The installation of two full-sized 80ton Deep Penetrating Anchors (DPA™) in August 2009 marked the end of a technological qualification process for this free-fall anchor concept for mooring of offshore floating structures. These anchors are 13m in length with four meter wide fins and were installated at the Gjøa Field in the North Sea off the West coast of Norway. Maximum pullout capacity is approximately 700tons. The anchors will be used by the mobile drilling rig (MODU) TransOcean Searcher for drilling and completion of the wells at the Gjøa Field. They are designed to take larger loads than what may be realized by the present MODU, but next generation rigs will be operating at the field in the future where 84mm chain will be used with an MBL in excess of 700tons.The anchors were installed with less than two degrees tilt and anchor tip penetrations 24m and 31m in a water depth of 360m using the AHTS vessel M/S Island Vanguard. Maximum velocity when they reached the seabed was 24.5m/sec and 27m/sec dropped from heights above the sea floor 50m and 75m, respectively. The successful installation and qualification of these anchors at the Gjøa Field has subsequently resulted in an anchor solution that can now be offered to the oil and gas industry as a viable alternative to present day solutions for anchoring of FPSOs and similar structures in soft seabed sediments.

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The Riser and Mooring project of the Norwegian Deepwater Programme (NDP) has undertaken a parametric study of marine growth on circular cylinders fitted with helical strakes. The study was performed at MARINTEK. The aim is to provide guidance on the effect that marine growth has on the dynamic response of the riser. The tests were conducted in a controlled laboratory condition where organic material is not allowed. Hence artificial marine growth had to be modelled and manufactured. Both hard and soft marine growth have been modelled. Further it is vital to document the properties of the marine growth. The present paper will discuss issues in modelling artificial hard and soft marine growth, as well as characterizing the growth for reference after testing. Results from the NDP test campaign will be presented in the paper.

The Riser and Mooring project of the Norwegian Deepwater Programme (NDP) has undertaken a parametric study on the influence of marine growth on the effectiveness of helical strakes to suppress vortex-induced vibrations on circular cylinders. Helical strakes are one of the most commonly used devices to suppress vortex-induced vibrations (VIV) on marine risers. The aim is provide guidance on the effect marine growth has on the dynamic response of a riser fitted with such VIV suppression devices. The tests were conducted at MARINTEK in a controlled laboratory environment. Artificial marine growth was modelled, manufactured and tested. Both hard and soft marine growth of various heights and coverages were tested. The present paper discusses some results obtained in this test campaign.

Riser fairings are designed to rotate freely about the riser axis and to passively align with the direction of incident flow so they will effectively streamline the flow and eliminate VIV (Vortex-Induced Vibrations). This rotational degree of freedom introduces the possibility of a complex dynamic phenomenon involving coupling between the hydrodynamic forces and the fairing / riser motions (e.g. cross-flow translation and rotation). Slocum et al. reported a scaled model test of a long flexible riser model with a freely-rotating riser fairing conducted at MARINTEK at OTC-2004. At low flow speeds, the test showed the fairings to be effective. However, at higher towing velocities they became unstable resulting in high displacements at its first bending mode. This paper presents the work related to a study of effectiveness of fairings and is one of several VIV research activities NDP (Norwegian Deepwater Programme) has conducted at MARINTEK in 2002–2007. The present work is partly a follow up activity to the work reported by Slocum et al. The purpose of the present work was to study possible higher order modal response of faired risers and in particular to find out if such response can occur in higher bending modes than the first. A vertically towed instrumented riser was tested with 2 different fairing designs in uniform current profile with different towing speeds. Fairing II was identical to the one used in Slocum et al (2004) while Fairing I represents an alternative design. The riser model was 9.32m long, had diameter of 20mm and was flexible. Both bare riser configuration and full coverage of the two fairing sets were tested. This study documented first-, second- and third-mode responses at high amplitudes (instability behavior) for Fairing II. Tests with Fairing I showed that the riser was stable, but the riser vibrations were found to be similar with respect to displacement amplitudes and frequencies to the bare riser VIV.