Audun Brandtzæg, MMT Sweden AB, outlines the importance of identifying

threats that put offshore pipelines at risk, and compares pipeline

inspection methods.

There are several methods available for external inspection of large diameter offshore oil and gas pipelines. Traditionally, the most dominant method that has been employed is the use of work class remotely operated vehicles (WROVs) to perform visual and acoustic inspections at relatively slow speeds but with high accuracy. Another well-established method is the use of towed side scan sonars

for documenting the exposure/burial status and freespan conditions of an offshore pipeline. Furthermore, the continuous development of high resolution multibeam echosounder technology has opened up the opportunity of undertaking hull-mounted inspections of pipelines in shallow waters. The last few years have seen the development of other pipeline inspection tools, such as survey autonomous underwater vehicles (AUVs) and more efficient survey ROVs (SROVs).

Both advancements in the industry and the wide range of options that are now available for offshore pipeline inspections have allowed companies to specifying the most appropriate survey method for the specific pipeline. This article presents the benefits and drawbacks of the various methods and tools that are available in the market. Furthermore, it highlights the importance of understanding the threat picture and risks posed to the pipeline asset in order to select the most effective inspection method for each specific project.

Risks facing offshore pipelinesLarge diameter offshore oil and gas pipelines are of great value to their owners, operators and customers. For safety, environmental, commercial, political and reputational reasons, as well as the need for secure energy supplies to customers, operational failures of these assets cannot be accepted.

pipeline threats

Regular pipeline inspection, risk evaluation and assessment is required to control the risks that pipelines face and to secure safe operation. The primary external threats for an offshore pipeline are defined during the design phase and updated during the operation. While the risks vary from pipeline to pipeline, typical examples are: structural integrity; corrosion; damages; third-party interference by anchors, dropped objects, trawling, etc.; and incorrect operations.

It may be assumed that the development of a pipeline threat into failure modus is relatively time consuming and, consequently, an early observation followed by continuous assessment of the

risk is important to allow for remediation prior to an incident developing into failure and consequential damage to the pipeline.

During a pipeline inspection, the status of the pipeline is reported to the operator in order to provide background data for risk assessment. The pipeline inspection data can either be reported as ‘as is status’ or as ‘changes from previous inspection’.

Below are some typical pipeline risks that are observed via different inspection methods in order to provide data for risk assessment. While corrosion risk can be assessed based on cathodic protection (CP) readings, anode consume and missing anodes, all of which are observed via visual inspection, third-party interference can be observed from either visual or acoustic inspection. Meanwhile, structural integrity risk can be assessed based on the following observations:

) Freespans – observed via either visual or acoustic inspection.

) Lateral buckling – observed via either visual or acoustic inspection.

) Upheaval buckling – observed via pipetracker data from visual inspection.

ROV visual and acoustic inspectionAs mentioned previously, running a ROV on, or just above, a pipeline is a traditional inspection method for providing visual coverage of the pipeline and the adjacent seabed. Typically, an ROV carries three video cameras, one centre camera and two side cameras mounted on booms. The three cameras provide an overlapping view of the pipeline.

ROVs are generally equipped with dual head multibeam echosounders mounted at the side of the ROV to provide high resolution data of the pipeline and the seabed 10 m either side of the pipeline. In addition, a pipetracker system tends to be used for detection of the burial depth of embedded pipelines. Dependent on the specific requirements, a CP system may be used for continuous field gradient measurements in order to report on the status of the CP.

While robust and traditionally regarded the best choice for providing pipeline inspection of high quality, this survey method is relatively slow and expensive. The videos taken from the three cameras are reviewed together with the acoustic multibeam echosounder data. They provide information regarding pipeline laying comfort, pipeline freespans, burial status and depth, anode status, pipeline damages, nearby debris/dropped objects, anchor and/or trawl scars, seabed features, and other events.

Acoustic pipeline inspectionIn principle, there are two ways of performing acoustic pipeline inspection – either based on side scan sonar or on multibeam echosounders. If a towed side scan sonar is used, it will typically run 20 m to one side of the pipeline, providing data about the pipeline and surrounding seabed from one side. There is also the option to cover the other side of the pipeline by performing another run.

The alternative method of acoustic inspection, which employs high resolution multibeam echosounder technology, runs on top of the pipeline (or slightly offset) either as a hull-mounted system at shallow waters or a ROV, AUV or remotely operated towed

Figure 1. A ROV visual pipeline inspection.

Figure 2. A large diameter pipeline with anode.

Figure 3. An example of photogrammetry taken during an inspection of a 42 in. pipeline.

World Pipelines / REPRINTED FROM JUNE 2017

vehicle (ROTV)-mounted system in deeper waters. Data collected using this method can be reviewed to provide information about pipeline laying comfort, pipeline freespans, burial status, anchor and/or trawl scars, debris/dropped objects and seabed features.

SROV – combined methodsAs an alternative to the abovementioned traditional methods, the use of purpose built SROVs may be considered. SROVs combine the fast speed and smooth manner of an AUV with the robustness, unlimited power availability and data access of a ROV. By combining a hydrodynamic shape, stern propulsion and front-mounted survey sensors with a small diameter steel armoured umbilical, the SROV is capable of running at least the same speed as an AUV at water depths down to 1000 m. Similarly, the hydrodynamic shape and the propulsion system is designed for stabile movement in a forwards direction, introducing very little noise in the survey data. Since there are no power limits, the SROV can be equipped with all high-end survey sensors and can access data in real time through the umbilical.

For pipeline inspection, the SROV will typically be equipped with dual head multibeam echo sounders and three high resolution still cameras. Side scan sonar may be utilised if required. When running on top of an exposed pipeline, typical speed can

be 4 knots running 3 - 4 m above the pipeline. In cases of buried pipelines, ongoing development of pipetracker systems based on gradiometers can be used on large diameter pipelines.

Since the speed of the SROV is approximately 3 - 4 times the speed of a WROV, the cost per kilometre will be significantly lower. Dependent on the pipeline configuration, all of the common external threats may be observed with a SROV. The following should, however, be considered from case to case:

) Length of survey – using a SROV may not be the best method for short lines.

) Pipeline freespans – marginal supports may not be observed correctly.

) In case of marginal seabed support, the support may be difficult to observe.

) Any requirement for CP stabbing will slow down the inspection speed.

) Traditional continuous field gradient measurements may not be available, but there are new technologies available.

) Traditional depth of burial measurements may not be available, but there are new technologies available based on things such as gradiometers.

Comparison of different methodsThe key difference between WROVs, SROVs and survey AUVs (e.g. Hugin) can be seen in Table 1.

Each type of survey method has positives and negatives. The ROV visual inspection method is, arguably, the most reliable and can provide observations of all external threats and CP measurements. This is, however, a slow and expensive method, and the acoustic data that is provided is not of the best quality.

Acoustic inspection offers a fast and cheap method of external pipeline inspection. While it is effective for observing large incidents, the acoustic inspection does not provide accurate pipeline damage observations. It cannot provide information regarding the depth of the pipeline’s burial and cannot measure CP

like ROVs can. SROVs are used for fast and cost effective

pipeline inspection. They have the potential to observe all kinds of external threat and measure both CP depth of burial at lower speeds. The key drawback of using an SROV is that its high speed may limit accuracy on some occasions.

Case studyThe following case study presents a project that required the most effective survey method for the specific pipeline to be selected. The case requiring inspection was a 110 km, 12 in. gas pipeline in the North Sea. Since the pipeline has been laid a year and an half before the inspection, it was not gas filled or taken into operation. Therefore, the objective of the inspection was to confirm that the pipeline was at an acceptable

Table 1. Comparison of WROVs, SROVs and AUVs

Item WROV SROV AUV

Survey speed 1 - 2 knots 3 - 6 knots 4 knots

Typical endurance >100 hrs >100 hrs 24 - 48 hrs

Data access Real time Real timeAfter data has been

downloaded on deck

Sensor availability No limits No limits Limited by power

Manoeuvrability No restrictions No restrictionsRestricted from rapid

changes in line plan

Attitude Potential noise in data Smooth Smooth

Acoustic noiseMay have a negative

impact on data qualityNo negative impact No negative impact

Potential view

below pipelinePossible Challenging Possible

Pipe tracker AvaliableTraditional methods

not avaliableNot avaliable

CP reading AvaliableTraditional methods

not avaliableNot avaliable

Figure 4. A SROV.

REPRINTED FROM JUNE 2017 / World Pipelines

status before filling it with gas. Since the pipeline had not been in operation, the risks were limited to the impact of third-party damages, and the risks caused by activities such as anchoring, dropped objects and trawling.

In this case, a SROV was used and the survey was completed in less than a day; this is a lot faster than traditional methods. The main survey sensors were a dual head multibeam echosounder, three still cameras and side scan sonar.

The pipeline was found to be in good condition with no damages. Consequently, the inspection concluded that the pipeline was ready for operation. Data from the multibeam echousounder and the cameras was used to document the position of the pipeline in the trench and to show burial/exposure status. The observations that were most clearly visible in the data were exposures and minor freespans. Furthermore, trawl scars were visible on the side scan sonar and multibeam echosounder, which indicated a changed trawling pattern since the preconstruction analysis, to be followed up on in later surveys.

ConclusionBefore selecting the most effective method for pipeline inspection, it is essential to have a proper understanding of the threats that may be imposed to the specific pipeline. It must also be evaluated whether a full inspection of the entire length of the pipeline is required or only short sections. It should also be judged if any tolerances in the data quality can be accepted.

As an example, there may be multiple freespans with marginal seabed support in soft sediments for some pipelines, which may be very difficult to observe correctly. The seabed may only be giving marginal support at the lower part of the pipeline, which may be difficult or even impossible to observe at a low speed. In such a case, whether to perform a review of the acceptance criteria or critical freespan lengths should be considered prior to selecting a method to determine whether these freespans may cause a risk to the pipeline or not.

In other cases, third-party damages are the most dominant threat to the pipeline. It would be worth considering and weighing up the inspection interval against the cost of each inspection in order to provide best value for money. However, in many cases, a fast and cost-effective inspection at short intervals will provide the most useful knowledge of the status of the pipeline and provide any unexpected observations at an early stage.

The most dominating third-party threat to pipelines is normally dragged anchors, and there have been a few incidents where anchors have hooked the pipeline and caused an initial damage to the pipeline, the anchor chain has been broken and nothing reported to the pipeline operator. In some cases, the initial damage has not caused any initial leakage to the pipeline, but after continuous operation of the pipeline over time, the initial damage has developed into a leakage and failure. It should be obvious that an early observation of these incidents is important to secure safe operation of the pipelines.

World Pipelines / REPRINTED FROM JUNE 2017