DEEPER - Deepwater Analysis Tools
(This article was published in MARINTEK Review No. 1 April 2000.)
New computational tools for the design and verification of deep-water floating production units have been developed by MARINTEK and Det Norske Veritas (DNV) under the Deep Water Analysis Tools (DEEPER) Joint Industry Project.
One important motive for the three-year project is the fact that offshore activities have moved into depths which cannot be properly simulated in model tests. Tests can provide acceptable verification at scales up to 1:150 – for modelling complete systems, with current, a maximum depth of 8-9 metres is available at MARINTEK’s ocean basin, equivalent to around 1,350 metres.
It therefore made sense to develop numerical tools which could be applied regardless of the water depth. Some model testing was used during the project for verifying these tools. Separately Marintek has worked on the development of hybrid modelling and testing in the VERIDEEP project.
The NOK 16 million project was supported by five oil companies – Mobil, Norsk Hydro, Petrobras, Saga and Statoil – and several engineering and contracting companies – Offshore Design, Aker Engineering, Babcock & Wilcox, Brown & Root, ETPM SA, Kværner Oil & Gas, Umoe Technology and Australian Maritime Engineering CRC. The Research Council of Norway provided 40% of the financing of the project.
Four objectives were set:
- to increase the accuracy in prediction/simulation of deep-water floating structures;
- to extend current software capabilities;
- to share costs in the development and maintenance of computer tools;
- and to offer a maintained and commercial software tool for the analysis of deep-water floating structures.
And four main subject areas were established:
- deep-water mooring and floaters;
- deep-water marine risers;
- development of an integrated analysis tool, DEEPFLOATER
- and the development of statistics and analysis procedures
Fully coupled analysis of a line breakage incident by DEEPFLOATER.
As the water depth increases, an increasing proportion of the cost of installing a floater goes into the mooring and risers. Increased current loads can also be expected, creating a potential problem of risers colliding with each other. An important focus was on the coupling effects between mooring and floater dynamics. It has been customary to separate vessel and mooring analyses, but in deep waters the influence of the current on the vessel, mooring and risers necessitates a simultaneous analysis of the complete system.
In fact performing design and verification analysis for deep waters is much more involved from a numerical point of view than for shallow waters, - it is necessary to solve the complete system dynamics all at once. The situation is further complicated by the fact that less statistical data is generated because of the slower motions in deep water, with the result that the reliability of the statistics is decreased.
The tools developed in the DEEPER project enable critical calculations to be made with respect to various phenomena which must be guarded against. One of these, for example, is ringing, which can be a hazard in terms of causing extreme loading on a structure. In deep water the natural period of heave increases to the point where the energy from steep waves causes high frequency loads.
The design must ensure high natural periods in the structure to cope with ringing. The issue came to prominence with the Heidrun tension-leg platform and the Draugen concrete gravity-base platform, both of which were designed linearly. During testing, however, it was found that ringing would cause additional loads of 30-40%, which meant that parts of the platforms had to be redesigned although construction of both platforms was already underway, Krokstad says.
Another critical calculation concerns the risers, which are mainly subject to current loads which can cause vortex induced vibration (VIV). VIV is normally considered a long-term fatigue problem, but it can also come into play during riser installation when there is no pressure inside the riser, thus making it much more sensitive to forces in the external environment. Risers may break down during installation due to VIV. A lot of experimental testing on VIV has been carried out at MARINTEK, and considerable experience has been gained. A new VIV program, VIVANA, has been developed.
A screening of coupling effects was carried out to check whether it was possible to rely on more standard analyses only. Three water depth scenarios – moderate (500m), deep (1,000m) and ultra deep (2,000m) were considered for TLPs, FPSOs and Spar platforms. The study concluded that coupling effects are pronounced for FPSOs in deep water while such effects are not substantial for deep-water TLPs. Spars had moderate coupled response in deep watersbut coupling effects could be high in ultra deep waters.
DEEPER’s main computer deliverable is the DEEPFLOATER program, which consists of a number of core programs: SIMO (simulation of complex motion and operation), RIFLEX (riser or slender body analysis tool), and ADVANCE (a more general program for detailed structural riser analysis). A handbook of analysis procedures has also been produced.
Considerable efforts have been put into making the tools more user friendly. A graphical user interface (GUI) has been developed in order to reduce the burden of modelling and make it easier to identify errors.
MARINTEK and DNV have cooperated on several recent joint industry project within deep-water technology. This has been a fruitful cooperation, bringing research results directly into well-proven commercial software products.
(This article was also published in Offshore Magazine's issue of April 2000).
MARINTEK ref.: Jorgen Krokstad