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Collaborating Authors
Mooring systems
The offshore oil and gas industry is continuing to push intodeeper water and onerous environments, using increasingly complicated vessels and equipment. This, combined with changing global weather patterns has increased uncertainty in offshore production operations. Monitoring mooring lines can help reduce this uncertainty by calculating fatigue damage estimates during major storm events. This greater understanding helps to optimise inspection and maintenance schedules and assess the likelihood of future mooring line failures. Added to this, if the deterioration or failure of a single mooring line is not detected, increasing loads on the remaining lines may result in additional failures. This is regarded as a system failure and could lead to significant consequences for both well control and riser integrity, resulting in huge costs for operators. Unless the operator inspects on a regular basis or monitors the mooring system in real time it is impossible to know for certain whether all mooring lines are in place. A popular technique for monitoring mooring systems is tomeasure mooring line angle (using accelerometer based inclinometers), to infer theoretical mooring line tension. Whilst these systems are effective at alerting operators to a line failure, the fact that tension must be inferred requires a certain amount of conservatism in the estimate. This conservatism can be difficult to quantify. This paper documents the sea trial of a well proven mooring connector combined with new mooring line technology capable of measuring both mooring line angle and direct line tension. This allows for a unique comparison between the two measurements. This paper will detail the results of this comparison, showing the accuracy of inclination based monitoring systems and the level of conservatism factored in when tension is inferred. The data published in this comparison can be used to reduce levels of uncertainty in offshore operations and thus reduce future levels of conservatism in design and analysis models. This can help save costs and increase efficiency for future operations, whilst also helping support safetystrategies.
- Asia (0.95)
- Europe > United Kingdom (0.68)
- North America > United States (0.47)
With floating facilities increasing in size and tending to deeper and harsher locations; the mooring system design for environmental loading becomes ever more challenging. Higher average line tensions and fatigue loads can present uncertainty when assessing the capacity and qualification of the individual components. In 2011 and in response to a client request, SRP investigated the availability of compact subsea mooring connectors with high bending and tension fatigue capacity. An absence of results led SRP to explore the potential of cross-pollinating the technology from deepwater drilling and production riser systems to mooring systems. The reasoning being that steel risers can be subject to very high combined loading; weight limitations and fatigue life often motivate the designer to use high strength steels and efficient, fatigue resistant connections. This paper discusses the process and design considerations arising from adaptation of an existing and proven technology from one application (wellhead connectors) to a new application (subsea mooring connectors). The design verification and qualification test program being carried out in 2012 and 2013 are detailed as an example of a design team's interpretation of riser, mooring and new technology standards. The results of the qualification test program, including static and fatigue testing, will also be presented as an indicator of technical success of the strategy.
Description of the Proposed Paper: The paper describes a reliable, effective, and practical, solution for monitoring and reporting fatigue damage of lashing lines on the Murphy Kikeh DTU. The solution is based on a ton-cycle method that uses measured line tension and calculated number of cycles when the critical line segment enters and exits the sheave. The method is calibrated using real data and then implemented on-board of the DTU. It provides indication of the integrity endurance level of a critical lashing line. The same method helped in re-designing the original lashing system, which reduced wear and prolonged the fatigue life by a factor of four. Application: The integrity and the fatigue life of the lashing lines that keep TAD unit closely connected to the DTU is carefully and continuously monitored by the Integrated Marine Monitoring System. Fatigue life of a line is monitored through cumulative ton-cycle value. When this value approaches the calibrated threshold, the line can be replaced at the most convenient time, avoiding unpleasant surprises such as when a line breaks during production, making the production safer and more cost effective. Results, Observations, and Conclusions: The lashing line ton-cycle method has been integrated with the other marine and structural monitoring parameters on board of the Kike DTU for several years.. When the values approach the 100% of the absorbed fatigue life, the results are displayed on the operator screens in the control room indicating that the line is in a critical condition with a high expectation to break. This indication helps the operator to plan, prepare and conduct the line maintenance or replacement in a timely manner and at the most convenient time without interrupting critical phases of operation. This process became a standard process for the lashing line integrity monitoring and maintenance and it has been working reliably for more than three years. Significance of Subject Matter: Developed and implemented ton-cycle method shows a simple yet effective and reliable method for increasing overall the safety of operations, as well as preventing costly interruption of normal production on the Kikeh DTU and the nearby TAD unit.
- Well Drilling > Drilling Equipment (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Mooring systems (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Floating production systems (0.94)