Offshore pipelines: a quantum leap in integrity management

August 9, 2010

Continuous registration of vessel traffic above pipelines via the proposed Automatic Identification System (AIS) represents a significant improvement on pipeline operators' emergency response to possible mechanical damage, says Jens Erik Thygesen, senior consultant with Dong Energy E&P. As well as adding value to the pipeline integrity management database and input for risk-based inspection planning, he argues that AIS will also provide valuable backup for pipeline operators looking to mitigate damage to their pipeline network, thereby minimising risk to personnel and the marine environment.

Onshore pipelines have been monitored for many years to avoid damage and associated risk to the surroundings. A similar monitoring of traffic across offshore gas and oil pipelines has been a strong wish of offshore pipeline operators for many years. One of the major threats to offshore pipelines is the risk of mechanical damage either by anchoring or other impacts.

The total failure statistics from the North Sea are shown in Figure 1. Targeted inspection for mechanical damage is therefore of high importance when maximising inspection and minimising cost.

Following the legislation that required an AIS (Automatic Identification System) system on every vessel above 300 BRT [gross register tonnage], the offshore pipeline Industry has now got a beneficial tool to monitor all offshore traffic in the area where their pipelines are situated as long as there is AIS coverage around them. This is the case in many areas, particular where the traffic is as dense as shown in Figure 2.

What is AIS?
AIS is a system that monitors all civil ship traffic, and it is based on VHF communication. Satellite coverage might in the future improve the coverage of the system. There are two versions of the system, Class A and Class B. Class A is the relevant system for offshore pipeline monitoring, as this is mandatory for vessels above 300 BRT – the vessels posing the greatest threat to pipeline integrity. The system has for some years been used for monitoring and controlling ship traffic around offshore platforms to avoid collisions, but it has yet to be applied for offshore pipeline monitoring. The system gives information on vessel name and type, its course and speed over ground and its position at all times.

Early in 2009 Dong Energy E&P started evaluating how this AIS system could be used for all activities around oil and gas transmission lines in the Danish sector of the North Sea. Over its 26 years of operations, the company had become aware of abnormal traffic across its pipelines, in some cases causing severe mechanical damage (Figure 4). Improved pipeline integrity monitoring methods were required.

The idea (illustrated in Figure 3) was to track abnormal behaviour of vessels crossing the pipelines or stopping inside a defined area around a specific pipeline.

A so-called ‘watchdog zone' is placed around and along the pipeline. In Dong Energy E&P's case this was defined as a nautical mile either side of the pipeline. The watchdog observes abnormal behaviour of any ships passing through this zone and logs such events as a ‘watchdog event'.

A watchdog event log will contain the following information:

  • actual time of event triggered;
     
  • actual time of registered entrance into the watchdog zone;
     
  • location of entrance into the watchdog zone;
     
  • expected exit of watchdog zone, time and location;
     
  • vessel identification, name, contact numbers and standard AIS vessel information;
     
  • observed location of vessel within the watchdog zone at the time of the triggered watchdog event; coordinates of the vessel in system; longitude and latitude coordinates for the pipeline; and
     
  • log of the specific vessel movements, and continuous monitoring of its route thereafter.

 

The accepted policy for the detection of mechanical damage to buried pipelines has always been to perform a calliper inspection every second year. Mechanical damage by anchors to buried pipelines might not show visible signs on the seabed. The inspection frequency is based on an arbitrary expectation that severe damage left undetected and exposed to the CP coverage in the marine environment could withstand two years of operation without degradation and eventual pipeline rupture.

A damaged metal surface in the sea environment which is only protected by cathodic anode system might suffer from hydrogen penetration into the steel, making it brittle, thereby losing its pressure containing strength and finally leading to a pipeline rupture.

Over the years many different justifications have been used to determine the frequency of inspection without success. Several years ago Dong Energy E&P implemented riskbased inspection and pipeline integrity management systems based on the DNV ORBIT system. Even with this system, the company still needed to improve the information database to determine an appropriate inspection philosophy and frequency for mechanical damage beyond the arbitrary two-year interval.

Based on damage suffered and analysis of near-miss situations, Dong Energy E&P;started developing the criteria (Figure 5) to be observed by an AIS system – the Watchdog – to track in real time manoeuvres around pipelines which in a worst case could lead to mechanical damage, thereby threatening security of supply.

Last year the company established contact with a Danish software company, GateHouse, which had been involved in developing ship traffic monitoring and management for the Danish Coast Guard. The principle of applying a detection zone around pipelines for detection of traffic was discussed and the GateHouse software modified to cover the criteria selected. The principle for this system is shown in Figure 6.

Early this year Dong Energy E&P;performed a pilot test on selected areas around the oil and gas pipelines network in the Danish sector to prove the concept. The pilot test was successful and showed that the principle was working.

Management improvements
Offshore pipelines play a crucial role in serving the energy needs of a modern society so it is vital they be secured against mechanical damage and, in cases where it is detected, that early action be taken to avoid sudden interruptions to supply without warning.

In extreme cases, early detection of a critical mayday around an offshore pipeline can through collaboration between pipeline operators and the rescue organisations minimise the risk of escalating risk to personnel and environment in an already tense situation.

The sorting and storage of observations, as illustrated in Figure 2, can be used for planning of inspection for possible mechanical damage instead of choosing an arbitrary frequency. In this way the AIS data provides a better focus on the high-risk areas for inspection, thereby leading to an improvement in inspection and value for money.

Registration and storage of traffic intensity across and along the offshore pipelines provides the best statistical data for risk-based inspection planning (Figure 7).

Once shipping traffic becomes aware that such an application is in place, Dong Energy E&P believes that the application of the AIS Watchdog principle around offshore pipelines will have a preventative effect on the risk potential for mechanical damage.

The AIS-base Intelligent Event Detection System will provide an alarm to the on-duty pipeline personnel within minutes and allow them to advise the particular vessel how to reduce the risk potential of the situation or, if the vessel is in distress, advise on rescue operations.

Repairing a subsea pipeline that has suffered extreme mechanical damage is not an easy task and can take more than one or two months, depending on the complexity of the damage, pipeline operating conditions and the specific water depth. If the damage has not lead to a rupture or leak, it is important to try to minimise the repair time and if possible minimise the influence on the pipeline operation.

Planning of a repair is time-consuming and needs a lot of information so that the severity of the damage can be assessed and an appropriate repair method devised and applied without long interruptions to pipeline use. In order to reduce the time for repair and prevent escalation of damage severity, immediate detection is vital for planning and to perform a proper repair, without risk to personnel and environment. OE

Earlier presentations on the application of AIS to pipeline integrity monitoring and the Dong Energy E&P pilot test programme, respectively, were given by the author at the 2009 Pipe Tech World Summit in Cyprus and the 3rd Annual Oil & Gas Pipeline Maintenance & Reliability Conference in Barcelona, April 2010.

Jens Erik Thygesen is senior management & technical consultant at Dong Energy E&P, having previously served as its head of offshore technology. His 30 years' experience in offshore production and pipeline transmission includes senior roles in project management, design construction and commissioning, operations and lifetime extension. He was VP for oil and gas transmission from the North Sea to shore plus Dong Energy's Siri inter-field systems. He also represented the Danish operator in Norway on the Ormen Lange operating committee, and lead the due diligence on the Norwegian Gassled system prior to merging the Langeled pipeline system into the total Gassled system.



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