Rewriting the subsea survey rules
Client: i-Tech 7
There is a need to be smarter and more efficient, reduce vessel days, increase safety and lessen our environmental footprint – all without losing accuracy and reliability in our subsea positioning. To make significant steps to address these requirements, we need to use the opportunities presented by technology to change the way we operate.
The underwater survey and construction industry can be quite risk-averse, especially to changes in procedures and methods. And rightly so. The potential negative impact of a downtime or rectification event on a subsea construction project means that trusted equipment and techniques are frequently preferred in the heat of an operation, instead of new technology and methods.
But, if you could both reduce the number of Compatts you need put down for a Long BaseLine (LBL) array for a pipeline installation project and reduce your calibration routine, saving not just hours, but days of vessel time, without compromising positioning accuracy, why wouldn’t you?
REDUCE TIME AND HARDWARE REQUIREMENTS WITH FUSION 2
By adopting Sonardyne’s Fusion 2 all-in-one software platform for inertial navigation system (INS) and LBL operations, i-Tech 7 have been able to do just this. Between August and November 2019, a minimum of 11 vessel days were saved across five separate deep water (1,000-1,500 m) projects in the US Gulf of Mexico, by taking advantage of Fusion 2’s real-time SLAM (simultaneous localisation and mapping) capability.
Choosing to operate a sparse LBL on paper delivers a reduction of Compatts. However if the sparse array cannot be calibrated using traditional LBL baseline calibration, the benefit can be lost due to the extra time that has to be spent performing additional ‘box-ins’ or collecting data and attempting to perform a post processed SLAM position of the Compatts.
This isn’t the case with real-time SLAM where you can reduce the number of Compatt transponders used to create the ‘sparse’ LBL array and then also calibrate those Compatts by flying the ROV through the array, acquiring the necessary observables to SLAM the array. With careful planning, this SLAM process can be done during other ROV survey operations, such as pre-lay surveys, thus removing an the entire baseline calibration work flow from the project schedule, by using real-time SLAM.
BACK TO THE FUSION FUTURE
But, first let’s take a step-back. How did we get here? At i-Tech 7, we are always looking for ways to improve our operations whilst managing risk and reducing complexity. That can mean taking a staged approach – assessing the options, undertaking trials and working with technology suppliers to ensure what’s introduced will work.
In recent years different combinations of subsea positioning technologies have been used in support of survey and construction projects that, whilst delivering an incremental improvement, have also resulted in increased complexity in integration and operation. There have been separate INS and LBL solutions requiring to be integrated via online navigation software, as well as the added complication of the physical mounting, calibration and interfacing of INS and Doppler velocity logs (DVL) on remotely operated vehicles (ROVs). The increased complexity adds cost in training and support, as well as increasing risk through having to control many separate parts from different vendors.
Co-locating a Sonardyne SPRINT INS with a separate DVL on to our ROVs was a first step in reducing the complexity. Using SPRINT enabled us to work more closely with Sonardyne to optimise sparse LBL operations, reducing the number of Compatts required for an LBL array by 50-66%, with Sonardyne post-processing SLAM calibrations for us. We were doing this back in 2017, with the support of Sonardyne while they refined their INS algorithms for both tracking and calibration. Then, in 2018, we introduced SPRINT-Nav on to our ROVs, further reducing integration complexity and increasing navigation and positioning performance.
All the experience and knowledge learned since our first use of SPRINT INS in 2012 has enabled Sonardyne to build SLAM calibration routines directly into Fusion 2. That’s the step that’s taken the simplification to a new level, enabling real-time SLAM operations in our 2019 projects – with more efficient trajectories.
The benefits of real-time sparse LBL SLAM are quite simple. Having already reduced the number of Compatts we need for an array – reducing how much time we spend deploying them and calibrating them – we can reduce the time allocated on project schedules for the lengthy box-in routines required in a traditional array.
To give an example, on one of our pipelay projects in 2019, we had three cut-to-length locations, over a 5 km-long corridor, which would have required 14 Compatts and three box-in routines in a traditional LBL array and baseline calibration. For an INS-aided sparse LBL array, which maintained four-Compatt arrays at the critical lay-down locations, we could reduce the total number of Compatts to nine, but would have required seven box-ins. With Fusion 2, and real-time SLAM capability, we were able to reduce the number of box-ins to two, while the rest of the Compatts were SLAM-calibrated during the pre-lay survey with an ROV fitted with a ROVNav 6+ and SPRINT-Nav, removing the need for a dedicated baseline calibration exercise.
In fact, following that pre-lay survey last year, an offshore manager asked one of our offshore surveyors how long the array calibration would take before pipe lay operations could start. The answer? It had already been done.
REDUCING TIME OFFSHORE
Simply looking at the time saved on deploying Compatts and performing box-ins, we estimate at least 11 vessel days have been saved over just five projects, all in 1,000-1,500 m water depth. That means operators can get to first oil faster.
In order to demonstrate the accuracy and reliability of real-time SLAM calibration, we deployed the technique on projects that had arrays with full LBL sections, which could be baseline calibrated, as well as sparse LBL arrays, which we SLAM calibrated. On several of these we ran the real-time SLAM calibration and also a traditional LBL calibration, where they overlapped, and compared the results. This showed centimetric agreement and proof that the real-time SLAM is giving us near traditional LBL levels of precision and accuracy.
SMARTER, MORE EFFICIENT, QUICKER, SAFER
Throughout the journey to successfully operating, and ‘real-time’ SLAM calibrating, sparse LBL projects, i-Tech 7 have proved we can be smarter, more efficient, quicker, safer and help lessen the overall environmental footprint of a major subsea installation project – all without losing positioning accuracy and reliability. This technique will continue to benefit our work with Subsea 7 and their customers and will provide a step change in performance wherever we require field-wide LBL accuracies.
The next challenge is enabling i-Tech 7 and Subsea 7 to bring these advantages onto all the projects we can. We’re often impaired by legacy industry specifications, which don’t always reflect the latest advances in technology, allowing us to conduct operations in a way that can realise the benefits that are available to us today. With real-time SLAM calibration we are shifting from over 30 years of prescribed ways of working and best practice and we now need to consider how we collaborate with Operators and validate and qualify the solution for the next 30 years and more.
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