Adekunle Peter Orimolade

Waves in polar lows

In a rather stationary fetch, one would not expect large waves in polar low situations. However, the picture changes when one considers a moving fetch. The significant wave heights that may be associated with the recorded polar lows on the Norwegian continental shelf from December 1999 to October 2015 are estimated using a one-dimensional parametric wave model. A comparison of the measured and the forecasted significant wave heights in two recent polar low cases in the Barents Sea is presented. The estimated significant wave heights show that the values could have been up to and above 9 m. The forecasted significant wave heights considerably underestimated the measured significant wave heights in the two recent polar low cases that are considered. Furthermore, a generalization of the fetch-limited wave equation in polar lows is proposed, which allows the wind field to vary in space and time, and is shown to give results that are consistent with the one-dimensional parametric model. This article is protected by copyright. All rights reserved.

Vessel stability in polar low situations

ABSTRACT An average of 12 polar lows develop yearly in the Norwegian and Barents Seas, when unstable air is accumulating over the ice–open water interphase. Polar lows are intense and rapid, accompanied by high wind speeds, large wave growth, and large snowfalls. There are uncertainties in polar low forecasts, and marine activities should not take place along the possible tracks of such events, even if the waves would be less than the extreme waves for the area. The metocean (meteorology and oceanography) conditions in polar lows suggest that a vessel at sea may experience severe sea spray icing. In this paper, we present vessel stability concerns, and the effect of snow and ice loads on stability, in polar low situations. Stability analysis results show that the additional snow and ice loads have more influence on smaller vessels’ stability compared to larger vessels.

Vessel Stability in Polar Low Situations: Case Study for Semi-Submersible Drilling Rigs

ABSTRACT The additional snow and ice loads that a typical sixth generation semi-submersible in polar lows may experience are estimated and the effects on the intact stability of the semi-submersible are analysed. The additional snow loads could be very significant if the snow accrete on all the horizontal surfaces of the semi-submersible, and could have more impact on the initial stability of the semi-submersible when in the survival from operating condition. Ice loads from sea spray icing may not exceed 300 tons in any of the semi-submersible loading conditions, except when the icing event extends to the main deck of the semi-submersible. The resulting asymmetric ice loads will lead to increase heeling of the semi-submersible, with maximum heeling occurring when the icing event extends to the main deck. Generally, the additional snow and ice loads in polar lows are deemed to represent operation and safety challenges.

On weather limitations for safe marine operations in the Barents Sea

The Barents Sea wave conditions and the weather conditions that often accompany fully developed polar lows are presented in this paper. The studies on the extreme waves showed that the extreme waves in the Barents Sea decreases as we move further north. The Barents Sea wave conditions are found to be more accessible for marine operations in the summer months compared to the North Sea and the Norwegian Sea wave conditions. However, the quality of actual weather forecasts and the contribution of long periodic swells to the total sea in the Barents Sea could negate the longer weather windows observed in the area. Further, the weather conditions that often accompany fully developed polar lows represent limitations to marine operations in the Barents Sea. It was found that the significant wave height in polar lows could be up to 9 m. In addition, snow and ice accretion in polar lows are deemed operation and safety hazards. Overall, marine operation may not be carried out within the period when a polar low is likely to occur.

Steel Lazy Wave Risers From Turret Moored FPSO for Deepwater Harsh Environment

Steel catenary risers (SCRs) have found greater applications in deep and ultra-deepwater developments. However, the deployment of SCRs in conjunction with a high motion deepwater floater such as the Floating Production Storage and Offloading (FPSO) system faces significant challenges due to their high motion characteristics, especially in harsh environmental conditions. The challenges posed by FPSO’s high motion characteristics include severe dynamic response on the SCRs and poor fatigue performance at the top section and the touchdown point (TDP) area. A number of alternative configurations of the SCR can be employed to decouple the FPSO’s motion from the SCR, thereby improving performance, and this include the steel lazy wave riser (SLWR) configuration. The lazy wave is achieved by introducing buoyancy modules along some lengths of the riser.In this work, a suitable SLWR configuration for deployment in conjunction with a turret moored FPSO was developed for a typical deepwater offshore West of Shetland environmental conditions. The optimum configuration is a low lazy wave configuration; this was achieved after several analyses using ORCAFLEX software program. In determining the optimum configuration, consideration is given to the SLWR sag and hog bend heights, the net buoyancy force, the buoyant section length, and the hang-off angle, among others.The extreme response, considering a combination of 100-year wave with 10-year current was satisfactory; the maximum stress was below the allowable stress level, and the maximum DNV utilization was less than unity, indicating a safe design. The wave-induced fatigue damage was calculated using a total of 216 load cases, resulting from 12-wave directions, and the wave-induced fatigue performance was satisfactory, with the minimum fatigue life observed at the riser’s TPD. Fatigue damage resulting from vortex induced vibration (VIV) was calculated considering currents in the in-plane and the out-of-plane directions to the riser, with a total of 22 load cases. The VIV fatigue performance was not satisfactory, and therefore fairings and strakes will be introduced to some lengths of the SLWR to suppress VIV. Detailed sensitivity studies also showed how the configuration can be further optimized.Overall, the results of this study showed that, the SLWR is a suitable riser concept for deployment from a turret moored FPSO, in deepwater, harsh environmental conditions such as offshore West of Shetland. The riser can be installed using Reeled-Lay installation method. The installation can be performed using pre-lay, abandonment, and recovery, as this offers advantages over the direct transfer approach.Copyright

Impacts of Cold Climate Environmental Conditions on a Riser System Design and Installation

Interests in exploration and production of oil and gas in cold climate areas has increased in recent times. This can be attributed to the continual depletion of reserves in mature fields, and recent discoveries of large quantities of oil and gas in the cold climate region, including the more recent discovery of the Alta Reservoir, in the Barents Sea. However, marine operations in this region are faced with challenges resulting from its arctic conditions.Knowledge of the physical environment is important in designing offshore structures, and in planning, and executing marine operations. Selection of a suitable field development concept may be influenced by the probability of occurrence of rare events, such as drifting icebergs. Furthermore, occurrence of mesoscale phenomenon such as polar low pressures may adversely affect planned marine operations. In addition, uncertainties in weather forecasting will reflect on the available weather window to perform installation and interventions works.This paper presents some of the challenges in designing and planning for marine operations in the cold climate region. A possible field development concept for the open water areas of the Norwegian sector of the Barents Sea is discussed. The current research work considers the need for further assessment of the probability of occurrence of drifting icebergs as of importance when selecting field development concept. The Floating Production Storage and Offloading (FPSO) is proposed, and this should be designed with an internal turret system that can be disconnected and reconnected. Some of the challenges associated with riser systems design when considering a turret system with the capability to disconnect and reconnect are discussed.This paper also propose the use of ensemble forecasts as an alternative to the use of alpha factors to estimate operational weather window when planning for marine operations in the Barents Sea. The unpredictability nature of the environmental conditions, especially in the early winter is considered a challenge to marine operations.Copyright