A mooring system optimization program has been developed to minimize the cost of offshore mooring systems. The paper describes an application of the optimization program constructed based on recently developed harmony search optimization algorithm to offshore mooring design which requires significant number of design cycles. The objective of the anchor leg system design is to minimize the mooring cost with feasible solutions that satisfy all the design constraints. The harmony search algorithm is adopted from a jazz improvisation process to find solutions with the optimal cost. This mooring optimization model was integrated with a frequency-domain global motion analysis program to assess both cost and design constraints of the mooring system. As a case study, a single point mooring system design of an FPSO in deepwater was considered. It was found that optimized design parameters obtained by the harmony search model were feasible solutions with the optimized cost. The results show that the harmony search based mooring optimization model can be used to find feasible mooring systems of offshore platforms with the optimal cost.

A mooring system optimization program has been developed to minimize the cost of offshore mooring systems. This paper describes an application of the optimization program constructed based on the recently developed harmony search (HS) optimization algorithm to offshore mooring design which requires significant number of design cycles. The objective of the anchor leg system design is to minimize the mooring cost with feasible solutions that satisfy all the design constraints. The HS algorithm is adopted from a jazz improvisation process to find solutions with the optimal cost. This mooring optimization model was integrated with a frequency-domain global motion analysis program to assess both cost and design constraints of the mooring system. As a case study, a single-point mooring system design of floating production storage and offloading (FPSO) in deepwater was considered. It was found that optimized design parameters obtained by the HS model were feasible solutions with the optimized cost. The results show that the HS-based mooring optimization model can be used to find feasible mooring systems of offshore platforms with the optimal cost.

The extreme responses of a turret moored tanker are sensitive to non-aligned wind, wave and current conditions. Such conditions commonly occur in the Gulf of Mexico during the passage of the eye of a hurricane. Conventional design practice often relies on a collinear or, at best, a "guessed" non-collinear combination of 100-year environmental return period wind, wave and current conditions. Hence there is a need to derive response-based design criteria, i.e. that particular combination of wind, waves and current which most likely yields the 100-year return period response. The long term response characteristics of a turret moored tanker in deep water Gulf of Mexico conditions are investigated through the use of a comprehensive hurricane hindcast database. The effects of turret location and wave spreading are considered. The 100-year long term responses are compared against the short-term 100-year design responses derived from a 100-year hurricane design analysis. Response-based design criteria are then derived. Introduction Turret moored tanker based FPSO systems are widely used in many deepwater areas. Conventional design of such systems often relies on the assumption of a design storm event comprised of a collinear (or at best a guessed non-collinear) combination of 100-year environmental return period wind, waves and current. However, it is well known that the extreme responses of a turret moored tanker are sensitive to non-collinear wind, waves and current (Refs. 6-8). With few exceptions, the effect of non-collinear wind, waves and current has received little attention. Such events commonly occur in the deepwater Gulf of Mexico during the passage of the eye of a hurricane. The resulting effects on the motions and mooring line tensions may be significant as such systems have a natural tendency to weathervane, i.e. align themselves against the prevailing direction of wind, waves and current. This then poses the question of how well the "conventional" design recipe really works for these systems in significantly non-collinear environments. In order to address this problem the actual long-term response characteristics need to be investigated and the 100-year return period responses need to be derived. Response-based design criteria may then be stipulated to capture specific response characteristics, e.g. the 100-year maximum offset storm is that particular combination of wind, waves and current that most likely yield the 100-year return period offset. Notice that the associated wave height, wind speed and current speed for such a non-collinear design event may well be lower than those normally referred to as 100-year return period environmental criteria. Also, the 100-year return period offset storm, say, is merely intended to estimate the 100-year offset while other responses (e.g. roll or mooring line tension) should be ignored. Theory The long term Gulf of Mexico environment is described by means of a hurricane hindcast database of 35 storms over an 85 year period since 1900 (Ref. 3). The original database contains some 240,000 records with the hourly values of wind, waves and current parameters.

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.

The paper focuses on the global analysis of FPSOs in shallow water, with an emphasis on vessel motions and offsets, mooring and riser design. The paper emphasizes the complex hydrodynamic interaction between the vessel and the environment, and the prediction of the vessel responses unique to shallow water. This done by using two examples of FPSOs moored in shallow water using an external turret mooring system and a tower yoke mooring system. Results are presented that illustrate the unique nature of both mooring systems and their sensitivity to variation in the environment and system damping. The design of compatible riser systems for both mooring systems is also presented and discussed.

This paper describes the global mooring analysis conducted for the Terra Nova FPSO, located on the Grand Banks, offshore Eastern Newfoundland, Canada. The FPSO has a disconnectable internal turret mooring system that must withstand the 100-year storm environment, and be disconnectable to avoid collision with large icebergs. The global analysis of the FPSO vessel and mooring system focuses on the response of the system in storm conditions, and during offloading, disconnect and reconnect operations. The analysis was performed using sophisticated analytical and numerical techniques, integrated with a comprehensive model test program primarily for verification of the analysis. As the Terra Nova FPSO is the first such platform to operate in an iceberg region, a focus of the analysis was in studying the interaction of pack ice and icebergs with the FPSO system. The methodology used to analyze the FPSO system is outlined in this paper and some key results are presented.

Squalls are mesoscale sudden wind-speed increases that can occur worldwide and are a design driver for FPSO systems in areas the other design environmental conditions are relatively benign, e.g. offshore West Africa. Squalls are transient winds which rapidly reach a peak wind speed (up to 50 m/s) and then decay to low speeds in a span of 60 to 90 minutes. As squalls are transient phenomena traditional steady-state analysis techniques cannot be used for the global analysis or the development of the extreme response estimates. This paper focuses on the characterization of the squall environment and the impact of various parameters on the response of FPSOs. The responses of both spread and turret moored FPSOs are presented and the difference in response is discussed. The paper then focuses on a parametric study on a representative single degree of freedom model of a spread-moored FPSO with an emphasis on the estimation of the extreme response and its dependence on sample size.

Mooring tensioning systems for offshore floaters have evolved from rotary windlasses on ships into multiple options nowadays. These options include fixed or movable winches, either linear or rotary, driven by electric or hydraulic, and the most recent in-line tensioners which remove the on-vessel equipment. Selection of a tensioning system directly affects mooring performance and installation, hull design, as well as overall project cost, schedule, operability and reliability. This paper compares a combination of seven types of tensioning system for the mooring system of a deepwater platform. The options under consideration for the tensioning system include fixed or movable, electric or hydraulic driven, and on-hull or in-line tensioner. The pros and cons of different alternatives are evaluated in terms of design, installation, and operating considerations, and are compared against criteria including Technology Readiness, Cost and Schedule, Installation, Layout, Maintenance, In-service Tension Adjustment, HSSE (Health Safety Security Environment) Risk, and Track Record. It is found that all options, fixed or movable, electric or hydraulic driven, and on-vessel or in-line tensioners have their advantages and disadvantages, and need to be evaluated systematically to fit different projects’ needs. Fixed hydraulic chain jacks remain the most popular choices for production semis, with 12 applications out of 24 since the year 1994. Movable options have merits over fixed ones in capital expenditure, especially with high numbers of lines. However, movable options require extra equipment and operations to relocate the tensioning system and thus have shortcomings in mooring installation, tension adjustment, and HSSE risk. An electric option has advantage in maintenance, because it does not require a HPU and has no hydraulic oil or flexible pipes to be replaced. However, electric options are heavy and large, with complicated gear boxes, and require a specialized team. Without on-hull tensioning and handling systems, the in-line tensioners may significantly reduce capital expenditure. Additionally, they eliminate the notorious problem of splash-zone corrosion since the top chain is completely submerged underwater. However, this system requires surface vessel intervention for tensioning and re-tensioning, and increases project execution and schedule risk. All of these need to be taken into consideration starting from early through execution phases of projects. As the offshore industry moves forward with emerging new technologies, projects usually involve multiple choices as well as technical uncertainties and financial risks. Most projects with mooring systems will encounter the similar challenges on selecting a reliable and cost effective tensioning system. This paper can serve as a reference for a major capital project that is going to select the most suitable tensioning system. With the state-of-the-art information and industry practice on mooring tensioning systems, this paper can also service as a reference for updating new versions of API and ISO station-keeping codes.