DP station-keeping: the next level

The new standard, released in July 2016, applies to both newbuilds and vessels in service and includes methods for assessing the station-keeping capability of dynamic positioning (DP) vessels as DP Capability Levels 1, 2 and 3 plus two sub-levels for site-specific assessment. Simultaneously a free web app supporting DP Capability Level 1 was released on DNV GL’s Veracity platform. “Since the release, the standard and the free web app have been used by ship designers, vessel owners, consultants, oil companies, equipment vendors, research institutions and universities,” reports Luca Pivano, Principal Specialist DP Simulations at DNV GL. “In general, the feedback on both the standard and the free web app has been very positive. We have also been involved in quite a few customer projects where we performed thorough Level 3 analyses using our time-domain tool DynCap, which also is available as a web app,” he points out.

When Total started planning DP drilling operations in a harsh environment dominated by extreme ocean currents, they soon contracted DNV GL to evaluate the vessel performance using DynCap. They found that running time-domain simulations with DynCap for comprehensive dynamic operability analyses allowed them to better understand vessel performance and limitations. The simulation results provided valuable and reliable input for the operational risk assessment and planning. The DynCap results were also compared to full-scale data from trials performed with the drill ship Maersk Venturer off South Africa in particularly harsh environmental conditions, including high current speeds and large waves. Despite the uncertainties on the models and environmental conditions, the results showed a realistic match and the correct trends between the full-scale data and the simulations. The results were published at the MTS DP conference in Houston. “With this project, we managed to demonstrate that DynCap is an important addition to other available tools, offering unique insight into the details of a vessel’s station-keeping performance,” explains Luca Pivano.

A drilling contractor in the Gulf of Mexico purchased the DynCap web tool for its entire DP drilling fleet. The aim was to run in-house analyses to strengthen their position in bidding tenders and to have a reliable tool for operational and contingency planning. The feedback has been very positive. “The customer received valuable results from DynCap, such as vessel footprints and complete vessel motion analyses which cannot be obtained with traditional static methods, in addition to automatically generated reports,” Pivano points out. “As DNV GL can log on simultaneously with experts from our customers to support the analysis, results can be achieved much faster. DynCap runs in the cloud, which also reduces effort for IT maintenance,” he emphasizes.

An International Exploration and Production company commissioned DNV GL to provide DP station-keeping analyses using DynCap. This included an assessment of the DP capability and operability for diving activity of a DP Class 2 vessels with two bow thrusters forward, two main propellers with rudders aft, and a single stern tunnel thruster. The DP station-keeping performance and reliability of vessels with similar configurations has known to be characterized by poor robustness, particularly in post-failure condition upon loss of the single stern tunnel thruster. Such characteristics have been identified in papers published and available in the public domain. The study for the diving vessel was carried out according to the standard DNVGL-ST-0111 – Level 3-Site which defines requirements for DP station-keeping performance testing using running time-domain simulations.

“Time-domain simulations have provided a more robust method of estimating vessel performance than traditional quasi-static DP capability analyses, which have been unable to predict the dynamic station-keeping footprint and excursions that may ultimately compromise operations,” explains Pivano. For manned diving operation, the robustness of the vessel’s post-failure DP station-keeping capability is of primary importance. DynCap offers insights into the details of a vessel‘s station-keeping performance that were hitherto unavailable. The data and conclusions derived in that project provided valuable and reliable input to operational risk assessment and planning.

However, a first release typically offers opportunities for improvement, and many suggestions based on user experience were received. “For us it is also important to react to changing and emerging requirements and to adapt to the users’ needs. That is why we collected feedback and suggestions from the industry regarding both improvements for the standard and new features, especially for the Level 1 web app. Some challenges on how to apply and use the standard and web app have also been addressed,” explains Aleks Karlsen, Senior Principal Specialist – DP Systems.

A common request from the industry has been to make Level 2 and Level 2-Site DP capability calculations available in the web app on the Veracity platform to support vessel and project-specific calculations.

A new version of the app including such a feature has been released on Veracity this year. The new release also allows users to perform DP capability computations for vessel shapes other than monohulls, such as semisubmersibles and catamarans. Users will be able to apply site-specific environmental conditions, include external forces, vessel-specific environmental load coefficients and thruster forces in the same view. A feature comparing the performance of up to four vessels in the same view has also been incorporated.

Enhancements and improvements

The main feedback regarding the standard was related to the calculation of forbidden sectors in thrust allocation in the presence of a skeg. In certain designs, the resulting forbidden zone did not allow the actuator to produce any net force. In the new version of the standard the interaction between a thruster race and a skeg will result in a thrust loss factor in line with industry practice, delivering more realistic results. Other improvements in the revised standard include clarified definitions and formulations.

Another concern raised was the relatively low results for some existing vessels when compared to prior assessments based on older methods and standards. This may be attributable to the fact that the legacy standards were too flexible and not conservative enough. The new DP Capability standard aims to remove some of the flexibility regarding the assessment requirements, deliver more conservative results and provide a better tool for benchmarking vessel performance. It is difficult to claim that the new standard results in data that are closer to reality; however, it will ensure consistency of the applied method and the results needed for benchmarking (particularly for Level 1). In addition, it is well known that previous approaches often delivered overly optimistic results.

One very important and clear conclusion based on discussions and experience is that the responsibility of evaluating the assessment with respect to the actual DP capability and capacity needed rests with the user who should not base his decisions solely on the vessel with the highest DP capability numbers or limiting wind speed. “All these are major improvements in the standard and the web tools that will provide customers with an easier, more reliable and transparent process when assessing a vessel’s station-keeping capabilities,” Karlsen emphasizes.