November 2015

De-risking offshore installation

Chris Jones

Chris Jones

Our recent experience has shown that deck handling may actually prove to be a period of particularly high risk to a subsea product during installation. These complex operations are often engineered through the use of standardised calculations or through simplistic modelling. With ongoing advances in Finite Element Analysis (FEA) software, these operations can now be modelled in significantly more detail. This can mitigate risks prior to mobilisation and optimise operability during handling and installation. Such detailed modelling can remove unnecessary conservatism and potentially allow installation in increased weather windows.

There are a number of software programmes currently available on the market for this purpose. The analysis software allows the procedure to be visualised throughout the various stages of the procedure and clearly highlights stages where there is any increased risk to the product. The aim is to help define offshore procedures through installation analysis, create lay tables which detail the methodology of each activity, and assess environmental limits such as wave heights, winds and currents.

Security and confidence
Xodus’ experience shows that, in many cases, the majority of the installation process can actually provide relatively low risk to the product since the installation stages where the product is in the water are often relatively straightforward and simply involve lowering and laying the product. There is often a good margin for error in terms of tension and compression, and relatively small curvatures are often seen, meaning a wide range of payout and vessel position are possible without damaging the product or lay equipment.

However, closer examination and experience from various types of installation analyses have shown that higher risk can be caused during deck handling, initiation, landing and tie-in operations. This part of the analysis is often by-passed or greatly simplified. It is therefore imperative when considering this critical area of installation, that there is a comfortable boundary in terms of product design limits in order that no damage occurs during the installation, and where there is uncertainty, consideration should be given to modelling these key stages in increased detail. This gives security and confidence in the procedure and ensures the operation is as risk-free as possible when it is eventually carried out.

Xodus recently carried out a detailed installation analysis for a client in which the first end Umbilical Termination Assembly (UTA) was sea-fastened to the vessel deck and then picked up and lifted over the overboarding chute before being lowered into the water. The UTA and bend restrictors needed to be lifted above and clear of the deck tensioner (which controls the umbilical payout as it is being installed) before the umbilical could be engaged into the tensioner. A similar procedure was required when passing the second end from the deck, over the chute, and through the splash zone. After passing beyond the tensioner, and before lowering into the water, the UTA was effectively free swinging in the air with very little opportunity to control the motion (e.g. via taglines). The process was therefore modelled in detail across the different stages of the lift, starting with the UTA laying on deck and continuing past the UTA being lowered through the splash zone, and was indeed found to be the highest point of risk to the product throughout the whole installation procedure.

Modelling the various steps as the UTA is lifted off the deck and into the water.
Modelling the various steps as the UTA is lifted off the deck and into the water.

During the installation, the offshore team was also able to compare screenshots from the analysis models to the reality of what was actually occurring throughout the procedure. This helped to confirm that the correct angles, motions and responses were being attained and that the process wasn’t going ‘off track’. The images below show a comparison of the models and the actual procedure and confirms that excellent correlation was achieved.

Comparison of the models and the actual procedure (photos courtesy of Ocean Installer).
Comparison of the models and the actual procedure (photos courtesy of Ocean Installer).

The environment in which an installation procedure is carried out can directly impact the success of the operation. Typical environmental criteria can include the sea state (wave height and period), wind speed and water currents. A key part of the installation analysis is to determine the allowable environments in which the operation can be carried out safely. Furthermore, modern software is able to represent the wave kinematics with ever increasing sophistication, such as the modelling of irregular waves, wave spreading, and vessel shielding. This enables the analyst to provide a more realistic picture of wave conditions. This will enable a more accurate response to give confidence in the lift and reduce risk to personnel, the vessel or the product.

New applications for detailed modelling
There are a number of operations which can now be modelled including:

  • Modelling load out and deck handling operations, including much greater detail of deck equipment, e.g. handling of equipment from reels or carousels over chutes and through tensioner
  • Detailed dynamic analysis of lifting through the splash zone
  • Complex subsea operations, such as upending and orientation of equipment using cranes and winches
  • Landing and connection to subsea equipment (e.g. mating of diverless connections)
  • Hinges and articulations, such as those seen on pipeline end terminations (PLETs) which can have significant effect on dynamic response
  • Rigging and lifting equipment, in as much detail as desired, e.g. including individual rigging components and the shackles connecting them together
  • Lift analysis of rigid spools and rigid risers (including modelling of the stresses within the pipe during the lift)
  • Full dynamic modelling of flexible and umbilical pull-ins through J-Tubes.

Analysis software capabilities are now becoming more wide-ranging. Though this involves increased complexity, the user experience is becoming easier and more detailed results are being achieved. This can generate better efficiency in the overall handling and installation of flexible pipe and umbilicals offshore and the resulting procedures. It can therefore markedly reduce risk and optimise operability during installation. The outcomes of such global analysis modelling are also extremely useful for future learning and application as more and more offshore operations are being carried out in hazardous and remote environments.