LESSONS LEARNED

Well decommissioning

• Front end work is relatively cheap and worth pursuing that extra mile. An accurate review of well construction records is essential for planning well decommissioning operations; Well production records are not always accurate or reliable and despite obtaining well construction records surprises did still occur, so early well access essential for effective planning;
• Contingency planning is essential for efficient abandonment operations and ongoing dialogue with the Functional Wells Group, Regulator and Well Examiner allowed the exploration of multiple “what if?” scenarios;
• Issues with obsolete wellheads and aging tooling are likely and should be planned for, because they will occur;
• Batching operations were an efficient way to perform the well decommissioning operations with good support from the drill rig personnel with respect to multi-well centre. Continuous operational reviews led to a relatively late decision to batch 13-3/8” recovery, but this proved an efficient option.
• Flexibility to suspend well operations and skid to another well centre was essential for efficient operations;
• Previously normally pressurised Chalk became recharged over time, possibly by Zechstein bleed off.

Facilities element

• ST-1 was the first of Spirit Energy’s platforms to be decommissioned and at handover from operations to the decommissioning the current status of the platform could have been better.
  Spirit Energy has now developed a Hazardous Area Declassification process recognised that there are advantages in handing over the facility from operations to the decommissioning project during late life operations rather than waiting until production has ceased;
• During pipeline cleaning several issues arose concerning the filtration system, oil in water sampling, including mistakes when dealing with the cleaning pigs and the initial wave of pipeline effluent during the cleaning process. This meant that the pipeline cleaning effluent being taken to shore in tote tanks rather than dealt with using the filtration system. As a result, we would recommend:
• Dispose of pigging fluids to a donor well where possible, but otherwise ensure that the first wave of pipeline effluent bypasses the filtration system and directed towards tanks to remove potential contamination issues;
• When performing the oil-in-water analyses using the temporary analysers the results were found to be inconsistent. Therefore, use the platform oil-in-water analysers as these will have been calibrated more consistently.
• Ensure that valves that will be required to maintain isolation of pipelines and plant while the installation has been put into ‘warm suspension’ have been operationally tested and maintained, and use a HAZID process to determine the need for pressure monitoring with a bleed-off facility;
• In previous decommissioning projects concrete mattresses had been successfully recovered using what is called the ‘curtain lift’ method which involves recovering the mattresses using the hoops on just one side of the mattress. It was expected that this method would be used again. However, in this case the mattresses were ‘link-lok’ type mattresses held together with smaller diameter (8mm) polypropylene rope. None of the mattresses could be lifted using the ‘curtain lift’ method and they were recovered to subsea baskets being handled multiple times. The lessons to be learned from this are:
• Carry out a trial lift using the method planned for the activity;
• Consider ‘what if’ scenarios, put contingency measures in place, and assume that they will be used.
• As is often the approach in executing decommissioning activities offshore there was flexibility built into the platform removal schedule, noting that well decommissioning and pipeline subsea disconnection activities needed to be completed before the platform could be removed. The idea is that the work could fit into a window of opportunity given by the removal contractor;
• In this instance a two-year window was agreed and as a result the project went into hibernation to reduce management costs. However, this proved to be challenging and probably was not helped by a change in the crane vessel being used, contractor’s resources being stretched and giving insufficient time to prepare for the offshore works. The lessons to be learned from this are primarily to ensure that the contractor has enough resources, and gives enough time to plan the work effectively and to carry it out;
• The preparatory works required the removal of paint, and some of the paint had been manufactured using the compound Cr(VI) that is a known carcinogen. In this instance, the company considered that preparatory work was poor. Lessons to be learned from this would include ensuring that the requirement for health monitoring are fully understood and prepared for prior to mobilisation, possibly involving a third party to verify preparedness of the contractor;
• Removal of the jacket involved excavating sediment from the pile guides. The removal contractor had installed the jacket several years ago, so should have been familiar with the requirements and should have been fully prepared, having analysed the geotechnical aspects, sourced the necessary excavation equipment, and have conducted field trials. Lessons learned from this are that the removal contractor should spend the time to be fully prepared for the work, if necessary, using a specialist contractor with a proven track record;
• Third party pipelines were connected to the Markham ST-1 platform and both a pipeline and umbilical needed to be severed before the platform could be removed. As the decommissioning programme had not been approved the work was limited to just cutting the pipeline and umbilical without remedial work. However, it was necessary to demonstrate that the 500m zone
  was clear of snagging hazards. Despite best endeavours the overtrawl demonstrated that a snagging hazard remained due to the third-party pipeline and understandably the NFFO could not issue a clean seabed certificate. The lesson learned from this would be to try and ensure that any third-party infrastructure can be dealt with as part of the same campaign so that the demonstration of a clear seabed can continue unhindered.