As any acoustic metrology surveyor will tell you, there are 4 fundamental steps to any acoustic metrology offshore. How the operator obtains this data will depend upon the software & hardware utilised to allow the data collection and recording of information.
This article will not describe the data collection or detail the post processing of the data after the acquisition stage. This is usually client specific, which cannot be shared for the purpose of this article. This article will merely share with the reader an understanding of the 4 basic steps to allow the reader, perhaps with confidence, undertake and QC the data collection of an acoustic metrology survey task .
- Structure Heading: The metrology surveyor is to determine the structure heading of both structures. This can be done in several ways. The most simple method is to measure the four faces of the structure with the ROV or a handheld subsea gyro and average the result, reducing the heading to the pre-determined structure north reference. The metrology surveyor is looking for repeatability and consistency in observations between the four observed measurements. An alternative to this, is to use a Gyro Compatt and align its heading reference with the pre-determined structure north reference, on the structure in question.
- Structure Attitude: An inclinometer Compatt, fitted with engineered metrology stabs and ROV handles will be installed in the metrology receptacle of the Structure. Observations of Pitch and roll will be collected at Structure North and then rotated through the 4 quadrants, taking observations of Pitch and Roll at each 90° rotation and then ‘closed’ out at Structure North. Again repeatability of observations and a reduction of each Pitch and Roll observation, transposed back to Structure North to show consistency of Attitude is what is required. If repeatability and consistency at any one of the quadrants is not seen, then the metrology surveyor is to repeat the measurements until this is achieved.
- Level Loops (depth observations): Using a high accuracy depth sensor (either a Paroscientific Digiquartz or Valeport mini IPS), measure the depths at each of the hub locations of the 2 structures. The first depth loop starting (opening) at one of the structure hub position, observing depth at the second hub and close out at the first hub location. This depth loop is repeated typically at least twice, assuming repeatability is observed and small deviation in depth readings achieved at both hub positions. The third depth loop, always starting at the same hub location involves observing to the 2 tripods at the pre-determined depth loop observation point and then to the second hub before closing out on the first hub. The final depth loop has observations taken along the seabed, at typically 5m intervals between the 2 hubs of both structures, incorporating both hub measurements as before. The metrology surveyor is to accommodate, through the use of the client provided’ Metrology Calculator’ the change of tidal information and the impact this has on the depth observations. Typically the depth loops are performed on the linear ‘flood’ or ‘ebb’ tide, to make allowances for a linear change in depth owing to tide. The depths are then reduced, subtracting the rate of change of tide over time.
- Braced Quad Acoustic Observations: The last of the four steps in any acoustic metrology is to measure the acoustic ranges between the 4 Compatts installed in the 2x tripods and both hubs, arranged in a robust braced quad geometric shape. The acoustic range between the 2 hubs is critical with minimal acoustic range error and low standard deviations. Acoustic line of sight is critical at this stage with errors seen acoustically caused by multipath, highly likely owing to the infrastructure on the seabed (ie Manifold’s, PLETs, Umbilical wet park frames etc).
Vertical Jumpers / Horizontal Spools / Inferred Metrology
To summarise, the most simple of metrologies in terms of data collection is a vertical hub to hub jumper metrology. The metrology surveyor populates the Hub of the structure (say PLET or Manifold) with an inclinometer Compatt. There are a whole host of stabs that are typically client specific and determined at the design stage to allow for the acoustic and inclination/depth observations, allowing the metrology surveyor to calculate the spool piece for the particular structures being measured.
Horizontal spools typically involve the use of a manufactured plate, fastened or referenced (inferred) to the mating face of the flange of the horizontal spool piece. This adaptive plate or inferred mating receptacle is ordinarily dimensionally controlled to millimetric measurement and rotation for the purpose of the metrology. The four basic metrology steps are still relevant, but different post processing steps are required to reduce the frame of reference of the mating face of the metrology tool to the mating face of the flange.
An inferred metrology suggests that the mating face of the flanges cannot be measured directly, either owing to access or orientation, so an alternative point of measurement is chosen on the structure to allow observations subsea which can be translated by offset and rotation back to the point of interest (ie flange mating face).
