MONITORING SYSTEM TESTED DURING LPG TANKER UNLOADING

May 14, 1990
A specially developed computer-based hazardous-materials monitoring system was successfully field tested late last year. The test of the portable system occurred during the unloading of 45,000 metric tons of LPG from a 740-ft tanker at the petroleum dock of a plant along the Mississippi River. The function of this system is to detect, report, alarm, and record unacceptable concentrations of hazardous vapors during marine-transfer operations.

A specially developed computer-based hazardous-materials monitoring system was successfully field tested late last year.

The test of the portable system occurred during the unloading of 45,000 metric tons of LPG from a 740-ft tanker at the petroleum dock of a plant along the Mississippi River.

The function of this system is to detect, report, alarm, and record unacceptable concentrations of hazardous vapors during marine-transfer operations.

SYSTEM CONFIGURATION

The wireless monitoring system assembled for this operation was configured by Robotic Guard Systems Inc. (RGS), Denton, Tex., in two primary components: the control center and the field unit (Fig. 1).

The field unit, placed onboard the ship, consisted of the following components:

  • Four combustible-gas sensors with detection scale from 0 to 100% lower explosive level (LEL), explosionproof housings, and sensor rain shield

  • Four custom-built explosion-proof portable detector-mounting assemblies with magnetic bases

  • RF-shielded signal cable linking the detectors to the field transceiver unit

  • A field transceiver unit containing a data radio transceiver and proprietary control components

  • A 24-v battery power supply capable of sustaining more than 80 hr of continuous field-unit operation.

To enhance portability, the field transceiver unit and the battery supply were housed separately in weather-proof carrying cases.

The control center is the operational brain of the monitoring system's electronic network. The portable version of the RGS control center is also built into a weather-proof carrying case.

The control center's primary components are: a data radio transceiver, proprietary control components; a 286 laptop computer; an ink-jet printer; a printer paper supply and feeder system; a power-supply system consisting of an ac-power converter and 12-v battery backup; and custom-designed software.

Detection signals in the system configuration for this test were sent from the sensors to the field transceiver unit via the signal cable. A proprietary device converted this signal into a format for radio transmission to the control center.

It is this wireless interface between the sensor and control center which enables RGS monitoring systems to operate in fixed, portable, or mobile applications.

For the purposes of this operation, the computer's programming allowed the control center to "poll" (send a request for current detector data from) the field unit at a selectable rate of either 5, 10, or 15 sec. The poll rate could be selectively higher or lower depending on the system configuration, the products monitored, and the operator's requirements.

Each field unit, when polled according to its unique address, responded to the control center's poll instantly (Fig. 2). The poll-response scenario permitted the operator to select which field units would respond, prevented a field unit from interfering with another's signal, and provided a system diagnostic test for outside disturbance or component failure.

This exchange of information transpired in milliseconds. In systems with multiple field units, the operator may select the poll-sequence format.

In addition to operating the monitoring system's radio telemetry poll-response sequence, the control center's computer serves as the system's real time display monitor, alarm initiator, and recording instrument. Custom software design can add numerous system enhancements as well.

Selectable field-unit response, detection values, alarm limits, graphic or numeric displays, disk data storage, hard-copy documentation, and discrete outputs to alarm devices are a few of the programming options available. The program format allows for operator selection of many system functions.

SYSTEM DEPLOYMENT

The supertanker's header system, located in the middle of the ship's 740-ft length, was the connection point for the dock's load arm. The majority of the foot traffic during the offloading work took place on the ship's port (left) side.

According to RGS, the monitoring system's field transceiver unit and battery supply were placed on the starboard (right) side of the upper deck (Fig. 3), below the header system's pipe rack, to be as unobtrusive as possible. This position also allowed the units to be exposed to the elements for weather-testing purposes.

One of the ship's control rooms was located to the bow (front) side of the header system. This was directly between the field transceiver unit and the dock's control room, which was located about 75 yd up the dock.

This arrangement prevented any direct line-of-sight radio transmission and provided a substantial test for structural interference.

Two primary areas for potential leaks were identified: on the header deck at the manifold-offloading pipe-flange connection adjacent to the vapor recovery system return and on the lower deck at the offloading pipe-load arm flange connection.

The four combustible gas detectors were numbered and divided into pairs, one pair each for the header and for the main decks.

Detector Nos. 1 and 2 were placed on the header deck approximately 3 ft to either side of manifold-offloading pipe connection and vapor-recovery return. Detector Nos. 3 and 4 were placed on the main deck below, approximately 8 ft to either side of the offloading pipe-load arm connection.

Viewed from the dock, the detectors were ordered numerically in a clockwise manner from the upper left. This way a leak source could be more easily identified when the computer display screen's detection information was viewed.

The RGS monitoring system's portable control center fit on an available 26 x 24-in. desk space in the dock's control room. With the carrying case open, the computer's screen was raised for viewing.

The printer, paper supply, and the rest of the control center's components were arranged in the case for portability. The power supply arrangement of batteries, kept at full charge by the ac-power converter, ensured 10-16 hr of operation in the event that ac power became unavailable.

Two persons were able to deploy and run a system and calibration check in less than 1 hr, says RGS. The detectors were calibrated and the field unit's battery level established for the starting point of the operation. Calibration and battery-level tests were scheduled for regular checks throughout operation.

SYSTEM OPERATION

Monitoring began when the offloading operation commenced on Day 1 at 6:18 P. M.

The computer screen displayed the detectors' readings graphically, updated every 15 sec. The alarm limits were set at 5% of LEL.

The reported information was printed once each minute, except in the case of alarm or system malfunction, in which case the message was printed immediately.

The 77 hr of operation produced 72 pages of printout material reflecting five significant events worthy of note.

In the early morning hours of Day 2, the warm temperature and high humidity resulted in a heavy dew. This concentrated the trace amounts of ambient hydrocarbons present in the background atmosphere into the water droplets forming on the detectors' metal housings.

Evidence of this phenomenon was the sporadic detection readings found on the operation's printout from 3:04 to 8:23 a.m. As precaution, a company safety technician scanned the monitored area with a hand-held combustible gas detector and found no evidence of hydrocarbon vapor. Each detector was checked for calibration and found operating within the manufacturer's prescribed tolerance.

By full daylight both the high humidity and the sporadic detection activity ceased.

A few hours later the morning of Day 2, the system's only hardware problem of the entire operation was discovered.

The power cable supplying ac power to the control center's battery system was found to have a short. As a result, the command center had been operating more than 16 hr on its backup battery supply.

The cable was replaced as quickly as possible. The system was brought back on line at 1:26 p.m. of Day 2 after about 1 1/2 hr of downtime. One positive note from the power-cable incident was confirmation of the control center's reserve-battery endurance, according to RGS.

A third incident of note also occurred on Day 2.

At 1:54 p.m., the system's self-diagnostic program displayed the first indication of radio frequency traffic interference.

The radios utilized in this system were tuned to an itinerant frequency in the UHF band. A preoperation radio scan of this frequency indicated a few infrequent transmissions possibly emanating from a nearby major university.

In the event of this type of interruption, the system repeats the poll request three times. Without receiving a complete response, the control center's computer displays the diagnostic problem message on screen and continues its polling sequence. When the interference subsides, normal operation resumes. The sporadic transmission from the outside source had very little effect on the system's communication, says RGS. Of the thousands of system messages transmitted over the course of the operation, only 17 interference indications were recorded. RGS notes that for permanent applications, a licensed frequency dedicated to the monitoring system should alleviate such occurrences.

GASKET FRACTURE, HIGH TEMPERATURES

The next few hours of the operation proceeded without incident. The routine was interrupted abruptly at 9:16 p.m. of Day 2 when the monitoring system's control-center alarm alerted dock personnel.

Detector Nos. 3 and 4 on the main deck indicated combustible vapors in concentrations above the 5% LEL alarm limit.

Dock personnel responded to the area to find two members of the ship's crew attempting to control a leak from the ship's offloading pipe. A flare gasket had ruptured on the offloading pipe's vertical section from the header to the main deck. Product spilled from the break to the main deck where Detector Nos. 3 and 4 sensed the vapor.

Dock personnel called for an immediate shutdown of the operation for repairs. Due to the quick response, the highest concentration of combustible vapor was only 8.8% of LEL. All detectable vapor had dissipated by 9:22 p.m., only 6 min 15 sec after the first alarm.

The LPG on board the supertanker was refrigerated to sub-zero temperature. The pipes involved in the transfer became covered with ice as the humid air condensed on the frigid pipe.

When the off loading operation switched products, from butane stored at - 43 F., the temperature shock caused the flange gasket to fracture.

The ice, which had formed on the offloading pipe, was washed away in order to disconnect the ship's offloading pipe for repairs. In less than 2 hr, the new gasket was in place and the product transfer resumed.

Routine operation resumed following the leak episode late on Day 2. Rare instances of radio interference and ambient vapor accumulation from condensation of the heavy humidity were documented.

The weather presented the most serious challenge. As temperatures neared 100 F., late afternoon thunderstorms were heavy and lightning forced the transfer operation to halt for 2 hr on Day 4.

The final event to note from the operation occurred as the transfer was completed near 10:30 p.m. of Day 4.

Residual product in the ship's off loading pipe and the dock's load arm spilled from the pipes during disconnection work. The RGS monitoring system recorded some vapor concentrations as high as 26.6% of LEL. At 11:09 of Day 4, the monitoring system was taken off-line. Two men recovered the portable system in only 45 min.

A final system check revealed the field unit's power consumption to be a consistent average of 0.6 v/24 hr. Throughout the operation the detectors remained within the manufacturer's specifications for calibration tolerance.

Copyright 1990 Oil & Gas Journal. All Rights Reserved.