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A Hybrid Drilling System for Deep Water
in the Arctic
By W. D. LOTH* and A. C. PALMERt
Summary
There is an established technology of offshore drilling and production in
Arctic areas where the water is
shallow or the ice
is landfast. Promising areas for exploration have deep water and constantly-moving ice, and there the existing techniques are not applicable. One alternative approach isto set the whole system on
the sea bed, and- to supply it by submarines: that is technically feasible,, but- would demand an extensive development, which the economics are unlikely to justify. The paper puts forward a less radical 'hybrid' scheme: the drilling system is on the bottom, but it receives power, control, supplies and maintenance from an icebrealcing surface ship, through a flexible riser-umbilical.
Introduction
There are known to be extensive proved offshore gas and oil reserves in several areas of the Arctic, among them the Beaufort and Chukchi Seas north of Alaska and western Canada, the Sverdrup Basin in the Canadian Arctic Islands, and off Sakhalin in the Soviet Far East. A number of other promising areas have been explored only superficially, or not at all: they include the Kara Sea west of the Yamal Peninsula (which has very large
gas fields onshore), the Barents Sea, Davis Strait, the seas
around Svalbard, and the Antarctic.
A few years ago, it appeared that an inexorably-increasing oil price would soon justify the high costs of
arctic offshore development, and some ambitious schemes were planned. As we now know, the underlying assumption was mistaken. Fortunately for operators and investors, none of the schemes were started before the oil price collapse. A widely-held opinion is that development of offshore Arctic resources is now unlikely
to be economically sensible before the end of the
century. That too could turn out to be wrong. A few developments do make economic sense at something like the current oil price. They are not the more grandiose schemes, and tend instead to reflect the economies that can be earned through effective use of existing
technology. More may become economic with the
application of advanced technology.
*W. D. Loth and Company Limited tAndrew Palmer and Associates Limited
Influence of the Arctic Environment
Arctic exploration and production are heavily
influenced by sea ice, which covers most of the sea for much of the year. It is tempting to think of the Arctic offshore as one geographic unit, but in fact there are wide variations in ice climate, at least as dramatic as the difference in wave climate between Lake Maracaibo and the Atlantic West of Shetland. These variations are decisive for the choice of technology.
A complete classification and description of sea
ice is complex, but progress towards a definition of technological needs in different areas can start from a simple distinction, between areas where the ice moves slowly (or hardly moves at all) and areas where ice moves rapidly. Slow-moving ice is found in bays and inside barrier islands, but can also occur in very deep water if islands stabilise the ice against wind-driven circulation.
A second factor is the size of the largest ice fragment that may move into the space occupied by a production system. If a fragment is small, it may be possible to make the system strong enough to break the ice. If it has an intermediate size (such as a small iceberg), it may be possible to deflect it by pushing, or to make it ground on a protective structure. In some areas, ice fragments are so large that neither option is practicable: close to
Svalbard, for instance, it is common to find ice fragments 201cm across, and 100 million tonnes mass.
A third factor
is depth, which influences design because it determines the cost of designing a structure to resist ice forces, and because it determines whether the ice will touch to bottom (which in turn decides howfar wellheads and pipelines have to be buried).
Figure 1 represents these three factors in different
geographic areas of interest to the offshore industry, and shows the wide variations that occur.
A complicating factor is variation with time.
Conditions can change rapidly, and ice velocities can
increase by a factor of 100 in a few hours. In some areas, such as the southern Beaufort Sea, the open water period is long enough to allow drilling and construction to proceed with conventional technology.
Development has generated a number of design
responses to the problems set by different ice and depth conditions. Figure 2 shows some of the responses to the
most pressing problem, that of securing a base for
Volume 13 Number 2
TEMINISCHE IIMVERSITEIT Underwater Technology
Underwater Technology
Fig 1 Depths, ice movement rates and maximum ice fragment areas in different areas of the Arctic
Winter ice movement rate
0.01 1 100 10,000 m/day I
II I
gravel ieland ice island gravel island caisson monoconehybrid drilling system
Fig 2 Bases for drilling and production systems
exploration drilling. In many instances the same
technology can be applied to development drilling, to pipeline construction and to production. Some of this is established technology: there is extensive experience of gravel islands in shallow water in the Beaufort Sea, and
of ice islands in the Sverdrup Basin.
There is increasing interest in areas where the ice
is highly mobile, to the right in Figs
1 and 2. The
conditions are severe. The incidence of ice is high, warning of approaching ice is limited to six hours or less,
and the masses can be many lcilornetres in width. The ice could extend far below the surface. There is often no reliable open-water season, and drilling from a conventional drillship or semi-submersible would be interrupted so often that progress would be very slow. Unless the water is shallow, a bottom-founded surface-piercing structure is unecononiic. The objective of this paper is to put forward an alternative scheme for this demanding environment.
A Hybrid Drilling System
Since it
is the ice on the surface that creates the
difficulties, an obvious response is to put everything underwater (Fig 3). That choice replaces the uncertain and ever-changing surface ice environment with a stable and totally-predictable deep-water environment under the ice. It eliminates many of the special problems of the Arctic, and allows the application of deep-water
technology developed elsewhere,
c--1
Fig 3 Autonomous underwater drilling system
This solution obviously generates new problems. An underwater system has to be installable, it has to be maintainable, and it has to have power. If a surface vessel can reach the site even for a few hours it
could set the structure on the bottom. If the ice climate is so severe that a surface vessel could never reach the site, the system could be set on bottom by a submarine.
Submarine operation under ice is an established technology, dating back to World War II, and there is far more experience of submarine operation in the
Arctic than there is with surface icebreakers. Submarines also provide a technology for transferring people to the system. The power problem is more severe, but could be
met by fuel cells or by a nuclear reactor (again an
established submarine technology).
None of this is beyond the reach of technology, but it would clearly demand a major development effort, over 10 to 15 years. We propose instead a less radical 'hybrid' solution, which transfers as much as possible of the drilling operation to the bottom, but keeps some of the system on the surface (Fig 4).
The objective of the system is to provide a year-round drilling capability which requires a minimum amount of technical innovation. The system would be able to move well off location while drilling proceeded but would be able to disconnect rapidly, in the event that very large floes approached. Some operations, notably running long casing strings and some completion activities, would
require the rig to maintain station.
The essence of the proposal is to remove much of the mechanical drilling system to the seafloor and connect the surface and seafloor with a flexible riser. Much of a floating drilling system is currently automated, semi-automated, or remotely controlled.
Automatic pipe handling systems have been around
for many years and are currently being improved.
Summer 1987 4
10
160
depth
1000
winter ice movement rate
0.01 1 100 10,000 m/day I I I I I I I barrier islands 10 Beaufort Sea Sakhafin 100
Sverdrup Basin Svalbard
depth Barents Sea
1000
Arctic Ocean
Fig 4 Hybrid drilling system
Pressure control is largely a remote function with only control being centralised on the drill floor. Drill pipe can be made up faster and more accurately with torque-turn devices than it can be by hand. There is no reason why all these mechanical devices could not be made to work on the ocean floor. While it may be possible to use a system, such as turbine drilling, that does not require a rotary table on the seafloor, moving the rotary to the seafloor (and powering it hydraulically) would not be a difficult task. The basic premise, then, is to move all these items to the bottom and mount them in moderate size modules which could be retrieved to the surface if
repair was required.
Lifting capability would pose a greater problem. One of the concessions which might be required is the handling of shorter stands of pipe and it might be more desirable to work with singles rather than triples.
Operations which are routinely performed by hand, such as setting slips, could either be automated or done with a robot. The robot proposed would not in fact be very complex. There would be no local control on the seafloor with the exception of mousetrap systems on the BOP. The robot would be operated telerobotically by a drill crew on the vessel deck who would need no skills beyond those used for normal drilling. Handling of shorter casing strings, at least the thirty and twenty inch strings, would be done using only the subsea system. In the event that casing had to be hung off in the stack it would be hung below a set of special blinds and the rig would have to move over to pick it up prior to resuming operations. Cementing could be done remotely but there is no apparent reason why it could not be done through the flexible riser extending to the rig. Cuttings could be handled in either location although development would certainly be easier if this system was kept on the surface. If it was found necessary to maintain very large levels of bulk supplies, it might be best to store these on the bottom to minimise the size of the surface vessel. In any event cuttings would be disposed on the bottom presumably using the single flexible riser. All power for
the system would be generated on the surface vessel.
A major
functionof the
drilling crew during conventional operations is directional control. In the proposed system this system would be remote. This could be done in several ways. The first is to use turbinedrills and an advanced position monitoring and telemetry unit, an offshoot of current MWD systems. The second is compatible with rotary drilling, and depends on a
robotic sub which
is programmed with the targetdestination. A self-contained inertial system monitors current position with respect to the target and then, with a hydraulic system contained within the sub, steers the bit. This system is currently in the early stages of
development.
A key element of the proposed system would be the flexible riser. The riser would be long enough to let the rig move off location to avoid ice. A horizontal swivel is not essential, but it may be necessary to have some provision to coil up the riser when the vessel moves over the well. The riser would contain hydraulic lines, lines to bulk storage if that is on bottom, and lines to route mud and cuttings to the surface and back to the bottom as required. The riser need not pass all casing strings and it may be possible to use a more conventional rigid riser
for those operations where the vessel is directly overhead.
This would be something of a liability if ice windows are so small that running and retrieving the riser occupies a significant portion of available time. The possibility of open hole completions would have to be thoroughly evaluated.
The floating rig would have a pipe handling system. It
is difficult to envisage that strings of production casing
could be handled by a compact subsea system. If suitably
simple completions can be devised it might be possible to run them from the subsea system but the additional complexities of dual string completions would pose space-out problems much easier solved on the rig floor.
The use of a system such as that described should allow drilling to proceed almost continuously. Some operations might require station keeping beyond that possible during some times of the year but in this case activities would be diverted to another well, which is in a stage of drilling which requires less demanding position control. Three to four wells might be completed each year. A rig such as this might be expected to be rather costly, and it is doubtful whether more than one such purpose built rig could be supported unless there were several potential applications. If the unit can work as
much as projected, however, it should generate a suitable
drilling programme.
A decision to develop this system would be bold but not reckless. Its components are existing technology, though it is innovative as a complete system.
Conclusion .
A combination of mobile ice and deep water precludes
the application of existing technology. In order to make progress, it is necessary to advance the state of the art.
This can be done by developing a hybrid drilling system.
There is of course no point in drilling for oil andgas
in a severe environment unless it would be possible to
produce them economically if they were found. A
broader investigation of a complete development
programme has shown that it would be possible to
establish viable production and transportation schemes in this environment. The schemes are based on parallel
concepts, and rest firmly on existing technology.
Underwater Technology
Underwater Technology
Summary
A review has been made of the current and future
research requirements relating to the design of the
foundations of offshore structures. In this context the term 'research' is used widely to cover position papers, literature reviews and the establishment of data bases as well as the performance of field tests, laboratory experiments and analytical studies.
Topics are classified according to priority ; alpha being
essential and urgent, beta being essential, gamma being necessary in the longer term. A number of prerequisites are identified if research results are to be quickly applied by industry.
Introduction
This document discusses the research needs concerning the behaviour and design of offshore structure foundations. The object is to encourage studies which will improve the reliability and economy of offshore structures. It would be counterproductive if these were done at the expense of more general and fundamental research in soil mechanics since this forms the basis on which specialist work such as offshore foundation design is built.
The document was prepared by the 1986 SUT
geotechnics committee for consideration by the
membership of SUT which includes government
departments, academic institutions, oil companies and suppliers predominantly in the UK and Europe. It is an update of a similar paper produced by the same
committee in 1982 (Ref 1) which played a part in
encouraging some of the ongoing research into pile design and jack-up rig performance in the UK.
The recent fall in the oil price increases the importance
of research to the offshore industry since new and more economic means must be found for field development.
*Committee Membership (October 1986):
Mr F. E. Toolan, Fugro Ltd (Chairman); Mr D. A. Ardus, British Geological Survey; Mr C. J. Barnwell, McClelland Ltd; Professor
J. B. Burland, Imperial College of Science and Technology; Dr E. M. Forster, Marine Technology Directorate; Mr T. J. Freeman, Building Research Establishment; Dr C. D. Green, Shell
International Petroleum; Mr J. W. Hayes, Hunting Surveys Ltd; Dr R. Hobbs, Lloyds Register of Shipping; Ms D. M. Lawrence, Shell UK Expro; Mr J. N. Mansfield, Department of Energy; Mr K. C. Mead, John Brown Offshore Structures Ltd; Dr C.W.
Swain, BP International PLC; Mr B. I. Tollin, Britoil PLC;
Professor C. P. Wroth. University of Oxford.
Statement of Research Needs in
Offshore Foundation Design
by
Offshore Site Investigation and Geotechnics Committee*
of The Society for Underwater Technology
Of course, at the same time budget restraints reduce the
amount of money available for research. The consequence
is that research must be directed towards priority prob-lems and duplication should be avoided. It is hoped that
this document will assist in achieving these ends, both for
those who fund research and those who perform it. The fact that the membership of the committee is predominantly British is reflected in the examples quoted and the topics discussed. Readers are invited to redress imbalance by writing to the Journal of the SUT.
Offshore Foundation Engineering Current and Future
Trends
Structure Types
Current offshore structures may be classified as piled jackets, tension leg platforms, compliant structures, gravity bases and jack-up rigs. Subsea structures are generally a special case of one of these structure types.
Many new platform concepts, which should improve the economics of field development, impose a higher proportion of dynamic to static load on the foundation
than traditional structure types. This will lead to a
greater emphasis on cyclic and dynamic soil testing, in situ and in the laboratory. Also analytical techniques will have to be modified to account for dynamic effects.
With regard to specific foundation types, concrete structures supported on caisson foundations are being considered for soft soil sites in deeper waters. In fact one such platform has been ordered for a field in the Norwegian Sector of the North Sea. Research concerning
the installation
and performance of this
type of
foundation is ongoing (Ref 2).
The development of Derrick Barges capable of lifting
8,000 to 10,000 tons means that platforms will be
completed in a shorter time frame, allowing less set-up to take place before the full dead load is applied. There is an obvious requirement to conduct more investigations into the relationship between pile capacity and time
after installation.
The widespread availability of underwater hammers is
encouraging the.use of vertical piles which means that all the horizontal load on a platform has to be resisted by the foundations in shear.
Site Investigation
The sophistication and productivity of offshore Site investigation methods have improved significantly over the last decade, with commensurate increases in the
