Centrifuge model study on spudcan footprint interaction 2

Chapter – Introduction Chapter Introduction 1.1 Jack-up unit Since the first jack-up, or also known as self-elevating mobile unit, was built in 1954, it has rapidly become the most popular mobile drilling platform used for offshore oil and gas exploration in relatively shallow waters. Owing to its mobility, a jack-up unit can be moved from one location to another with relative ease. A jack-up unit consists of multi-leg, the 3-leg unit is being the most common. There are two basic types of leg design: one has columnar legs which are simply big steel tubes, and the other has open-truss legs, as shown in Figs. 1.1(a) and (b). Open-truss legs have an advantage over columnar legs as they can withstand higher bending stresses. Regardless of columnar or truss type, each of the leg is typically supported by a footing, known as spudcan foundation (Fig. 1.2). A spudcan is approximately circular in plan with a shallow conical base, often with a pointed spigot at the bottom of the conical base. This protruding spigot is to facilitate initial positioning and provide improved sliding resistance. Table 1.1 shows examples of jack-up units with the number of leg and the equivalent spudcan diameter up to 1970. The increase in spudcan equivalent diameter has been coupled with the decrease in the number of legs. Fig. 1.3 shows evolution of spudcan footings for jack-up rigs built from 1955 to 1982. In general, the spudcan size has increased from an equivalent diameter of 4.8 m to 20.1 m. The foundation of jack-ups is not Chapter – Introduction custom-designed for a specific site. Therefore, they must be designed to remain stable, regardless of the soil conditions (Poulos, 1988). The legs pass through openings in a barge hull where its deck serves as the platform for drilling equipment and other machinery. The legs are firmly gripped by the jacking device at hull yet the leg must be able to move up and down through it. In some cases, rack and pinion (Fig. 1.4) is used as gripping system. The basic function of leg-hull connection is to allow forces to transit between the legs and the hull. Each connection commonly consists of a pair of upper and lower guides and a jacking system and/or fixation system (SNAME, 2002b), see Fig. 1.5. Some degree of rotation movements are allowed in this kind of connection. 1.2 Typical spudcan installation modes Generally, four (4) typical modes are involved when a jack-up unit is to be installed at a designated site. Firstly, the unit is towed to the site floating by the buoyancy of its hull and with the legs elevated out of the water, as illustrated in Figure 1.6(a). On location, the rig is first soft-pinned into the seabed by its selfweight. The running anchors are then installed after meeting the installation procedures. The tension test is performed on the anchors one by one. After the anchors are proved to be functioning, all three legs are raised up. The rig is moved to the final intended position facilitated by the mooring anchors. The platform legs are then jacked down until the footings are in contact with the seabed. The hull is slowly elevated out of the water, causing the spudcans to penetrate under the self-weight of the jack-up unit. Preloading is initiated and the spudcans are proof loaded with a vertical load that is higher than the Chapter – Introduction operative load. The objective of preloading is to provide sufficient safety margin against the worst environment loading that the rig is likely to experience throughout the entire operational period. The preloading is carried out by pumping seawater into the ballast tanks allocated in the hull to increase the weight of the rig (see Fig. 1.6(b)). Fig. 1.7 shows the record of spudcan bearing pressures for rigs built from 1974 to 2008. Within the popular rig classes, the maximum vertical installation bearing pressure has increased from a range of about 200 to 400 kPa for old rigs, to around 400 to 600 kPa for modern rigs (Osborne et al., 2006 and 2008). The settlement of each leg is monitored throughout all the preloading phases. The rig is held at full preload for a couple of hours (dependent on individual rig operational procedures) and the installation is approved when the settlement of each leg is stabilised. After a stable condition is achieved, the preload water is dumped out and the hull is elevated to an air gap of typically 12 to 15 m to proceed with the drilling operations (Purwana, 2007), see Fig. 1.6(c). Depending on the type of work involved, the operational duration of a mobile jack-up rig in the field can be from weeks to as long as years (Purwana, 2007). Upon completion of operational task, spudcan extraction is performed, as shown in Fig. 1.6(d). After the spudcan is completely pulled out from the seabed, a spudcan footprint is formed. 1.3 Explanation of footprint in the context of spudcanfootprint interaction After a jack-up unit is removed from a site, depressions consisting of disturbed soils are left in the seabed. In the guidelines by the Society for Naval Architects and Marine Engineers (SNAME, 2002a), the depressions are Chapter – Introduction termed as footprints. However, there is insufficient emphasis on soil condition underneath the depression in this terminology. It is likely that the interaction between spudcan and footprint is not only affected by the depression but also changes in soil properties beneath the depression. The soil beneath a depression is highly non-uniform due to back flow of remoulded soils during and after spudcan penetration and extraction, and reconsolidation of the soil. The footprint feature is dependent on various factors such as footing shape and size, soil types, the footprint penetration that had been achieved, methods of extraction and time (for both operational period and elapsed time after a footprint is formed). Its shape is also affected by the local sedimentary regime with time after its formation (SNAME, 2002a and b). In normally consolidated clay with deep spudcan penetration, the footprint can be larger than the spudcan diameter. In this thesis, the footprint is a term used to describe a seabed condition with  Changes in physical profile (existence of depression) and  Changes in soil properties (shear strength of soils beneath a footprint is highly non-uniform). 1.4 Problems concerning jack-up installation at site with old footprints A jack-up unit may be re-installed at the same site for drilling additional wells or enhancing production of existing wells. The presence of footprints due to former jack-up activities may pose threats to new spudcan installations when it is installed at or close to existing footprints. A statistical study by Berg (2004) indicates that within Shell EP Europe, roughly 1200 footprint points had been registered in geotechnical and footprint datasets, of which 800 footprint points Chapter – Introduction are on the UK side and 400 points on the Dutch side. In addition, there are approximately 80 new single footprint points added to the existing datasets every year. Thus, it can be seen that existence of footprints are not uncommon. Fig. 1.8(a) shows a bathymetry of an established site with a permanent piled jacket platform and pipeline. Fig. 1.8(b) shows a cross-sectional view illustrating a potential footprint problem. In this case, one leg installation interacts with an existing footprint. The crater and non-uniform bearing resistance may cause the spudcan to slide towards the footprint. The tendency of sliding is resisted by both structural stiffness from the legs, the leg-hull connections and the hull, and foundation stiffness from the other two legs that are installed in the intact ground. The restraint in movements is reflected in horizontal force, H and moment, M acting at the spudcans and the leg-hull connections. When a jack-up unit is installed, the two forces acting on the spudcans are the soil resistance and preload from the structure. If a spudcan is installed in a uniform ground in the absence of environmental loads, the preload is equilibrated by the soil bearing resistance and the spudcan would penetrate vertically into the seabed. On the other hand, if a spudcan partially overlaps with an existing footprint during its installation, the spudcan may slide towards the footprint due to the uneven seabed profile as well as non-uniform soil strength beneath the footprint. Such spudcan-footprint interaction would induce additional forces on the penetrating spudcan leg which have to be resisted by the jack-up structure. This could result in excessive stresses acting on the structures that potentially lead to structural damage of the rig. If the resisting force from the jack-up unit is insufficient to take the induced forces, Chapter – Introduction the spudcan would slide towards the footprint until equilibrium is achieved. The resulting displacement may cause severe structural damage, injury to personnel and at worst lead to catastrophic failure (SNAME, 2002a). The sliding magnitude may exceed positional tolerance and the situation could be complicated if the new installation is in close proximity to a fixed platform or wellhead. It is apparent that the interaction between a new spudcan installation and an existing footprint is not only dependent on the soil conditions but also on how the structure responds to it. 1.5 Case histories and footprint-related reported incidents A statistical survey on the geotechnical jack-up incidents shows a trend of increase in frequency of jack-up incidents associated with spudcan-footprint interaction from 1979 to 2005 (Osborne, 2005), as shown in Fig. 1.9. The growing concern of the industry on this issue is also reflected by a jointindustry project initiated by ten oil and gas companies to investigate the problem associated with footprint in 2002 (Sumrow, 2002). All of these indicate the significance of this problem and the eagerness of the industry to understand and subsequently resolve this problem. According to Research Report 289 (MSL, 2004), one-third of jack-up accidents are associated with foundation problems. Of which 15% of incidents is due to uneven seabed/scour/footprint (Fig 1.10), the second highest rate in incident cause. It should be noted that there might be many other incidents that have not been reported in the public domain. The reasons causing the increasing frequency in spudcan-footprint interaction issues were surveryed by Global Maritime (2002) (a confidential Chapter – Introduction document that only disclose to the JIP’s participants) and reported by Osborne et al. (2008). Some of the reasons are:  Increasing reliance on jack-up rigs for drilling and work-over activities for unmanned and subsea facilities.  Increase in jack-up drilling capabilities and environment capabilities  More choices of jack-ups for use at a single location  Reduction of operational period (in some cases it can be as short as 10 days at a time). This leads to increase in opportunities for alternative units to operate over the individual field lives. 1.6 SNAME guideline (2002) Spudcan re-installation very close to or partially overlap with existing footprints is generally not recommended in the guidelines (SNAME 2002a). In a situation where this is inevitable, the guidelines recommend the use of an identical jack-up (same footing geometries and leg spacing) and locating it in exactly the same position as the previous unit, where possible. However, it is unlikely that two jack-up units have an identical design as the structures of most units are often custom-made and the deployments of units are subject to availability. In such a case, the guidelines suggest careful positioning of the jack-up on a new heading with one footing locating over a footprint and the others in virgin soil to alleviate the potential of spudcan sliding. If it is not possible to avoid spudcan-footprint interaction, the guidelines suggest infilling the footprints with imported materials. However, the suitability and Chapter – Introduction effectiveness of infill to ease the footprint problem in various soil conditions are still questionable. 1.7 Needs for research It is evident that existing guidelines are not adequate for rig operators to install jack-up units in close proximity to existing footprints safely. Though the consequences of spudcan-footprint interaction can be hazardous to a rig installation, there are still no guidelines available to help operators to achieve a safe installation (Dean and Serra, 2004). The current understanding on this topic is still insufficient. To date, only a small number of studies (e.g. Stewart and Finnie, 2001; Jardine et al., 2001 & 2002; Foo et al., 2003; Dean and Serra, 2004; MSL, 2004, Teh et al., 2006, Gan et al., 2008b and Cassidy et al., 2009) on the footprint problem are available in the public domain. The studies on the footprint characteristics and its influence on spudcan-footprint interaction are still lacking. 1.8 Scope of study and outline of thesis In this thesis, a systematic study to qualitatively and quantitatively investigate footprint problems has been conducted using centrifuge modelling technique. The first part of the study focuses on the investigation of factors affecting footprint characteristics and how these characteristics influence the off-centre spudcan installation in clay of varying shear strength profiles. The study on footprint characteristics includes i) shape of seabed depression (or crater) and ii) soil shear strength profile before and after a footprint is formed. To study how these footprint characteristics affect off-centred spudcan installation, a rigid connection is modelled. The interaction between a spudcan and a Chapter – Introduction footprint is evaluated in term of V-H-M profiles where V is vertical force, H is horizontal force and M is moment. Half-spudcan tests were performed to examine the soil failure patterns for the initial and re-penetration of the spudcan foundation. The findings obtained leads to a further investigation of the footprint soil characteristics with consideration of time effects. The time, herein, refers to the simulation of operational period and also elapsed time after a footprint is formed. By combining the findings obtained from the above investigations and existing published results, dimensional analysis is performed and a framework is then proposed to estimate the load responses during spudcan-footprint interaction. The limitations for tests presented in this thesis include: i) the spudcan was installed and extracted using displacement controlled, ii) a rigid leg-hull connection was modelled, and iii) stiff leg. The outline of this thesis is as follows: Chapter reviews the existing literatures on spudcan-footprint interaction including conceptual soil-structure interactions, experimental investigations, numerical simulations, industry adopted precautious/mitigation measures and SNAME guidelines. The spudcan foundation behaviour in clay and the T-bar and ball penetrometers are also reviewed. Chapter explains the methodology of the present centrifuge model tests. This includes the experimental setup, soil sample preparation and test procedures for tests done using the geotechnical beam centrifuge in National University of Singapore and also the drum centrifuge in University of Western Australia. Chapter – Introduction Chapter reports the experimental results for the investigation on footprint characteristics. The effects of these characteristics on spudcan re-penetration are also reported. The practical implications of these findings are then evaluated. Lastly, the soil failure mechanisms for both spudcan initial and repenetration are also examined. Chapter reports the effects of time on spudcan-footprint interaction. Firstly, the changes in soil shear strength beneath a footprint with time are studied. Secondly, the effects of these changes on spudcan-footprint interaction are discussed. Chapter discusses some other factors that may affect the spudcan-footprint interaction. The results from dimensional analysis are presented. Chapter summarizes the main findings established in this research and its practical implications. In addition, recommendations for further studies are also made. 10 Chapter – Introduction Table 1.1 Example of early jack-up units Rig name Mr Louie Offshore No. 52 Offshore No. 54 Julie Ann Penrod No. 55 Ocean Master I Minimum nominal water depth (m) 40 30 42 47 50 100 Number of footings 12 14 3 Footing equivalent diameter (m) 6.7 4.9 6.1 11 11 12 Approximate footing load (MN) 10 4.5 8.8 19 25.3 43.3 * data extracted from Gemenhardt and Focht (1970) 11 Chapter Introduction (a) Columnar legs (b) Open-truss legs Fig. 1.1 Two basic types of jack-up (courtesy of Salen Offshore Drilling Co.) Drilling machines Cantilever beam Hull Spudcan footings Fig. 1.2 Jackup supported by spudcans (after Reardon, 1986) 12 Chapter Introduction Fig. 1.3 Examples of spudcan footings (after McClelland et al., 1981) Fig. 1.4 A rack and pinion consists of a rack and two pinions that mesh with the rack (after Bennett et al., 2005) 13 Chapter Introduction Upper guide Lower guide Fig. 1.5 Representative leg-hull connection (after SNAME, 2002b) 14 Chapter Introduction Fig. 1.6 General installation modes for jack-up rig (after Purwana, 2007) 15 Chapter Introduction Fig. 1.7 Record jack-up spudcan bearing pressures from 1979 to 2008 (after Osborne et al., 2008) New rig 1’ Footprints Piled jacket platform (a) Plan view (b) Section view 1-1 Fig. 1.8 Bathymetry of an established site 16 Chapter Introduction Fig. 1.9 Comparing recorded geotechnical jackup incidents for 1979 to 1988 and 1996 to 2005 (after Osborne, 2005) Fig. 1.10 Case histories classified according to cause of failure (after MSL, 2004) 17 . is apparent that the interaction between a new spudcan installation and an existing footprint is not only dependent on the soil conditions but also on how the structure responds to it. 1.5 Case. domain. The reasons causing the increasing frequency in spudcan-footprint interaction issues were surveryed by Global Maritime (20 02) (a confidential Chapter 1 – Introduction 7 document that only disclose. Engineers (SNAME, 20 02a), the depressions are Chapter 1 – Introduction 4 termed as footprints. However, there is insufficient emphasis on soil condition underneath the depression in this terminology.