The tank gauging system is intended to be installed on semi submersible drilling rigs as part of the rig stability monitoring system, or on jackup rigs as part of a preload system. The system is intended to measure the level of:
      - Ballast or preload tanks
      - Clear water/industrial water tanks
      - Fuel oil tanks etc
Depending of the type of tank and the system configuration, the tank gauging system can give various alarm signals, such as tank "full" or "empty", in order to alert the operator of a critical condition. The system also permits the monitoring and recording of water or fuel consumption over time, thus helping to improve water and fuel refill management for example.

Measurement principle

The level of liquid in each tank is determined indirectly by measuring the hydrostatic pressure at the bottom of the tank via a bubbler system. Air bubbler lines are running from the level sensor (in control console) to each tank compartments. The system is maintaining enough airflow in each line to keep them full of air at all time. Knowing the shape of the tank and the fluid it contains, this pressure signal is used to compute the volume and weight of the liquid. The control system also provides for basic and daily maintenance for the pneumatic system, such as pneumatic line purging and facilitated calibration procedures.

System overview

Control system Architecture

A typical tank gauging system consists of:

• An instrumentation cabinet for the monitoring of tanks on the PORT side and the STBD side of the facility
• One or several control screens in the control room (PC based)

The PORT and STBD cabinets are the core of the tank gauging system. Each cabinet is housing the control equipments: the LevelCom100 sensors and a Programmable logic controller (PLC). The cabinet is also housing the pneumatic system interface to the rig air supply and the internal tubing. The Human Machine Interface (HMI) screens located in the control room are connected to the PORT and STBD assembly PLC via an Ethernet network for system supervision.

NOTE: configuration may be different according to rig topology Instrumentation cabinet

The cabinet is housing the pneumatic gears necessary for the connection of the rig air supply and the sens lines to the tanks. Each cabinet is housing the LevelCom 100 smart sensors for tank level measurement and a programmable logic controller for data collection to the HMI units.

Programmable controller

The PLC housed in the instrumentation cabinet is based on the Allen Bradley ControlLogix system. This rack controller features various communication interface and remote I/O modules and is very flexible. The PLC is the gateway between the LevelCom 100 sensors and the presentation system (HMI).

LevelCom100

The LevelCom 100 is a microprocessor based tank level indicating and process control system, which utilizes pulsed pneumatic (bubbler) depth sensing technology.

The device connects directly to the sens tube in the tank it is monitoring. It actually measures the static pressure at the end of the sense line, and convert the pressure value into level indication, based on user configured data.

Furthermore, each LevelCom100 units features its own set of solenoid controlled air admitting and purging valves, and is capable of self diagnostics.

The LevelCom 100 tank level measuring system provides the tank level measurement via a serial link to the programmable logic controller..

Control room display

The Human Machine Interface or HMI system can range from a single stand alone computers to a network of computer, when level data are to be accessed from various locations.

In the main control room, a wide LCD screen can be fitted showing an overview of the rig tanks and their respective level data.

At runtime, the presentation system displays the measured values.
At configuration time, a menu based interface permits the calibration of the system.

Philosophy of operation & display

Controller

The controller is gathering the LevelCom 100 data and monitors the system for alarm conditions.

An audible alarm signal generator (buzzer) can be fitted to the system if required for generating an alarm signal in case the level measured in one tank is too low or too high.

All these data are available for reading on the display (HMI).

HMI screens

The HMI control system provides:

• A main screen with the real time tank level data represented on a large mimic, as well as global data of interest such as overall quantity of specific liquids. The graphical symbols on the screen are laid out according to the system topology and the rig architecture.
• Process specific screens such as liquid level trends for specific tanks
• Alarm log screen
• A configuration screen where the operator can perform the following action:
  • Configure the unit of measurements for the system (typically ft and m)
  • Enter alarm set-points.
• A diagnostic screen where the operator can perform the following actions:
  • Monitor the system status in case of fault. (detector faults)
  • Maintain and calibrate the system (Advanced)

The HMI screens are configurable and customized according to the project specific requirements.

The following figure is an illustration of an HMI screen at runtime. It represents the foot print of the semi-submersible rig with a scale on each corner, representing the draft.

The following figure is another example of an HMI screen at runtime, representing the daily consumption for different types of liquids.