Subsea control system adapted for growing deepwater constraints

C.Vigne
ECA

• General TOV module organization for a xmas tree. [5,570 bytes]
• The TOV skid being lowered into the water during testing. [15,923 bytes]
• The skidded TOV assembly for the recent 13-month test program off Bergen. [16,635 bytes]

Subsea developments today must overcome the difficulties posed by deeper sites, greater distances from host installations and restricted budgets. Taking these constraints into account, conventional techniques and components quickly reach their limits. It is therefore of prime importance that the industry prepares for completely new concepts.

By replacing the usual electrohydraulic control pod with an electrohydraulic network linking standard light modules, Toulon-based ECA's TeleOperated Valve (TOV) system allows for a very reliable, ROV-operable control system. The module, being dedicated to the basic function (control of one valve) becomes the standard and considerably decreases the amount of engineering needed by Xmas tree equipment. Furthermore, the required logistics support is extremely limited since no heavy running tool is necessary, and spare parts are kept at the module level.

Current flaws

A typical control pod, rated for 150 meters diverless use, is 2 cu m in volume, weighs a couple of tons, requires heavy installation tools and has a mean time before failure (MTBF) of two to five years. Furthermore, the concentration of underwater connectors on a single pod demands improved positioning accuracy. All these parameters have a bad effect on cost, ease of intervention and reliability.

The TOV, by contrast, weighs just 30kg in water and can be ROV installed or retrieved in water depths down to 2,000 meters using a simple and lightweight (50kg) interface. Communications capability is up to 1Mb/s, with a reliability rating of 1 million hours MTBF. It can also be reused subsequently on other fields.

With the TOV, the general organization relies on the fact that this is a distributed control system as opposed to a centralized pod. To ensure control of a xmas tree, one TOV module is needed per actuator to be controlled. Each module is linked to the electrohydraulic network constituted by the umbilical.

In order to keep compatibility with any field layout, and to keep umbilical numbers to a minimum, dispatcher units have been developed, either active or passive. Electrical and hydraulic power is normally distributed passively. Data distribution depends on the number of TOVs connected, plus length and characteristics of umbilicals.

The TOV's small size and weight allows it to be installed at any location in the Xmas tree. However, a panel arrangement has been developed in order to standardize the interfaces. Hydraulic and electrical distributions between the umbilical and the TOVs (the network) are made within this panel and are not subject to disconnection subsea. Links between the panel and the subsea valves and sensors are made by the integrator (usually the Xmas tree manufacturer) on land.

By reducing to nil the cable connections to be made subsea (except when the arrangement requires the umbilical to be connected to the panel via jumpers), overall reliability is further increased.

Design aims

ECA's main design objectives with the TOV system have been standardization - each module has the same hardware, allowing series manufacture - plus small volume and light weight. Main functions of the TOV module are:

• control of one hydraulic valve
• data acquisition from two external sensors and one internal sensor with, if necessary, two limit switches indicating the valve position
• network interface management
• integrated safety functions and self-testing.
• TOVs are diver or ROV-installable and retrievable with adapted tools, by means of an electrohydraulic connector. The modules are pressure balanced, filled with dielectric oil. The electronic subassembly is fully contained in a pressure-resistant tank (600m standard) which is 90mm OD and 180mm long. This subassembly includes regulated low voltage power supplies from the network power and only one hybrid circuit (64 x 64mm) specially designed to effect performance of the following functions:
• communications with the surface unit via the network, using HDLC protocol, at a programmable speed from 1,200b/s to 1Mb/s
• opening and closing of the TOV bistable solenoid valve
• sensors, data acquisition, digitalization and calibration
• integrated self testing
• local safety automation, programmable via the network, such as valve closure in case of communication loss or sequential shutdown.

Hydraulics

The hydraulic subassembly is composed mainly of one bistable solenoid valve with two stages, the power stage being of a shear seal technology. Pilot stages are redundant and pulse operated.

A safety device ensures the valve closes in case of loss of hydraulic pressure. Output to the Xmas tree valve is filtered (with an additional pressure relief valve) to avoid solenoid valve pollution by the Xmas tree actuator. This output is equipped with a pressure sensor, allowing monitoring of both the pressure and valve position by pressure footprinting.

Water-based or mineral fluids can be adopted with a maximum service pressure of 5,000psi. The solenoid valve is plate mounted on a drilled block, including filter and pressure relief valve and self-sealing couplers (hydraulic input, valve output and return port).

Development status

Several TOV modules have been built for a recent 13-month test program in a fjord in Norway, due to be completed this month. The project has received funding from the European Community's Thermie scheme. Aside from the TOVs, three key control components were tested:

• a subsea high pressure unit (HPU)
• a fieldbus transmission system
• a wet mateable optical connector.

The subsea HPU comprises a pump and an electric motor installed in a pressure-balanced vessel (300mm OD, length 700mm), rated for 600 meters depth. An electric converter supplies the motor with voltage necessary to reach the programmed output pressure.

The connector is based on an existing dry mateable, underwater optical connector designed by ECA for its minehunting ROV. The technical basis is that of the expanded beam, as each extremity of the fiber is equipped with aspherical lenses providing expanded collimated beam, thus ensuring a very good immunity to pollution.

For the long-term test that began on July 1, 1996, four TOV modules, the HPU, several communications links and four actuators were mounted on a skid which was submerged in Fusa (Bergen), first in the Framo Engineering dock, then in the fjord and finally in the dock again for convenience of demonstration. A control sequence was programmed in the surface computer, simulating an `accelerated life' for both the TOV and the pump, six months of this sequence representing around five years of operational life.

As expected, the pump was tested until failure, which occurred in the seventh week of this year. This represents a five-year lifetime in operational conditions. Even towards the end of the test period, the TOV network was still working perfectly, based on the pre-programmed sequence, operating on a daily basis one actuator twice and two actuators once, while leaving the other one in the same position until the end of the test in order to check reliability after a very long standby.

Oil companies involved in this project included BP, Elf, Norsk Hydro, Statoil and Total. The TOV panel will be exhibited at Offshore Europe in Aberdeen next month.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.