Pile Monitoring of Two South China Sea Platforms, Xijiang Field.
Authors: Somehsa P. , Doyle E.H. , Ming Chen , Pus C.P. , Miner R.F. , Thomas R.L. 
Published: October 2008

Pile monitoring, using strain and acceleration gages mounted near the pile top during driving, was carried out during the installation of two offshore platforms in the Xijiang Field, in about 325 ft water depth off the southern coast of China. The soils in the area are generally normally consolidated clays intersected by dense sand strata. The measurements assisted the contractor to identified hammer problems and improve hammer performance, and assisted the owner in checking pile adequacy and wave equation analyses. POSI installation analysis of the data using signal matching techniques resulted in insights about damping and quake factors appropriate to the software, as well as soil resistances to driving (SRD) during continuous driving and after delays in the mixed sandand clay profiles encountered in this area, The findings should aid future installation planning in this area. Thismonitoring effort reflects a growing recognition of the value of dynamic monitoring to engineers and installation contractors involved with construction control or assessment of foundation adequacy, and for supplemental information in the event of unexpected conditions.

Introduction

Phillips Petroleum International Corporation Asia (PPICA) together with its partners, China Offshore Oil Nanhai East Corp. (CONHE) and Peeten Orient Company (POC), comprise Ihe PPICA group. PPICA, as operator for thePPICA group, recently completed the installation of two platforms, 24-3 and 30-2, in a water depth of 325 ft in their Xijiang Field concession, in the South China Sea. The platforms are approximately 13 km apari and about 128 km due southeast of Hong Kong, as shown on Fig 1, and were installed in 1994 and 1995, respectively, Soil conditions and pile capacities at the two sites are summarized on Figs 2 and 3, The two platforms are similar, eight-leg jackets supported by eight 72-in diameter piles at the corners, four 84-in skirts and four 60-in piles at the inner legs. Selected 72-in comer piles from each platform were instrumented with strain and acceleration gages mounted near the pile top to monitor pile driving. The wall thickness of these piles varies between 1.25and 2,50 in, The target vertical penetrations are 307 fi for the 24-3 jacket and 375 ft for the 30-2 jacket. The design compression capacity of these piles are 14,500 and 14,700kips, respectively.

The purposes of the instrumentation were to measure driving stresses and transmitted energy in order to evaluate pile installation and to assess soil resistance characteristics and pile adequaey. Pile monitoring at the Xijiang 24-3 platform was carried out by GRL and Associates, while for the Xijiang 30-2 platform, monitoring was carried out by Somehsa Geosciences. This paper presents the results as obtained in the respective reports and therefore permits not only a comparison of the raw data to determine differences inthe installation and ground conditions, but also of the analyses carried out to determine soil parameters and resistance.

The paper also describes the pre-installation planning, field instrumentation and pile installation control, and the results of a post installation study of soil resistances to driving.