Data points

Older subsea technology provides little data to operators, meaning they often behave cautiously to minimise risk. Wireless systems can now be installed at significantly less cost – harnessing this data can provide multiple benefits

The predominant consideration driving the development of today’s oil and gas industry is cost reduction. In the field of subsea monitoring, next-generation wireless systems have been shown to reduce costs by 70 per cent in comparison with alternative hard-wired systems. Retrofitting these wireless-monitoring technologies has the potential to extend field life or increase production.

“In general, subsea fields or tiebacks are considerably cheaper than their platform based counterparts,” said Brendan Hyland, Chairman of WFS Technologies. “Their long-term extraction efficiency, however, can be lower and operating costs, higher. Many point to the increased cost of subsea well intervention as the main reason for this, however, this is only part of the story. Just as important, is the relative lack of good quality data without which, it is very difficult for the operator to make informed decisions.”

Modern subsea systems incorporate numerous sensors that can allow the operator to regulate flow, etc. Unfortunately, these represent a very small proportion of the world’s installed subsea production base. Older generation subsea installations provide very little information, making it difficult for the production engineers to understand what is happening.

“A simple analogy is running a car without a fuel gauge,” said Hyland. “This does not change the way the car runs, but it has to be operated much more conservatively. The driver may fill up every quarter of a tank to avoid taking risks. Similarly, running a subsea operation with too little instrumentation means that the operator naturally reverts to very conservative behaviour which ultimately, adversely effects production.”

A practical example of this is an operator deciding to choke back production to avoid mechanical damage to pipework from slugs forming in the wellstream. Retrofitting an ultrasonic sensor conversely, would provide an invaluable warning when the wellstream was about to go into slug flow.

“Like the fuel gauge analogy, it doesn’t tell the operators what to do; it just gives them more control of the production process,” said Hyland. “Unfortunately, the subsea industry has historically spent far less in monitoring than most other process industries. This culture has changed over recent years, but many older fields are devoid of most basic instrumentation.”

The industry routinely uses water and gas injection as core elements of enhanced oil recovery. Water is pumped from the surface into a manifold from where it is distributed to multiple injector wells. Many systems, however, were installed without individual flow meters on each ‘branch’ exiting the manifold.

The operator, therefore, could not know if water was equally distributed or simply finding the path of least resistance and favouring some injector wells in preference to others. WFS has been recently involved in a programme to retrofit ultrasonic flow monitors on such a field to ensure that the correct amount of water goes into each well as specified by the geologists. They reasonably expect to see production increases of five per cent to ten per cent.

“Even simple monitoring can be advantageous. Not only can systems identify potential hazards at an early stage, but they can potentially extend the life of the entire field,” said Hyland.

“When fields were originally designed, the strength and integrity of the rigid or flexible flowlines were estimated using fatigue models. These models were based on assumptions that were inherently conservative.

“Getting better information on fatigue and corrosion etc, can allow today’s engineers to challenge these original assumptions, potentially prolonging asset lives by effectively allowing the models to be recalibrated.”

Costs
So why are all fields not retrofitted with monitoring devices? The main reason is cost. A typical arrangement envisages a remote measurement device connected to a surface monitoring station by cable. It is these cables, or more specifically, installing them, that dominate the costs.

Cables are used to supply the signal and power to the monitoring device, however, they bring with them, a number of intrinsic problems. These include is the possibility of damage during installation and long term integrity issues associated with water ingress.

Most fields have a main subsea electrohydraulic umbilical running from the platform, terminating in some sort seabed junction box. Jumper cables carry the power/signal from this termination assembly to individual subsea devices. Retrofitting new sensors would require new subsea jumper cables to be tied back to this junction box.

Assuming there are free ports for these additional lines, the jumper cables are very expensive to install, requiring large intervention-class ROVs to deploy. The surface spread may also require heavy lifting gear.

Cable-based systems are not without their advantages. They can provide a continuous stream of data in real time. In many applications, however, this is simply unnecessary. Most applications only require measurements being taken daily or when enough points have been recorded in order to recognise a trend or provide an alert warning.

Wireless
WFS has argued convincingly that in many applications, replacing these hard-wired systems with the latest wireless technology has the potential to reduce costs considerably. There are also additional safety benefits from eliminating the divers needed for the cable installation.

A typical system simply consists of a sensor and ter package powered by battery. These systems have undergone significant technological advances in recent years. The wireless instrumentation that WFS builds and deploys, weighs less than 10kg. This means that even the smallest vehicles can deploy equipment and harvest data. An ROV used to deploy cables would cost up to £200,000 per day. By working with light ROVs off the platform, this brings the cost down to nearer £5000.

Originally, battery packs had to be swapped out after six months. Today, they can last for up to ten years. These performance improvements are not only a function of advances in battery technology, but also the incorporation of processing systems within the sensor.

“If you take 10,000 temperature data points, in most circumstances, 99 per cent of those will be all within a band,” said Hyland. “Only small percentages are of real concern. Within the intelligent subsea sensor, we use basic algorithms to process and compress the raw data, to only transmit useful information.”

The distance that Seatooth based wireless systems can be sent through seawater is dictated by the laws of physics, but a typical limit is five to 50 metres. This means that any diver or lightweight ROV flying within 10m can interrogate the sensor without having to dock onto it. A small AUV can harvest data across an entire field.

Another significant advantage of Seatooth wireless systems is that they can send data across the air/sea interface at the splashzone. “It has been historically difficult to monitor fatigue in mooring chains,” said Hyland, “because any cable would have to pass through the attritional environment of the splash zone. In a wireless arrangement, however, the sensor could be installed in the relatively calm subsea environment and the signals picked up at the surface.”

Wireless systems are suitable for a wide range of monitoring applications. A well-known issue with metal structures (offshore platforms) in salt water (seawater) is that an ionic imbalance causes corrosion. One solution has been to weld sacrificial anodes onto the structure but an alternative cathodic protection technique often used in large platforms, is to use an impressed current across the asset.

“Traditionally, the effectiveness of an impressed current is taken annually at a fixed data point,” said Hyland. “We are presently working on a system that allows us to take 10,000 data points distributed across the structure. This gives a much greater depth of understanding.

“By going wireless, in the correct applications, we can take up to 70 per cent of the cost versus hard wired alternatives. At present hard wired systems are not being installed because operators don’t have the budgets. Using wireless systems, however, would only require 30 per cent of the cost, bringing monitoring into the cost effective development of more subsea fields.”

WFS TECHNOLOGIES
WFS Technologies is a global leader in the delivery of subsea wireless instrumentation, control products and communication solutions to the offshore oil and gas industry. Its Seatooth wireless technology enhances subsea monitoring by coupling a range of intelligent sensors with reliable data transmission systems.