Accurate offshore dimensional control is essential for a variety of offshore projects including setting of equipment, damage assessments, modification, refurbishment, and integration of structure, piping, and other components.
Traditional offshore measurement methods include use of physical tape measures and drawings with field verifications or as-built drawings. 3D point cloud scanning with lasers is now commonly used throughout the industry but does not provide real-time feedback, requires powerful computers for post-processing, and is only accurate to +/- 2mm (varies depending on the machines used).A more precise and accurate method of measurement is use of a total station device with auto-leveling features turned off. This method is accurate, portable, and versatile, and it is also more cost effective than traditional 3D point cloud scanning. Placing control point targets around the objects of interest allows an engineer with a standard computer to quickly take measurements from multiple locations and then reference them into a single global coordinate system. Advantages of this method include real time feedback of measurements, means to measure dangerous areas from a safe position, increased accuracy, and increased precision. Total stations are a common sight in ship yards, but they are usually operated by a field technician (not a surveyor and not an engineer). Total stations are not commonly used offshore since they require special consideration for use. They also require a small amount of post-processing in a spreadsheet to be relatable to global coordinates.
This article will discuss and compare current options for offshore dimensional control and describe the use of the total station method for offshore work. The article will also present several case studies, drawing from offshore dimensional control projects, where use of a total station benefited the project beyond the traditional methods of offshore dimensional control.