Subsea Control System with Integral HPU

The Location

This system has been designed to control and monitor two pipeline end manifold (PLEM) ball valves used when offloading liquid petroleum gas from tankers to onshore storage tanks. The PLEM structure is located 5.5 km from the coast of Colombo in Sri Lanka in approximately 15 meters of water and was installed in 1999 for Royal Dutch Shell.

The Requirement

The control system is designed to be installed on the seabed, with the Master Control Station (MCS) located in the gas terminal control or switch room. The decision to install the control system on the seabed was one of security, this ensures that the equipment cannot be tampered with. The system is quite different to other control systems because all hydraulic power for actuation of valves is created subsea. The only tie to the shore is a 5.5-km armored 4-core 4mm2 subsea cable. This supplies the dual 240-V AC power and high frequency communications, super imposed on to the control module power line by use of a power line carrier using 'LONWORKS' multi drop open architecture network communications.

The Solution

This eP cost saving initiative consists of:

• A subsea control system utilizing a subsea hydraulic power unit, combined with a multiplexed subsea control module. This eliminates the need for hydraulic umbilical hoses.
• The system can be used as an economical solution to control subsea wells, manifolds or remote isolation valves, located some distance from the process or host facilities.
• A single subsea electric cable provides the power through the subsea control module to a subsea pump to control and pressurize the subsea valve actuators. The pressurized hydraulic fluid is then stored in a bank of subsea accumulators. All pump and process controls are autonomous subsea, and together with instrumentation data is transmitted to, and adjustable from the topside control panel.
• Controls and communications can be achieved over a distance of 30Km or more.
• Dual redundant communications over power conductors reduce the cost of cable.
• Dedicated dual channel power supplies providing fault detection from topsides to the subsea control module.
• Topsides control panel has hot standby PLC's with external control system via RS-485 dual mode connection. Units are supplied with a 30 minutes back-up uninterruptible power supply.

The PLEM control system consists of the following equipment.

Subsea Control Module Mounting Base

This is a painted rectangular carbon steel structure which has the opposite mating stabplate connectors used on the Subsea Control Module (SCM). Correct engagement of all electrical and hydraulic connectors is achieved by the use of a guide pin bush assembly located on the SCM and mounting base. On three sides of the unit are diver mateable hydraulic and electrical connectors. One side receives the incoming subsea cable; the other two either supply electrical power or hydraulic pressure to the unit or receive electrical signals from either the subsea hydraulic unit or the PLEM monitoring instruments.

Subsea Control Module

A standard diver or ROV installed electro/hydraulic subsea control module can be supplied according to water depth or the client's specification.

Subsea Hydraulic Power Unit

This essentially consists of four bladder accumulators with isolation valves and pressure transmitters on both hydraulic and nitrogen lines. It is these accumulators that supply the hydraulic pressure to the PLEM valve actuators. The system has been designed to supply enough stored energy to completely function all valves open and closed a minimum of four times without the hydraulic pump being employed. A 30 liter reservoir is installed from which the hydraulic fluid is supplied and returned after functioning. Below the hydraulic reservoir is mounted a 100 watt 24-Vdc hydraulic pump which automatically recharges the accumulators to a set pressure once a low pressure signal has been triggered. This unit works as a closed loop system, but there is the facility for topping up fluid or nitrogen if required. Subsea supply, reservoir pressure, reservoir level, control module and pump status are monitored by the MCS.

Master Control Station and Uninterruptible Power Supply

• A fully enclosed, proprietary powder coated steel enclosure with front access.
• Standard design suitable for safe area, i.e. non-hazardous gases, operation and air-conditioned environment.
• Dedicated dual channel power supplies including fault detection to the subsea electronics' module.
• Control and monitoring of the subsea valves.
• Single operator workstation for control and monitoring interface, configured for Wonderware "Intouch" for Windows "NT" and "Windows 95" graphic displays.
• Dual redundant plc configured for "Hot Standby" central processor units, with associated input, output and communication modules.
• Communication to external control system via RS-485 Dual mode system.
• Dedicated dual channel power supplies, including fault detection to the subsea electronics module.
• Modems and signal isolation to effect the "communications on power" transmission system.
• Control and monitoring to the Master Control Station.
• Electrical power back-up for 30 minutes in the event of a power supply outage to both the Master Control Station and Electrical Power Units.

Subsea Power and Signal Cable

This consists of a 4 core 4mm2 screened and double steel armored polyethylene outer sheathed cable 5.5 km in length terminated on to a Hydro-Bond subsea electrical connector.

The Benefits

The subsea control system and HPU is a cost effective solution which meets all of the customers requirements.

Other benefits of the system are:

The smaller diameter of the cable compared to traditional umbilicals.

• Reduces the weight.
• Reduces transportation and installation costs
• Reduces ongoing maintenance costs
• Less expensive components exposed to failure
• Easily retrievable
• Reduces CAPEX costs.

No need for a hydraulic umbilical, a significant cost saving can be achieved over a 5.5 km step out.