mat supported rigs and dredging & partial gravel filling. Dredging and gravel placing for levelling the surface at the isopach to the hard layer (depth to change in soil strata) provides a level, hard and permeable surface at the bottom of the spudcan during the next deployment of the jack-up rig. Since this ensures uniform strength over the diameter of the spudcan it obliterates the leg slippage problem. Also, gravel being permeable, no suction develops under the spudcan resting on gravel. Hence this approach solves all the geotechnical issues related to jack-up rig deployment. Keywords: Jack-up, No-Go locations, Punch Through, Slippage, Retrieval Prakasha Kuppalli, Engineering Specialist, Saudi Aramco, Dhahran, Saudi Arabia, prakasha.kuppalli@aramco.com

2 TACKLING JACK-UP RIG NO-GO LOCATIONS Prakasha Kuppalli, Engineering Specialist, Saudi Aramco, Saudi Arabia, ABSTRACT: Jack-up rigs are deployed for drilling and work-over (maintenance) of offshore wells. Deployments of jack-up rigs often face problems of punch-through, slippage due to previous pugmarks and leg retrieval. In addition, small jack-up barges have the problem of insufficient leg length. These can be classified into problems related to bearing capacity and those related to contrast in soil strengths. Punch through occurs due to contrast in soil strength in the vertical plane, slippage occurs due to contrast in soil strength in the horizontal plane, excessive leg penetration occurs due to low bearing of soil strata and leg retrieval problems occur due to very high capacity of the soil strata below the spudcan. The problems are compounded when a thick soft clay layer overlies a hard clay layer and the rigs with conical spudcans have been deployed earlier. Locations where rig owners refuse to go to certain platforms due to these problems are popularly known as No-Go locations. Though considerable research has been carried out, solution to such problems require a comprehensive understanding of the various rig and geotechnical issues. One such solution is dredging and gravel placing for levelling the surface at the isopach (depth to change in soil strata) to the hard layer, which is explained in this paper. INTRODUCTION Oil production involves drilling and maintaining the wells. In offshore environment, this is being carried out by using jack-up rigs and smaller barges. While the Jack-up rig legs can have high leg length, even of the order of 70m, while the barges have limited leg length of 45m. Figure 1 shows a typical jack-up rig deployed close to an offshore platform. The first time deployment is generally analyzed thoroughly for any possibility of punch through (sudden uncontrolled penetration of one of the legs of the rigs relative to the others leading to tilting and instability of the rig), before deploying the jack-up rig. Nevertheless, punch throughs happen even to this date. Figure 2 shows the tilted rig after an punch through incident (gcaptain.com). But, redeployments do not receive the same care as for the first deployment. Rigs with different foot prints are deployed based on availability without any consideration to geotechnical issues. This sequence of operation creates a seabed with lateral contrast in strength and an undulating isopach. This increases leg penetration in subsequent deployments and the legs slip into the earlier created depressions in the hard layer. So, Geotechnical issues related to Jack-up rig deployment have to address issues related to having enough leg length, punch throughs and sliding of rigs and bending of legs due to the presence of pugmarks left during earlier deployments.

3 Prakasha Kuppalli Jack-up Operation Jack ups are towed barges carrying oil drilling rigs. They are anchored at any location by three (rarely four) legs which are supported by spudcans at their bottom. The legs can be lowered or rised using a rack and pinion arrangement. The jack-up anchoring operation typically includes the following steps: Figure 1. Jack-up rig positioned close to a platform When the jack-up reaches the location the legs are lowered to touch the seabed. Then the hull of the barge is lifted using the rack and pinion arrangement transferring the self-weight of the barge to the legs. The selffight of the barge and equipment on it is generally known as the light ship weight (LSW). When the hull is completely in air the preload is applied onto the legs by filling the ballast tanks in the hull. The preloading is carried out leg by leg, by ballasting the tanks close to each leg. The penetration at full preload is the final leg penetration. Then drilling is carried out by extending/cantilevering the rig out from the barge. After the drilling, the hull is lowered in to the water. The legs are pulled out from the soil, by depressing the hull into water (by rack and pinion arrangement) to take advantage of the buoyancy to apply an upward force on the legs. If the force of buoyancy is found insufficient to pull the legs off the seabed, suction at the bottom of the spudcans is released by jetting water through the nozzles (if provided) at the bottom of the spudcans. The barge is then towed to the next drilling location. Geotechnical issues related to jack-up rigs Figure 2. Tilted Jack-up rig after punch through The geotechnical issues related to jack-up operation include; estimating the leg penetration and Saudi Aramco: Company General Use

4 confirming the jack-up has sufficient length, punch through and leg getting stuck in the sea bed. Estimating jack-up leg penetration into the seabed Generally, the seabed soils are layered and it is common to have 3-4 layers of soil within the depth of jack-up leg penetration. SNAME 5-5A ( ) recommendations are generally used to estimate the penetration depth as well as the variation of bearing capacity with depth. The estimated penetration for the problem analyzed (see next section) is shown in figure 3. The estimated penetration is 13m. Too much penetration If the legs penetrate considerably into the seabed and the available leg length is not sufficient to push the spudcans to a depth sufficient to bear the preload, the barge has to be demobilized. This is generally problem related to smaller barges. Generally barges have leg lengths less than 45m, which limits the penetration in the present case (see next section) to 11m below seabed, as the water depth is 34m. Punch through Punch through is the phenomenon in which one of the legs penetrate suddenly during the preload causing the hull to tilt. These can be catastrophic. Leg getting stuck in the seabed This problem is encountered when the spudcans rest on a clay layer. After the jack-up rig is anchored, drilling operations are carried out, which generally takes 6-9 months. This gives enough time for the pore pressures in the clay layers (at least close to the spudcan) to dissipate. After the drilling operation, the negative pore pressures in the clay layer below the spudcan, resist pulling out of the legs. Some of the rigs have been provided with water jetting system to release the suction, but their effectiveness has not been consistent Depth (m) Capacity (MN) Punch Throu Capacit y at p (MN) Figure 3. Bearing capacity and punch through So, it becomes necessary to provide enough pull to ensure a reverse end bearing failure below the spudcan. Since the pull that can be mobilized is limited to the buoyant force on the hull, it is not always possible to provide enough pull and the legs get stuck in the seabed. This is shown schematically in Figure 4. Problem definition One of the real-life situation is presented below, which illustrates all the geotechnical problems associated with jack-up rig operation. The location had a water depth of 35m. The soil stratigraphy consisted of soft clay followed by a thin

5 Prakasha Kuppalli medium sand layer underlain by hard clay as shown in Table 1. The sub-bottom profiler survey after 3 deployments shows an undulating seabed and completely disturbed sand layer with large depression in the hard clay layer (refer Figure 5). La yer No. Dept h to top (m) Dept h to botto m (m) Laye r Desc riptio n Wat er cont ent (%) Shear strength/phi(kpa /degrees) Very soft clay Very soft clay Fine to medi um sand Figure 4. Typical conical spudcan and mechanism of leg retrieval Hard clay Table 1. Soil stratigraphy and parameters Figure 5. Cross section from a sub-bottom profiler survey This was caused by the following sequence of events: The first deployment remolded the top soft clay layer. The conical spudcan penetrated the sand layer and rested on the hard clay layer After the drilling operation the leg was retrieved by pressing on the hull, without applying suction (older rigs didn t have the arrangement for jetting at the base of the spudcan to relieve the suction between the spudcan and the hard clay layer). The hard clay layer failed due to reverse loading and a wedge of the hard clay layer Saudi Aramco: Company General Use

6 came out sticking to the spudcan due to low permeability of hard clay layer. A depression was created in the hard clay layer Subsequent deployments had different foot prints which both widened and deepened the depression in the hard clay layer This led to deeper penetrations and more slippage of the legs into the earlier depressions during subsequent deployments at the second isopach. The location was finally declared a No Go location. The paper enumerates the various possible solutions, their merits and demerits and proposes a feasible solution. Possible Solutions 1) Dumping rocks or concrete blocks on the seabed this is feasible but is highly risky as it creates a contrast in strength with depth, which is known to increase punch through possibilities. See Figure 3, which shows the bearing capacity curve after surface strengthening, demonstrating the possibility of punch through of about 12m. Rigs can generally tolerate a sudden penetration of 1-2m and hence such a punch through of 12m would be catastrophic. 2) Improving the soft clay with stone columns - Though this could increase the bearing capacity, it has been shown to be insufficient to sustain bearing pressures in excess of 250 kpa exerted by the spudcans on the seabed. 3) Using mat supported rigs These have very limited drilling and work-over capabilities and the pressures they exert on the seabed exceed the bearing capacity of remolded soft clay at the seabed. 4) Using anchored barges It is difficult to deploy them at the location due to presence of cables and pipelines on the seabed 5) Dredging and levelling the hard clay layer by gravel filling This appears to be a feasible option and can solve all the rig deployment problems a. It can solve the rig slippage problem as it eliminates lateral contrast in strength of the hard layer at the bottom of the spudcan b. It resolves the leg retrieval problems, as the gravel is highly permeable and does not offer any reverse bearing resistance. c. It solves the leg length issues as the thickness of gravel filling can be adjusted to suit the leg length. Implementation of dredging and gravel filling This involves dredging out the soft clay layer first and then filling the hole with gravel to the required thickness. Dredging: This can be accomplished either by dredging and removing the dredged material by fluidizing the soil in-situ. Dredging can be carried out by mechanical dredgers or by fluidizing the soft clay layer and the thin sand layer and pumping them out as slurry. Since the clays are of soft consistency they can be easily fluidized and pumped like sand. While the mechanical dredgers are easy to operate, caution should be exercised while deploying them close to the offshore platforms for their limited dynamic positioning capabilities. However this is a feasible option. Other disadvantage with dredging is that the piles of the platform loose the soil support up to the level of dredging. Since remolded soil strengths are used in the design, this is not expected to be a serious issue. Nevertheless the dredging should be carried out during calm weather conditions to ensure minimal environmental loading on the platform and dredger stability and eliminate possibility of any accident.

7 Prakasha Kuppalli The other approach is to fluidize the soft clay layer through jetting in-situ and place gravel at the bottom of the slurry through a pipe. This involves high volume and high pressure jetting to excavate the soft clays and sand up of 11.7m. These can be directly deployed from the offshore platform and do not require a vessel. However ensuring proper placement and levelling of gravel in the fluidized clay could be a concern (Jan De Nul). Typical dredging operation is shown in Figure 6. length is expected to be a concern, the thickness of the gravel can be increased accordingly. However there could be some problem in placing though fluidized slurry and properly levelling it. The dredged hole is left open and will be filled by natural back-filling. Figure 7. Gravel placing in progress Conclusion Figure 6. Dredging in progress Gravel placing: Gravel dumpers that can accurately place gravel through a pipe are available. Figure 7 shows the gravel placing operation. These vessels have a good dynamic positioning system and can easily be deployed very close to the platform. They are equipped with echo sounders to ascertain proper levelling of the gravel layer. Hence the thickness of the gravel fill can be accurately controlled to bring the level up to the first isopach (say 2-3m). If leg Dredging and gravel filling only over a small thickness (2-3m) is the most economical and feasible solution to ensure safe deployment of jackup rigs and barges at No Go locations. References 1. Jack-Up Collapses at Maersk-Operated Al Shaheen Field Off Qatar. gcaptain. Mike Schuler, 7 July Web. 29 July < 2. The Society of Naval Architects and Marine Engineers (2005), Guidelines for Site Specific Saudi Aramco: Company General Use

8 Assessment of Mobile Jack-Up Units, Technical and research bulletin 5-5A. 3. People and global expertise. Jan De Nul. N.p., n.d. Web. 29 July <