Estimating installation loads while deployment or removal of equipment offshore is complex in nature due to the motions of the vessels and the hydrodynamic behavior of the subsea equipment in the water column. Dynamic amplification factors (DAF) are used to estimate the installation loads for lifting operations. DAF calculated using standard hydrodynamic coefficients and formulations is intended to be conservative to account for uncertainty, although this may not always be the case in practice.
Hydrodynamic performance of simplified scaled structures, circular plates of various porosities were tested in the lab for a range of parameters, including Keulegan-Carpenter (KC) number varying from 0.03 −2.5 and β parameter range from 20,000 to 72,000. The forced oscillation (FO) technique was implemented to study the variation of hydrodynamic forces due to proximity to the free surface boundary. The KC parameter was restricted depending upon the proximity to boundaries, such that structure was always under the free surface. The added mass and damping coefficients are found to be linearly dependent on KC number. The hydrodynamic coefficients variation with KC and β parameters is also found to be dependent upon free surface proximity. The added mass and damping coefficients are found to increase by a factor of about 2.5 and 4 respectively when the plate is oscillated very close to the free surface as compared to when deeply submerged. This increase is possibly due to the increased pressure difference below and above the plate that arises due to water cavity formation on top of the plate while oscillating close to the free surface.
There is an increasing number of offshore developments that utilize subsea based separation and processing. Modules of different shapes and weight (e.g. processing modules, manifolds, templates, suctions anchors, drag anchors, pumps) contribute to the seabed infrastructure. A number of large crane vessels that are used for heavy lifting, are purpose designed and equipped to perform heavy lift operations at sea. Some of these vessels are also capable of performing these operations in rather severe environmental conditions, Wouts et al (1992). The shapes of these modules are dependent upon the field requirement and are not always streamlined for easy installation purposes. The hydrodynamic behavior of only a few simple structures is available in literature, but not for all the complex structures used in the real offshore world. Rough estimations of hydrodynamic coefficients may lead to disasters if they turn out to be under predictions. Adam et al (1986) reported that installation of these structures is one of the main concerns. The conventional type of installation faces problems even though the structure is installed very slowly and safely. The non-conventional type of installation becomes even more challenging, as the degree of freedom of the structure is higher and the dynamics is far more complicated. These operations have limited tolerance in terms of final positioning of the structure. Environment can bring sudden unpredictable challenges on the load hanging off the crane wire, therefore it is obvious that these operations involve high level of risk. A sound knowledge of hydrodynamic coefficients is required and in-depth study using numerical and experimental analysis should be performed as a mandatory step towards the operation, Rawstron & Blight (1978). Oristland and Lehn (1987) analyzed added mass and drag coefficients of several generic structures while being lowered. Wadhwa et al (2009) published their findings on the hydrodynamic behavior of structures oscillating close to the seabed and showed the seabed proximity affects the hydrodynamic performance. The motion of the structure is found to be strong enough to move the artificial sand seabed. Added mass and damping coefficients are found to be increasing with KC parameter. Ireland et al. (2007) concluded that the heave added mass of the suction can is reduced with decreasing KC number, but also found it to be independent of the frequency of oscillation.