The challenge

Houston headquartered engineering specialists Trendsetter Vulcan Offshore (TVO) were set just such a difficult deepwater intervention challenge. They were brought in to help an operator rectify a completed wellhead at a deepwater site that had suffered from misalignment. The wellhead, which was in more than 2,000 m water depth, had been bent and there were concerns other components might have been damaged.

TVO were tasked with finding a solution to bringing it back to a vertical position. This would allow intervention access to the well for remediation and abandonment operations, so it could be permanently sealed. A key challenge was to carry out this work extremely carefully and in a controlled manner, in order to prevent any further potential damage to the wellhead.

The solution

TVO came up with an innovative solution to the problem. It proposed a ring of six subsea tensioning systems, mounted on suction piles positioned in a ring on the seafloor around the wellhead. Each one would be connected via a rope to a Trendsetter-supplied lower riser package (LRP) installed onto the wellhead. By carefully tensioning the ropes, they would be able to then bring the wellhead back into an upright position.

To do this in a safe and controlled way, TVO required a monitoring system which would provide:

• Near real-time feedback of the tension data at each of six load pins installed on the LRP (one each for each of the six tensioners).
• Near real-time inclination data from the wellhead itself

Conventional deepwater intervention solutions

Traditionally, inclination data is acquired by visually checking subsea bullseye levels attached to a structure using a diver, or, in this water depth, a remotely operated vehicle (ROV). Similarly, visual displays on the tensioning systems would be the source of the tension data, also read using an ROV.

However, these techniques add a significant amount of time, given the 30-40 m radius of the circle of tensioning systems, even if, as was the plan, only two would need to be actively tensioned during the righting operation. Bullseyes can also be mis-read.

Another alternative option is to deploy battery powered accelerator and inclinometer measuring bottles, using an ROV. But these would have to be retrieved to the surface after each measurement, so would also mean a lengthy operation, potentially taking days.

Taking an alternative, SMART, real-time intervention approach

Instead, TVO proposed and successfully deployed a wireless, intelligent, digital underwater monitoring solution. This provided near real-time inclination and tension data to the operations team at the surface (around three minutes delay). It comprised of two of Sonardyne’s Subsea Monitoring, Analysis and Reporting Technology (SMART) and six Compatt 6 transponders.

The internal inertial measurement unit in each SMART was used to calculate pitch and roll data from two points either side of the LRP, allowing TVO to calculate the wellhead’s bend angle in the local coordinate frame.

This data was then transmitted acoustically, using the SMART’s internal modem, to the surface at three-minute intervals. The Compatt 6s were interfaced with the load cell shackles at each tensioner point on the LRP and transmitted the tension readings to the surface at less than one-minute updates.