In this paper a comparison between model basin experiments and results of diffraction computations on side-by-side moored LNG carriers is presented. The computations are based on a new lid method in diffraction codes to suppress non-realistic high wave elevations between the two floating objects. This lid method was originally formulated by Chen (2005). In this method a damping value is added to the free surface by means of a damping parameter. Since no theoretical solution can be found to establish the required value of the damping parameter, model basin experiments have been performed to determine this value. However from the results of the model basin experiments it is shown that it is difficult to obtain one unique value of the lid damping (for the 4m or small gap). The way of tuning the damping value of the lid is crucial. Tuning the damping based on first order results, like motions or wave height RAO’s will lead to a much larger variation in the estimate of the second order sway wave drift force transfer function.

Estimation of green water occurrence and the associated loading on exposed structures is an important design consideration for FPSOs [Floating (Production) Storage and Offloading] in harsh environments. As FPSO concepts are being developed for deep and ultra-deep water Gulf of Mexico, this phenomenon must be understood for hurricane conditions and designs developed to alleviate its impact. This paper provides a mechanism for understanding and evaluating the green water phenomenon for FPSOs in harsh environments, both at the bow of the vessel and along its sides. The paper utilizes recently developed analysis and design methodologies, and presents criteria specifically developed for the Gulf of Mexico hurricane environment. A case study is used to discuss an FPSO for the Gulf of Mexico designed to resist green water occurrence, and the development of design environmental criteria from a longterm response analysis. The case study is also used to illustrate the design methodology developed, and to show the sensitivity of the green water loading as a function of freeboard exceedence allowed. Finally the paper presents guidelines that provide a strong foundation for the evaluation of green water for FPSO concepts in the Gulf of Mexico. Introduction With the expansion to deep and ultra-deepwater developments in the Gulf of Mexico, several Floating Production System (FPS) concepts are being considered to safely and effectively exploit the fields. FPSO [Floating (Production) Storage and Offloading] systems are a mature floating system technology for harsh environments, having been successfully deployed in the North Sea, the Grand Banks, and the South China Sea. These systems are attractive for the deepwater Gulf of Mexico, especially where the pipeline infrastructure is limited or non-existent. This is demonstrated by the interest in this floating production technology over the past five years and the fact that they are considered in most concept selection studies for the deepwater Gulf of Mexico. The main focus of most floating deepwater production facilities is on the subsea, mooring and riser systems, and other challenges associated with deep and ultra-deepwater. However, some important aspects in the design and operation of Floating Production Systems (FPS) are still related to the interface with the environment, especially in harsh environments. For FPSOs this applies to the weathervaning performance, tandem offloading with shuttle tankers, vessel motions, and green water loading. Obviously offloading to shuttle tankers and weathervaning performance are FPSO specific issues and do not apply to other FPS. However, vessel motions and green water related issues are shared between all floating systems, albeit to different degrees. A major design consideration for platforms in the Gulf of Mexico is the wave run-up and deck overtopping of deep draft caisson and semi-submersible platforms, and the airgap between wave elevation and the bottom of the deck for Tension Leg Platforms (TLP), semisubmersibles, and jacket platforms.

In the past seven years, joint industry project ‘Offloading Operability’ has been executed. The project developed a tool ‘Shuttle’ that can reliably simulate complex offloading operations, including those of future operations by LNG carriers loading at floating LNG production installations. The tool allows simulation of side by side moored vessels as well as in tandem configuration. Significant research was done to correctly simulate the wind and current shielding, wave resonance in between two vessels in close proximity and damping on the complicated natural modes of motions. The validation tests and full scale reality checks executed showed that the tool is capable to predict typical behavior and associated mooring loads of vessels during offloading operations. The paper discusses choices made in the simulation model, marine operations involved and feedback from mooring masters. Results of these simulations are compared to model tests experiments. The chosen simulation methods allow a reliable evaluation of critical weather conditions and the related downtime figures. Safety of the operation and possible contact problems can be evaluated during normal operation as well as during special safety cases. Introduction Environmental considerations limit allowance to flare gas. Combined with rising energy demand, transportation of LNG is considered around the world. Escalating construction costs and high local opposition to onshore LNG facilities coupled with geopolitical issues are key drivers to offshore offloading operations. Considering offloading operations in an early stage of the design increases the safety and reliability of gas transfer. Over the years, mooring masters gained significant experience in offloading of oil and gas from offshore terminals like Floating (Production) Storage and Offloading (F(P)SO) vessels to transport vessels and back to land based receiving terminals. With the increasing transportation of LPG and LNG, side by side offloading gained significant interest. The side by side operation is a proven methodology for ship lightering and is successfully implemented for the first few permanently moored structures in relatively benign waters. Computer programs are developed to study and increase availability of safe and reliable offloading operations. Offloading from one vessel to another involves operational and hydrodynamic challenges and requires focus on both man and mathematics. The weather limits are affected by experience of mooring masters and by assistance from tugs to reduce the hawser loads and relative motions. But loads on bollards and fairleads are also driven by complicated hydrodynamics and proper design of the mooring configuration (both between the vessels and to the seabed). Offloading operations have large impact on the design and operation of offshore terminals, because:–possible weather downtime of the offloading operation affects the overall economic performance of the terminal–the choice of the offloading system affects capital costs as well as operational costs of the terminal significantly–safe day-to-day operation requires clear procedures and trained personnel Finally, offloading operations have a large safety impact, because they involve by definition the operation of two or more structures in close proximity. The safety of personnel and the offshore structure, plus the possible environmental impact (pollution) has been of concern of regulating authorities as well as the industry itself. The subject of offloading plays e.g. an important role in the investigations of the US authority MMS related to FPSOs in the Gulf of Mexico.

Since 1996 Spars have been used as production platform in the Gulf of Mexico. Spar Vortex Induced Motions (VIM) in strong currents like the hurricane and loop currents are an important consideration for the design of the mooring system and risers. This is important for the extreme offsets as well as fatigue in risers and the mooring system. This paper compares the VIM behavior of a truss Spar in sheared currents, like the Hurricane current in the Gulf of Mexico, with tow test results. Experiments have been carried out on a scaled model in both a complete mooring system and in a towing set-up with a simplified horizontal mooring. The Spar model consists of a hard tank with removable helical strakes, a truss section and a square soft tank. The results of this model test program show that both the choice of the mooring system and current profile have a significant influence on the VIM response of the Spar. The paper discusses the results of this research and also addresses important issues and considerations for VIM model tests.

This work aims at characterizing the probability of wave impact and determining the position of impact on an FPSO (floating production storage and offloading platform) bow geometry. In order to determine the instants when impact occurs, an experimental program was performed on a specific bow shape. The bow was instrumented with pressure transducers and the test program, also making use of video recordings, was designed such that it was possible to determine the correlation between undisturbed wave shape and the impact pressure time traces. It has been found that the wave impact at the bow is highly correlated with the local wave steepness, which for very high waves has at least second-order effects. A comparison between the probability distributions of local wave steepness of the experimental undisturbed wave time trace and numerical simulations of second-order wave theory is provided and it confirmed that the latter is very adequate for calculations. The experimental results were further used to determine how the probability of impact varies with free surface vertical velocity. It was found that the significant wave height of the sea state itself does not have significant influence on the result and a regression model was derived for the bow type in the experiments. The proposed model for determining the probability of having an impact is based on combining distributions, adjusted a priori to the numerically generated second-order free surface vertical velocity, and the experimental probability of impact of a known certain seastate and free surface velocity. The analytical description makes it fast and easy to expand to other cases of interest and some example calculations are shown to demonstrate the relative ease of the procedure proposed. The position of the impact is determined by the nonlinear wave crests and the ship motions. The ship motions can be determined based on a linear response to the nonlinear waves considered.