Accurate depth registration and high quality continuous sampling of seabed sediments at multiple locations from mundline are essential for shallow foundation design. This is particularly true for the siliceous silts and sands of the Taranaki basin seabed off New Zealand?s west coast.
OMV New Zealand recently commissioned the use of Benthic Geotech?s Portable Remotely Operated Drill (PROD) to carry out a detailed confirmatory site investigation at the Maari platform site. The PROD was able to make use of two short weather windows to complete successfully the required scope of work.
This paper describes how the PROD was use to deliver the site investigation results and illustrates how accuracy and quality were assured.
The Maari Field is situated in the Tasman Sea offshore the west coast of Teranaki, New Zealand, as shown in Figure 1. The field is approximately 80k m from the coast in a water depth of 101m and is the most south westerly of the other existing developments in the area. The Maui B platform is 36km to the weather events direct from the Southern Ocean, or funneled through the Cook Straits.
The development, currently in the project execution phase, comprises a not-normally manned wellhead platform (WHP) with associated floating production and storage off-loading facilities and flow lines. The WHP supports 12 conductor slots for the first eight wells that will be drilled from a jack up drilling rig.
Maari is a marginal field in a challenging environment. There are no precedents for platforms on shallow gravity foundations in this region. The site investigation had to deliver sufficient quality evidence to ensure verifiable design for a skirted shallow foundation without another investigation. This paper describes how quality and accuracy were achieved in the geotechnical site investigation.
Prior Regional Experience and Selection of Platform Concept
The Taranki Basin experiences challenging metocean conditions, which have combined either directly or indirectly with ground conditions to impact adversely on a number of projects. While these are not often publicly documented, Rennie et al. 1 have given detailed account of experiences at Maui A, while four pile were successfully drive to 70m, 24 piles met refusal in sand layer at around 30m. hammer setup on site led to less efficient performance, higher blow counts and ultimately pile refusal. The complex sequence of sand and silts encountered at Maui are shown in Figure 2 in term of pore pressure ratio, Bq and cone resistance, qc
A similar geology of inter-bedded sequences of sands and silts were anticipated for Maari, and the potential for similar shallow pile refusals was considered high. In addition, the high costs of mobilizing large pile driving equipment to this remote region, couple with the lack of suitable weather windows for the area, contributed to the decision for Maari to favour a self-installing gravity-based platform.
The platform concept chosen is from Arup?s range of ACE self installing steel gravity based platforms 2 and is similar in concept to the Yolla A Drill ACE platform installed in the Bass Strait, Australia 3