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12 Baseline » Issue 3

Technology

Acoustic Subsea Control Systems

A Wideband BOPSystem;Dunkingtransceiver with cabledrum, portable controlunit, Subsea ElectronicsModule and either side,two DARTs

Cuttingthetiesof

Baseline » Issue 3 13

SONARDYNE BEGAN

DELIVERING acousticBOP (Blow-Out Preventor)control systems more than10 years ago. Thisemergency back-upsystem allows a subseawell to be shut in and

the riser disconnected if control via the riserumbilical is lost. With systems installed inBrazil, West Africa and the Gulf of Mexico,an excellent track record for reliable operationhas been established.

Building on this reputation, Sonardynehas now incorporated its Wideband acoustictechnology into the latest generation of acousticBOP controllers with the first batch of newsystems being delivered to Noble Drilling for

installation on the new-build rigs ‘DavidBeard’, ‘Danny Adkins’ and ‘Jim Day’.

A typical installation consists of a surfacecontrol system, which may be portable orpermanently installed on the vessel, andsubsea components fitted to the BOP stack.

The permanently installed surface systemcan operate either via a dedicated hull-mounted transceiver or a Sonardyne WidebandUSBL positioning system, if one is fitted.The portable surface system is a self-contained,battery powered control unit that is used witha lightweight acoustic transceiver and a strain-relieved deployment cable. It can be operatedfrom a small workboat or even from a life raft.Acoustic commands and their replies useForward Error Correction techniques combinedwith Sonardyne’s proprietary Wideband >>

Well known for providing pinpoint accuracy in positioningsystems, Sonardyne’s Wideband® acoustic technology isnow being used as the foundation for a range of highintegrity wireless control systems utilised in critical subseaapplications.The same signalling and receiver processingtechniques that offer outstanding positioning performanceby allowing precise time-of-arrival measurement, alsoenable highly reliable acoustic transfer of data throughwater and Sonardyne has incorporated these techniquesinto its latest generation of wireless wellhead controlsystems. Baseline speaks to Ted Kenny, Business Manager,Subsea Control Systems.

umbilical control

14 Baseline » Issue 3

Technology

Acoustic Subsea Control Systems

acoustic telemetry protocol to ensure thehighest integrity through-water data transfer.

Subsea hardwareThe subsea components mounted on theBOP stack consist of a Subsea ElectronicsModule (SEM) and two acoustic transceivers,referred to as DARTs (Deep-rated AcousticRemote Transceivers). The DARTs are locatedon opposite sides of the riser to ensure that thestructure cannot mask the acoustic signals fromthe surface system.

The SEM contains dual redundantelectronics and battery packs for drivingthe pilot valve solenoids within the BOP andreading back pressure switches to indicatecorrect actuation of the valves. The SEM candrive up to 12 pilot valve solenoids and read12 status switches and four 4-20 mApressure sensors.

The dual redundant SEM can operate withup to four DART transceivers to provide multipleacoustic communication paths and ensurereliable communication even under the most

demanding conditions. This level of acousticand functional redundancy, combined with theinherent robustness of the Wideband signalsachieves an acoustic link with reliability that iscomparable to that of a cabled link and maybe considered as a direct alternative to anumbilical cable.

This means that the use of acoustics is nolonger restricted to that of a back-up systemand it is equally suitable for use in applicationswhere it provides a primary control function,for example, as seabed shut-off device, usedin conjunction with a Surface BOP system(see opposite). BL

The inherent robustnessof the Wideband signalsachieves an acoustic linkwith reliability that iscomparable to that of acabled link and may beconsidered as a directalternative to an umbilicalcable.”

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Sonardyne began delivering acoustic BOP control systems over 10 years ago.This emergency back-upsystem allows a subsea well to be shut in and the riser disconnected if control via the riser umbilical is lost.

Baseline » Issue 3 15

The latest delivery of Sonardyne’sWideband Subsea Acoustic ControlSystem is to the Helix ‘Q4000’ vessel,operated by Well Ops (A Helix company).

This will be the first such system to gainregulatory approval for use in the Gulf ofMexico and marks a milestone in theacceptance of wireless acoustic control inmission-critical applications.The acousticsystem includes extensive self-diagnosticsand a regular, automated ‘health check’to ensure constant availability.

The Q4000 is a multi-purpose semi-submersible vessel currently undergoinga drilling upgrade to include theinstallation of a Surface BOP drilling system.

Surface BOP drilling differs fromconventional subsea drilling in that theBOP is located at the top of the riser(typically in the moonpool) rather than atthe seabed.This requires the use of a highpressure riser, of smaller size than theconventional 21 inch marine riser.

The use of a smaller riser means lessriser and mud weight, so reduced risertension and buoyancy requirement.Thissignificantly extends the operating depthcapability of the rig, compared toconventional drilling, and is increasingly

Helix Q4000 Acoustic Control System

used to allow the depth rating of 3rd or4th generation rigs to be increased, sothat they can be used in place of moreexpensive 5th generation rigs in locationswith suitable metocean conditions.

With the BOP at the surface, it isnecessary for environmental protection,to have some form of shut-off device atthe seabed to allow the well bore to beisolated and the riser to be disconnectedunder emergency conditions.

This seabed disconnect device typicallycomprises two shear/blind rams and twolatches, to unlatch the riser or the completepackage. Pre-charged accumulatorsprovide hydraulic power and control isvia a single lightweight umbilical andan acoustic control system.

While connected, the cable providesthe primary control, with the acoustics asback-up, but if a disconnection is carriedout, the acoustics must provide primarycontrol and monitoring, until re-connectiontakes place.

This places greater demands on theintegrity of the acoustic control systemthan would be the case for a conventionalback-up system.