Dropped object assessment

Client:
Operator
Year:
Business Unit:
Consultancy

A conductor falling at terminal velocity and impacting the jacket is the last thing anyone wants to see or hear about.

That is why our client approached us to perform a dropped object assessment for one of their conductors with critical integrity concerns. The client required an urgent assessment of the risks posed to the structural jacket and vulnerable subsea infrastructure if the conductor were to detach and fall to the seabed. This client routinely manages ageing North Sea infrastructure and has to make informed decisions on how to best proceed where risk and cost are both considered.

Apollo has extensive knowledge and experience in dropped object risk assessments and by combining our pragmatic approach with our automated tools and processes, we were able to react and deliver for our client quickly.

We delivered an initial high-level assessment which summarised the energy of the dropped object, indicated the vulnerable equipment and structures that could potentially be impacted, and the possible damage that could result. We then followed this up with a technical report, formalising the study.

The Problem

  • A conductor is at risk of detaching and falling to the seabed due to the potential failure of a repair clamp, initiated by environmental loading.
  • If the conductor does detach then it has the potential to impact the subsea infrastructure or the jacket structure resulting in time and cost implications to the client as well as a safety

Our Solution

  • We reviewed the information provided to identify the detached conductor size, drop path and vulnerable equipment/structures.
  • Given our extensive knowledge and pre-existing standard calculations, we were able to perform a dropped object impact assessment following the methodology given in “DNV-RP-F107 Risk Assessment of Pipeline Protection” in a reduced time and cost whilst ensuring a fit for purpose response with good engineering. This enabled our client to act quickly.
  • Provided the client with an initial high-level summary of the results as they became available, this allowed the client to stay up to date with their asset as if we were an extension of their own team. Our engineers are routinely in direct contact with our clients to help project run efficiently.
  • Delivered a comprehensive technical report to the client formalising the findings of the assessment. This included the subsea infrastructure impact probabilities and resulting consequences, and identification of vulnerable jacket members and an assessment of the local damage caused by impact so that the client had all the consequence information on their asset
  • It was found that if the conductor were to directly impact a jacket member, then it would cause local damage, significantly denting the impacted member. It was therefore recommended that a review of the jacket redundancy be performed to assess how critical damage to a single member would affect the overall integrity of the jacket.

And for all you engineers who want more details:

The methodology

Details of the conductor section at risk of detaching and the vulnerable subsea infrastructure were determined using the information provided by the client. The methodology provided in DNV-RP-F107 was applied to determine the drop path of the conductor. A range of concentric circles drawn at 10m intervals, centred on the conductor’s initial location, were marked onto a plan view of the platform and shaded based on the probability of the conductor landing within each area. From this, the probability of the conductor impacting the identified subsea infrastructure was calculated.

Utilising experience, we were able to rapidly turn around the results and provide the client with the answers they were urgently seeking.

The solution

We provided the client with an assessment of the impact of a detached conductor with the subsea infrastructure. This analysis modelled the conductor detaching and accelerating to the seabed as it fell through the water column. The kinetic energy of the falling conductor was determined and used to assess an impact with the subsea infrastructure and jacket members. The elastic deformation of a pipe was calculated for a potential collision and used to determine the impact severely were a worst-case scenario was a full rupture leading to a loss of containment. Impacts with jacked members considered local member buckling. Other subsea infrastructures, such as umbilical’s and flexibles were also considered.

The fall excursion path of the conductor was also determined, where the potential distance it could travel from its start point was calculated. This allowed the probability of a range of impact scenarios to be assessed. The results were provided to the client as clearly marked up layout drawings, where concentric circles indicated the likely excursion limits.

Presenting the results in a readily understood form allowed our client to quickly assessed the situation and make an informed decision on how to manage their assets.

What did the client think?

The client was very complimentary at the speed we turned the job around, it allowed them to make a swift and informed decision on their asset.

This has led to introductions to other parts of the clients business and allowed Apollo to become a trusted engineering partner to deliver quick and accurate engineering answers.

If you have a pressing issue and need engineering support, let us know on consultancy@apollo-oe.com

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