Focus on Ancillary Equipment and Fatigue in Coiled-Tubing Deepwater Commissioning
The installation of flowlines in ever-deeper and -more-remote areas requires specific technologies for precommissioning. Coiled tubing can be a solution, but long durations may affect tubing stress and fatigue.
The installation of flowlines in ever-deeper and -more-remote areas requires specific technologies for precommissioning. Coiled tubing can be a solution; however, offshore precommissioning can require coiled tubing to be deployed several times for durations sometimes exceeding a month and requires larger diameters. Therefore, a campaign was initiated to characterize the behavior of coiled tubing under combined plastic and elastic fatigue. In addition, an innovative bend-stiffener design was developed to control the stress levels in the coiled tubing at the hang-off location.
Precommissioning is a critical part of flowline installation and operation. This process occurs at the end of the installation to validate the integrity and performance of any system before commissioning.
When no direct access is available from a surface facility, precommissioning is, most of the time, performed from a multiservice vessel (MSV) using a flexible line. However, as water depth and the diameter and length of the flowline to be precommissioned increase, the submerged weight of the line becomes an issue, requiring the installation of buoyancy modules to limit the loads transferred to the vessel. To simplify offshore precommissioning activities, the company decided to use coiled tubing.
Coiled tubing offers the advantage of having a higher diameter/weight ratio than flexible lines, allowing for large-diameter tubing to be deployed from an MSV while keeping loads transferred to the vessel at an acceptable level. In addition, coiled tubing has an extensive track record onshore and most of the required components are available off the shelf. However, onshore applications of coiled tubing are essentially static while offshore precommissioning activities are dynamic. The coiled tubing hangs from the vessel and, therefore, is subject to loads from the waves, currents, and vessel motions. This means that the coiled tubing and the associated equipment should be designed to withstand extreme loads and elastic fatigue.
The typical scenario for offshore precommissioning with coiled tubing consists of deploying a small-bore rigid line (i.e., the coiled tubing) to approach the location of the pipeline end termination and pig launcher and receiver that will be used. A flexible hose is then connected between the coiled tubing and the pig launcher and receiver. The hose allows the system some flexibility to avoid transferring loads to the pig launcher and receiver from vessel movement and environmental loading.
Topside equipment for coiled-tubing offshore precommissioning is identical to that used for conventional coiled-tubing operations, with two exceptions: The injector that usually snubs the coiled tubing into the well serves as a tensioner, and a dedicated deployment system is required for the flying hose. The flying hose is usually a noncollapsible hose that can tolerate only very limited tension loads but has a very low minimum bend radius. This hose is usually equipped with ancillary equipment and cannot be deployed directly connected to the coiled tubing. Overboarding operations are required to set up the system. An overview of the topside equipment is presented in Fig. 1.
The exit of the tensioner is clearly the critical location. That is where the tension loads are at their maximum. Just below the tensioner, the coiled tubing has to support the weight of a significantly long line, typically 2000 m. This is also where bending loads are at their maximum. At the exit of the tensioner, the coiled tubing can be considered as being clamped on an infinitely stiff structure, which means that bending loads will be concentrated at that location. As is the case for riser applications, a mitigation device is clearly needed. As highlighted in Fig. 1, a bend stiffener is required.
The following guidelines were considered in the design of the bend stiffener, which could be tailored for each project:
- The bend stiffener should be made of a single piece to be as simple as possible. Therefore, a connector allowing passage through the bend stiffener is required.
- A straightener is needed so that the coiled tubing is straight enough to pass through the bend stiffener.
- The bend stiffener has to be large enough to accommodate large ovalization of the coiled tubing.
- Precautions should be taken to prevent the coiled tubing from being scratched during the insertion.
Once a suitable design has been achieved, the design life of the coiled tubing has to be assessed. The main challenge lies in the combination of plastic and elastic fatigue.
Typical offshore precommissioning requires several deployments and recoveries of coiled tubing and lasts approximately 600 hours. Consequently, coiled tubing will be exposed to high levels of stress and plastic fatigue during deployment and recovery and, between each deployment, will have to sustain a significant number of elastic fatigue cycles.
Plastic fatigue of coiled tubing is well-documented because of its extensive use for onshore operations. Elastic fatigue of coiled tubing is far less documented. Few data on the combination of plastic and elastic damage are available, making the combination process quite uncertain.
Fatigue-Assessment Methodology. To address the various hurdles, a fatigue-assessment methodology was developed to verify that the design life of the system overmatches the foreseen duration of operations. The considered model differs from standard fatigue assessments in that it aims to take into account the effect of plastic damage on the elastic fatigue behavior of the system. Please see the complete paper for a description of the model.
Implementation of the Model. A specific tool was created to apply the proposed model. The tool is based on the time series of the environmental data in the area where precommissioning is going to be performed. It computes the overall damage for each operation starting date with a step of 3 hours over several decades, corresponding to tens of thousands of load cases. The tool accounts for any reeling and unreeling operations for coiled-tubing deployment and recovery required in precommissioning and for each time the system has to be recovered because of wave height exceeding the maximum allowed value.
The procedure begins with computing the number of allowed plastic-fatigue cycles by use of a dedicated coiled-tubing software. Then, preliminary elastic-fatigue calculations are made. The time series is transformed into a scatter diagram, and the elastic damage is computed for each sea state within the scatter diagram. This damage is normalized with regard to the number of waves within the sea state, then the number of applied cycles and the number of allowed cycles within a sea state are extracted. Plastic and elastic fatigue are then combined. Considering the operational scenario, the damage induced by both plastic and elastic fatigue is computed with the model for each sea state met over the duration of the required operations.
Once the damage has been generated for all load cases from the time series, statistical data can be derived from the results.
Fatigue-Testing Program. The company decided to conduct fatigue tests on actual coiled tubing. The tests were carried out on coiled tubing made of CT-80 grade material. In-house plastic-fatigue testing allowed for the assessment of fatigue performance with a single methodology, reducing bias. Samples repeatedly were deformed and straightened plastically until a given plastic-damage level was reached. The samples were then transferred to the elastic-fatigue-testing bench and were cycled until failure. This step allows for fitting the power law on the damage evolution. In order to assess the sensitivity of the model to alternating plastic- and elastic-fatigue cycles, a final batch of tests will be performed in which a fatigue scenario will be repeated until failure. The samples will be deformed plastically once, then elastically loaded, then deformed plastically once again and so on until the samples break. The testing is currently ongoing and, at the time of publication, all data were not yet available.
Elastic-Fatigue Testing. The first step of the program consisted of performing elastic-fatigue tests on actual coiled tubing. A testing bench was purpose built for the tests. The bench was a conventional four-point bending bench with particular attention paid to the clamping system to limit the risk of breaking the pipes at those locations, which would make the results irrelevant because it is practically impossible to assess the stress at the clamp locations. The main reason for using the four-point bending testing instead of resonant fatigue testing is that the same bench will be used on plastically deformed pipes.
Samples were tested to failure, or testing was stopped when reaching the run-out that was set to 1.5 times the target.
Nine samples were tested. They all overmatched the run-out.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25665, “Adaptation of Coiled Tubing for Deep- and Ultradeepwater Commissioning: Focus on Fatigue and Ancillary Equipment,” by François Lirola andFrançois-Régis Pionetti, Saipem, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. Copyright 2015 Offshore Technology Conference. Reproduced by permission.