Copyright 2004, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE Asia Pacific Oil and Gas Conference and Exhibition held in Perth, Australia, 18–20 October 2004. This paper was selected for presentation by an SPE Program Committee following review of information contained in a proposal submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract This paper addresses the latest developments in remote field control strategy and focuses on long distance umbilical control versus the use of a control buoy located over the field. The paper provides an insight into both the technology and also the commercial key drivers for the selection of control strategy. The historical development of umbilicals and control buoys is summarised. The relevant technologies are described, covering aspects such as buoy design, telecommunication options, onboard power, fluid storage and injection capabilities, operational issues including access and maintenance, and opportunities for workover activities. The discussion is supported by case examples drawn from a number of fields around Australia.

Introduction In the Asia Pacific region there are many known remote oil and gas fields in environmentally hostile areas or at deep-water locations. Such fields can typically only be made economical to develop if low cost subsea solutions can be implemented. At such remote locations where there is no existing platform infrastructure, the support of subsea facilities by umbilical can be difficult and expensive to implement. An alternative such as a control buoy for monitoring, control and injection may provide a better solution. Umbilical Solutions Development of Umbilical Technology. Since Xmas Trees first went underwater in 1961, the industry has been struggling with reliable and cost effective ways of operating them. There is a requirement for controlling valves on the Xmas Tree and downhole, and also for monitoring key parameters (such as annulus pressure).

The early subsea completions involved single wells in shallow water at short step-out distances, and direct hydraulic control of the tree valves from the surface facility was

straightforward, using one hose per valve. Pressure in the well annulus could also be measured at the surface via another hose. The first umbilicals were little more than bundled hydraulic hoses with a "cheap and cheerful" philosophy. Reliability was not a concern as repair and replacement were easily carried out.

As the industry matured, longer step-outs made direct hydraulic control less attractive. Umbilical cost increased with length, and the response time of the valves became longer. This was cause for concern, because it took longer to shut in a Xmas Tree in an emergency, and the stroking time of the valve became so long that significant wear could occur.

Multiple completions resulted in the umbilicals growing significantly in size, because one hose was needed for each valve on each of the Xmas Trees. Also, when the subsea Xmas Trees were manifolded together, the chokes had to be relocated from the platforms to subsea. This presented problems because the opening/closing time for stepping chokes became excessively long, and resulted in an unacceptable delay when opening up Trees for production.

Advances through Control Engineering. Control engineering provided a solution for these difficulties. Techniques such as sequenced control and hydraulic or electrical piloting allowed the use of smaller umbilicals and sped up valve response times. The availability of directional control valves from the aerospace industry allowed these techniques to be implemented in a cost-effective way. Nevertheless, the increased size and cost of umbilicals was causing operators to become increasingly concerned, especially as a single failure in one of the hydraulic or electrical cores would lose control of the valve, which could potentially result in loss of control of the Tree.

The next major step forward for umbilicals again came through advances in control engineering, using equipment from the electronics industry. The introduction of integrated circuits allowed a number of signals to be multiplexed onto a single electrical control line, transmitted subsea, and then decoded into separate signals to operate Tree and downhole valves. So the umbilical was now reduced to a signal cable to carry the valve commands, a power cable to operate the subsea electronics module (SEM), and three hydraulic hoses, for low pressure (LP) and high pressure (HP) hydraulic supplies and hydraulic fluid return. Best of all, these cables and hoses could be doubled up to provide redundancy, with the supplies being combined in the subsea control module (SCM). A single failure would no longer cause loss of production.

This technique (Multiplexed ElectroHydraulic, or Mux-EH) allowed a number of trees to be controlled through a

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Remote Field Control and Support Strategy - Umbilicals versus Control Buoys P.E. Christiansen, S.A. McKay, K. Mullen, L.R. Upston, (INTEC Engineering)

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single umbilical, and also allowed signals to be directed from subsea back to the surface. Signals from transmitters measuring wellhead pressure and temperature, annulus pressure, flowline pressure and flowrate, and downhole pressure and temperature could be transmitted to the surface on the same signal line. The majority of subsea completions (around 90%) are Mux-EH.

The drive for low cost to make marginal fields feasible gave rise to some new techniques: • Signal on Power (SOP) allow the signals to be

superimposed on the power line, cutting the cost of the umbilical, but is only suitable for medium step-outs with a few wells

• Subsea Pressure Intensifiers (PI) allow the HP line to be eliminated from the umbilical, with the PI regenerating the HP subsea from the LP supply

• Use of higher voltages for power transmission allowed a reduction in size of the power cores, with step-down by a subsea transformer

• Use of open hydraulic systems, with spent hydraulic fluid being expelled to sea, was facilitated through the use of environmentally friendly control fluids, allowing the hydraulic fluid return line to be eliminated from the umbilical

The thermoplastic hoses of the original umbilicals suffered various problems such as leaks at fittings, methanol permeation, and hydrostatic collapse, which could be solved with new materials and designs. However, thermoplastic hoses are inherently prone to volumetric expansion, which means that long umbilicals take in large amounts of hydraulic fluid when they are pressurised up, and the charging and discharge times are prolonged.

Game Changers. The volumetric expansion problem inherent in thermoplastic umbilicals was largely overcome with the introduction of steel tube umbilicals. Teething problems were experienced with welding and quality issues, but steel tube umbilicals can now be regarded as a mature technology. Significantly longer step-outs became technically possible, though not always economically feasible.

The limitations of signal distribution by copper wire were overcome with fibre optic signal distribution. Fibre optics gave a breakthrough with an increased range of up to 200 km, greater bandwidth, and also immunity to electrical interference, allowing signal distribution alongside electrical power cables for submerged pumping etc. Innovations in technology allowed fibre optic distribution from the shore to a subsea optical/electrical conversion pod. This minimized optical power losses, and allowed standard electrical distribution to standard SCMs on the subsea Trees (Ref 1).

State-of-the-Art. The present state-of-the-art in umbilicals is illustrated by the Statoil Snøhvit umbilical which connects the subsea installation to chemical pumps, power generators, hydraulics and fibre optic controls onshore, over a distance of 145 km.

The subsea installation will eventually number 20 wells, in water depths of 250-345 metres.

The umbilical comprises: • Three one-inch hydraulic lines • Two fibre optic cables, each of 12 fibres

• Two power cables (three phase, delivering six kilowatts at three kilovolts)

• One half-inch chemical line Both subsea production in the field and pipeline transport

will be monitored and controlled from the LNG plant control room.

Umbilicals in both thermoplastic and steel tube are now used for static applications (on the seabed) and in dynamic mode (as risers to floating facilities).

Local Issues. A particular difficulty for umbilical systems in Australasian waters is the "Tyranny of Distance". Small umbilicals can be shipped on Coflexip reels, but large umbilicals need mid-length joints or the mobilisation of carousel vessels at the umbilical factories, usually in the USA or Europe. Consequently, Australia is disadvantaged by the distance that umbilicals must be shipped.

Future Trends. The following developments may occur in the near future: • Improvement in bandwidth of electrical signal

transmission - like modern modem technology • Development of hydraulic fluids with lower viscosity

and higher temperature capabilities • Improvement in reliability and water-blocking

capabilities of electrical connectors (Ref 2) • All-electric umbilicals to suit all-electric Trees • Improvement of the existing 11 kV electrical connectors

to suit a transmission voltage of 33 kV at 500 amps, giving a maximum rating of 30 MVA

Development of Integrated Service Umbilicals, which have a large bore internal conduit for production or chemical injection, is unlikely to be relevant to Australasia because of the difficulty and expense of transporting such a product.

The major changes may come from new technology, e.g. the development of room temperature electrical superconductors.

Existing Control Buoy Solutions Four existing control buoy solutions are known to the authors of this paper. A summary of details are included in Table 2 at the end of this paper. Regnar Control Buoy. The first control buoy operated subsea development was the one well Regnar field in the Danish sector of the North Sea (Ref 3), which started operation in September of 1993. Although the design life was originally estimated to be only 1-3 years, the field is still in operation and producing (April 2004) about 300 BOPD. The control buoy concept selected was based on the opportunistic use of an existing loading CALM buoy already in the operator’s possession. The operator states that the system is still functioning satisfactory and boat visits to the buoy take place once every two months, the main activity of the visits being refueling of diesel with a duration of a few hours. The operator only stated one serious event when one line parted during a storm, however, given that the buoy design life was only 1-3 years and the mooring system was only designed for the 10 year return period storm this was not entirely unexpected. East Spar NCC Buoy. The East Spar development was brought on stream in December of 1996 and is also still

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producing (Ref 4 & 5). Originally only two production wells were drilled but recently a third well has been added. This field adopted a purpose built tension leg buoy design primarily to provide a very stable platform with benign motions for the UHF radio link. The offshore system is operating satisfactorily and the design availability of 98.5% has always been exceeded. The buoy concept was tested during the passage of Cyclone Vance, which traversed right over the East Spar location with wind speeds approaching 300 km/hour. 4-ALS-39. A very simple one well subsea development with control buoy was installed by Petrobras in Brazil (Ref 6) in September of 1996. Although little information is available for this development it appears the buoy used is a small disc type buoy originally designed for collection of meteorological data. The current status of this development is unknown. However, initial experience revealed excessive heave and badly located system modules which were hard to access, together with manufacturing problems that resulted in various failures, e.g. pump breakdown, and a connector short circuit resulting in a battery explosion. After retrieval of the buoy and a complete rebuild, most of these problems were solved but the excessive heave still caused wear and premature need for replacement of the mooring chains. However in all other aspects the system has performed better than expected. Mossel Bay EM Control Buoy. The Mossel Bay EM Field was brought on stream in April 2000 (Ref 7). This development uses a control buoy of almost identical design to that used on the East Spar development. There is no published data to the authors' knowledge on the performance of this buoy, but it appears the production targets from the field are being met. Operator Comments. The operators for all the above buoy controlled subsea developments quote cost and schedule as the main reason for selection of the buoy solution in preference to an umbilical to the host facility. Also, risk of fouling with fishing gear has been quoted as adding additional risk to umbilical solutions. Umbilical risk associated with crossing coral reefs and shallow water sections has been listed as arguments for the buoy solution for subsea development with host facilities on shore.

It appears from the published data and discussions with operators that an intervention interval of 4-6 visits per year is achievable once the operation is stabilised and initial start-up problems have been solved. All the buoys are accessed by boat and this has proved satisfactory even though both the North Sea Regnar and Mossel Bay EM buoys are located in very hostile environments. Control Buoy Concept Selection Buoy Design. A control buoy is a permanently moored floating structure that remotely controls and supplies subsea systems. The buoy provides an alternative to long electrohydraulic control umbilicals, which can be costly. The buoy has a shorter dynamic control umbilical, but offers the same service, including: • Wellhead control • Provision of corrosion inhibitor and/or hydrate inhibitor

(methanol or MEG)

The buoy includes systems for operation of the subsea equipment and other provisions for maintenance, including: • Pull-in and hang-off of the dynamic umbilical to the

seabed • Personnel Access, by boat or helicopter • Subsea Control Systems (including hydraulic power

generation) • Communication systems (radio and/or satellite) • Electrical power generation • Ventilation (and possibly air conditioning) for

maintenance personnel A control buoy generally has no hydrocarbon processing

ability, although hydrocarbons may be present either through a vent/flare, pig launcher/receiver and fuel for the power generation. Although not present in control buoys to date, the following services can be designed into the buoy system: • Flare and/or Vent for hydrate remediation • Pig launcher and/or receiver • Well Workover Systems

The buoy is designed to meet classification society requirements, regarding strength, mooring and stability. It has motion performance to allow communication systems to operate in even the most extreme environment and analysis have been conducted to ensure that the motion performance during operating seastates allow personnel to work without suffering seasickness. Hull Arrangement. The control buoy hull arrangement is driven by required payload, environmental conditions and access requirements. The following describes three different control buoy solutions.

Small Payload Control Buoy Solution. A simple control buoy with small payload is proposed for minimal developments (Figure 1). As the power requirements are small, the buoy can utilise fuel cell technology or solar power.

Figure 1 – B-DEOS Mini Control Buoy

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This hull arrangement (Figure 1) has been proposed for the B-DEOS (British - Dynamics of Earth and Ocean Systems) project to study links between the physical, chemical and biological processes regulating the earth-ocean-atmosphere-biosphere system.

The complete buoy is modularized, internal compartments have no access, and fluid storage facilities are not available. For maintenance or repair it is proposed that the control buoy be recovered to the deck of a suitable vessel and any modularized components replaced.

Large Payload Control Buoy Solution. For larger

payloads in benign or moderate environments a ‘Disc Control Buoy’ is proposed as a suitable hull arrangement (Figure 2). The ‘Disc Control Buoy’ is currently configured to carry a 50 tonne equipment payload and a 100 tonne variable fluid payload.

The complete buoy is a single level consisting of three subdivided compartments. The internal compartments have watertight doors between rooms, and fluid storage facilities are available at the perimeter. Independent access is through each of the three ‘Castles’ to the compartments below, allowing two alternate exits in an emergency.

The ‘Disc Control Buoy’ is principally intended for application in slight to moderate environments where access and egress can be easily managed by a supply vessel. The buoy can also be fitted with a helideck, as shown; however the environment must be relatively mild during normal operation to ensure the buoy motions (roll/pitch of between 3 and 4 degrees) allow helicopter operations.

Conventional naval architecture design methods are employed to minimize motions, this includes bilge keels to increase drag at the edges of the buoy, minimal freeboard to allow water to wash across the deck and a ‘step’ above the external compartments to entrain water, hence increasing damping.

Figure 2 – Disc Control Buoy

Large Payload Control Buoy Solution – Extreme Environment. For moderate to extreme environments, an alternate control buoy system that still offers the advantages and payload of the ‘Disc Control Buoy’ is proposed. The proposed hull arrangement, with a deep draft, results in significantly less motions compared to the ‘Disc Control

Buoy’ thus allowing helicopter access in seastates where boat operations may not be practical.

To achieve a significantly more stable platform, one that is much less susceptible to wave loading, a completely different configuration is proposed. The ‘Caisson Spar’ structure is illustrated in Figure 3.

Equipment payload and fluid capacity (diesel, corrosion inhibitor and chemical injection) is identical to the ‘Disc Control Buoy’. The buoy hull arrangement consists of a dual level topside, each level consisting of a single open compartment and a deep caisson with solid ballast at the keel. Independent access is through a watertight door at each of the levels to the compartment, allowing two alternate exits in an emergency.

Natural periods of the ‘Caisson Spar’ are sufficiently larger than the likely wave periods ensuring little excitation due to wave loading. In addition, the large mass/inertia of the ‘Caisson Spar’ and the small water plane area in the wave zone ensures that there is very little wave response.

Although Spar/Caisson structures themselves are well understood, the innovative approach proposed here is to incorporate the topside structure into the hull of the ‘Caisson Spar’ allowing load-out, wet tow and installation of the complete unit to be carried out readily without a costly installation vessel which is of significant benefit in remote areas.

Figure 3 – Caisson Spar Control Buoy

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Telecommunication Options. In addition to copper cable and fibre optics, a number of different technologies are available for communications from land to an offshore location, and can be divided into four categories: • Line of sight radio links (e.g. Unlicensed 2.4 GHz

Spread Spectrum equipment, VHF/UHF Telemetry links, Microwave 2 GHz link)

• Over the horizon radio links (e.g. ground wave, high power tropospheric scatter link, meteor burst communications)

• Low Earth Orbit (LEO) satellite links (e.g. Iridium and Globalstar networks)

• Geosynchronous satellite links (e.g. VSAT, Inmarsat M and C)

Copper cable, fibre optic and radio links can be expensive to install, but have low operating costs. Techniques using satellites can be expensive to operate, although communication with LEO satellites operated on an occasional dial-up basis may prove cheap to install and operate.

Line of Sight Radio Links. The range that can be achieved with a single line-of-sight link depends upon the height of the buoy antenna and the height of the onshore antenna. A nomogram is shown in Figure 4 to provide guidance on the antenna heights needed for a particular range. For example, a shore station located at 200 m ASL can communicate with a buoy with an antenna at 10 m ASL over a distance of 75 km. This nomogram assumes that the intervening distance is flat. Range can best be increased by raising the height of the buoy antenna or by taking advantage of elevation at the shore location. In certain circumstances, an intervening island may be used as a repeater.

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Figure 4 – Line-of-Sight Range depends upon Heights of Shore Station and the Buoy Antenna

The buoy antenna height needs to take into account

extreme seastates. High waves will reduce the effective

antenna height, and wave-riding buoys such as ‘Disc Control Buoys’ will also sink in the trough of waves with the antenna trying to see over the next wave crest. Rather than designing antenna height for extreme waves, the buoy may instead utilise a non-directional satellite antenna for communications when the radio link is frequently interrupted.

Over the Horizon Radio Links. Radio links that rely upon ground wave and tropospheric scatter offer ranges that are on the order of a few hundred kilometres but they require large amounts of power and have large antennas. The high power tropospheric scatter link between North Rankin A platform and the onshore LNG Plant supports 30 voice circuits and a 1 Mbps data service. The HF spectrum, 3-30 MHz, can sometimes provide communications up to a few thousand kilometres, but propagation varies through the day and can be disrupted by solar activity.

Meteor burst communications provide long-range signalling, by reflecting bursts of compressed data transmitted at high data rates off the ionised trails left by micrometeors as they enter the earth’s atmosphere. The maximum length of a single-hop link is about 1600 kilometres, a distance determined by the height of the meteor trail and the curvature of the earth. A typical meteor trail has an average duration of a few hundred milliseconds, while wait-times between suitably located trails can range from a few seconds to minutes depending on the time of day, time of year and system design factors. The sporadic nature of meteors means that they are only suitable for communications with a low average data rate.

Geostationary Satellites. Geostationary satellites orbit at an altitude of 36,000 km, so that their orbital period is the same as the rotation of the Earth. Geostationary satellites receive microwave transmissions from ground stations, and retransmit the same signals back to the Earth. Thus they are a means of communicating from one point on the Earth to another with little or no regard to distance. Subject to reliability, and attenuation of the microwave signals by intense rain and hail, geostationary satellites provide high-capacity communication links for digital and analogue voice and data signals.

Telecommunications links with geostationary satellites require an earth station with either a traditional large dish (3 - 15 metres), or a smaller VSAT (Very Small Aperture Terminal) dish (0.5 - 2 metres). Both types need careful alignment towards the satellite, and operation from a buoy with significant motions requires detailed consideration.

While it is possible to use compact and non-directional antennae to communicate with geostationary satellites, the height of the satellite requires a high power level. Mobile use of satellite communications are increasingly based on Low Earth Orbit (LEO) satellites.

Low Earth Orbit (LEO) satellites. Because the low height of LEO satellites, simple low power transmitters are adequate, with antennae that do not require pointing. But because the satellites are so close to the ground, their footprint or coverage is small. Globalstar, Iridium and Orbcomm are three variants of LEO satellite (see Table 3 for a comparison), and these three networks transmit messages to the destination in distinctly different ways. Iridium has the capability of passing a message from one satellite to another, until it reaches the final destination. Orbcomm satellites store the message

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received from the originating station until it flies over the intended land station, so there can be a latency of several minutes. Globalstar is limited to stations within reach of the satellite above the transmitting station.

Hardware costs are quite low (1000 - 3000 US$) for all the LEO satellite options, but the operational costs should not be underestimated. Call rates can be as low as 20 US cents per minute, but a permanent connection will rack up 100,000 US$ per annum.

Savings can be made on satellite costs by using an occasional rather than a permanent connection. The frequency of connection must be acceptable to both the operators and the regulator. A "heart-beat" from the offshore system on a frequent basis will assure the onshore control facility that everything is satisfactory.

The operators are competing and introducing new messaging schemes such as the Iridium Short Burst Data (SBD) service. This is designed for applications that send and receive data messages of up to 1960 bytes in size, which could be ideal for many subsea control applications. The buoy control system communicates with the Iridium L-Band Transceiver (LBT) over a serial connection. The LBT transmits and receives data messages across the Iridium satellite network utilising inter-satellite links to reach the Iridium gateway where it communicates with the onshore computer system through e-mail or a secure landline.

All the LEO satellite companies have suffered financial difficulties and filed for "Chapter 11" bankruptcy protection, though Iridium was rescued and resurrected. Even when the operating company goes under, the satellites and infrastructure are still in place, and smart investors can pick them up and continue to operate them. It should be expected that any satellite carrier will undergo some corporate reshaping during the life of an offshore project.

Telecommunication Installations on Buoys. Some of the drivers with regard to telecommunication installations on buoys are: • The buoy may be subject to pitch and rotation, which

will interfere with precise pointing of antennas. Non-directional or low gain antennas are preferred for this situation

• Waves can affect the height of the buoy antenna above sea level, and cause drop-out of communications in adverse weather

• Antennas and equipment should be located outside of any hazardous zones on the buoy

• The equipment offshore should be duplicated to maintain availability and a back-up should be provided through another satellite network for additional redundancy

• For communication with vessels offshore a VHF marine band radio should be provided operating on the International IMO channels

• A GPS receiver with a suitable data output is required to provide buoy location information

A meteorological station on the buoy is required for remote monitoring, so that conditions at the buoy are known before boarding by vessel or helicopter.

Power Generation Options. The larger control buoys to date have utilised marine diesel generators, which although reliable still require scheduled maintenance activities to be carried out. Actual power requirements are small, and economising on power can lead to alternate power solutions, minimising the payload requirements and hence the hull weight and cost. Viable alternatives are given below.

Mini Turbines. Alternatives exist to Marine Diesels requiring less maintenance, and increased reliability. These include, but are not limited to, the proprietary systems of ORMAT (ORMAT Energy Convertors) and Mini Engine. These units are operated offshore (unmanned platforms) and in remote regions and have a proven track record, requiring little or no maintenance and would be well suited to extended operation offshore on the control buoy.

Fuel Cells. A fuel cell converts hydrocarbon fuel (e.g., natural gas or methanol) and oxygen into electricity through electrochemical reactions, not combustion. The complexity of the fuel processor depends on the fuel cell type, but these devices basically convert hydrocarbon fuel into the hydrogen-rich gas that the fuel cell consumes. A fuel cell is similar to a battery, but does not run down. It generates electricity as long as it receives hydrogen fuel and oxygen, with water (H20) as its byproduct.

Figure 5 – Fuel Cell Process Figure courtesy of www.fuelcells.org and USDOE

Renewable Energy. Several concepts regarding renewable

energy have been considered, depending on the power requirements. The most common renewable energy source is solar power, and the Petrobras 4-ALS-39 control buoy successfully used solar panels. Advances in both wind turbines and wave energy systems make these other concepts worth considering early in the development.

Energy Storage. A key part of designing a power system for a buoy is specifying energy storage, in batteries, or in the accumulators of the hydraulic power unit. Stored energy can allow the buoy to continue operating during failure of the primary power source, and can assist with peak-shaving at times of high power demand.

Chemical Storage and Injection. In addition to the control equipment for the subsea trees and manifolds, there is often a requirement to store fluids in the buoy, including:

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• Diesel (for power generation) • MEG (for continuous injection into the pipeline or for

starting/restarting a ‘cold’ well) to prevent hydrate formation

• Methanol for hydrate management or melting hydrate plugs

• Corrosion inhibitor to protect the pipeline from corrosion The ‘Disc’ buoy can readily accommodate large storage

volumes. Although the ‘Spar’ buoy has a high payload capability, variable storage payloads may be limited due to excessive change in the spar draft, depending on the loading condition. For a ‘Spar’ buoy facility requiring large fluid storage capability it may be worthwhile considering a ballast compensating system to control draft variation.

Fluid replenishment is generally achieved by surface vessel via a delivery hose. The recharge cycles are generally of the order of months and may be scheduled to occur with maintenance and inspection visits.

Personnel Access. A control buoy facility will generally be normally unmanned and designed for unattended operation. With the exception of the small payload buoy described above (which can be maintained by boat recovery), a control buoy facility will need to be boarded from time to time for inspection, maintenance, repair and fluid replenishment operations.

The ‘Spar’ buoy has very low vertical motions and can be designed to be accessible by helicopter in severe conditions. Boat access is possible in suitable seastates, and is facilitated by access ladders with small rungs for grasping and outer buffer bars providing a safe zone for personnel transferring from the workboat. The workboat can be driven hard into the buffer bars to stop the boat moving up and down, easing transfer from the back deck to the buoy.

The ‘Disc’ buoy moves with the wave motions and is suitable for boarding only in relatively benign conditions. It is considered that the inclusion of a helideck would be feasible so enabling fast access for very remote locations.

Well Intervention. Apart from the simple control of subsea facilities, additional operations are frequently requested for ‘Control buoys’. Most recently operators have questioned whether a floating system, capable of controlling the subsea systems, could also undertake limited well intervention.

INTEC, in conjunction with Leading Edge Advantage (LEA), Advanced Well Technologies (AWT) and SEATRAC, proposed an alternate development solution involving a floating hull solution (Figure 9). The hull was configured to be inherently stable during the extreme weather conditions experienced in the Bass Strait allowing well workover in severe weather conditions.

The workover system proposed by LEA/AWT/SEATRAC had successfully been utilized by a semi-submersible in the North Sea, including under balanced drilling to improve reservoir performance.

Pig Launcher/Receiver. For the more stable platforms, namely the ‘Caisson Spar’ (Figure 6) and the Well Intervention floating structures, there is also the capability to

accommodate steel catenary risers in deeper water allowing pig launcher and receiving capabilities.

Figure 6 – Multihull Caisson Spar Umbilical vs. Control Buoy – Key Selection Drivers Cost. Over short distances, an umbilical system is undoubtedly cheaper, though even very short umbilicals can have a significant cost due to design and the mobilisation of installation vessels. As stepout increases, the umbilical cost rises steadily. A step change occurs when the length of umbilical becomes too large to go onto a single reel, and joints must be incorporated, with consequences for cost, installation and testing time, and system integrity. Alternatively, the decision may be taken to load the umbilical onto a carousel vessel, but the mobilisation cost to Australasia may be prohibitive.

For ultra-long umbilicals, the decision may be taken to simplify the umbilical. Other means may be used to assure continued operation in the event of failures, e.g. Snøhvit has backup power line signaling should the fibre optics fail, and a Back-up Intervention Control System which can be deployed on a vessel over the field through a dynamic umbilical in the event of complete umbilical failure. CAPEX could be significantly reduced by simplifying the umbilical through the elimination of the dual redundant cores.

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shown in Figure 7. The cost for satellite communication increases slightly with the stepout, due to the cost of access for replenishment and maintenance. Radio communication becomes infeasible beyond around 70 km due to the necessary height of masts, unless there is elevated ground at the shore location or an intermediate site for a repeater. The costs indicated in Figure 7 are for ‘Disc’ buoys, and the cost for ‘Spar’ buoys will be higher. The cost of the Petrobras 4-ALS-39 control buoy is quoted as only 1 million US$ (Ref 6).

There is always a degree of uncertainty in the cost of umbilicals as the marketplace is driven by supply and demand, and the number of suppliers of large umbilicals is limited. On the other hand, the material and equipment for buoys is readily available from a number of sources, and the fabrication can take place at many sites.

Schedule. The availability of umbilicals can sometimes be prolonged (in excess of 12 months) due to market conditions and the order books of the umbilical manufacturers. In slack periods, delivery may be as short as 4 months. For long umbilicals in the Australasian region, the availability of a carousel vessel may also have to be considered.

The schedule for control buoys is more predictable, and inclusive of up-front design work, is around 12 months.

Risk. There is a perceived risk in any new technology such as control buoys which fails to take account of the fact that buoys are built from proven off-the-shelf sub-systems. On East Spar, for example, it is documented that there were four innovations used subsea, but the buoy itself only used existing technologies combined in a novel manner. The lessons learnt from previous buoy projects are available and indicate primarily that extensive qualification, type testing and integration testing will reduce offshore commissioning and infant mortalities (Ref 6 & 9).

Risks incurred in the use of umbilicals include: • Danger of bad weather occurring during the lay, which

may take several days for a long umbilical, or even longer if a shore crossing or a riser pull-in precedes the

lay. In the Bass Strait or on the Northwest Shelf, weather windows may be too small for comfort.

• Difficulties in trenching or the expense of rock-dumping for stabilisation and protection in shallow water or adjacent to platforms

• Risk of the umbilical being damaged by dropped objects, dragged anchors or trawling

Umbilicals will inevitably be sourced from overseas, so there may a risk on currency fluctuations, whereas control buoys can be entirely fabricated in Australia using local content materials and labour, an increasingly important consideration.

The main risk with buoys is an over-run during construction, leaving insufficient time for comprehensive onshore testing.

Support. It needs to be recognized that buoys require a level of support which exceeds that of subsea umbilicals. Much of the control system equipment required for buoy or umbilical solutions is identical, whether it is located on a buoy above the field, or situated on land or a platform and connected to the field through a subsea umbilical. The technicians and equipment needed are similar for both scenarios, but supporting the facilities on the buoy is more difficult due to the expense of visits by workboat or helicopter, the short duration of visits (constrained to daylight hours), and the seastate occasionally prohibiting boarding.

Availability. There should be no difference in the availability of buoys vs. umbilical systems. Both should be engineered to provide an adequate level of availability for the field development. One key difference is that the repair time for topsides control system equipment will be longer for the buoy option due to the mobilisation time, but this can be compensated for by increasing the level of redundancy to attain an appropriate Mean Time To Failure.

Case Examples Two case examples are presented below to illustrate some of the considerations for developments with medium and extreme step-outs.

70 km Tieback, 130m Water Depth. This case focuses on a medium step-out development, located in shallow water offshore Australia. The case considers: • Two subsea wells with provision to expand to four • 70 km to the onshore facility • Significant wave heights of 8.8 m survival, 6.9 m

operation and communications, and 2.0 m (access and maintenance)

• Chemical injection requirements 250 litres/day (peak) Buoy Selection and Design Features. A disk buoy

concept is proposed as an alternative to the East Spar type control buoy as this concept promises significant cost advantages over the TLP type control facility with savings in both the mooring installation and buoy fabrication.

The buoy is 17 m diameter with an inner shell 14 m diameter (Figure 8). The annular section between the inner and outer shells is divided into small compartments which

SPE 88546 9

provides robustness, protection in case of collision from a vessel, and also provides storage for approximately 40 m3 of hydrate and corrosion inhibitors.

Figure 8 – Disc Control Buoy

Three towers or castles enable access to the buoy and

provide location for the communication masts and buoy ventilation outlets. The castles remain clear of waves while allowing green water to wash over the buoy in extreme conditions.

A catenary mooring system is selected which comprises 3 groups of 2 chain legs (550 m long x 38 mm dia) with 6-8 tonne Stevpris drag anchors.

Telecommunication system. Line of sight communication is suitable for this case, due to the relatively short (65 km) transmission distance, assuming availability of an onshore antenna site at high elevation (300 m). A telecommunications study indicates that an availability of over 99.99% could be achieved for a data rate of 19.2 kbps, with a system consisting of the following: • A full duplex synchronous data link • Dual redundant 450 MHz UHF units • Low gain (3 dB) buoy antenna • A high level repeater station with a 14 dB gain antenna

Buoy pitching and roll motion has to be accommodated by the beamwidth of the antenna. A relatively low gain antenna was selected as it has a beamwidth of ±20°. This will permit uninterrupted operation through the 5 year return period storm which has a maximum pitch and roll of ±19°. It is possible that there could be an interruption to communication in the 50 year return period storm which results in a pitch and roll of ±21° but there is sufficient gain margin to make it likely that communication will still continue even in these extreme conditions.

The communication limitation in the survival storm is not due to buoy pitch and roll but is driven by the height of the waves. When the buoy is in a trough between waves the antenna on its elevated mast may be below the level of the wave crests around it which will interrupt the line-of-site path to the onshore repeater station. To allow operation through the 50 year storm a primary antenna height of 16 metres was

chosen. This was made up of a 10 metre tower on top of a ‘castle’ at 6 metres above sea level.

Buoy Systems and Utilities. The subsea structures are connected by electrohydraulic umbilicals, which allow electrical control and monitoring of equipment and valves, and the delivery of fluids including hydraulic control fluid, corrosion inhibitor and hydrate inhibitor. The buoy is connected to the subsea facilities by a dynamic electrohydraulic umbilical.

Equipment can all be housed within the watertight compartments of the buoy. Access for maintenance and inspection is through entries in the castles.

Electrical power is generated on board using four diesel driven generator sets. One of the four generators is run continuously while the other three generators are provided as backup to achieve high reliability. Each backup generator set is started and run periodically to ensure maximum availability should the duty generator fail.

The DC system is charged directly from the generator sets, has 16 individual charging units which maintain trickle charge to 24 volt DC battery banks. The batteries would total 10 tonnes in weight and in the event of AC failure will provide power for 5 days.

The buoy monitoring and control includes: • Fire and smoke automatic isolation and purge • Water ingress alarms for compartments • GPS monitoring and excursion alarm • Buoy accelerometer, inclinometer signals and alarms • Metocean readings; temperature, wind and wave data • Battery charge levels and electrical supply integrity

Buoy compartments will be adequately vented to maintain temperature and humidity conditions suitable for the equipment and for personnel visits.

Operations. Motion analyses were undertaken to assess buoy access and maintenance operations. A 2.0 m Hs sea was considered as the upper limit for personnel transfer to and from the buoy and therefore adopted for working within the buoy. The buoy motions for the 2.0 m seastate were determined and compared with limits of motion under which personnel can undertake various levels of work. It was concluded that an average person would be able of undertaking ‘heavy manual work’ in the buoy and the probability of seasickness would be around 20% after 1 hour on board and 30% after 2 hours. It is preferable that people servicing the buoy would be experienced marine technicians with less susceptibility to seasickness than average.

Construction and Installation. There are a number of yards in the Australian region that have the capability to construct, test and commission a buoy. Budget estimates were obtained from one of these yards. Transportation to the field was assumed to be by a wet tow.

In order to minimize the offshore spread mobilization costs a single anchor handling vessel (AHV) was intended to install and pre-set the anchor legs despite inadequate storage capacity for all six anchor leg components. The AHV would pick up a pair of anchors and mooring legs from a staging site, proceed to the field and install each anchor before returning in turn to collect and install the second and third anchor pairs. The AHV will apply the design preload and survey will be used to

10 SPE 88546

determine the final position of the end of the chain and hence identify which link is to be set in the buoy chain stopper.

Following installation of the chain legs and confirmation of burial of the anchors the AHV will return to the staging site and pick up the buoy. Pennants will be pre-slung through the hawse pipes. On arrival to the site the AHV would recover each of the anchor legs in turn and connect to the pennants. The pennants would then be pulled through the chain stoppers and the appropriate chain link set into the stopper.

Estimated Buoy CAPEX and OPEX Costs. The estimated CAPEX cost for the ‘Disc’ buoy including design, fabrication, systems and integration testing, commissioning, transportation and installation was 12 million US$. Not included in this sum were the dynamic control umbilical and the subsea control equipment on the seabed.

OPEX costs were estimated at 340,000 US$ per annum which was estimated on the basis of: • No additional operators required at the onshore plant to

operate the buoy • Two planned and three unplanned service visits to the

buoy carried out per annum involving mobilizing a local supply vessel and including two technicians for two days per visit

• Underwater surveys that are required every 2.5 years would be carried out in conjunction with the scheduled maintenance visits

Umbilical Design and Cost. A 70 km steel tube Mux-EH umbilical with additional tubes for chemical injection could cost in the region of 11 million US$. There would be additional costs for transportation and installation. This length of umbilical is on the upper boundary of what can be stored on a Coflexip reel, so the project could be facing the inconvenience of a mid-line joint, or the mobilisation of a carousel vessel at around 9 million US$. Further costs could be incurred in the shore approach, where rock dumping could be necessary, at a cost of possibly 7 million US$. Other problems for the umbilical may include coastal dunes, cliffs or reefs, where installation by horizontal directional drilling and pull-in may be necessary near the shore.

250 km Tieback, 1000m Water Depth. This case considers an extreme step-out in deepwater, offshore Australia. Features are: • Six subsea gas wells • 250 km to the onshore facility • Significant wave height of 13.0 m survival and 5.5 m

1 year return period non-cyclonic storm • MEG injection requirements 1.6 tonnes/hour

MEG Supply Options. Several options can be considered for the delivery of MEG to the wells including:

a) Storage and injection from the floating control facility, with periodic chemical replenishment. Buoy storage required is 1150 tonnes assuming a monthly replenishment cycle or 3500 tonnes for a 3 monthly cycle.

b) Bulk supply from shore in dedicated pipeline/s with compression and injection facilities located on the floating control facility.

c) Bulk supply from shore in dedicated pipeline/s at the required pressure for direct injection into the subsea facilities.

For a spar type buoy the large variable MEG payload would require a ballasting system to avoid substantial changes in draft. After comparing CAPEX and OPEX costs for the delivery options and considering the HSE issues associated with transferring large quantities of hazardous chemicals to the buoy by boat, the best option selected is to use a separate 4" dedicated MEG service line run from shore. This MEG line could be piggybacked onto the pipeline.

A nominal quantity of 50 m3 of methanol is stored on the buoy which may be used for hydrate remediation in the event of a hydrate blockage.

Buoy Selection and Design Features. A spar buoy concept was proposed for this case. It consists of an 11 m diameter cylindrical column reducing to 6.6 m diameter section through the splash zone. The overall draft is 60 m. The spar concept is selected for its stable motions enabling helicopter access in severe conditions. A key feature of the design is that the topsides is fully enclosed and watertight. This means that it can be fully integrated with the hull, towed horizontally to site and upended without the requirement for a crane vessel to install a separate topside structure.

Permanent iron ore ballast of about 400 tonnes is provided at the keel. This is preinstalled in the fabrication yard. Permanent water ballast is provided by flooding the lower portion of the hull during offshore upending.

The mooring is a semi-taut system consisting of six mooring legs at 60° angle. Each mooring consists of 10 m of chain at the fairleads, 3700 m of 70 mm structural strand and 40 m of chain connecting the strand to the anchor. Drag, suction pile and plate anchor options were considered and either may be best suited depending on soil conditions and installation preferences.

Buoy Systems. The systems on the buoy for control of the subsea facilities include electrical power generation, Hydraulic Power Unit, Master Control Station, and interface to the telecoms system. The transition to the seabed is made with a dynamic umbilical, with distribution to the subsea wellheads by static umbilical and Umbilical Termination Assemblies.

Telecommunication System. Line of sight communications are generally not possible for fields this far from shore, as the onshore antenna would need to be at a height of 3000 m. Over the horizon radio systems are not recommended because of the high power requirement.

Satellite systems are appropriate, and the small bandwidth needed for subsea control systems allows use of LEO satellites with a small power demand rather than geostationary satellites which would consume more power, and need a stabilized platform or steered dish.

The non-directional antenna used with LEO satellites allows freedom in the design of the buoy, as buoy motions are not a constraint for the telecoms system.

Significant savings can be made in satellite costs if connection is not continuous, and is made on a dial-up basis on demand or every 15 minutes. The newly-introduced Short Burst Data services should also be investigated. The quantity of data being transmitted back from the buoy is within the 1960 bytes limit for SBD, even without data compression. The

SPE 88546 11

data requirements for buoy to onshore transmission is only 728 bytes for a 6 well configuration (Table 1). "Heart-beat" transmissions can be made from the buoy every 30 seconds, including an update for any parameter which has changed significantly, using the SBD minimum packet size of 30 bytes. The back-haul section may be made by email over the internet, but an alternative secure means of connection should be available in the event of internet crashes, such as a land-line or a satellite terminal at the onshore control room.

The ground-based equipment to connect to LEO satellites is inexpensive, and can be duplicated for redundancy. In addition, equipment for a second satellite constellation should be provided as a hot standby in case there are problems with the primary network. The Iridium network has coverage in all parts of the globe, and Globalstar has coverage in Australia out to the 200 nautical miles (370 km) territorial waters limit so is suitable for all but the most remote fields.

Operations. The spar buoy will be not normally manned and designed for unattended operation. The required period between interventions is expected to be approximately six months. Personnel access will normally be via helicopter, though boarding from a surface support vessel is also needed.

Motion analysis was undertaken to assess acceptability for helicopter access and for onboard working conditions (susceptibility to seasickness). The adopted maximum operating condition was the 1 year return non-cyclonic storm. The rms heave at the topside was determined to be less than 0.15 m/s2 which is better than the maximum expected for a cruise liner. Probability of seasickness is around 3% after 2 hours on board and 4% after 3 hours.

The controlling motion for helicopter operations is the maximum pitch or roll. Motion analysis for the spar buoy predicts a significant pitch of 4° as a result of steady state and dynamic motions. It is noted that the APPEA guideline (Ref 10) provides a range of pitch limitations for helicopter landing on different vessel types. Typically for semi-subs and large ships the allowable pitch/roll is 4-5°.

Construction and Installation. Hull construction is assumed to be in the Singapore region where there are ample yards with fabrication capability and loadout facilities. The spar buoy with integrated topsides is designed to be transported horizontally to the field by either wet tow or dry tow on a semisubmersible barge. If dry towed the structure may be floated off in a sheltered region and then wet towed to the field. The spar buoy is self upended by flooding the buoy compartments. No lift vessel is required to install the spar buoy.

The installation spread will be dictated by the requirements to install the anchors and moorings.

Estimated Buoy CAPEX Cost. The estimated CAPEX cost for the spar buoy including design, model testing, fabrication, mooring system, systems and integration testing, commissioning, transportation and installation is approximately 20 million US$.

Umbilical Design and Cost. Interestingly, an umbilical system is technically feasible over a distance of 250 km. Pressurising up the umbilical system could take two hours or more, and an extended period would be needed between valve operations to allow the hydraulic system to recharge, but nevertheless, the tree valves could still be operated at this

distance. Electrical power transmission at high voltage can minimize power loss in the electrical cables. Fibre optic signals can be transmitted without repeaters to distances in excess of 300 km using Raman optical amplification and remote erbium-doped fibre amplification (Ref 11).

However, the cost and size of this umbilical would be prohibitive. A conventional umbilical of this length would cost around 40 million US$, but this alone would not be suitable as it would need much larger hydraulic cores and electrical cables. There would be additional costs for transportation and installation, needing a large carousel vessel. The duration of lay would need a large weather window, entailing a high level of risk, and this could be further extended by pull-in at the shore or at a platform. Stabilization and protection from fishing activities or dragged anchors might be needed near shore. Conclusions Some of the driving factors for selecting umbilicals vs. buoys have been presented. For ultra-long stepouts, the wisdom of using buoys is self-evident, but for intermediate or short distances, each case should be assessed on its own merits.

New developments in satellite communications and in the hull design of buoys are providing more competitive and cost-effective alternatives to umbilicals.

Acknowledgements The authors thank Leading Edge Advantage (LEA),

Advanced Well Technologies (AWT) and SEATRAC for their contribution and advice on well intervention technology.

Credit is given for figure 5 to Fuel Cells 2000 (www.fuelcells.org) and the US Department of Energy, who funded its preparation.

References

1. Collins, J.: "Optical Tieback Umbilicals Solution to Scale Problems in Adding Fields", Offshore, September 2000.

2. Theobald, J., Painter, H. and Barlow, S.: "The Development of a Cable Termination System for Deep Water Applications", OTC 15364, 2003.

3. Thalund, K.M., Brodersen, F.P. and Rolgaard-Petersen, B.: "Regnar – Development of a Marginal Field", SPE 28859, 1994.

4. Boyle, E.C.: "East Spar Development - NCC Buoy, the Vertical Submarine", SPE 38524, 1997.

5. Boyle, E. C., Coral, J. B.: "East Spar NCC Buoy - Advancing Remote Technology" OTC 8638, 1998.

6. Pinho, O. J. de, Euphemio, M. L. L. and Correia, O.B.: "Autonomous Buoy for Offshore Well Control and Monitoring", OTC 8793, 1998.

7. Offshore Technology – Mossgas Gas Field Project http://www.offshore-technology.com/projects/mossel/index.html#specs, 2004.

8 "2002 Australian Computer Crime and Security Survey", AusCERT, Deloitte Touche Tohmatsu and NSW Police, www.auscert.org.au/download.html?f=11, 2004.

9. Campbell, P.F., Miller, G.L. and Speechley, A.E.: "East Spar Development: First Offshore Gas Field in Australia Developed by Alliance Approach", SPE 38461, 1998.

12 SPE 88546

10. "Guidelines for Management of Offshore Helicopter Operations", Australian Petroleum Production & Exploration Association Ltd (APPEA), February 2000.

11. Alcatel - Unrepeated Systems http://www.alcatel.com/submarine/products/ur/, 2004.

Parameter Number of bytes

Time Stamp: Date and Time of data collection 10 Subsea Wells (6 off): Downhole pressure and temperature 48 Wellhead pressure and temperature 24 Annulus pressure 12 Choke valve position 24 Mass flow (instantaneous, hourly and daily total) 15 Valve status indication 48 Control signals to all valves. 48 SCM data including hydraulic supply and return pressures 36 SCM house keeping data 48 Supply voltage at SCM 12 EPU: Power Circuit status 10 Alarms generated 25 HPU: LP and HP Supply Pressure 20 Reservoir Levels 10 Pump Status (Operating/Available/Isolated) 20 Master Control Station: Alarms generated 20 Alarm settings 100 Wells on Workover 10 ESD shutdown inputs 10 Production Facilities Topsides Equipment Fire/smoke detection 8 Water ingress alarms for compartments (compartment sump overflow) 4 GPS monitoring 20 Accelerometer 8 Buoy inclinometer analogue signal and alarm 8 All fluid reservoir status indication and high/low level alarms 10 Battery charger levels 6 Electrical supply integrity 4 Metocean readings (temperature, wind speed and directions, waves) 10 RAYCON (Radar intensification) status 6 Chemical injection flows, pressures, daily consumption 20 Alarms generated subsea, topsides, and chemical injection 10 ESD System status 10 Power Generation demand and unit status 20 Intruder detectors and alarms 10 Overview / Safety System Power generation overview (status/loads of all generators) 10 Fire pump status 10 Checksum 16 bit Cyclical Redundancy Check 4 TOTAL number of bytes to be transmitted 728

Table 1 – Data Requirements for Buoy to Onshore Transmission, for a 6 well configuration

SPE 88546 13

Regnar East Spar 4-ALS-39 Mossel Bay EM Field Location Danish Sector Southern

North Sea Australia – North West Shelf

North East Brazil Offshore Alagoas

South Africa – Bredasdorp Basin

Operator Mærsk Oil and Gas Apache Petrobras Moss Gas Design Production Capacity

20,000 BOPD 100 MMscf/d Unknown 195 MMscf/d

Production Start-up September 1993 December 1996 September 1996 April 2000 Still in Operation Yes Yes Unknown Yes Water Depth 42 m 95 m 25 m 95 m Wave Height (100 yr) 25.7 m 21.9 m Unknown Host Facility Dan F Platform Varanus Island Onshore Onshore, 80 km backhaul

through modem via dial-up cellular connection

FA Platform

Distance to Host 13 km 63 km 22 km + 80 km 49 km Pipeline Diameter 6 in 14 in Unknown 18 in No of Wells Design / Actual

1 4 / 2 1 10 / 5

Subsea Control Direct Hydraulic Multiplexed ElectroHydraulic

Direct Hydraulic Direct Hydraulic

Chemical Injection Capability

None Continuous corrosion inhibitor and intermittent hydrate inhibitor

None Continues corrosion inhibitor and intermittent hydrate inhibitor

Subsea Monitoring None Temperature, pressure, valve and choke positions, multiphase flow metering and pipeline corrosion monitoring

Temperature, pressure, valve positions

Buoy Type Converted CALM Loading Buoy

Tension leg submerged buoy with slim column continuing to above water

Disk buoy Tension leg submerged buoy with slim column continuing to above water

Buoy Size Diameter 11 m Height 5.3 m

Diameter 7.5 m Height of main body 17 m

Diameter 3.5 m Diameter 8.0 m

Mooring System 3 catenary legs with pile anchors

2 x 4, 56 mm spiral strand cables connected to steel gravity base

Unknown 2 x 3, 50 mm spiral strand cables connected to concrete gravity base

Method of Boarding By boat By boat By Boat By Boat Communication with Host 2 x 19.2 kbps UHF radio

link 2 x 9.6 kbps UHF primary link with a 2.4 kbps satellite backup. Separate UHF and VHF link for voice communication

1.2 kbps UHF link UHF/VHF with satellite backup

Power Generation 2 x 7 kW 100% redundant diesel generators with battery backup

4 x 6.8 kW triple redundant diesel generators with battery backup

Solar Panels with battery backup

Diesel generators with battery backup

Schedule from Conceptual Design to Installation

Unknown 14 month 6 month

Operator quoted key decision points for selecting control buoy concept

Cost, and availability of CALM Loading Buoy

Cost and schedule saving Cost and schedule saving plus high risk with umbilical crossing coral reefs

Reusability

Table 2 – Existing Control Buoy Details

Globalstar Iridium Orbcomm Services Mobile Voice and Low Rate Data

(9.6 Kbps) Mobile Voice and Low Rate Data (2.4 Kbps), also Short Burst Data

Store-and-Forward Messaging (up to 300 bps)

Constellation Number of Satellites 48 66 36 Altitude (km) 1400 780 770 Satellite Transponder Bent Pipe Processing Bent Pipe Inter-satellite links No Yes; 4 crosslinks @ 25 Mbps;

22.55 to 23.55 GHz No

Table 3 – LEO Satellite Constellations

14 SPE 88546

Figure 9 – Concept for Well Intervention from ‘Control Buoy’