Marine Growth on North Sea Fixed Steel Platforms: Insights From the Decommissioning Industry

It is reputed in the Oil & Gas industry that marine growth is overestimated in the North Sea. Can we quantify this overestimation to better tackle the challenges marine growth incurs? This paper provides insight into the intrinsic and diverse nature of marine growth. It documents the biofouling development and the factors that affect it globally. Focus is made on fixed steel platforms in the North Sea approaching the end of their economic lives. A methodology to quantify the weight of marine growth is proposed and illustrated. 28 years after its installation, the weight of marine growth accumulated on Valhall 2/4G jacket located in Southern North Sea is evaluated based on the proposed method. It is confirmed that the current regulations overestimate the weight of marine growth in the North Sea.

Advantages of Using a Time-Domain Approach for Dynamic Positioning (DP) Pipelay Studies

Normally, the DP capability of a vessel is calculated through the use of static force equilibrium programs in which the dynamic effects are either not taken into account or taken into account by empirical load amplification factors. However, competitive and safe S-laying of large diameter pipelines in deep waters lead to large and long pipe lay vessels for which DP requirements are demanding. The power/propulsion requirement of the vessel needs to be considered from an early stage especially when accounting for the pipe laying equipment demands. This imposes a need for detailed dynamic analysis of the lay vessel. This analysis needs to include the slow drift oscillations counteracted by DP and the analysis in entirety needs to ensure the pipe string integrity is maintained. To this purpose Saipem developed in-house a time-domain simulator (FIPLA – Fully Integrated Pipe LAying) that employs all environmental forces (i.e. wind, wave, current) as well as the pipe tension on the vessel. It is used to assess the DP performances and laying capabilities of pipe lay vessels in harsh dynamic environments as well as critical areas for operations. The software can also be used to assess the performance of the vessel in case of failure of thrusters, power generators or bus bars, study DP Control System improvements, and assess the interaction between vessel, tensioner and pipe in deep and shallow waters. This paper focuses on the advantages of using dynamic simulations as an enhancement to the static DP capability charts, to produce detailed information for the DP Operator (DPO) in terms of setting of the DP parameters and to analyze critical laying events. This information can be used together with the weather forecast and can help getting the best performance out of the DP system in harsh environments, reducing downtime, improving operability and ensuring a safe operation.

Consideration of Epistemic Uncertainty in Extreme Wave Height Estimation

Extreme value estimation of significant wave height is essential for designing robust and economically efficient ocean structures. But in most cases, the duration of observational wave data is not efficient to make a precise estimation of the extreme value for the desired period. When we focus on hurricane dominated oceans, the situation gets worse. The uncertainty of the extreme value estimation is the main topic of this paper. We use Likelihood-Weighted Method (LWM), a method that can quantify the uncertainty of extreme value estimation in terms of aleatory and epistemic uncertainty. We considered the extreme values of hurricane-dominated regions such as Japan and Gulf of Mexico. Though observational data is available for more than 30 years in Gulf of Mexico, the epistemic uncertainty for 100-year return period value is notably large. Extreme value estimation from 10-year duration of observational data, which is a typical case in Japan, gave a Coefficient of Variance of 43%. This may have impact on the design rules of ocean structures. Also, the consideration of epistemic uncertainty gives rational explanation for the past extreme events, which were considered as abnormal. Expected Extreme Value distribution (EEV), which is the posterior predictive distribution, defined better extreme values considering the epistemic uncertainty.

North Sea Mooring Systems: How Reliable Are They?

In recent years a number of high profile mooring failures have emphasised the high risk nature of this element of a floating structure. Semi-submersible Mobile Offshore Drilling Units (MODUs) operating in the harsh North Sea environment have experienced approximately 3 mooring failures every 2 years, based on an average population of 34 units. In recognition of the high mooring failure rates, the HSE has introduced recommendations for more stringent mooring strength requirements for units operating on the UK Continental Shelf (UKCS) [17]. Although strength requirements are useful to assess the suitability of a mooring design, they do not provide an insight into the question: what is the reliability of the mooring system? This paper aims to answer this question by evaluating failure statistics over the most recent decade of available data. Mooring failure rates are compared between the Norwegian Continental Shelf (NCS), the UKCS, and with industry code targets to understand how overall reliability is related to the strength capacity of a mooring system. The failure statistics suggest that a typical MODU operating in the UKCS would experience a mooring line failure in heavy weather approximately every 20 operating years. This failure rate appears to be several orders of magnitude greater than industry targets used to calibrate mooring codes. Despite the increased strength requirements for the NCS, failure rates do not appear to be lower than the UKCS. This suggests that reliability does not correlate well with mooring system strength. As a result, designing to meet the more rigorous HSE requirements, which would require extensive upgrades to existing units, may not significantly increase mooring system reliability. This conclusion needs to be supported with further investigation of failure statistics in both the UKCS and NCS. In general, work remains to find practical ways to further understand past failures and so improve overall reliability.

Non-Linear Wave Runup Along the Side of Ships Causing Green Water Problems: Experiments and First CFD Calculations

Large relative wave motions along the side of a ship can lead to green water on the deck. With a simplified test setup of a thin plate under an angle with the wave direction (to separate non-linear wave run up from motion effects), the non-linear wave reflection along the side of ships is studied in the present paper. These pilot tests with regular and irregular waves gave new insight in the process of non-linear wave run up with plunging and spilling breakers close to the plate. The complex processes observed made clear that linear or second order models will not be able to predict this behavior accurately. Previously [1] it was concluded that CFD methods that allow wave breaking are necessary for a prediction of these important effects. In the present paper a first pilot study is presented with an improved Volume of Fluid (VoF) Method. It is concluded that the method is in principle able to present these relative wave motions, but that a finer gridding is necessary to study the detailed flows.

Lifting Operations for Subsea Installations Using Small Construction Vessels and Active Heave Compensation Systems: A Simulation Approach

Sub-sea installation operations require a high level of accuracy and control in order to avoid misalignment and possible collisions between modules on the sea bed. To reduce costs, smaller and lighter construction vessels are now performing these operations. The most critical parts of the operation are lift-off from the deck, passing through the splash zone, and landing sensitive equipment on the sea bed. The hazards are: high dynamic loads, resonance effects, and slack line snap. Therefore, in this study, modeling and simulation are applied to optimize design parameters and develop operational procedures for each operation to reduce risk of failure. Further, the same models can be used in operator simulator training. Modeling and simulation of interactive multi body systems is a rather complex task, involving the vessel as a moving platform, lifting equipment such as cranes and winches, guiding devices, lifting cables and payload behavior in air, all while partly to fully submerged. It is a multi-physics problem involving hydrodynamics, mechanics, hydraulics, electronics, and control systems. This paper describes an approach to link the different models to simulate the operations including interactions between the sub-systems. The paper focuses on the modeling approach used to connect the various dynamic systems into the complete operating system. The work is in its initial phase, and some of the sub-systems models are not complete. The models are described in this paper and will be included in future work. Some initial operational examples are included, to show how the models work together.

Semi-Active Hydropneumatic Heave Compensator

A hydropneumatic heave compensation system and a semi-active control are proposed according to the requirements drilling offshore of 6 km deep. The goal of this semi-active control is to maintain an acceptable performance of the system when the drill string mass changes, which also changes the performance of the hydropneumatic system. The control action is executed just by a servo valve, which modifies the system damping to optimize the compensator performance. This servo valve is the only moving part of the control system and therefore, this system is more robust regarding mechanical/ electrical failures and it has low consumption energy. The results show a satisfactory frequency response of semi-active control when the drill string mass is changed.

Long Pile Upending Lifting Study for Fixed Platform on Offshore Oil and Gas Field

Fixed platform have been widely employed in the offshore oil and gas reservoirs development. Pile foundation reliability is critical for these platforms where drilling, production and other functions are integrated. The lifting operation for the long pile, being a key step in the jacket installation, has been considered for further developments. With deep water developments, the sizes and weights of long piles are reasonably bigger. The corresponding process and equipment employed are subsequently altered, which brings challenges to developing a cost-effective, easy-operable approach for lifting operation. In this paper, the technology for the offshore long pile upending lifting operation including pile feature, installation methodology, lifting rigging and analysis model, covering water depths ranging from shallow to near deep water zone (60–300 m water depth) has been suggested. In addition, the applicability of the adoptable novel approaches has been discussed considering the practical project experience.

Logic Sequence Prevents Launching Devices Downhole Out of Sequence

Within the oil and gas industry, significant costs are often incurred by the operating company during the well-construction phase of drilling operations. Specifically, the operators cost to drill a well can cost tens or hundreds of millions of USD. One specific area where significant changes in drilling operations have occurred is in the offshore environment, specifically operations from mobile offshore drilling units (MODUs). With the ever-increasing demand for oil and gas, operators globally have increased drilling budgets in an effort to meet forecasted demand. However, the increased budgets are often eroded or offset by increasing drilling costs. Therefore, operators are continually in search of new technology, processes, or procedures to help improve drilling operations and overall operational efficiencies. One Latin America operator identified a common operation as a possible area where operational cost could be easily reduced through the implementation of systems that allow the manipulation of valve manifolds remotely. Additionally, operating such valve manifolds remotely enhanced operational safety for personnel, which was an equally important consideration. This paper details the evaluation of existing equipment and procedures and a process used to develop a new remote-control system using a machine logic control (MLC) that has been designed, built, tested, and deployed successfully on MODUs operating in Latin America.

An Experimental Investigation of Cylindrical Floater VIM in Current and Waves

VIM (Vortex Induced Motion) is one of the important issues in the safety evaluation for cylindrical floating offshore structures. The VIM is basically placed as a phenomenon to occur in strong current, but that also appears in current and waves in the sea where offshore structures are installed. The authors have recognized the phenomenon that the motion amplitude of a cylindrical floater in current and irregular waves together is larger than the sum of the motion amplitude in current and in irregular waves respectively in a VIM experiment. This VIM amplification phenomenon in the current and waves is remarkable when wave height is relatively low that has high occurrence frequency in the sea. It is, therefore, expected that the amplification phenomenon has large influence on the accumulative fatigue damages of the offshore structure’s mooring system. In order to make clear this VIM amplification phenomenon, the authors have conducted detailed VIM experiment in waves using a circular cross sectional mono-column floater model. The results of the VIM experiment in current and waves are described in detail in this paper. The results of the experiment in irregular waves show different characteristics for VIM amplitude in current. The results in regular waves show the effect of wave height and wave period on VIM amplitude in waves. Using those results, the mechanisms of the VIM amplification in waves are investigated.