In 2000, Halliburton Energy Services developed a Subsea Well Intervention System known as the Swift Riser System for use in very deep water. This system was designed to combine all slickline, electric line, and coiled tubing well intervention capabilities into a single, "smart" composite riser, with embedded conductors and fiber optics.

The Swift system includes a Lower Riser Package (LRP), with the lower component of the Pyramid Up Guidance System (PUGS) installed on top. The LRP with the PUGS is located on the Xmas tree and remains attached during the intervention with a control umbilical to the surface. The lubricator and the upper component of the PUGS is run from the vessel on a compliant flexible riser system with the close tolerance coiled tubing intervention string inside.

The lubricator is fully equipped with the tools required for the work to be done. The mating of the PUGS to the LRP is done without guidelines. Therefore, a means to provide precise relative positioning between the two units was required to enable the mating of the two.

The positioning requirement for the PUGS system is to provide precise relative positioning between the LRP and lower PUGS frame and the incoming lubricator with the corresponding connection. The system enables accurate guidance and docking before the actuation of the critical connector seal between the two.

The requirement to build a system for deepwater and the desire to provide a self-contained solution made traditional methodologies impractical. Halliburton Subsea, sister company to Halliburton Energy Services, was asked to design the means to provide the accurate relative positioning of the PUGS and the LRP during mating. Halliburton Subsea devised an LBL solution and approached Thales GeoSolutions to see if it could be implemented within WinFrog.

Thales' WinFrog integrated navigation software is widely used in oil and gas exploration, survey, marine construction, submarine cable installation, and fleet management markets around the world. Halliburton has been using WinFrog on sophisticated underwater construction projects since 1996 and has often collaborated with Thales GeoSolutions in the addition of advanced capabilities to WinFrog. In this case, Thales GeoSolutions refined Halliburton's design and developed the models and algorithms, implementing them within WinFrog as the Inverted LBL application. This is a completely new positioning methodology that addresses the requirements for the Swift Riser System and is now commercially available as part of the WinFrog LBL module.

The LBL Solution
The Swift Riser System is deployed in water depths of up to 10,000 feet (3,000m). It is necessary to accurately monitor the PUGS position with respect to the LRP during its descent to the seafloor. During the final stages of the descent, it is also necessary to monitor the orientation and attitude (pitch and roll) with respect to the LRP. The PUGS is tracked from the surface vessel during the descent using an USBL acoustic system. Positioning during the final 65 to 100 feet (20 to 30m) of descent and the docking itself is done using Long Base Line Extra High Frequency (LBL EHF) acoustics.

The conventional approach to LBL acoustic positioning has been to deploy transponders on the seabed and static structures to form a fixed network array and determine their positions through calibration. The interrogating unit (transceiver or smart transponder) is then mounted on the dynamic vehicle and its position is determined by measuring ranges to the fixed transponders. The concept proposed by Halliburton and developed within WinFrog is the inverse of this.

Given an array of static transceivers whose spatial relationship is a relative network of known points, range measurements from each of these to a single dynamic transponder can provide the data required to solve for the transponder position using a least squares adjustment.

As with any adjustment, there are several important points to consider. First, the number of observations (measurements) and, therefore, the number of static transceivers in the network, must be equal to or greater than the number of unknowns that are being solved for. In this case there are three unknowns, position (X and Y) and depth (Z), and therefore a minimum of three observations is required. The use of more than three measurements improves the results of the adjustment.

Second, there must be strong geometry in the relationship of the static transceivers network and the dynamic transponder. Third, the position computed for the dynamic transponder is relative to the static transceiver array. It is only accurate in the absolute sense if the static transceiver network is accurately positioned in “real world” coordinates.

It is important to consider the relative spatial relationship of the static transceivers. Given a vessel, all positional information must be referenced to a specific arbitrarily assigned point called the Common Reference Point (CRP). It must also be referenced to a specific arbitrarily assigned orientation, generally a line parallel to the respective vessel center line from stern to bow. This orientation and CRP define a local vessel-based coordinate system. Applying this to the static transceiver array, a CRP is assigned to a central point within the network and a reference for the orientation is selected. The static transceivers are then accurately measured within this coordinate system. The adjusted position of the dynamic transponder is also relative to this coordinate reference frame.

The positioning of the single dynamic transponder can be expanded to include an array of transponders that are attached to a dynamic but rigid structure. This structure is assigned a CRP and reference orientation to create its own local coordinate system. The transponders are fixed and their spatial relationship within this local reference frame can be accurately measured with respect to the CRP and with each other. Range measurements from each of the static transceivers to each of the transponders produce an independent solution for the position of each transponder. Adding the known baseline lengths between each dynamic transponder pair as constraints to the adjustment results in improved, correlated solutions for the transponder positions.

The last stage in the process is the solution of six more unknowns, specifically the attitude (pitch and roll), orientation, and CRP position (X, Y and Z) of this dynamic structure. A least squares adjustment is used. There must be at least six observations. The observations in this adjustment are the spatial relationships between the dynamic transponders, three for each transponder pairing. This adjustment requires a minimum of three transponders in the dynamic array. This provides two transponder pairings (not three, as one might expect). The third pairing is the equivalent of combining the other two, not an independent observation. Including a fourth transponder results in redundancy in the adjustment, improving the results, the expected accuracy, and the quality control and monitoring options.

To reiterate, the results of these adjustments are relative to the local coordinate reference frame of the static transceivers. From them, the distance from the static transceiver CRP to the dynamic structure CRP in terms of XYZ relative to the static transceiver local coordinate system can be calculated. This approach is particularly well-suited for application to the Swift Riser System since it is required that the PUGS be positioned relative to the LRP. The results can be transformed to an absolute sense by referencing the static transceiver network to what is referred to as “real world” coordinates.

Implementing the Inverted LBL
In WinFrog, the term PUGS was assigned to the lubricator/PUGS assembly and the term LRP was assigned to the LRP/PUGS assembly. Each was considered a vessel in WinFrog and, as such, each was designated a CRP and a line of orientation reference. The horizontal center of the bottom of the upper PUGS component being lowered was assigned as its CRP. The horizontal center of the mating surface of the LRP's PUGS component was assigned as its CRP.

The acoustic system selected for positioning the PUGS relative to the LRP during the final stages of the mating was the Sonardyne EHF ROVNAV Mk4 system with Sonardyne EHF transponders. The ROVNAV system is an acoustic control unit housed in an underwater casing with connections for two transceivers and a communications cable to a peripheral computer.

Among other features, the two ROVNAV transceivers can be sequentially instructed to interrogate transponders. The proposed Sonardyne transponders are known as Mark 4 Computing and Telemetering Transponders (COMPATT) with the “Simultaneous” feature. These COMPATTs can be instructed by the ROVNAV to interrogate other transponders and then send the observed range data back to the ROVNAV. Communications between the ROVNAV and this Simultaneous COMPATT is done via acoustic telemetry. EHF is required to provide the measurement resolution necessary to enable the determination of the transponder positions to the accuracy required, especially for determination of the orientation and attitude of the PUGS.

In the original design, one ROVNAV system with two transceivers and one Simultaneous COMPATT were mounted on the LRP. The ROVNAV transceivers would interrogate the transponders on the PUGS and then instruct the Simultaneous COMPATT to do the same. The communications cable from the ROVNAV was connected to the surface and the WinFrog computer. In the final installation, this would be via the cable connection from the LRP to the surface support vessel. Four COMPATTs were mounted on the bottom of the PUGS pointing down, one on each corner of the structure.

The WinFrog package incorporates multiple plan and profile graphic windows for the presentation of vessels to facilitate their navigation. While these provide the basics required for the positioning of the PUGS and LRP, it was felt that a more graphical display would be of benefit in assisting the bridge crew during the docking operation.

Rather than incorporate this display into WinFrog, it was decided to implement it as a separate Visualization Utility. This utility was developed by Halliburton Subsea using the Microsoft DirectX toolkit. The display shows the relative locations of the LRP and PUGS. The LRP/PUGS components are represented as a rendered 3D model, displayed in three windows (plan, elevation, and perspective), in which the scale and orientation can be adjusted. WinFrog constantly updates the Visualization Utility via a serial connection, passing the current set of position data for the PUGS and the LRP, including orientation and attitude.

Testing the Application, Part I
Two sets of trials were conducted in testing and validating the Inverted LBL approach. The first was a tank trial performed at Halliburton's facilities in Aberdeen, Scotland. The objectives of the tank trials, from the perspective of the LBL component, were to confirm the concept for this unique application of the LBL acoustics for determining the position, orientation and attitude of the PUGS with respect to the LRP; to confirm the operation of the software developed to implement this concept; and to evaluate location of the acoustic hardware on the PUGS and LRP units for optimum performance and protection.

It was determined early in the trials that the acoustic environment of the tank made the required telemetry communications with the Simultaneous COMPATT unusable. It was also decided that the cycle time required for a complete set of measurements was too long when it included the telemetering of the data from this COMPATT. Therefore, the COMPATT was replaced by another transceiver. The location of the acoustic hardware was adjusted occasionally through the trials to determine a balance between performance and protection.

The trials also indicated areas where the software required modifications to improve both ease of use and the display of data and results for the purpose of monitoring and quality control. Most of these modifications were made on-site and tested during the trials.

These tank trials proved that the Inverted LBL solution provided steady and reliable results when the acoustic data was reliable, thus validating the concept. It was also shown that the measurement cycle time and the approach taken for the processing of the data resulted in reasonable response times to changes in the position and orientation of the PUGS.

Testing the Application, Part II
The second set of trials was performed from a barge with a crane in Loch Linnhe, Scotland. The primary objective of this set of trials was to evaluate the LBL positioning and the actual mating of the PUGS to the LRP in open water. The first phase was performed with the barge dockside, while the second phase was conducted in open water. These trials were supported with an ROV.

The first phase was similar to the tests performed at the tank trials. First, the LRP and PUGS were suspended in the water. Next, the LRP was placed on the bottom and the PUGS lowered to it. The second part of the test required that the barge move away from the pier to a water depth of 300 feet (91m). The LRP was deployed to the bottom and again the PUGS was lowered to it. This trial included the use of USBL to monitor the initial descent to the bottom. During both operations, position, attitude and orientation of the PUGS relative to the LRP were monitored with WinFrog and the ROV.