SUBSEA METROLOGY

THE METROLOGY CHALLENGE

Short sections of the pipeline are required to connect subsea infrastructure together. These short sections of pipeline are commonly referred to as ‘spool pieces’ or ‘jumpers’. Due to the product they carry, they are generally made of a rigid material, and therefore must be constructed on land, after the subsea infrastructure has been installed.

Highly accurate measurements are required between the hubs on the subsea structures and pipelines in order for the spool piece to be constructed within the tight tolerances required in order to join the two hubs together. The method of collecting these measurements is called ‘Metrology’. Subsea metrology is the process of acquiring accurate and traceable dimensional measurements for the design of subsea structures, primarily interconnecting pipelines. Pipeline interconnections are required to join subsea assets to complete the flow of hydrocarbons from the reservoir to processing and storage facilities. The objective of the subsea metrology survey is to determine accurately the relative horizontal and vertical distance between subsea assets, as well as their relative heading and attitude. There are several types of techniques currently available to support metrology surveys including; acoustics, inertial, and dynamic laser scanning.

Long baseline (LBL) acoustics is the most commonly used subsea metrology technique in use today. This method is most widely used because it is adaptable, has redundancy and the results can be processed within hours. It is also attractive because the results can be referenced to an absolute datum. The disadvantages are that it is susceptible to subsea noise and it is equipment and time-intensive.

Diver taut wire metrology is essentially a tape measurement of the direct distance between hubs. This method was the first subsea metrology procedure employed by divers and was designed primarily for diver operations on horizontal spools. It is still widely used.

The digital taut wire is a more sophisticated version of the diver’s tape measurements. Additional sensors provide a more accurate distance measurement; depth is also resolved with pressure sensors and relative hub attitude with digital inclinometers. However, it still requires a line of sight and is not redundant. There is a limitation on the length of spool measured once the weight of the wire causes sagging giving a linear distance error.

Photogrammetric survey has only recently been developed successfully for subsea metrology applications.
The basis of photogrammetry is to build a three-dimensional model based on a sequence of two-dimensional photographs. Measuring bars placed on the seabed and reflective markers on the structures provide scaling and reference. The processed images are used to derive a three-dimensional model of the positions of the hubs, the seabed, and any other points of interest on the subsea structures. The main advantage of this system is that in a single survey a very high quantity of information can be gathered. The image processing required makes very intensive demands on computer time. Photogrammetry requires good subsea visibility.

INS metrology is relatively new to the offshore industry. The use and availability of inertial navigation systems have greatly increased in recent years. Inertial navigation systems (INS) use three accelerometers and three gyros to compute a position based on a known start point and the measured changes in velocity and attitude. Unaided INS does not need an outside signal or reference to compute a position; because they are self-contained they do not require line of sight, nor are they affected by poor subsea visibility or a noisy subsea acoustic environment. The main drawback of INS metrology is that inertial sensors have drift associated with them. This sensor drift increases over time and requires some form of correction, generally provided by input from other positioning
systems.

Acoustic metrology is the most common method used for acquiring precise measurements underwater in the offshore oil and gas industry today for metrology and installation. Long baseline (LBL) techniques are employed to provide an accurate hub to hub range, a pressure/depth survey then determines the hub depths, and subsea gyros and instrumented transponders are used to measure the hub pair’s altitudes.

An acoustic metrology system comprises a number of Sonardyne’s Compatt 6 transponders placed in a network on the seabed and on the structures and/or hubs. Depths of the Compatts are accurately measured and Sonardyne’s Fusion 6G Wideband acoustic ranges collected between them. The distance between each Compatt is often referred to as the ‘baseline’. This allows a mathematical least-squares network adjustment to be performed which positions the Compatts relative to each other. Additional instrumentation such as GyroCompatt 6 will provide the required information regarding the hub attitude.

SHG provides the following LBL metrology services:

  • Array planning
  • Frequency planning
  • LBL calibration
  • High accuracy subsea installation and ROV tracking
  • Spool tie-in
  • Inertially aided acoustic subsea positioning
  • Provision of ISO drawing

Direct

  • Observations conducted directly at and between the hubs in question
  • No offsets required

Indirect

  • Observations conducted at different locations, related to the hubs with dimensional control
  • Acoustic ranging using Sonardyne 6G transceiver and compacts
  • The sound velocity required – profile or direct logging

ROVs equipped with Sonardyne’s SPRINT (Subsea Precision Reference Inertial Technology) technology can perform hub-to-hub metrology using the acoustic inertial SLAM (simultaneous location and mapping) technique. This solution combines trusted 6G acoustic technology, the field-proven SPRINT INS platform and Janus post-processing software. A SPRINT-equipped ROV places 6G transponders directly into each metrology hub and then maneuvers around the hubs to collect acoustic ranges between each one. The attitude of each hub is measured using a GyroCompatt or a Compatt 6 transponder with an inclinometer endcap. Janus post-processing software is used to determine the position of one transponder relative to the other. When combined with the hub orientation measurements, the hub-to-hub horizontal and depth difference along with the relative bearing between the hubs is then calculated.

Image result for lidar metrology picture

Mapping the ground using lasers from the air (known as LiDAR) has revolutionised the efficiency of on land and shallow water survey projects, both in terms of the speed, accuracy and coverage that can be attained. Baseline datasets of both ‘Static’ and ‘Dynamic’ Subsea LiDAR Laser can be merged into precise 3D models of infrastructure, seabed, spools and pipelines. These models can be used to measure accurate distances, angles and volumes. Asset managers, LoF managers and subsea engineers are able to visualise an exact digital twin of the subsea offshore asset. 2D charts are now a redundant deliverable. 3D digital copies of the offshore subsea infrastructure reduces interpretation, reduces mistakes and allows cost effective and precise repair solutions.

Seabed structures and connecting infrastructure can be captured at installation or at any specific time and that spatial relationship between structures and seabed is preserved at that point in time.
4D temporal data collected year over year can be used to precisely determine any differential settlement between drill centre structures, pipeline out of straightness, pipeline ovality and the volume of any scour, over time. Annual datasets can be viewed simultaneously using colour as differentiation, making changes easy to locate and then measure.
Exact location of flying leads can be recorded and bend radius measured. Anode depletion rate can be calculated by measuring anode volume. Centreline splines can be fitted to any pipeline, riser or spool to calculate precise bend radius. These splines can be exported to CAD or Finite Element Analysis (FEM) software packages for further analysis.

Mobile mapping is similar to multi-beam echo sounder surveying, but provides a much greater resolution. Such is the increased resolution, high quality metrology measurements can be made from the 3D laser point cloud. Mobile mapping is faster than static laser scanning and can cover a much wider area and is less affected by turbidity. The ability to dynamically position a laser in close proximity to the structures allows operations to be conducted in reduced visibility, reducing delays, and allowing a greater point density over targets. High resolution point cloud data can contain a wealth of information which can be utilised for various engineering requirements. One survey operation can accomplish multiple tasks and also reveal previously unknown engineering features. SHG is able to support these projects with tightly integrated, acoustically-aided inertial and Doppler underwater navigation, together with all the equipment, planning and operational services that you need to ensure success.