INTRODUCTION MOORING DESIGN

Transcription

1 DIVING IN SUPPORT OF BUOY ENGINEERING: THE RTEAM PROJECT Sean M. Kery Woods Hole Oceanographic Institution Woods Hole, MASSACHUSETIS U.S.A. Diving has been used as a valuable tool in the development ofseveral key buoy systems. One application where diving was utilized is described in detail. A RealTime EnvironmentalArcticMonitoring (RTEAM) system has been developed by the Woods Hole Oceanographic Institution. This mooring uses a variable buoyancy ascent module, connected to an instrumented cable to transmit data to a satellite. The module fills an internal ballasttankand rises to the surface once a day, transmits, andthen vents the ballast tank and descends to a rest position. Diving was used in the development phase to assess andfine tune the sea-keeping, ballasting and valve timing of the ascent module. The buoyancy and hydrodynamicsofthe module change continuously during the ascent and descent. Diving and underwater photography were used to identify and correct problems that developed during sea trials. INTRODUCTION The KillAM mooring (Fig. 1) (Bocconcelli, 1987) is specifically designed to daily telemeter oceanographic data from locations that may be covered by ice. Data from current meters, hydrophones and the elevator module are carried through the electro-mechanical cable by an FSK-SAIL loop and stored in an Integral Bus Controller (IBC). When the module is not transmitting, it descends to a rest position to avoid damage at the ice interface. The module rises to the surface, or ice interface, once a day, transmits the data and then sinks back to its rest position. If the surface is ice free, the data is transmitted directly through the ARGOS system back to a shore station. When ice covers the mooring site, telemetry is accomplished through a radio frequency (RF) floating antenna which is attached to the module. The data is received by a nearby shore based RF receiver (150 nautical miles range) and relayed from there to the ARGOS satellite. MOORING DESIGN The ascent module contains a variable buoyancy tank, valves and driver assembly, the IBC and telemetry packages housed in a fiberglass frame (Fig.2.) The ascent module is tethered to the subsurface mooring by an umbilical, carrying electrical power, data and the supply of high pressure purge gas. The main batteries and gas storage tanks are mounted on top of the large syntactic foam sphere that is also the principal buoyancy element for the instrumented mooring cable. Data is collected, internally recorded and telemetered from a series of current meters and hydrophones. The balance of the mooring components consist of three conductor 191

2 ~CiO._. Diving for Science electromechanical cable, backup recovery, an acoustic release and mechanical length members (kevlar rope and chain). Figure 1. Site 'D' prototype mooring. \/,-, - >- d Y 0- ',J ~ ~~f.v:!.2ll S1"... T.f'~"'M,"LC"'T!:.(':"O~) c-flf\ 40 ~--- -~~ _"---:--:'" "'h... ~."..<".V~ ". /'.,; ' ;--1l«... 1" ~S.I1"O"" '/ : _.. \- ~000"'~ a r..' c.._ "',... "..." 75_. l,)... a_ c r'\.c c: (_,-"lc"") ""''''',,2.~' 200 \1...(" los If...'" # 2,'7 "Y1)ROPWONE 1 C,, STU... C:'" ""'&'fo.{c,,7'''o) 1I IfM(H,.31 r-==== 2."'. 3/;" C... A,I... p- c"". "", or. I 1 7M J/S"O-tAI"l _ L.!t' SVNTAcnc COA.M ~PW RE "----- \ ft )11" ( /(,,-..., ",,0'.." 3...'....;1' rt. fir..-...,.a( t.o,(: _/J~G.T 1 L95H. 5/1'" 1(10"',,-",," llop.a. " {'"',_C.'''fo _...Cw"' '... ~ ('<.) " "~,,.. "o.l.~ l:ll~---' M.OVSTIC. P. L :ASE: i---- ~'"'. If~' C... N IlJ COU.s1'IL R.(.Lli:... S.t. 1 5,.. t:t." '"AI''' t1 SII(;'K'-VLAR.ROP (AD.JvST~OLE: 5.J -._----- SO M. 5/1'- Kk:I/L.A~ ROPE "'DJ""'ST...~i s.) M. I- NYLON ROPE: loo'o'lii:1.. """'_'u: _co < "-"'-) _. ONe..f)ooO,... ~o T".. Prior to building and testing a prototype, a computer program "DIPPER", (Kery, 1986) was developedto model the forces on and the geometry ofthe umbilical andvariable buoyancy module. A study of the umbilical geometry was made involving variations of current speed, module buoyancy, umbilical length and depth of point of attachment. Results from this analysis were then used to design the ascent module. DEVELOPMENT PHASE ONE A prototype ascent module was built and a series of tests off of the WHOI dock were undertaken (Fig 3.). To initiate the ascent process, a valve at the end of the high pressure gas is opened. The gas forces the water out of the variable ballast tank through 192

3 Kery: Diving in support of buoy engineering holes located at the bottom of the tank and the module begins to rise. These holes also allow any excess gas to escape as the module ascends and the gas expands. Figure 2. Variable ballast elevator. VENT VALVE. ~\G\\. ~R 'Eobu R E. \-\OSE \fligh PR 5SURE \ VALVE. \ ARIABLE BUOyANCY TANI' ARGOS ANTENNA ( :. l. \ I '/.~... I c. \LI~.:.--_ J...!.._._----t1 I-I-----i-----_-_-----=-= ,II I TERMINAT\D"-l BOOT t"lec.rrlc.j>.l DJ>.TA CONTROLLE.R I ['L~CTRON ICS PRESSURE HouSING BALLAST-' 1 MF ANT NNA Divers were used to study the behavior of the ascent module in the rest position. The module hangs on the umbilical with the bow higher than stem. In one of the earliest tests, the rest angle allowed some of the purge gas to escape out of the bottom vent holes. This was corrected by moving some of the ballast weights forward. Angle measurements were made underwater using a plumb bob tied to the center of a protractor. The quantity of gas left in the variable buoyancy module after the purge valve closed was also measured (Fig.4). This measurement was used to fine tune the valve timing. The module ascended from its rest depth of 60 ft to the surface in several seconds. The descent was slow enough so that divers could keep up. A shallow watertest mooring was deployed nearby in Martha's VineyardSound to validate the computer predictions and assess system reliability. Purge gas was supplied by (4) 80 ft 3 aluminum scuba tanks manifolded together, mounted on the anchor.(fig.5). The mooring was deployed in a depression 90 ft deep off of a point called Jobs Neck. A second mooring with two S4 current meters was deployed close by to monitor current velocities. A VHF radio transmitter with a surface detecting switch transmitted whenever the ascent mcxlule was on the surface. A pressure transducer recorded the depth of the ascent mcxlule. Data was recorded over a period of several weeks with the mcxlule cycling once an hour. The current meter and depth data were used to modify the buoyancy and drag coefficient estimates for the Dipper computer program and consequently to implement the next prototype design. 193

4 Diving for Science Figure 3. Prototype module being lowered Into the water for sea tests. Figure 4. Divers hand pointing to the ballast tank air level as the module starts to submerge. A full scale test mooring was designed and deployed at site D (39 N,700W) on June 2nd,1987 in 2700 m of water. Liquid C02 was used instead of air for the purge gas because of its greater volumetric storage efficiency. After 40 days of operation, a faulty 0 ring allowed high pressure C02 to leak into the valve body and rupture the diaphragm. Sea- 194

5 Kery: Diving in support of buoy engineering water then leaked in to the valve body and shorted the main power supply. The mooring was recovered after 62 days on station. Figure 5. Shallow water together for gas supply. test anchor showing scuba tanks manifolded DEVELOPMENT PHASE TWO The phase one tests proved the feasibility of the system. Various elements of the system were modified in preparation for the full scale arctic deployment including the following: o New ascent module design: A new, more ruggedly streamlined ascent module was built and tested. The new configuration featured lower purge gas consumption, and greater ice impact resistance. o The valve mechanism was redesigned and built into a separate pressure case from the main electronics. o The software for the IBC controller was refined for greater reliability and power efficiency. The ascent cycle timing was programmed to coincide with predicted satellite passes. The new elevator module was extensively tested at the WHOI dock and in shallow water prototype moorings prior to deployment. During the several ascent/descent routines performed by the module off the dock, divers were deployed in order to monitor the trim of the elevator and the umbilical trajectory (Fig. 6 & 7). As a consequence of the divers' observations, a ballasted keel was mounted on the module to improve its stability. A fiberglass horizontal flap was also added on the stem to achieve a correct ascent when in transit from bottom to surface and vice versa. The divers filmed many cycles of the module before and after modifications and through the film footage it was possible to optimize the 195

6 Diving for Science final module configuration. Diving observations turned out to be crucial for the [mal design ofthe ascent module and its control system. Figure 6. Final module prototype In rest position with added keel. Figure 7. Rear view of final module prototype. FINAL DESIGN The final mooring design was fabricated and tested. The mooring was successfully deployed in the arctic on August 24, 1988 at N and W in 1300 m of water depth (Clay, 1988). 196

7 Kery: Diving in support of buoy engineering ACKNOWLEDGEMENTS The author wishes to express his gratitude to fellow divers S. Longworth, T. Rioux and W. Spencer and to the other members of the RTEAM project team for their assistance in preparing this manuscript. Special thanks to Mr. Henri Berteaux, Mr. Alessandro Bocconcelli and Mr. Alan Fougere for their critical review of this manuscript. The work described herein received support from the Office of Naval Research under Contract No. NOOOI4-86-C This paper is WHOI Contribution No LITERATURE CITED Bocconcelli, A (Ed.) Real Time Environmental Arctic Monitoring (RTEAM) Interim Report. WHOI Kery, S.M DIPPER a BASIC computer program developed for the RTEAM project. Moller, D A Computer Program for the Design and Static Analysis of Single Point Subsurface Mooring Systems: NOYFB. WHOI Clay, P Real Time Arctic Monitoring (R-TEAM) Deployment Cruise. Melbourne G. Briscoe, December 1988 WHOI

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