Installation of a Production Manifold in 2000 ft Water Depth Offshore Brazil

Abstract

Abstract This paper describes the installation and related analyses of a subsea production manifold in 620 meters water depth at Campos basin, offshore Brazil. A pre-installed mooring system was used to moor the crane barge which lowered the structure. Time domain and frequency domain computer programs were used to evaluate the response of the system A sensitivity study was conducted in order to assess the added mass, damping, stiffness and excitation forces influences on the system response and the importance of such parameters in the computer model calibration is discussed. Model tests were performed in order to determine the manifold hydrodynamic coefficients to be input to a computer program to evaluate the displacements and forces during the descent of the structure. Offshore measurements were performed on the hoist forces, vessel motions, crane boom tip vertical motions and manifold vertical motions. The anticipated responses are compared with some of these measurements. The method used for lowering the manifold, the mooring system and the manifold orientation and positioning system are described. The design and installation strategy considered to adapt a conventional crane barge for installation beyond its nominal specifications, but considering safety and cost-effectiveness, are also described. Introduction In December 1995 the DL-2 production manifold was installed in 620 meters water depth at the Albacora field on the continental slope at Campos basin, offshore Brazil. The contract for fabrication was granted to a Brazilian company and the operator of the field, Petrobras, undertook the installation of the manifold through its owned 122 by 30.5 by t3.5meters pipe lay and crane barge BGL-1, The structure was first loaded out on 18* of November onto the deck of Petrobras transportation barge BS-5, transported to BGL-1 inshore and then lifted by BGL-1 crane and sea-fastened to the crane barge deck. The manifold was landed on the sea bed on 19th of December. The manifold dimensions are 19 m by 14.5 m by 7.5 m high and it weighs 4120 kN in air, which includes a foundation substructure, the manifold itself and a installation lifting tool. Shallow foundations were adopted and four 40 m2 footings were utilized to provide the required bearing capacity. For details about the functional features of the manifold please see OTC paper 8236. The seabed has a 5 degrees slope and the substructure was built so as to have the mudmats at the same inclination. The manifold had to be landed within +/- 10 degrees in heading, in a target area with 10 meters radius and within +/- 2 degrees in leveling. Should the later not be achieved, the manifold leveling system would be used. In this case anothervessel with drill pipe handling capability would be required. Installation planning The installation alternatives considered for lowering the manifold involved a drilling rig, a dynamically positioned crane vessel and a conventionally moored crane barge.