This paper describes a series of model tests aimed at gaining insight in the tension variations in the export risers and mooring lines of a CALM buoy. The test result were therefore not only analysed carefully, but were also used as input and to validate a numerical tool that computes the coupled motions of the buoy and its mooring system. The tests were carried out at a model scale of 1 to 20. Captive tests in regular and irregular waves were carried out to investigate non-linearities in the wave forces on the buoy for example from the presence of the skirt. Decay tests were carried out to determine the damping of the buoy’s motions and to obtain the natural periods. Finally, tests in irregular waves were carried out. The dynamics of the mooring system and the resulting damping have a significant effect on the buoy’s motions. A numerical tool has been developed that combines the wave-frequency buoy motions with the dynamical behaviour of the mooring system. The motions of the buoy are computed with a linearised equation of motion. The non-linear motions of the mooring system are computed simultaneously and interact with the buoy’s motions. In this paper, a comparison is shown between the measurements and the simulations. Firstly, the wave forces obtained with a linear diffraction computation with a simplified skirt are compared with the measured wave forces. Secondly, the numerical modelling of the mooring system is checked by comparing line tensions when the buoy moves with the motion as measured in an irregular wave test. Thirdly, the decay tests are simulated to investigate the correctness of the applied viscous damping values. Finally, simulations of a test in irregular waves are shown to validate the entire integrated concept. The results show that: 1. The wave-exciting surge and heave forces can be predicted well with linear diffraction theory. However, differences between the measured and computed pitch moment are found, caused by a simplified modelling of the skirt and the shortcomings of the diffraction model. 2. To predict the tension variations in the mooring lines and risers (and estimate fatigue) it is essential that mooring line dynamics are taken into account. 3. The heave motions of the buoy are predicted well. 4. The surge motions of the buoy are predicted reasonably well. 5. The pitch motions are wrongly predicted.

Recently, the phenomenon of out-of-plane bending (OPB) fatigue of mooring chain links emerged as an important parameter in the fatigue assessment of mooring lines. Vessel motions induce a bending moment at the top chain of a mooring line. This bending moment induces alternating local stresses in the link and thus contributes to fatigue damage of those links. High pretension mooring systems are particularly sensitive to this phenomenon, since a small vessel motion combined with a high tension results in a relatively large bending moment in the upper mooring chain links. In mooring systems with high pre-tensions, this damage is of much greater magnitude than the fatigue damage induced by tension-tension loading only. An extensive study has been executed to investigate the fatigue life of mooring chain in deep water systems. This paper presents the calculation procedure to include the effects of local chain bending in the overall mooring line fatigue analysis. It was concluded that despite the complexity of the OPB issue, it is a phenomenon that can be incorporated in the mooring analyses by means of numerical procedures. The developed method is based on extensive Finite Element Method (FEM) analyses of chain links. Models of multiple chain links have been used that take into account the plastic-elastic properties of the material and contact friction between chain links. The FE models are used to derive empirical relations, between load angles, interlink angles, bending moments and stresses. These calculations were made for different combinations of line tension, interlink friction and chain size. The results were stored in a database to gain insight in the out-of-plane bending phenomenon. This database provides empirical formulas to lead to the local stress in different points on a chain link. These empirical formulas are used to translate floater (vessel or buoy) motions into local stress variations and fatigue damages in chain links. The long-term motion behaviour of the floater is known, the long term tension and bending stress ranges can be obtained and thus a fatigue damage of the chain links can be calculated.

The Shtokman Gas Condensate Field (SGCF) is located 610 km from Murmansk in the Barents Sea. The water depth at location is around 340 m. The offshore facilities of the SGCF Phase 1 development will include ice-resistant ship shape disconnectable turret moored floating platform (FP). Significant sea ice invasions occur at Shtokman in approximately 3 out of 10 years, on average. Icebergs may also occur in the SCGF area. Ice and iceberg management are planned to support the FP operations. The present paper describes the methodology to assess performance, operability and risk of the FP in waters where occasional invasion of sea and glacial ice is anticipated. Introduction Challenges with ice-related design and operating philosophy for the Shtokman FP have been described in [Ref. 1]. In brief, the main challenge is to achieve an appropriate reliability level and at the same time minimize operational downtime at acceptable costs. Table 1 summarizes the main ice-related challenges as identified in the pre-FEED and updated during the FEED, categorized by the accidental scenarios. Actions have been taken in order to address the challenges and quantitatively assess the risks. It shall be emphasised that no attempt was made to target the design of individual components to achieve the target reliability level for the entire system, i.e. floating platform with respect to ice actions. This is simply because it would have been too speculative given all the uncertainties involved. On the contrary, it was decided early to establish challenging but realistic design targets for the main systems (e.g. hull, mooring and disconnection system) and in parallel, work on design of the operational measures, including assessment of their efficiency and reliability. Further, quantitative operability and risk assessments of the entire system have been performed to evaluate potential needs for optimisation. The objective of this paper is to present the approach developed and used for assessing performance, operability and risk of the Shtokman Floating Platform with regard to sea and glacial ice. The content of this paper is based on numerous studies performed to provide input for the Final Investment Decision.

Offshore oil and gas exploration and production comes with risk of release of hydrocarbons in to sea. Today, the probability of such an event is relatively low, but the consequences are nevertheless significant and can cover a vast geographical area. As such, it raises the question as to whether liability and compensation in oil & gas related incidents should be covered under international law. Current international legislation addresses the issue mainly in terms of shipping. This paper attempts to shed light on the topic in relation to oil and gas exploration whilst investigating notable events in the UK and the USA. The findings show that domestic laws of these countries cover the matter sufficiently. However, the question of whether the regulation should fall under international regulation can unfortunately not be answered with confidence as it would require a test-case of a situation where an oil spill affects multiple littoral states.

The combination of oil, LNG and LPG present at offshore fields may require a multi purpose production and storage facility that can produce the different hydrocarbons. In order to achieve feasibility, the required vessel size to handle multiple hydrocarbons is significant. HSE, technical and economical considerations led to the investigating of using two floaters to split the LNG / LPG storage from the production vessel. This paper deals with the intention to deploy two vessels in tandem: One producing unit that has storage capacity for produced oil and one barge that stores the LNG and LPG. Challenges to be met in the design of the tandem moored vessels relate to relative motions between the vessels: The relative motion is restricted by the transfer lines for products and the flexibility of a gangway that enables safe passage between the vessels. The mechanism that connects the two vessels should be capable of withstanding the forces to keep the vessels at the intended distance without growing into enormous proportions itself. This paper provides the technical background and the feasibility of the concept to connect two large barges in benign conditions. The concept has been developed based on existing tower mooring principles that is combined with a spread moored mooring arrangement. Additionally the paper discusses the sensitivity of the connector and mooring design on the overall performance.