The effect of moonpool geometry on a ship hull’s calm water resistance was investigated numerically using fully viscous CFD simulations with 2D and 3D computational domains. Simulations were performed using a Series 60 hull form with a block coefficient of 0.65 as the base case. The effect of changing the cross-sectional shape of the moonpool recess was examined by comparing the predicted resistance of the original hull with the result of the model fitted with various moonpool configurations. The kinematics of the separated flow within the moonpool and downstream of the underside of the moonpool recess revealed a relationship between the length of the recess and added resistance. Additionally, the impact of varying the recess height was investigated by comparing the free surface elevations between simulations. It was determined that shallower recesses increased the amplitude of the moonpool wave motions and therefore the overall vessel resistance. It was also determined that 3D effects were necessary to adequately capture moonpool hydrodynamics.
Scale buildup due to water production can choke oil production and require repetitive scale treatments across entire fields. In subsea wells, the common solution employs a deepwater rig to conduct either workover operations or large-volume scale inhibitor squeezes. Less frequently, coiled tubing (CT) is used from a moonpool vessel. However, current oil prices required a custom solution for subsea well treatments that was more cost effective than either a rig or a moonpool vessel.
Similar previous operations successfully used 1 ¾-in. and 2-in. (44.4 mm. and 50 mm.) CT at the same time from a moonpool vessel. A remotely operated vehicle (ROV) in the open water connected the CT to the subsea safety module (SSM) through a dynamic conduit and connected the SSM to the wellhead. An engineered solution to change to 2 7/8-in. CT and use high-rate stimulation pumps was planned to deliver subsea treatments at up to 15 bbl/min. The equipment layout was designed for a multipurpose supply vessel with chemical storage tanks; to increase the available selection of vessels, the CT was designed to run overboard rather than through a moonpool.
This project was initiated after accelerated scale buildup occurred because of a pressure decrease close to the bubble point, which happened when the drawdown was increased for aggressive production targets. To effectively inhibit scale in this environment, treatments required thousands of barrels of inhibitor. For wells with more-severe scale conditions, acid treatments were planned. These treatments were delivered with one complete CT package, stimulation pumping fleet, and subsea equipment, which were all installed on the spare deck space of the available vessel.
A custom overboard CT deployment tower was designed. The new tower improved the dynamic bend stiffener (DBS) placement, which allowed the clump weights to be deployed with the bottomhole assembly (BHA) and simplified the rig-up. The chosen vessel worked well for the operation; however, the equipment layout and the local weather conditions combined with the response amplitude operator (RAO) of the vessel shortened the projected fatigue life of the CT.
CT integrity monitoring with magnetic flux leakage (MFL) measurement was introduced here, and the vessel’s motion reference unit (MRU) provided an input to a fatigue calculator, based on the global riser analysis (GRA). The measurements and the analysis were utilized successfully to prevent CT pipe failures in the open water and deliver the required well treatments. To allow further improvements in deepwater operations, the new engineering work-flow was carefully documented.
In recent years, large diameter internal turret systems are increasingly becoming a viable solution for large FPSOs and FLNGs in harsh environmental conditions, because of higher vertical load capacity and flexibility in the riser number. The large volume of water in the moonpool introduces unique design challenges in extreme operating conditions, due to complex interactions between the water and the turret structure. The water trapped inside the moonpool exerts considerable dynamic loads on the turret and moonpool structures. These loads are combined with other environmental loadings and need to be considered in the design of the system.
In the present work, computational fluid dynamics (CFD) models were created for simulating complex entrapped water dynamics in a generic large turret design. The CFD model in this study used fully 3-D Reynolds-Averaged Navier-Stokes (RANS) equations and the
The CFD simulation started with grid and time step sensitivity studies for an open moonpool. The CFD model was further validated with free decay cases by comparing the results with empirical estimates. After that, a chain table and a turret system were added into the model to perform free decay cases and forced motion cases. That CFD model was also assessed with the vessel motions in regular and irregular waves. Simulation results revealed sloshing and piston modes and natural decay rates in the moonpool, as well as temporal and frequency characteristics of water motion in response to wave forcing.
The CFD simulations clearly captured the effects of the moonpool, the chain table and the turret structures on the dynamics of entrapped water. The simulation results offered valuable insights into the flow patterns of entrapped water in the moonpool as well as the resultant dynamic loads, which are crucial to the moonpool and turret design.
Edward, Ceasar (Newcastle University in Singapore / Frigstad Engineering Pte. Ltd.) | Deshmukh, Rahul (Frigstad Engineering Pte. Ltd.) | Prampakov, Velizar D (Arun Dev, Newcastle University in Singapore)
Offshore drilling for exploration and scientific purposes in deep waters and subjected to harsh environments due to high current or extreme waves, or a combination of both has been a challenge for decades and lacks a complete and robust solution to allow reliable and continuous drilling operations. A unique riser restraining system is presented in this paper for improving the operability windows of drilling rigs in harsh environments. The restraining system restricts transverse deflection of the risers due to high current drag forces and coupled dynamic motions due to extreme waves that cause potential clashes with the moonpool. The restraining system proposed in this paper uses a modified tensioning ring to act as a centralizer, which is connected to hydro-pneumatic tensioners in the lateral plane for providing retention resistance and attached to removable support structures fixed to the moonpool decks. The customized upper and lower restraining joints are used for interfacing the riser system with the centralizing tensioner ring along with treble slips joint, intermediate flex joints and PUP joints to ensure smooth load transfer and reduce stress concentrations on the connection interface.
A thorough review was carried out on some of the potential offshore drilling locations that are exposed to extreme environments. These assessments were used to generate wave and current combination and three different water depths were defined to establish the design basis for analyzing the system. Numerical analysis using time domain simulations were carried out to check the global and local strength integrity of the system. The results were utilized to benchmark the improvement in operability of the riser system with the restraining solution as compared to conventional systems with and without fairings. Simulations with the proposed restraining system show that the exceedances of key parameters like flex joint angles and riser clashes with moonpool deck are reduced to significant levels while maintaining the stress envelopes within allowable limits.
Generic modifications required for implementing on existing drillships and semisubmersibles are also highlighted. The overall global and local strength integrity of the system and significant improvement in operability window are presented to demonstrate the benefits of the proposed system.
AbstractThe offshore drilling market is pushing for more cost efficiency, more efficient drilling packages and more safety and redundancy, but also have the desire to be able to drill and develop more complex wells in deeper water. New concepts and designs will be needed to meet these challenges and make drilling complex and deeper wells commercially attractive.A rig design solution to provide increased drilling efficiency is to set up the rig for full dual drilling featuring two fully equivalent well centers. This will combine the benefits of dual activity with additional rig flexibility and redundancy. In the existing configurations well centers are mostly not equivalent, as for example with the commonly used main or auxiliary hoist, specific operations are be limited to only one well center. When aiming for full interchangeable well center functionality, at all times full accessibility to both well centers is required, at drill floor level as well as in the moonpool. This arrangement does not exist on earlier or contemporary drillship designs. In general a drillship features ample deck space, but the vessel deck areas and the well centers are longitudinally arranged. This causes the forward well center to block access from the forward deck area to the aft well center, and vice versa.By breaking with traditional naval architectural thinking patterns, an innovative approach to the drilling process in a moonpool arrangement is introduced. It comprises a transversely oriented moonpool, named the Euryale moonpool concept. The Euryale concept allows the well centers to be transversely arranged, and thus providing the right accessibility to both well centers on the drill floor as well as in the moonpool from forward and aft deck areas, ensuring all advantages of full dual drilling and full redundancy on a drillship.The (patent pending) Euryale moonpool features not only full dual drilling accessibility, it also mitigates sloshing motion in the moonpool during transit due to the significantly reduced length of the moonpool. The calm water in the moonpool not only creates a safer working environment for high value equipment and personnel, it saves fuel cost and reduces emissions as well. The Euryale moonpool is integrated in the ship's structure ensuring the overall integrity of the drillship, and does not contain any failure sensitive underwater mechanical parts nor protruding parts in the moonpool that might impose damage on the deployed riser.A patent has been applied for the Euryale moonpool.
Guo, Xiaoxian (Shanghai Jiao Tong University) | Lu, Haining (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration(CISSE),) | Yang, Jianmin (Shanghai Jiao Tong University) | Peng, Tao (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration(CISSE),)
As the exploration of offshore oil and gas resources are moving into larger depth, many mobile drillships have been widely applied to the offshore drilling operation, thanks to the good mobility, large variable deck loads and storage volumes. Large motion responses of the drillship and the water motions inside the moonpool were reported, which introduced safety concerns. In this paper, both numerical simulations and physical experiments have been carried out to investigate the hydrodynamic performances of a newly designed deep-water drillship, and the resonant water motions inside the moonpool with submerged recess structure. The three modes of water motions coexist in the moonpool even when there is only one external excitation source provided by a regular wave train. The submerged recess structure clearly introduces the obstacle effects leading to exacerbation of the water motion inside the moonpool.
Moonpools are designed as vertical openings through the decks and the hull structures to support the required underwater operations on marine vessels and offshore platforms. The water inside moonpool is required to provide a moderate environment for operation rather than being exposed to the violent metocean conditions. However large water motions may occur inside the moonpool under some resonant conditions, including the vertical piston motions, and the longitudinal sloshing motions. The effects on 6 degrees of freedom (DOF) RAOs of the drillship which are caused by moonpool also need to be investigated. Recently, the recess type moonpool appears in drillship’s moonpool design, which has advantage on drilling equipment arrangement. The impacts introduced by the recess type structure on relative water motions inside moonpool need to be clarified.
Fukuda (1974) carried out a set of physical experiments to investigate the water behavior inside the moonpool and the effects on the vessel motions in a towing tank. He obtained an empirical formulation which could be used to identify the resonant frequency of the water motions. A mathematical model was developed later to describe the water behavior inside moonpool , and also corresponding model tests with respect to the damping mechanisms and motion behavior were conducted (Aalbers, 1984). Molin (2001) treated this problem in the framework of linear potential theory under the assumption of a infinite length and beam of a barge equipped with a moonpool, and brought out the oscillation natural frequency formulation. It was believed that the water motions response inside the moonpool is overestimated with respect to the experimental results without considering the fluid viscosity (Kristiansen and Faltinsen, 2008). Kristiansen and Faltinsen (2008) set up a fully nonlinear numerical wave tank coupled with an inviscid vortex tracking method to investigate the impacts caused by fluid viscosity and the nonlinear effects that associate with free surface. More recently, computational fluid dynamics (CFD) has been utilized to study the sloshing phenomenon. Zhang (2012) applied Moving Particle Semi-Implicit (MPS) method to study the sloshing phenomenon. Large impact pressure was observed, and it was shown a periodic impact had two pressure peaks in each period.
Ma, Peifeng (Keppel Offshore & Marine Technology Centre) | Xing, Xiuqing (ABS) | Yan, Deguang (ABS) | Chien, Hung-Pin (ABS) | Lee, Sing-Kwan (ABS) | Gu, Hai (ABS) | Xu, Haihua (Keppel Offshore & Marine Technology Centre) | Choudhary, Ankit (Keppel Offshore & Marine Technology Centre) | Hussain, Anis (Keppel Offshore & Marine Technology Centre) | Merchant, Aziz Amirali (Keppel Offshore & Marine Technology Centre)
During transit, a drillship moonpool creates added resistance that can be more than 50% of the total resistance and hence significantly increase the fuel consumption. This paper reports a real case study for reducing moonpool added resistance by model tests and Computational Fluid Dynamics (CFD) simulations. Model test data show that, unlike conventional ships that experience nearly constant resistance, a drillship resistance presents largely fluctuating behaviour. In CFD validation, the physics based CFD best practices are applied for resistance predictions. The largely fluctuating resistance is well captured in simulations. It is found from flow visualizations of the large amount of CFD simulation results that the high moonpool induced added resistance is mainly attributed to the vortices shed from the moonpool front wall, which enter into the moonpool and impinge on the rear wall. The CFD predicted mean resistance are in good agreement with the model test data, within 3% difference for a wide range of speeds. The well validated CFD tool is applied to study the effects of moonpool dimensions on the added resistance. The results of parametric study reveal a design principle that smaller moonpool dimension results in smaller added resistance. Through comprehensive CFD parametric study, a universal design principle and CFD best practice are established for industry applications.
Deepwater drilling is under continuous development with ever evolving design requirements for rigs as a consequence. The need to drill deeper and longer wells, in always increasing water depths, and to perform well completions in always more complex wells has had its effect on the rig functional demands and has led to bigger sized and heavier equipment and consequently larger ships. These new rigs need to be ready to apply the new and advanced drilling techniques and to keep up with the growing safety standards.
An analytical approximate approach for determining periodic solutions and identifying parametrically nonlinearity of sheltered riser vessel (SRV) motion equations involving three degrees of freedom is investigated. The cubic stiffness and square damping involved nonlinearity describing both of sloshing and piston phenomena of SRV motion are taken into account in motion equations. The iteration scheme of motion is established by incremental harmonic based method. The nonlinear parameters are identified according to deduced iteration scheme, which shows good convergence with numerical solution. Utilizing the developed IHB based method, the nonlinear parameters involving in motion of SRV are identified. The developed parametrical identification methodology exhibits good computational effectiveness and can be utilized in strong nonlinear and multi degrees of freedom occasions.
"PROXIMA" is an Ultra Deep Water Drillship concept designed for the future, sporting a revolutionary and innovative drilling rig based on two independent towers integrated in a very performing and efficient platform. The concept follows the experience matured in 2013 during the design development of the Ultra Deep Water Drillship "OVERDRILL", which represented a natural evolution compared to the present drillship fleet exploited through the deep integration between the hull and the traditional drilling system. On this new project are maintained all the positive results achieved with the "OVERDRILL" philosophy, in particular in the integration process, but the traditional lattice structure derrick is substituted by a completely new drilling rig based on the double tower concept which creates the premises for a general improvement of the unit performances in general. The main targets of the project are: - To design a completely innovative drillship anticipating the future needs for offshore ultra deep water exploration - To answer to most of the requests/expectations expressed by operators of the sector during the world wide presentation of "OVERDRILL" project in 2013 improving safety and efficiency of drilling operations.